Ducted Fan or Shrouded rotor Aerodynamics and its applications in VTOL Miniature Aerial Vehicles

Ducted Fan or Shrouded rotor Aerodynamics and its applications in VTOL Miniature Aerial Vehicles

AE 416 Project Kailash Kotwani

Contents Abstract 1.

Introduction 2.

MAVs- Introduction 2.1 MAV- Definition 2.2 MAV-Applications 2.3 The need for focus on SDR-VTOL MAVs 2.4 Issues involved in aerodynamic design of SDR-VTOL MAVs 3.

Low Reynolds Number Aerodyanmics 4.

Mini Propeller Aerodyanmics 4.1 Experimental Measurement 4.2 Blade Element Theory based analysis 5.

Shrouded/Ducted Fan Aerodyanamics 5.1 Qualitative Understanding 5.2 Quantifying using simple theory 6.

Current examples and future 7.

Conclusion References

Abstract The goal of this paper is to review the applications and need of shrouded/Ducted rotor based VTOL MAVs (SDR-VTOL MAV) and aerodynamic challenges involved in its design. Initially through logical reasoning it has been emphasized that SDR-VTOL MAV proves to be superior platform for MAV class applications. The discussion in this paper has tried to generalize MAV class platform suiting to almost all sizes and configuration in this class of vehicles. Major aerodynamic concepts involved in its design: understanding low Reynolds number flows, Characterization of MAV class propellers/rotors and understanding duct-rotor interactions. Throughout the text different configurations of SDR-VTOL MAVs have been discussed.

Keywords: Rotor VTOL configuration, Shrouded/ducted rotor Aerodynamics, Unconventional Mini Aerial Vehicle, Low Reynolds Numbers Aerodynamics, Actuator Disc theory, Blade element theory, Vortex theory Abbreviations: MAV = Miniature Aerial Vehicle (synonymous for Micro or Mini Aerial Vehicle) VTOL = Vertically Take-off and Landing SDR-VTOL MAV = Shrouded/Ducted Rotor based VTOL MAV RPM = Revolutions Per Minute BET = Blade Element Theory CP CT D R J n Ps ηP PA TA V∞ ρ

= = = = = = = = = = = =

coefficient of power coefficient of thrust diameter of propeller radius of propeller advance ratio RPS or rotational frequency shaft power efficiency of propeller power available thrust available freestream flow velocity density

Introduction Recent advances in electrical and mechanical system miniaturization have spurred interest in finding new solutions to an array of military and civilian missions. One such solution is mini aerial vehicle (MAV). The applications are mainly reconnaissance, surveillance, traffic monitoring, meteorological studies, hazard mitigation etc. The advantages of these vehicles are small size, less weight, low cost, low operation cost etc.

Literature survey has revealed that tremendous amount of research is being carried in the area of design and development of MAVs for above mentioned mission goals. Most of it is focused on development of conventional fixed wing configurations whereas rotary wing vehicles have significant advantages over fixed wing vehicles for these types of missions particularly when the vehicle is required to remain stationary (hover) or maneuver in tightly constrained environments. For example, intelligence gathering around or within buildings requires a hovering vehicle with good maneuverability characteristics.1 Deployment of shroud/duct augments thrusts by 25 to 30 % than what is produced by open rotor of same diameter. This concept offers potential advantages for mini/micro class vehicles where every extra gram in total weight of vehicle plays critical role in performance. The shroud around rotor is multi-functional: it supports rotors, produces a portion of the lift, and contains propulsion, avionics, fuel, payload and other flight related hardware. This configuration also enhances vertical and operator safety for operations in confined areas by protecting the rotor from tip strikes.

Increase in thrust is due to several factors and aerodynamics here is much more complicated than that of open rotor. One of the reasons is due to small clearance between rotor tips and shroud; this helps in reducing rotor tip losses and hence increases rotor thrust. Second reason, upstream and slipstream flow of rotor creates pressure distribution on shroud in such a way that net effect creates extra thrust in upward direction. For a given rotor, this extra force can be optimized by varying parameters like size and shape shroud, location of rotor along vertical axis and tip clearance.

Adjacent Fig. shows Cypher an example of two shrouded co-axial counter rotating based vehicle. This vehicle is very innovative because it is the only vehicle which uses collective and cyclic pitch variation on its rotor blade to control life and moments. The result is highly maneuverable platform. The difference between shroud and duct is shroud has high width to length ratio. Comparatively duct are thin and very long.

Overall this project will discuss about important features of MAVs, complexities involved in understanding of Low Reynold’s number aerodynamics and its implementation to propellers and rotors, important aspects of duct/shroud rotor aerodynamics and its application in different concepts of VTOL MAVs. This paper also presents some of the successful examples of SDR-VTOL MAVs and what can be future advances and challenges for them.

