Elevator Trim Tab Deflection Angle

[wantobe]

I am designing an experimental two seater tractor, conventional horizontal tail. What is a normal range for the elevator trim tab deflection angle?

[martymayes]

What is a normal range for the elevator trim tab deflection angle?

A cessna 172 trim tab has a total travel range of 41 degrees. 28 up, 13 down.

[rv8bldr]

A cessna 172 trim tab has a total travel range of 41 degrees. 28 up, 13 down.


You need to be careful, though. It will depend upon the size of your trim tab, the size of your elevator, and the aerodynamic authority that your elevator will have. The original Bearhawk trim tabs were enormous and needed very, very little movement to have en effect. Bob Barrows has since essentially halved the size of them and they are still very effective, and don't use nearly all of their available travel.

All that said, I'm not an aeronautical engineer, nor do I play one on TV...

[WLIU]

I will offer the suggestion that there are multiple factors that go into sizing a trim surface.

1 - You obviously want it to be large enough so that you get the effect that you want with minimum deflection. More deflection means more drag which is bad. So from that point of view, larger is better. You also want the trim to be effective enough to be able to fly the airplane using only the trim system in case you have a partial failure of the main control system. I understand that one of Sean Tucker's bail-outs was in part because his elevator trim did not have sufficient authority to fly a stable approach to landing. I know of a case where an Extra 300 pilot broke the control stick off starting a maneuver and did fly home on trim only. So you want enough, but not too much.

2 - Flutter and failure modes have to be taken into account. If a larger trim tab gets loose with age, its uncontrolled movement has a larger effect. You will feel more through the control system. And if it jams after you have set it to push the main control surface strongly one way or another, you want the airplane to be flyable. I once had a Piper Arrow jam its elevator trim full nose down. Took a lot more muscle than usual to fly the airplane. Some folks were surprised I was that strong. Food for thought.

3 - You want the trim control linkage, and the hinges, to have zero slop. Modern designs use a double linkage to the trim surface so that the play in each cancels out. This is another guard against flutter.

So against that general backdrop, my best advice is to go look at some airplanes on the flight line. Measure the size of their horizontal stabs and elevators, the size of their trim tab surfaces, and calculate the % size of the trim surface to the control surface for each airplane. A spreadsheet of those values will give you a real good idea of what works for Piper/Cessna/Mooney against the real world yardsticks of FAR 23 and their flight test departments' work.

Alternately, some texts speak to this subject but you will come up with a good answer faster by taking advantage of the work the big companies engineering teams have already put into practice. Imitation is the most sincere form of flattery.

Best of luck,

Wes
N78PS

[Bob Dingley]

I am designing an experimental two seater tractor, conventional horizontal tail. What is a normal range for the elevator trim tab deflection angle?
One way is to plagerize a cerified design that is close to what you are designing. Both Wes and RV8 have good advice. You could measure areas on the flight line then go to the Type Certificate Data Sheets (TCDS) where (among other things) all control surface deflections are published. You start on the FAA home page, scroll down the left side and there is the the link to "TCDS". For example, I found that the CE 150 trim tab is 10 deg up and 20 down. The CE 206's is +15 and -25 deg +/- 1 deg. And so on. I flew one type that had bungee cords.

That leads me to a story that a fellow commercial pilot told me. Somewhat OT, but what the heck. He previously instructed in T-34Cs at NAS Whiting near by. He sometime demonstrated a non-sylabus maneuver that permitted lower than normal landing speeds and distances. The idea was to get more up elevator authority. If you trim nose up, the trim tab goes down and that reduces the elevator area available to pitch the nose up. This also fights the problem. My buddy said that he would instead trim full nose down. Tab then goes up. That increases the elevator area that pitches the nose up and results in slower, shorter landings. He says that he would sometime do the re trim on short final. The nose wheel would touch the pavement at about walking speed.

The T-34C is basicaly a 400 HP Bonanza with Baron wheels and wings and weighs in a little over 4,000 lbs. I will accept that he could overcome full trim at slow speed, we Marines being fit and all that. But I am still suspicious of his story because after all he was a fighter pilot. As the Mythbusters say: "Don't try this at home."

Bob
(ps) I once had lots of trouble rotating a Baron when I screwed up and started TO trimmed for cruise.

[wantobe]

3 - You want the trim control linkage, and the hinges, to have zero slop. Modern designs use a double linkage to the trim surface so that the play in each cancels out. This is another guard against flutter.



Can you provide a picture showing this double linkage trick?

[wantobe]

But I am still suspicious of his story because after all he was a fighter pilot.

