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Thread: Not Enough Nose Down Trim...

  1. #1
    FlipFlop's Avatar
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    Not Enough Nose Down Trim...

    This guy needs some help!... This sounds like a scary situation!...

    Date: Tue, 18 May 2004 09:10:39 -0800
    From: Keith Imel <keithimel@alaska.net>
    To: piper-cub-builders@yahoogroups.com
    Reply-to: piper-cub-builders@yahoogroups.com
    Subject: [piper-cub-builders] trim problem
    Group Members,
    My Cub does not have enough nose down trim. My trim system is assembled correctly. I have a 1946 J3C with a O235 Lycoming with 115 horsepower. I have Flaps installed and a PA18 fuel system. Also I have PA18 tailfeathers.
    If I leave the Flaps up then I have enough trim authority. But when I pull on the Flaps I quickly run out of nose down trim. At full Flaps with a passenger it requires about 20 pounds stick push to maintain 45 MPH indicated!
    Also I have a PA18 swing mount with PA18 pressure cowl. I wonder if the thrust line is not down enough?? But at low power settings it is just as bad. I did install a spring under the rear seat pulling the stick down and took care of about half the problem. I have the option of increasing the spring tension even more. Any ideas on the wisdom of doing this? Or any ideas of other ways to solve this problem?
    Thanks Keith

  2. #2
    cubunltd's Avatar
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    Sounds to me like your trim yoke is in upside down. The "Y" of the yoke should be pointing down. If you have it installed like the illustratyed parts manual, then it is in upside down. cubsunlimited@verizon.net

  3. #3
    Crash's Avatar
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    Trim

    I bet his tail is (bent) up 2 inches from where it should be. The bottom longerons should be flat as a flitter. The more the tail post is up the less nose down trim you will have. I flew one like this once. Got out and looked at the bottom longerons, the tail was up, way up. Crash

  4. #4
    fobjob's Avatar
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    ouch

    My plane had this problem when I got it. It had been flipped over on it's back once....(no mention of this in the logs)...the wing was at zero angle of incidence, taking away a lot of pitching moment....crashing it and replacing the top deck cured it....it spun very, very, well: vigorously. Micro vortex generators restored a great deal of control to it, until I got around to wrecking it later, had to find just the right mountain....

  5. #5
    Luke_theDrifter's Avatar
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    Greetings......

    I aggree w/ Crash....most likely the fuslage is bent near the tail.

    I've owned a couple J-3's, and several PA-11's, have many hours in each. The lower tubes are apt to bend, espesially during a hard landing, and re-peated "harder than you'd like" landing over many years. Couple this with some rust from the inside that you can't see, and you got a bent tail section.

    You might try, grabbing the lower longerons and lifting on the plane. Apply around 20-30 lbs of pressure, you don't have to lift the plane, just lift enough to hold some weight. You can do this from about 3 feet in front of the handles, and then work you're back until you get to the tail. Take notice while you're doing this is anything feels "soft" or "spongy". Also take the inspection plates off the bottom of the fuselage and take a GOOD look in there with a bright flash light. Another great place to look from is through the openings on each side where you're tail moves, look down past the jack-screw w/ a bright light. You're looking for any obvious rust, paint chips, bent tubes, ect. I'd bet the tail is in worse shape then you might like, in fact after a GOOD INSPECTION you might not want to fly it anymore until its' fixed.



    The J-3/PA-11's are week to start with (in the tail section), add some weight to the baggage area, then fly it, and use it like an 18 and you'll bend it everytime, I know from experince...>laugh!@@!

    One of my favorite ol' planes was a PA-11 that was all fixed up like a Cub, just like the one you decribed. Had the -18 tail, big gear, flaps, wing tanks...that same old 0-235 that you have, the one that doesn't really develop 115hp, it more like an honest 100-108 ponies (on a good day).
    Anyway, my point is that I don't think your problem is all the mods you've done, mine enver had the trouble you decribe, until I bent the tail by driving it like a -18, Cub.

