Elev push pull tube support needed or not?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by ramjetwiebe, Dec 5, 2019.

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  1. Dec 5, 2019 #1

    ramjetwiebe

    ramjetwiebe

    ramjetwiebe

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    In the just build the thing and most answers will come to you department, I finally have a fresh thought...

    I have completed 90% of the wood construction of Roger Mann's RW4,5,6,7 with about 90% to go. (I have tail feathers to complete and any fuse accoutrements beyond the elegant box.)

    I have left tail feathers til last as I am undecided on the final design. (Church Midwing or the Duster or even the Heath Parasol...). I keep changing my mind.

    Regardless of such as in that I am now carving the metal bits and ordering some more that I don't have the resources to carve (all the exotic AN stuff...) I finally have a non critical question.

    The design calls for an elevator push pull tube. As I see it, the tube is unsupported (difficult with some vertical deflection along the entire length due to elev movement) for the entire length. Is it common practice to have the tube merely suspended from the horn at one end and the control hardware at the other?

    If you study Roger's plans closely you find a side drawing of an elevator bracket (a short piece of 4 x 2 aluminum tubing) for use rather than a conventional horn which is an interesting idea. I see no advantage over the horn except it would take five minutes to carve and not an hour to fab the horn and you would save the cost of a rod end but introduce a point of wear to preflight inspect. I might just go down this road with the addition of a sleeve bushing through the tube for the bolt to pass through. If I'm lucky, the tube will whistle at certain airspeeds.... A worry about next year issue.

    Screenshot 2019-12-05 at 12.07.11.png
     
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  2. Dec 6, 2019 #2

    ScaleBirdsScott

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    I can't make much sense of how the joint works without more context/detail.
     
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  3. Dec 6, 2019 #3

    BJC

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    That is common, but whether or not you should do it depends on the length, diameter, wall thickness, material, end connections and force in the tube.


    BJC

    Edit: Vibration, usually excited by the engine/propeller, can also be a factor. One common solution is to use two (shorter, higher natural frequency) tubes.
     
    Last edited: Dec 6, 2019
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  4. Dec 6, 2019 #4

    TFF

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    First it depends on how long a tube and how thick. Mooneys have long tubes with supports. Some planes have a reverse bell crank at mid span so there is two short tubes. Tailwind W10 has long unsupported tube it while the W8 has a reverser. W10 pulls up elevator so there is no flex in normal flying. If it was aerobatic, the reverser is better. Same amount of flex each way.
     
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  5. Dec 6, 2019 #5

    Tiger Tim

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    I forget if the PT-19 has an idler or other support on the elevator push tube but I do remember it stepping up in diameter a bunch of times to be quite wide in the middle and prevent bowing that way.
     
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  6. Dec 6, 2019 #6

    ramjetwiebe

    ramjetwiebe

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    I can't see how to support the tube without introducing flexing as there will be some vertical movement with the changing position of the horn.

    Obviously the RW planes are UL or near UL so weight would be a consideration in the original design. With the light forces a long tube may suffice but I'd be interested in some intermediate support.

    If I ever design an aircraft, I'll use pull for up elevator
     
  7. Dec 6, 2019 #7

    ramjetwiebe

    ramjetwiebe

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    Hah! I never even thought about resonance induced vibration!
     
  8. Dec 6, 2019 #8

    ramjetwiebe

    ramjetwiebe

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    I had to look and think about this too for some time.

    I had to do some thinking on this too.

    Instead of a conventional horn being actuated by a rod end inserted into a solid plug at the end of the tube there is this:

    A piece of rect alum 4"x2" tube cut about 2" long is mounted to the elev - open fore and aft - in the horn's position to stand tall in the same manner as the horn.

    No plug or rod end is used. Instead the push pull tube enters the rect section and a bolt (missing from drawing) is installed through the section - across the section at the design height of horn hole - and through the push pull tube, which is inside the rectangular section.

    A two sided horn if you like.
     
  9. Dec 6, 2019 #9

    wsimpso1

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    You are describing how the thing is built for us by referencing another piece of the same airplane, which is also unknown to most of us. Look at it from the perspective of not knowing anything about the design and try describing the whole thing. Or take photos of the plans that include the details.

    As to a single push-pull tube, why not? Lots of designs do this. Others put an idler of some sort in the middle and use two shorter pushrods. And a number of designs use long slender tubes that would buckle at high control forces except that they are supported mid span with a set of rollers or a nylon bushing that keeps it from buckling. These that have support do pick a spot for this support arrangement with a minimum of non-linear movement, but they do usually have some bow in them at one or both control extremes. In that case, so what? The pushrod is known to want to buckle and get out of line there anyway, and it is supported so that it can not get badly out of line. It works.