2.

MAVs – Introduction

2.1

Definition: its miniature, micro or mini?

In different resources MAV has been referred to convey platforms of varying sizes. At many places it has been mentioned as Mini Aerial Vehicles (replacing outdated RPV remotely piloted vehicles term) having span between 1 and 8 ft and weight of the order of 1 to 10 kg. At many places it has been referred as Micro Aerial Vehicle. The definition employed in DARPA's program limits Micro Aerial Vehicle’s to a size less than 15 cm (about 6 inches) in length, width or height 18. Lets not get bogged down with the definition based on size or weight.

Binding definition of MAVs based on number will never bring all these class of vehicle under certain limits of weight or size and will always be ambiguous. Mini usually represents scale of the order of 10 -3 m and micro is for 106 m though none of the important dimension (like span or height) in these class of vehicles is of that order. Whether micro or mini all these vehicles possess few following similar characteristics.

1) They are very portable. The whole system can be carried, launched, deployed and controlled remotely by single person.

2) They are completely autonomous, semi-autonomous or remotely piloted eventually eliminating the need of human on the site of danger or site to be observed or sensed.

3) They are low cost and dispensable vehicles (or very easy to repair) carrying a particular or many sensors depending on given military or civilian mission.

Any Vehicle satisfying all of these characteristics will be broadly termed as Miniature Aerial Vehicle and lets keep this definition of MAV for our purpose of text. MAVs are designed, ranging from backpackable systems to insect-sized "mesicopters", and miniature "smart dust" sensors. They can be launched by hand, deployed by larger UAVs, or ejected from artillery or mortar projectiles, as expendable sensors and all done by one person. Fig. 1 shows backpackable Casper MAV designed for military surveillance purpose hand-launched by a soldier on battlefield. Fig 2 shows tiny Black Widow of span 6 inch whereas Fig. 3 shows bigger Aersonde. Table 1 compares their strikingly different parameters though both comes under miniature class vehicle satisfying characteristics mentioned above.

Fig. 1: Soldier launching Casper

Table 1: Comparing size: Black Widow and Aerosonde Characteristics Black Widow Aerosonde 1. Wingspan 6 inch 9.5 ft 2. Length 6 inch 5.4 ft 3. Weight 56.5 gm 30.9 lbs 4. Altitude 500 ft 20,000 ft 5. Range 0.5 nmiles 1800 nmiles 6. Endurance 33.4 min 40 hrs 7. Cruise Speed 25 mph 70 mph Fig. 2: Black WidowBattery powered motor Fig. 3: Aerosonde 8. Propulsion Gasoline Engine 9. Application Capturing real time images Collecting atmospheric data 2.2

Applications Military Interest: MAVs are not small versions of larger aircraft, but they are fully-functional, militarily capable, six-degree-of-freedom aerial robots. Their mobility provides the capability of deploying a useful micro payload to a remote or otherwise hazardous location where it may perform any of a variety of missions. Such missions may include reconnaissance and surveillance, targeting, tagging, and bio-chemical sensing. Initial missions for MAVs would include reconnaissance and surveillance, but they also could encompass targeting artillery and mortars, assessing battle damage, carrying acoustic sensors to listen for the movement of heavy equipment, and transporting detectors to sense radiation or biological and chemical weapons (As shown in Fig. 4, 5, 6).

Initial development of the MAVs has been spurred on by military interest in producing miniature intelligence-gathering planes. Various defense programs hope that the devices will give small military units direct access to reconnaissance data. MAVs could help them in battling an enemy just over a hill or in engaging them in streetto-street fighting in an urban setting. MAVs are envisioned as an asset at the platoon level or below giving the individual soldier on-demand information about his surroundings and for providing increased situational awareness which results in greater effectiveness and fewer casualties.

Fig 4: Over the hill reconnaissance

Civilian Applications:

Fig 5: Detecting chemical weapon

Ultimately, MAVs could also be adapted to many civilian applications including guiding fire and rescue operations, monitoring traffic, forestry and wildlife surveys, border surveillance, observing crops, real estate aerial photography, and furnishing information to police patrols.

2.3

The Need for VTOL MAVs

So far we looked into the applications of MAVs here we will VTOL MAVs that is special class of the factors that give them advantage fixed wing MAVs. Applications will require a vehicle with very slow stationary as something is to be sensed Fixed wing vehicles have certain stall flightworthiness below that velocity is hovering vehicle will do better job intelligence gathering around or within hovering vehicle with exceptional characteristics (as shown in 6).

definition and focus on the need for MAVs. We will explore over the conventional explained in Fig. 4 and 5 loiter velocity or almost or captured using camera. velocity and maintaining almost impossible. Hence here.

