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.

By Nihad Daidzic, PhD, ScD
ATP/CFII, MEI, CFIG, AGI/IGI
Pres, AAR Aerospace Consulting
Professor, Minnesota State Univ


Leonardo AW609, jointly developed by Bell Helicopter and AgustaWestland, holds promise to be the 1st tiltrotor aircraft certified for civilian operations. Configuration options include corporate, SAR and EMS. Projected max range is 1100 nm with aux fuel tank, cruise speed of up to 275 kts.

On Oct 25, 2016, Boeing was awarded the US patent # 9,475,585 for a particular tiltrotor vertical-lift regional transport aircraft design capable of carrying 100 passengers. The patent priority/filing date was June 25, 2014 and Boeing's original claims and intellectual property will be thus protected until 2034. This may signal the intent of Boeing to build such aircraft in the future.

However, that will depend on many other internal (company) and external (market) factors, and Boeing may also choose never to build such aircraft, sell the patent, license the invention to another manufacturer, or simply abandon the patent.

Due to its ability to take off and land vertically and/or use short horizontal distance (V/STOL), such powered-lift aircraft could theoretically operate from urban helipads or very short runways, cruise at about 275 to 300 kts and land in V/STOL mode in another city center.

One of the principal innovations in the published patented solution is the low-wing design to prevent heavy high-wing and engines from possibly crushing the fuselage during crashes (crash survivability) and to simplify maintenance, servicing and airport operations offered by low-wing designs. All previous experimental tiltrotor aircraft have utilized high-wing designs.

Powered lift

Bell XV3 aircraft 1st flew in 1955 with a single P&W R985-AN-1 radial engine powering the wingtip tiltrotors via driveshafts.

If pure trust-vectoring (or rocket mode) is discounted, only 2 terrestrial atmospheric flight principles are practical for long-duration air travel. One is the flight utilizing the buoyancy principle, which applies to lighter-than-air aircraft such as hot/gas air balloons and airships that are hybrids of air/gas-balloons and powered aircraft (airplane). The other is heavier-than-air, the most com­mon and important flight principle, based on the angle-of-attack (AOA) and the relative motion between the lift-generating surfaces and the surrounding air. Both fixed-wing or rotary-wing aircraft generate AOA.

Helicopters are powered rotary-wing VTOL aircraft that can hover, fly vertically up and down, and fly forward, sideways and backward – simply amazing machines. Helicopters produce thrust as a vector component of the total lift force by effectively tilting the main rotor disc. Gyroplanes, on the other hand, 1st flown in 1923, are hybrids between helicopters and airplanes in which case the lift-producing rotary-wing is in the constant state of autorotation (windmilling), while horizontal thrust is normally produced by an internal combustion engine driving conventional propellers.

So why do we need yet another airplane-helicopter crossbreed? We need it because helicopters and gyroplanes are limited by low cruising airspeeds and ranges, while airplanes are not VTOL-capable.

Gyroplanes, though marvelous STOL machines that are far simpler, lighter and cheaper than any V/STOL concept, have currently little commercial use and are not VTOL-capable either. They cannot sustain pure vertical flight or hover without auxiliary energy storage systems, and if they do it is only for short periods (Pro Pilot, Sep 2013, p 78). And although helicopters do have great VTOL utility, they are still limited by low cruising airspeeds (around 200 kts max) and short operational range.

NASA-funded Bell and Boeing-Vertol XV15 twin-engine tiltrotor technology demonstrator from 1977.

Some success has been achieved in designing high-speed helicopters, but these are really special and expensive machines of complex designs. So why not marry airplane and helicopter again? This time let's a utilize tilting propulsive rotary-wing with passive fixed-wing lifting surfaces in a convertible design. Then we will call this new V/STOL system powered lift. Or we could tilt the entire wing with fixed attached propulsion engines.

The propulsive force in heavier-than-air powered lift does not have to come from proprotors only. Indeed, few existing powered lift aircraft include specialized military jet airplanes which also exhibit V/STOL capabilities. For example, subsonic military jets such as the British Hawker-Siddeley AV8A Harrier (1967) and the Soviet/Russian Yakovlev Yak38 Forger (1971), and the supersonic Mach 1.4 Yak41/141 Freestyle (1987) use extreme thrust-vectoring concepts with special vectoring nozzles that allow aircraft to lift off and land vertically.

However, such thrust-vectoring military designs eject pilots automatically if the aircraft becomes unstable (or engine failure) during VTOL operations, and therefore are not practical for large aircraft designed to transport passengers, equipment and materials.

Tiltrotors or tiltwings?

The history of tiltrotors and tiltwing designs is rich and exciting. In tiltrotor designs, the main wing(s) remain fixed while the engines with proprotors switch from vertical (helicopter mode) to horizontal (airplane mode) position.

On the other hand, in tiltwing designs the engines and proprotors are fixed to the wing, and the entire wing rotates from vertical to horizontal in airplane mode.

Tiltwings have 1 big advantage over tiltrotors in that the wing does not interfere negatively with the propwash. For example, in the Bell-Being V22 Osprey, significant download on the fixed-wing is generated during hover and it is the reason why flaperons are deflected vertically (90°) to reduce the interfering surface area.

Boeing-Vertol VZ2 tiltwing aircraft flew for the 1st time in 1957, while the Hiller X18 tiltwing 1st flew in 1959. And a more recently canceled (1993) tiltwing concept was the TW68 twin turboprop commuter designed and built by the Ishida Group from Japan. So at the moment, it seems as if the tiltwing idea has been abandoned.

In tiltrotors, the same propulsion unit – very appropriately called proprotors – provides vertical lift (rotors) in helicopter configuration, and horizontal thrust (propellers) when in airplane configuration. So tiltrotors can operate in both VTOL and airplane modes. Such metamorphosing aircraft are generically called convertiplanes.

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