    Good Flying...>>Byron
    .

  6. #6
    cubunltd's Avatar
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    You guys are right, If the trim yoke is in correctly, then the tail section has to be bent. That plane should fly right even with the mods.

  7. #7
    Crash's Avatar
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    Tail

    PA-18's, PA-12's and PA-14's are also not as strong in the last section of the tail as they should be. The last section before the tail post is the only one that only has two tubes (lower longerons) where as the rest of the bottom sections have three tubes (including the diagonal tubes) taking up the longitudinal shock of a hard tail landing. The lower longerons in the last station will stretch over time allowing the tail to move up and out of position.

    My experience has been, that a plane with the tail bent up will cruise faster then normal, but will also "slow fly" poorly, land hot and run out of nose down trim with a load in the back.

    I modify the last station on a Cub by welding in a 5/8" .035 cross tube (about 3" long) about 9" forward of the tail post to tie in with the inner liners (doublers) of the lower longerons. Then I weld in a 5/8" .035 diagonal tube from the corner of my little cross tube to the opposite corner of the first section cross tube (diagonal) so it's like all the other stations. This gives "twist" strength as well as longitudinal strenght to this area. This helps if you drop your tail wheel into a goffer hole while you are powering around in a turn, the tail is less likely to"tweak".

    Another thing builders will do is replace the entire lower longerong with .049 instead of the stock .035 tubing. This is OK but adds more weight.

    Take care. Crash

  8. #8

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    You should also check to see if the upper longerons are installed with the bend in them that "should" be there. They should not be straight all the way back like PA-12, but should be bent downward from the trim tower to the tailpost. I have seen rebuilds where someone rebuilt the tail section and missed putting this bend in and it causes just the problem you are describing.

    Frank

  9. #9
    behindpropellers's Avatar
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    Re: Tail

    Quote Originally Posted by Crash

    I modify the last station on a Cub by welding in a 5/8" .035 cross tube (about 3" long) about 9" forward of the tail post to tie in with the inner liners (doublers) of the lower longerons. Then I weld in a 5/8" diagonal tube from the corner of my little cross tube to the first section cross tube so it is like all the other stations. This gives "twist" strength as well as longitudinal strenght to this area. This helps if you drop your tail wheel into a goffer hole while you are powering around in a turn, the tail is less likely to"tweak".

    Take care. Crash
    Crash-

    Do you have any pictures?
    While on the topic, what's the best way to align a new jack screw tower? My J-5 jackscrew tower was cut off...??


    Tim

  10. #10
    Crash's Avatar
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    Upper Longerons

    Quote Originally Posted by Frank T
    You should also check to see if the upper longerons are installed with the bend in them that "should" be there. They should not be straight all the way back like PA-12, but should be bent downward from the trim tower to the tailpost. I have seen rebuilds where someone rebuilt the tail section and missed putting this bend in and it causes just the problem you are describing.

    Frank
    On a PA-12 the longerons actually cant up 3/4" from the last cluster to the tail post (lay a straight edge on your Piper blue prints, its there, I had this arguement with Charlie Center and lost). On a PA-18 they cant down about 1 1/2" from the last cluster to the tail post. The big thing is to level the plane, hang the stabilizers with the trim system assembled and check your up and down degrees of travel and make sure it matches the factory specs. If it doesn't and the tail is not bent up (bottom longerons are flat), then it is time to cut the upper longerons loose from the tail post and adjust them up or down to get back to the trim specs.

    Take care. Crash

  11. #11
    Crash's Avatar
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    Re: Tail

    Quote Originally Posted by behindpropellers
    Quote Originally Posted by Crash

    I modify the last station on a Cub by welding in a 5/8" .035 cross tube (about 3" long) about 9" forward of the tail post to tie in with the inner liners (doublers) of the lower longerons. Then I weld in a 5/8" diagonal tube from the corner of my little cross tube to the first section cross tube so it is like all the other stations. This gives "twist" strength as well as longitudinal strenght to this area. This helps if you drop your tail wheel into a goffer hole while you are powering around in a turn, the tail is less likely to"tweak".

