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Dual Servo Electric Elevator Trim

WrightsRV7

Well Known Member
Well N56GZ has flown through phase I and out of the coral, now the fun phase. I installed dual electric servos on the elevator trim and have found them to be quite nice, effective, and from all indications working extremely well and safely. At least for my style of landings, with two forward occupants, 2/3 full fuel, and a case of oil in the baggage compartment, I have about 1/4" of up trim (trailing edge) on each tab (and yes, they move and stay well in sync) at landing with full flaps. In cruise, the trim tab is neutral. I do not trim out all stick forces on final, I prefer to have some light back pressure on the stick, so I am sure others will or may have more "up" trim in play than I. These are very big trim tabs for a small and light plane and it is no wonder most find them very sensitive. I may look at testing out one trim tab down the road, appears from my flying of the RV10 a very small fraction of the dual trim tab movement is utilized.

I will start to collect some video of the trim tab, with particular interest in the servo attach bracket, looking for any significant flexing as the trim tab is placed in more demanding positions (simulating a runaway trim etc...). Beefing up the attach plate (as done in the RV14) will be a rather easy task, if needed.
 
How are they synced? I've wanted to do this too but can't help but think that unless they were synced via some position "brain" that they'd diverge from each other over time.
 
Soapbox

I am sorry but this is a very bad idea. With no way to sync the tabs or deal with a single tab servo failure it is not if you will have a scary/deadly situation but when. I think this idea needs more cognitive work before it is ready for prime time.

Gary Specketer
 
But...

Just another perspective...

Many, many aircraft out there use only a single trim tab.

I should think that syncing the trim tabs with dual servos would not be an issue, as the system that is stock on the -10 does not "sync" the tabs either.

Kudos for trying something different! Keep us updated on how it is working out...
 
tabs "out" of sync issue

Good question...Ray Allen supplied me with two servos having very close, but not identical run speeds. What I have found that after a flight of many touch & goes, some cruise (2 h), and then landing, they are still very very close to identical L/R trim tab positions. They can be reset easily by running to full down trim, then back to take off position (~neutral for me, I like to pull the airplane nose up and off the runway).

I am certainly NOT claiming this is a done deal, I did build an experimental aircraft, and I do think my "ideas" through, and carefully, and with 35 years of Ph.D. research in chemistry, explosives, composites, and polymers, AND take safety very, very seriously. Have an equal number of years flying single (conventional & three-leg), twin engine aircraft, and have worked under a host of very talented IAs and A&Ps, with great respect for their experience and knowledge and the FAA's AC 43. AND still learning and listening as time goes on. If any believes or thinks a "std RV10" trim installation DOES NOT have up to 20 deg of trim tab mismatch then they need to run it to full down trim and take a look. The fact is with the std trim installation (single servo) rarely are the trim tabs in alignment and this is fine by design. We ALL need to consider a "runaway" trim and having one versus two is different for sure, but having one "runaway" trim tab move is less demanding in the cockpit than dealing with both moving (e.g. to full up...).
 
Six years ago I contacted Van's about doing the same thing. Both elevators are set up to take a servo just like the other RVs, and the unnecessarily complicated RV-10 trim tab set up with cams and cables violated my engineer eye.

The short story - I got my head handed to me by Van's. It seemed I hit a sore point with them.

I did follow up on why on earth this design came about. The second/third/fourth hand story was the RV-10 was originally to have one elevator trim tab with a standard servo set up just like the other RVs. The other elevator trim tab was to have a servo that operated only with the flaps - so when you put the flaps down a fixed amount of nose up trim would come in. I don't know why it was changed out.

That's the story I pieced together - anyone else?

Carl
 
I guess my question is what is the benefit of going with 2 servos over the stock setup of one servo (esthetics aside)? The stock setup works just fine so I don't understand the problem. If someone was going to mod the elevator trim system I'd have bet it would have been to ditch the 2nd tab, not add a 2nd servo.
 
The other elevator trim tab was to have a servo that operated only with the flaps - so when you put the flaps down a fixed amount of nose up trim would come in. I don't know why it was changed out.

That's the story I pieced together - anyone else?

Carl

I heard the same story.
I can only guess at the issues, but one guess would be that airloads help the flaps retract a bit faster than extend - and over time the servo would walk to one end of its range.
 
Very Good points...Thank You....

I too got an ear full from Vans, and I fully listened and appreciated their point of view. We all appreciate Vans gives us incredible support and what a superb plane, no matter what model. In fact I downloaded the RV14 "beef up" and have it ready. Most Cessna aircraft use a single trim tab, my twin aero commander (6000 lb aircraft) uses a single trim tab and with less surface area than the two RV10 tabs...:)

Why do it? Good question Todd...:) I was a bit bothered having two trim tabs working out of sync...just seemed wrong...and your right...it works well and will continue to do so, no doubt. Vans is full of bright and sharp guys and gals. I will test out what one trim tab does, now I can just unplug one connector at the back fairing and go for it...will be an interesting set of data points and I will post, and I am sure I will get some "feedback", but I am science guy, I like to think things through, and then push a bit forward, even when people say I might fail. From my data and flight experience to date, which has covered very forward CG limits (109.3), I just see no need for the two trim tabs.

Always appreciate the caution, questions, and dialog, makes us a great community of builders and pilots!!! Lets face it, Vans Aircraft has changed general aviation forever, and in such a positive way.

Cheers, Mike
 
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From my data and flight experience to date, which has covered very forward CG limits (109.3), I just see no need for the two trim tabs.

Cheers, Mike

I have a 'stock' 10 with a fairly forward empty cg. Solo and part fuel, I need to carry 15 lbs ballast. On a full flaps landing I end up with full nose up trim, and still nose heavy (but manageable). That's with two stock trim tabs.
 
elevator trim at landing...

I too have a very stock RV10 (IO-540 D4A5, Hartzell metal 2-blade, typical Dynon/Garmin avionics) and surprised with the difference, but really good to hear about your data point, thank you Bob. Wish we had the planes in the same area of country, would be interesting to have you fly N56GZ and see where the trim tabs end up with similar CG loading. We (i.e. pilots) all seem to have slightly different "styles" on final approach and flare, not implying one is better than the other, and I am sure that can and/or could account for some difference.

Would love to hear about other RV10 pilot/plane trim tab positions at landing with CG limits around 110 or less (typical of one or two in front with considerable fuel, light baggage).

Cheers & Happy Flying....Mike
 
When I'm solo or plus 1 I fly with a 5 gallon collapsible water container in the baggage area giving me 40 lbs of ballast that I can easily adjust down to zero if I pickup pax or cargo enroute.

In the past I've always kept a mimimum of 10-15lbs back there when I'm forward CG. As a result, I've never had to run my trim to full nose up.
 
I am curious why you did not just attach both cables to the same point on the lever arm and use one servo instead of two?
 
equal attach points

Thought about that option and think it is a good one too. I choose the two servo option for a bit of redundancy and thinking about an option to switch one out one for takeoff-cruise operations and use, if needed, for landings with forward CG. Help decrease the sensitivity of the trim as well. I always have wondered why Vans made them move with different rates/positions, one of those questions I am almost afraid to ask them....:rolleyes:

Cheers, Mike
 
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