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Thread: Exp cub out of nose down trim....Ideas please?

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    Exp cub out of nose down trim....Ideas please?

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ID:	46999I have an experimental cub that runs out of nose down trim. I think the builder increased AOI but I'm not sure how much. Had thrustline mod on and took it off. Consistently fly over 2000lbs, 2300 gross. 1235 empty on 35s and a pod. I do a weight and balance for many flights and I'm well within the cg envelope but run out of nose down trim. 14.35 is CG on wheels and 15.33 on skis. Attached are pictures of jack screw on my cub and I believe it is set up correctly and has full travel. I have 27 turns or so on the trim handle for the entire range. I had a mechanic weld and change tail section longerons to help this issue but still not fixed. I've thought about a small trim tab on elevator to give me a little nose down push on the elevator? A buddy of mine said I need to raise leading edge of stab. Not sure how I would do this as it would hit jackscrew tower. Any help would be appreciated. I've done many heavy flights where I'm pushing the stick forward in flight and do a w/b and I'm still within cg range but ran out of trim. Skis it is much worse and Im out of trim even quicker with the CG shift. I'm never in the neutral-nose up half of the trim... If i could get 3-4 more turns nose down life would be much better. Not unsafe as control forces aren't much on final when landing but have been concerned about a tail stall with lots of pushing down of the stick? Thanks
    Last edited by adamgrenda; 02-13-2020 at 01:29 PM.

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    www.SkupTech.com mike mcs repair's Avatar
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    airfoil tail/flying tail surfaces

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    It would help to know what your baseline is now. Put the cub in level flight as you would for rigging and get the wing angle at the fuselage and you tail travel up and down.
    DENNY

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    www.SkupTech.com mike mcs repair's Avatar
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    what prop? use heavy one?

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    www.SkupTech.com mike mcs repair's Avatar
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    to raise stabilizers, you would leave jackscrew/torque tube installed and make some sort of adaptor/ maybe as simple as bolts up to new stabilizer position.... what's the strength of bolt/nut in that position??? will it bind stabilizer jackscrew? hmmm

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    www.SkupTech.com mike mcs repair's Avatar
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    or move engine forward.... (think like the lower thrust line parts, but on all... not sure that would be strong enough, since top would be out also.. sag) have custom engine mount made, slide cowl forward, extend boot cowl forward

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    Quote Originally Posted by DENNY View Post
    It would help to know what your baseline is now. Put the cub in level flight as you would for rigging and get the wing angle at the fuselage and you tail travel up and down.
    DENNY

    so get angle of bottom of the wing at the root? What do I measure on the tail?

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    Mike that is what Dave C. Recommend was to move leading edge of stab but any further up would hit jackscrew I think. Catto prop and moving engine forward seems like a nightmare. Do you know what the angle of the tail should be in respect to the wing as Denny mentioned? That def seems like the answer to me but Iím not a mechanic.....

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    skywagon8a's Avatar
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    Do a new empty weighing. With the loaded loads as you describe, at 2300 lbs the CG should be further aft than 14.35". Or is that your empty CG? Your description suggests that you are flying at a CG further aft than the 20.0" limit.
    N1PA
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    Quote Originally Posted by skywagon8a View Post
    Do a new empty weighing. With the loaded loads as you describe, at 2300 lbs the CG should be further aft than 14.35". Or is that your empty CG? Your description suggests that you are flying at a CG further aft than the 20.0" limit.
    that is empty weight cg. At 2300 lbs I can with the pod I can be around 19.0-19.5 and be needing 2-3 more turns nose down. On skis 1/2 tanks and 30 lbs in co pilot seat I cruise at 7 turns back from full nose down trim. Skis are rigged 3 degrees nose up could that be making a big difference as opposed to 0-1.5 degrees nose up rigging ?

