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Thread: Rigging

  1. #1
    Administrator Steve Pierce's Avatar
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    Default Rigging

    Went and picked up a Clipper recently from Ft. Worth to do an annual inspection. Shortly after take-off I realized this airplane had some serious rigging issues. I held right rudder and aileron all the way to Graham and any time i let go of the controls I went in to a descending left turn. Also discovered there were no bungee spring system between the elevators and the jack screw. Spoke with the owner and we decided to rig the airplane after I got the bungee system installed, new jack screw and yoke and turned the stabilizer link assembly over the way it is supposed to be.

    Started off with Piper Service Memo No. 18 Model PA-16 Rigging Procedures. (No. 20 is for the PA20 and No. 21 is a supplement to PA-20 rigging procedures for the PA-22) These Service Memos are available from Univair in a book called "Service Aids, Bulletins, Letters and Memos for Piper PA11, 12, 14, 15, 16, 17, 18, 20 and 22". I have found this to be a very good resource for lots of information on our Short Wings. With service memo, plumb bob, 1 3/8" block and 6" scale, 30" level in hand I'm ready.

    Hang the plumb bhttp://www.shortwingpipers.org/forum/asset.php?fid=82&uid=4&d=1262718016ob from the hole in the upper door channel and raise the tail till it lines up with the dimple in the cross tube under the front seat.

    Pull a string tight across the front spar from tip to tip and check the dihedral by measuring from the top of the fuselage spar attach fitting.


    Check to make sure the dihedral is equal on both wings by placing the level on the leading edge skin over the front spar on each wing.


    Now I check the angle of incidence of each wing at the root. Sometimes these are different from previous repairs. I use a piece of square tubing between the spars to measure incidence because the butt ribs are usually not straight as can be seen in this picture.[attachment=3:vfp09ehs]100_2955 (Medium).JPG[/attachment:vfp09ehs][attachment=2:vfp09ehs]100_2962 (Medium).JPG[/attachment:vfp09ehs][attachment=1:vfp09ehs]100_2956 (Medium).JPG[/attachment:vfp09ehs]
    Then I check the washout of each wing with my 1 3/8" block at the outboard aileron bay rib. I have figured out if I can get both ribs the same using a digital level and by adjusting the rear struts, the airplane normally flies hands off on the first flight.[attachment=0:vfp09ehs]100_2961 (Medium).JPG[/attachment:vfp09ehs]
    Square up the tail feathers using the same level making the wires deflect 1/2" with a 12-15 lb. pull with a fish scale and it's done. The barrels were frozen solid to the wires on this airplane. Soaked with Aero Kroil overnight and one still broke so they got replaced. Flew hands off needing a little right rudder to keep the ball centered so I pushed the leading edge of the vertical stabilizer to the left slightly and that remedied it. Hope this helps someone and would love to know about others rigging tips and tricks as well.

    More pictures here. [https://photos.app.goo.gl/ViX93NJPfnvnTTNT8
    I have also included Piper Service Bulletin SB966 on rigging the tail brace wires on the PA18 which is the same procedure as most all rag and tube Pipers with a jackscrew trim and tail brace wires.
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    Last edited by Steve Pierce; 06-25-2019 at 04:32 PM.

  2. #2
    Lownslow's Avatar
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    Default Re: Rigging

    It's always nice seeing someone doing things "by the book". Thanks for sharing the pic's with us.

  3. #3
    Frank Green's Avatar
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    Default Re: Rigging

    Pictures are worth a thousand words someone once said. Nice job. When I reassembled the 22 it was way off, dihedral, washout, bad. Very pleased with the way it flies now except the speed. Only getting 108 indicated at 2300 but will climb like an angel. What have others found on deviating the 1 3/8 washout for speed vs. performance? I know there is a limit to maintaining stall characteristics but how far can we go and what will it do?
    -Super Stub-

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    Jinkers's Avatar
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    Default Re: Rigging

    Good information for the un-initiated. Being a low time pilot on my first airplane I have never really thought too much about having to keep a little pressure on the right rudder and right aileron... some rentals flew straight and level and others needed a little help.

    After reading your post I took my PA-22/20 out and did some testing trimming it in cruise at 2400 rpm and 105 mph indicated at 7000 ft. I flew all four points of the compass and my Pacer requires some right rudder and aileron to keep it co-ordinated and level. It's not alot but If I go hands free on the aileron and keep it co-ordinated with rudder I am in a 30 degree left bank in 20 seconds or so and starting to lose altitude. If I keep my feet off of the rudder and just keep it level I end up 1/2 to 3/4 ball off of plumb. This seems to be what you described.

    So, does this sound like an airplane in need of having it's rigging checked? Should any A&P be able to do this correctly or do I need to find an Short-Wing guru?

    Thanks!

  5. #5
    d.grimm's Avatar
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    Default Re: Rigging

    Scott,
    Yes and Yes.
    Your performance will probably improve on both ends of the spectrum also.
    Dave

  6. #6
    deandayton's Avatar
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    Default Re: Rigging

    Loading also makes a difference. I fly a 108hp Colt so I usually keep my right tank fairly empty for improved performance. With the left tank full and just me in the left seat the airplane is a little left wing heavy. My solution is to fly from the right seat when solo.

