Minnesota State University, Mankato

From the SelectedWorks of Nihad E. Daidzic, Dr.-Ing., D.Sc., ATP, CFII, MEI

February, 2017

VTOL hybrids: 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 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 and 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 courtesy Photo we need yet another -heli- Leonardo AW609, jointly developed by Bell and AgustaWestland, holds promise to be copter crossbreed? We need it be- the 1st 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 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 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 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 ? 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 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. Flight control computations are performed by 3 independent redundant light control computers (FCCs) that inter- face via MIL-STD-1553B data buses. Naturally, each engine is equipped with a full-authority digital engine control (FADEC) system. The V22 uses a mid-wing gearbox as a main accessory gearbox pow- ering 5000-psi hydraulic pumps, 2 constant speed (CF) AC generators rated at 40 kVA each, and 2 variable/ wild frequency (VF) AC generators each rated at 50/80 kVA.

Advanced tiltrotor V/STOL aircraft designs

Bell-Boeing V22 Ospreys have complex flight control systems that enable pilots to seamlessly Proposed future tiltrotor conigura- transition between helicopter (VTOL) and airplane modes. tions include, for example, Bell-Boe- ing Quad Tilt-Rotors (QTR) and Joint 6000 to 6150 shp. They were linked Flight controls of V22 Osprey Heavy Lift QTR (JHL QTR) as seen via individual gearboxes with inter- in Figure 1. The intention is to build connecting drive shafts emanating V22 uses cyclic/stick and collective upon the success of the V22 and its from a mid-wing gearbox, so that controls similar to ordinary helicop- many variants in service, but many if 1 engine failed the other could ters (swash-plates). Since proprotors of these future programs are uncer- power both proprotors. Hovering is are counter-rotating, there is no need tain. China is also showing interest not possible with 1 engine inoper- for separate antitorque control. Con- in developing and building fast and able. Failure of both engines in ver- trolled yawing motion is achieved by heavy vertical lift aircraft. Howev- tical light can theoretically result using differential cyclic (using rudder er, formidable technical challenges in emergency landing. pedals), roll is achieved by differen- must be irst overcome. Low blade-inertia makes autorota- tial collective, and pitch control and tive landing characteristics relatively longitudinal motion is like in most Civilian tiltrotor – Leonardo poor, but if suficient forward speed helicopters. Lateral translation is AW609 exists, the aircraft can glide and land achieved by separate inching control horizontally as an airplane (HOTOL) on top of cyclic applying coordinat- Leonardo is planning to market where engines must be tilted about ed cyclic tilt on both rotors. Thrust is its AW609, formerly known as the 45° above horizontal to prevent prop controlled by throttle/collective. Bell-Agusta BA609 (see title pho- strikes. Vortex-ring state (VRS), where In airplane mode, the stick/cy- to). It is a twin-engine tiltrotor V/ vortex rings surround the rotors re- clic operates conventional eleva- STOL aircraft similar externally to sulting in loss of lift, has been blamed tor(s), roll is achieved with - laperthe Bell-Boeing V22. However, it in at least 1 V22 accident although ons (laps+ailerons) mounted on is quite a different aircraft in many there are opposing opinions about the trailing edges of the main wing, respects. It is smaller in size and the severity and importance of VRS. rudder pedals operate conventional designed to carry 6 to 9 passengers tail-mounted rudders for yaw con- in a pressurized cabin up to about 1980s 1990s 2000s 2010s 1989 Figure 1: Quad Tiltrotor aircraft •฀2฀sets฀of฀V22฀ 1999 concepts (QTR and JHL QTR). rotors,฀drive 2005–2007

V22 Osprey •฀19,000฀lbs฀VTOL useful฀load JHL QTR Initial Quad TiltRotor (QTR) •฀60,000฀lbs฀VTOL฀ •฀31,000฀lbs฀VTOL฀useful฀load useful฀load฀needed

98 PROFESSIONAL PILOT / February 2017 low-wing design so that the main landing gear can be retracted into the wing structure and not into fu- selage where it usurps needed pay- load space. If such future civilian transports could ultimately operate between city centers and avoid hav- ing passengers commuting to large airport hubs, many air transportation congestion problems could be re- duced and convenience for travelers could be achieved.

