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VTOL Hybrids: Tiltrotors Are Gaining Acceptance Nihad E Daidzic,, Ph.D., Sc.D

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VTOL Hybrids: Tiltrotors Are Gaining Acceptance Nihad E Daidzic,, Ph.D., Sc.D Minnesota State University, Mankato From the SelectedWorks of Nihad E. Daidzic, Dr.-Ing., D.Sc., ATP, CFII, MEI February, 2017 VTOL hybrids: Tiltrotors are gaining acceptance Nihad E Daidzic,, Ph.D., Sc.D. This work is licensed under a Creative Commons CC_BY-NC-ND International License. Available at: https://works.bepress.com/nihad-daidzic/30/ VTOL HYBRIDS Tiltrotors are gaining acceptance Led by Bell’s XV3 and XV15 and military service-proven Bell-Boeing V22, Leonardo will market the AW609 with other wing & rotor hybrids forthcoming. ry-wing VTOL aircraft that can hov- er, ly vertically up and down, and ly forward, sideways and backward – simply amazing machines. Heli- copters produce thrust as a vector component of the total lift force by effectively tilting the main rotor disc. Gyroplanes, on the other hand, 1st lown in 1923, are hybrids between helicopters and airplanes in which case the lift-producing rotary-wing is in the constant state of autorota- tion (windmilling), while horizontal thrust is normally produced by an internal combustion engine driving conventional propellers. So why do Photo courtesy Leonardo we need yet another airplane-heli- Leonardo AW609, jointly developed by Bell Helicopter and AgustaWestland, holds promise to be copter crossbreed? We need it be- the 1st tiltrotor aircraft certified for civilian operations. Configuration options include corporate, cause helicopters and gyroplanes SAR and EMS. Projected max range is 1100 nm with aux fuel tank, cruise speed of up to 275 kts. are limited by low cruising airspeeds and ranges, while airplanes are not By Nihad Daidzic, PhD, ScD cipal innovations in the published VTOL-capable. ATP/CFII, MEI, CFIG, AGI/IGI patented solution is the low-wing de- Gyroplanes, though marvelous Pres, AAR Aerospace Consulting sign to prevent heavy high-wing and STOL machines that are far simpler, Professor, Minnesota State Univ engines from possibly crushing the lighter and cheaper than any V/STOL fuselage during crashes (crash surviv- concept, have currently little com- n Oct 25, 2016, Boeing was ability) and to simplify maintenance, mercial use and are not VTOL-capa- awarded the US patent # servicing and airport operations of- ble either. They cannot sustain pure O9,475,585 for a particular fered by low-wing designs. All pre- vertical light or hover without aux- tiltrotor vertical-lift regional trans- vious experimental tiltrotor aircraft iliary energy storage systems, and if port aircraft design capable of carry- have utilized high-wing designs. they do it is only for short periods ing 100 passengers. The patent prior- (Pro Pilot, Sep 2013, p 78). And al- ity/iling date was June 25, 2014 and Powered lift though helicopters do have great Boeing’s original claims and intellec- VTOL utility, they are still limited tual property will be thus protected If pure trust-vectoring (or rocket by low cruising airspeeds (around until 2034. This may signal the intent mode) is discounted, only 2 terres- 200 kts max) and short operation- of Boeing to build such aircraft in the trial atmospheric light principles are al range. Some success has been future. However, that will depend on practical for long-duration air travel. achieved in designing high-speed many other internal (company) and One is the light utilizing the buoy- helicopters, but these are really external (market) factors, and Boeing ancy principle, which applies to special and expensive machines of may also choose never to build such lighter-than-air aircraft such as hot/ complex designs. So why not marry aircraft, sell the patent, license the gas air balloons and airships that are airplane and helicopter again? This invention to another manufacturer, hybrids of air/gas-balloons and pow- time let’s a utilize tilting propulsive or simply abandon the patent. ered aircraft (airplane). The other is rotary-wing with passive ixed-wing Due to its ability to take off and heavier-than-air, the most common lifting surfaces in a convertible de- land vertically and/or use short hor- and important light principle, based sign. Then we will call this new V/ izontal distance (V/STOL), such pow- on the angle-of-attack (AOA) and the STOL system powered lift. Or we ered-lift aircraft could theoretically relative motion between the lift-gen- could tilt the entire wing with ixed operate from urban helipads or very erating surfaces and the surrounding attached propulsion engines. short runways, cruise at about 275 to air. Both ixed-wing or rotary-wing The propulsive force in heavier- 300 kts and land in V/STOL mode in aircraft generate AOA. than-air powered lift does not have another city center. One of the prin- Helicopters are powered rota- to come from proprotors only. In- 96 PROFESSIONAL PILOT / February 2017 Bell XV3 aircraft 1st flew in 1955 with a single P&W R985-AN-1 radi- NASA-funded Bell and Boeing-Vertol XV15 twin-engine