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Thread: rigging Cub wings

  1. #1
    hotrod180's Avatar
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    rigging Cub wings

    I helped a friend of mine hang & rig the wings on his SC restoration project the other day.
    Here's how he did it:
    levelled the fuselage at the wing attach fittings.
    Hung the wings.
    Re-levelled at the wing butt rib (tail needed to be lowered a bit).
    Installed front struts & adjusted for 3-1/8" dihedral.
    Installed aft struts f& adjusted for 3/8" washout at the end rib.

    I knew from experience that the TCDS calls out for plumb-bobbing down to a prick-punch mark for leveling:

    http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/ab203ab0e89895af862572090071f7cd/$FILE/1A2.pdf

    He didn't do this, nor did he check to see that the (stock gear) MLG axle was +2" (aft) of the leading edge datum.
    This page from Atlee Dodge also calls out an alternative means of levelling the airplane:

    http://www.fadodge.com/fad_pdfs/Corr...a8ztqFncHAgdU4

    Didn't do any of that either- just levelled the butt rib.
    I'm curious if the butt rib is indeed also level when the airplane is levelled per TCDS,
    neither the TCDS or the rigging procedures page from Dodge mentions that.
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    www.SkupTech.com mike mcs repair's Avatar
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    butt rib should have been be 1 degree trailing edge down, to be in level flight position, then set washout...(as I measured on a surplus new piper fuselage that had never been flown, using the plumb bob and mark in door)

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    I use a smart level. Seems quicker and more accurate.

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    Never could find the prick punch mark on mine
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    skywagon8a's Avatar
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    When all else fails, plumb the firewall between the upper and lower engine mount bolts. The firewall is straight up and down on the fuselage drawings.
    N1PA
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    hotrod180's Avatar
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    Quote Originally Posted by mike mcs repair View Post
    butt rib should have been be 1 degree trailing edge down, to be in level flight position, then set washout...(as I measured on a surplus new piper fuselage that had never been flown, using the plumb bob and mark in door)
    "Level flight position" being leveled as per TCDS: plumb-bob down to punch mark on hinge?
    So having levelled at the butt rib, his washout would be incorrect.
    It would be 1 degree less than specified.
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    hotrod180's Avatar
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    Quote Originally Posted by bob turner View Post
    I use a smart level. Seems quicker and more accurate.
    Where would you zero your smart level for rigging the washout?
    Smart levels are great indeed-- quicker & more accurate.
    However, I've heard it said that smart levels are responsible for a lot of airplanes being rigged incorrectly.

    Kinda reminds me of a guy I used to work for--
    he loved computers, and anything that was computer-generated was just naturally better than anything hand-written.
    Unfortunately he didn't seem to know about the "garbage in, garbage out" factor.
    Last edited by hotrod180; 08-10-2017 at 11:29 AM.
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    skywagon8a's Avatar
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    Quote Originally Posted by hotrod180 View Post
    Where would you zero your smart level for rigging the washout?
    On the firewall between the upper and lower engine mount bolts.
    N1PA
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    PerryB's Avatar
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    If you're you're using a smart level to set washout, airframe level isn't critical. If the outboard rib is 2.5 deg less than the butt rib, washout is correct. The biggest "problem" with a smart level is it shows you all the imperfections the plumb-bob and bubble doesn't. If used properly, a smart level gives you a better chance of getting a perfect hands-off airplane on the first try. The trick is to check the twist in a couple points down each wing and do some averaging. Even then you may need to take in a half turn on one rear strut after the first flight.
    After Monday and Tuesday, even the calendar says WTF !

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    PerryB's Avatar
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    Elaborate a bit when you have a minute. The relationship between the butt rib and the fuselage cannot be changed, so all that actually matters is the relationship between the butt and outboard aileron. That IS the definition of twist/washout. I just can't see how fuselage level has any bearing when measuring wash with a smart level. Now that having been said, checking the butt rib against fuselage level WILL tell you if the incidence is in spec.
    I CAN see how a discrepancy in the engine mount lugs (assuming that was used as the level reference) would derail all those other figures, but I'm only talking about setting wash/twist.
    After Monday and Tuesday, even the calendar says WTF !
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    Quote Originally Posted by hotrod180 View Post

    ...I knew from experience that the TCDS calls out for plumb-bobbing down to a prick-punch mark for leveling:
    This is used for leveling the airframe for weighing and subsequently computing the Weight/Balance, nothing to do with hanging the wings as PerryB points out.