    Let me ask you this: Has this design been around a while? How many are flying, and how many have had problems with the elevator control system? Any crashes attributable to elevator control system? Has this detail design for elevator control ever been a known issue with this airplane design? If it has been a problem, is there a known fix? If it has not been a problem, what makes you think it will be a problem in your airplane?

    If the bird in question has a good history when built to the plans, it is a good bet that the design is OK as is. If, on the other hand, you have reason to believe that the design has any serious flaws, why would you build it?

    I am a serious believer that the vast majority of us who are planning to build an airplane should:
    • Select a design with a good history, and no bad behaviours;
    • Make sure that it has no known issues unless there are also known proven fixes for the known issues;
    • Build to the plans unless there is a known issue with a known good fix, and then build to the known good fix.
    The reasons for this are multifold, but the biggest reason is that few of us are qualified airplane designers/engineers, experimental fabricators of new designs, test engineers, and test pilots. And since most of us are not qualified in all of these areas, how good a job do you expect we would do at spotting defects and figuring out how to fix them? If you are changing the design, you are taking on all of these roles, and you had better be really good at all of those roles...

    Billski
     
    Last edited: Dec 6, 2019
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  10. Dec 6, 2019 #10

    ScaleBirdsScott

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    Ok. From this finally figured out the rectangle tube was being viewed on-end and that the bolt was just invisible and not also on-end. So as in the top diagram of my sketch:

    c68c6cf8-6ed8-4b27-a148-109b88f2d17d.png
    For a second I was imagining the square tube might be bolted to the end of the round tube like on the bottom diagram.
     
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  11. Dec 6, 2019 #11

    TFF

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    Mooneys are very long, the flex is restrained but not completely stopped. They let it flex probably a half an inch in any direction. Same with the ailerons.
     
  12. Dec 6, 2019 #12

    ramjetwiebe

    ramjetwiebe

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    Bill,

    Thank you for your response.

    I apologize for not better defining my project design along with my question. I did leave that side of the Internet research open to those wishing to reply to my question, of which several people, including yourself, were very kind to do. I very much appreciate the detail your explanation provided. In future I will strive to provide more background info.

    This is, however, not my first rodeo. My question was not meant to cast shade on the design I chose to build but rather to seek enlightenment on what else may be out there, or in my case "are push pull tubes generally left unsupported?".

    Perhaps I misunderstand the intent of amateur building, but I assume - for myself at least - that the purpose is to learn by doing - hopefully with guidance and support from those who have gone before. I can buy a very good radio, computer, chair, airplane, etc. from the store but instead I choose to build so as to enhance my personal knowledge and skill set. I personally feel that the best place to ask questions or seek advice is NOT the Internet and so, until now, I have mostly not.

    Kindest regards, Robert.
     
  13. Dec 6, 2019 #13

    ramjetwiebe

    ramjetwiebe

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    Such talent!

    Your little green man has it. I had been looking at the plans for some time before I saw your check marked solution in my tired brain . Only my elevator is wood. haha. Or will be when I finally choose the shape to make it in. Either way, I'll be laminating up a small pile of 1/8" thick spruce sticks to make the shape. It will, I promise you, not come out as light as your lawn chair tubing covered in Dacron. OH! Did I just say that out loud?! (click thumbnail image for larger image.)

    Screenshot 2019-12-05 at 21.17.10.png Screenshot 2019-12-05 at 21.17.50.png

    More rounded is for Church Midwing and Heath Parasol far-off scale while right is a more modern looking "Duster." http://rogermann.org/ragwing/designs/rw7/


    There are two alternative fin and rudder combos as well. Red pill?
     
  14. Dec 6, 2019 #14

    ramjetwiebe

    ramjetwiebe

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    Is green guy devil horning?

    https://thumbs.dreamstime.com/b/woman-s-hand-giving-rock-roll-sign-devil-horns-gesture-78058521.jpg

    Rock ON!

    c.f. https://en.m.wikipedia.org/wiki/Sign_of_the_horns
     
  15. Dec 6, 2019 #15

    ScaleBirdsScott

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    Yeah, he was excited.
     
  16. Dec 6, 2019 #16

    ramjetwiebe

    ramjetwiebe

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    Well, hey! If this is the kind of stuff that he gets him excited, he's my kind of dude.

    Green guy for
    President
    of Mars!
     
  17. Dec 6, 2019 #17

    wsimpso1

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    Hmm, I did not mean to be condescending... tone is hard to transmit in this manner. I was attempting to caution someone new to us on the scope and risk involved in airplane design.

    A caution I frequently give around here is that you will usually elicit more and higher quality responses when asking higher quality questions. When you have pictures, plans excerpts, internet addresses, etc, please provide them so that we can understand your question. When we put effort into a thought out and well presented question, it does inspire more of us to both a better focused set of answers and to more effort in our responses.