I do not see the logic. Are you questioning the integrity of a fighter pilot.

Nice advice. I am dashing to TCDS now.

[wantobe]

Here are some data. Positive means trailing edge up, negative means trailing edge down, unit is degree:

Cessna 150/152 +10 , -20
Cessna 172 +28 , -13
DA 40 +12, -39
DA 42 +17, -35
SR 22 +17, -10.5


My conclusion is that the range is set from flight testing results, that is why it appears to be semi-random.

Is there a place for such data on LSA?

[WLIU]

You are missing important points. The specs for deflection only make sense when combined with relative size of the control surface. That is why your numbers do not correlate.

I do not have a photo handy, but look for a picture of the Pitts S-2C elevator trim surface. The two push-pull cables are adjusted so that there is zero slop in the combined linkage.

Best of luck,

Wes
N78PS

[martymayes]

My conclusion is that the range is set from flight testing results, that is why it appears to be semi-random.

Correct, I believe that is addressed in certification regs. Full nose up trim is not supposed to stall the plane and full nose down trim is not supposed to exceed Vne.

[WLIU]

It might be appropriate to explain how the engineering process works in industry. In the case of trim surfaces, the designers of each airplane looked at history and theory, and committed their trim design to paper. That design was reviewed by senior engineers and approved as part of the entire drawing package for the new aircraft. Sheet metal gets cut only after the calculations are reviewed and drawings are created. Once the entire airplane is documented and parts built and assembled, the flight test dept gets to demonstrate that the prototype aircraft has the behavior and performance that the engineers calculated it should have. Flight test is a design verification and quality control function. The designer has calculated and committed to paper parts that are expected to result in the prototype aircraft meeting performance expectations and certification standards. If the flight test demonstrates different behavior or performance, that means that the design engineer's calculations are off and there is some head scratching as to root cause of that "delta".

So the different numbers that you have identified likely reflect the different design calculations, not tuning by the flight test department.

I offer this perspective from 40 years in the engineering world.

I used to work with a gentleman who was the chief designer at Piper for 30 years. Jim has some interesting stories about the engineering process and flight test issues.

Hope this info is helpful,

Wes
N78PS

[Ron Blum]

There is a lot more to this than meets the eye originally. Regulations require trim down to 1.4Vs (US) and 1.3 Vs (EASA). Designers try to shoot for 1.1Vs to 1.2Vs on the slow end. The high end is maximum cruise speed. Trim systems are also bounded by runaway conditions (if they're electricly driven) and failure modes (jammed and free). Sizing is very hard to estimate, even with advanced CFD, as it is based on hinge moments of the control surface and trim tab and control system friction of the primary flight controls. Hinge moments vary with the hinge line and the manner in which the surface is hinged. It is "normal" for sizes and travels to be adjusted in Flight Test.

An earlier suggestion of looking at airplanes like the one you're designing is a really good idea. In other words, if you're designing an airplane similar to a Piper cub (large hinge gaps, elevator hinged on the leading edge, ...) look at Cub-like airplanes. In this case, looking at a Cessna isn't going to put you in the right direction. Pitching moments from large flaps is another consideration. The airfoil (on the tail surfaces) is also a big driver. Look for airplanes like your design; you won't be far off. Plus, you're designing an experimental airplane; modify it if you don't like the way it feels. There are LOTS of things you can do aerodynamically.

[Richard Warner]

Aeronca Champ is 20 degrees up, 34 degrees down.

[prasmussen]

Anyone know why a Cub adjusts the whole horizontal stab rather than using a tab? Seems like it could produce less drag but then drag was never the issue for a J3.

[Ron Blum]

You're correct, adjusting the stabilizer is more efficient. It also allows the airplane to be flown to an airport if the elevator jams or one of the elevator control cables comes loose. The early Cessna 180s also had a trimmable stabilizer, as do most of the bigger airplanes/jets.

[WLIU]

The challenge with adjusting the horizontal stab for trim is that the stab can put large forces back into the trim linkage. My anecdotal understanding of the early C-180, 182, etc trim systems was that the prototype suffered from the trim setting creeping due to the forces feed back into the linkage by the movable horizontal stab. My understanding is that the "ratchet" mechanism part of the trim wheel provides just enough resistance to prevent the creep.

I will note that Pitts and Sukhoi's have resistance built into their elevator trim mechanism to resist the forces fed back by the elevator trim tab. If you allow some looseness to occur, you might pull or push 6G's and discover that the trim lever has moved to full up or down. A nasty surprise.

Best of luck,

Wes
N78PS