Quote:
Originally Posted by BobDDuck  Best I can make of it is that it trims the nose up as the speed increases irregardless of control input when the autopilot servo is NOT engaged. |
That fits in with my impressions. You see a lot of pilot literature talking about mach tuck being an "instability" issue, when it's really a "too much stability" issue. But along with that extra stability is a trim change, which is what the trimmer is designed to compensate for.
One of the guys over at PPrune posted the following, and I know the poster to be very knowledgeable:
A Mach trim system will, as the aircraft accelerates, progressively input a aircraft nose-up trim input (whether stab or elevator, stab is easier to visualise).
Suppose you start at M0.80, stab 0 degs, elevator 0 degs (just for simplicity) - column neutral.
Now, push the throttles to the detent and wait.
If the aircraft has good, natural, static stability, it should start to pitch up as you accelerate, which will force the pilot to push the column more and more to keep the aircraft in level flight as it accelerates.
If the aircraft has bad static stability it won't do so as strongly. If it has NO static stability (easiest case to visualise) the aircraft will remain in trim as it accelerates.
We want the first case, not the second. So, if our aircraft is like the second one, you have to find a way to induce the pilot to apply forward column as the aircraft speeds up. One way (typical Mach Trim operation) is to feed in a stab trim motion proportional to the speed increase. The stab is moved to induce a nose-UP motion (stab T/E UP; if it were an elevator it would be elevator T/E UP also). In order to counter that, the pilot pushes forward on the column to get a elevator angle to cancel out the Mach trim stab input. The total lift force on the tail is the same as when we have no Mach Trim, but now it's one force due to the stab, and another opposite force due to the elevator. So it looks like the aircraft requires forward column when it accelerates, and everyone is happy, and the regulations are met.
So in one sense, the mach trimmer does increase the
static stability, when you measure it by the control forces needed to get a change in airspeed. And that's the way the regulations DO measure it, which is why the guy referred to "regulations are met."