Very interesting!
That’s not a “servo” in any normal sense. That’s just a DC brush motor with a gearbox, and the big cam
is just screwed onto the gearbox output shaft. The motor+gearbox is itself like that used inside servos – this is the general idea.
There is no slipping clutch so how would you override a runaway situation?
If I understood it all correctly you’d have to pull the CB. And also there’s two contacts (see them?) that stop the motor.
The A/P uses THIS trim motor mainly in cruise flight – and the S-TEC trim motor (with clutch) is used only for big attitude changes and is connected to the control cables. I do not yet understand what the pitch servo will do in such a scenario where the pitch trim motor (that is connected to the elevators by a SEPERATE linkage) runs away…
Will read tonight ….
If I understood it all correctly you’d have to pull the CB.
Sure, that will stop the motor, but it will leave the linkage (wherever it goes to) stuck in the final position.
These types of motor+gearbox units can sometimes be back-driven – it depends on the gearbox ratio. If the gearbox is something like 1000:1 then you won’t move it. If it’s say 50:1 then you probably can, with some force.
__ If the gearbox is something like 1000:1 then you won’t move it__
Can you simulate that on ground (Hangar)? Trim let’s say full nose up, pull the C/B and then try to move the elevator in the opposite direction.
Can you simulate that on ground (Hangar)? Trim let’s say full nose up, pull the C/B and then try to move the elevator in the opposite direction.
Surely the elevator will move anywhere you want on the ground, this motor only controls the trim tab, am I right? For certification they will have demonstrated a procedure for pitch trim runaway, it’s probably quite feasible to overpower the out of trim control force.
No, there is no trim tab on the SR22, the motor only controls the 0 position of the elevators. Of course you can overpower the trim, but it’s no fun. What I don’t know (yet) is what the pitch SERVO will do if the PITCH TRIM SERVO runs away. I guess it should overpower the pitch trim, but maybe the clutch would slip? I have to ask this one on COPA … will report back
it’s probably quite feasible to overpower the out of trim control force.
You are probably right but I am not sure. Try trimming your plane all the way to one end or the other and see if it is still landable, or even flyable. I know for sure that larger planes (jets etc) cannot be overpowered in pitch if the THS is all the way to one end.
You cannot overpower a runaway pitch trim of a Piper Seneca at cruise speed, I am pretty sure.
Found two things in the POH of the SR22:
- the trim can always be overpowered
- the A/P disconnect button will stop a runaway trim aswell
the trim can always be overpowered
I thought that too, after I write the last post. Since the SR22 trim is electric-only, a loss of electrical power could kill you. So they must have limited the trim tab authority so that a certification-specified-strength pilot can overpower it.