For example, buildings requires a maneuverability Fig.6: Urban intelligence or warfare

Based on discussion so far, at first sight miniature version of helicopter will appear as plausible solution for these applications. But operations in confined areas raise questions about the protection of rotor blade. Secondly, noise generated by rotor will not suite for intelligence gathering requirement. All these arguments makes shrouded/ducted rotors based VTOL MAVs (SDR-VTOL MAVs) as the optimum solution for these applications.

Potential advantages are 1) Augments the lift/thrust produced by rotor- - very crucial miniature class vehicles 2) Protects the rotor from tip strikes 3) Safe for people operating in confined areas 4) Damps the noise of rotor 5) Shroud/Duct can be used as housing for various avionics, sensors, propulsion system and other equipments Allied Aerospace developed the iSTAR (Fig. 7) family of high speed, vertical takeoff and landing (VTOL) MAVs utilizing Lift Augmented Ducted Fan (LADF) system in response for the US Army Organic Aerial Vehicle (OAV) program. The design concept is simple and efficient and makes use of lightweight composite construction techniques.

The structure is comprised of an outer duct enclosing the fan system, centerbody containing the avionics and engine, fixed stators and movable vanes operated by actuators, performing thrust vectoring. The engine is housed in the centerbody, and fuel tanks are located in the forward section of the duct. A variety of payloads may be carried in the nose, tail or duct of the vehicle. Unlike other VTOL UAV’s, the iSTAR utilizes the airfoilshaped duct to provide augmented lift during low and high-speed cruise.

Vehicle control is provided by thrust vectoring resulting in a highly stable and controllable vehicle during all phases of flight

Fig 7: iSTAR SDR-VTOL MAV

2.4

Issues Involved in Aerodynamic Design of SDR-VTOL MAV

Here we will briefly discuss issues involved in aerodynamic design of shroud/duct rotor combination of SDRVTOL MAV. In following chapters we will discuss them in detail.

1. Low Reynolds number SDR-VTOL MAV operates in low Reynolds number regime (usually 1000 to 0.1 million). The low Reynolds number regime is significant in that it projects a fundamental shift in physical behavior at MAV scales and speeds an environment more common to the birds and the largest insects. While naturalists have seriously studied bird and insect flight for more than half a century, our basic understanding of the aerodynamics encountered here is very limited. Neither the range - payload performance of bees and wasps nor the agility of the dragonfly is predictable with more familiar high Reynolds number aerodynamics traditionally used in UAV design. And if our understanding of low Reynolds number effects is limited, our ability to mechanize flight under these conditions has been even more elusive.

2. Unsteady Effects Unsteady flow effects arising from atmospheric gusting or even vehicle maneuvering are far more pronounced on small scale MAVs where inertia is almost nonexistent, that is, where wing loading is very light.

Sufficient aerodynamic data at various unsteady conditions will be required to achieve an autonomous platform.

Shroud or duct will behave as extremely bluff body for atmospheric gusts.

3. Aerodynamics of Propulsion System Small-scale propulsion systems (shroud/duct rotor combination) will have to satisfy extraordinary requirements for high energy density and high power density. Acoustically quiet systems will also have to be developed to assure covertness. To better understand some of the propulsion issues, consider the power equation for a rotor/propellor driven aircraft as shown in Figure 8. This relationship provides insight into ways to reduce the power required for propulsion. First, we need good aerodynamics (high lift to drag ratio). But low Reynolds severely affects the lift to drag ratio of these class vehicles. Propeller aerodynamics must also be efficient, but propellers smaller in diameter have poor efficiency, on the order of 60 percent or less. So low Reynolds numbers affect propulsion in two ways: Poor lift to drag ratios increase the power required, and propeller efficiencies are low.

Fig. 8: Simple Power Requirement Equation for isolated rotor

Finally, still in reference to the power equation, three is nothing more effective than low weight to reduce power requirements. Technologies like MEMS, low power electronics, and component multifunctionality will help.

High energy density (i.e. light-weight) power sources are essential. Battery-based systems will likely power the first generation MAVs, but more exotic technologies like fuel cells are being developed for follow-on systems.

4. Shroud/Duct Aerodynamics Apart from other reason to deploy a duct or shroud, main reason is duct-rotor or shroud rotor combination provides more lift than that of open rotor. Shroud/Duct rotor interaction play very important role in augmenting this lift. Some of the issues which need to be understood before sizing are 1) Tip clearance between rotor and Duct/Shroud. Open rotor faces tip losses due to tip vortices. Here tip losses are reduced due to presence of duct/shroud 2) Pressure distribution over duct or shroud is such that it adds into lift 3) Location of rotor along vertical direction inside duct/shroud is chosen in such a way that it optimizes the performance that is lift/power ratio 4) Shape and size of duct or shroud affects the lift augmentation 5) It needs to be decided between optimum way to control the direction of vehicle. Whether tilting the rotor or thrust vectoring using vanes. Anyway will make aerodynamics much more complicated though both are significantly different.

