Originally Posted by Dave Jackson
I'm not against augmenting inefficient aerodynamics.I'm against inefficient aerodynamics.
The gyro would not be there as a band-aid to poor aerodynamics.
A light helicopter suffers inherent DYNAMIC control instability, because the pilot only has direct control over the pitch/roll torque moment - ie acceleration. That is why it is so hard to hover at first, since the tiniest cyclic input sends the machine careering off. The pilot is actually learning to differentiate his input. Adding lateral and longitudinal dihedral (you would need additional rotors for this) will improve STATIC stability, but the machine does not become easier to fly.
A fixed wing is stable because the pilot controls pitch/roll velocity, since the elevator and aileron are forcing a steady-state change in flow. The machine has DYNAMIC stability because the machine responds linearly to a given stick deflection. Lateral and longitudinal dihedral also give the machine STATIC stability, so that in addition to being easy to fly it will return to a wings level ground state with no input.
I am not saying that there is anything inherently wrong with the interleaving config. I am saying that it does not overcome the basic source of instability in a helicopter. For efficient hover you need the lowest disc loading, and for any given landing box a single disk will always win. If the need is to unload the retreating blade then coaxial or intermeshing are the next best options. Intermeshing anhedral will reduce stability, but if there is already a gyro then so what.
General thoughts on this config in roll yaw coupling: If the machine is in a sideslip, and the tail vertical stabiliser converts this to yaw which way will it roll? The rotor gyro precession favours correct sense roll, but considering which rotor effectively speeds up and which slows down the machine also suffers adverse roll. So you see that outboard advancing, which you need to make the machine fly efficiently at high speed, may reintroduce all the problems that Flettner rotation solved. Dihedral will help by sideslip correct sense roll before yaw, but the sideslip roll out then produces adverse yaw by gyro forces and correct sence yaw by aero forces. Which wins out sounds more like fixed wing the spiral divergence over dutch roll scenario.
The practical upshot is that ANY aerodynamic machine is a dynamic and static stability compromise. The only way forward is to accept this, and devise the simplest/cheapest practical control system - i favour the original Lockheed mechanical approach in a light heli. This then frees up the design for the best package compromise.
At any rate this is all academic. Sikorsky are already paving the way with the X2...
Mart
BTW: I enjoy our discussions, since it helps me formulate my ideas!
