Originally Posted by
Ascend Charlie
IBut does it really matter, other than giving you something to think about, and divert your mind from the lack of Lou Rawls and the closure of Dan Murphy's?
Good morning everyone. It is only the exceptional circumstances I find myself in that have tempted me out of my shell. I hope I can be forgiven for raising a bit of diverting controversy.
I have been away looking for more information and I now have some doubts about the cause of the roll in the hover.
I am sure about the physics: the balance/equilibrium of forces and couples in the hover; the need for correct terminology; the absence of a pivot.
I looked back at what Nick Lappos had said in earlier threads and he is telling us that the torque applied to an articulated or semi-rigid rotor head by a tilted rotor disc is big. I was ignorant of this in the 70s and it is something I learned here. I could not find that he explicitly said this was enough to cause the hover roll and he also said that the position of the tail rotor was significant in determining the degree of the roll. I am also largely ignorant of the very sophisticated design of modern helicopters - the last I had some small knowledge of was the Lynx.
Here is where I stand at the moment. There is tail rotor drift and this is corrected by tilting the rotor disc laterally. The mechanism by which the disc is tilted is a combination of design choices and pilot control. The design choices will be a compromise between providing for the hover and cruise. In nearly all cases tilting the disc will result in a roll couple, which is from the torque on the rotor head and/or the offset between the tail rotor and main rotor. As the roll progresses the roll couple is balanced by the couple formed between the vertical component of the rotor thrust and the weight acting through the CG.
My real doubts are about where the roll couple comes from. The hover is a state of static equilibrium and in comparison to most helicopter aerodynamics it should be relatively easy to analyse. However, I am aware that there are a lot of things contributing to the forces and couples and that Nick Lappos may know of interactions that have completely escaped me.
Lastly, I think that the simple explanation may be true for teetering rotors, but I am even questioning that.
