RCM&E 20 PAGE FREE PREVIEW August issue

FREE SLINGSBY PRO-PLAN! LAN! Slingsby Prefect PART 2

Avro Shackleton www.modelflying.co.uk

VOL.66 NO.8 AUGUST 2023

OWN DESIGN MASTERCLASS

RED HAWK

XFLY T-7 FLIGHT LIIGHT TEST

Shadowed

by Giants Thrrotttle Up LMA COSFORD STATIC SHOW DIY DIIES SEL CARB

PLUS II MORE WALSALL WONDERS - F/F SCALE INDOOR NATS II FINELY BALANCED - FLIGHTPLAN C.G. STAND

WELCOME Editor: Kevin Crozier. Mortons Media Group, Media Centre, Morton Way, Horncastle, Lincs LN9 6JR kcrozier@mortons.co.uk

W

elcome to the August 2023 issue of RCM&E. I have just returned from my usual weekend morning flying session and the conditions couldn’t have been more perfect. The sun was shining and the wind, whilst a bit breezy, was more or less straight down the strip.

And, boy, what a strip! The chaps at the club had done their usual sterling work (many thanks to Frank and his crew) and the grass was, I think, the best I have ever seen it. It’s the nearest to a bowling green that I’ve ever flown from at that particular site and even the usual sheep deposits and divots from their hooves had all but disappeared. In short, it was a lovely day for model flying.

There was only one dampener on proceedings as for quite some time I was completely on my own! (Where do all the aeromodellers go on the best flying days?) That was until, thankfully, one of the regulars turned up for a chat and a short fly.

The weird thing is that unusually in this day and age this club normally has a very good attendance at weekends and so although it is quite a journey for me, and much farther to travel to than some of my other local clubs, I like to visit on a regular basis rather than fly on my own elsewhere.

Having said this, the following morning, for which the forecast was not that good, turned out to be another fine one so the locals turned out in force. Seems that I just got my timing wrong!

Returning to my train of thought above - ‘in this day and age’ - what I was really building up to comment on is how what I would call the ‘usual club flying days’ seem to have shifted away from the weekends and now take place during the week. I guess this is another sign of our ageing aeromodelling population and points to the fact that many model pilots are now retired or semi-retired and so can fly on any day of the week that suits them. Combine this with a host of different means of communications such as club Facebook and WhatsApp groups, rather than just relying on the old traditional ‘See you next Sunday morning!’ way of doing things means that it’s much easier to announce a flying session when the weather looks good and be sure of being joined by a few clubmates at the patch.

It’s a bit galling though for those modellers still at work and trapped in an office, factory or other workplace while their chums are out enjoying a flight or three with their models. Never mind lads and lasses, hopefully we will all get there someday – unless some bright spark decides to put up the pension age once again!

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I can’t complain though as I’m doing a job I love, and I do get to nip out mid-week occasionally for a bit of test flying and photography. Yes, I know, it’s a hard life…

It’s time now to take a brief look at what we have lined up for you in the August issue of RCM&E, starting with our regular columns.

In his latest Bench Blog, Tim Hooper devotes his column to turning up an R/C carb for a shelf borne diesel using his new machine tools. In One Man & His Shed, Dave Goodenough does without a milling machine but still manages to repair a glow motor. Then it’s over to Chris Williams for more Scale Gliding, this time from the White Sheet club’s spring meeting. Later on, Keith Jackson’s Aerobatic Scene column looks at some new models being prepared for the forthcoming F3A World Champs in Australia before Graham Ashby and Kick the Tyres in which he broaches the thorny subject of accident-prone pilots then lightens the mood with a few inspirational models.

For this month’s centre-fold Pro-Plan it’s back to Chris Williams who introduces the second part of his article describing the build and flying of a classic one sixth scale vintage glider, the Slingsby T.30 Prefect. This completes the Prefect plan and article which started in the June issue.

Our feature articles start with Creating Your Own Model in which Martin Hardy offers words of encouragement to any experienced modellers wishing to step up and design their own large scale model. Next, Shaun Garrity visits the RAF Museum Midlands at Cosford for the LMA Static Show to see a fine collection of large models on display amongst the full-size exhibits. Finally, we join Danny Fenton as he continues his report from the Scale Indoor Nationals; this month Danny concentrates on the free flight events.

