Russia accidentally leaks image of high speed rotorcraft

[KenV]

Originally Posted by MPN11

I always thought that blade tip speed was a limiting factor. Relative to airflow, advancing tip goes supersonic, retreating blade stalls, or simething like that. How does this design overcome that law of aerodynamics to achieve such a speed? Or is it relying on the stubby wing and jet propulsion, with freewheeling rotor blades, in high-speed cruise?

Sikorsky has been working on a coaxial "ABC" (Advancing Blade Concept) for decades. They got it to work, but ran into serious vibration issues. The idea is that only the advancing blades, one on each side of the aircraft produce lift. Thus no need for the retreating blade to produce lift. The rotors essentially feather while retreating. At high speeds the advancing blades significantly unload because they only produce lift, not lift plus thrust because forward thrust is provided by a pusher prop. The cool thing about this is that unlike a conventional helicopter which rotates nose down to accelerate, the ABC rotor system allows the aircraft to accelerates in a flat attitude, much like a fixed wing aircraft. This is especially useful in an armed craft which keeps the weapons pointed ahead all the time, rather than pointed down whenever the aircraft accelerates.

[MPN11]

Thanks, Ken V.

[VX275]

Looks like a jet powered Rotodyne without the tip jets to me.

[KenV]

Originally Posted by VX275

Looks like a jet powered Rotodyne without the tip jets to me.

A rotodyne has a conventional main rotor except they are tip powered and thus there is no counter torque and hence no tail rotor. The rotodyne's rotor disc is also lightly loaded in forward flight because it is producing only lift and not lift plus thrust. So the rotodyne solves two of three problems. However, rotodynes still have the problem of different advancing vs retreating blade lift which fundamentally limits airspeed. The point of the coaxial ABC system is to eliminate that third problem: retreating blade lift. In the ABC system, the retreating blades produce zero lift during high speed forward flight. In addition, because the retreating blades do not need to produce lift, the rotor can be significantly slowed down at speed which reduces the advancing blade tip mach number with huge benefits in drag rise, noise, and blade tip buffet.

[JohnDixson]

MPN11, you have a point re the advancing tip mach number, thus the X2 and 97 use a reduced Nr schedule at higher aircraft speeds. I imagine that the rotor speed control schedule is now automated on the 97.

BTW, handling tip speed for compounds involves the same situation, and they will obviously take the same design approach. If you plan on flying year round, implying colder ambient, and do the math for Mach 1* it becomes clear that you have to adjust Nr, not so much for rotor stability questions, although even that can become an issue if there is unacceptably low torsional blade stiffness, but for drag rise and performance impact. Beneficial effect on acoustic signature too, as you’d imagine.
*During the UTTAS development, the Army made a special request to clear the rotor to a free stream mach number of 1.0**, which was done on a cold day flying out of Burlington Vt. The rotor behaved well, and the bonus was that we got some terrific skiing in while the data was being reduced to ensure all was good.
**not an original spec requirement. I do not know if Boeing received the same request. Now that I think about it, that request may have come after the competition was over.

[chopper2004]

https://defence-blog.com/news/tsagi-...elicopter.html

The Central Aerohydrodynamic Institute named after N.E. Zhukovsky (TsAGI) has confirmed the development of a new Russian advanced high-speed helicopter.

[unmanned_droid]

I've done a fair amount of reading around about this, and have decided to adopt a similar approach for an Unmanned Aircraft Design Study.

For a small vehicle, the main emphasis is about what the best compromise is in terms of vertical takeoff and forward flight, whilst accomodating the fact that humans have to interact with it. So the design below is a fixed pitch tandem 'bi-copter' which works in vertical flight and slow forward flight. The transition to forward flight is carried out with fixed 'thrusters' and structure not yet added to the model below. The aim is to offload the rotors as fast as possible to increase overall vehicle efficiency.