    Quote Originally Posted by hotrod180 View Post

    ....He didn't do this, nor did he check to see that the (stock gear) MLG axle was +2" (aft) of the leading edge datum.
    This page from Atlee Dodge also calls out an alternative means of levelling the airplane:
    This is a fixed relationship, if it's wrong, you've got problems.

    PerryB wins

    Quote Originally Posted by PerryB

    If you're you're using a smart level to set washout, airframe level isn't critical. If the outboard rib is 2.5 deg less than the butt rib, washout is correct.....
    .....The relationship between the butt rib and the fuselage cannot be changed, so all that actually matters is the relationship between the butt and outboard aileron. That IS the definition of twist/washout.

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    www.SkupTech.com mike mcs repair's Avatar
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    Quote Originally Posted by Helmetfire View Post
    This is used for leveling the airframe for weighing and subsequently computing the Weight/Balance, nothing to do with hanging the wings as PerryB points out.
    ..
    guess you never read the PIPER SB on rigging......

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    PerryB's Avatar
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    I wasn't out to stir the pot, and wasn't interested in "winning" anything. I was asking Mike if he could point out something I'm not seeing.

    Helmetfire, with regard to the axle being 2" aft of the leading edge, that is one of the definitions of airframe level. You drape a plumb over the leading edge and lift the tail until that relationship is reached. I didn't know the actual number (I think on my 12 it was 1" or 1.5") but the proceedure is a common alternative used when the often elusive punch mark can't be found.
    Last edited by PerryB; 08-12-2017 at 06:38 AM.
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    Quote Originally Posted by PerryB View Post
    ... all that actually matters is the relationship between the butt and outboard aileron. That IS the definition of twist/washout. I just can't see how fuselage level has any bearing when measuring wash with a smart level. . ....
    Piper does call out to level the fuselage as part of rigging the dihedral and washout.
    I agree that the washout is "the relationship between the butt and outboard aileron", what I'm curious about is the actual difference.
    Maybe it's on the drawings which I don't have?
    I see in a later post that you stated that the difference between butt and end rib is 2.5 degrees.
    Is that on the drawings,
    or did you measure it after rigging the wings via the "level the fuselage" method?
    Last edited by hotrod180; 08-12-2017 at 09:54 AM.
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    Read service memo 19 and it will tell you everything you need to know about rigging a supercub and pa-12 http://www.fadodge.com/fad_pdfs/Corr...a8ztqFncHAgdU4
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    Quote Originally Posted by PerryB View Post
    I wasn't out to stir the pot, and wasn't interested in "winning" anything....
    Sorry, bad attempt at levity

    Quote Originally Posted by PerryB View Post
    ...with regard to the axle being 2" aft of the leading edge, that is one of the definitions of airframe level. You drape a plumb over the leading edge and lift the tail until that relationship is reached.
    Agreed, but this is only useful for ensuring a level airplane for weighing, correct? Seems to me that once a person is at the point of hanging wings on a cub, any measurements for dihedral and washout are done relative to a (hopefully) square airframe and there's not much you can change...at least not without major surgery. Again, sorry, a feeble attempt at funny, as you can see I'm not really good at it, especially after a few ...with regard to hanging wings, the actual distance between the leading edge and axle relative to the longitudinal axis is fixed....I'll shutup now

    [QUOTE=mike mcs repair] guess you never read the PIPER SB on rigging......
    [
    /QUOTE]

    I haven't

  17. #17

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    read the whole thing page 3 tells you the how's and whys of using a digital level

  18. #18
    www.SkupTech.com mike mcs repair's Avatar
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    Sent from my iPhone using SuperCub.Org mobile app