  7. #7
    Jinkers's Avatar
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    Default Re: Rigging

    Quote Originally Posted by d.grimm
    Scott,
    Yes and Yes.
    Your performance will probably improve on both ends of the spectrum also.
    Dave
    Thanks Dave, does the second "yes" mean that any A&P should be able to do this or that I need a short wing guru?

    Quote Originally Posted by deandayton
    Loading also makes a difference. I fly a 108hp Colt so I usually keep my right tank fairly empty for improved performance. With the left tank full and just me in the left seat the airplane is a little left wing heavy. My solution is to fly from the right seat when solo.
    I didn't consider this while doing my testing. I had ~12 gallons in each tank (per the dip stick) when I started and switched to the right tank as soon as I reached cruising altitude before I started testing. I weigh 225(+) and my instrument panel is probably heavier on the left side of the center line. How much of an effect would this have on the tendency to turn left?

  8. #8

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    Default Re: Rigging

    [/quote]Thanks Dave, does the second "yes" mean that any A&P should be able to do this or that I need a short wing guru?

    I didn't consider this while doing my testing. I had ~12 gallons in each tank (per the dip stick) when I started and switched to the right tank as soon as I reached cruising altitude before I started testing. I weigh 225(+) and my instrument panel is probably heavier on the left side of the center line. How much of an effect would this have on the tendency to turn left?[/quote]

    First quote: I think it means that any A&P CAN do it (or maybe "SHOULD be able to") but rigging is a real Art. Not so much you need a "ShortWing Guru" as you need a "rigging Guru".

    Secondly, all sorts of thing can add up to make a wing fly low (over and above being slightly "out of rig" anyway). You have landing/taxi lights, yeah? Four pounds (Piper says), but they are "out there" on a pretty long Arm!!! One extra coat of dope on one wing, more than the opposite wing, and you have a "significant" weight difference. If you don't think that EXACTLY where a weight is located "matters"...get trimmed, then lean forward. Flying hands off... move to the right on your seat a couple silly little millimeters and watch the airplane roll right. Its AMAZING what a "minor" weight shift does. It is "vitually unimportant" in a Light Plane, but that is EXACTLY why airliners have three axis trim. And "fuel loading"??? Jeez, fuel burning off a tank changes "wing level" ALL THE TIME. On test hops, when I rig an airplane, I balance the fuel load (plus a couple gallons on the left side, of a ShortWing), and once stabilized in still air, in cruise, I loosen the seat belt and slide over into the middle of the seats and for rudder input I use my left foot on the Pilot's left rudder pedal and my right foot on the co-pilots RIGHT rudder pedal (taking someone "with" that weighs [practically] the same as the pilot does the same thing). This may well be called "overkill" (but I'm an Engineer, too so we don't hafta go there, do we?) and unless the Mechanic is really picky enough to take BOTH ends of the strut being worked on off the airplane to tweak the rig...you are "limited" to one full thread "adjustment".

    It really ISN'T necessary to get this persnickety, because like I said...fuel load is ALWAYS changing...but then that is why Piper and other airplane Manufacturers -as well as CFIs, but many of them probably never "though about it"- mention switching tanks on a long cross country. To keep the weight in perspective concerning rig about the longitudinal axis. This takes "workload" (Legwork load?) off the Pilot. But as close to "ball in the cage" you can rig the airplane, the better it treats you as far as "fatigue factor". Flying an airplane "dog tracking" when there is no wind to crab into (and even when there IS!) costs you both speed AND fuel. Maybe that doesn't matter to most people, but they oughta do some math and see what that costs them over twenty years, or several thousand hours.

    You can get as "retentive" as you want (or, not), but if you ALWAYS have to hold in rudder to keep the ball near the center...then that's the tell that the airplane requires rigging. Flying IS "more fun" when you don't get cramps in one leg!

    I'm as "proud" of my "initial rigging skills" as anyone. But when someone says that their airplane flew "hands off" the first flight...well let's be a little "realistic". Sure, it is indeed "possible", but it is dumb luck, when it happens. There are SO MANY "variables" possible, and the airplane is SO "sensitive" to imbalance...it just isn't POSSIBLE that "one rigging solution" (even using a Pro360. or whatever SuperDigiLevel, du jour) will result in "perfect rig". In this one case it's..."close wins the cigar". "perfect" under more than ONE SINGLE LOADING SCHEDULE is not possible without a variable input trim system. But in RagBag airplanes, "close" is a real pleasure to fly.

  9. #9

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    Default Re: Rigging

    John,
    My Kitfox flew straight and level first flight. It wasn't until a few flights later when the blood returned to my knuckles and I could breath steadily that it did require rigging adjustment. Strange,eh? Larry

  10. #10
    Frank Green's Avatar
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    Default Re: Rigging

    Interesting- First flight was around the pattern a few times, too busy watching gauges to even think about rigging. Picked up father in law and flew to Frankfort, over the Thruway as much as possible. 2 equal passengers, half tank both sides, dead center ball. Flew to Cooperstown Sat. Filled left tank, me only in left seat and it never occured to me why I was 3/4 ball off, and had to do a little left leg. I found it made about 5 mph airspeed difference, 110 @ 2300 vs 105. Interesting, very interesting.
    -Super Stub-

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