CONTACT! ISSUE 101 PAGE 1
PO BOX 1382 Hanford CA 93232-1382 United States of America 559-584-3306 Editor@CONTACTMagazine.com
Volume 17 Number 6 March - June 2010
Issue #101 MISSION CONTACT! Magazine is published bi-monthly by Aeronautics Education Enterprises (AEE), established in 1990 as a nonprofit corporation, to promote aeronautical education. CONTACT! promotes the experimental development, expansion and exchange of aeronautical concepts, information, and experience. In this corporate age of task specialization many individuals have chosen to seek fresh, unencumbered avenues in the pursuit of improvements in aircraft and powerplants. In so doing, they have revitalized the progress of aeronautical design, particularly in the general aviation area. Flight efficiency improvements, in terms of operating costs as well as airframe drag, have come from these efforts. We fully expect that such individual efforts will continue and that they will provide additional incentives for the advancement of aeronautics. EDITORIAL POLICY CONTACT! pages are open to the publication of these individual efforts. Views expressed are exclusively those of the individual authors. Experimenters are encouraged to submit articles and photos of their work. Materials submitted to CONTACT! are welcomed and will become the property of AEE/CONTACT! unless other arrangements are made. Every effort will be made to balance articles reporting on commercial developments. Commercial advertising is not accepted. All rights with respect to reproduction, are reserved. Nothing whole or in part may be reproduced without the permission of the publisher. SUBSCRIPTIONS Six issue subscription in U.S. funds is $25.00 for USA, $35.00 for Canada and Mexico, $47.00 for overseas air orders. CONTACT! is mailed to U.S. addresses at nonprofit organization rates mid January, March, May, July, September and November. Please allow time for processing and delivery of first issue from time of order. ADDRESS CHANGES / RENEWALS The last line of your label contains the number of your last issue. Please check label for correctness. This magazine does not forward. Please notify us of your date of address change consistent with our bimonthly mailing dates to avoid missing any issues. COPYRIGHT 2010 BY AEE, Inc.
It’s now been 18 months since EAA and CONTACT! Magazine published the first issue of Experimenter. The electronic newsletter is being received very well and it’s bringing CONTACT! some muchneeded exposure. Those of you who subscribe to Experimenter may have noticed some recycled content from CONTACT! published in the pages of Experimenter. This is being done intentionally as an effort to entice Experimenter readers to consider subscribing to CONTACT! by showing them what they’ve been missing. In that same manner, this issue of CONTACT! Magazine contains an article that was published exclusively in Experimenter, to hopefully entice you to consider subscribing to Experimenter. For those of you who are already subscribers to Experimenter, please note that this article is not exactly as-published in Experimenter. In traditional CONTACT! Magazine fashion, it contains far more information than the Experimenter version of the same article. To subscribe to Experi-
menter, you don’t have to be an EAA member, but you do have to subscribe to the newsletter by going to the EAA website. Please don’t consider Experimenter a replacement for CONTACT! Magazine. We still need your support to be able to produce both publications. Just consider Experimenter an extension of CONTACT! There will always be higher quality articles in CONTACT! to make it worth the subscription fee, but with the addition of Experimenter, please consider that the value received from your subscription has tripled.
LOOKING FOR DONATIONS With the cost of living and the cost of doing business ever rising, the last thing We want to do it increase any of our fees. The US Postal system is about to raise their fees once again, and in order to hedge against all the overhead increases, I’m asking you to consider making a tax-deductible, charitable donation to CONTACT! Magazine. Your donation doesn’t have to be in the form of cash, it can be virtually any material object we can resell. Aircraft, engines, instruments, automobiles, or even frequent flyer miles. We probably spend over $1500 each year on airfare, traveling to the shows and if you have miles sitting there, please consider helping us out with them. ~Pat
Jake Jaks Pober Junior Ace.— We caught up with Jake at Sun ‘n Fun 2009 and bring you his story of building his Corvair powered and the associated problem solving.
Revmaster R-2300 Conversion.— The fine people at Revmaster Aviation have done it again, increasing the bore and stroke of their renown R-2100 to up the horsepower safely.
Talk On Tailwheels.— Scott Weinberg of Iron Design LLC has paid a lot of attention to the mass production of a quality and durable tailwheel assembly for the experimental market.
Propeller Blades: Number, Pitch and Planform.— Paul Lipps expounds in great detail in an effort to describe his propeller design theories in a simple manner.
On the cover: Jake Jaks Pober Junior Ace
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Patrick Panzera It's always an honor to complete an experimental aircraft and have the designer look it over and give it the thumbs-up. Such was the case with Jacob Jaks and his Pober Junior Ace when he had the opportunity to show it to Paul Poberezny at the 2009 Sun n' Fun fly-in. That meeting is only a small part of this story of building the plane from scratch, deviating from the plans just enough to successfully install a Corvair engine. Jake Jaks is the second of three generations of pilots, starting with his father who earned his wings in 1945, at the age of 17 and while still in high school. He graduated early to enter the Army Air Corps flying cadet program but never actually flew for the military. After the war, college, and marriage, he took over her family’s farm near Wheatland, CA and raised his family. Jake was born in 1959 and grew up on that same family farm. The flying gene however comes from both sides of his family tree, as by the time he was a toddler, Jake's mother had obtained her private pilot’s certificate, going on to fly in a couple of the Powder Puff Derbies in the mid 60’s. Jake's first airplane ride was in the back seat of his dad’s T-6 Texan when he was only two. By the time he was in grammar school, his grandmother obtained her private as well. As with most of us, Jake's youth was spent building model airplanes, dozens and dozens of them, both plastic displays and airworthy balsa control-line types powered by the venerable Cox .049, many of which saw airto-air combat with his younger brother's creations. Jake took up structured flying lessons at the age of 15, soloed on his 16th birthday, and had his private a few days after his 17th birthday, just like his dad. With a $1,500 loan from the local bank, Jake was able to buy half interest in an Aeronca Champ. When Jake was a junior in high school, his dad began construction of a Volksplane II and enlisted Jake's eager help. Once airworthy, Jake was able to fly it and the champ quite a lot, logging somewhere around 200 hours by graduation. Jake applied to only one place for college, the Virginia Military Institute, and with help from a Marine Corps scholarship, he was guaranteed a flying slot upon graduation. Jake's military career was short but fulfilling, doing an 18 month stint as a T-2C (Buckeye) instructor and then on to flying F-4 Phantoms in South Carolina, Japan, South Korea, the Philippines, and Okinawa. Jake www.ContactMagazine.com
met his bride Donna while in school in Pensacola and they had two sons during their military stint, Sam and Jacob. In 1987 Jake returned to civilian life and moved his young family to Oroville, CA, where he began working as a civil engineer, his major at VMI. Jake’s and Donna's third son, Doug, was born in 1989. In 1990 the family bought their first Mooney and within a week had the entire family loaded and flying east to visit Donna’s family in Pensacola. The plane saw many family flights until sold in 1997. A newer Mooney replaced it in 2005 and being very happy with this plane, Jake can see owning it for quite a while. This brings us to the third generation of military pilots, Jake's sons. Sam is now an F-15 Crew Chief at Bagram AB, Afghanistan. Jacob. just reported to NAS Pensacola for flight training. Doug, has taken some flying lessons, but is presently stymied by college, girlfriend, and finances, (Jake's not sure about the order), and is delaying his training. Jake has accumulated roughly 2,300 hours, holding commercial, multi-engine, and instrument ratings
BUILDING A PLANE Like many of us, Jacob (Jake) Jaks was a little undecided when it came time to choose a plane to build. He knew he wanted to build something, from scratch, but really didn't have a firm mission in mind except 'low-andslow' was a main criteria. Of course that term should immediately conjure the visualization of the venerable
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Pietenpol in any aviation enthusiast’s imagination, which is the direction Jake initially took. Plans in hand, but no material purchased, Jake and his wife Donna flew their trusted 1963 Mooney (which has since been replaced by a 1982 version) from their home in Tallahassee Florida to Oshkosh in 1994. While at OSH they saw Paul Poberezny's Pober Junior Ace sitting on the flight line. The coziness of side-by-side seating (and relative roominess as compared to the Piet) as well as the overall lines sold them on it. Although the plane didn't have much of a commute to get to the event, it was flown for exhibition nearly every day. Returning to Florida, Jake ordered a set of Junior Ace plans. The Pober Junior Ace is credited Any craftsman can appreciate the artistic blending of wood with metal for the launch of an industry, as well as the Experimental Aircraft Association, when the “The spars were a thing of beauty when they arrived,” plans to build it were published in Popular Mechanics in Jake told us, “17’ long sections of vertical grain clear the early 1950’s. It is lightweight, considered by many to spruce. You can bet Jake was careful when planing the be easy to build and fun to fly, and requires minimal edges to fit the ribs. All of the wood Jake used, except maintenance. It’s not an original design however, but for the maple blocks that hold the internal bracing, is airrather a modified version of the Corben Baby Ace. A few craft grade. The ribs have mahogany plywood gussets of the modifications include widening the fuselage, reand spruce members. designing the horizontal stabilizer to ease construction, and the addition of modern aircraft wheels and brakes. THE ENGINE With nearly a 34’ wingspan and Clark Y airfoil, the Pober Construction was slow for Jake, putting family and inJunior Ace, designed by EAA founding president Paul come above building. Jake had it extra difficult having Poberezny, is docile in stall and landing speed. The airjust moved from California and was setting up shop as a frame is constructed from 4130 steel tubing. The strutself-employed civil engineer. But with the support of his braced parasol wing is made from all wood construction. family and help from his sons and his father, the project The recommended powerplant is a Continental C-85. eventually got to the point of needing an engine decision.