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    If your a welder, cut the tubing off at the tailpost just aft of the rear horizontal stab mount and splice them back on lower down...then patch fabric. That will change the aoa of your whole set up.
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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Dan Gervae View Post
    If your a welder, cut the tubing off at the tailpost just aft of the rear horizontal stab mount and splice them back on lower down...then patch fabric. That will change the aoa of your whole set up.
    and you may need new tail wires then.....don't ask how i know this.....

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by adamgrenda View Post
    Mike that is what Dave C. Recommend was to move leading edge of stab but any further up would hit jackscrew I think. Catto prop and moving engine forward seems like a nightmare. Do you know what the angle of the tail should be in respect to the wing as Denny mentioned? That def seems like the answer to me but Iím not a mechanic.....
    on a NORMAL -18, firewall vertical 90.0 deg, then wing bottom surface will be 1 deg trailing edge down... and tail surfaces travel will be as indicated in the TCDS

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by adamgrenda View Post
    Mike that is what Dave C. Recommend was to move leading edge of stab but any further up would hit jackscrew I think. ...
    you can compensate for that as you make whatever adaptor

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    skywagon8a's Avatar
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    Quote Originally Posted by Dan Gervae View Post
    If your a welder, cut the tubing off at the tailpost just aft of the rear horizontal stab mount and splice them back on lower down...then patch fabric. That will change the aoa of your whole set up.
    A friend was getting flaps certified on his J-3 when the FAA had him make a "Repair". The engineer even told us where to cut. We took 1/2" out of both lower longerons ahead of the flying wires. I don't recall the exact flight issue, but that solved the problem.
    N1PA
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    Dave Calkins's Avatar
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    Quote Originally Posted by adamgrenda View Post
    ....Skis are rigged 3 degrees nose up could that be making a big difference as opposed to 0-1.5 degrees nose up rigging ?
    I bet it looks goofy and I am sure its slowing your cruise speed. why did yu not go to 1.5?

  17. #17
    Mauleguy's Avatar
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    You can gain about a 1/4" or more by taking that off the top of the yoke, not sure if that would be enough to help but it is something. Javron just recently made a few yokes this way so Jay knows what to do.
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    fobjob's Avatar
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    Is your wing angle of incidence at +1.843 deg? (At the foot rib) Sounds like it’s flat....lack of pitching moment of the wing will run your nose trim at least half way down. A stall will result in an instant and vicious spin. Seven turns minimum recovery in mine when the wing was flat....the stab HAS to be generating negative lift for positive stability.

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    Quote Originally Posted by fobjob View Post
    Is your wing angle of incidence at +1.843 deg? (At the foot rib) Sounds like itís flat....lack of pitching moment of the wing will run your nose trim at least half way down. A stall will result in an instant and vicious spin. Seven turns minimum recovery in mine when the wing was flat....the stab HAS to be generating negative lift for positive stability.

    I stall the cub often. It breaks left wing first but nothing violent Iíve flown much worse.

  20. #20
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    Quote Originally Posted by skywagon8a View Post
    A friend was getting flaps certified on his J-3 when the FAA had him make a "Repair". The engineer even told us where to cut. We took 1/2" out of both lower longerons ahead of the flying wires. I don't recall the exact flight issue, but that solved the problem.
    Looked up the 337. The 1/2" was taken out of the lower longerons 32" ahead of the tail post.
    N1PA

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    Quote Originally Posted by adamgrenda View Post
    so get angle of bottom of the wing at the root? What do I measure on the tail?
    With the airplane level, run trim so stabilizer is also level, that is your ď0Ē. Now run the stab all the way up and measure, then all the way down and measure. Compare your measurements to the stock -18 and see how close you are.


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    Basically you need more information especially if someone has already cut the tail once. Is this problem only with skis or just worse with skis? Changing the rigging on the skis should only take an hour or so. The rest may take a few days. I think I might have met you a year or two ago at lake hood fuel pumps. It is a nice looking cub.
    DENNY

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    Quote Originally Posted by DENNY View Post
    Basically you need more information especially if someone has already cut the tail once. Is this problem only with skis or just worse with skis? Changing the rigging on the skis should only take an hour or so. The rest may take a few days. I think I might have met you a year or two ago at lake hood fuel pumps. It is a nice looking cub.
    DENNY

    It's worse on skis as CG is almost an inch further aft. I've met you at Mitch's hangar a time or two (he's the one that did the cutting originally and I plan to take it back to him just seeking opinions from builders).