  19. #19
    hotrod180's Avatar
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    Quote Originally Posted by PA-22/20-160 View Post
    Read service memo 19 and it will tell you everything you need to know about rigging a supercub and pa-12 http://www.fadodge.com/fad_pdfs/Corr...a8ztqFncHAgdU4
    It does describe using a smart level, but in conjunction with levelling the fuselage.
    It calls out the washout at the end rib at 2.5 degrees but isn't clear whether that's with the airframe levelled, or vs the butt rib.
    It does call out the wing incidence of bot the PA18/19 and the PA12,
    but those are measured at the centerline of the attach bolts.
    I'm not sure if that is square to the butt rib or not.
    From the linked page:
    "e. The correct wing washout of the Piper PA-18 model series and PA-19 airplanes is 2 ˝ degrees, the same as that of the Piper PA-12, PA-12S and PA-14 airplanes. The PA-18 model series and PA-19 airplanes, however, have a wing angle of incidence of +1.843 degrees at the wing root (inboard end; i.e., the centerlines of the wing butt hinge bolts), wile the PA-12, PA-12S, and PA-14 airplanes have a wing root angle of incidence of -0.060 degree."
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    cubflier's Avatar
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    This is yet another thread full of misinformation and nonsense about using a digital level to rig cub washout. You guys crack me up.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!

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    PerryB's Avatar
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    Quote Originally Posted by hotrod180 View Post
    It does describe using a smart level, but in conjunction with levelling the fuselage.
    It calls out the washout at the end rib at 2.5 degrees but isn't clear whether that's with the airframe levelled, or vs the butt rib.
    It does call out the wing incidence of bot the PA18/19 and the PA12,
    but those are measured at the centerline of the attach bolts.
    I'm not sure if that is square to the butt rib or not.
    From the linked page:
    "e. The correct wing washout of the Piper PA-18 model series and PA-19 airplanes is 2 ˝ degrees, the same as that of the Piper PA-12, PA-12S and PA-14 airplanes. The PA-18 model series and PA-19 airplanes, however, have a wing angle of incidence of +1.843 degrees at the wing root (inboard end; i.e., the centerlines of the wing butt hinge bolts), wile the PA-12, PA-12S, and PA-14 airplanes have a wing root angle of incidence of -0.060 degree."
    The 2.5 deg. is butt rib vs. outboard aileron.
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  22. #22
    cubflier's Avatar
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    The 2.5 degree reference is between the centerline of the fuselage and outer edge of the wing tip. The difference between the butt rib and the outboard aileron rig is 2 degrees.

    Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!
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    BC12D-4-85's Avatar
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    A previous discussion. Pick your points of reference but also note Gordon Mandell's info: http://www.supercub.org/forum/showthread.php?46289

    Gary
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  24. #24
    hotrod180's Avatar
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    Quote Originally Posted by BC12D-4-85 View Post
    A previous discussion. Pick your points of reference but also note Gordon Mandell's info: http://www.supercub.org/forum/showthread.php?46289
    Gary
    What I get from that is a 2 degree difference between the butt rib and the aileron rib = correct washout.
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    hotrod180's Avatar
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    Quote Originally Posted by cubflier View Post
    This is yet another thread full of misinformation and nonsense about using a digital level to rig cub washout. You guys crack me up. Jerry
    So can you enlighten us with the correct way to use a digital level to set washout?
    Or is this impossible to do except by using a plumb bob, a whiskey stick, and a 3/8" block?
    After a lifelong career in the building trades,
    it's my opinion that anything that can done that way can also be done (usually quicker and easier) by using a laser & a smart level.
    It's just a matter of knowing how to apply them.
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    Why not just level the airplane and use the .07 degrees called out in memo 19 it takes less time to level an airplane than to type up a question as to what the appropriate number is for the butt rib and then you know the number is true as it comes from Piper and not some 14 year old internet troll. ( got to go my mom is calling)
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    BC12D-4-85's Avatar
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    Quote Originally Posted by hotrod180 View Post
    What I get from that is a 2 degree difference between the butt rib and the aileron rib = correct washout.
    I'd refer to Mandell's analysis and Post #22. As a former Cub owner of a few (2 PA-18, PA-12, and PA-11) and not an A&P I found the initial washout to be a starting point for rigging the planes. Next comes determining wing heaviness, longitudinal alignment (ball in center), desired stall behavior, and available range of elevator trim at forward and aft CG with various gear configurations. All and more are influenced by the rigging and should be explored if desired.

