Page 1 of 2 12 LastLast
Results 1 to 10 of 16

Thread: Elevator Trim Tab Deflection Angle

  1. #1

    Join Date
    Dec 2011
    Posts
    67

    Elevator Trim Tab Deflection Angle

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

  2. #2

    Join Date
    Jul 2011
    Location
    Clarklake, MI
    Posts
    1,288
    Quote Originally Posted by wantobe View Post
    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.

  3. #3
    rv8bldr's Avatar
    Join Date
    Nov 2011
    Location
    Pakenham, Ontario
    Posts
    30
    Quote Originally Posted by martymayes View Post
    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...
    -------------------
    Mark
    EAA 367635 VAF 185
    RV-8 80965 C-GURV (Flying since Nov 2004) RV-8 Build Log
    Bearhawk #1078 Bearhawk Build Log
    The Strawbale house we built Strawbale House Build Log

  4. #4

    Join Date
    Oct 2011
    Location
    New Hampshire
    Posts
    1,038
    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

  5. #5

    Join Date
    Jul 2011
    Location
    NW FL
    Posts
    263
    Quote Originally Posted by wantobe View Post
    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.

  6. #6

    Join Date
    Dec 2011
    Posts
    67
    Quote Originally Posted by WLIU View Post

    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?

  7. #7

    Join Date
    Dec 2011
    Posts
    67
    Quote Originally Posted by Bob Dingley View Post
    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.

  8. #8

    Join Date
    Dec 2011
    Posts
    67
    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?

  9. #9

    Join Date
    Oct 2011
    Location
    New Hampshire
    Posts
    1,038
    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

  10. #10

    Join Date
    Jul 2011
    Location
    Clarklake, MI
    Posts
    1,288
    Quote Originally Posted by wantobe View Post
    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.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •