Dual electric elevator trim servos.

[Nev]

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.

Hey thanks - very much appreciated. Just to clarify my understanding, are these pics showing the elevator, the servo is being installed inside the elevator? One or two servos ? Many thanks.

[huntaero]

Hi Nev

I will mock it up today and take a couple pics for you and yes it is in the elevator basically right against the fwd 7/8 tube.

[Nev]

Thanks to everyone who has contributed to this thread so far. Really good to get all input whether for, against or otherwise. If you have any concern or advice then I’d really appreciate knowing.

Runaway trim

Preventing runaway electric trim altogether is probably difficult. Others have mentioned a number of ways to reduce the effects already.

One issue is that runaway trim doesn’t always look like runaway trim. An example is a blocked pitot/static in IMC with an autopilot coupled. No major deal in level flight, but in a climb or descent (which is often a “speed on elevator” mode) the autopilot will pitch up (or down) continuously in a self perpetuating loop to try and maintain incorrect airspeed. Reacting by using the wrong checklist isn’t uncommon in the heat of the moment.

In a pure VFR aircraft, turning off the master switch may be a simple solution.

One aircraft I used to fly had an audible (and very annoying) beep that sounded whenever you ran the trim. Simple and effective, but you did tend to get used to persistent beeps and block them out.

Both Garmin and Dynon incorporate some runaway protection into their EFIS panels, although one RV user experienced a runaway on his first flight and traced it to having the Garmin incorrectly installed via the normal RAC trim relay. Removing the relay resolved the issue - which if I understood it correctly it needed to be one or the other, not both.

But I do think there’s many advantages to electric trim including simplicity (yup), low weight, speed of operation, and keeping hands on the controls.

All systems have pros and cons and mechanical systems come with their own set of pros and cons too - I’d like to consider all aspects. Keep the ideas rolling in.

[zkelley2]

I doubt an autopilot servo can overcome the forces of a bearhawk out of trim. The motor will stall. There's no airplane I've flown in recent memory that has the control forces in pitch the bearhawk does when it's out of trim. It's two hands on the yoke heavy.

[Battson]

The shear linkage should break before the motor stalls? IDK. I know you can overpower the shear linkage with one hand in the Dynon system, if you fight the motor. That is a deliberate design choice.

[Nev]

Zkelley2 - would that be heavier because of the mechanical servo tab? If the trim servo was direct drive to the tab (removing the servo action) would it still be as heavy to control in an out of trim situation?

In other words the servo tab is designed to make the control forces lighter for normal flight. In an out of trim situation is this still the case, or do they then become heavier?

[zkelley2]

I don't think so. The idea behind the servo tab is to make the control forces lighter, to add "power steering" to the elevator. I could be wrong though.

[schu]

Yes, my electric system could malfunction so I'm taking every precaution to minimize or eliminate that (did anyone look at the safety trim box?). At the same time, if I take off out of trim (a much more likely scenario) then I can put both hands on the stick while holding a button (or in my case pressing the go to neutral button once). I'd argue that's safer than trying to wrestle it around with one hand while trimming.

I'm very comfortable with what I'm doing and why, but it wasn't without really thinking about it and designing solutions to deal with the issues. I posted to try and help Nev, I feel I did, so I don't feel the need to continue other than to say: I think it can be done and done safely, but you absolutely need a very solid servo mount, you need to make sure that you wire it in a way where a ground short won't cause runaway, you need to be able to suddenly and without notice fly an out of trim airplane , and finally you probably should have a controller of sorts to limit servo travel to 2-3 second bursts as well as a way to deal with faulty buttons.

schu

edit: grammar.

[Nev]

I took a look at the safety trim box Schu. Looks good and appears to offer a good level of protection.
My understanding is that the Dynon will also offer runaway protection too. I gather the safety trim box is a stand-alone device. One great feature is the ability to isolate the fault, and regain some normal level of trim. I assume that I would want one protection system but not both i.e. either the safety trim box, or Dynon trim protection.

[Mark Goldberg]

Just a quick note: I have no data or scientific reason to be cautious about electric trim. I had a friend crash and die from a runaway trim (it went to full nose down trim). So I am less than objective about this subject. Mark

[schu]

Very sorry to hear that. That's very sad and it's understandable why you would dislike it after going through that....

