cl12pv2sYou are correct.

I was thinking in terms of 'Absolutly' Rigid Rotors (zero phase lag). The previous reply in this posting has been removed.

Dave

Zero Phase Lag?
 

Surely an absolutely rigid rotor is an theoretical construct only? Any real world (physical) rotor system is going to have some flex to it. Even if you don't have flapping hinges, the blades themselves will flex and act like flapping hinges.

Dan

Deemar

You are correct ~ as well.



For more; http://www.UniCopter.com/0815.html

Dave

crab said, "Matthew, I think you are saying that you agree that flapping/coning is likely to be the initial reaction to a lowering of the lever. I think that is the answer to the original thread question and all the stabiliser stuff is a secondary effect once the RoD has been established."

I probably did, but that's not exactly what I meant to say. I think most that are in favour of the flapping/coning argument are correct if you limit the reaction to a very short time period. If you're a designer or a tester, limiting it this much is needed, but if you're just trying to understand the helicopter I think you should go beyond the instantaneous reaction and look at a larger picture.

The larger picture is that when you lower collective, you reduce the blade pitch, create a rate of descent, and throw the helicopter out of equilibrium. From those you get reduced coning which changes the disc angle, changes in lifting and dragging surfaces due to the rate of descent, and moments about the center of gravity due to being out of equilibrium.

By answering the question with all the above effects (and even more that could be added) you make it very difficult for a designer to do anything about it, but you help the helicopter pilot understand what is happening to his machine.

Matthew.

crab is right,

the pitching motion is instantaneous, the horizontal bits may add to it but it is secondary,

the pitching of the disc, up or down, occurs whenever you raise or lower the collective or whenever you accellerate or decellerate the aircraft. a simple aerodynamic fact.

if you lower the collective and hold the cyclic position the nose will pitch down and the aircraft will accellerate, the nose will then pitch back up, flapback, and pass through its original position in a descelleration, this is static stability the nose will then pitch down and accellerate again, flapback, these up and down oscillations will increase to destruction if not controlled, dynamic instability.

flapback, the tendency of the disc to go back to where it was, occurs because of simple aerodynamics whenever you raise or lower the collective or accellerate or descellerate and don't compensate with cyclic.

Simple: It's because when you lower the collective the weight of it is nearer the front, you see, simple C of G thing :ok:

thread revival - I've had my nose in this traditional pprune thread for the last hour or so and was interested to see if anybody had anything to complement it with?

In most helicopters, the horizontal tail surface is operating to a greater or lesser degree in the downwash from the main rotor in powered flight. Surely this must mean that as soon as the collective is lowered, the airflow over the surface is modified and causes a nose down pitch before the descent is established. Unfortunately, in practice it is impossible to disentangle the various effects. However, it is noticeable that larger helicopters still exhibit the nose down pitch even though their inertia means that they don't alter the flight path into a descent quite so quickly as for instance an R22.

Lowering collective = nose down
 

It is kind of amusing in retro spect to see how speculative and complicated answers tend to be.

I would use the following example: what happens to a plane if you remove the forward wings....

You can of course start a whole thread on how the tip vortices of the forward wings react with the tail, but the fore most important thing is that you remove upward lift at the front of the air craft, so all other things equal, the nose must come down, that in fact is exactly the purpose of the tail.

Perhaps a more serious result of that: high speed low level flight in a heli is more dangerous than planes: if you lose your engine your nose may point faster to the ground than you are able to recover by putting cyclic backward while flaring...

m2c, d3

Differential Flapback
 

This long running thread is good. But only Thomas coupling seems to have the answer. It’s all to do with differential flap back. While there is no relative V change made by raising and lowering the collective, there is a difference in percentage change of alpha. On the advancing side, a lever raise changes alpha by far more in percentage terms than on the retreating side, because the advancing side has a lower alpha to begin with. Across the fore/aft axis, when phase lag is applied, the disk climbs more on the advancing side than the retreating side, causing a ‘flapback’ effect leaving the disk higher at the front. The opposite is true for a lowering of the lever - lever down, ‘flap forward’, disk lower at the front.

Woah. 14 years. :D

I think I said that in post #54

Must be a new record.

24 years since it started - almost quarter of a century!

I figured it was none of the above but due to gyroscopic precession of the tail rotor.

And apparently M. Vuichard has a much better technique for stopping it:E