On test this month David Ashby straps in to test fly the new T-7 Red Hawk EDF jet from XFly-Model. Finishing off, yours truly looks at another of the laser cut accessories offered by Flightplan Models. This time the spotlight falls on their C of G Balance.

I hope you enjoy reading it all.

Happy Flying!

www.modelflying.co.uk | August 2023

CONTENTS

RADIO CONTROL MODELS & ELECTRONICS I VOLUME 66 ISSUE 08

ONTHECOVER

Photo: Chris Williams This month Martin Hardy begins a two-part article on Creating Your Own Model. Part 1 covers aircraft selection, making a practical flying model, sources of information, determining the best size and what you may need by way of a plan. Part 2 (September issue) provides, as an example, an overview of the build of a 7’ 6” span electric powered Mk.2 Avro Shackleton.

22

Reviews

10 XFLY T-7 RED HAWK

Regulars 8

XFly-Model have released a larger retract equipped version of their popular T-7. David Ashby straps in for a flight test

44 FINELY BALANCED

SWITCH ON

The Editor looks at another of the laser cut accessories offered by Flightplan Models. This time the spotlight falls on their C of G Balance

Our latest round up of model flying news.

59 COUNTERPOINT

A selection of new gadgets, kits and bits for you to buy

60 ALL WRITE

86

Have your say in RCM&E’s monthly chat room

92 GOING PLACES

Our monthly list of model shows, events and competitions for you to visit over the next few months

94 MARKETPLACE

Sell off your unwanted airframes and engines or maybe buy a few new ones

95 NEXT ISSUE

Take a look at what’s coming in the September ‘23 issue of RCM&E

98 PARTING SHOT

A pair of DH.60 Gypsy Moths soak up the sun, artfully captured by Chris Williams

10 6 | RCM&E

44 www.modelflying.co.uk | August 2023

36

16

Features

22 CREATING YOUR OWN MODEL

Maybe you’ve reached the point of wanting to design your own large scale model. If so, Martin Hardy offers some words of encouragement

42 SUBSCRIBE…

... and receive Luftwaffe Fighters & Supermarine Secret Projects FREE!

Columns

16 BENCH BLOG

Tim Hooper machines up a replacement R/C carburettor for a longshelved diesel engine

64 LMA STATIC SHOW

Shaun Garrity visits the RAF Museum Midlands at Cosford to see a fine collection of large models on display amongst the full size exhibits

80 THE ROAD TO BULGARIA

Alex Maxfield prepares to compete as part of the GB F5J Team at the 2023 World Championships

86 SCALE INDOOR NATIONALS

Danny Fenton’s second report from Walsall concentrates on the free flight scale events

64

28 ONE MAN & HIS SHED

Dave Goodenough continues his refurb of a bargain SC glow engine

36 SCALE GLIDING

Chris Williams and chums defy the relentless depressing spring weather to enjoy a fine day of scale soaring from White Sheet Hill

72 AEROBATIC SCENE

This month Keith Jackson looks at new F3A models being aired prior to the World Champs being held later this year in Australia

28

free pro-plan

48 SLINGSBY T.30 PREFECT

48 August 2023 | www.modelflying.co.uk

Chris Williams introduces part two of his article describing the build and flying of a classic 1:6 scale vintage glider

All reasonable care is taken in the preparation of the magazine contents, but the publishers cannot be held legally responsible for errors in the contents of this magazine or for any loss however arising from such errors, including loss resulting from negligence of our staff. Reliance placed upon the contents of this magazine is at readers’ own risk.

RCM&E | 7

SWITCHON

Get in touch...

kcrozier@mortons.co.uk

BMFA PAYLOAD CHALLENGE 2023

This year’s BMFA School and University Payload Challenge was held at the BMFA National Centre on the 14th and 15th of June. It was attended by thirteen universities and one high school from across the world. The competitive spirit and the buzz was palpable throughout the site and it was good to see so many young people collaborating and working together on their models and designs.

The standard of competition was high and with a high rate of success. Most of the models were constructed to a very good standard and had been extensively tested before the competition.

There were three categories in this event: Distance - with two teams from I School Chenmai (China) and AGH University Krakow (Poland). Although there were only two teams entered for this challenge the competition was closely fought and AGH University Krakow won by one point. It was interesting to note that I School Chenmai closely matched the standards of the university undergraduates and for future years a win would not be impossible if the judging pointers around drawing standards were taken note of.