Jake and his father putting the finishing touches to the wing structure.
Taking the completed wing structures to the airport for assembly prior to covering.
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Being discouraged that the certified engine route would be cost-prohibitive, Jake started looking at auto conversions. Jake was already familiar with the Volkswagen conversion (flying his father’s Volksplane) but it would not be an appropriate replacement for the C-85 specified for the Pober. While perusing EAA’s Sport Aviation for engine options, Jake came across an ad for William Wynne's Corvair conversion manual. So he bought it. Using a tip found in the manual, Jake located the local CORSA Corvair Club (a national organization dedicated to keeping the Corvair automobile on the road) and one of the members invited him for a visit to his ranch to pick a core engine. What Jake was looking for was a 110 horsepower engine, preferably from a 1965-1966 automobile. Armed with the serial number codes that identify that range, Jake made the trip to his new friend's compound. I use that term as what Jake found when he arrived was a family-owned parcel of about five acres with multiple homes located on it, surrounding a good-sized barn. The barn contained not less than 40 Corvair engines lined up just sitting there waiting for a new home. "I just went around until I found the serial number that I wanted." Jake said. Armed with a three-quarter inch box wrench (another Wynne tip) Jake was able to make sure the engine rotated two full revolutions by torquing the bolt on the end of the crankshaft. Happy with his selection, Jake handed the owner $125 and went on his way.
CORVAIR COLLEGE Right about that same time, Wynne started inviting those who purchased his manual to come to his hangar in Port Orange, FL to use his tools and to take advantage of his tutelage to open, inspect, clean, rebuild, and potentially run their engines. Opening one of these core engines that have been sitting in a barn, or bolted into a wrecked car for who knows how many years (decades in many cases) is akin to opening an oyster hoping to find a pearl. They all look horrid on the outside, and many look the same (or worse) on the inside, but the majority contains that pearl, each of a varying degree of gem quality. I've seen some people open their engine to find all new parts inside, while others have basically had to toss it and start over, with the majority somewhere in between. As a digression, William Wynne's happening (to use an old hippy term) started becoming a regular event and were soon dubbed "Corvair College." The concept has been a huge success, with each one being more successful than the previous. The most recent event was Corvair College numbered 17 (March 18-21, 2010 in Orlando) and was a tremendous hit. Corvair Collage #18 will be in Livermore CA in late 2010. Visit www.FlyCorvair.com for more info. Jake, alumnus of the very first Corvair College, arrived there with his engine already disassembled and cleaned and with fresh new parts in hand. New parts for the Corvair engine are easy to come by, most of which are just a phone call away. His heads had been remanufactured by a local (trusted) machine shop so when he arrived on campus, it was just a matter of assembly, which went www.ContactMagazine.com
swimmingly. By the end of the weekend, the old serviceable engine parts had been inspected, painted (where applicable), joined with the fresh parts, and fully assembled under the guidance of William or one of the other experienced attendees. Jake took his freshly refurbished engine back home at the end of the weekend. Not ready to be installed, Jake thought it best to keep his engine in his air conditioned office where it quickly became a conversation starter with the curious customers of his engineering business.
Jake’s sons Sam (left) and Doug, helping Jake get the Corvair engine to the airport to mount on the firewall.
Eventually Jake got to the point where he couldn’t hold off covering his airframe any longer. Everything else that could possibly be done before covering was complete, except installing and running the engine, so that was the nest step. With a cobbled “hood” as a cooling plenum (not shown) and all the necessary systems installed and ready to be monitored with the proper instrumentation, Jake ran the engine for the first time. Once the hiccups were ironed out, the plane was completely ready for cowling, fabric, and paint.
Donna Jaks, inspecting the workmanship of the initial engine installation.
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READY TO FLY Jake and his father next to the Junior Ace skeleton with the running engine installed. Nothing left to do except cover and paint.
The covering process was poly-fiber. Jake was planning to go with a dark blue and yellow, like the Volksplane of his youth, but his son Dougâ€™s favorite color was green, so forest green and federal yellow. Now, whenever Jakeâ€™s father sees it, he asks how much John Deere paid to have him paint it those colors.
Taxi testing began in 2004. Soon after, the FAA inspection was completed and NX1028A was ready to fly. The original aluminum-skinned cowl had a hand-made fiberglass nose bowl and traditional aluminum baffling with silicone seals.
The plaster mold used to create the fiberglass nose-bowl.
Tail feathers hanging for spraying/drying in the backyard.
The engine was not set up with a starter and had no true electrical system other than just enough to keep the electrons flowing to and from the ignition coils. The plane was flown in this configuration for approximately 30 of the requisite 40 hours.
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Wings suspended from hangar roof, getting ready to take the fuselage to William Wynne's to install the refurbished engine w/starter, alternator, and new crankshaft.
For the first 30 hours of flight time, the homemade wind generator provided enough electricity for the automobilestyle ignition system used in a William Wynne Conversion.
The hand-propping aspect began to become bothersome, especially with the overly thin Warp Drive propeller not giving much to grip. Coupling this with the propeller's low moment of inertia and the engine's relatively high compression ratio, it was becoming quite the chore to get the engine started. The elevation of the centerline of the propeller, as measured from the ground, was not very ergonomic either. Jake had enough. A call to William Wynne resulted in the decision to convert the engine from hand-prop to electric start. This is really a straight-forward, bolt-on conversion, with the majority of the work being more electrical in nature than mechanical. In essence, the CNC prop hub is changed out for one that incorporates a bolt circle for the flex-plate and a pulley for the fan belt that drives the John Deere permanent magnate dynamo that is bolted to the front of the engine. The gear-reduced starter is modified by Wynne to keep the profile as low as possible and is also fitted with a different gear, one with teeth compatible with the flex plate’s teeth. With the installation of his custom bracket, the starter is bolted to the top of the engine and aligns perfectly with the flex plate. Then it’s just a matter of wiring, which as [previously stated, is probably more work than installing the hardware. At this same time there was (in essence) an airworthiness directive mandating that any Corvair conversions that were currently being flown or destine to fly in fast aircraft (such as a KR-2, Dragonfly or even a CH 601 XL) must have the crankshaft inspected (Magnaflux) and be treated with a surface-hardening process called nitriding. (see CONTACT! issue #85) So when William was presented with an opportunity to help one of his customers convert to electric start and to change the crank for the sake of safety (even though it wasn’t necessary for the “low-and-slow” Pober), he went out of his way to help Jake. The decision was made; the engine was pulled and delivered to William’s hangar. www.ContactMagazine.com
The revised installation with several new items including an electric starter.
William has designed, tested and manufactures a complete line of bolt-on parts for converting Corvair engines so it really wasn’t necessary to bring the engine to William's shop. Jake could have easily ordered the individual parts and sub-kits William offers (even the new nitrided crank) and converted his engine himself, just as any Corvair engine builder can. It was just more convenient for Jake to truck his fuselage to William and leave it in his capable hands. Unfortunately, the installation of the starter on the front of the engine necessitated a new cowl, which also meant a new baffling system and a new spinner, which gave the airplane a completely different look. When Paul Poberezny saw the plane with the new nose bowl (another off-the-shelf William Wynne item, see the photo on page 3), he commented with an uncontainable grin, “That’s one streamlined-looking nose on that thing.” That made Jake’s day for sure. Along with the new starter and the permanent magnet dynamo (generator), Jake revised the intake system, and extended the exhaust to compliment the revised cooling air outlet. The electrical system also afforded him the luxury of a transponder and radio.
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So with the new cowl completed and phase one still in effect, Jake began taxi-testing. All this time Jake was using the stock (approved) oil cooler with satisfactory results. But as many of us know, a small change to the airflow under the cowl can net huge results – some good and some not so good. Testing showed that the oil wasn’t getting cool enough for Jake’s comfort level. Cylinder head temps were getting a bit high, too. “I ground tested it a lot; probably ground tested it more than I should have when it comes right down to it. I eventually decided that I had enough testing, it’s time to change some things around and get more air flowing through this thing.” So Jake removed the cowl and stared to rework the baffles when he discovered an oil leak and worse still, a blown head gasket. One phone call later to William seeking at a minimum a shoulder to cry on, netted the offer by William to bring the engine back, still installed on the airframe, so that’s what Jake did. Once William inspected the engine, the blown head gasket was verified, the root cause being detonation. Jake left the engine and airframe with William and went back to Tallahassee and started modifying the cowl with a little tweak to the William Wynne nose bowl, opening the inlets a little and adding a lip to the underside of the exit seemed like it should increase the flow through the engine compartment. Meanwhile, William was getting the engine back in top form. Once William was finished with the engine, Jake made the trip back to pick it up, but not before both of them ran the engine on William’s test stand ensuring everyone was satisfied with its operation.
Doug Jaks, Paul Poberezny, and Jake Jaks, SnF 2009.
Completing phase-one just in time for Sun ‘n Fun 2009, Jake set his sights on making that trip with his youngest son, and that’s where we caught up with them. We had a rare opportunity to coordinate the introduction of Paul Poberezny to Jake’s plane early one morning. I’m not sure who was more excited to make this meeting, Paul or Jake, but they hit it off like long lost buddies. It seems that they had already developed somewhat of a relationship during the build.
Back home with the resurrected engine, Jake continued to refine the installation with the addition of a larger oil cooler by removing the stock cooler from the stock location (just behind the #2 cylinder) to outside the cowl on the side of the fuselage. With some successful taxitesting, Jake was back in the air with some pleasing temperatures. Oil is still a bit high, hovering around 230 degrees (most likely as a result of poor sensor location) but cylinder head temps are down below 350 at all times.
