Tilt-rotor Ducted Fans and their Applications · PDF fileTilt-rotor Ducted Fans and their Applications ... uses an in-wing ducted propeller design. ... by using a ducted propeller

Tilt-rotor Ducted Fans and their Applications

Jacob A. Wilroy University of Alabama, Tuscaloosa, AL 35487

Introduction

Ducted fans are capable of producing more efficient thrust, as well as decreasing

the noise induced by multiple rotating blades. More efficient use of energy can lead to

lower fuel consumption and faster speeds. Decreasing noise also has several benefits,

whether it is a military aircraft for use in wartime or civilian use such as emergency

medical lifts. The inefficiencies of non-ducted blades can arise when tip vortices form on

the end of rotating blades. This induced drag can affect the blade creating the vortex as

well as the blades rotating behind it. To increase the amount of thrust produced, turning

vanes can be used before and after the blades to help straighten the flow and cause the

rotating momentum of air to become straight. By creating more efficient thrust, power

required to achieve flight can be reduced, and therefore smaller engines can be used or

the useful load of the craft can be increased. Using multiple small, ducted fans can also

allow for the transformation from lift devices to propulsive devices. This has been given

the name tilt-rotor. An advantage to a ducted tilt-rotor is that, like a helicopter, the

aircraft does not need a wing to generate lift. Unlike a helicopter, however, a craft with

ducted fans cannot auto rotate increasing the risk of operating the craft. Listed below is

some of the history of tilt-rotor craft and how the tilt-rotor was evolved.

Past Vehicles and Current Technology

Bell XV-3

The Bell XV-3 was created to explore convertiplane technologies and was one of the

United States’ first attempts at a tilt-rotor craft. Convertiplane aircraft are capable of

vertical takeoff and landing (VTOL) and high-speed forward flight relative to helicopters

of the time. The craft was first completed in 1955 and had two wing mounted proprotors

capable of rotating the plane of rotation 90° from a vertically upward position, used for

takeoff and landing, to a forward position for high-speed forward flight. Power was

transmitted from a Pratt & Whitney R-985-AN-1, a radial reciprocating engine, via shafts

to the outboard rotors. The rotation of the rotors allowed the craft to hover like a

helicopter and fly forward at relatively fast speeds like an airplane, combining the best of

both worlds. These characteristics provide great military advantages. They also opened

the door to solutions of airport over-crowding problems. As mentioned by Dick Spivey in

the book The Dream Machine: The Untold Story of the History of the Notorious V-22

Osprey, this technology would lead to large VTOL crafts capable of carrying large

amounts of passengers.1 The XV-3 made its first rotor conversion from hover to forward

flight in 1958.2 During its testing period it made 110 successful transitions before being

severely damaged in a wind tunnel accident in 1966.3 The technological breakthroughs

brought on by the XV-3 would soon be used by the XV-15, which would then pave the

way for the world class V-22 Osprey.

Figure 1. The Bell XV-3 during fully converted flight.

Hiller X-18

The Hiller X-18 was another attempt at a tiltrotor concept aircraft, except this craft

was also capable of rolling short takeoffs making it a Vertical/Short Takeoff and Landing

(V/STOL) craft. Instead of only tilting it’s rotors, or massive propellers, the X-18 also

tilted its large wing, which could be used to generate lift given enough airspeed.

Unfortunately, when in the vertical lift mode, the wing tended to act like a sail due to its

large planform area making it susceptible to uncontrollable situations due to wind gusts.

This meant that a craft without large tilting wings must be used. Another issue with the

craft was that its two contra-rotating propellers were not cross-linked, meaning that each

set of propellers was connected to its own engine. With this being the case, if one engine

failed during hover the craft would become uncontrollable due to loss of thrust on one

side. Test engineers also learned that control of thrust levels through throttle control of

the engines was not ideal due to the slow-revving nature of turbo machinery. This would

lead to the method currently used today of keeping the engines at a constant and

optimized revolutions per minute (RPM) and control thrust using the pitch of the blades.

Figure 2. The Hiller X-18 in its VTOL orientation.

Bell X-22A

The Bell X-22A was the next viable attempt at a V/STOL aircraft. Adding yet another

stepping-stone to the technological advancement of V/STOL aircraft, the X-22A used

four ducted propellers for lift and thrust. Like the VTOL aircraft before it, these

propellers were designed such that they could be rotated from a vertically upright

position to a fully forward position for forward flight. Unlike the X-18, the X-22A had

turbojets that were cross-linked to provide power to all four propellers incase one engine

failed. The X-22A was one of the first V/STOL to employ the use of ducts in its

propulsion system. The VZ-4 built by Doak in the 1950s was a VTOL that used two

ducts to generate lift, but was canceled by the army due to a change in funding interests.

Before, aircraft used small open rotors (or large propellers) as a way of generating lift.

However, the engineers of the X-22A decided to use small 7 ft. diameter propellers

instead of larger diameter rotors like those used on the XV-3 (25 ft.) and X-18 (14 ft.).

