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! )!" = 1 2a! (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 1 Whittle, 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]. 3 Kiley, 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. 5 Leishman, J. G., Principles of Helicopter Aerodynamics, 2 nd Edition, Cambridge University Press, New York, 2006, Chaps. 2, 5, 6. 6 Srivastava, R., Time-Marching Euler Analysis of Ducted-Propeller, Journal of Propulsion, Vol. 12, No. 1, January-February 1996, pp.134-138. 7 Oleson, R. D., and Patrick, H., Small Aircraft Propeller Noise with Ducted Propeller, AIAA Journal, 1998, pp. 464-472. 8 Yilmaz, 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. 9 Gilmore, A. W., and Grahame, W. E., Research Studies on a Ducted Fan Equipped with Turning Vanes, Aerophysics Group, Grumman Aircraft Engineering Corporation, IAS 27 th Annual Meeting VTOL Section, New York City, January 1959.