Sikorsky's Reverse Velocity Rotorcraft

[Dave Jackson]

http://www.unicopter.com/temporary/Sikorsky_RVRC.jpg



Nick,

The above sketch was in the latest issue of Vertiflite. I also understand that a Mr. Ashby presented a paper on this craft at the AIAA Powered Lift Conference two weeks ago.

It looks like a very interesting and potentially viable concept. Do you have any information about it that would be of interest?

Thanks

Dave J.

[donut king]

Was there not a Piasecki test vehicle similar to this drawing???

Was it not an actual flying concept???

Correct me if I'm dreaming!!!!!

[ShyTorque]

Donut King,

Are you thinking of the Piasecki 16H Pathfinder?

That was a compound helicopter, a 5 seater with a 3 bladed main rotor. It had a wing mounted beneath the fuselage and a ducted airscrew in the tail, with vertical vanes to deflect the efflux laterally to counteract torque reaction.

It was flying at up to 170mph in the early 1960s, powered by a 510 hp Pratt & Whitney PT6B-2. There was a proposed twin engined version but I'm afraid my spotter's knowledge finishes here! :D

I would be interested to hear how this "new" (it's been considered before) design of aircraft gets over the age-old problems of excessive retreating blade stall and advancing blade compressibility at the proposed high speed of 350 kts......:confused:

[Dave Jackson]

All compound helicopters, to my knowledge, require a wing. This has meant that the wing is excess baggage in hover and the rotor is excess baggage in fast forward flight. This concept uses the rotor for lift in all modes of flight.

Assume that the helicopter in the sketch is flying at a fast forward speed. It can be seen that the retreating blade has negative pitch at its root. The root of this blade is in reverse airflow, therefor it is contributing to the lift. The root of a convention retreating blade opposes lift.

This is the basic concept. There is much more that can be said, and hopefully Nick will be able to contribute some Sikorsky input.

Shytorque ~ The rotor rpm will be significantly lower than that of conventional helicopters and this will postpone the advancing blade compressibility until a much higher air speed has been reached. I suppose that, theoretically, both the blade and the craft could eventually go supersonic.

Dave J.

[DeltaFree]

This idea would surely run into problems with the blades when they are in the forward sector of rotation, with slow rotational speeds and high airspeeds they effectively become forward swept wings. I know that with clever aeroelastic tailoring forward swept wings can be prevented from divergent fluttering and ultimate destruction, but this can surely only work either on the advancing side or retreating side not both.

[ShyTorque]

Dave,

Yes, I thought that a slow rotor might be the case! Not sure where it can be seen that the retreating blade has a negative pitch though, perhaps because the picture you have posted is not detailed enough. Has the designer come up with a new idea for radically adjusting the inputs on the retreating blade to give the required negative / reverse angle of attack?

I think it's probably true to say that it has been tried before. "BERP" Blades, "Advancing Blade Concept", "Convertiplane", blade rigidity / stability problems at low Nr and high forward speed, "X-wing", blown blade surfaces, plumbers nightmare, etc etc.

And then of course, in the not-too-distant past, the larger manufacturers filed it in the all too difficult drawer and designed the tilt-rotor as a possible alternative. As we know, that has it's own problems too, some of which make it not such a good alternative to the conventional (but slow) helicopter because of handling / manoeuvrability problems in certain flight regimes, as was tragically demonstrated on at least two occasions not too long ago.

Sorry to sound sceptical but I am so! If it works though it would be a great thing.

What say you, Nick?

[Dave Jackson]

DeltaFree,

You are correct. If, or as, this concept is developed for faster and faster flight, the whole rotor assembly, including blades, will require higher levels of rigidity.

ShyTorque,

This concept is based on the principle that the blade tips and the blade roots have separate pitch controls (say separate swashplates).

This is an oversimplification, but may help in grasping the principle;
In forward flight, on the advancing side, the pitch of tip and the root are the same. In forward flight, on the retreating side, the pitch of the tip will be positive (upward, as is conventional) but the pitch of the root will be negative. It is negative so that the reverse airflow over the root will provide lift as well.