6) Limiting high tip mach numbers Understanding aerodynamic design of SDR-VTOL MAVs involves following three basic areas 1. Low Reynolds Number Aerodynamics 2. Propeller aerodynamics 3. Shroud/Duct Aerodynamics and its interaction with rotor Following three chapters address these topics in detail and will finally sum-up this whole project.

3.

Low Reynolds Number Aerodynamics

The rotor blade and duct is composed of basic airfoil structure in SDR-VTOL MAVs. These vehicles are small and therefore operate at chord Reynolds numbers usually below 200,000 where very little data is available on the performance of lifting surfaces.

Figure 9: Reynolds number range for flight vehicles

All low Reynolds number vehicles share the ultimate goal of a stable and controllable vehicle with maximum aerodynamic efficiency. Aerodynamic efficiency is defined in terms of the lift-to-drag ratio. Airfoil section Clmax, Cdmin, (Cl/Cd)max as a function of Reynolds number are shown in Figures 10a, 10b, and 10c after McMasters and Henderson (1980). It is clear from this figure that airfoil performance deteriorates rapidly as the chord Reynolds number decreases below 100,00.

Fig. 10: Airfoil Performance

Although design methods developed over the past 35 years produce efficient airfoils for chord Reynolds numbers greater than about 200,000 these methods are generally inadequate for chord Reynolds numbers below 200,000 especially for very thin airfoils. In relation to the airfoil boundary layer, important areas of concern are the separated regions which occur near the leading and/or trailing edges and transition from laminar to turbulent flow if it occurs. It is well known that separation and transition are highly sensitive to Reynolds number, pressure gradient, and the disturbance environment. Transition and separation play a critical role in determining the development of the boundary layer which, in turn, affects the overall performance of the airfoil. The aerodynamic characteristics of the rotor, wing and other components in turn affect the static, dynamic and aeroelastic stability of the entire vehicle. Therefore the successful management of the sensitive boundary layer for a particular low Reynolds number vehicle design is critical.

The survey of low Reynolds number airfoils by Carmichael (1981), although almost two decades old, is a very useful starting point in the description of the character of the flow over airfoils over the range of Reynolds numbers of interest here. The following discussion of flow regimes from 1,000<Re<200,000 is a modified version of Carmichael’s original work.

1. In the range between 1,000<Re<10,000 the boundary layer flow is laminar and it is very difficult to cause transition to turbulent flow. The dragon fly and the house fly are among the insects that fly in this regime. The dragon fly wing has a sawtooth single surface airfoil. It has been speculated that eddies in the troughs help keep the flow from separating. The house fly wing has large numbers of fine hair-like elements projecting normal to the surface. It is

speculated that these promote eddy-induced energy transfer to prevent separation. Indoor Mica Film type model airplanes also fly in this regime. It has been found that both blunt leading and trailing edges enhance the aerodynamic performance.

2.

For chord Reynolds numbers between 10,000 and 30,000 the boundary layer is completely laminar and artificial tripping has not been successful. Experience with hand-launched glider models indicates that when the boundary layer separates it does not reattach.

3.

The range 30,000 to 70,000 is of great interest to MAV designers as well as model aircraft builders. The choice of an airfoil section is very important in this regime since relatively thick airfoils (i.e., 6% and above) can have significant hysteresis effects caused by laminar separation with transition to turbulent flow. Also below chord Reynolds numbers of about 50,000 the free shear layer after laminar separation normally does not transition to turbulent flow in time to reattach. Near the upper end of this range, the critical Reynolds number can be decreased by using boundary layer trips. Thin airfoil sections (i.e., less than 6% thick) at the upper end of this regime can exhibit reasonable performance.

4.

At Reynolds numbers above 70,000 and below 200,000 extensive laminar flow can be obtained and therefore airfoil performance improves although the laminar separation bubble may still present a problem for a particular airfoil. Small radio controlled model airplanes fly in this range.

5.

Above Reynolds number of 200,000 airfoil performance improves significantly and there is a great deal of experience available from large soaring birds, large radio controlled model airplanes, human powered airplanes, etc.

Laminar separation bubbles occur on the upper surface of most airfoils at Reynolds numbers above about 50,000 (shown in Fig. 11). These bubbles become larger as the Reynolds number decreases, usually resulting in a rapid deterioration in performance, i.e., substantial decrease in L/D. In principle the laminar separation bubble and transition can be artificially controlled by adding the proper type of disturbance at the proper location on the airfoil.

