Overlooking the difficulty in landing such a craft, would a helicopter with the main rotor below the center of mass, differ significantly in stability from normal one with the main rotor above the center of mass?
sophiecentaur said:BTW, I can't imagine anyone is being really serious about this idea!
sophiecentaur said:Yes - true but the blades curve upwards* when providing lift (they are flexible) and the cm will be lower, wrt the centre of effort than you might imagine.
*they may curve up much more than they droop down when 'resting' - which is an appreciable amount in pictures of some helicopters I've seen.
BTW, I can't imagine anyone is being really serious about this idea!
sophiecentaur said:I imagine a "serious" proposal would include a mention of where the wheels go. Please enlighten me. I have tried drawing one and there seems to be a topological problem.
sophiecentaur said:With the cm above the rotor, you must also be in a seriously unstable situation - as was the 'flying bedstead' and the Lunar lander. There's no 'pendulum effect' from those arrangements ( no restoring force when perturbed).
Doesn't the lift pass through the centre of the rotor?Phrak said:There is no restoring torque to keep a helicopter stable, such as in the case of a boat, whether the blades are above or below the center of mass.
mgb_phys said:Doesn't the lift pass through the centre of the rotor?
In that case it's equivalent to the helicopter being suspended from a rope attached to the hub
Phrak said:That being said, I was hoping someone would come up with something to show a difference between the mass being located over vs. under the rotor. After all, paragliders are stable (after a fashion) because the lifting surface is above the center of mass.
A helicopter is a dynamic system.
Imagine a stable paraglider with the pilot proped up above the sail on compressive structural members. I don't think that would work.
Why is a paraglider stable, and an inverted paraglider (apparently) unstable? The same answer should apply to an inverted helicopter where the distance between rotor and center of mass is large.
I don't know the answer to this question.
sophiecentaur said:One definition of stability is that, for stable equilibrium, a perturbation will increase the potential. For a mass hanging on a string or a mass hanging below whatever is suspending it (i.e. rotor or wing) the potential increases with tilt.
If the cm were above the centre of effort of the blades (if that's all that's involved) then any perturbation will decrease the potential - so it's unstable.
So called Low (fixed) Wing aircraft mostly have dihedral, giving a centre of effort higher than it may appear and a cm which is below where it may appear to be (like double decker buses).
They will mostly be inherently stable (you could hold them up by the wing tips and they would not pitch) but, even if not, when moving through the air, the tail and other surfaces will tend to keep them stable (roll is self corrected).
Dihedral, itself provides more lift on the side that is tilted down, which gives a righting moment so this may also apply to a helicopter rotor, which is tilted up - more lift on the appropriate side to help with stability.
More manouverable aircraft have no dihedral and are much less stable (can be put into a spin, for instance).