[Battson]

More information:

https://bearhawkforums.com/forum/bea...im-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.

Here are the up and down measurements (attached). I have never used full nose down trim, I use full nose up almost every landing.

Note that the trim tabs do servo, and I have measured these in the elevator neutral position.

[zkelley2]

What hole on the trim tab do you have the arm attached to?

[schu]

That helps a lot. I suspect I'll have plenty of trim. I have roughly double that much travel in the middle hole. I'm starting to think about covering, so hopefully next summer I'll know more.

[Battson]

I am using the lowest hole, i.e. least sensitivity, but I have moved it around before. In my opinion, you wouldn't require any more trim tab travel that this.

[TimTall]

I've had a runaway electric trim situation on my Murphy recently. Not very pleasant. And that's in a plane with very light control forces that requires almost no trim adjustments. Definitely wouldn't want to deal with that at a critical moment in a plane with heavier control forces.

I've flown Hughes 500 heli's with electric trim. During the type rating we trained quite a bit to deal with it. Not that fun removing one hand from a control to pull a breaker when you're 3 feet off the ground with a stick trying to pull out of your other hand. They installed that system in that particular heli to avoid the need for a hydraulic boost system. Add a complicated system to avoid a more complicated system. That makes sense. Adding a complicated system to avoid a less complicated system is tough for me to understand.

[zkelley2]

The main reason for electric trim in the bearhawk, at least to me, is to remove the servo action from the trim tab.

[Nev]

Tim did you discover the cause ?

[TimTall]

Looks like just a faulty switch. Have to pull it out this week and order a new one. The motor works so its probably just the switch sticking.

[Bcone1381]

Bob Nuckolls says
1) "Things Break"
2) "Systems shall be designed so that when things break, no immediate hazard is created."
3) Things needed for comfortable termination of flight require back up or special consideration to insure dafe operation."

The Swearingen SA-227 Metroliner was a 14,000 pound 19 seat commuter aircraft. It had an electric trim and each yoke had a trim switch. It was a split switch. First flight of every day we would test each half of the trim switch. With a single switch activated, we should see no trim reaction. Then we would move both switches and look for normal trim movement. Then there was a trim master switch on the center column. It was a dual switch as well. If any switch failed closed, we found out first flight of the day.

The Metroliner took seriously the idea that switches fail. Apply Bob's principle to your trim and it will be safe. If any single failure point causes anything more than a ho-hum experience, can it be rethought and made safer? For example, "If my trim switch fails closed the motor is active and out of control, is that acceptable? Can it be made acceptable?

[Nev]

Same aircraft that had the beeper for pitch trim movement. The double trim switch is a pretty good idea. (The cargo door proximity sensor - piggy backed off the nose locker light circuit - not so much).
It was a pretty “pitchy” aircraft thought so I guess a runaway pitch trim would have needed immediate attention,

[svyolo]

Has anyone done the simple test of trimming all the way nose up or down, and see how she flies at various speeds, in case of runaway trim? I guess the limiting factor would be how it lands, with full nose down trim.

All of my last 30 years was with a trim-able horizontal stabilizer. On what I was flying the stab trim was much more powerful than the elevator. It would be very easy to lose control. Stab trim cutout switches were close at hand to shut the trim motors off. I just don't see that being nearly as bad when the trim is accomplished with tabs on the elevator. I could be wrong.

If runaway trim only causes a few pounds of force on the stick, is runaway trim that big of an emergency?

[zkelley2]

Has anyone done the simple test of trimming all the way nose up or down, and see how she flies at various speeds, in case of runaway trim? I guess the limiting factor would be how it lands, with full nose down trim.

All of my last 30 years was with a trim-able horizontal stabilizer. On what I was flying the stab trim was much more powerful than the elevator. It would be very easy to lose control. Stab trim cutout switches were close at hand to shut the trim motors off. I just don't see that being nearly as bad when the trim is accomplished with tabs on the elevator. I could be wrong.

If runaway trim only causes a few pounds of force on the stick, is runaway trim that big of an emergency?