Quantity - with five teams, including four from universities in China (I School Chenmai, Hebei University of Science and Technology, University of Hong Kong and Beihang University). The fifth team came from Loughborough in the East Midlands.

The speed and precision of some of the teams loading and unloading the payload was fascinating to watch as some teams were like F1 mechanics in their speed and accuracy. This event was exciting to watch and the level of innovation and imagination was high.

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Weight - 18 teams were registered but, in the event, only seventeen competed. Again, there was an international entry comprising of three teams from China, two teams from Krakow, one team from Munich and eleven teams from UK Universities.

This was an exciting and very competitive event and most teams managed to get at least one round underway with their designs. The gusting winds made for some very nervewracking flights but thankfully there were few model casualties that could not be repaired.

In conclusion this year’s Payload competition was a great success and was enjoyed by all. There were some excellent model designs which flew beautifully and successfully met the brief. The only thing to do now is to look forward to next year with an even greater entry – we hope!

Helen Jones, BMFA Outreach Director

www.modelflying.co.uk | August 2023

MÁRIO’S MARS In the last issue we gave you a glimpse of Mário Vilaça’s latest seaplane project, ct, a Martin JRM Mars, courtesy of our Iberian correspondent Arnaldo Correia.

Arnaldo has recently been in contact again and says, ‘Just a short one to let you know Mário has finished his Martin Mars. What would be a major project for me is ‘just a quickie’ for Mário, to unwind after that gorgeous Fairey IIID (see Santa Cruz aricles, Dec 22 & Jan 23 RCM&E). He is looking forward to flying it at a meeting in July although there has already been a successful first test flight. I’ll keep you posted.’

Thank you, Arnaldo. Here’s the vídeo of the Mars’ test flight. She flies every bit as good as she looks: https://youtu.be/12qV8wJtcM0

MODELRETRIEVAL

Sad news has reached us of a fatal accident at a model flying site in Spain at the beginning of June when a modeller retrieving his aircraft from the runway was hit by another model which was either on a low pass or landing.

It would seem that the accident occurred because the pilot of the second aircraft was unaware that his fellow pilot had entered the runway to collect his model.

In light of this tragic event we asked the BMFA for comment and we were told that swift action was being taken to update the relevant section of the Members Handbook,

specifically section: 13.3 Radio Control Flying Safety Paragraph 13.3 (n) will now read: (n) NEVER assume that the landing area is clear even if you have called landing. In emergency situations call for help from your fellow flyers and always be prepared to land in a safe place off the landing area if necessary.

Pilots retrieving models from the active runway must call out their intentions ‘loud and clear’ and GET CONFIRMATION from any pilots still flying that it is safe for them to do so. In ALL cases, the safety of people is paramount.

exposed scale radial engine, three channels (rotor x 2 and ESC) and 400g flying weight.

It is a nice model but is quite delicate. As with all autogyros it’s important to spin the rotors up to flying speed before launch.

The distinctive whoosh and flight characteristics add to the fascination of these aircraft, which

contributed so much to the later success of the helicopter.’

Thanks for sending pictures of your model, Alisdair.

LAheli’s autogyro kits can be viewed in the Autogyro section at: https://www.laheli.com

CIERVA C.30

Alisdair Clark was quick to respond to our recent request for pictures of radio controlled Cierva autogyros: ‘I noticed in the June issue the anniversary of the first successful flight of an autogyro designed by Juan de la Cierva and your interest in any scale R/C versions of these unusual aircraft.

Please find two photos attached of a C.30. This is from the LAheli kit, with a 690 mm rotor diameter, August 2023 | www.modelflying.co.uk

RCM&E | 9

T-7A RED HAWK X-Fly have released a larger retract equipped version of their popular T-7. David Ashby straps in words & photos » David Ashby

Y

es, I had to look it up. But now I know that Boeing-Saab’s T-7 Red Hawk is an advanced jet trainer selected to replace the Northrop T-38 Talon. It bears more than a passing resemblance to the F-18 and the fifth-generation fighters to which pilots will advance. Or, as some have colloquially commented, it’s a a bit like what you’d get by mating an F-16 with an F-18.

A newish brand XFly may be but the releases to date have impressed reviewers. This is their second T-7A and it joins the smaller fixed-gear 64mm fan version. The design suits an electric ducted fan (EDF) model, particularly thanks to a wide fuselage that’ll happily accept larger 6S LiPo batteries.

