Double bay elevator trim tabs

Gary Wallace

Gary Wallace

Well-known member
Looking for some input before covering..my elevators have the older double bay trim tabs..should o just desensitize these by extending trim tab horn, or weld the left tab fixed, and just use the right trim tab? Or both? Thanks
 
Have you covered the parts yet? If not, I'd sure be tempted to get out the torch and shorten them.
 
Not covered yet....id prefer if possible to use just one side and fix other side.....less work. My cessna 140 has a trim tab on right side only and it seems to work fantastic.....
 
Just for the record.....i called Bob regarding this and his input was greatly appreciated....so nice that he is available as quick as a phone call for some input. I was nor sure which route to go due to previous posts regarding elevator trim sensitivity etc....Bob stated that any of my above options are acceptable...Bob mentioned he has still a double bay trim tab flying as well as the latter. He mentioned it takes some getting used to....but does like the extra trim in case of elevator cable issue ...something to consider,,, i decided to keep both trim tabs, double bay, but i will have a longer bell horn inside fuselage....adjustable for more or less trim....
 
I disabled one of my double bay tabs and have been very glad I did. It isn't the trim sensitivity. It was the excessive servo action of the tabs that I couldn't configure out. The plane refused to fly hands off, except in the most steady air, until I changed it.
 
Kestrel....so by disabling one side you basically decreased the servo action 50 percent which you found had better trimming characteristics. Is this correct? You left operational trim tab as it was with existing servo action? Thanks and which side did you disable???
 
Correct. The primary goal was to reduce the servo effects of the trim tab. I made various compensating changes to the sensitivity of the trim. It is important to know that mine was built with electric trim so discussion of trim sensitivity doesn't cross-over.

I happen to have disabled the right side, but I don't think it matters.

When I bought mine from a prior owner, it had thinner links between the trim torque tube in the horizontal stab and the trim tab. That was a known problem that Mark helped me update with newer, thicker links.

I would seriously consider updating yours to match the new standard of smaller tabs. I'm happy with mine, but the disabled tab is ugly. If you're going to make yours look good, your going to weld something before covering.
 
I have also electric t2-10A ray allen servo....thinking after your input to weld up right trim tab and have single double bay tab on left side. I have added an extra long horn in fuselage with many holes to adjust throw of tab ...do you agree with this ? I appteciate your input
 
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Gary, if you are planning to use a Ray Allen servo to drive the pitch trim, by installing the servo in the elevators (not through the horizontal stabilizer) it will eliminate the servo action. I very nearly did this, although I’m now building it to plan and I may well alter it as above after I’ve got it flying. To do this I need to remove the current trim linkages which is easy enough. I’d then need to weld a bracket inside the elevator to mount the servo on. The main issue is the weight aft of the elevator balance line.

In theory this method should eliminate the servo action, whilst retaining full trim movement. It does put a slightly higher load through the controls (as it’s now unassisted).
 
Nev said:
Gary, if you are planning to use a Ray Allen servo to drive the pitch trim, by installing the servo in the elevators (not through the horizontal stabilizer) it will eliminate the servo action. I very nearly did this, although I’m now building it to plan and I may well alter it as above after I’ve got it flying. To do this I need to remove the current trim linkages which is easy enough. I’d then need to weld a bracket inside the elevator to mount the servo on. The main issue is the weight aft of the elevator balance line.

In theory this method should eliminate the servo action, whilst retaining full trim movement. It does put a slightly higher load through the controls (as it’s now unassisted).
Click to expand...

Nev - I’ve watched most of your videos, and with great interest! Thanks for your service to the community. Regarding this thread, will I find your final configuration on one of your vids? If not, would you kindly update us here? Did you end up making the alteration after first flights, or did you decide not to?
 
I was thinking long and hard about changing my full trim flap to a half, all said that the servo action is to much, I changed mine and am happy with how it came out. Better now than later. 5FC73782-1EE7-4E91-9DE0-2FBD67E50DB1.jpgDDD136C8-5C5D-428A-B217-D44E5201BAD7.png
 
Hi David, yes the top and bottom of the horizontal is covered with aluminium 020. The pilot side rear window frame is also aluminium
 
Nev - I’ve watched most of your videos, and with great interest! Thanks for your service to the community. Regarding this thread, will I find your final configuration on one of your vids? If not, would you kindly update us here? Did you end up making the alteration after first flights, or did you decide not to?
Click to expand...

Randy - thanks. In the end I built with the trim system stock standard. I don't think I've discussed this in my videos, although I did mention that I moved the CG center point to provide more nose up trim.

I've now got nearly 100 hours on my Bearhawk and I'm thinking of revisiting the trim to tone down the servo action although it's a double edged sword in my opinion. The underlying issue is the wide CG range, coupled with the airspeed range. The trim has to be able to provide sufficient nose up trim at the forward CG limit, at say 50kts. It also has to cope with an aft CG at 125kts. As a result, the trim becomes very sensitive at higher speeds. Part of the issue is that Bob designed an aircraft powered by an O360, and many of us have put an IO540 in it that increases the speed range, changes the thrust vector, moves the empty CG forward, and increases the full power pitch-up moment. Everything is a compromise.

I've noticed a slight "pitchy" feel at normal cruise speeds that could perhaps be improved perhaps by lengthening the trim tab arms that the rod ends bolt to. This shouldn't change the trim range, but it should dampen the servo action. Perhaps others might chime in if they have done this.
 
When I was cutting my trim tabs out I considered only doing one as an A model 4 place I had flown seemed very trim sensitive to me. I consulted with Mark and Bob. The advice was to stick to the plans as Bob had designed the aircraft so it could be flown on the trim alone if required in pitch axis. That was good enough for me! I’m not an experienced builder or designer so I’ve found sticking to the plans to be a good position to take. In my view the 4B is a very well designed aeroplane that handles magnificently, it’s well balanced on the controls, if anything a little sensitive in yaw however, as with most nuances of different aircraft types we adjust quickly to them. I’m very pleased I stuck with the standard trim configuration.

My 2 cents worth would caution deviating from the plans without referring to Mark and Bob.
 
I haven't finished my 4pl A model yet, but I lengthened the trim arms on the tabs another hole as well as the arms on the trim tab tube. This will allow me to fiddle with the amount of movement of the tabs once I get into flight testing (a year or two away....)

I have put in electric trim (Ray Allen servo) that drives the standard trim tab mechanism. The servo is mounted in the tail just below the hstab leading edge attach.

Trim Servo.jpg
 
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The discussion about flying the airplane with trim if you loose elevator is interesting. In a servoing system, the more trim you put in, the more the elevator moves, which adds more trim. If my math is right, that makes this exponential which makes the center very sensitive. I think that what drives the ability to fly with trim only is the total throw of the servos, not the sensitivity around the center.

If I'm right, then that would make lengthening the trim arm less sensitive around center, but also less total throw.

Given my mission to use the entire CG of the airplane and even occasionally fly IFR I have removed the servoing and mounted two servos directly in the elevators. I have some level of redundancy given the two servos, and the total throws are about what I have measured on other bearhawks and more than what is on my 170.
 
No doubt you can fly the airplane with the trim alone. In cruise this is what you should be doing anyways.

For landing, trim and power would make the descent ok. I'm skeptical you could successfully do much of a flare with the trim. It's too sensitive. At that point it'd probably be best to just have a rather hard landing then potentially stall it at 10ft.
 
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