BACK TO THE AIRFRAME Part of Jake’s success can be attributed to sticking to the plans. With rare exception, Jake followed the plans to the letter, even as updates (addendums) arrived in the mail, most of which were caught in time. “The Jr. Ace was built strictly to the plans, except for the use of the Corvair engine, which is itself on conformance with William Wynne’s installation recommendations. The fuselage was welded using oxy/acetylene. “I would have liked to have used a TIG outfit, but it was very expensive, and I would have needed to have the cutting torch capability anyhow.” Jake told us.
Looking out the windscreen from the left seat, one can easily see the new oil cooler.
Only one addendum caused some rework as the plans contained an error that at the time, seemed wrong to Jake, but he was bent on following the plans verbatim.
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OVERALL OPINION The entire process of building his own plane, as somewhat of a family affair, has been very rewarding. When he started, his three children were three, six and seven years old and demanded a lot of Jake's time, but as they got a little older, they were enlisted to help the build process. Fifteen years later they are all adults, having grown up in a home where an aircraft was built from scratch. Jake's youngest son, Doug, was fortunate enough to have flown with his dad to SnF meet Paul Poberenzy. On the other hand, maybe it's Jake who is the fortunate one, having the opportunity to build a plane with his family and to make the Sun 'n Fun trip and the meeting with Paul with his son. Jake's other two sons were certainly influenced by the build. His eldest is currently serving his second deployment in Afghanistan as an F-15 crew chief and his middle son is at Pensacola for flight training.
Jake taking his father for his first flight in the plane after phase one was completed .
As it turned out, his instinct was correct and one of the welded-in cross members had to be removed and repositioned. There were a couple of times during the build where Jake had a question. “A phone call to the EAA they put me right through to Paul every time.” “I probably talked to Paul at least three or four times, with different questions, and there were a couple of times during the building when I’d see him at either Sun ‘n Fun or Oshkosh and I’d get a chance to talk to him again about my project. Paul was valuable encouragement for me during the building process.” With no active builder's group that Jake was aware of, he pretty much felt alone during the build process, so speaking with Paul the few times he did went a long way. Along the way Jake met a couple of Corbin Ace builders, but there’s enough difference between the Corbin and what Jake was building that he didn’t glean much in the way of technical support. But when it came time for the engine, the peer support can hardly be matched.
PERFORMANCE Overall Jake is pleased with the performance, hitting the advertised numbers with relative ease. “It stalls about 35, cruises about 70. Empty weight is 822 pounds and gross is 1,320 all within the sport pilot limits. The one thing I would change is the angle of incidence on the wing; I think it’s got too much so I ended up having to shim the tail surfaces to counter that, to give me a little more nose flying down attitude.” It turns out that after Jake completed the aircraft and was amid phase-one flight testing, it was brought to his attention that there is a mistake in the cabane drawings. The aft members should have been an inch and a half taller, which would reduce the angle of incidence. www.ContactMagazine.com
But as the saying goes, all good things must come to an end, as the same is true for this project. The Pober is one too many planes for Jake's hangar and it must go. He's of course not thrilled about getting rid of it, but if you are looking for such a plane, send him an e-mail and maybe you can work something out. Jrjaks@jaksengineering.com Tallahassee, FL 850-386-2058
Pober Junior Ace Specifications Top Speed
288 statute miles
Rate of Climb
168 sq ft
No. of Seats
Steel, wood, fabric
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Patrick Panzera Editor@ContactMagazine.com Introduction by Tim Kern email@example.com Revmaster Aviation has finished development of its latest upgraded engine and the results are in: more horsepower at any usable RPM. The new Revmaster R-2300 (2331 cc) engine main- Revmaster’s breakthrough R-2300 engine offers more horsepower at a Lower cruise tains Revmaster’s renowned pro- RPM that previous versions. prietary systems and parts including its RM-049 heads that feature large fins and hemiRevmaster has been in the engine business since 1959, spherical combustion chambers. It maintains the earlier starting out as a remanufacturer of the early 36 HP enR-2200 engine’s maximum 82 horsepower at only 2950 gine that was introduced in the Volkswagen beetle. In RPM continuous, but offers 85 ponies for takeoff at 3350. 1960 the VW was upgraded to the 40 HP engine that has become the cornerstone of VW flight engines. Around The additional power ultimately comes from a 94 mm that same time, Revmaster developed a 2000 cc version bore plus lengthening the stroke to 84 mm, but that’s of the VW for the experimental aircraft market by first oversimplifying things. “We’ve put a lot of energy into this manufacturing target drone engines for Northrop Corporedesign,” says Joe Horvath, president and founder of ration. Revmaster spent about two years in this endeavor Revmaster Aviation. “On paper it looks like just a few before discovering thrifty and resourceful homebuilders minor modifications, but we’re really closer to a complete were using some of these drone engines in experimenrework of the internals: crank specification, connecting tals. With many of the installations being highly successrods, pistons and cylinders are all new.” The longer ful, Revmaster decided to go in that direction. Now with stroke results in greater displacement, longer connecting well over 40 years experience in the homebuilder market rods yield better vibration and power characteristics, the and literally thousands of engines sold, Revmaster is lower cruise RPM allows the use of longer propellers, announcing the latest addition to their successful line-up, and the higher peak horsepower can be felt in shorter the R-2300. takeoffs and steeper climbs. Although the Revmaster is based on the VW engine, not Strength and reliability are boosted by Revmaster’s fourmuch of the original engine remains. From a proprietary main-bearing, 4340 forged steel crankshaft (boasting crankshaft to proprietary heads, including a modern elecnitrided journals) that runs on huge (as compared to a tronic ignition with individual coils for each of the eight stock VW) 60 mm center main bearings. Thrust is hansparkplugs, this is not the old shake-and-bake VW condled by the custom-installed 55 mm #3 bearing at the version of yesterday. It is more a purpose-built aircraft prop end of the crank, formerly found at the other end. engine than it is an automobile engine conversion no Fully utilizing its robust, proprietary #4 main bearing, the matter how it's measured. Revmaster crank has built-in oil-controlled variable-pitch THE CRANKSHAFT propeller capability, a feature unique in this horsepower Connecting the propeller is always the most difficult part range and exclusive to Revmaster VW conversions. of adapting an automobile engine to aviation use. Not Unlike other VW conversions, props other than wood are that it's particularly difficult to physically accomplish, but usable on any Revmaster engine of any vintage. www.ContactMagazine.com
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rather that the loads imposed on the crank by the propeller are considerably different that those in an automobile. Potentially, the worst of these loads are gyroscopic in nature, although some might argue that torsional loads, especially harmonics that can be amplified by the propeller, are much worse if not a close second. Throughout the years, ever increasing displacements have multiplied the strength of the power-pulses and have amplified the propeller effects. And through trial and error, it's generally accepted throughout the VW engine community that the propeller used on a Volkswagen conversion should be wood and be as light as possible. This is not the case with the Revmaster and carbon fiber, aluminum and even variable-pitch propellers are open for consideration. Throughout the decades that experimenters have been flying behind the VW engine, there have been a number of different ways used to attach the propeller hub to the crankshaft, most of which have been to simply bolt a custom hub to the pulley end of the otherwise stock crankshaft using the same method that the generator fan belt pulley is retained. It wasn't until Revmaster cleanslated the crankshaft design to include a precision taper fit of the hub to the crank that the VW conversion crankshaft was made robust enough to handle props other than wood. This fourth bearing rivals that of any certified horizontally-opposed aircraft engine. Just behind the steel cam drive gear is the #3 main bearing. Where this would be a normal plain bearing in a stock VW, Revmaster has machined the case to accept one of their custom thrust bearings they manufacture in house.
substantially more surface area than the original three combined. It replaces the oil slinger, the ignition timing gear and the comparatively insignificant automotive front bearing that's designed to carry only the fan belt loads. The case is line-bored to accept the new fourth bearing as well as the larger-than-stock Revmaster main bearings, and the adjacent engine case web is machined to accept the otherwise stock VW thrust bearing that normally resides at the opposite end.
Where the stock VW crank steps down twice to smaller diameters and has two keyways, one for the distributor drive gear and the other for the fan belt pulley, the Revmaster crank has been beefed-up and then precisionground for the 3 degree taper.
Other VW engine conversion companies have tried to emulate the design (a short list would include HAPI, Great Plains and now AeroVee), but none have come close to the total package Revmaster has developed. This package includes, among other things, a left-hand threaded retention bolt that tightens with vibrations (not one thatâ€™s prone to loosen) and the elimination of the stress riser inducing keyways that others still use. The total package is rounded out with the installation of the previously-mentioned fourth main bearing that has www.ContactMagazine.com
The new, one-piece, bearing-grade aluminum alloy tubular fourth bearing is machined from billet stock and is slid over the crank prior to the hub being fitted in place and the cases closed up. The bearing is held in place by essentially an interference fit between it and the case halves, locking it into place. The prop hub bolt is installed but not tightened until after the case halves are bolted together and torqued to specification. The prop hub itself is machined from a single piece of 4130 or 4310 (steel) billet stock that is then heat treated for hardness to ensure the locking effect of the precisionhoned 3 degree taper. The previously mentioned left handed retention bolt is Âž-inch in diameter and is torqued to 160 lb-ft, locking the taper so securely that any form of externally applied puller will destroy the hub before it can be removed. However, through the use of carefully placed internal threads and the properly sized
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Deep inside the steel propeller hub are a set of threads into which the drive bolt is installed. When driven in far enough, the bolt bottoms out on the nose of the crank, forcing the hub off with symmetrical loads, safely removing the hub with no damage to any of the parts involved.
drive-bolt, the hub can be removed and replaced numerous times with no damage to the hub, crank or bolt. When I visited Revmaster during the build of the engine for this article, the technician slid the hub into place and secured it by patting lightly with the palm of his hand. He then asked me to pull it back off, which I couldn’t do. He had to use the drive bolt to pull it back off as a demonstration of the strength of the taper fit.