The out of trim forces are very high. Two hands on stick high.

[svyolo]

Does your BH have the original, larger, trim tabs, or the later ones?

[Mark Goldberg]

Even a BH with the smaller two trim tabs has high pitch forces when out of trim. Where you might see this is if you forget to re trim for take off after landing. Taking off with landing trim is something most people do only once. It is an arm full. Mark

[JimParker256]

One of the tests for certified aircraft is to move the trim to full nose up position, and verify that the airplane (without any forward stick pressure) does not stall. Because of this, in most certified aircraft, you can use full aft trim to set up Vg (max glide speed) following an engine failure. It significantly reduces pilot workload at a time when you really need your focus.

I learned about this from an AOPA article years ago (it's still out there if you search the AOPA site for it), and it has worked on every certified airplane I've owned or flown since. Of course, most of those certified planes have a limited CG range when compared to the Bearhawk line...

[Bcone1381]

I believe you are correct about trimming to best glide speed, Jim. (My belief follows and is not backed up with data) If I trimmed a small GA aircraft full nose up, then applied full power for a Go-Around and sat back and watched, I think it would stalll in a rather spectacular fashion. I envision a large increase in pitch authority from the prop wash induced airflow over the H-stab.

[svyolo]

I guess my question is aimed at whether electric stab trim cutoff, and a manual backup trim is absolutely required, or just desired. As long as the aircraft is controllable for a certain length of time(???), and safely landable, I would not say it is required, even if it is desirable.

With a large, powerful, THS, cutout switches and manual backup trim is required. The aircraft would be uncontrollable without both if you had runaway trim.

I like the idea of dual actuators for redundancy. Another simple redundancy "add" might be having the two stick mounted trim switches "switchable". Either individually on or off, or R or L. If the switch is causing the runaway condition, you could turn it off.

That could also prevent accidental trim actuation from a passenger from their side, or yours if you are trying to join the mile high club.

[Nev]

That’s a very good idea having switchable L/R trim switches.

[jaredyates]

One lesson to learn from the 737 MAX is to not give the electric trim system more authority than it needs. Rig the linkages geometrically so that full servo travel provides no more than full desired trim travel. It would not be prudent to be getting the most trim travel you'd ever want to have, and still have any further servo travel after that. In the 737 case, the system was a stall protection movement of the pitch control and not plain old elevator trim, but the concept is the same.

If there are any doubts about the possibility of a runaway, consider this case. One of the most robust systems you can imagine, with split switches, automatic switch lockout logic, two trained career pilots doing exactly what they were trained to do, in a modern transport category airplane, and still they just about lost control of the airplane:

http://avherald.com/h?article=4cef2f7b

The short version of the story is that incorrectly-applied safety wire caused intermittent wire shorts in the control column, which led to an intermittent runaway plus an intermittent trim disconnect button. It's the classic error chain, all the way from the design phase, maintenance, inspection, etc. There were committees of very smart people devoted to making sure this didn't happen, at many stages in the life of the airplane, and for untold tens of thousands of hours it didn't happen, until the one time it did. Not to speak for the many wise people that we are glad to have here, but speaking for myself, there's no way I'd presume to design a system that is more runaway-proof than one that many full-time professional engineers can make. Yet, even their systems can fail. So we need to have good failure tolerance like Brooks says. I think this is a good situation to consider a pull-able circuit breaker on the panel, even on an airplane that otherwise uses inaccessible fuses. Back when I had an autopilot installed, the breaker for the servos was right above the autopilot control panel. I was rounding up parts to install electric trim, because the Dynon system would automatically trim in pitch, but after abandoning the autopilot I never continued on to install it.

Taking off at landing trim is a syllabus item in the transition training course, in part because it is very likely that your first-ever takeoff will be at least slightly out of trim.

When designing the trim system, it's worth noting that the CG determines how much trim is required. A forward CG requires more trim change with speed change, and a rear CG requires less. In theory, a Riblett airfoil (model B, Patrol, LSA) should require less trim change than the original airfoil. This seems to play out in the Patrol/LSA fleet, with more data pending on the Model B fleet.