Ten servos - a mix of 9g and 13g units - are scattered across the airframe. There are LED wing tip and landing lights too but it’s the electric retractable undercarriage that really

10 | RCM&E

impresses. If there was a league table for readyto-fly electric retracts then the beautiful items fixed to FMS’ F-7F Tigercat would be near the top, alongside those you’ll find here. They’re scale, all-metal, very strong, beautifully made and accompanied by sequenced bay doors. I could watch ‘em go up and down all day.

If you’ve flown a few foamie jets, particularly those with retracts, then you’ll appreciate some of the small details that contribute to a model’s success. Reassuringly there are plenty here.

For starters there are the strong horns with ball-socket links, while the all-moving stabiliser arms and mounts, potentially an area where problems may occur, are very sturdy. The wings use a quick connector to engage the aileron and lighting electrics so removal for transportation and storage is quick and painless. The same goes for the fins.

www.modelflying.co.uk | August 2023

TESTING, TESTING | EDF jet

This is the Boeing-Saab T-7A. I had to look it up too.

August 2023 | www.modelflying.co.uk

RCM&E | 11

It’s a contemporary design. Amongst others, there’s a bit of F-16 and F-18 in there.

“A newish brand XFly may be but the releases to date have impressed reviewers” ASSEMBLY

A bespoke foam tray protects the airframe sections very nicely but don’t confuse the temporary wing joiner (that’s designed to support the fuselage in transit) with the longer example also included. The latter is the one you’ll need for flying.

Initial inspection revealed a loose wooden cover over the ESC but accessed from the fuselage underside it was nothing that a little UHU Por glue couldn’t fix. That aside, everything was nicely finished and superbly protected by the packaging.

Guided by a very thorough manual, a 2mm hex wrench is all that’s required to put everything together. Amusingly, the fins seem little bigger than the wings, but I guess that’s contemporary military jet design for you. Everything slips together reassuringly well though. If your car is small, or storage space tight, then the model could be quickly and easily disassembled thanks to the integrated wing root connectors. Don’t bin the protective packaging tray as it could be used to store the sections between flying sessions.

An all-moving stabiliser needs strong mechanics and that’s certainly the case here.

13g and 9g digital metal gear servos do the pushing and shoving.

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DATAFILE Name: Model type: Manufactured by: UK distributor: RRP: Wingspan: Fuselage length: All-up weight: Wing area: Wing loading: Power system: Connector type: Req’d to fly:

T-7A Red Hawk Ready-to-fly 80mm EDF jet XFly Model CML Distribution www.cmldistribution.co.uk £549.99 975mm (38.4”) 1360mm (53.5”) 3150g (111oz) 21.5dm2 (333 sq.in.)

146g/dm2 (47oz/sq.ft.)

3280 size 2200kV inrunner, 80mm 12-blade fan, 100A ESC XT90 Receiver, 6S 40005000mAh LiPo

Wing servo arm protection. Another nice touch.

www.modelflying.co.uk | August 2023

TESTING, TESTING | EDF jet

A full-length canopy and cockpit moulding covers the battery and radio bay.

Battery bay is commodious and easily accepts the 6S 4000-5000 LiPos suggested.

SET UP

Apparently, Boeing and Saab intend to offer an attack fighter version of the T-7 in due course so XFly have included plenty of underwing pylons and ordnance. They slot into place easily enough although I suspect that few will fit such a draggy payload. The low-slung drop tank will impede take off long on grassy strips too.

Besides, I think the model looks far better flying clean.

It’s important to calibrate the ESC before flying. The manual explains how that’s done while also providing general programming instructions, which is handy should the ESC find itself in another model down the line.

There ware no flaps, although flapperon (both ailerons down) deflection measurements are provided, along with the necessary compensatory down elevator. The aileron servo Y-lead must be removed in order to do that of course.

Retracted, the undercarriage shifts notably forward, so I balanced my model with the undercarriage down. Although the C of G moved forwards a few mm when the gear was up it’s not enough to worry about.

A mixing board and my Rx sit just behind the nose cone.

“XFly have included plenty of underwing pylons and ordnance”

The retracts are beautifully done. Note the powerful landing lights.

Likewise, the nose leg is a substantive bit of kit.

Underwing stores are provided although I suspect few pilots will fit them. Note the absence of flaps.

