The instantaneous reaction to lowering collective is for the lift on the advancing side to reduce MORE than the lift on the advancing blade. [The amount of blade angle reduction on adv and ret sides is reduced by the same but the ratio of lift is different].
Thomas,
This doesn't seem to be the case when I run the numbers through a simple 'lift formula' spreadsheet. A change of pitch doesn't appear to change the ratio of lift between the adv and rtg sides, seeing as the only parameter in the lift formula being changed is the pitch and that is being changed the same all over. (That's why its called a 'collective'!) In fact the the opposite of what you said happens... the amount of lift reduction is different but the ratio is the same. That's because of the V-sqaured in the equation.
The only thing that changes the ratio of lift is the forward airspeed of the aircraft. Of course this would make sense as the whole point about dissymetry of lift is the difference in forward speed. .i.e. in a hover no dissymety of lift! If you are in a hover and pull much collective you still don't get dis-of-lift! Forward speed is the key.
Here's how I see it!
In forward level flight for a 'given' speed, there is a 'given' amount of blowback / flapback (whatever you call it)...thus the pilot will need a 'given' amount of foward cyclic to balance that dis-of-lift which gives rise to the 'given' attitude of the aircraft. It's all in balance; a system in equilibrium. (It could be argued that actually there is no dis-of-lift in steady forward flight, but that's for another thread!)
Lowering the collective simply un-balances the equation. Now there is less lift. To bring the system back into equilibrium, there must be a change in the other parameters. If the pilot does nothing, the speed of the aircraft will initially begin to bleed away, thus reducing the dis-of-lift. The cyclic position is now too far foward and the nose of the aircraft drops, sacrificing the level flight in order to maintain airspeed.
What happens next is up to the pilot. If he wants to maintain level flight with the new collective setting, then speed will be sacrificed and aft cyclic must be used in order to balance the dis-of-lift once again.
You don't notice much airspeed change while all this happens, due to the 'law of momentum'. The reduction of pitch does not instantly cause a reduction of airspeed, but it does cause an inequilibrium between the cyclic setting and the degree of flapback (dis-of-lift). The momentum fights against the 'in-equilibrium'. The momentum is lost at the same rate as the equilibrium is restored.
If one adds in the effect of drag (which would be different at different attitudes), then there is going to be some change in airspeed. Let's say the aircraft was originally in level flight at the most aerodynamically efficient attitude and speed. Then lowering the collective here will cause all the same stuff to happen as above. However, as the aircraft reaches a less efficient attitude, then the speed will be lost, causing a greater pitch down if not corrected by the pilot.
Conversely a pilot who is flying above the 'efficient' attitude / speed, will find the opposite. Lowering the collective will produce the inequilibrium state, but this might not be so marked as the aircraft adjusts to a more efficient attitude. In fact initially the aircraft may not seem to pitch down at all.
This is noticeable in aircraft which have definite speeds (VToss / VBroc). The amount of collective needed to initiate a descent can be very different depending on what speed you are at and what speed you are going to!
I am wary of the explanation regarding the downwash on the horizontal stab, as I think this doesn't obey the premis of 'action and reaction'. The downwash action must be complimented by a reaction at the rotor. Sit on a child's swing with your legs out in front of you. Then reach forward and try to push your legs down...it doesn't work! Just like if you were to get a huge fan and try to blow your legs down. That's why you can't put a huge fan on the back of a sailing boat and hope to go any faster!
The only part the horizontal stab might play in the whole affair is that when you drop the forward speed the stabilizer (depending on its shape and AoA) will work differently, and I don't discount that this may have some affect.
Well that's food for thought anyway,
cl12pv2s