Electric trim servo.

[Mark Moyle]

Matthew Schumacher and I are collaborating on a new electric trim mounted within the elevator/s. Comprised of a servo stepper motor and planetary gear motor. Acme thread/screw, using PTFE bushings and thrust washers. I'm doing the machine work and assembly design. Matthew is doing the electronics. The trim servo uses a programmable set point for one button take off trim, and infinitely adjustable. We will be load testing and continuous cycling of the assembly until failure or sufficient time has elapsed W/O failure.


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[N3UW]

That looks great. what are you planning for weatherproofing? Are you planning on installing it in the actual elevator or the tail? will that stepper motor deal with moisture? What are you using for feedback for position of the trim motor? What is the weight so far? Nice work. Let us know how it goes.

[whee]

Great work! Was going to reply on BCP but saw your post here. You going to have only one trim tab?

[Mark Moyle]

That looks great. what are you planning for weatherproofing? Are you planning on installing it in the actual elevator or the tail? will that stepper motor deal with moisture? What are you using for feedback for position of the trim motor? What is the weight so far? Nice work. Let us know how it goes.

Thanks,
The gear motor is sealed. The servo motor....need to check on that. Installed in the elevator. I'll post a picture of the position components. Haven't put it on a scale yet. Is the first iteration and is for testing..... second will go on a diet...


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[Mark Moyle]

Great work! Was going to reply on BCP but saw your post here. You going to have only one trim tab?

Just one big one.


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[Mark Moyle]

Just one big one. Schu's quick build kit has two of the small trim tabs...he's the instigator of the project and an electronics genius.


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[Mark Moyle]

Here is the trim arm position sensor set up.
Started life fuggly

Little welding

Little lathe work




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[Mark Moyle]

Finished the trim lever assembly. Not exactly what I want....but it will work for testing. I'd like to make it out of block of 6061 including the lever/arm. If made out of a chunk of aluminum I could machine pockets on the side to contain PTFE inserts to maintain lever alignment as the acme thread shaft turns.....and would eliminate the block of PTFE shown in the picture.


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[Bdflies]

Pardon my inquisitive nature, but I have to ask; why don't you use a Mac servo?

Bill

[Battson]

Fair question.
Their servo's are small, unbelievably light, and powerful for their size.

[Papa Foxtrot]

If the goal is to fit the servo inside the elevator, the Ray Allen servos are slightly over an inch tall.

[Battson]

Given the aerodynamically profiled horizontal stabilisers that everyone wants to be using these days - it's a non-issue.

[Mark Moyle]

Schu and I talked about the Ray Allen trim servos.... Will have to get back to you.... I don't recall right now.


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[Mark Moyle]

Done. Mailing to Schu tomorrow. Weighs 10.8 ounces. 2-3/8" travel. Over all length is 8" long, 1-3/8" wide, and one inch thick.




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[whee]

Looks good Mark.

[Mark Moyle]

Thanks....
Spoke with Schu a few minutes ago and asked him to write up a list of reasons why we're building this thing.
Subject: Servo information.

So the question is: Why would we take the time to make a completely new
servo for experimental aviation. Well, simply put, we couldn't buy what
we wanted, and had the knowledge and experience to built it ourselves.

One might ask what's wrong with the Ray Allen setup, well there are a
few things I didn't like about it:

1. The servo is plastic.

2. The servo motor only has one speed unless you buy another $45 speed
control box

3. The pushrod setup is plastic.

4. There is no ability to set take off trim without holding a trim
button down and manually watching the indicator.

5. If a button gets stuck you can have a run away trim.


Many of these problems are addressed with the TWC safety trim box. It
deals with the run away trim and has an option for the speed control but
the speed control is only two speeds and the box is $245.

So add that to the ray allen T3-12A servo ($165) and the indicator ($95)
and you have a $505 servo system, and it's still plastic, and you
haven't bought any wires or switches yet.


Our design uses a stepper motor like an autopilot instead of a dumb DC
motor. That gives us some advantages.

1. We can control the steps and speed them up or slow them down. If
you press the trim button we can make it start out very slow, and
gradually speed up to full speed over 3 seconds. So there is no need
for an additional servo trim speed, the longer you hold the button the
faster the trim adjusts.

2. The stepper motor holds everything into position while not activated
because there is an electromagnetic field.

3. Because the motor is a stepper, shorting out wires or buttons cannot
cause a run away servo. The only way you could have a run away trim is
if the stepper driver keeps stepping the motor, which depends on the
microcontroller generating a step signal. The failure mode is always
stuff stops working.

4. Detecting a faulty button is pretty easy, if there is a button press
more than 5 seconds we simply ignore it.

5. We can offer a 3rd button that looks at the linear position sensor
and returns the servo to take off trim with the press of a button.

6. The stepper has a metal gearbox and a metal lead screw. It's much
stronger.

7. The servo motor has a tab that sticks out making it easier to
install in a control surface.

So...going in a box today. Did some quick calculations before talking with Schu. Lifting force is 67 pounds. Holding for is 274 foot pounds. The trim tab ain't budging once it's set. And I like the idea of having trim set points at the push of a button. One thought was an additional button. You'd have the take off trim button plus one push a button for short field trim... On my pacer I'd trim for full nose up to insure I didn't run out of elevator during the flare... Then again I'm moving the stabilizer surface so.....Don't know how the Bearhawk behaves during a short field landing? Does it run out of elevator?


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[whee]

Sounds good to me Mark. FYI, you can get a servo with a metal rod (Bristol SG B6-11T).

I have the small trim tabs, I don't want to do any welding on my elevator and in my opinion with the small tabs you need both. Think your servo could be mounted in the fuselage and drive both tabs through push/pull cables kinda like Pat F. did?

[Battson]

A slight aside:

In terms of the set-point for take-off, I have noticed that the best T/O trim setting varies depending on where the CG is. These days, I set my T/O trim depending on the CG.
At forward CG locations, having the trim tabs set to trail in-line with the elevator yields the best results.
The further aft the CG, the more 'nose down' I set the trim for T/O. This prevents the aircraft wanting to "zoom" upward immediately after you break ground, and reduces pilot workload post lift-off.