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Thread: Elevator Bungee Question

  1. #1

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    Default Elevator Bungee Question

    I ordered univair part number U81411-002 which is what was called out for the rear bungee system (former PA-22, now bushmaster in progress). The PMA information that came with the springs states that they fit J5, PA-12, PA18, PA-20 and PA-22.

    When comparing the old springs to the new ones there is a huge difference. Old springs are almost 7 inches long. New univair parts are approximately 4 3/4 inches long. The old springs are also much stiffer taking nearly 30 pounds of pressure to stretch spring one inch. The new springs take about 17 pounds for the same amount of stretch.

    I am stumped. I can certainly make these springs work but am concerned I got the wrong part or cant find the callout for the length of the upper and lower cable assemblies. Currently my cable assemblies are the same lenght as stock which of course would cause the springs to be stretched significantly.

    I would be interested in any insight you all might have. Thanks in advance.

  2. #2
    Nathan Hiebert's Avatar
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    If a PA-22 is what you are working on, 10940-02 are the bungee springs you need. Univair's number is U10940-002 (same spring, extra letter and number). I just ordered and received 2 of them for my PA-22 and they are about 7 inches long, and very stiff! The spring you ordered is the tensioning spring for the elevator trim system.
    Last edited by Nathan Hiebert; 07-07-2011 at 09:18 AM.

  3. #3

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    That's right, that's wrong. The cables themselves are Piper p/n 40123-61 and 40123-70. The exact specifications for making these cables can be found on Piper Drawing Number (wait for it...) 40126!

    And, as answered, the correct Piper p/n for the springs individually is indeed 10940-02. The part number for these items is very easily found in the PA-22 Parts Manual, which also clearly identifies the spring you ordered (working from the parts manual, and using the Numerical Reference in the rear, which tells you what sketch number this part number is illustrated on) as being the tensioning adjustment spring for the FORWARD END of the trim cable [system]. You'll need ONE of those, anyway. You can't "do this stuff" by squinting at the little pictures on the Univair WebSite. Univair "adds" a "U" to items that they manufacture for resale with PMA Approval as direct replacement parts, and add an extra "0" to the FRONT position in the "dash number". Only PIPER can build a p/n "12345-67", but only Univair can build a p/n "U12345-067" to directly replace it. Makes the reprint that they sell of the Piper PC (Univair p/n "22PM" worth every cent!). For anyone reading this does does not already know this...Drawings such as PAC Dwg No 40123 (a BUNCHES of other ones, although NOT, for example, Dwg No 10940. Unfortunately) are available from the SWPC.

    THAT spring drawing can be "picked off the Internet. Here it is: PAC Dwg No 10940.pdfPAC Dwg No 10940.pdf

  4. #4

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    Thank you both for your responses. I have ordered the correct springs.

    For what its worth, I do have the complete PA-22 parts manual. As I sit here, I cant recall where I got the part number but it wasnt from univair as I cant make heads or tails of their web site without part numbers. Anyway, now I have one extra spring for the forward end and two correct springs on their way for the rear of this whole equation.

    I still cant quite get my head wrapped around why these things are necessary (I have read a bunch of threads) but they are going in my plane just the same. Apparently someone who is smarter than me decided they were a good idea so I am going to do it just the way it is supposed to be done.

    Thanks again for the help.

  5. #5

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    Of all the things I'd ditch when building an experimental SW, the elevator springs would be #2; right after converting to sticks and just before bolting on PA-18 tailfeathers. They are a carry-over from a time when they were trying to make aircraft as spin/stall resistant by restricting elevator authority and a few other means.
    My opinion of course. Its not like the aircraft become divergent without them so they aren't there for stability.
    -Grant

  6. #6
    Gilbert Pierce's Avatar
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    Quote Originally Posted by Grantmac View Post
    Of all the things I'd ditch when building an experimental SW, the elevator springs would be #2; right after converting to sticks and just before bolting on PA-18 tailfeathers. They are a carry-over from a time when they were trying to make aircraft as spin/stall resistant by restricting elevator authority and a few other means.
    My opinion of course. Its not like the aircraft become divergent without them so they aren't there for stability.
    -Grant
    I flew a Clipper the the bungee springs loose. I did not like the way it flew. It did not feel stable.
    Here is what Leighton Collins had to say about the Clipper bungee springs in AIR FACTS Magazine when it was introduced in 1949.

