Fixed Trim Tab vs Servoed Trim Tab

Enewton57

Enewton57

Member
From: Budd Davisson
Subject: fixed trim tab versus servoed trim tab

BH'ers, I just came back from flying Pat Fagan's rehabilitated Smokey Bearhawk and I just want all of you to know that his modification to the trim tab system has made what was a good airplane into one that is as nearly perfect as an airplane ever gets.

It's never been a secret that even though I've always thought the Bearhawk to be the most practical, best performing four-place homebuilt ever designed, I was never crazy about the fact that the trim tab was servoed. To clarify for any who may not recognize the term, a servo tab differs from a straight trim tab in that, when adjusted, it is not fixed in position in relation to the elevator: the trailing edge of the tab moves in relation to the elevator, when the elevator moves. The direction it moves determines whether it is "helping" the elevator, thereby making felt pressures lighter (like power steering), or "slowing" the elevator and making it feel heavier. In the case of the BH, it is making the elevator lighter. Bob will tell you that he didn't design it to servo, that's just the way the geometry of the linkage turned out. The amount of servo action is slight and something anyone can get used to quickly. However, I thought it detracted from the airplane's overall handling and gave it a vaguely "abnormal" feeling.

As designed, when in level flight and the elevator is pressured one way or the other, the trim tab servo action multiplies the effect of that pressure giving more elevator action than is expected. The result is a "pitchy" feel to the airplane. So, in cross country you're better off not touching anything or you'll slightly upset the stability. This isn't a huge deal, but in turbulence it can keep you busy. And it's not in keeping with the strong stability of the airplane on the other axis.

Pat's system uses a push/pull shielded cable to move the trim tab via the trim wheel and it stays stationary in relation to the elevator. So, right at neutral, when you move the elevator, you aren't getting the "power assist" from the servo tab. Where a BH usually feels unusually light in pitch, when compared to Cessna/Piper/Beech products, Smokey now feels just like any of the aforementioned Spam cans: it is now very normal in pitch.

I flew the airplane at the same time that long time BH pilots Kevin Deuscher (spelling?) and Scott Williamson did and we were all wildly enthusiastic about the improved handling.

On downwind coming back in to land, I said to Pat, "I now don't see why anyone would even consider building a four-place other than a Bearhawk."

I'm certain that given the reaction of three BH pilots known to him, Bob B will come out with a similar fix soon. He has already done the preliminary engineering for it so it should be a fairly easy project for him.

Anyway, just thought ya'll would like to know. Smokey is alive and very, very well!

Budd Davisson

PS Pat's airplane is so smooth and slick handling, it's hard to describe. He has about 700 hours on it and is approaching 50 since the rebuild.
 
Ignore my signature at the end of the last post. It was written by Budd Davisson and posted here at his request.
 
I would like to point out one thing about Pat's Bearhawk. Two major changes were done at the same time. The tail ribs on the Hor Stab and Vert Stab were changed to airfoil shaped ribs like on the Patrol, and also the trim system was changed to eliminate the servo action. Bob feels that to weigh the benefits of a fixed trim system - these two changes need to be evaluated individually. Which Bob intends to do. Harry Ribblet told Bob that changing the tail ribs to airfoil shaped would in essence make the tail behave like it has 20% more area. More stability with more tail area.

There are now two Bearhawks flying with the airfoil shaped tail ribs, and Pat's has the modified trim system as well. Bob is going to evaluate this mod and do some testing. Mark
 
Also, I believe that Pat did one other smal modification at the tail. I put in gap seals between the Horz. Stabilizer and elevator. Who knows which changes did what.
 
I am told that gap seals make the elevator more effective (more twitchy?). Not sure if that's true as it's not first-hand, but doubtlessly it does something. We'll be flight testing with the profiles ribs soon enough, so we'll be able to offer some comparison at that stage, although we have set -3 degrees h.stab incidence to start with as a risk mitigation to the profile change (too much or too little force from the h.stab is obviously bad), but we may also trial -4 depending on how the plane responds.
 