    Take care. Crash
    Crash-

    Do you have any pictures?
    While on the topic, what's the best way to align a new jack screw tower? My J-5 jackscrew tower was cut off...??


    Tim
    Tim, I don't have any pictures as everything is covered and flying.

    On your jack screw mount question. I would assemble the stabilizers, connector tubes, jackscrew yoke, jack screw, etc, with the tower setting in place. Turn the jack screw so the yoke is about 1/2 way up the screw. Make sure the yoke is not bound up against the front connector tube "carrier" and just out a little ways from it. Tack (small tack)weld the tower in place and operate the jack screw up and down to check for binds.
    The main thing is, the rear mount (if it is where it should be) will establish where the tower will go. Just make sure the jack screw turns freely and you have full travel up and down without anything binding up. Re-check while you finish welding things together, things can move.

    Take care. Crash

  12. #12

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    Maybe.......

    Maybe nothing is wrong? I used to own a J3 that would also run out of down trim at cruise, I had to hold a little down elevator. What speed do you see at cruise? A standard cub is designed to cruise at around 70-80mph with a 65hp engine. Using a 115 hp engine is a lot more power and speed. If you are seeing cruise speeds in excess of 90mph, I really doubt ANY J3 standard trim system would have sufficient movement to stabilize at that speed.

    Mike

  13. #13
    fobjob's Avatar
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    down trim

    To add a bit to my earlier post....my plane had zero angle of incidence on the wing, measured at the root, with the plumb bob hanging from the screw to the dimple in the rear door hinge. This meant that the pitch trim was normally positioned half way between neutral and full 'nose-down' under all loading conditions. I never had to adjust the trim. The elevator was always deflected down. The horizontal stabilizer was always producing an 'up' lift, which makes pitch stability negative and produces a savage stall, (and instant spin) and a possible stabilizer stall on a slow approach with a full load. The trim change was due to the wing angle not being high enough to be producing (nose-down) pitching moment. After bashing around with longeron/tail alignment for a year or two, I finally realized that zero incidence on the wing was NOT normal....

  14. #14
    FlipFlop's Avatar
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    Looks like you guys have Keith going in the right direction...

    "Would you please post this to the supercub.org group. Hopefully I can
    check the responses thru the link you sent me.
    I really looked at my Cub close today. First thing That I checked was if
    the tail was bent. I eyeballed down the bottom longerons and also stretched a string along them and they appear to be nice and straight Whew!!! That worry is gone. Also I did not see any rust or blistered paint. I lifted on them along their entire length and prodded and poked at them and they seem sound. Second worry gone!
    Then I checked to see if my Yoke was pointing down. It is not. It points
    up and attaches to the leading edge of the horizontal stabilizer at the top.
    This surprised me because several " experts " have told me the tail was
    assembled correctly! I then drove around our Airport and looked at 5 other Cubs. 4 were assembled the same as mine! The 5th one had the Y of the yoke facing up but was attached at the bottom of the horizontal stabilizer. I then looked at an old worn out J5 Cub and the Y on it`s yoke faced down and it attached at the bottom of the horizontal stabilizer. So I am assuming that I need to flip my yoke around to face down and roll the attach bracket around to let the yoke attach at the bottom??
    The big question is how to do that? How do I get the screw jack out of
    there? I hope like crazy it doesn`t have to come out the top! That will mean cutting the fabric out of the way. I have aerothane paint,,, not easy to patch.
    Well guys sorry for such a long post. I do really appreciate all your good
    advice. By the way I am based in Juneau Alaska and my Cub is on PK1500 floats.

    Thanks Keith"

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