To: Rotor Nut,
In rereading Outside Loops’ description of inflow I think, maybe, perhaps, I know what he is talking about. He mentioned that if the advancing blade flaps up due to the increased relative and rotational airspeed of the blade then gyroscopic precession dictates that the disc will tip up and this requires the application of more forward cyclic to counter the raising of the disc at the front.
This sounds logical but I think it is incorrect. Somewhere in the Rotorheads forum, I described the effect of gyroscopic precession and I mentioned that you do not think of individual blades but that you think of the disc as being solid and the change in position of which is caused by the change in forces across the disc. Just like the rotor on a gyroscope that is mounted in dual gimbals.
Again I must use the tail rotor analogy. When the blades flap in or out they change pitch from the pitch that the pilot commanded using his anti torque pedals. The same is true for the main rotor. The pilot inputs the necessary pitch to get the helicopter off the ground (collective). He then incorporates cyclic input which changes the lift forces across the disc(cyclic). Now the helicopter starts to fly forward and the advancing blade because of its rotational speed is exposed to a higher relative wind, which it seems would cause the advancing blade to lift. However the advancing blade is also at a lower pitch than what was inputted for collective hovering. Meanwhile, the retreating blade is at a higher pitch. This causes a lift differential that results in the tipping of the disc, which is what set up the increased wind velocity over the advancing blade in the first place.
Now we get to Outside Loops’ posting. Due to the oncoming high velocity airflow the advancing blade will tend to flap up. If it does, the pitch link is still attached to a rigidized swashplate and if the blade flaps up or tends to, the pitch link will subtract pitch causing the blade to return to the in track position. This is aerodynamic and does not relate to precession. It is sometimes referred to as a Delta Hinge Effect.
I have said it before and I’ll say it again. With all of this pontificating, I can still be totally wrong.
I will add this point. If in effect what Outside Loop says is correct then when does this condition end. It would be like a short tailed dog chasing its' tail. As the forward speed builds up the lift on the advancing blade increases and precession sets in and causes the disc to raise in front then the pilot adds in more forward cyclic to counter the condition and the helicopter speeds up and the advancing blade lifts and the pilot adds in more forward cyclic until he hits the stops. The next thing that happens is the onset of retreating blade stall.
------------------
The Cat
[This message has been edited by Lu Zuckerman (edited 11 October 2000).]