August 2023 | www.modelflying.co.uk

RCM&E | 13

SKYWARDS

Try as I might, I’ve been unable to fly this model on better days this spring. The weather has just been so windy. In some ways that’s good as a decent headwind does help an EDF jet get away during take-off, especially one with a high wing loading. This T7-A needs a good long take off run, which is something to bear in mind if your take-off surface is grassy and on the shorter side.

I don’t drop the wing trailing edge (T.E.) for take-off, but she unsticks with a push of elevator and climbs away comfortably.

On occasions, when the end of the runway approached and I’ve had no option but to bite the bullet and pull elevator, its lifted and rocked the wing, but never stalled or fallen out of the sky. With a 12-blade fan spinning away the sound is great and the model behaves predictably.

The low rates suggested should be fine for most flyers and while the manual makes no mention of exponential it’s best to fly the T-7 first and see what you think before dialling that in. Just a little, 15% across ailerons and elevator, suits me. It rolls quickly at low rates but not uncomfortably so.

EDF jets like this are best flown smoothly; carve big lines and you’re doing the right thing.

Low passes look and sound impressive and loops can be flown from level flight.

I deploy flaperons for landing, with ‘servo slow’ enabled at the Tx, so making sure the transition is smooth. The compensatory down elevator mix mentioned in the manual does prevent the nose from rising as a result. She’s very well behaved with the T.E. lowered and the effect certainly helps to scrub off speed on Push the throttle and it’ll move along at an appreciable rate of knots.

I tend not to drop the trailing edge for take-off.

approach, even on a windy day. I guess the undercarriage and wide gear bay are doing their bit to slow the model by adding drag too, but it means I haven’t really noticed the absence of flaps.

ALL DONE

I’d really love to report that this model rewards with an impressive endurance but my 6S 5000mAh packs (700g) deliver no more than four minutes of economical flying. I set the timer for three minutes, so ensuring I have some reserve for a go-around if necessary.

Plenty of good things are going on here though; it’s well made with a fine finish, has a strong retractable undercarriage and it flies well with no vices that’ll catch out experienced EDF pilots. If your flying field is small or rough then it’s probably not for you, although the same can be said for many comparable machines.

Landing flapperons and a slight nose up attitude help scrub some speed for landing.

“…it’s well made with a fine finish, has a strong undercarriage and flies well with no vices”

Below: This is XFly’s second T-7A and it joins the smaller fixed-gear 64mm fan version.

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www.modelflying.co.uk | August 2023

GIVE THE PAW A

CARB Tim Hooper machines up a replacement R/C carburettor for a long-shelved diesel engine words & photos » Tim Hooper

This month’s victim is this PAW .2.49 diesel, seen here sporting its shiny new carburettor.

16 | RCM&E

www.modelflying.co.uk | August 2023

BENCH BLOG | Column

A

s the custodian of a small collection of diesel engines, I’ve often wished that more of them could be brought back into active service i.e., doing what they should be doing - powering model aeroplanes around the skies - as opposed to sitting on a shelf.

So, why aren’t they back in active service?

Well, unlike my hardier free flight brethren, I’m an R/C man and having access to an inflight throttle control is, for me, a necessity.

Unfortunately, most of my little collection of vintage diesels pre-date R/C throttles and have a fixed-size induction venturi which precludes any form of speed control in the air.

Aeromodellers of a certain, senior, age will know all this already so forgive the preamble, okay? However, if you’re of fresher (electric) blood then a model without a speed control is maybe a scenario you’ve not encountered before.

If you’ve been following my adventures in metal machining over the last few episodes, you might recall that one of my ultimate aims is to manufacture a working two-stroke engine from scratch. And my route towards that goal is to start by making replacement parts for some of the engines I already own. A little while ago we looked at making a replacement aerofriendly cylinder head for a .75cc DC Merlin, which worked out well.

Now, we’re getting a bit more serious. I have two PAW 2.49 diesels - one has a factory-made R/C throttle and has found its spiritual home in the nose of my Murg vintage model, which it’s been hauling around the air for a few years now. The other, alas, has no throttle and sits forlornly on the shelf.

The plan is dead simple: copy the R/C carb from the functional engine and then fit the copied carb onto the shelf sitter. And, yes, before anybody says it, I know that you can buy a nice new carb direct from PAW (still alive and well in Macclesfield, by the way) for a paltry £25 or so, but that’s not really the point, is it? If I wanted an easy life, then I certainly wouldn’t be an aeromodeller!