    There's an interesting feature of this ship's longitudinal stability. They had trouble with it at first because if you keep adding power to an airplane without increasing its size the propeller finally begins to have a de-stabilizing effect at high power, as in a climb. You can think of it in terms of a projection of the propeller blade in the horizontal and vertical planes, in effect, adding a fin and stabilizer surface to the nose of the airplane which would have the same effect as reducing the size of the fin and stabilizer on the tail. In addition to this effect, they also had the problem that the bungee spring they have to use to get enough up-elevator in a hands off power-glide to meet CAA trim requirements is within itself a de-stabilizer. The problem was cured by using a double-acting bungee, which, fortunately not only cured their lack of stability but actually increased the stability. If you run the overhead trim crank sitting on the ground sometimes you'll notice that the stick follows the trim and that any time you move it either fore or aft it goes against the bungee springs. The airplane can now be trimmed to climb hands off at any speed you want up to the stall and holds its trim speed quite closely, returning to it if disturbed. And, of course, its speed-keeping tendency is even better, power-off.

  7. #7

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    Quote Originally Posted by stretchedpacer View Post
    Apparently someone who is smarter than me decided they were a good idea so I am going to do it just the way it is supposed to be done.
    Phewww! I find that to be a personal mantra. The LAST question I will ask somebody is "Why ya doin' it THAT WAY?" (unless I clearly know it to be the WRONG WAY!). It irks me to no end when somebody "smarter" than Piper Engineering thinks for himself. That usually means a PROBLEM is coming. Not that every possible item couldn't maybe be improved somehow...just, Piper didn't do ANYTHING without a good reason. If they didn't feel it was "necessary", they DIDN'T PUT IT ON. When I can't understand "why Piper did something", I defer to the Engineering Department (I do it the way they did it). They got it pretty darn good the first time (and improved it along the way).

  8. #8

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    Quote Originally Posted by JohnW View Post
    Phewww! I find that to be a personal mantra. The LAST question I will ask somebody is "Why ya doin' it THAT WAY?" (unless I clearly know it to be the WRONG WAY!). It irks me to no end when somebody "smarter" than Piper Engineering thinks for himself. That usually means a PROBLEM is coming. Not that every possible item couldn't maybe be improved somehow...just, Piper didn't do ANYTHING without a good reason. If they didn't feel it was "necessary", they DIDN'T PUT IT ON. When I can't understand "why Piper did something", I defer to the Engineering Department (I do it the way they did it). They got it pretty darn good the first time (and improved it along the way).
    Very true with certified aircraft.

    But the OP is going well outside the engineering envelope building a stretched version of the aircraft. The springs were put in place to meet certain certification requirements specifically regarding backpressure to maintain glide and holding trimmed speed. Those springs were designed to do that with the specific tail moment of the certified aircraft, by stretching his he's completely changed the tail moments and its unlikely the standard springs will be appropriate.
    Depending on what size tailfeathers he's using and how far he's extended he may actually need to look more at what the longwing Pipers did to meet those requirements.

