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Thread: Elevator Trim Tab Deflection Angle

  1. #11

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    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

  2. #12

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    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.

  3. #13
    Richard Warner's Avatar
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    Aeronca Champ is 20 degrees up, 34 degrees down.
    Last edited by Richard Warner; 11-29-2012 at 11:20 PM.

  4. #14
    prasmussen's Avatar
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    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.
    The journey is the reward.

  5. #15

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    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.

  6. #16

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    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

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