The methodology was going to be quite simple: strip the existing carb, make a note of all the salient dimensions and then just manufacture a new set of components on the lathe and milling machine. It shouldn’t take more than an hour or two, right? Well, no, as it turned out…

MAIN BODY

Electing to start with the main body of the carb, I used the bandsaw to hack an appropriate lump of aluminium from a larger block, after which it was milled on all six faces down to size, using a digital vernier gauge to check on dimensions.

Still on the mill, I cross-drilled a 10mm hole through the block to accept the throttle barrel later and then turned it through 90° so I could drill a 6mm hole through its entire length. All of the block’s outside edges were deburred with a fine file and an old scalpel blade took care of the burrs around the holes.

The barrel needs to be a close, smooth fit in

August 2023 | www.modelflying.co.uk

No need to re-invent the wheel.

I already had a PAW with an R/C carb on which to base my rendition.

the body of the carb. If it’s slack and leaking air, then the carb won’t be able to suck fuel through from the tank. I popped a length of 12mm brass rod into the lathe’s chuck and turned down the end by 15mm so that it was just too large to enter the barrel. The final sizing was down to using a strip of abrasive paper, a little at a time, as the piece spun in the chuck. Lightly holding the two ends of the paper strip between the fingers was the order of the day, so keeping my clumsy digits out of the range of the chuck’s spinning jaws.

After a minute or so the brass rod was a snug, rotating fit in the body, so I resisted any temptation to polish it any further and attacked it with a centre drill, prior to boring the end with a 4mm drill to accept the spraybar later.

The entire brass rod was then mounted up in the mill’s vice, the centre line found and a transverse 6mm hole bored through it to become the throttle’s venturi. When fitted into the throttle body this new hole lined up nicely

A BIT AT A TIME

As an aside it’s been a bit of a revelation to learn that an ordinary drill bit is not a precision tool at all. Apparently, it’s rare to find one where the two cutting edges at its tip are perfectly equal and centred, which means that a drill used alone will tend to wander in a circle as it chews its way through a work piece, thus producing a slightly larger hole than desired. The trick is to use a small centre drill first to produce a tiny indentation and then use progressively larger drills until final dimension is reached.

with the hole through the carb’s body.

The end of the brass rod was gently bevelled for neatness, and then parted off. The resulting barrel was flipped end over and, in the chuck, it was faced and bevelled so that its finished length was just slightly greater (by 0.5mm) than the width of the carb body.

“The plan is dead simple: copy the R/C carb from the functional engine and then fit the copied carb onto the shelf sitter.”

RCM&E | 17

FOUR JAW CHUCK

Using PAW’s own carb as a guide I milled the start of the new ali body and brass throttle barrel.

The heart of the carb is the smooth, snug fit between body and barrel to allow leak-free operation. Precise measurements help get it right first time.

Using a dial test indicator (DTI) within the carb’s bore ensures that the four-jaw chuck is centred correctly.

18 | RCM&E

The next part of this little adventure was to place the carb body in the chuck and turn down the ends - not as easy as it would seem. That four-sided block is just not going to be held neatly in the standard three-jaw chuck.

Luckily the Chester DB8 lathe comes with quite a few extras, one of which is a chuck with an extra fourth jaw. So, what’s the big deal with that, then? Well, on the three-jaw chuck, all three jaws operate together when the chuck key is rotated, just the same as the chuck on a standard electric drill. Not so with the four-jaw unit, where the jaws are completely independent of each other and need to be adjusted one at a time.

This property of the four jaw is both a blessing and a total pain in the nether regions.

Yes, you can secure four-sided (or irregularly shaped) work pieces very accurately, but there’s quite a process in getting the piece centred to be exactly concentric with the centre line of the lathe itself.

There are quite a few ways to centre a part in the four-jaw chuck and after making initial adjustments by spinning the chuck at a low speed and checking by eye, I chose to mount a dial test indicator (DTI) in the tool holder.

Then, using the adjustment on the cross slide, I positioned its probe within the central bore of the carb body, just bearing on one side of the bore. Rotating the chuck by hand it was obvious by the erratic movements of the dial’s needle that the carb was nowhere near accurately centred. The procedure is to work out which way to adjust the jaws to centre the part, which was not at all intuitive as far as I was concerned.

The jaws must be treated as opposite pairs.

When the jaw nearest you is loosened then the chuck must be rotated by 180° and the opposite jaw tightened by a corresponding amount to move the part over very slightly. And when I say ‘slightly’ we’re talking about hundredths of a millimetre here, so a light touch is definitely required.

