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Thread: Trim Assist Kit Option

  1. #21
    Senior Member Dave S's Avatar
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    Default Re: Trim Assist Kit Option

    Jim,

    Curious which version of pitch trim you have. Do you have the manual servo tab type trim common to the early S7's or the electric jackscrew on the HS which is currently used?

    Wondering if these systems behave differently with the spring assist?

    I have the manual servo tab type trim with the spring trim assist and have been happy with it; but, haven't experimented as you have with regard to potential drag effects.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  2. #22
    Senior Member jiott's Avatar
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    Default Re: Trim Assist Kit Option

    I have the electric jackscrew on the HS.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  3. #23
    Senior Member jrevens's Avatar
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    Default Re: Trim Assist Kit Option

    Jim,

    In my opinion I think it may not be valid test. If you applied the force duplicating the spring force by feel then the possible, and very probable fact is that 1 mph is not enough to prove anything one way or the other. There are too many possible variables.
    Last edited by jrevens; 10-10-2017 at 01:47 AM.
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  4. #24
    Senior Member jiott's Avatar
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    Default Re: Trim Assist Kit Option

    You are right that accuracy is questionable. A better way would be to measure on the ground with a spring scale the actual force applied by the spring assist. Then knowing this, go fly and use that same spring scale to apply the forward stick pressure. Thinking thru the process of what is occurring with the spring assist, I think it is logical that the spring tends to add drag in a trimmed cruise configuration. The amount is the only question in my mind. Are we in agreement with that?
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #25
    Senior Member jrevens's Avatar
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    Default Re: Trim Assist Kit Option

    This is an interesting discusion... ponder this, Jim - without the spring, the elevator drops down towards the ground because it is not a balanced surface and gravity does it’s thing. In the air (as long as you are upright) there is still that gravity effect, but the air flowing over the surface causes it to come into alignment without the spring. That creates a certain amount of drag, which is eliminated or diminished if the spring is there already aligning or trying to align the surface. So, as long as the spring isn't too strong and “over-aligning” the elevator, it seems that you would usually have less drag with the spring. I believe that the opposing forces and drag created are variable, and at any particular condition there might be more, or less, or even the same amount of drag, with or without the spring. “If you like your spring, you can keep your spring.”
    John Evens
    Arvada, CO
    Kitfox SS7 N27JE
    EAA Lifetime
    Chap. 43 honorary Lifetime

  6. #26
    Senior Member Esser's Avatar
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    Default Re: Trim Assist Kit Option

    Quote Originally Posted by jiott View Post
    You are right that accuracy is questionable. A better way would be to measure on the ground with a spring scale the actual force applied by the spring assist...
    Can you remove the spring for a test flight?
    ------------------
    Josh Esser
    Flying SS7
    Rotax 914iS
    AirMaster Prop

    Edmonton, AB, CWL3

  7. #27
    Senior Member jiott's Avatar
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    Default Re: Trim Assist Kit Option

    Yes I could easily disconnect the spring, but it would be impossible to compare two separate flights with exactly the same power setting, etc. if you are looking for only 1 or two mph differences.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  8. #28
    Senior Member jiott's Avatar
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    Default Re: Trim Assist Kit Option

    I think you may be right John, as long as the spring exactly offsets the gravity effect on the elevator it may reduce drag somewhat. In my thinking I was not considering gravity. Apparently Kitfox has done a pretty good job of designing a proper spring force because with flaps up it appears to be quite close to offsetting gravity. A little hard to tell due to friction in the system. Yes, an interesting discussion, and I think we all understand this is a molehill and not a mountain.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  9. #29
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    Default Re: Trim Assist Kit Option

    Here's some food for thought.

    When you pull in flaperons with the spring, you are deflecting the elevator
    to alleviate the back pressure. This changes the camber of the tail to increase
    the downforce. Which to the pilot feels good (i.e. no back pressure), but
    is this really a good thing? I think it is an artificial good feeling.

    Now with that spring increasing the camber, and removing that back pressure
    if you then next trim out the pressure the spring is holding, you are changing
    the tail incidence back into a less likely to stall angle. So everything is good
    (in this condition).

    The downside is ... That if you don't trim, and the spring holds the pressure
    that camber is increased, and you are much closer to stalling the tail. Which I
    believe is what happened to me, when the tail likely flew into a bubble of
    turbulated air.

    So I am asserting that the spring is a bad thing, because you trick yourself
    into thinking the plane is trimmed. Which if you fly along and don't then
    trim out what the spring is holding you're on the edge of a tailplane stall.

    No matter what the tail can stall at some point, but I think that with
    increased camber and a high loading the stall point is at a lower number
    than the flatter tail at higher angle of attack.. I might be wrong, but
    that's what I believe based on my experience.

    In any case my plane flew so horribly with 22 degrees of flaperons
    since the control harmony is so bad I've never missed the changes
    anyway. I can get down scary fast using a slip anyway so it's not really
    needed.

    So my humble opinion is that the spring assist is a bad thing.

  10. #30
    Senior Member jiott's Avatar
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    Default Re: Trim Assist Kit Option

    Quote Originally Posted by n85ae View Post
    No matter what the tail can stall at some point, but I think that with
    increased camber and a high loading the stall point is at a lower number
    than the flatter tail at higher angle of attack.. I might be wrong, but
    that's what I believe based on my experience.
    Yes, the answer to this question is I believe the crux of your scary incident. If you are right, then the spring may have increased the onset of tailplane stall; however, I don't think the spring effect is nearly as much as you may think. If it really puts us all "at the edge of tailplane stall" then surely there would be many more incidents such as yours. More than likely you experienced a rare combination of flap angle and air turbulence that caused the stall with maybe a very small amount of increase from the spring. I do tend to agree with your reasoning though.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

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