    -Grant

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    Grant; That may be true as all getout and if I were stretching a shortwing fueslage myself, I would absolutely do the math to see what the tail loadings actually ARE, but as a "rough gauge" let's remember that the same tail trim bungee spring p/ns are applicable to the Super Cubs (including the 90hp Sweet Flying "little tailed one" as well) and I really have some little doubt that the stretched "Pacer" exceeds the arms and moments of the Cubs. I suppose "all this" remains to be seen, but I am literally right in the middle of assembling an engine and can't take the time out to chase it down... b'sides, this isn't MY Experimental endeavor! Remember too that the tensions are also THE SAME "top and bottom" on the springs, so the difference in the "pull" they exert is basically nulled. The salient premise is whether they actually require substantially more -or less- "input power" from the operator to "overcome" the trim should he see the need to do so. I would think that would be "negligible" and in practice, inconsequential to the operation of the aircraft. I also see it as a hat's off to Piper's Engineering. They knew what they were doing!

    But what do I know? I still think that stretching the fuse of a shortwing will make the tail surfaces MORE effective due to their increase in arm, and that the idea (although I don't know if this is applicable in this instance) of "going with larger tail surfaces" is something of a "wrong idea". The stock shortwings sure fly nice just as Piper built them (IMHO), and they never saw the need to increase the area of the tail feathers up through the 160 hp versions! Increasing the arm should have a linear effect. Now, where I just said "wrong", I do not necessarily mean to imply "wrong dangerous"; it's just I haven't considered EVERYTHING when I apply myself to what will happen to the "nice flying characteristics of the shortwing" -one way or the other- when all the geometry "corrections" are considered. Hey, if "finding out" is what trips any particular Gentleman's trigger, more power to him, and full speed ahead! He's having the recreational and educational experience, and if and when I ever get the free time and the inclination all at the same time, I may very well relegate my time to a similar (or entirely different!) project.

  10. #10

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    Quote Originally Posted by JohnW View Post
    Grant; That may be true as all getout and if I were stretching a shortwing fueslage myself, I would absolutely do the math to see what the tail loadings actually ARE, but as a "rough gauge" let's remember that the same tail trim bungee spring p/ns are applicable to the Super Cubs (including the 90hp Sweet Flying "little tailed one" as well) and I really have some little doubt that the stretched "Pacer" exceeds the arms and moments of the Cubs. I suppose "all this" remains to be seen, but I am literally right in the middle of assembling an engine and can't take the time out to chase it down... b'sides, this isn't MY Experimental endeavor! Remember too that the tensions are also THE SAME "top and bottom" on the springs, so the difference in the "pull" they exert is basically nulled. The salient premise is whether they actually require substantially more -or less- "input power" from the operator to "overcome" the trim should he see the need to do so. I would think that would be "negligible" and in practice, inconsequential to the operation of the aircraft. I also see it as a hat's off to Piper's Engineering. They knew what they were doing!

    But what do I know? I still think that stretching the fuse of a shortwing will make the tail surfaces MORE effective due to their increase in arm, and that the idea (although I don't know if this is applicable in this instance) of "going with larger tail surfaces" is something of a "wrong idea". The stock shortwings sure fly nice just as Piper built them (IMHO), and they never saw the need to increase the area of the tail feathers up through the 160 hp versions! Increasing the arm should have a linear effect. Now, where I just said "wrong", I do not necessarily mean to imply "wrong dangerous"; it's just I haven't considered EVERYTHING when I apply myself to what will happen to the "nice flying characteristics of the shortwing" -one way or the other- when all the geometry "corrections" are considered. Hey, if "finding out" is what trips any particular Gentleman's trigger, more power to him, and full speed ahead! He's having the recreational and educational experience, and if and when I ever get the free time and the inclination all at the same time, I may very well relegate my time to a similar (or entirely different!) project.
    The tail surfaces should be more effective with the longer arm. The aircraft will also generally be more pitch stable with the longer arm. Whether those two tendencies balance out I don't have the grey matter to figure out.
    If the same springs are used in the longwings then its really a no-brainer, put them in.

    I sure wish Piper put the balanced tailfeathers on the Clipper, that is really my only complaint; its a little heavier in pitch then roll. Of course its still one of the best balanced aircraft I've ever flown and has spoiled me forever compared to the spam cans I learned in.

    -Grant

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