Mach Trimmer

[MLS]

Hello all, I’m researching a question that I was asked in the first interview on how a mach trimmer actually works? I knew why it was there, but I couldn’t explain the mechanics of it at all. Is it a function simply built in to the stab trim system that will activate all the time when a preset Mach number has been reached? My effort so far to find the answer has failed.

Can anyone help?

[Beechdrivr]

hi MLS i hope this helps

The mach trimmer is a system that artificially corrects for tuck above the a/c's mcrit by sensing the speed of the a/c and signaling a proportional upward movement of the elevator or variable incidence stabilizor to maintain the a/c's pitch attitude throughout it's speed range up to its max Mach demonstrated diving speed (MDF).

Good luck with the 2nd interveiw:ok:

[Blacksheep]

Just to add a bit to the above reply, in aircraft with autostabilisers - such as most large jet transports - 'Mach Trim' is just a mode of the pitch trim system that may apply up elevator as speed increases (or vice-versa if slowing down) with the auto-stab function following up to trim out the elevator displacement. In some types the mach mode only operates when the autopilot is not in use and the aircraft is being hand flown. In this case, manual trim takes priority over mach trimming.

[MLS]

Thanks guys for taking the time and helping me out!

MLS

[Medwin]

Beechdriver,

Are you sure its the upward movement of the elevator not downward. Remember, Bristow's book is not the greatest book out there.

Medwin

[Mad (Flt) Scientist]

It's correct.

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.