Reducing Elevator Trim Sensitivity

[Jenny]

I'm doing some fix-ups on a Bearhawk 4-place that's been flying for a long while. One complaint about the trim system is that it's way too sensitive. I'm wondering about gearing it down at the control . Has any one come up with a solution for this, or for improving the trim system in general? This is one of the earlier Bearhawks with twisted trim cables up above. All the fabric is currently off the fuselage as I'm recovering it, so it's a great time for any modifications. Any suggestions are appreciated, thanks!

[jaredyates]

Welcome! There have been some changes made to make the trim less sensitive, such as lengthening the arms back in the tail.
14q3x.jpg
If it is an old one, the elevators may have trim tabs that span two rib bays, vs the more modern single bay tabs. Not sure if you are recovering those parts too, but if you are, it may be a good time to consider that change, along with adding airfoil shapes to the ribs of the horizontal stabilizer.

[Isilverone]

You can also extend the tab coming off the actual trim tab. Some have bolted on little extensions to create an extended arm etc. Also, my 4 place used to have a trim tab on each elevator but 1 of them has been disabled by removing the connector arm and bolting it in a fixed position to the elevator.

[zkelley2]

You need to change it's basic design from a servo tab to a non servo action. The only reason I can see that it's a servo tab is because that's easier to build.

A servo trim tab is like adding power steering. Necessary on an MD80 where to pilot's a still using muscle to move the control surfaces. Not so much on a bearhawk.

To do this, the actuating arm needs to be attached to the elevator and not the horizontal. A solution some other people have thought up that achieves this is putting ray Allen servos in the elevator and ditching the cable trim altogether.

[jaredyates]

Based on an unfortunately small data set of a few minutes of stick time in a plane without it, I find I prefer the servo version. The sensitive trim is a briefing point for new pilots but not something I've been troubled by after a few hours in the plane.

[zkelley2]

It's not just trim sensitivity that a removing the servo action would fix, it also should fix the elevator sensitivity.

[marcusofcotton]

As well detailed by Tim Babcock in the 2019 1st quarter Beartracks, servo action can also be reduced by extending the arm of the trim tab further down (point of linkage hook up to trim tab farther from trim tab). I seem to recall some drawings/detail about this from before as well, possibly from Bob.

[Mark Goldberg]

"You need to change it's basic design from a servo tab to a non servo action. The only reason I can see that it's a servo tab is because that's easier to build."

I think the above statement to be brash and uninformed. Zach - you might not understand why Bob designed the system like he did. If Bob told you this I stand corrected. But I strongly believe there was other design criteria Bob used and not just ease of construction as you see it. And - have you ever flown a BH set up like you are saying everyone needs to do? There are two Bearhawks flying like this. I suspect you have never flown either. And more than 150 flying as designed.

While you are 100% entitled to do whatever you want with your Bearhawk project - please give recommendations to others that have basis in fact and maybe real experience with a Bearhawk. Mark

[zkelley2]

I've been in two bearhawks with the normal trim system. One was with the two bays per side, one with one. Both got extremely pitch sensitive loaded towards the aft C.G. Like I've never seen a certified airplane do. The former plane wouldn't meet the longitudinal dynamic stability requirements for certification, as least with that C.G. I didn't test the newer one.
You could get used to it, certainly in VMC. But it's not a very stable platform, to the extent that I'm not sure I'd be comfortable entering IMC with an aft C.G. in that airplane. That's a problem in my opinion.
From my understanding there's really one main thing that causes that. The location of the C.G. relative to the aerodynamic center of the aircraft. Having not actually done the math, I'm guessing we're awfully close to having the C.G. at the A.C. with a fully aft loading. If that's the case, then any airplane with that configuration will go from a positive stability through neutral and into negative as you decrease(and then move aft of) the arm between the C.G. and the A.C.
The lessening stability is exacerbated by the force the servo tab induces on the elevator, causing the control forces to be lighter in a less stable condition.

As far as the easier to build comment, that's the best I could come up with for it existing. Having been around and flown a lot of airplanes, for the life of me I could not come up with a reason for it, especially with the known aft C.G. stability. Usually when you have longitudinal stability issues you put an anti-servo tab on.

I'm actually having a hard time thinking of a single light aircraft with a servo tab. Almost all of them are variable incidence horizontal stabilizers, anti-servo or just trim tabs since this isn't an issue unique to the bearhawk.

[Mark Goldberg]

I just called Bob to get his input on this question. First, the servo system (versus fixed pitch moved with Bowden cable) is actually harder to design and build. The purpose of Bob designing the trim system like this was to keep the pitch forces light like the ailerons & rudder. Most pilots prefer balanced control forces.

The ways to minimize pitch "twitchiness" using the Bob designed servo system are a few. Bob changed the size of the actual trim tabs from two rib bays down to one bay on each elevator. Some time ago.

When you hook up the trim pushrods - connect them as high as possible on the front (under the hor stabs) and as low as possible on the aft side (under the trim tab). If you reverse this you will have a worse case scenario.

Next - have your trim cables TIGHT. Sloppiness in the 1/16" trim cables makes the whole system behave poorly. Recently Bob went to see a builder who had finished his 4 place BH and was complaining about how hard it was to keep trimmed. Bob checked his trim cable tension and saw it was way too slack. So the builder tightened the cables and is now happy.

Just a few tips from Bob to help you make your BH fly as good as it can. Mark

[whee]

We got a bit off topic here; only the first two posts really answered the question Jenny was asking (“fixing” the trim sensitivity) and the pretty much provided all the information required.

We’ve hashed out elevator sensitivity in enough other threads that we don’t need to do it here since it has nothing to do with the OPs question. But since we are talking about it I’ll quickly mention that I pretty much agree with Mark G; the trim system as designed works well, will suit most and you will get used to its function quickly. For very specific reasons I installed a different trim system that eliminates the servo tabs and I really like it aside from not eliminating the trim sensitivity which is my fault because I didn’t consider it enough during the design.

[Jenny]

Thank you everybody! Very helpful! An extension either place looks like a great simple way to go.

[PFalkner]

I am going to install a gear reduction trim wheel as used on a maule

[Nev]

........ I installed a different trim system that eliminates the servo tabs and I really like it aside from not eliminating the trim sensitivity which is my fault because I didn’t consider it enough during the design.

Whee, I’m interested in what you did with your trim system. Could you elaborate with more details ? Are you happy with the results ?

[whee]

Whee, I’m interested in what you did with your trim system. Could you elaborate with more details ? Are you happy with the results ?

I installed a “Pat Fagan” system. You can find all the details here on the forum. I like it. It could use some additional tweaks but they’d be hard to accomplish on a finished plane.

If you want electric trim Schu has a sweet setup going on his plane.