This increases the disc loading, which put more stress on the blades and also increases the

amount of debris throw up during maneuvers near the ground. Ducts can also help to

create more efficient thrust and reduce noise due to the elimination of propeller tip

vortices. It is stated that the ducts of the X-22A were biased to provide good static and

low-speed operation and that they provided very effective aerodynamic lifting surfaces.4

The ducting system also allowed designers to place elevons inside of the duct to direct

the flow of air for control of the craft, but this system was only used during forward

flight. Differential control of the blade pitch was used for craft control during hovering.

The X-22A was also a test bed for a new variable control and stability system needed for

V/STOL aircraft since their transition from hovering to forward flight back to hovering is

very unnatural.

Figure 3. A rendering of the Bell X-22A showing both the forward flight and VTOL ducted

propeller positioning.

Bell XV-15

The Bell XV-15 would be a step away from ducted propellers as a means of lift in

favor of large proprotors. The achievements and breakthroughs made during the XV-15

program would eventually be placed into the well know Bell Boeing V-22 Osprey, which

had its first flight in 1989, twelve years after the first flight of the XV-15. The Osprey is

where funding is now being placed as this aircraft is seen as a viable solution to the

V/STOL problem. All of the V/STOL technologies up until this point have built on

themselves to create the V-22, and they continue to build as a Bell plans to produce a V-

280 Valor by 2017. The V-280 will be of a similar design to the V-22 but will be lighter,

faster, better, and stronger in many ways.

Figure 4. The XV-15, predecessor to the V-22, in flight.

AgustaWestland Project Zero

Another project that was recently finished was the AgustaWestland Project Zero.

Since the XV-22A, V/STOL aircraft have moved away from ducted fans/propellers as a

means of propulsion in favor of large rotors, presumably for higher efficiency during

hover due to lower disc loading (Fig. 6). Large rotors associated with low disc loading do

not usually utilize ducts since the large size would create structure challenges that may

not benefit the overall propulsion system. Therefore, ducts tend to be used on smaller

systems.

Figure 5. AgustaWestland Project Zero whose first flight was in December 2010. The aircraft

uses an in-wing ducted propeller design.

Figure 6. Chart produced by Leishman that compares the hovering efficiency with effective disk

loading of several aircraft types.

Ducted Fan Advantages and Disadvantages

Introduction

The use of a duct on a fan or propeller is known to provide advantages and

disadvantages to the propulsion system. It can be used to produce increases in

performance and safety when optimized. By using a duct, tip losses can be greatly

reduced by designing a small tip clearance between the blade and duct.6 Ducts allow for

the use of devices like stators and pre-rotators as well to further increase the performance

of the system. Ducts can also provide a decrease in the noise produced by a rotating

blade.6, 7 They do, however, have their disadvantages. As stated above for the X-22A, the

duct system was designed for low speed and static performance, and this is typically the

case since the benefits of a duct can be greatly reduced at higher speeds due to the

additional skin friction.6 Weight can also be a factor when designing a ducted versus an

open rotor system. Since weight becomes an issue, the size of the fan also has to be

considered and cannot be too large, and an increase in disk loading leads to a decrease in

hovering efficiency. To take advantage of the benefits of a ducted system, it would seem

that use on low speed crafts would be most suitable.

Performance

Ducts can provide many performance advantages over open rotors, but they

typically require smaller diameter fans or propellers, meaning higher RPMs, which

requires stators to remove the flow swirl so that its benefits can be used. By doing a

simple analysis and using the techniques produced in Chapter 5 of Leishman, it can be

determined that a craft with multiple small rotors will require more power in hover than a

craft with one large rotor like typically seen on helicopters of today. Using the Sikorsky

UH-60 as an example, the total power required in hover can be as high as 1,715 hp. By

changing the propulsion from a main rotor and tail rotor design to four small 7 ft.

diameter propellers, the increase in required power can be as twice as high. This can be

seen in Fig. 6 where hovering efficiency is presented as a function of disk loading.

However, by using a ducted propeller instead of an open system, the required power can

be reduced. For material in Chapter 5 of Leishman, many of the equations have been

developed from a momentum theory discussed in Chapter 2. In Chapter 6 Section 10 we

find a discussion of “Other Anti-Torque Devices” where the “fan-in-fin” design is

discussed. Here an analysis is presented for determining the thrust produced by one of

these systems. Ducts are typically designed with converging and diverging sections

before and after the fan, which changes the flow characteristics and requires a slightly

different derivation of the momentum analysis used earlier. In an open rotor system, it

has been found that the flow naturally converges to a radius that is 70% of the rotor

radius (Pg. 63) in the far field wake since the flow is moving at a higher velocity. With a

ducted system, the duct has a direct effect on this wake contraction since it provides a

boundary for the wake to flow through. Observing this we see that the mass flow rate

becomes  

𝑚 = 𝜌𝐴𝑣! = 𝜌 𝑎!𝐴 𝑤   (1)

where 𝑎! =!!!.  From Bernoulli’s principle and momentum analysis of an open rotor we

know that this value is 0.5. When comparing the induced power of a fan versus a

conventional tail rotor and increasing 𝑎! to one (no contraction of the wake), then the

power required by the ducted fan of the same area will be 30% less than that of an open

rotor. This equation is derived in the text on page 323 and is given below.    