    Gary
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  29. #29
    cubflier's Avatar
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    Quote Originally Posted by hotrod180 View Post
    So can you enlighten us with the correct way to use a digital level to set washout?
    Or is this impossible to do except by using a plumb bob, a whiskey stick, and a 3/8" block?
    After a lifelong career in the building trades,
    it's my opinion that anything that can done that way can also be done (usually quicker and easier) by using a laser & a smart level.
    It's just a matter of knowing how to apply them.
    There is plenty of information in old threads that have discussed the subject in detail. There are some misinterpretations that are constantly offered up in this forum as facts when it comes to rigging with digital level. The most common is that the twist of a cub wing between the butt rib and the outer aileron bay rib is 2.5 degrees. Virtually all people that read the alternative rigging methods in AC 43-16 get this wrong because of how poorly the AC is written. Another issue is how to correctly measure wing incidence at the butt rib. If you take a 30 inch level and measure the incidence at the butt rib it should read around .5 degrees less than spelled out in the AC. Another common misunderstanding is "the only relationship that matters is that between the but rib and outer aileron bay rib so why bother to level the fuselage". Fundamental to rigging is the leveling of the aircraft at the fuselage horizontal reference. Rigging washout in the level position compensates for any variation in wing incidence that can occur, especially in older airframes.

    As a Supercub knowledge base I would think that by now this forum would have it right and a curious newcomer could get rigging advice that is accurate. I think that the correct interpretation of AC 43-16 would be a great starting point. This thread is a very comprehensive discussion about how that poorly written document came about and the correct way it should be read (http://www.supercub.org/forum/showth...out-from-where ). But in the end cub rigging tolerates all kinds of variation so understanding how to read AC 43-16 is optional.

    Take care - Jerry
    If it looks smooth...it might be

    If it looks rough...it is!!
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    McCarthy Matthew's Avatar
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    Hello I have a Pa-12 which had new sealed wing struts installed. The mechanic apparently put the new struts on after setting them to the exact length as the old ones. I now have a plane that flies very nose heavy. In fact I've run out of trim and have attached 8 lbs in the rear just to get it to cruise solo without back pressure on the stick. With the help of a less experienced a&p I tried to level the plane and measure the dihedral and washout as per the published pa-12 rigging procedure. The results were not confidence inspiring- with a 4 ft level the left wing was showing a 1" rise compared to a5/8" on the right. Furthermore the right washout read low- 7/8" over 4'. The overall dihedral and left washout checked ok. The plane flies pretty darn close to wings level/stick centered/ailerons flush to wings like this. I'm questioning my ability to level the plane properly as I can't find the plumb bob mark that is supposed to be in the door frame/rear seat support. Also the rear seat front support is made out of wood on this plane so the alternative leveling method didn't seem advisable. I leveled laterally using the rear seat rear cross support and longitudinally off the wood floorboards (insert derogatory comment regarding my intelligence here). Any advice about how to properly level this bad boy? Can I use a plumb bob off the engine mounting bolts on the firewall as skywagon8a suggested above for a pa-18?

    I have heard pa-12 owners with 150 hp engines and a bunch of mods say they need a lot of weight added to the rear to keep it stick/trim neutral. This is a pretty much stock-plain Jain bird with an 0-235-c1 engine. About all that isn't stock is a battery bet the engine and firewall, a skytec starter, the seats, and the tail wheel springs. This plane was a bit nose heavy when I was slowing to under 80 mph while soloing before the struts were switched out. I used to go trim all nose up, then three 360 deg turns back for me and 2 more turns for each rear px- that would get me pretty close for take off. Now I'm always trimmed nose all up even with the 8 lb weights I added (unless I have 2 rear px). I'll finally have a few days in sept to try to fix this as my crazy summer work schedule will be slowing down. Any comments on how your stock pa-12 is normally trimmed for take off and cruise? Thanks....

  31. #31
    skywagon8a's Avatar
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    A couple of comments.
    Just changing the struts with new of the same length should not effect the nose heaviness one way or another.

    When you trim your stabilizer all the way nose up (leading edge down) does the stabilizer leading edge rest on the top fuselage longeron? If not your fitting which joins the yoke may not be correct. It should be hanging below the yoke attach bolts. It can be incorrectly installed above the attach bolts which would make the plane feel nose heavy because the travel is limited.
    N1PA
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    McCarthy Matthew's Avatar
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    Thanks for the comment Skywagon. I suspect the struts ended up not being the exact same length although that was supposed to have happened.

