Advanced Rotorcraft Concepts
In this Section
- Rotorcraft Concepts
- Advanced Concepts for the Future
- Tilt Rotor
- Variable Rotor Diameter
- Compound Thrust
- Selected References
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.
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.
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.
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 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.
Up-to-date news and press releases are available from Aviation Week,
Flight International, and Aerospace America.
Advanced Concepts for the Future
Tilt Rotors
Variable Rotor Diameter
Compound Rotorcraft Thrust
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Selected References