Wires, tape strips, grooves, steps, grit, or bleed-through holes in the airfoil surface have all been used to have a positive influence on the boundary layer in this

critical Reynolds number region. The type and location of these so-called “turbulators” and their actual effect on the airfoil boundary layer has not been well documented.

Furthermore, the addition of a turbulator does not always improve the airfoil Laminar Separation Bubble performance. In fact,Fig. how11:the disturbances produced by a given type of turbulator influence transition is not completely understood. As a result of this critical boundary layer behavior, several important issues must be addressed:

a) The free stream disturbance level and flight environment for a given low Reynolds number application. b) If the flight conditions are known and a suitable design technique was available, the resulting vehicle or component should be adequately evaluated in a wind tunnel which, in general, has a different disturbance level and environment than the flight condition? c) The hysteresis in aerodynamic forces observed in low turbulence wind tunnel experiments present in powered applications (i.e. Structural vibrations originating with the propulsion or drive system affect boundary layer transition) d) Because the critical quantities measured in wind tunnel experiments are very small, the level of accuracy should be improved in design and analysis methods.

4.

Motor Gearbox

Propeller/Rotor

Propeller

Various theories have been proposed so far to of propellers and rotors. Mostly used among theory, blade element theory and minimum We have to figure out how far these predict the performance mini propellers and applications. For this purpose we will measurement with blade element theory chapter will explain briefly about performance characteristics using equation 1, 2, 3 and 4 gives the performance η

P T ×V C ∞ =J× T = A = A P P P C s s P

(1)

(2)

PS ρn 3 D 5

(3)

CP =

V J= ∞ nD

RPM Sensor

a) Thrust Set up Torque Sensor

RPM Sensor

(4)

Aerodynamics

analyze the performance them are actuator disc Perspex base induced loss vortex theory11. conventional theories fit to Load Cell rotors used for MAV scale compare the results of experimental based analysis. Also this Rigid Clamp measurement of propeller experimental set-up. Given below parameters for propellers.

DC Motor

Bearing

TA CT = ρn 2 D 4

Motor Clamp

Rubber Shaft

DC Motor

Channel

b) Power Set up Fig. 12 Experimental Apparatus

4.1 Experimental Testing Thurst set-up and Power-setup shown in Fig. 12 is mounted inside wind-tunnel one by one. During experimental testing, velocity of wind tunnel is kept constant. Using a DC motor propeller RPM is varied from zero to maximum to cover certain range of J. Experiments are repeated for different tunnel velocities to give values of parameters (Thrust and Power) for full range of J from zero to near wind-milling condition. At each J, C T is computed using measured thrust [Eq. (2)] whereas CP is computed using measured torque [Eq. (3)].

1. Thrust Measurement Thrust measurement set-up (Fig 12a) uses a load cell of capacity suitable for class of propellers being tested (here 60 N load cell was used). DC electric motor clamped between two C-clamps fixed on perspex bench is mounted on one end of load Cell. Other end of load cell is clamped to rigid support. This load cell works as cantilever beam.

Thrust produced by propeller acts normal to the vertical axis of load cell. RPM is measured using optical sensor as explained in previous section. This set-up is mounted inside the tunnel and produces drag. Hence drag coefficient of set-up is obtained separately and used in correcting the thrust measurements. Order of magnitude of drag produced by setup is 1 N at 20 m/s freestream velocity whereas order of static thrust of propeller is 10 N. 2. Power Measurement Power measurement set-up (Fig 12b) uses a torque sensor at the centre with the propeller and motor mounted on either side of sensor. A central shaft on which the torque sensor is mounted has four bearings, two on either side of the sensor. Motor on one end works as prime mover and sensor measures the torque transmitted to the propeller.

Product of this torque with rotational velocity gives the shaft power (P s). Power losses in bearings are measured by running the motor on set-up shown in Fig. 1b under no load condition (without propeller) and characteristics of power loss as a function of RPM is obtained. Lower block shown in Fig. 1b is a channel, selected to minimize the blockage behind the propeller. Inside this channel a small optical RPM sensor is mounted. RPM sensor consists of an infrared light transmitter and a receiver. Light emitted by transmitter is reflected from white patch painted behind the propeller. Receiver generates a count each time it receives reflected light, which is used to obtain RPM value.

4.2 Blade Element theory based

Analysis

A useful method of propeller behavior is blade element which is used here to estimate the

understanding theory28,29 (BET) characteristics

Fig 13: Rotor Blade Elements

of propeller analytically. In this method the propeller is divided into a number of independent sections along the length (Fig 13). At each section a force balance is applied involving 2D section lift and drag with the thrust and torque produced by the section. At the same time a balance of axial and angular momentum is applied. This produces a set of non-linear equations that can be solved by iteration for each blade section. The resulting values of section thrust and torque can be summed to predict the overall performance of the propeller. The inputs for analysis are chord and twist variation along the blade radius and aerodynamic characteristics of airfoils at different blade radius.

