Copyright © A. Filippone (1999-2003). All Rights Reserved.

Advanced Rotorcraft Concepts

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Rotorcraft Concepts

The main rotorcraft concepts developed over the years include single or double main rotors. In the latter case the two rotors can be in tandem on two different shafts (CH-47 Chinook), co-axial and counter-rotating (Kamov Ka 52), or inter-meshing (Kaman K-1200), with number of blades variable ranging from 2 (most Bell helicopters) to 8 (heavy lift Mil 26).

Tail rotors are available on most helicopters, excluding the Kamov designs with 2 counter-rotating rotors and a few more (ex. Boeing-MD 520). The single rotor helicopter with rear vertical tail propeller evolved directly from the early studies of Igor Sikorski.

Advanced Concepts for the Future

After the radical concepts developed in the early 1950s, rotorcraft technology has evolved steadily. Now the best performances seem close the the limit of the flight envelope. For this reasons old and new concepts are back to the design boards. Such radical ideas include the tilt rotors, the tilt wing, rotors with variable diameter, and the compound thrust.

Tilt Rotors

The tilt rotor performs a conversion of VTOL aircraft into a more ordinary aircraft by tilting the propeller from vertical to horizontal to achieve horizontal flight. The first such conversion was achieved with the experimental aircraft Bell XV-3 (1958).

The great technological advances produced since that time have led to the development of the military utility tilt rotor Bell-Boeing V-22 Osprey. This vehicle has a flight envelope merging those of rotorcraft and aircraft mode. The concept is making its entry in the civil aviation with the Bell-Agusta 609.

Variable Rotor Diameter

The possibility of adjusting the diameter to optimal take-off and forward flight is an attractive idea, albeit a risky one. A complicated mechanical system must be tested to assess safety and reliability.

The convertibility concept also has some alternative options, like for example folding the rotor blades once hover conditions are reached off the ground, and then switch the propulsion system from turboshaft to turbofan.

Compound Rotorcraft Thrust

Compound thrust would be obtained by adding a thrust vectoring to a conventional helicopter rotor. The compound thrust would alleviate the rotor loading, would decrease its rotational speed, and with it all the problems associated to the extreme tip speeds (transonic and supersonic Mach numbers, dynamic stall effects, fatigue, structural vibrations, noise generation, etc.).

Performances will be improved at the high end of the flight envelope. The compound system can be rendered yet more compound by driving exhaust gas to canards/wing systems and the speed increases (over-the-wing blowing).

This idea was first proposed in 1967 for the Lockheed AH-56A Cheyenne (after the demonstration program, the project was aborted, apparently because too ahead of its time, and for the escalating costs).

The AH-56 featured a 4-bladed main rotor coupled to a 3-bladed thrust propeller, slender wings and retractable landing gear. It achieved the remarkable speed of over 405 km/h (252 mph). Recent designs include those of Piasecki aircraft, and the re-engineering of some Sikorski vehicles.

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Selected References

Up-to-date news and press releases are available from Aviation Week, Flight International, and Aerospace America.

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Copyright © A. Filippone (1999-2003). All Rights Reserved.