tiltrotor technolo- al engine powering the wingtip tiltrotors via driveshafts. gy demonstrator from 1977. deed, few existing powered lift air- TW68 twin turboprop commuter de- the Leonardo AW609 and other sim- craft include specialized military jet signed and built by the Ishida Group ilar aircraft that may be produced in airplanes which also exhibit V/STOL from Japan. So at the moment, it the future. FARs (§61.157, §61.163, capabilities. For example, subsonic seems as if the tiltwing idea has been and §61.165) provide ATP certiica- military jets such as the British Hawk- abandoned. tion requirements for powered-lift er-Siddeley AV8A Harrier (1967) and In tiltrotors, the same propulsion category. Flying tiltrotors requires the Soviet/Russian Yakovlev Yak38 unit – very appropriately called pro- the piloting skills for both helicopters Forger (1971), and the superson- protors – provides vertical lift (rotors) and airplanes and very few pilots are ic Mach 1.4 Yak41/141 Freestyle in helicopter coniguration, and hor- currently certiied in the powered-lift (1987) use extreme thrust-vector- izontal thrust (propellers) when in category. ing concepts with special vectoring airplane coniguration. So tiltrotors nozzles that allow aircraft to lift off can operate in both VTOL and air- A little bit of tiltrotor history and land vertically. However, such plane modes. Such metamorphosing thrust-vectoring military designs aircraft are generically called con- The idea of tiltrotors is actually rel- eject pilots automatically if the air- vertiplanes. Airplanes need cruising atively old. It was conceived shortly craft becomes unstable (or engine thrust-to-weight (T/W) normally in after 1st successful helicopters: The failure) during VTOL operations, and the 0.05 to 0.15 range due to rela- Focke-Wulf Fw 61 (1936) and Igor therefore are not practical for large tively high aerodynamic eficiency. Sikorsky’s VS-300 (1939–1940). Bell aircraft designed to transport passen- But high-performance ighter jets Helicopter apparently started con- gers, equipment and materials. require T/W in excess of 1 to ac- sidering tiltrotor concepts as far back celerate in vertical climb as well as as 1944. Their 1st operational (ex- Tiltrotors or tiltwings? during VTOL operations if in a pow- perimental) tiltrotor aircraft, the Bell ered-lift coniguration. VTOL mode XV3, 1st lew in 1955 with a 450-hp The history of tiltrotors and tilt- of conventional helicopters or tiltro- fuselage-mounted P&W R985-AN-1 wing designs is rich and exciting. tors also requires T/W in excess of 1. radial engine transmitting torque/ In tiltrotor designs, the main wing(s) Tiltrotor aircraft typically utilize 2 power out to the wingtip rotors via remain ixed while the engines with wingtip engines that operate as main heavy driveshafts. The wingtip rotors proprotors switch from vertical (heli- rotors in helicopter mode and 2 pro- with associated gearboxes and blade copter mode) to horizontal (airplane pellers in ixed-wing mode. Tiltro- pitch change mechanisms were de- mode) position. On the other hand, tor blades are longer than propeller signed to tilt 90° (vertical to horizon- in tiltwing designs the engines and blades but shorter than helicopter tal position). proprotors are ixed to the wing, and rotor blades. The tiltrotor wingspan With NASA funding, Bell and Boe- the entire wing rotates from vertical must obviously be longer than a ing-Vertol produced the XV15 tech- to horizontal in airplane mode. helicopter’s rotor disk diameter. Ac- nology demonstrator, which lew Tiltwings have 1 big advantage over cordingly, the rotation of proprotor successfully in 1977. Unlike XV3, the tiltrotors in that the wing does not in- blades will be somewhere between XV15 moved the 2 engines from fuse- terfere negatively with the propwash. the helicopter main rotor rotational lage out to the rotating wingtip pods, For example, in the Bell-Being V22 speeds (300–500 rpm) and the com- which were then directly coupled to Osprey, signiicant download on the mon turboprop propeller rotations the rotors. Ultimately, the successful ixed-wing is generated during hover (1500–2000 RPM). For example, the XV15 became the military Bell-Boe- and it is the reason why laperons are Leonardo AW609 blades rotate at ing V22 Osprey tiltrotor. delected vertically (90°) to reduce about 570 rpm in rotary-wing mode the interfering surface area. and at about 480 rpm while in air- V22 Osprey Boeing-Vertol VZ2 tiltwing aircraft plane mode, primarily due to unusu- lew for the 1st time in 1957, while ally long prop mode blades and high The V22 Osprey 1st lew in 1989. the Hiller X18 tiltwing 1st lew in cruising airspeeds. It was powered by 2 Rolls-Royce 1959. And a more recently canceled The powered-lift category was spe- AE 1107C (T406) turboshaft engines (1993) tiltwing concept was the ciically developed by the FAA for mounted at the wingtips developing PROFESSIONAL PILOT / February 2017 97 trol, and forward thrust is controlled by the thrust control lever (TCL) changing the pitch of constant-rpm proprotor blades collectively.
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