THE CRANKCASE The crankcase starts life as a stock off-the-shelf Brazilian-made magnesium VW part. Although Revmaster can obtain aluminum cases, magnesium cases are far lighter and have thinner cross sections in various places. Once received at the Revmaster facility in Hesperia CA, the crankcase undergoes extensive machining to allow it’s integration with Revmaster's other components. Due to the relative distance between the centerlines of the crank and the cam, it's easy to see that the engine can only be "stroked" so far. This is one reason that Revmaster opted to make their own case for the larger R-3000 engine we wrote about in CONTACT! issue #82. That same case can be used for the "tweener" 2500cc engine, but Revmaster feels comfortable tweaking the stock case to the 2300cc being featured in this article. Since the global market is so unreliable, the future availability of the Brazilian magnesium cases is always in question and that's one of the many reasons Revmaster developed their own case. For now, however, it's more economical to buy the off-the-shelf case and modify it, but with their own case, Revmaster is not locked in to a sole source should it ever dry up.
traveled by the piston from zero degrees to 180 degrees of crank rotation. Another way to look at it is the distance between the crank pin centerlines as measured when 180 degrees apart, so moving the pin’s centerline farther away from the crankshaft centerline increases the stroke. When the stock diameter pin is moved away from the crank centerline, as it rotates toward the cam the clearance between the rod and the case is decreased. New stroke Old stroke
New centerline Old centerline
Crank main journal
Old diameter New diameter
Not to scale
BORE AND STROKE
What Revmaster does is to not move the stock diameter pin farther away from the crank centerline, but rather to grind the stock pin smaller in diameter, removing material from the surface of the pin that’s closest to the crank centerline, resulting in the pin’s centerline being moved outward as shown in the illustration above.
As previously mentioned, the stroke is ultimately limited by the cam location, but Revmaster has found a way around that. Stroke is usually defined as the distance
There are other clearance issues such as interference between the connecting rod cap or bolts and the crank-
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the stroke, care needs to be taken to provide enough support for the piston skirt. Revmaster handles this with the custom manufacturing of cylinders with longer spigots that enter farther into the case than stock cylinders. The deeper spigots do create other interferences inside the case that have to be dealt with, but Revmaster has refined solved all of them.
CONNECTING RODS Forged 4340 steel I-beam connecting rods have 100% machined surfaces and utilize 9mm ARP 2000 rod cap bolts. They are balance-matched into weight groups of +/- 3 grams. The “big end” carries pressure-lubed plain bearings from a General Motors application, rotating on 2” polished and radiused journals. The small end (with bronze bushing) connects to full floating VW wrist pins that are retained by spiral circlips. Splash-style lubrication is used effectively to get oil into the wrist pins and piston lands via strategically-placed orifices in the piston interior and the rod end.
CAMSHAFT case or even the opposing piston skirt. These are addressed with traditional, proven methods, but the use of proprietary connecting rods with streamlined bolt lugs goes a long way toward solving these issues. There are other issues that arise when stroking the engine, the most obvious being taking care of the compression ratio, but in this instance, there is one issue that’s not so obvious. When the piston is at the bottom of
On the left is the special Revmaster cylinder designed to support the piston all the way to bottom dead center of the bored and stroked R-2300. Contrasting on the right is a stock VW cylinder. What’s not shown is the additional machining to the spigot end of the cylinder that’s necessary for clearance.
The camshaft is a chilled cast-iron unit with a lobe hardness of 60 HRC. In the casting process a “chill” (a metal piece placed in the sand mold) is used. These “chills” act as quenches which remove or “wick” heat rapidly from a specific area in the mold. The rapid cooling makes the metal near the chill much harder than the surrounding material without the chill. The hardening depth goes significantly beyond any other hardening process. The custom grind of the R-2300 is not particularly noteworthy (270° duration with a .390” lift), being on par with a lift and duration for low RPM/high torque as one might suspect. It performs well between 2500 and 3400 RPM, with peak torque at 3200. Revmaster services the entire spectrum of automobile applications for the VW engine and will grind one of their camshafts for just about any profile for any application. The stock (aluminum) VW cam gear runs against the otherwise stock VW crank gear at the front of the engine, while the cam itself turns in pressure-fed plain VW bearings.
Note the substantial differences between the crank-end of the stock VW connecting rod in the foreground and those attached to the crankshaft.
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CYLINDER HEADS Revmaster, through their history with EMPI, has had a big part in the creation or evolution of the 044 heads. These heads are pretty much the benchmark for aftermarket high performance VW heads and aviation conversions but Revmaster has taken the evolution even further with the development of their proprietary 049 heads. With thicker sections where needed and opened air passages for better cooling, these heads are capable of dissipating the torturous heat that has otherwise peaked the VW aircraft engine at the 65-75 hp limit for sustained power.
LIFTERS AND VALVE SEATS
Although a lot of parts that go into the Revmaster conversion are proprietary, they try to use off-the-shelf parts where ever they can. The pistons are high performance forged Mahle parts. Note how short the piston is and that when itâ€™s at BDC the rings are inside the case.
One of the frequently mentioned issues that plagued early VW conversions was the persistent (though incorrect) belief that the solid lifters in those engines required adjustment of the valve lash every 25 hours. The truth, discovered after exhaustive research by the Revmaster technicians, is that the original valve seat material was inadequate to tolerate the heat created in the combustion chamber once the displacement grew beyond the Volkswagen factory specifications. The hot valves had begun to lift tiny particles of metal from the seats and that erosion, in addition to destroying the efficiency of the combustion chamber, allowed the valve to sit deeper in the head. Testing proved a loss of about 0.001" of erosion per hour. The clearances at the rocker arm would then diminish or disappear altogether, causing some to believe that what was happening was that the valves were "stretching". Frequent valve lash adjustments (typically every 25 hours) became the common practice and continue to this day on VW engines that still utilize inferior valve seat material.
The areas around the combustion chambers have been beefed up to easily accommodate 92-94 mm bores. This particular head was built for automobile use and has only one set of spark plugs.
Installing the solid lifters in the Revmaster R-2300.
Rough casting as it arrives at Revmaster Aviation.
Other folks choose to modify the engine to accept hydraulic lifters, which takes care of the need to constantly adjust the valve clearances, but does not remedy the cause of the problem occurring at the valve seats. The solution is to use stainless steel valves paired with valve seats with a very high nickel-content alloy, which is precisely what Revmaster has been doing since 1985. Solid lifters work just fine and are very easy to set to the
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Joe Horvath showing us the inner-workings of his high volume oil pump that includes the ability to install a spinon filter. The pump, which is driven by the camshaft, also runs an eccentric for driving the fuel pump.
proper clearances. No additional maintenance beyond “check at annual” is required and the valve train has proven to be very durable since the improved hardened seats were adapted. For installations that are tightly cowled or if high power is used for extended periods and higher than normal cylinder heads temps are an issue, another anomaly may show up besides the need to constantly readjust the valves: a repeated need to tighten the head bolts. 18 lb-ft of torque on the head studs equates to .011" of stretch. As temperatures rise and the thermal expansion of the head kicks in with the aluminum expanding more than the steel stud, there could be another .004" of stretch. Since the stud lengths are not equal, the amount of stretch is not equal either and asymmetrical pressure can be concentrated on the portion of the head where it meets the cylinder. Coupled with the softening of the aluminum head as the temperatures rise and the high concentration of pressure from the point-loading of the cylinder spigot on the mating surface, the cylinders can work their way into the head and the bolts will begin to lose their torque. If the engine is continued to be pushed during these conditions, the mating surfaces of the spigot to the head can begin to leak hot combustion gases and a hole begins to be cut into the head from the concentrated stream of hot gases, like a cutting torch. To hedge against this anomaly, Revmaster developed for their earlier turbocharged models that used standard cylinders, a "power belt", which is a band of steel that's installed over the cylinder head end of the spigot, helping to maintain the cylinder’s concentricity and increases the contact area by 60 percent, reducing the point-loading enough to hinder the initial distortion of the softened aluminum. The test to see if the "fix" is needed that the lower head studs will need periodic retorquing. The R2033 however uses 94mm cylinders with sufficient wallthickness already built in to the spigot, eliminating the need for the band. www.ContactMagazine.com
Tucked neatly under the engine is the oil cooler. Efforts have been made to allow for draining the oil without having to remove the cooler. Since the engine uses a quality oil filter, there’s no need to drop the pan and clean the screen with each oil change. Braded stainless steel hoses plumb the oil to and from the cooler.
OIL SYSTEM At the front of the engine, below the prop hub and driven from the end of the cam is the lubrication system sourceanother proprietary Revmaster casting. It includes the oil-pump cover section, the mounting location of the spinon, full flow oil filter (available at any auto parts store), as well as the mount for the optional add-on diaphragm-type mechanical fuel pump. The oil pump uses 38mm gears as opposed to the stock 30mm VW gears, and can flow nine gallons of oil per hour. Besides servicing the normal oil passages for the internal engine components, pressurized oil is also plumbed to the propeller shaft housing via external braided hose and threaded fittings. Additional lines are routed to the oil cooler, which is usually mounted in a horizontal plenum positioned beneath the crankcase. Other styles and mounting locations for oil coolers can be specified by the customer.