Conclusions It has taken a long time, but it seems that civil transport tiltrotors are inally arriving. Tiltrotors com- bine the good VTOL characteristic of the rotary-wing aircraft and the fast and eficient cruise speed of ixed- Figure 2. Boeing’s low-wing vertical lift tiltrotor design could transport more than 100 passengers wing turboprop aircraft. The price directly between city centers. of that helicopter-TP blending is air- craft complexity. The short tiltrotor 25,000 ft. It features 2 Pratt & Whit- tiied in the powered-lift category re- transition phase includes conversion ney Canada PT6C-67A turboshafts quiring both helicopter and airplane during which the powerplant tilts rated at about 2000 shp each and practical and theoretical knowledge from the fully vertical to mostly hor- turning 3-bladed proprotors via re- with about 10 unique tail-rotor pilot- izontal position, moving the aircraft spective gearboxes. It has no rudder ing tasks. forward where the main wing pro- on its vertical stabilizer. Yaw control Expectations are that FAA and duces enough lift in forward motion is achieved by differential collec- EASA will award certiication in to sustain level light. tive pitch change (slight asymmetric 2017 after 10 years of delays. Emergency landings are possible thrust) of the proprotor blades during in both tiltrotor modes, with con- horizontal light. Boeing VTOL tiltrotor transport ventional autorotations possible in In helicopter mode, the yaw con- design helicopter mode and forward gliding trol is achieved by using differential descent possible with airplane-like cyclic. AW609 has no lateral cyclic A drawing of one of Boeing’s sev- horizontal landings. actuators and cannot perform lat- eral patented inventions of tiltrotor An “exotic” powered-lift category eral translations (sideways motion) vertical lift aircraft proposed for ci- has been introduced by the FAA to that are possible in V22. AW609 vilian transport is shown in Figure certify future tiltrotor pilots. It seems is quite capable of autorotations 2. Boeing’s unique tiltrotor design that, for now, tiltrotors have won the as has been demonstrated during has several ixed aeropropulsion contest against tiltwings, but that light testing. Apparently, it was also units mounted on the wings, which may also change in the future. I also found during light tests that VRS contribute torque and power to a would not totally discount autogy- (settling with power into downwash) common power transmission system ros, which with the hybrid power-as- is not an issue and that both rotors for the wing-mounted nacelles, me- sist solutions could even hover for will not simultaneously enter VRS. chanical-coupling gearboxes, and short periods. AW609 incorporates a triple-redun- proprotors. Many competing powered-lift de- dant digital ly-by-wire control sys- Boeing has not been very specif- signs may be explored in the coming tem, FADEC, and head-up display, ic in disclosing what kind of pow- years. It seems that a bright future and its hydraulic system operates at erplants would be mounted under awaits airplane-helicopter hybrids. a conventional 3000 psi. AW609 is the wing (turboprop is mentioned planned to be certiied for known-ic- in the patent), but very likely it will ing conditions. be turboshaft engines. Placement of Nihad Daidzic is The AW609 is capable of landing several engines (say 2 per proprotor president of AAR vertically and is planned to have via gearboxes) would provide re- Aerospace Consult- a range of 800 nm and a cruising dundancy in the case of engine(s) ing, L.L.C., located speed of almost 300 kts – far better failure. Also, moving the engines in Saint Peter, MN than any conventional helicopter. inboard and distributing weight and and has worked for The AW609 could weigh up to about power production should reduce the many years on the 18,000 lbs on takeoff and is primari- bending moments in the wing roots, US and European ly targeting the civil market, requiring smaller spars and saving space programs. He is also tenured including VIP transport and offshore weight. full Professor of Aviation and of oil and gas operators. It will be cer- Boeing’s innovation employs a Mechanical Engineering.

100 PROFESSIONAL PILOT / February 2017