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Good thing I've been slow to build lately. I'd sure like to make this mod before I cover the tail. I guess I'll just have to stop construction until Bob puts out the mod :-) I built the horizontal stab as the plans show but now plan to put the wood strips on to form the airfoil stab. I have the wood strips from Avipro to make the original stab into an airfoil stab. Glenn Patterson wrote an article on how to do this in the 2011 4th quarter Bear-Tracks for anyone else that is interested or might be considering this now.
 
Would be great to get a detailed write-up of the changes in the next bear-tracks. Maybe some nice pictures/drawing to go with it.
 
I would be interested to see this change developed more, and some sketches approved by Bob. I already have some extra work to undo some changes by the original builder of my project. I would rather not make the mistake of "playing" aircraft designer. I will likely go with the airfoil tail ribs, as Bob has already approved this.
 
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I talked to Bob about this a few weeks ago. He has no plans to make a change to the trim system. He did say if I was going to fly the plane at far aft CG then I could benefit from doing what Pat F. did. I agonized over whether I should make a change but decided not to.
 
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IMHO, if you are going to fly long distances without an autopilot, you will prefer the change.

--
Bearhawk
 
Circling back on this... Does anyone know where to find documentation of Pat's push-pull cable installation? Or some photos?
 
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I'm told this is Pats install.

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I bought Dan Riffee's project, and he had one of the earlier plan sets, with two elevator trim tabs. The work he did was excellent so I was hesitant to undo anything, however, when I rigged the controls with some temporary connections, I was surprised at the amount of servo action from the trim tabs. And it was not linear, changing from positive to negative servo action as the elevator is moved. As a result, I have removed the right-hand trim tab and am in the process of copying a Citabria's trim mechanism, which uses two control cables. I expect it to work very similar to Pat's.IMG_1771.JPG
 
Anyone know what kind of cable Pat used on his trim? I've been surfing around the Aircraft Spruce web site and can't find anything that resembles it. It kind of looks like Bowden cable with good quality threaded forks on the ends ....

Thanks
 
Pat used Morris control cables... Mcmaster-Carr is much cheaper than ACS. I don't have the part numbers on this iPad...


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Mark Moyle said:
Pat used Morris control cables... Mcmaster-Carr is much cheaper than ACS. I don't have the part numbers on this iPad...


Sent from my iPad using Tapatalk
Click to expand...

Thanks, Mark.

Unfortunately, I'll probably have to get them from ACS. McMaster-Carr refuses to ship anything to Canada. Not sure why, maybe the cold ? :-)

-29C here at home this morning, -40C with the wind chill.


Sent from my SM-G920W8 using Tapatalk
 
I just got two 5 ft A-920 cables in the mail from ACS. If you fix one end from moving, the other end has 1/8" of play. That's fine for a throttle or carb heat, but I believe it's too loose for a trim tab and will invite flutter. I will be returning them.
 
Those cables will be just fine, I think. Easy for me to say, I know...

My trim tabs have plenty of free play, over 3/8s at the trailing edge of the tab from memory. Maybe a lot more. This causes no problems whatsoever, they are not inclined to flutter.

You could add friction to damp the system if you are really concerned.
 
Great job Paul!. I think this is a worthwhile mod but I don't have enough ambition to take it on. I did modify the original tabs to the smaller ones. My trim wheel is mounted on the floor next to the flap handle, seems more natural to me not to reach down for the flaps and then up for the trim wheel. Looks like you are making great progress.
 
Has anyone thought about using the old style, longer trim tab but just installing it on the right elevator instead of both elevators? That way you would only need one cable. One thing that may need to be considered is the stress that it may impose where the two elevators bolt together. May have to run this idea by Bob.
 
We have talked in the past about even using one small trim tab, but I'm not sure if anyone ever got around to asking Bob about the twisting stresses. The number of cables is the same (1) with one or two tabs, but a single tab would save a pushrod and a couple of other pieces.
 
One of the Hales Landing crew drew one up. We didn't write it up for the Beartracks because Bob wasn't a fan of that level of modification and complexity. I don't recall him having safety objections, just elegance objections, if you will.
 
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