𝑃! !"#

(𝑃!)!"=

12𝑎!

 (2)

Therefore, by using the duct to modify the far field wake, the power required to produce

thrust can be reduced. It is noted that because of the additional structural weight and drag

penalties in forward flight, the overall benefits of a fan-in-fin can be reduced. However,

tip losses are greatly reduced by the duct decreasing the induced power requirements. The

above method and characteristics can be generally applied to any ducted system used on

V/STOL aircraft. In a study conducted by Srivastava, it was concluded that the thrust

coefficient along the blade of a ducted propeller was slightly less than that of an unducted

propeller, except at the tip where it was much higher.6 His results contradicted two other

studies, which showed significant overall improvement. Another study also showed that

by using an airfoil shape for the duct, flow can be accelerated through the duct, and

additional thrust can be produced from circulation around the airfoil shape.8 This study

also showed that viscous effects diminished thrust produced by the propeller at low RPM,

therefore requiring the use of a higher RPM.

For V/STOL aircraft, ducts can provide great benefits. It would appear that these

benefits are better suited for aircraft flying at low speeds, but an in-depth analysis would

provide the results needed to determine the overall gains. For ducted system, stators are

typically needed to straighten the flow since the rotor is required to spin at higher RPMs,

but this can be a great advantage since all flows tend to rotate due to the spinning rotor.

Gilmore et al shows us that by using variable pitch stators and pre-rotators, the efficiency

of a duct can be improved by straightening the flow and helping to create symmetric flow

over the blade length.9 Ducts also take advantage of modifying the flow through the use

of duct geometry and cross-section shape such that it increases or decreases flow

velocities for additional thrust.

Noise

Reduced noise is also another advantage of ducted rotors over unducted systems.

By using a duct, pressure waves produced by the fan blades can be shielded.6 Noises that

are typically produced from the creation of vortices from the blade tips are eliminated.

On the contrary, it was reported by Oleson et al that the use of a duct on a propeller

increased the noise by 6 dB when compared to an unshrouded propeller.7 They attributed

this noise increase to rotor-stator interaction, but stated that further understanding of this

interaction could lead to a reduction in noise. It was also stated by Leishman that “efforts

to reduce the noise of fan-in-fin design through phase modulation using unequal blade

spacing have made the fan-in-fin sound subjectively less noisy – see Vialle & Arnaud

(1993)”.5 An example of this is show in Fig. 7.

Figure 7. This image shows the design of the Fenestron or fan-in-fin. Uneven spacing between the

blades can be seen through careful observation.

Conclusion

Since the 1950s work has been going on to produce the worlds next V/STOL or

VTOL aircraft that is capable of efficiently hovering while also being able to fly at fast

forward speeds. Aircraft created by companies like AgustaWestland and their Project

Zero or Bell and Boeing and their V-22 Osprey lead to further advancement of tilt-rotor

technology. Crafts like these and the X-22A take advantage of ducted propulsion

systems, but under the current understanding, these systems are limited in their

performance to slower speeds, which makes them specifically designed for certain

aircraft design goals. Since the creation of the X-22A, V/STOL tilt-rotor craft have

moved away from ducted propulsion in favor of “proprotors” for efficient hovering and

high forward flight speeds, but ducted rotors could always make a return if improvement

in designs are shown.

References

1Whittle, R., The Dream Machine: The Untold History of the Notorious V-22

Osprey, Simon and Schuster, 2010

2“Bell Helicopter Textron XV-3,” National Museum of the US Air Force, URL:

http://www.nationalmuseum.af.mil/factsheets/factsheet.asp?id=10326 [cited 30

November 2014].

3Kiley, D., “The Tiltrotor. Aviation’s square peg?”. Flight Safety Information

Journal, Special Edition, July 2003. Accessed on 30 November 2014.

4“Bell X-22A: Analysis of a VTOL research vehicle,” Flight International, 23

March 1967, pp. 445.

5Leishman, J. G., Principles of Helicopter Aerodynamics, 2nd Edition, Cambridge

University Press, New York, 2006, Chaps. 2, 5, 6.

6Srivastava, R., “Time-Marching Euler Analysis of Ducted-Propeller,” Journal of

Propulsion, Vol. 12, No. 1, January-February 1996, pp.134-138.

7Oleson, R. D., and Patrick, H., “Small Aircraft Propeller Noise with Ducted

Propeller,” AIAA Journal, 1998, pp. 464-472.

8Yilmaz, S., Erdem, D., and Kavsaoglu, M. S., “Effects of Duct Shape on a

Ducted Propeller Performance,” AIAA Aerospace Sciences Meeting including the New

Horizon Forum and Aerospace Exposition, AIAA, Washington DC, 2013.

9Gilmore, A. W., and Grahame, W. E., “Research Studies on a Ducted Fan

Equipped with Turning Vanes,” Aerophysics Group, Grumman Aircraft Engineering

Corporation, IAS 27th Annual Meeting VTOL Section, New York City, January 1959.