Can a contra-rotating helicopter simply tilt its entire rotor set instead of using a cyclic?

Question

I was reading this interesting question and, per answer #1, a normal helicopter cannot do it because gyro effects would probably mean the body of the helicopter would tilt more than the rotor.

So now I think about contra-rotating helicopters, like the Kamov Ka-50. They don't need tail rotors, because the contra-spin cancels out reaction torque (except when they want to yaw, for which they extend little things at the blade tips of one rotor, producing more torque there and therefore opposite yaw).

Anyway, can't a contra-rotator just tilt its whole hub instead of using traditional cyclic (swash plate, scissors link, which in fact are a lot more complex for a contra-rotating helicopter).

Answer 1

Juan de la Cierva, inventor of the autogyro, on the C.30 went to a tilting rotor. Most autogyros tilt the entire rotor for cyclic control. Some autogyros apply engine torque to the main rotor to assist in achieving short takeoffs. Now for this brief time, the autogyro is functioning like a helicopter, and the main rotor is driven by the engine prior to takeoff. There is now a torque from the engine acting on the airframe, that requires more pilot hand force to control the rotor while the engine is driving the rotor. Counter-rotating rotors on a helicopter would neutralize the reaction torque into the airframe. So yes for a very small helicopter, it should be possible to tilt the main rotor for cyclic control, without hydraulic power assist. If one is going to use a swashplate for collective control, it is easier to just control cyclic conventionally. Here is one example of a helicopter that tilted the rotor for the cyclic: http://www.aviastar.org/helicopters_eng/bensen_zipster.php

Answer 2

The cyclic does not "tilt" the disc. It cyclically alters the blade pitch to vary the angle of attack which causes an increase or decrease in the lift produced cyclically around the blade orbit. The blade "flies" into position with the blade reaching a maximum where the swashplate is at it's maximum and a minimum 180 degrees later. For complete accuracy, due to precession, the cyclic input is made 90 degrees in advance of the desired lift change.

The disc is titled by aerodynamic forces and therefore from the power input from the engine. It is not moved by the forces applied by the cyclic control and links.

The cyclic on a light helicopter is generally connected directly to the swashplate with mechanical links. The disc tilt can be controlled with the lightest of touches. In a hover in no wind, you can move by applying pressure to the cyclic with the cyclic barely moving. A tiny force on the swashplate results in a large force on the disc.

If you used links to mechanically move the entire disc, rather than causing the large aerodynamic changes, it would have to oppose them.

Answer 3

I don't understand your idea of "tilting the shaft", how would you do that? With the cyclic control it is very easy to tile the aerodynamic force but not the whole helicopter, why would you use a more complex mechanism of tilting the shaft (and the engine), and how would you do it? The gyroscopic effect will work against you and I have in mind no way to move a so big part of the Mass of the helicopter.

Moreover, since you are referring to another question that already has an answer, do you think it is easier to tilt a single shaft or the double coaxial shaft of the Ka-50?

The cyclic system gives a direct response to input and it is a very simple system. To prove it, try to Google AW609, that is a convertiplane working with the principle you talk about. See how complex, risky and expensive a convertiplane is and you will realise it in few seconds!

The V-22 Osprey (Like the AW609) tilts the whole engine, and it is a hell of a system, very complex, expensive, risky and not competitive. The advantage there is to have something that can be VTOL but can flight faster than an helicopter. A pure helicopter that tilts the rotor would have no advantage and only disadvantages!

Answer 4

It's the twin-rotor that can't do it!

To control the attitude you have to create a torque. You do that by offsetting the aerodynamic force (lift) from the centre of mass, and whether you do it by changing the lift distribution or shifting the centre of mass does not matter too much. Except you can achieve larger change by changing the lift distribution (via cyclic control).

Now however helicopter rotor is a gyroscope and that does complicate things quite a bit. The gyroscopic effect means that applying sideways torque makes the craft pitch and forward and aft torque makes it bank—the force must be applied 90° ahead of the direction you want to bank in the direction of the rotor rotation.

For cyclic, the links actually ride the swashplate ahead of the blades, so when you tilt the swashplate forward, the advancing blade pitches down, which makes it start to descend and reach the lowest point in front and everything pitches down nicely.

But when using weight-shift to get the same effect, with counter-clockwise-rotating rotor you'd tilt the rotor right (shift weight right, effectively). It would be rather weird, but with the controls properly rigged probably possible.

However on twin rotor, shifting weight to the right will make one rotor want to pitch forward, but the other one want to pitch aft. And they'll just twist and stress the shaft, but not do anything! The cyclic easily solves this, because is decreases the lift at different place on each rotor so they both want to tilt the same way, but weight shift can't do that.

So you could do it on a single-rotor craft. It would not really be simpler (tilting driven, spinning axles is not easy) and allow less manoeuvrability, but it would work. With counter-rotating rotors you can tilt each differently to get the desired effect (the tiltrotors do that). But with contra-rotating rotors you can't tilt each differently as they are on the same axis, so you can't do it this way at all and cyclic with a swash-plates and scissor-links is the only option.