Dual electric elevator trim servos.

[Nev]

I’m considering installing electric elevator trim inside the elevator, similar to an RV. It would probably require two trim servos. There appears to be several pros and cons, so I’m interested in any ideas, thoughts etc.

This type of installation would:

1. Remove the servo tab action (that’s not why I’d be doing it, but it’s a consideration)
2. Having two servos will require a way of controlling them in tandem. Has anyone done this before ?
3. Flutter becomes a possible issue if a linkage were to break.
4. Having two servos will double the risk above.
5. Using a Ray Allen servo with Dynon Skyview, my understanding is that no relay is required.
6. Additional elevator counter-balance is required forward of the hinge-line, effectively doubling the servo weight, at a long moment arm.

Any thoughts at all are appreciated.

[PaulSA]

Sounds unnecessarily complicated to me. Why not fit the standard cross link and drive it with a servo fitted in the rear fuselage rather than cables and a wheel?

[Nev]

The installation is similar to that used on the RV10, and seems pretty straight forward. Uses a direct drive linkage to the trim tab and eliminates much of the additional hardware.

Turns out it’s not a new idea and I won’t be the first to do it on a Bearhawk. The servo weights are very light at 115 grams. However you’d need two servos (and mounting hardware) and then counterbalances, taking the total additional weight to around 0.5 kg (minus the weight of removed hardware), at aft C of G.

Ray Allen publishes a wiring diagram for wiring both servos together in parallel and it’s been done in practice with minimum differential in trim tab position, even though the servos are not governed together.

[Battson]

Hi Nev,

After we talked about this - I have thought about it carefully. I don't think the dual servos directly linked to the trim tabs are a good idea.

I am still supportive of replacing the bearing-less bolted joints with something longer-lasting. But I think it needs to be made of steel with nylon bearings at least.

The 4130N steel parts which rotate on the bolts (on the trim direct drive horns) have worn down quite quickly. They were showing considerable wear and tear after just a few hundred hours, however it doesn't seem to be getting much worse than that - no considerable worsening after 750hrs now.

I can see a small / lightweight servo motor with lots of plastic parts wearing out very quickly.

The oscillating forces on the trim tabs from the prop cause a lot of stress cycles too, aluminium parts would be at risk of failing too. Fatigue cracking.

I think the stuff directly coupled to the trim tabs needs to be made of 4130 steel.

[svyolo]

Hey Jon. Would welding a small tube into the trim tube keep fix this? The bolt would then be inserted into that tube, and when you tightened the bolt, it would tighten up properly, and not just distort the trim actuating tube. Maybe it wouldn't even need to be welded. Just act as a spacer/bushing.

Looseness in the controls isn't good. It can change the harmonic properties.

[zkelley2]

What you're likely looking for is the B6 servos that don't use plastic.

http://www.rayallencompany.com/products/B6newspecs.html

What you are describing is my plan B depending on how the rest of my flight testing goes.

[Nev]

Hi Jon,

You make a very good point about the fatigue stresses that I hadn’t considered. I wonder if vibrations from the tailplane, fuselage and elevator are acting cumulatively via the servo linkage and being concentrated on the trim tab linkage and perhaps resulting in excessive wear on the bushing and bolt. Any movement between the fuselage/tailplane, tailplane/elevator, elevator/trim tab may be cumulatively acting via the servo linkage.

If so, Pat Fagan’s mod using Bowden cables should minimise this effect to a large degree. It won’t stop vibrations, but it should stop them from acting cumulatively on one point.

I’m wondering if mounting an electric servo inside the elevator may also isolate vibrations from the tailplane
/fuselage so they’re not transmitted to the linkage - they won’t be amplified.

Interested in everyone’s thoughts.

[Nev]

What you're likely looking for is the B6 servos that don't use plastic.

Yes I’m definitely looking at those. Although interestingly the RV’s have had good success with standard servos (plastic parts) mounted in elevators.

[schu]

Lots of other airplanes use those servos in a tail that vibrates a lot. The rans s-7 and highlander have had those installed in elevators.

The servos themselves are designed to hold 500lb static load, and the metal gear ones are tight.

I'm going to use them and I'm not at all worried. If you make the tray in the elevator right, then the forces on the push rod straight in/out, meaning very little flex on the mount as everything is in tension/compression. It is so close to straight that I debated on not having any hinge on the servo side, but the arc is just enough that it binds ever so slightly.

I want electric trim and a non-servoing trim tab. If I use a single servo to push pull cable, it's more weight and more complexity as the cable weighs more than the servo. If I use two servos, then I have some redundancy.

As for the counter balance, it doesn't add that much because the tray and servo are light and mounted within 4" of the hinge. One solution could be to use oratex to cover the elevator. I suspect that will completely offset the counter balance weight.

[Battson]

Lots of other airplanes use those servos in a tail that vibrates a lot. The rans s-7 and highlander have had those installed in elevators.

The servos themselves are designed to hold 500lb static load, and the metal gear ones are tight.