Corrections for tip losses are incorporated in BET analysis. Correction in thrust term Eq. (5) was used whereas power term was adjusted by 4 percent for the same (Ref 9). Drag of hub is subtracted assuming hub as circular cylinder of known radius.

0.12 0.1 0.08 CT

One customized tool available for propeller Javaprop30 a tool for analysis and design of Adkin’s iterative scheme12. Both codes were propeller31 of known geometry that uses Clark11x7 propeller (11 inch in diameter and 7 been chosen as a candidate for experimentation chord lengths and twist angles at six equally one propeller blade were measured with the help height gauge. Later the blade was sliced at same contours of airfoils which were digitized to give co-ordinates were smoothened and fed into analysis tool32 for generating aerodynamic Convergence was a problem in XFOIL at many for these airfoils. Hence it was decided to airfoils to nearest standard 4-digit NACA measuring maximum thickness, camber and its XFOIL. The Lift and drag characteristics of airfoils at low Reynolds number were then XFOIL and were used for BET analysis.

Comparison between Experimental and has been shown in Fig. 13. This exercise shows up and right tools one can estimate performance used for MAV applications.

0.06

0.02 0 0

Qualitative Understanding

0.2

J

0.4

0.6

0.8

0.05 0.04 0.03 0.02 0.01 0 0

Shrouded/Ducted rotor

Here we will try to acquire brief qualitative physics of flow involved in ducted fans (rotor) some simple results will be provided to

Expt BET

0.04

Efficiency

5.

(5)

tip _ chord 2× R

CP

loss _ factor = 1 −

0.2

J

0.4

0.6

0.8

analysis is propellers based on applied to a Y airfoil. inch in pitch) has and analysis. The spaced sections of of caliper and sections, giving co-ordinates. These XFOIL airfoil characteristics. of angle of attacks approximate these classification by locations using these six NACA generated using analytical results that using right setof mini-propellers

Aerodynamics

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

understanding concept.

Later quantify.

0

0.2

J

0.4

0.6

Fig. 14 Characteristics of 11x7 Propeller

0.8

There are two major fields of application for ducted fans, at the high and low ends of the subsonic speed range respectively. The field of application that we have all experienced is the one least useful to us as light aircraft designers, namely the high speed range exemplified by the turbofan engines that propel the airliners on which we have all flown at one time or other. In these machines, low speed thrust/horsepower is sacrificed in order to make a ducted fan operate with reasonable efficiency at speeds approaching that of sound. Rarer in our experience are low speed applications, typically VTOL aircraft and air-cushion vehicles. Here we will concentrate on static and lowspeed thrust, because that is where the advantage of a duct is greatest.

We start with the familiar diagram (Fig. 15) of an open propeller immersed in a stream of fluid; this is how cruising flight looks from the airplane’s point of view. The flow has three regions: the inflow into the propeller, the slipstream behind it and the free stream which doesn’t go through the propeller.

Propeller design calculations are all carried out in this regime.

But before a ’plane can cruise it has to take off, and its takeoff performance depends on the performance of the propeller at low speed and at rest. With the propeller immersed in a static air mass, the picture we saw earlier changes radically (Fig. 16).

Fig. 15: Cruise Condition Now there are two regions. The slip stream behind the propeller is familiar, but there is no more free stream; everything that isn’t slipstream is in the inflow region.

This means that the propeller is aspirating air from behind it as well as from in front. Consider now the plight of an intrepid air particle starting just outside the slipstream, behind the propeller. It has to go forward to the propeller disk, make a 180degree turn, accelerate instantly and enter the slipstream.

As real air particles have mass and therefore inertia, and as open propellers have to be lightly loaded at the tips, the result is that a region of reversed flow exists near the propeller tip. This Fig. 16: Open Propeller-static Condition diminishes the effective disk area of the propeller and restricts the amount of air able to flow through it, just when the highest mass flow is needed! Now take the same propeller and surround it with a close-fitting shroud having a nicely rounded leading edge, and see how the picture changes (Fig. 17a and b).

