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Thread: Elevator/stab trim

  1. #1
    Senior Member jmodguy's Avatar
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    Default Elevator/stab trim

    Got a question for those that have experience flying both adjustable stab AND elevator trim in a Series 5 thru 7... I’m looking for a comparative analysis please!

    What were the differences in trim adjustment and “flyability”?

    I have read that the adjustable stab is a bit sensitive in that it requires quick jabs to find the right spot. Any issues like this with trim tabs?
    Jeff
    KF 5
    340KF

  2. #2
    Senior Member Dorsal's Avatar
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    Default Re: Elevator/stab trim

    The sensitivity of the adjustable stab can be mitigated by slowing the trim motor. I did this by putting a power resistor in series with the motor, others have used PWM to achieve similar results.
    Dorsal ~~^~~
    Series 7 - Tri-Gear
    912 ULS Warp Drive

  3. #3
    Senior Member Dave S's Avatar
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    Default Re: Elevator/stab trim

    Jeff,

    A couple thoughts on the issue.

    Most of the 5-7 KFs seem to have the electric jackscrew to effect trim by moving the HS. The early S7s (like ours) have the manually adjustable trim tabs on the elevator.

    Personally I like the manual trim tabs - maybe because that is what I have and use and have gotten accustomed to. I prefer manual controls over electric controls because there is one less failure mode. The thing I really like about the manual trim is the ability to go full up and full down in an instant since the control is just a friction locked lever; and, fine adjustments are no problem at all.

    On the other side of the coin, it has been explained to me by the factory personnel that the electric jackscrew is more effective since the full face of the elevator is available to round out on landing as opposed to a portion of the elevator being blanked out when the manual trim and the elevator are both in the full nose up position. Apparently the HS trim is more helpful with bushwheels and backcountry flying so a person can get the tail down further if they have the greater elevator effectiveness because the trim tabs aren't reducing the elevator area in the nose up position.

    One person that I know of took off with the electric jackscrew trim in the full nose up position and it was a pretty frightful experience for the person. But that scenario is common to other HS trim aircraft also.

    I recall that the 5s came out with the electric jackscrew HS trim; and, the manual trim tab version was used when the S7s first came out - John went back to the electric jackscrew when he got the kitfox back in production.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  4. #4
    Senior Member jiott's Avatar
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    Default Re: Elevator/stab trim

    I have taken off in my SS7 with the electric HS trim in the full nose-up position. It was not a frightful experience, just a rather hard forward push on the stick until I retrimmed. I believe the airplane could be easily controlled in flight and landed with the HS trim stuck in either Nose-up or down position. In fact, I believe this is a requirement by the FAA for certification (SLSA). Also remember that if the trim fails in full nose-up, just slow down and deploy full flaps-this will immediately give a strong nose-down pitch to counteract the stuck trim.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #5
    Senior Member Dave S's Avatar
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    Default Re: Elevator/stab trim

    Jim,

    I should have mentioned - the individual who had this experience also was set up with a continental engine in an S 5 so there were some differences with regard to HP and W/B compared to a Rotax powered craft.

    I experimented with full nose up and nose down with our manual tab trim and that was no problem to handle out of trim in either direction nor with any power setting.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  6. #6
    Senior Member jmodguy's Avatar
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    Default Re: Elevator/stab trim

    Thanks for the inputs. Guess I should be less vague about my intentions or why I'm asking about this.
    I'm running a Lyclone and need a little more weight in the tail. I can put bigger batteries in the tail area and get my W/B correct without adding dead weight (Lead). To do this I need the area taken up by the HS trim servo. I guess my question is this; Is the elevator trim tab effective? I will be installing vortex generators across the windscreen to help control the air disturbed over the windscreen/H stab in high AoA situations.
    Cheers!
    Jeff
    KF 5
    340KF

  7. #7
    Senior Member Dave S's Avatar
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    Default Re: Elevator/stab trim

    Hi Jeff,

    That helps with understanding your original question.

    Don't know if this may be helpful; however, of the KF builders I know of who did an S5 with a heavy engine, the battery was located approximately below where the fuselage handle is attached to the upper longeron - so it was ahead of where the jackscrew mechanism is located. I have not heard of anyone using a heavy engine with the manual trim tab system.

    I am hesitant to speculate that my experience concerning elevator effectiveness with the manual trim (and a lighter Rotax) on a Trigear would translate to effectiveness of the manual trim system on an aircraft with a heavier engine in the tailwheel configuration.
    Dave S
    Kitfox 7 Trigear (Flying since 2009)
    912ULS Warp Drive

    St Paul, MN

  8. #8
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    Default Re: Elevator/stab trim

    Jeff I have the Series 5 with the Continental IO-240 engine (weighs at least 250 LBS) and the trimmable stab , which I believe is a necessity for the heavier engines. I don't think the trim tab option would have near enough controlability for your situation. The problem shows itself at approach speeds with full up trim . Early on that was a problem on my plane , where I would need aft stick pressure with full up trim coming down final. This was not an acceptable characteristic for me , so I went about the fix. First , like you said , get as much useable weight aft as you can. (I didn't want dead ballast weight, that to me is a sin) so in the empennage I have my battery (odyssey,) and my ELT behind the battery. Any item that could be mounted behind the seats practically , i did. This included a holley fuel pump , A/P servos , strobe box etc. Next is the trim assist kit-when Kitfox came out with this mod , I added it and it was a game changer. Last is gap seals on the stab the elevator. While it is hard to quantify the overall results , I did notice a difference especially in the flare with enhanced elevator authority . I also have a wiring diagram for a fast /slow trim speed switch for the trim , that I could send you if you go this route. I think Kitfox sells a speed controlled version of this as well. Bruce N199CL

  9. #9
    Senior Member jmodguy's Avatar
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    Default Re: Elevator/stab trim

    Thanks all.
    My batteries are currently just in front of the trim servo. I have dual Odyssey High performance batteries stacked back there due to the EFII system. I'm also in the "dead weight is a sin" camp. That's why I'm chasing this. Bought a couple slightly larger batteries that will fix the dead weight issue and they are slightly too large... So I was digging around back there (took the elev/stab out because I ordered the larger elevator) and thought "Hmmm If the servo was gone they would fit". If I can't fit them I will keep the batteries I have and add some lead shot or a dive weight. I'll stay with the servo as installed. Good to hear the elevator gap seal is somewhat effective. Never understood why the rudder was faired but the elevator was not.
    I picked up the elevator spring kit when I was at S&R last April and it is installed. Took the spring out for now though.
    Wish it would warm up - need to get back to the BossFox and get it finished.
    Jeff
    KF 5
    340KF

  10. #10

    Default Re: Elevator/stab trim

    You can fit the battery just in front of the Trim actuator In the doodle 5-7. No need to convert to manual trim. As far as a heavy engine full up trim takeoff goes, I actually had a trim actuator failure on take off where it failed from takeoff position to full up trim. Let me tell you it is startling but completely controllable, in fact the airplane can be controlled in all trim positions it will just have heavy stick forces. I have attached the trim failure video, skip to 9:38 in the video.
    Last edited by redbowen; 02-14-2020 at 07:26 PM.
    Bryan
    Project Kitfox
    Bowen Aero LLC
    Kitfox Model 5 Lycoming O-290 D powered
    Building a Model 7 Apex powered
    Redding, CA
    http://www.youtube.com/c/ProjectKitfox

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