Bik_116.80
No, I meant what I said.
I know most people tend to think in terms of a T/D couple and a L/W couple, the reason for this is probably that this is the way most basic PofF texts / notes tend to break it down, normally with the intent of discussing the balancing forces required on the tail.
But it is a little off the mark. You don't have to have a pair of opposed forces, applied to different lines of action, in order to produce a rotational effect.
All you need is one force that doesn't act through the C of G.
In our example we are imagining an aircraft in the cruise. The forces are all cancelling one another out, so the aircrafts flight path is steady. The rotational effects of all the forces are also cancelling one another out - we know this because the aircraft attitude is constant.
i.e. in Newtons laws terms there are no unbalanced forces (so no acceleration) and in angular terms there are no unbalanced moments (so no angular acceleration).
i.e. you can forget about them.
All we are saying is: if we add some more thrust, what is the effect of the extra. You might as well be looking at a space ship in deep space (so forget all the other forces).
If the thrust line doesn't pass through the C of G you'll get angular acceleration.
Like I said, its basic mechanics.
Imagine a glider. No thrust so there is no thrust drag couple. Now stick a pole 5 feet up from the canopy with an airbrake on it. Deploy the airbrake. Glider pitchs nose up, because the line of action of the force is above the C of G of the object.
Hope that helps.
Must dash. Mrs Pit Bull is announcing the readiness of dinner.
CPB.
[This message has been edited by Capt Pit Bull (edited 09 November 2000).]