I'm going to use them and I'm not at all worried. If you make the tray in the elevator right, then the forces on the push rod straight in/out, meaning very little flex on the mount as everything is in tension/compression. It is so close to straight that I debated on not having any hinge on the servo side, but the arc is just enough that it binds ever so slightly.

I want electric trim and a non-servoing trim tab. If I use a single servo to push pull cable, it's more weight and more complexity as the cable weighs more than the servo. If I use two servos, then I have some redundancy.

As for the counter balance, it doesn't add that much because the tray and servo are light and mounted within 4" of the hinge. One solution could be to use oratex to cover the elevator. I suspect that will completely offset the counter balance weight.

With every iota of respect for the points you're raising, there are some differences. The tailplanes of the S-7 and Highlanders - they are more like a Kitfox than a Bearhawk. Both in terms of size of the tail plane, and hp delivered from the prop to the tail. Both of those things are considerable factors, about twice the tail area and more than twice the prop wash.

I cannot say the servo will cause a problem - I don't know that. That's why we have to try these things.

I can say that one plastic servo is doing just fine after 750hrs driving the whole system via the torque tubes.

[schu]

As always, I appreciate data points, especially when they come from flying airplanes.

Since there isn't a good way to know, I'm going to make an educated guess, just like you did when you installed the servo in your airplane.

I'm betting that 750 hours of the plastic servo driving a horn, to a torque tube, to a push rod, to both trim tabs working fine will mean that two METAL geared servos will be able to directly drive the tab with similar reliability. If for some reason a servo fails, I won't be completely without trim as I have two.

I think by far the most risky thing is the mount itself. But the angles work out in such a way where the forces push/pull against the mount butted up against the LE of the elevator.

Besides, people are putting 300HP in the highlander, and soon kitfox. What could possibly go wrong?

[huntaero]

Hi Nev This is what I did on mine, my partner used one servo and connected it to the Bearhawk linkage. I used a piece of 1/4" 2024 T3 alum rod drilled and taped both ends to 10-32 and used Ray Allen's clevis kit for the push-pull rods which is lighter than the 4130 tube.

[schu]

Hey Hunt, not meaning to be critical, but absolutely make sure that your mount is up to the task. If that cracks or breaks then you will have a very bad day.

Make sure you have a way to inspect it and I'd even lightly push/pull on your trim tabs as part of your pre-flight.

[huntaero]

Hi Schu the bracket is welded into the elevator, along the rib and a 90 degree flange welded around the torque tube so it is structurally sound. No need to be sorry we all have to watch out for possible problems and identify them.

[Mark Goldberg]

In your back and forth about the way to install electric trim best - I want to state my hesitation concerning any electric trim. That is a runaway trim situation. With the trim forces as powerful as they are in a 4 place Bearhawk - a runaway trim could really ruin your day. Especially if it happened at an inopportune time.

I am not going to post again about this. So no need to defend your decision about using electric trim and how the runaway trim situation could never happen with your installation. But as electric trim is being discussed, I wanted to make this comment so others might take it into consideration. Mark

[zkelley2]

I have the same hesitation Mark. The out of trim forces are high enough the runaway trim even caught right away would make for an unpleasant rest of the flight.

[schu]

More information:

https://bearhawkforums.com/forum/bearhawk-4-place-quickbuild/770-electric-trim-in-a-qb-kit

Heya Battson, can you do me a huge favor and tell me what kind of throws you have in your trim system? Measuring the space between the elevator and trim tab at the trailing edge at both full up trim and full down trim will tell me what I want to know.

I want to make sure my ranges are reasonable.

[Battson]

In your back and forth about the way to install electric trim best - I want to state my hesitation concerning any electric trim. That is a runaway trim situation. With the trim forces as powerful as they are in a 4 place Bearhawk - a runaway trim could really ruin your day. Especially if it happened at an inopportune time.

I am not going to post again about this. So no need to defend your decision about using electric trim and how the runaway trim situation could never happen with your installation. But as electric trim is being discussed, I wanted to make this comment so others might take it into consideration. Mark

Procedures for handling trim runaway are normally published for larger aircraft. This was true whether the system is mechanical or electrical. No trim system is infallible, therefore we need the next best thing, a way of managing the risk.

Do private pilots flying light singles know the procedure, and practice it? Not normally, because it happens so rarely in GA aircraft.

So many newer light single engine aircraft are fitted with electric trim these days, and we are not seeing this failure mode coming up more commonly in electrical systems than mechanical systems for smaller aircraft. Can we be certain that one system is more reliable than the other?

The electrical trim system is ergonomically better and practically safer / faster than manual trim. It's also more accurate / repeatable. Overall, I think we each have our own personal preferences. I don't think it's appropriate to suggest one is categorically safer than the other, unless we have data. This only drives the conversations into the private messages (which is happening), and the group learns less as a collective - people are still installing electric trim in a large number of Bearhawk aircraft.

I think the most constructive outcome here - is a thread about practical trim runaway testing (which I have done, for one) and the procedure for controlling the aircraft under those circumstances. Then people only need to adopt the procedure and practice it.