ACCESSORY CASE This accessory housing package accommodates four items critical to engine and aircraft operations. It contains three major operating systems: the dual alternators, the self-energized ignition source, and the electric starter, and it also provides the physical mount to the airframe. The R-2300 model is nearly identical to the proven unit currently used on the R-2100 (more than 60 of those units are now in use) and is yet another product made exclusively by Revmaster. The three electrical subsystems are independent but function as an integrated unit within one compact aluminum case. Let’s look at each component separately for the sake of clarity. The precision machined alloy casting fully encloses the dual 18 ampere alternator package. Mounted to the interior face is a stationary twelve-pole stator ring. An aluminum flywheel incorporates twelve neodymium iron-boron magnets that are attached to the interior of the flywheel. These magnets, the strongest magnets commercially available, rotate around the 8½” diameter stator. Any
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movement of the flywheel sends its magnets orbiting in close proximity to the stators, with 12 feet each of copper wire windings exciting the electrons and creating electrical energy. There are two groups of five alternator coils, each set functioning as an independent 18 amp alternator. The current generated from these coils is sent to solid state regulators and then to the aircraft’s battery and operational power bus. In the unlikely case of a failure in one system, the other would remain unaffected.
With the accessory case installed, timing the ignition is next on the list.
The ignition advance is set at a maximum of 25° before top center. This would normally be identified as a fixed timing position but in reality the “effective advance” behaves as if the low RPM timing is at 15° BTC. This desirable situation is created by magnetic precession in the self-energized design. Lower voltage exists in the system
IGNITION The two coils which make up the ignition power source are located 180 degrees apart at the 12 and 6 o’clock positions (see photo above), separating the previously mentioned five-left and five-right alternator stator coilgroups. The ignition coils are also creating power whenever there is rotation of the flywheel, but their energy is dedicated exclusively to the CDI package.
The “brains” behind the brawn. Once the timing is set, the electronics are wired up and bolted in place,
The back side of the accessory case, showing the ignition trigger sensors.
A triggering sensor mounted to the center area of the housing’s interior receives a signal from a device attached to the end of the crankshaft, acting as the “distributor” and telling the CDI when to transmit the power to the eight mini coils which are positioned near the upper and lower spark plugs at each combustion chamber. Once the engine has been started, the battery is not necessary to operate the ignition. www.ContactMagazine.com
when the engine is turning slowly, reducing the current flow at the timing triggers. The engine likes 15° BTC for easy starting and comfortable idle but as RPM rises, so does the voltage and the ability to “snap” the timing, and the advance moves quickly to its maximum setting. Experience has proven that 25° BTC, while possibly leaving a few horsepower untapped, is a smart place to limit the spark advance because it greatly reduces the possibility of destructive detonation.
STARTER The aluminum flywheel includes a steel starter ring that is heat-shrunken onto it. The geared electric starter motor is a compact 6” long model that weighs 8.5 pounds. Experience has established a long service life for this economical unit, which is mounted in an aft cantilever
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style. Previously installed starters were proprietary and were designed by Molt Taylor of Mini-Imp fame. Machined locations for the polyurethane-cushioned engine mounts are located in the “corners” of the accessory case casting. With the symmetry of this design, the unit can be rotated 180º to facilitate the starter motor being positioned at either the top or bottom. This can be a particular bonus for airframe designs such as the Zenith 601, whose firewall angles aft at the top, and the Sonex, whose firewall rakes aft at the bottom. Slanted firewalls present unique challenges when installing engines other than those that were originally planned by their creators, so the ability to place the starter motor in the location with the most surplus space can be a huge advantage.
DETAILS The Revmaster R-2300 comes complete, ready to run and in fact, test run. While it’s not a kit like others offer, Revmaster will provide it as a kit if you so choose. While I was at the Revmaster facility conducting the interview and photo-shoot for this article, the entire engine was built from beginning to end (with many interruptions for questions and photos) in a little over two hours. At the peak of production back when Quickie Aircraft Corporation was ordering 100 units per month, Revmaster had a full-time staff of five engine builders knocking out 2-3 engines per day. In the big picture, absorbing the cost of building each engine right the first time, and checking by actually running it, far outweighs the headaches and problem solving with a builder who could be thousands of miles away, not to mention the damage to a company’s reputation that might come from a builder making mistakes and the product being blamed. Everything pictured in this article is included. There is nothing more for the builder to buy or supply to make the engine run except fuel to the RevFlow throttle body injector (carb) and electricity to the gear-driven starter. Airframe-specific items like the exhaust system, baffling, engine mount, and propeller are of course not sold with the engine, but in some cases are also available from Revmaster as separate items. Revmaster Aviation has a customer base that is pushing 40 years old. The name is trusted and respected throughout the aviation insurance community as an "approved" automobile conversion or alternate engine. Each engine (and most of the individual components) has a unique serial number and Revmaster has the complete records going all the way back to the raw materials. If anyone has ever considered the purchase of a 25 year old Revmaster, a quick phone call to Joe Horvath at Revmaster has probably netted the history of that engine, especially if each subsequent owner contacted Joe when they became the owner of the engine. When contrasted with a home brewed conversion with various parts collected from various vendors (some of whom may have gone out of business), it is easy to see how the insurance industry considers the engines from Revmaster to be more like certified engines than auto www.ContactMagazine.com
conversions. This is no accident, since back in the early days the plan was to create a certified engine from the proprietary parts and since that time, all parts have been handled with the same paper trail and quality control standards as certified parts. The R-2300 and the R-3000 may still some day be FAA certified, and will most likely be ASTM compliant for use in factory-built special light sport aircraft. The Revmaster facility is as complete as a manufacturing shop can be, shy of having a foundry. From the initial drafting through the entire manufacturing process and including final assembly, testing, crating and shipping, everything is done in-house. Customer service is paramount and the people at Revmaster are prepared and available to assist the user in any way imaginable, from supplying all parts that they manufacture (including new old stock) to completely rebuilding an entire engine. Revmaster is also fully equipped for thorough testing, as evidenced by the patina on the dynamometer and the Magnaflux equipment, both of which have been in use for as long as I've been alive. Numbers published by Revmaster are well documented through actual testing, not by guessing. For the sake of time, production engines are tested outside with a test club of known performance. If the engine can't swing the club to the prescribed RPM with the anticipated manifold pressure, it doesn't leave the shop. In this way, testing can be done in under an hour rather than a full day in the test cell. The test cell is mostly reserved for specific performance research and development such as proving porting experiments after flow bench testing, intake and exhaust manifold designs, and carburetion and bore-and-stroke combinations. With the dyno, 1-3 horsepower gain or loss is easy to document and is repeatable in the controlled conditions of the cell. Those interested in the Revmaster R-2300 or any of the products they offer should feel free to contact them at their Hesperia office. Joe Horvath will be presenting forums at the upcoming COPPERSTATE EAA Regional Fly-in, KCGZ Casa Grande, Arizona October 21-23. From their website, www.revmasteraviation.com: We welcome visitors; a phone call in advance is appreciated. 760-244-3074 Revmaster Aviation 7146 Santa Fe Avenue East Hesperia, CA, 92345 (Located across the runway from Hesperia Airport)
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Scott Weinberg of Iron Design L.L.C., a fabrication shop operating from a little country shop in Northeast Iowa, produces a very robust but simple tailwheel. As a general industrial designer (or conceptual engineer if you prefer) Scott is a student of form function, and the connection between product and user. Iron Design does not design or manufacture the gears or motors that make machines move or the circuits that control the movement, but they can affect technical aspects through usability design and form relationships. They usually partner with engineers and marketers to identify and fulfill needs, wants and expectations. In some cases where production is specialized, they produce the product as well. Weinberg does the latter part. Scott’s love for flying emerged when he was in his mid teens, but when he turned 25 it really kicked in. As a passenger in a single engine aircraft during a three hour flight (outbound leg during the day, return at night) conducted by a potential employer, the diversity of the flight solidified his desire to become a pilot. Raising a family had to take center stage, but once kids were grown there was finally time for flying. Scott learned in the traditional path of Cessna 150-152-172's, earning his IFR rating in a Piper Warrior. “I just love flying in Champs, Super Cubs and of course the Bearhawk Patrol.” (an outstanding example of a 2 seat tandem STOL aircraft) Scott said. “I love the low side windows that only high-wing aircraft can incorporate with ease. Of course RV's are great too.” Scott was in college when he discovered computers as a tool. About 15 years ago, a CadKey program set the stage for being able to accurately draw 2-D sketches and compare them with plans that were on the market. That has evolved into a 3-D SolidWorks CAD program enabling the designer to draw and visualize each and every part and test-fit related parts on the screen, long before production. This capability includes load testing, lofting, form shapes and modeling. Working with this program has allowed Scott to evolve many designs into what he feels is a sustainable production level without wasting time, energy and material.
