Electric Trim Motor - Speed Control

[Esser]

Glad I ran into this tip. It makes me ask the question though, does anyone run an trim switch interrupt on their plane? Maybe I am over thinking this but in my mind I can think of all the terrible things that would happen if the trim switch ever screwed up and did not stop turning the trim motor.

[Dorsal]

In my experience you can run the trim motor to its stops without breaking anything and you can fly and land the plane at any trim setting (not convenient but doable).

[HighWing]

One thought. I wonder if the (loose) heat sink is really necessary. I suspect the heat sink allows a continuous rather than intermittant use of the controller. I wonder if a fifteen second run (stop to stop) with mostly momentary inputs would generate enough heat to be a concern. I had a commercial trim motor controller that was about the size of two nine volt batteries. I had to think of why one was so robust and the other so small. The intermittant trim use of the little and the continuous design of the big one one was the only thing I could come up with.
Lowell

[Tom Waid]

When I took a demo flight with John McBean I asked the same question. He trimmed all the way nose up and let me fly it for a while. He then let me fly with the trim all the way nose down. The plane was completely manageable in both conditions. (Not very comfortable though.)

[jtpitkin06]

A runaway trim with a shorted switch or wire is probably going to be a momentary condition. As soon as the trim starts to run away, you will usually trim in the opposite direction. That will short circuit the speed controller and pop the breaker.

A trim cutout switch is a nice feature, or you could simply pull the circuit breaker. Iprefer the simple approach.

The most likely scenario is a trim switch or trim motor fail where the trim doesn't run at all.

The aircraft is flyable with the trim run to the up or down limit. It's just going to be a pain if you have a distance to go. If the trim runs away nose up you can use the flaps to relax some pressure. If it runs nose down, you'll be hanging on for a while and landing without flaps.

JP

[jdmcbean]

OK... didn't want to let the cat out of the bag just yet as there is some more testing to do..

We have designed and are testing a switch that does several things.. mainly it is setup so that when the airspeed exceeds 80 the trim motor speed will automatically reduce to a much slower rate (thats what is being tested now) then when you return to speeds 80 and below it will operate at full speed. There are some other enhancements like time limited operation to reduce the possibility of a runaway trim.

Hopefully it will be available in the real near future.

[Dorsal]

Sounds cool, please keep us posted.

[Esser]

John, that sounds great. The fact that I know the plane is still flyable with the trim all the way up or down makes me feel better about not putting a interupt switch. Thanks for all the great answers