Fig 17: Ducted Fan (a) Cruise Condition (b) Static Condition

We still have only two flow regions, but now there is a solid boundary between them in the neighborhood of the propeller—the shroud or duct—and our air particle has a much easier time doing what is expected of it, because it need only flow around a duct lip or leading edge of finite radius. We can even provide the duct lip with a slot to help keep flow attached, as suggested and tested by Krüger. The duct also shields the propeller from the harsh realities of the outside world, and the propeller “sees” air flowing in only one direction—front to rear. In fact, from the propeller’s point of view it is not at rest at all, merely cruising at some fraction of its maximum speed. What is more, the endplate effect of the duct wall allows the propeller to carry a non-zero load right out to the tip. The effects of all of this are that: • Fewer design compromises are required; the ducted propeller operates nearer its ideal operating point throughout the aircraft’s speed range • The effective diameter of the ducted propeller is larger than its physical diameter. To understand why, look at the diagram of the open propeller at cruise and note that the slipstream contracts behind the propeller.

Now look at the ducted propeller and note that the slipstream diameter is that of the duct exit. That larger diameter represents a smaller delta(v) and a larger mass flow—and by now we know that means higher thrust/horsepower.

• While the propeller develops about the same thrust as before, there is now a second force acting on the duct. If the duct is shaped correctly, this force is additional thrust.

At cruise, the advantages are less pronounced and limitations more obvious, but we can still note that the effective area of the propulsor is approximately that of the duct inlet, which may be considerably larger than the swept area of the propeller itself. We can note here a subsidiary advantage of the ducted propulsor, namely that it offers the possibility of thrust vectoring and even thrust reversal.

Another area in which ducts offer advantages is noise suppression. Ducts allow noise to be reduced three different ways: · Running the propeller under optimum flow and loading conditions eliminates the propeller-tip "buzz," a substantial component of the noise of a propeller-driven airplane. · Enclosing the propeller in a duct allows various acoustic treatments to absorb noise before it can impinge on the ears of bystanders. · By offering improved static and low-speed thrust, ducted fans make possible steeper climb out, which in turn reduces perceived noise at the airport boundary, an important public-relations advantage.

Quantifying the effect of duct using simple theory To estimate the maximum static thrust, first the idealized momentum theory is used. This theory gives the following expression for the maximum static thrust of an open propeller: T =(Pio)2/3 *(2ρ A)1/3

(6)

where Pio is the static induced power and A is the propeller disk area.

A correction to this value needs to be made for obtaining the static thrust of a ducted propeller. For the power to thrust ratio, a simple analysis based on the momentum theory gives32

[

P 1 = 3V + V 2 + 4ϕ ( T / ρA) T 4

]

(7)

where V is the axial air velocity into the duct and φ is the ratio of the propeller disk area to the exhaust area, φ = A/Aexhaust

(8)

For an open propeller, φ is equal to 2, and for a constant area duct, φ is equal to 1. At the static condition, V =0, T = (4P2ρ A/φ)1/3

(9)

Lets assume constant area ducted propeller with φ equal to 1. For the same power, the static thrust of a ducted propeller can be obtained from the static thrust of the corresponding open propeller by using the following equation: Tducted (φ =1) = 21/3 Topen(φ =2) = 1.26 Topen(φ =2)

(10)

6. Examples and Future (Few beyond the scope of MAV but all uses ducted rotor)

1) The adjacent fig. Shows Hummingbird considered to be most agile craft currently underdevelopment stage. It uses ducted rotor in configuration. Expected that it will be able to kinds maneuvers imagined so far.

concept wing perform

Fig. 18: Hummingbird

2) Adjacent Fig. shows NASA’s idea of spiral vehicle. When highways will become extremely vehicle will be best for door to door air

duct commuter congested, this commutation.

Fig. 19: Spiral duct commuter Vehicle for personal aviation

Fig. 10: Airfoil Performance

all

3) This experiment incorporates both autonomous and manual control of SADTU (Self Automated Dynamic A single ducted fan propulsion unit is used to uniform levitation. Stability and control is maintained simple electronics and the fundamental principles of and fluid mechanics. This research will prove that a body can maintain controlled, stable levitation changing gravitational environments; and compensate factors using a single propulsion unit. This research to VTOL craft and can be appreciated by those who daunting aspects of Vertical Take-Off and Landing.

Thrust Unit). maintain by usefp of aerodynamics free-floating through for external can be applied challenge the Fig 20: SADTU

7. Conclusion 1) MAVs have proven to be excellent solution for military and civilian reconnaissance and surveillance purposes 2) Shrouded/Ducted rotor based VTOL MAVs are better platforms than conventional fixed wing MAVs for several applications.

3) This class of vehicles operating in low Reynolds’s regime shows different behavior. This opens up a lot avenues for research in this area to have accurate experimental data and better analytical tools to understand physics in this regime 4) Experimental set-up and analytical scheme was explained to characterize mini propellers and rotors.

5) Simple qualitative and quantative analysis was provided to explain the duct-rotor interactions.

6) More research and understanding in duct rotor concept is leading to open up applications in other fields of aviation.