When one pair of jaws is getting close to centred, then we’ll rotate the piece by only 90° to engage the other pair of jaws and do it all again. When we’re close, we’ll check the first pair again, then the second to finish up. Phew!

If all this sounds very tedious, then you’d be right. Centring the carb body for the first time took me about twenty minutes but the result was that, as the chuck was rotated, the DTI needle in the central bore barely quivered. This meant that the DTI could now be swapped for a cutting tool and the bottom of the carb body could be turned down to match the intake port on the PAW’s crankcase.

It wasn’t possible to try the carb for fit on the crankcase in situ, so I rechecked my measurements yet again before releasing the carb body from the chuck by slightly loosening two adjacent jaws and trying it for real. Luckily the body slid home into the crankcase pretty perfectly, otherwise I’d have had to centre it up again for more machining to get it right.

Time for the other end of the carb body, which was destined to form the air intake.

www.modelflying.co.uk | August 2023

BENCH BLOG | Column

Inlet and outlet stubs are machined to size. On the right is the fixed venturi that was the engine’s original fitment.

As the chuck jaws were still where I’d left them, it only took a few minutes to flip the carb around and true it up again. I’ve watched videos where experienced machinists can centre up a four-jaw chuck from scratch in a couple of minutes, so practice obviously makes perfect. We can only hope so.

BLOWING A TRUMPET

The carb on an original PAW has a parallel intake tube but, feeling brave, I thought I’d have a bash at machining a tapered, trumpet shape, just for appearance’s sake really.

Going back to basics for a moment: the lathe’s big saddle (carriage) moves along the lathe bed and the smaller cross slide moves perpendicularly to the saddle and moves across the bed. On top of the cross slide sits the much smaller compound slide, which can be set at a wide range of angles - handy for producing tapers, angled bevels, etc.

I set the compound slide at a 10° angle and, using a miniature boring bar, I widened the carb’s 6mm centre bore to give an internal bell mouth. Changing the cutting tool again let me cut the outside of the bell, as well bevelling the top of the carb body too. This was a particularly rewarding operation as it let me stamp a hint of individuality on a machined part.

NEEDLE VALVE ASSEMBLY

The next components to consider were the needle valve and spraybar assembly (NVA) and here’s where things got a bit fiddly.

If you’re not IC conversant, then here’s how it works. Fuel is sucked from the tank by the engine, which acts like a suction pump. As the piston moves up and down it creates a partial vacuum in the crankcase, which encourages the outside air to enter the carb and then into the engine. On the way through the carb the air passes either side of the spraybar and is compressed by the restriction. This restriction causes the moving air to speed up a bit, resulting in an area of lowered pressure. This low-pressure acts on the fuel filled hole in the spraybar, thus sucking fuel into the engine.

That’s a very simplified way of explaining what goes on in the carb, and represents the limited range of my own level of physics

August 2023 | www.modelflying.co.uk

and fluid dynamics, so don’t deluge me with corrective emails, okay?

The spraybar is a turned bar of brass, bored through from either end. On the fuel inlet side there’s a 1mm hole, whilst on the other is a 1.5mm hole. Where the two holes meet a tiny internal shoulder is formed. There’s an additional 1mm hole bored through the side of the spraybar, just adjacent to this shoulder.

The pointy needle valve fits through the larger 1.5mm side and seats against that little shoulder, thus cutting off any fuel flow. As the needle is unscrewed the inlet hole becomes progressively opened, so letting the fuel through. The released fuel can now exit the spraybar through that hole in its side and be sprayed into the venturi and thence into the engine. That’s the theory, as I understand it.

In practice a 35mm length of 8mm brass bar was chucked up and bored with a puny 1mm drill. Another part of the learning curve with this machining malarkey was to learn that the smaller the drill bit, then the faster it must rotate. I’ve always run small drills at slow speeds to save them from breakage and then wondered why they actually did break so often!

The trick is to run small drills at high speed (1500 rpm in this case) and to feed them in very slowly, using regular drips of oil to ease their passage. The drill’s flutes soon clog with swarf the deeper it gets so it needs to be retracted and cleaned repeatedly. The spraybar was then flipped around in the chuck, and the other end was drilled with the bigger 1.5mm drill to create that tiny internal shoulder.

The outside of the 8mm brass bar received attention next. The reasoning behind such a ‘large’ diameter bar is to be able to leave a portion near to one end at full diameter, which will butt up against the throttle barrel.