DEVELOPING THE PROTOTYPE TAILWHEEL Starting with a set of plans from R&B Aircraft Company (Bearhawk), Scott and his brother Gregg began pondering how they could make the assembly of parts detailed in the plans come together in a production-based environment, keeping in mind that any two of the same parts should be able to interchange for years to come. They reworked assemblies consisting of multiple components to create single-piece parts, utilizing modern CNC production methods. By doing so, the virtual axis lines can be maintained without piecemeal welded parts. Although www.ContactMagazine.com
there is absolutely nothing wrong (mechanically) with built-up parts, for consistent quality of components and speed of production, there is no beating machining from a single piece. With that in mind, the main body of the steering arm, large cam body, and kingpin are each machined as completed components. With align-boring, the parts slide together right out of the machining center, producing parts with less than 1% scrap rate after welding. Read that as being very low. It’s for these reasons that Scott and Gregg took on the tailwheel project at the suggestion of two friends. They had drawings of a wheel they felt would work and they challenged Scott to see if he could mass-produce it and if so, they would be one of his first customers. That was the beginning of a great building experience. The building and marketing of the tailwheels has an added benefit, one that a price can’t easily be placed upon. Scott and Gregg have been able to form a great number of relationships with people from all over the world, most of whom started out as customers. Their tailwheels have made it to the far corners of the world, from New Zealand, South Africa and most states or provinces in North America to Europe and the Orient. A short listing of aircraft is as diverse as the countries in which they were built and include Bearhawk, Patrols, Champ and Pacer clones, Super Cub clones, GlaStar, Rebel, Kitfox (and the many variants), Peg 100 and Highlanders. Even an RV-8 with large main tires has been fitted with an 8” model with stinger. The stingers have been getting a fair bit of attention in the last few years, but any flat spring model plane with gross weights up to 2,750 pounds and standard spring angle work well. Any plane with a Scott 3200 or similar will have the correct angle for the flat spring version. “We really knew we were on the right path when we flew a Champ-like aircraft with a old Scott 2000 wheel and a 6" hard rubber tire and in 15 minutes put on our 8" assembly with a pneumatic tire.“ Scott said. “You wouldn't even know it was the same plane on the ground”. When he made that
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first turn onto the runway at the end of the field, it swiveled as it should and then locked up in sync with the rudder. “It really ended that ‘happy feet— hoped its locked’ dance on landings and turn arounds.” The tailwheel is locked with the rudder for standard taxi work and then goes into full swivel operation with additional directional change. It has a very simple locking pin and cam action. Locking and unlocking is not done with any control inside the cockpit.
MAINTENANCE The sum total of all replaceable parts for the Iron Design tailwheel costs less than $30 and in six years, they have only sent out a total of $20 worth, proving the longevity of the piece. Thorough annual inspections of the assembly should take less than 30 minutes, including cleaning the steering arm/locking pin which can be done on the plane. Silt from repeated landings in soft dry sand or the dusty Southwest (USA) is about the only foreign material that has worked its way into the lubricated surfaces, but can be dealt with in minutes. Latest revisions in the design (June 2010) have greatly aided in keeping the single critical locking pin very clean.
APPLICATION Any experimental aircraft that uses an 8” or 10” Scott 3200, Maule or Matco, can use the Iron Design wheel. Most general homebuilt aircraft have been built to this standard attach angle. A Tundra version, (5.5"x11") is also available. In 15 minutes, any of the three yokes sized for these tire sizes can be interchanged, so the tailwheel can match the mains if or when tundra tires are installed or replaced. The complete assembly arrives ready to bolt in place with minimal work to the tail spring to ensure a perfect fit. Those who have been following the progress of the Iron Design tailwheels may have noticed that they seem to come in a multitude of different finishes. That’s not necessarily so. Working with various coatings and platings, Scott is still trying to find the holy grail— a combination of durability and ease of application. All tailwheels get dirty, but the goal is to use a finish that will keep them looking great for as long as possible.
OPTIONS The “stinger” version, designed to work with a round tapered rod spring, is the newest addition to the lineup and is working well. The round tapered rod is usually lighter than a traditional leaf spring and is just as strong (some even being made from titanium). It’s also more capable of handling side load shock, allowing the tailwheel itself to be a little lighter to boot. Iron Design’s titanium rod saves up to five pounds in spring weight alone over a standard triple leaf flat spring. In many cases, the cost of this rod will only be $50-80 more ($180 vs. $100) for the titanium rod.
INNOVATIONS Iron Design is on the verge of debuting their conversion system to adapt airframes fitted for leaf springs to accept the tapered titanium rod they now offer. Even considering the extra weight of the adapter, there will be a net weight savings and coupled with the potential handling enhancements, Scott believes the installation to be a win-win deal. Iron Design considers research and development to be a never-ending process, as they keep tweaking new ideas and new manufacturing processes. They seem to be getting closer to a 6" pneumatic to serve the smaller, lighter experimental aircraft market. Their sights are set on weight savings and longevity, the same thing they have accomplished with the larger assemblies. The wheel will of course be steerable, while still being fullcastering when kicked past the lock. At this writing the actual wheel for the tire is found through a common source but they are close to manufacturing their own. The idea is that they can save some weight and ensure control over production and quality.
NOT JUST TAILWHEELS Current projects include the fabrication of motor mounts, prefabricated firewalls, a double fold front seat, a super strong but low-cost sling seat, and a streamlined process for window frames and baggage doors. On the back burner is a wing building process, completely designed in SolidWorks, that will create a completed wing assembly where the labor savings will pay for the material. The company has a refreshing policy that seems to be rare among experimental aviation vendors. They will never market an unfinished, untested product and will never solicit funds for development. For more information and for specific details on wheels and design projects, please visit Iron Works on the web at www.irondesign-airparts.com or call Scott Weinberg at (319) 404-4401 “Made in America, Spent in America”
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Photo: Frank Harris
By Paul Lipps
TIP LOSS... DOES IT EXIST, AND WHAT DOES IT LOOK LIKE?
I'm sure that when you've been discussing propellers with others, the number of blades that go into making up a good propeller has come up, right? And somewhere during the discussion someone mentions that a single-blade propeller is the most efficient, right? Somewhere in this exposition the nebulous idea of "tip loss" is introduced, with all parties nodding their heads in agreement. Because after all, if there is a tip loss demon then it's quite obvious that the more tips there are, the more losses there are, right? But somehow, the description of how this nefarious loss occurs is avoided; it's just taken on faith that it exists! And everyone seems to know that three or four-blade props give better takeoff and climb performance, but they are not as good in cruise, right?
ONE BLADE = ONE WING Now I want you to picture in your mind what would happen if you have only one wing sticking out the side of the fuselage on an airplane. I'll bet you conjure up the image of that plane rolling round and round as it plummets toward earth. And why might that be? Is it because the center of the wing's lift and the center of mass of the whole contraption don't coincide? So if you have a single-blade propeller, won't it just produce thrust on whichever side it happens to be at the moment? And if that's the case, isn't it going to pull the engine back and forth and up and down on its mounts, making it whirl around? It doesn't seem like that would be very comfortable to fly behind, even if you could keep the engine from departing the aircraft. If the single-blade propeller is all that it's cracked up to be, why aren't Sensenich and Hartzell and other propmakers turning them out by the hundreds or thousands? And what about the military? Wouldn't you love to see a Huey hovering with a single-blade rotor? There are some motor gliders currently flying with retractable engine pods that necessitates the compactness of a single blade, but given the option of two blades (which would probably weigh less than the additional heroics taken to use the single-blade prop) I’m sure the designers would prefer to go that way. Then there’s the Everel Propeller that flew successfully for a few years and even went into a short production period. If it were the next best thing since sliced bread, where is it now? Why hasn’t someone taken the idea and run with it? www.ContactMagazine.com
You know quite well that you can't have a pressure differential existing across the upper and lower surfaces at tip of a wing. If you did, what would keep the air molecules from filling in the disparity? You've seen and heard that "nature abhors a vacuum". That anxiety of the molecules to fill in a void is what causes those in the higherpressure air on the bottom of a wing to flow up and around to the region of lower-pressure air on the top, thus creating the tip vortex. But what if we were to somehow gradually decrease the pressure differential as we go outboard on the wing until it reaches zero at the tip? I'll bet there would be no vortex! And another by-product of this is that the vortex generates noise and induces drag, so by getting rid of it the noise decreases or goes away. Well guess what; by gosh and by golly, one of the greatest minds in fluid dynamics, German physicist Ludwig Prandtl, discovered how to do this by using an elliptical lift distribution. The resulting elliptical planform that produces this lift distribution has the least induced drag of any planform. Picture the beautiful elliptical wing shape on Spitfires, Culver Cadets and P-47 Thunderbolts! Now consider this: if the lift goes to zero at the tip, irrespective of the shape, then what good is any area at the tip? Well, it produces drag, that's what! But that's not good, is it? As an illustration we'll take the case of the last inch of a two-blade 73" diameter propeller with a 4" wide tip, spinning at 2700 RPM on a 160 HP plane going 130 MPH at sea level. The mid-point of this tip, at a radius of 36.5", will have a rotational velocity of 860 feet per second (fps). Combining this with the forward velocity gives a total velocity of 880.9 fps, M (Mach) 0.79 at 59 degrees Fahrenheit (F). Correcting for compressibility gives a dynamic pressure of 1,073 pounds per square foot (psf). The drag coefficient of a NACA 2312 airfoil at a typical coefficient of lift (CL) of 0.4 is about 0.06. Multiplying drag coefficient by dynamic pressure and by the area of 4 sq in/144 sq in gives a drag of 1.79 lb. Multiplying this by the radius in feet, 36.5/12, gives 5.36 lb-ft. Finally, multiplying this by the RPM and dividing by 5252 gives 2.76 HP per blade. For two tips that's 5.52 HP, or 3.5% of the engine's HP! And that's just the last inch! And it's producing very little thrust because of the minimal pressure differential. Remember that pressure differential goes to zero at the tip and it’s that differential that makes lift on a wing or thrust from a prop.
