flapback/flapforward and collective?

[Stuck_in_an_ATR]

Please forgive another anorak question from me, but so far I haven't found a better place to ask them and get in-depth answers... :)

In the books I have read, flapback and flapforward are explained as being associated with forward speed and flapping of the advancing and retreating blades to equalize lift. Seems logical... :)

Now, why do you get flapforward when rapidly reducing the collective (like when entering autorotation)? Conversly, why do you get flapback when rising the collective? (eg. bottom of an approach)?

My guess is that changing the pitch angle by the same amount on both sides (when moving the collective) will result in greater flapping (be it up or down) on the advancing side, because the effect of blade pitch change is "amplified" by V squared...

So, eg. when reducing collective, the advancing side will "see" higher lift loss than the retreating side and thus will flap down causing the disk to tilt forward. Opposite would happen when rising the collective.

Is that correct?

Stuck.

[[email protected]]

Yes it is :)

[RVDT]

To look at it from another perspective -

All you are ever doing is "changing the flap".

[helisphere]

Hmmm... seems like I remember a fairly long thread on this very subject some time ago.

Anyone have the link? I'll look...

[helisphere]

Ok here it is:

http://www.pprune.org/rotorheads/224249-why-does-nose-pitch-forward-when-you-lower-collective.html

[delta3]

Stuck_in_an_ATR

There is a simple analytical formula for rotors that calculates the blow back. It boils down to:

Blow back is proportional to the coning angle
Blow back is proportional to the forward speed

Manipulating the collective will influence the total lift/load on the rotor and as such the coning angle: the coning angle is proportional to the MR load, ie collective , so the blow back is proportional to the collective (please do not extrapolate this relation into the acrobatic regimes so to speak)

To put it figurative : think of an umbrella and the effects wind has on it. Rotor is of course a bit more complex, but it has a similar behavior.


Remark : this relationship has nothing to see with nose up or down, which is the consequence of the rotor lift on the hull and hull aerodynamics (horizontal stabilizers)

d3