As mu [forward velocity / tip velocity] increases, the location on the retreating blade which experiences no airflow will move outward towards the tip. At mu => 1 the whole blade, at 270-degrees azimuth, will be in reverse airflow. Therefor at mu=>1 the whole blade, at 270-degrees azimuth, will have negative pitch.

IMHO
Dave J.

[ShyTorque]

Dave,

The concept surely calls for an engineering paradox - very high longitudinal blade rigidity with good flexibility in twist.

How exactly will they control the blade pitch at the tip end? There must be something for the swashplate to act against, unless there is to be an aerodynamic "tab" arangement. If so, this would cause complication with a reverse airflow on the retreating side. It also requires some way of actually delivering the control inputs to the tip end of the blade.

How about an annular ring around the outboard end of the rotor disc to house controls for the outer part of the blades? This could be made to an aerodynamic profile so it would contribute to the lift in forward flight. We could then call it the "Advanced Rotor Speed Engineering" (AR$E) ;)

[Dave Jackson]

ShyTorque,

You're right, it ain't going to be easy, but as technology advances, the RVRC's day may come.

Who knows, the blades may eventually be made of composite silk. Silk is the strongest thread known, and last year they discover how to produce it artificially.

Dave J.

[DeltaFree]

That will be the day a helicopter that flies as smooth as silk!

[sprocket]

Rotorcraft??? Then it aint necessarily a helicopter. I may have missed something in the discussion, but …..
….With no tailrotor illustrated, autorotational forces must power the main rotor. Therefore we already know the concept is proven and works in the autogyro.

Two questions Dave.

1. Can a retreating blade stall if it is being driven by airflow?
2. Is disc solidarity the key to this rotorcraft? ie many blades with a wide chord?
:confused:

[DeltaFree]

In a conventional helicopter lift is shared by either side of the rotor, flapping to equality. As the airspeed goes up more and more of the retreating blade ends up going backwards and producing no lift leaving a small section to produce the required lift, when this becomes too much of a burden it stalls.
If the inner section could be made to produce lift then the outer section would not have to create so much lift hence delaying or preventing retreating blade stall altogether.
Wider blades may be necessary to carry all the control gubbins, as these rotors are clearly not going to work conventionally. In general wider blades reduce efficiency, but I guess the benefits of higher speeds must come at a cost.
I can see how the hover will work and can see the high speed bit may be feasible, but how it gets from one to the other will be interesting. And they said the V22 has interesting transitional problems!
In answer to the two questions an autorotating rotor would definitely hinder high speed flight and while retreating blade stall is possible on a driven rotor I think not here. High rotor solidity is likely to be a requirement for the complexity of this machine rather than an aerodynamically desirable feature.

[Dave Jackson]

sprocket,

This is only a guess, but the yaw is probably controlled by thrust vectoring of the ducted fans.

It must be more than a gyrocopter because it is intended to travel at 350 knots. It also has to be more than the CarterCopter because of the apparent reverse twist on the blade at 270-degrees azimuth, plus the lack of wings.

Any answers to your questions are only speculation, but here goes.

1/ I don't think that the blades should stall if the 'reverse' angle of attack is not excessive. The airflow will be moving from the thin edge of the blade to the thicker edge and this will increase the drag on the retreating blade, but this drag will assist with rotation.

2/ The large number of blades should offer a number of advantages, such as;
~ a/ Improved lift during hover.
~ b/ Better lift on the retreating side. This is particularly important at mu = 0.5 because the outer half of the blade will have a 'conventional' positive pitch (with positive angle of attack) and the inner half will have a negative pitch (with positive angle of attack), BUT, the middle of the twisted blade will not be providing any lift.
~ c/ Less rotor induced vibration.
_____________

The key to this concept is probably its name [Reverse Velocity Rotorcraft Concept].
For me the concept very interesting, particularly since I started to work and post Internet pages on the same idea, [Negative Pitch], about four months earlier.

I believe that this concept will be more effective if it is applied to an interleaf configuration. If anyone is interested, the reasons can be found at; Negative Pitch (http://www.unicopter.com/1108.html#Interleaf)

Dave J.