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Prior to the 2009 Reno Air Races, I modified the planform of the tips of the propeller on a race biplane. This planform change consisted of a curve starting about 9" in from the tip at the leading edge to a point at the tip at the trailing edge. This plane picked up a 5.5% speed increase from qualifying at 201 MPH in 2008 to qualifying at 212 MPH in 2009. The team received the award for the most speed increase from one year to the next. Of course, as could be expected, with the increased speed the RPM also went up by the same amount. A 5.5% increase in speed is like having a 17.4% increase in HP. But since the increased RPM allowed the HP to increase by the same 5.5%, the efficiency increase amounts to 11.3%— so tip shape definitely matters!
MORE BLADES So let's increase that prop from two blades to four. Will the tip loss double? No! Why not? Because if we keep the diameter the same, the four blades will need the chord reduced by half to keep the overall area the same as the two-blade, making each blade's tip area half the size— so the loss is the same. But doubling the number of blades allows the diameter of the prop to be reduced by as much as 30% and still produce the same thrust, since reducing the diameter to 70% will sweep out half the area on twice as many blades so the result is the same. Now because the dynamic pressure at the tip is a function of velocity-squared, reducing the diameter will result in lower tip velocity with reduced dynamic pressure and reduced high-Mach drag coefficient, so the loss will be less! So maybe the five-blade props on commuter turboprops, or six-blade props on the C-130J, or the eight-blade props on the European A-400M, or the 18-to24-blade fixed-pitch props (fans) on the front of a fanjet engine make a little more sense now. And how about the multiplicity of blades that make up the compressors and turbines in gas turbines? If these had the tip loss demons that so many proclaim, they would be so inefficient as to not even be viable. So why haven't traditionally-built multi-blade props produced as much cruise speed as two-blade props? Very simple; just look at the shape of the blade in the root region. Is it streamlined? No! Just round shanks with high drag coefficients both whirling around and being pushed forward. That gives both rotary and forward drag that the engine must overcome. Some WWII planes were equipped with streamlined cuffs that fit over the blade roots to decrease not only rotational and forward drag but also to allow the root region to contribute to the blade's thrust. Now here's another aspect of multi-blade propellers. Lift is a product of mass times velocity. With wings or propellers, the downwash velocity imparted to the air is energy thrown away. So to increase efficiency it is necessary to increase the mass flow to decrease the velocity for a given lift. As a wing goes through the air, it intercepts a mass of air that has the volume of a circle with a diameter of the wing span times the forward speed. That means that for a given wing area, increasing the span www.ContactMagazine.com
will increase the volume or mass of air that it passes through, reducing the downwash. That is why sailplanes have such long, slender wings. Since the downwash velocity is decreased, the wing has a lower induced angle of attack. Thus the lift angle is tilted back less, lowering the induced drag. You can do the same thing with a propeller by increasing its diameter, but then you run into the problem of increasing the tip velocity at a given RPM. In order to take advantage of a larger diameter without having extra tip loss, it is necessary to turn the prop at a lower RPM so now you have to add a gearbox with its increased complexity, wear, weight, and cost. On the other hand, every blade on a propeller sweeps out an equal mass of air, so by having more blades, there is more mass flow and less downwash. So it should be possible to reduce the diameter a little to decrease tip drag, but still have more mass flow so you get the best of both worlds— lower tip drag and lower downwash. That means a multi-blade prop can actually be made that is more efficient than one with fewer blades! How about that!?! Boy! I'll bet that statement gets the "fewer blades, thinner blades, longer blades" Luddites in an uproar! At Reno in 2003, Tom Aberle (featured in CONTACT! Magazine issue #79) raced at 220 MPH with a twoblade, in 2004 an additional 20 MPH put him at 240 MPH with a three-blade, and in 2006 another 12 MPH upped it to 252 MPH with a four blades. That really makes the case for "the fewer blades the better", doesn't it! ☺ And now, folks, for the No. 1, absolutely dumbest argument against multi-blade props of all time: "more blades will interfere with each other, with following blades working in the dirty air from the previous one". That blade with a 72" effective pitch will have a following blade (of a four-blade prop) 18" AHEAD in space of where the previous blade once was, not behind it! These blades follow a helical path in space with each one having its own individual path apart from the others. And propeller blades don't blow the air out sideways, they give it a slight angular deviation in the direction of rotation from straight back. That is the slight swirl of maybe 1 to 3 degrees of the air behind the prop in cruise.
PITCH No, this is not a treatise on baseball or a black, tarry substance. This has to do the relationship between the propeller's forward advance and its revolution. Pitch and diameter are the two data points that are normally used to characterize a propeller and which cause someone considering a propeller purchase much consternation and anxiety. "Joe uses a 72-68 on his VR-15 but Herb uses a 68-72 on his and says he gets the same speed.
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What to do, what to do! But Joe's prop was made by Ronnie Hintz and Herb's by Greg Castro and I wanted to get mine from Red Helix. What numbers do I tell Red? And then there's this guy who comes up with these strange props who says pitch doesn't matter! What to do, what to do!" People like to think that a prop is like a screw and that it screws its way through the air. They liken the pitch to some obscure angle on the blade, often the angle of the flat portion of a blade that uses a somewhat flat-bottom airfoil or maybe even the chord of the airfoil. But unlike on a screw, that angle changes all along the blade. Now let's see; should I use the angle from that flat bottom, or the angle from the chord-line? And then, what do I do with that angle? Oh, yeah; I'll use trig! I'll take the propeller's radius in inches where I measured that angle, multiply it by two-pi to get the circumference of the arc at that radius, and then multiply the arc length by the tangent of the angle I measured and voila!- I've got the pitch in inches! That's pretty simple since my calculator has the trig functions. Now all I have to do is multiply this by the RPM and divide by a certain number (which I've heard is 1056) and I've got my airplane's speed. So what if I order a prop with 156" pitch; will I really be able to go 400 mph at 2700 rpm??? I don't think it really works that way, since I've seen that a certain RPM in a climb gives a different speed than the same RPM in cruise. Oh! I know; I hear this anomaly is called slippage! But then that means I have to know how much slippage I'm going to get. And I'll bet that must have something to do with drag and horsepower or else only the prop's pitch would limit my plane's speed. So here I am back at the beginning. What pitch do I tell Red I want? I see why the weird prop guy says pitch is not a good measure and why the phrase “black art” is tossed around so freely. What does a propeller do? It turns horsepower (a combination of torque and RPM) into thrust. And an airplane, with its parasite and induced drag, requires a certain amount of thrust to counteract the total drag at a given speed. The propeller has some losses for this same reason, which means that not all of the power produced by the engine will be converted into thrust. So the thrust horsepower divided by the engine horsepower is the efficiency, which is multiplied by 100 to turn it into a percentage. Some props are as much as 92% efficient, throwing away only 8% of an engine's power to merely turn the prop, but some are only 65% efficient, wasting 35% of the engine's potential. What is it that determines a prop's efficiency? The equality or inequality of the thrust-totorque ratio all along the blade's span, nothing else! When that equality is maintained, every bit of the blade is doing its portion of the work at the best that can be done, leading to high efficiency. But back to the meaning of pitch. There is really only one measure of pitch that has any meaning, and that is the effective pitch. That value is obtained by multiplying the www.ContactMagazine.com
true airspeed (TAS) in MPH in level flight by 1056 and dividing by the RPM. Now a given high-efficiency prop on several different aircraft of the same type and horsepower will have the same effective pitch, but because of differences in the drag of each plane, they will not have the same TAS and RPM. The plane with the least drag will have a higher RPM and TAS, and the plane with the higher drag will have the lower RPM and speed. Now this is only valid over about a 10% to 20% speed range and less efficient propellers won't show quite this kind of performance since they don't have the necessary equal thrust/torque ratio. The wing on an airplane is required to operate over a speed range of more than 3:1, from stall to top speed. When it is at stall, it is operating at a peak CL of about 1.5, whereas in top speed at low altitude CL will be more in the range of 0.1 to 0.15— about a 10:1 range. This is because the lift is related to speed-squared, so a 3:1 speed range means a 9:1 dynamic pressure range to produce the necessary lift. A propeller, on the other hand, has most of its velocity determined by rotation rate, except in the root region. The lift or thrust is a function of dynamic pressure, lift coefficient, and area. At the higher RPM range at which propellers operate, the velocity over the majority of the blade, and its attendant dynamic pressure, are more constant. This allows the airfoil section to operate at a higher CL. Increasing CL allows the chord to be reduced, which reduces parasite drag. Some of the laminar flow airfoils have L/D ratios approaching 150:1 at a CL of 0.5. A typical airfoil section used on propeller blades resembles a NACA 4415. One of the characteristics of an airfoil is alphaZL, the angle of attack (alpha) at which the lift is zero. On the 4415 this is about -4.0 degrees relative to the chordline (the line from the most-forward point of the leading edge to the tip of the trailing edge). This means that with the airfoil pointed downward at a negative angle of 4 degrees, the lift is zero. The flat bottom portion of this airfoil has a slope of about -3.1 degrees relative to the chordline. This airfoil also has a slope of CL vs. alpha of about 0.10/degree. To have a design CL of 0.5, the airfoil would have to be at an alpha of 5 degrees relative to alphaZL. So the required alpha is 5 degrees below the +4 degree alphaZL or at 1 degree below the chordline, a required alpha of +1 degree. But this angle is 2 degrees above the flat slope on the bottom surface. As the airfoil generates lift, it causes the air to have a downwash angle, which reduces the apparent alpha; this is known as the induced alpha and requires the airfoil to tilt up further to get back to the necessary 5 degree alpha. This could result in the airfoil chordline having to be inclined upward about another 3 degrees. This is the actual angle that the design alpha is inclined above the actual path of the blade as it screws its way through the air. The result of all of this is that the actual angular path of the blade ends up 1 degree above the slope of the flat bottom of the airfoil!
