Why does the nose drop when you reduce the 'throttle' / thrust?

[cl12pv2s]

A topic is raging on the 'Rotorheads' forum (not for the first time) asking the same question.

The problem is that I think the complexity of helicopter aerodynamics is clouding what is probably quite a simple concept.

Thus, I put the same question to y'all in the hope of a simpler explanation!

So, why does the nose drop when you reduce throttle?

cl12pv2s

Same question to the Rotorheads! (http://www.pprune.org/forums/showthread.php?t=224249)

[Speedpig]

So that you can see where you're going?

Seriously though, I like the CofG theory, and logically, would they not design it that way so that you can indeed see where you're going?

coat on

SP?

[john_tullamarine]

Two considerations

(a) speed stability.

(b) (perhaps, depending on Type) thrust line pitching moments

[F4F]

or equationally expressed:
Certain speed = certain amount of lift =
Less speed = less lift
+
CG ahead of CL
= "nose drop" :)

[SpiralStability]

On an aeroplane with underslung wing mounted engines it is the moment created around the CofG by the thrust line of the engine....known as the thrust pitch couple.

I don't think that the CofG in relaton to Centre of lift has any bearing - this affects the force required on the tail plane to balance the moments and maintain a stable flight path.....


Hope that makes sense and is right!! ...I'm not always the best at explaining technical concepts!

[Rainboe]

.....particularly for helicopters!

[barit1]

On an aeroplane with underslung wing mounted engines it is the moment created around the CofG by the thrust line of the engine....known as the thrust pitch couple...

Other side of the same coin: There's a related thread here (http://www.pprune.org/forums/showthread.php?t=224184&highlight=nosewheel) about the MD-11 & DC-10 wherein chopping #2 throttle causes a pitch UP.

[VNAVSPD]

It's due to the fact that the forces of Lift, weight thrust and drag are not acting through exactly the same axis, mainly because the centre of pressure, and centre of gravity rarely coincide. Weight acts through the centre of gravity, where as lift acts through the centre of pressure.

In most aircraft the CofG is slightly forward of the CofP, and therefore the opposing forces of lift and weight create a nose-down pitching moment. Normally this force is balanced by the fact that the thrust line is slightly lower than the drag line, and so when the a/c is straight and level, with thrust=to drag and lift=weight, there is no pitching moment.

If you cut the power, you reduce the nose-up pitching effect of the thrust-drag couple, and so the lift-weight couple causes the nose to pitch down.

Most light aircraft are designed this way so that if the engine fails, the aircraft will begin to adopt a glide attitude without the tendency to loose speed.

I hope this makes sense, it's really hard to explain without a diagram. Can anybody help me out here?