    I can change the rigging this weekend with some bad weather coming is should be fairly quick task. Will that help considerably with the nose up pitch? Should I stick to 1.5 degrees or go down to zero on ski check cable rigging?

    I also met you at Port Alsworth Fly in last May. Red/White/Black cub on 35s.

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    Put slats on it.

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    fobjob's Avatar
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    Again I ask: What is the wing angle of incidence?

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    Quote Originally Posted by fobjob View Post
    Again I ask: What is the wing angle of incidence?

    I can measure this weekend. Set plane to level flight and then measure at the root across the spars?

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    Quote Originally Posted by stewartb View Post
    Put slats on it.
    I've thought about it but I thought that would intensify the problem wanted to increase the lift moreso….Never flown a slatted cub so school me up..

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    fobjob's Avatar
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    Micro vg’s reduced my spin recovery from seven turns to about one third of a turn....

  29. #29

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    Slats move the aft CG limit 1" back. In my opinion they do the same for the forward limit. Maybe that's just my plane.

    Isn't a typical Supercub empty CG around 13? I can't remember.

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    fobjob's Avatar
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    Wing center should be +2 deg, but no good way to measure it, wing root rib at +1.843 deg. Leveling airplane usually uses bottom of door, but in a home built fuselage, that’s no guarantee of being level....use plumb bob from rear door screw to detent in rear door hinge...but in home built, that may not even exist....wing angle measurement usually along bottom of inboard rib, since airfoil is mostly flat, but that’s not a guarantee either....wing tips should be at - 0.5 deg.
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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by adamgrenda View Post
    I can measure this weekend. Set plane to level flight and then measure at the root across the spars?
    bottom of butt rib, but same...

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    Quote Originally Posted by fobjob View Post
    Micro vg’s reduced my spin recovery from seven turns to about one third of a turn....
    Do you start thinking about when your chute was last packed after 6 1/2 turns?
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    I would go to 1.5 on the ski and see what it does (it should help). Mitch is the guy to fix it if you do end up cutting the tail again.
    DENNY

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    Quote Originally Posted by KevinJ View Post
    Do you start thinking about when your chute was last packed after 6 1/2 turns?
    chute?

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    AkPA/18's Avatar
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    Adam,
    Fuselages can be a pain to measure for angle of incidence. Just an idea if you want to simplify it. Measure your wing angle across the bottom of the spars, then put your trim full flight nose down trim. Measure the stabilizer angle. You should have a positive number greater than the wing number. This will give you an idea of the trim relationship to your wing. I would do both wings

    Take my advice with a grain of salt. No pedigree IA or A&P
    Last edited by AkPA/18; 02-14-2020 at 09:08 PM.
    http://thrustline.com/

    Takeoffs are optional--Landings are mandatory
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    Bill Rusk's Avatar
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    Adam

    Trust me on this....AkPA/18 above knows what he is talking about. That is your answer.

    Bill
    Very Blessed.
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    Gordon Misch's Avatar
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    Measure your wing angle across the bottom of the spars, then put your trim full flight nose down trim. Measure the stabilizer angle. You should have a positive number greater than the wing number. This will give you an idea of the trim relationship to your wing. I would do both wings
    Could you please clarify "positive number greater than the wing number"? Which direction is "positive", and in reference to what? The Horiz Ref Line? or ? Thank you!
    Gordon

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  39. #39
    AkPA/18's Avatar
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    Leading edge up is positive. Stabilizer in relation to wing.
    http://thrustline.com/

    Takeoffs are optional--Landings are mandatory
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    BC12D-4-85's Avatar
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    Do a rigging survey of the airplane versus stock Piper. Info above and via TCDS. Report. If Experimental determine the difference between Piper and yours.

    Gary

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