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Let's say that the speed that Joe is getting is 185 MPH at 2700 RPM. That says his effective pitch is 72.4". Typically, geometric pitch is measured at 75% of span, which on his prop will be 27". The rotational velocity at this radius and 2700 RPM will be 636.2 fps or 433.8 MPH. Since the pitch angle's tangent is the forward speed divided by the rotational speed, the effective pitch angle (the actual helix angle that the blade is moving forward through the air) will be 23.1 degrees. But if we used the angle from the bottom of the blade, which is 1 degree less than the helix angle, that would give a pitch angle of 22.1 degrees, which would give a geometric pitch of 68.9 degrees for a 3.3" pitch error. Therefore, the pitch resulting from this method is only an approximation as to how the prop will really operate. But what if the designer chose a lower design CL of 0.3 for his prop rather than 0.5, and used increased chord? Since chord times span is area (which would have to be greater with reduced CL for the same lift/thrust), that would result in the desired alpha of 0.3 degrees being 1 degree above the chordline, meaning that the airfoil is pointed down at an apparent negative angle of attack, at least as concerns the chordline. Again using the 3 degree induced alpha, in this case the bottom slope would be 3 degrees less than the actual helix angle. This would give a geometric pitch angle of 20.1 degrees and a pitch of 42.3"! Not a very good approximation! Now, can we move on to better subjects, such as baseball or that black, tarry, stuff?
PLANFORM Now that we've looked at the number of blades and pitch of a propeller, let's consider the planform. With a nonswept wing all of the wing moves through the air at the same speed, so the dynamic pressure is pretty much the same all along the span. Even so, the pressure difference generating the lift drops off toward the tip because (as previously discussed) there can be no pressure difference at the tip. But now consider the propeller, whose velocity increases along its span. Additionally, the forward speed of the plane adds to the speed of the blade above that which occurs from rotational velocity alone. A propeller acts along a helical angle in which its speed through the air is on the hypotenuse of the angle formed by the plane's speed in the forward direction and the rotational speed perpendicular to it. Let's put some numbers to this, similar to those we looked at for the number of blades discussion. Consider a 72" diameter propeller spinning 2700 RPM on a plane going 200 MPH. At the surface of a 12" diameter spinner the prop has a forward speed of 293.3 ft/sec and a rotational velocity of 141.4 ft/ sec for a helical velocity of 325.6 ft/sec. At the tip it also has the forward speed of 293.3 ft/sec but a rotational velocity of 848.2 ft/sec for a total velocity of 897.5 ft/sec. At 59F standard sea level temperature that is M 0.80. Correcting for compressibility, the dynamic pressure on the blade just outside of the spinner is 128.7 psf, and at the tip it is 1112.2 psf, an 8.7:1 ratio. So if you had a propeller with a constant chord, neglecting the lift drop-off at www.ContactMagazine.com
the tip, the lift and drag would be 8.7 times as much at the tip as at the root. Think of this as a wing that has a chord 8.7 times as wide at the tip as at the root where it joins to the fuselage! That doesn't look like it would be very efficient, does it? Just think how much bending force this would generate at the root! A propeller has another effect which is not present with a wing. Because the velocity increases toward the tip, eventually the velocity at a larger radius begins to get closer to the speed of sound, where the parasite drag goes up considerably. This parasite drag and the liftreactive force, usually referred to as "induced drag", acts at a distance out from the center of rotation and opposite to the rotation. What do we call a force perpendicular to the radius, i.e. force times distance? Torque! So the farther out on a blade that you have a force resisting rotation, the greater the torque will be that the engine must overcome. And since torque times rotation-rate is how horsepower is measured, more horsepower will be required to overcome the force. The laminar airfoil 66-210, operating at an angle of attack of 4 degrees at M 0.8 will have a drag coefficient of 0.04, whereas at a lower Mach the drag coefficient will be only 0.005. That's an 8:1 drag increase ratio. So let's say a prop is similar to the above and has a tip chord of 3.5". The last inch of that blade will have an area of 1" X 3.5" / 144 or 0.024 sq. ft. The dynamic pressure at the center of that last inch, 35.5" radius, is 1088.4 lb/ sq. ft. Since drag force is dynamic pressure times drag coefficient times area, the drag will be 1088.4 X 0.04 X 0.024 or 1.06 lb. Multiply that by the radius of 35.5" and we have 3.13 lb-ft of torque. Converting to horsepower at 2700 RPM gives 3.22 X 2700 / 5252 = 1.61 HP or 3.2 HP loss from two blades. That's a 2% loss just from the last inch of a propeller blade on a 160 HP engine and that's only if the prop tip is clean and smooth and not rough as you see on so many propellers after hundreds of hours of use! So what's the answer? As we saw before, Prandtl showed that an elliptical lift distribution with its constant downwash along the span has about the lowest induced drag. That requires that a plot of the product of dynamic pressure, area, and lift coefficient versus span follow an elliptical shape from root to tip. For a wing, having constant dynamic pressure and lift coefficient along the span, all that is necessary is to have the chord distribution (planform) follow the shape of 1/4 of an ellipse from root to tip. In mathematical terms that means that the chord, y, at any distance x from the root outward, must follow the equation y = b/a X (a^2 - x^2)^1/2 where b = the distance from root to tip, a = root chord, the symbol "^2" means to square the value, and "^1/2" means to take the square root. So if we had a constant dynamic pressure times chord (that is, for any point along the span the value of the chord would always be the inverse of the dynamic pressure), what would result would be an apparent constant
CONTACT! ISSUE 101 PAGE 23
lift distribution such as with a constant chord "Hershey bar" wing as on an RV or an older-model Piper Warrior. We could then take that model and multiply it by the ellipse equation to get the desired elliptical lift distribution. Obviously the chord cannot be expected to keep increasing to the amount dictated by the equation of the ellipse, both from a practical standpoint and because of the finite width and thickness of the chosen propeller blank. So the chord will eventually reach a maximum, and then because of the blank dimensions, it will somewhat reduce as it reaches the spinner radius. Now here's a positive effect which results from this shape; the tip chord will go to zeroâ€” that is, a point. Well, if the chord goes to zero, then the drag must go to zero, and the pressure difference across the tip must also go to zero. If this happens, then there will be no pressure-induced span-wise flow outward on the bottom surface and inward on the top surface and no flow around the tip and voila, no tip vortex and no noise! This is not just theoretical, it is actual! Propellers that have been made this way don't generate the incredible noise that you hear with a Skylane or T-6 on takeoff. One of the first things that the crew of Tom Aberle's racing biplane noticed when he made a low pass down his runway with a propeller made this way was that for the first time they could hear the engine exhaust noise that had previously been obscured by the prop noise. This same effect was noted at the Reno air races when the two planes equipped with this type of prop flew by and they had no propeller noise. It was very easy to hear if the engine was running smoothly or had a miss. Is there a drawback to this design? Yes, because of the reduced blade area in the outboard portions, the static thrust and initial acceleration are somewhat low. However, by the time the airspeed reaches about 20% of design speed, the propeller really gets into its own. And by the time the plane is stabilized at its best rate of climb, the propeller is already on its way to the high efficiency that it will give in cruise. In fact, its efficiency in a climb is better than that of most fixed-pitch propellers in cruise.
SWITCH ON! Continued from page 2
Some influence on where the point of the span at which the chord reaches a maximum can be made by the choice of the blank dimensions. By this means the widest chord can be situated somewhat farther out to enhance static thrust and initial acceleration. Another aspect that must be considered in maintenance and operation is the fact that the tip point is more subject to breakage than is a wider chord, and can also be a hazard to human flesh if care is not exercised, and a nasty puncture wound may result. For this reason it is recommended that a slotted tennis ball be placed over the tip when on the ground with the propeller positioned vertically.
DETERMINING TRUE AIRSPEED Here's a set of GPS ground-speed numbers based on a 30 MPH wind from different directions and its affect on a 200 MPH airplane flown on two courses to obtain true airspeed. On one course the plane is held on north and south headings, and on the second it is held on north and south ground-tracks. 30 MPH Wind Direction
171.2 229.1 200.2 170.9 228.8 199.8
174.7 226.5 200.6 173.5 225.4 199.4
180.0 222.2 201.1 177.7 220.1 198.9
186.8 216.6 201.7 183.3 213.3 198.3
194.4 209.8 202.1 190.1 205.7 197.9
202.2 202.2 202.2 197.7 197.7 197.7
So even with a 30 mph wind 45 degrees off the groundtrack the error is still only 1.1 mph, or 0.055%! The major error is still going to be from the pilot's ability to hold the plane at a steady altitude during the runs and the repeatability of the winds. It really doesn't make a lot of sense to do those three-way runs where the results aren't known until you get back on the ground and put the numbers through equations or a spread-sheet program. The two-way run, using pre-flight forecast winds and GPS ground-track runs will give as good or even better results than the three-way since there is more chance for pilotinduced errors when having to hold the plane steady for three runs than for two. Plus, you have the data at hand to see the results immediately and be able to do a second set for data redundancy and repeatability! But there are some that think that because they use a computer to do the number-crunching that somehow their results have more significance than just jotting down numbers in flight. They have sort of a techno-snobbishness or pedantry that somehow their numbers are better or classier! Paul Lipps
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