Speed trim and what it does.
First lets assume that the working line of thrust goes trough the CG, so no moments created by the thrust.
1. Suppose you are flying at 250kt and you reduce your speed to 220kts. You will have to trim nose up to do that for aerodynamic reasons only. Remember the thrust line goes trough the CG.
2. If at constant speed you select flaps i.e. increase the camber, then you have to trim nose up again.
If you combine both, flaps and speed reduction the you have to add both trim requirements. If the aircraft is light and the CG aft then the moments created by these forces are smaller than if heavy with forward CG.
Now since the engine thrust does not go trough the CG it may be so that; when reducing speed and setting flaps and increasing thrust to compensate for the increase in drag that the pitch up created by the engines does the trimming work mentioned under 1 & 2 for you.
Theoretically it is even possible that it does it so well that you have to trim FOREWARD iso aft or that the trim requirements become so small that it becomes difficult.
That is why the FAR amongst other reasons sets a min req for stick force / g.
The speed trim takes care of this in a way to trim in the other direction of the required trim. It does so not to trim the aircraft but to fix the Stickforce/g requirement. You can then trim again as usual. Once you get away from this condition the process starts all over again when the system senses that the stick force/g is too low.
Without speed trim the flare would become very difficult bcs the stick force / g would change in a way that makes the required control input a function of too many parameters f(thrust change, rate of decent, runway slope, inertia, wind changes etc)
The reverse happens when you acc.
There is a bit of confusion that arises bcs of the name they gave to this system. On the B777 it works all the time. Fly by wire has the advantage that you can play with this easily. On the B777 the pilot no longer feels the pitch created by the engines at all times. The aircraft behaves as a glider with respect to trimming requirements. In other words you trim for speed changes only.
In fact you see that these things were introduced when engine thrust went up on acft with the engines below the CG.
Bigger issue on the B747 where CG moves horizontal as well as vertical.
Last edited by Pitch Up Authority; 21st Feb 2012 at 11:36.