    The hor stabilizer is resting (full down) against trim pieces that have always been in place when full nose up trim is selected. These trim pieces cover the hole exposing the jack screw. It has been verified that the jack screw is full down in this position. Not sure about the longerons but I will take another look. I'm not sure what the yoke attach bolts are. I have taken a close look at the control cables, yoke, trim cables, as well as the elevators and hor stabilizer and everything seems in order. A mechanic has verified that my elevators are traveling all the way up. No work has been done on any of the above or any of the airframe in years except for the struts- that's why I'm suspecting them. I hope to make some small adjustments to the wing struts bringing them closer to the bulletin and see if this helps.

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    skywagon8a's Avatar
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    The white tube hanging down from the yoke is in the correct position.



    This one on the yellow plane is above the yoke and is wrong. This particular PA-12 reported the same condition that you describe. When he changed it to match the upper picture the problem disappeared.

    N1PA
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  34. #34
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    From the PA-12 TCDS:
    "Leveling means: plumb bob from machine screw at door frame channels near upper rear corner of door to hole in plate near rear seat."

    The Atlee Doge rigging sheet linked to earlier in this thread doesn't give an alternate leveling means like it does for the PA18.
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  35. #35
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    Quote Originally Posted by cubflier View Post
    ....Another common misunderstanding is "the only relationship that matters is that between the but rib and outer aileron bay rib so why bother to level the fuselage". Fundamental to rigging is the leveling of the aircraft at the fuselage horizontal reference. Rigging washout in the level position compensates for any variation in wing incidence that can occur, especially in older airframes....
    My thought is that when rigging an airplane, you should try hard to get everything exactly right.
    Whether or not you can, who knows?
    My joke is that I strive for perfection but hope to at least achieve mediocrity.
    What if the butt rib angle of incidence is off even when the fuselage is correctly levelled?
    My impression is that adding washout to the wing is to insure that the butt end stalls before the outboard end,
    thus maintaining some aileron effectiveness right down into the stall.
    So IMHO getting the difference in AOA between the butt rib and the aileron rib is more important that the AOA difference between the aileron rib and the fuselage.
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  36. #36
    hotrod180's Avatar
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    Quote Originally Posted by cubflier View Post
    ....Fundamental to rigging is the leveling of the aircraft at the fuselage horizontal reference. ...
    This is a key remark.
    A buddy of mine was having trouble levelling his PA-22/20--
    the door had been modified to a seaplane type door, upper channel replaced and no hole in the new one,
    so the TCDS "leveling means" was no good.
    He tried plumbing down from the wing leading edge datum and measuring to the MLG axles,
    then checked a couple other things that were supposed to be plumb when the fuselage was level.
    The trouble was that he couldn't get any two of the three measurements to agree.

    Finally he checked the Piper drawings and discovered the "horizontal reference line" (aka "water line).
    This line runs between two points, one at the firewall and one at the tail, which he was able to level using a laser.
    Bingo!
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    McCarthy Matthew's Avatar
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    Well I did double check the trim screw and yoke- seems to be installed correctly. Thanks for the lesson though skywagon. The alternative leveling I read for the pa-12 is to use a spirit level on the floor between the main gear attach points- as my cabin floor is old plywood I'm a bit suspicious....
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    Good Matthew, that item is OK. Changing strut lengths has no effect on nose or tail heaviness whether they are correct or not. It will effect how the fuselage hangs under the wings and whether or not there is a wing heaviness. Piper was very consistent in how they made their airplanes in that they used the KISS method. Keep them as similar as possible to keep production costs down. So while I'm not certain about the -12, the -18's horizontal reference line (water line) is perpendicular to the firewall. I assume that it is the same on the -12. Even on new fuselages all of the other measuring locations can have slight differences in accuracy. Use the firewall to level it in pitch. For roll use the main cross tube above the windshield to which the wing front spars attach. This is likely to be the most accurate location. Since the airplane is 70 years old, who knows how straight the other locations may be? Even on new fuselages it is difficult to find accurate locations to measure.

    Look in my upper picture above. Notice the cable which is attached to the aft side of the yoke which passes over a pulley going aft to the up elevator attachment where it is connected through a spring. This is on an -18, I'm not certain about a -12. This cable applies up elevator pressure when the trim is "nose up" alleviating a nose heaviness feeling. mikemcs can verify if this cable should be on a -12. Carrying a few pounds in the tail is not a bad thing and is permissible for CG control.
    Last edited by skywagon8a; 08-18-2017 at 05:39 AM.
    N1PA

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