References 1) Douglas Murphy, James Cycon, ”Applications for mini VTOL UAV for law enforcement”, SPIE Proc. 3577: Sensors, C3I, Information, and Training Technologies for Law Enforcement, Boston, MA, 3-5 November 1998 2) H.G. Nguyen, W.C. Marsh and W.D. Bryan, "Virtual Systems: Aspects of the Air-Mobile Ground Security and Surveillance System Prototype," Unmanned Systems, Vol. 14, No. 1, Winter 1996 3) Van Houten, “Analysis of Ducted propellers in steady flow”, ARMC Report No. 4.76-1, February, 1986.

4) McCormick Barnes W., “Aerodynamics of V/STOL Flight”, Academy Press New York.London, 1967.

5) Erdin Altu¸ James P. Ostrowski, Robert Mahony, “Control of a Quadrotor Helicopter Using Visual Feedback”, Proceedings of the 2003 IEEE International Conference on Robotics and Automation, Taiwan, May 2003.

6) Profs Ilan Kroo and Fritz Prinz, “The Mesicopter: A Meso-Scale Flight Vehicle” NIAC Phase I Final Report 7) Profs Ilan Kroo and Fritz Prinz, “The Mesicopter: A Meso-Scale Flight Vehicle” NIAC Phase I Interim Report.

8) Thomas J. Mueller (Editor) “Fixed and Flapping Wing Aerodynamics for Micro Air Vehicle Applications”, AIAA publications, 2001.

9) F. Marc de Piolenc & George E. Wright Jr., “Ducted fan Design”, Volume-1, Millennial year edition-2001.

10) Adkins, C. N.: Design of Optimum Propellers, AIAA-83-0190. Also published in the Journal of Propulsion and Power, Vol. 10, No. 5, September-October 1994.

11) Larrabee, E. E.: Practical Design of Minimum Induced Loss Propellers, SAE paper 790585, Business Aircraft Meeting and Exposition, Wichita, April 1979.

12) Bramwell, A.R.S. (1976) Helicopter Dynamics. Edward Arnold 13) Johnson, Wayne (1980) Helicopter Theory. Princeton University 14) Larry A. Young, Edwin W. Aiken, “Engineering Studies into Vertical Lift Planetary Aerial Vehicles”, AHS International Meeting on Advanced Rotorcraft Technology and Life Saving Activities, Utsunomiya, Tochigi, Japan, November 11-13,2002.

15) Raymer Daniel P., “Aircraft Design – A Conceptual Approach” AIAA Education Series third edition.

16) Mattingly J D, Heiser W H, “Aircraft Engine Design” AIAA Education Series, 1987.

17) Short course on design, build and fly-Miniature Aerial Vehicle, CASDE, IIT Bombay webpage: http://www.casde.iitb.ac.in/IMSL/index.html 18) Humming bird project webpage: http://www.ae.utexas.edu/design/humm_mav/introduction.html#introduction 19) Felipe Bohorquez, Paul Samuel, Jayant Sirohi, Darryll Pines, Lael Rudd “Design, Analysis and Performance of a Rotary Wing MAV” 20) The humming bird- Worlds most maneuverable aircraft, http://www.esotec.co.nz/hb/HTML/HomePage2_F.html 21) Johann Borenstein, Department of Mechanical Engineering and Applied Mechanics, “The HoverBot An Electrically Powered Flying Robot” 22) VTOL (Vertical Takeoff and Landing)WEB, http://easyweb.easynet.co.uk/~pppf6/Research/UFOphenomena/UFOlogy/VTOL/VTOLweb.html 23) AHS International Vertical flight society, http://www.vtol.org/index.html 24) ACDL projects and research, http://raphael.mit.edu/projectsresearch.html 25) Airfoil design and analysis, http://www.designfoil.com/ 26) UIUC airfoil data site, http://www.aae.uiuc.edu/m-selig/ads.html 27) Aerodynamics for model aircraft, http://www.mh-aerotools.de/airfoils/index.htm 28) Glauert, H., Elements of Airfoil and Airscrew Theory, 2nd ed., Cambridge University Press, England, UK, 1959, Chap 16

29) McCormick, B. W., Aerodynamics Aeronautics and Flight Mechanics, 2nd ed., John Wiley and Sons, Inc., USA, 1995, Chap 6 30) Hepprle M., “JavaProp – Design and Analysis of Propellers,” Technical Database for Model Aircrafts, [online database] URL:http://www.mh-aerotools.de/airfoils/javaprop.htm [cited 17 November 2004] 31) Adkins, C. N., “Design of Optimum Propellers”, Journal of Propulsion and Power, Vol. 10, No. 5, SeptemberOctober 1994 32) Newmann, S., The Foundations of Helicopter Flight, Edward Arnold, London, United Kingdom, 1994, Chaps. 3, 4.