The part of the spraybar that will live within the barrel was turned down to 4mm and then

treated to a matching 4mm external thread for part of its length, which will accept the needle shroud.

The central part of the spraybar was turned down to 3.5mm and then the side hole was drilled on the mill. Two flats were milled on the 8mm portion to allow the use of a 6mm spanner. Confession being good for the soul, I must admit to getting the location of the side hole all wrong and having to solder it up and re-drill it in the proper place. But don’t tell anybody, okay?

Back in the lathe again, the other end of the bar was turned down to create a nipple for the fuel tube to cling onto. I also turned up a brass disc, with an internal thread and external flats to suit a 10mm spanner. This disc acts as a nut that will trap the throttle barrel on the spraybar shaft.

NEEDLE POINT

The steel needle came next and it started as a short length of 16SWG piano wire. I’d been pondering the best way to taper one of its ends to create the pointy bit. The point is destined to penetrate that tiny shoulder within the spraybar. As the needle is unscrewed it retreats out of the central hole, progressively opening the fuel inlet as it goes.

Now, piano wire is tough, springy stuff, so I figured that the best way to taper one end would be to grind it down whilst it rotated at high speed. In a burst of clarity, I figured out the easiest way to achieve a shallow taper on such a skinny length or wire would be to mount it in the chuck of the Dremel, spin it up to top speed and then bring it into contact with the side of the rotating grinding wheel of my bench grinder.

This may sound a bit dodgy, but it worked so well that I ground down another couple of needles for future use whilst I was at it.

“This was a particularly rewarding operation as it let me stamp a hint of individuality on a machined part.”

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BENCH BLOG | Column

Whilst the needle was still in the Dremel’s chuck I polished it smooth with everdecreasing grades of wet ‘n’ dry paper. The completed needle was soft soldered into the brass thimble shroud.

THROTTLE ARM

The very last piece to make was the throttle arm, which serves to rotate the barrel when wiggled by the appropriate servo. This was cut from thin ali sheet and drilled.

When the carb was assembled, I found that there was a little bit of lateral play between the barrel and the carb body, as the barrel protruded on either side.

To counter this side play I milled a shallow (0.5mm) groove in the end of the barrel. Not only did this take up the unwanted play but it also gave the throttle arm a definite location so that it couldn’t turn without turning the barrel too.

The piano wire needle valve was spun in the Dremel, then gently pressed against the side of the rotating grinding wheel.

Flushed with success I made two further needles whilst I was on a winning streak. Abrasive paper got them to an acceptable polished finish.

Needle fully inserted in the spraybar and being soft soldered into its brass thimble.

I even got to play with my new knurling tool on the thimble!

Out came the phone and there’s a short video on YouTube: https://youtu.be/v6r-d3e9zVE So, that’s that. The next step is to sort out a suitable airframe to give the PAW a home but that can wait a while.

occupies the whole of a Bench Blog column so apologies to all who would rather read about balsa work. I have to confess that the level of detail and focus needed to produce this little carb has taken any previous thought and planning required to a whole new level. For a relative newbie like me it’s been a fairly intense experience, but very enjoyable for all that!

TESTING TIME

At the end of all this the new carb was installed on the PAW engine, which was itself installed in the test stand to await a trial run. I didn’t have long to wait for a suitable day.

I’d only run the PAW once before and that was over a year ago when it first came my way. Since then, it had sat on the shelf, dosed up with automatic transmission fluid (ATF), which is my usual after-run preservative. I’d like to report that it fired up straight away, but it took several minutes to flush the ATF out and get it saturated with new diesel fuel, when it started to cheerfully pop away.

So yes, it started and ran on the few drops of fuel in the ‘tank’ (an old syringe). Refuelled, it started readily so I jiggled the throttle pushrod and was happy to see it respond.

FINAL THOUGHTS

Well, it’s not often that a single project

Parts of the new carb all present and more or less correct. The saw cut on the thimble let me press the threads together to give a friction fit on the spraybar, to stop it vibrating loose when the engine is running.

Old and new. Original PAW carb features a throttle stop screw but I’m happy to use the Tx to limit the travel as required, so I didn’t bother fitting one.

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These things don’t just happen by themselves. There’s a lot of working out to be done and that’s part of the joy of it. Here’s the sketch of the needle and spraybar.

www.modelflying.co.uk | August 2023