    I answered your question about the practice of using a single push-pull tube. It is done and so is breaking the tube into two tubes. I also commented that some designs use a single tube with supports in the middle. I will say that they use a support on a tube that is long enough and slender enough that it would go into column buckling without the support. Look up column failure theory and Euler criteria online or in Shigley (see the design FAQ's). Designing your own on this topic will require knowledge of column failure and perhaps bench tests of mockups to make sure that your modified scheme behaves as you are hoping...

    BJC mentioned another failure mode. That is one of resonance. A simply supported beam (push-pull tubes qualify) will have a vibration mode where the center of the tube vibrates back and forth across centerline. If this vibration's natural frequency coincides with other input frequencies, the system will resonate, perhaps destructively. Usually this frequency must be put at least one-quarter octave above the highest input frequency it will see to avoid resonance in use. Bone up on vibration theory or come back here for more help on this topic. The FAQ does not have a text on vibration - perhaps we should add one....

    As to my cautioning folks to build to plans, that really does avoid a lot of problems and allows completion in far less time than when you are scheming stuff out for yourself. Completing and flying an airplane is quite an accomplishment by itself, but rolling your own design much more than doubles the time and effort required, and greatly reduces the likelihood of finishing. Once this is understood, if one chooses to become an airplane designer, well, knock yourself out! You are in good company around here, a number of us are doing just that. We will talk design approaches, methodology and education.

    The alternate scheme presented in the plans and identified by ScaleBirdsScott (good job dude) is a simplification that we normally do not exercise in primary control circuits in airplanes. Self-aligning rod end bearings are commonly used to terminate push-pull rods for a bunch of reasons. The main reasons are to allow for less then perfect alignment between the mechanisms at both ends of the push-pull rods. This is hardly a slight on the build quality, but a recognition that airplane structures are relatively soft and never perfectly built. There is always some variation from intended plans when a part is manufactured and installed - build tolerances exist and must be accommodated. Then you fly it and flight loads cause deflection of fuselage, tail surfaces, and control surface parts - whatever level of alignment was built in is not maintained. Self aligning bearings in the rod ends are commonly used to transmit the intended loads without adding bending or torsional moments to the connections that can be destructive, enlarging holes, damaging bolts or pins, fatiguing control horns, etc. There are places where simple fork or clevis ends are used and AN spec parts are available for these applications. Usually these are for short controls and low forces, where the elements are all quite overbuilt.

    As to learning by doing, yes, we do a lot of that in homebuilding of airplanes. Mostly folks are learning skills and understanding how our aero structures are put together, what good practice looks like, and seeing the character of the designer in the process. How to redesign things is not in the plans and is rarely imparted by the experience. We have to learn that elsewhere and bring that to the table. Elsewhere can be other plans sets, our observations of how someone else solved a problem, engineering education, etc. The FAQ's cite a bunch of books, articles, and posts on the pertinent subjects. Then we have the help we fellow forum members bring to the table.

    Enjoy,

    Billski
     
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  18. Dec 6, 2019 #18

    ScaleBirdsScott

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    I would say in general a chunk of rectangular tube as a control horn isn't bad for quick and dirty. But you can quickly and easily turn it into a U without loosing any strength, and profile it some with fairly common tools and thus reduce weight. But as a quick starting point to ensure some squareness and paralelleism in the horn it's actually a good idea IMO.

    But I would stick to using a common spherical rod end bearing (aka the good ol HEIM joint) at the tube end for reasons mentioned above.

    I wouldn't just trust a bolt thru some tube as a hinge, not a good bearing surface esp when not a static one. It might technically work, but for how long?

    Push tube construction is pretty well solved by the state of the art for what, 50+ years? Any major advancements in revent memory?
     
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  19. Dec 6, 2019 #19

    BoKu

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    To address the issue of how much support the push-pull tube needs, that is well-addressed by Euler long column buckling. If you know the maximum force in the push-pull tube, you can get the formulas to evaluate that off of Wikipedia. For the Young's Modulus, use 10m for aluminum or 30m for iron. Don't worry about alloy or temper; those are pretty much irrelevant. All aluminums and all iron alloys (including 4130 steel) have near enough the same Young's moduli as makes no difference.

    There's also an easier way; Stan Hall once wrote an article for Homebuilder's Hall that includes simple lookup table for tubes of various stiffnesses, (primarily iron and aluminum), ODs, and wall thicknesses. You either plug in your maximum axial force and get a go/no-go for a given tube, or plug in the tube and see what the maximum axial force is. The article is in the Soaring Magazine archive, and also in the _Collected Works of Stan Hall_ sold by the Experimental Soaring Association.

    --Bob K.
     
    Last edited: Dec 6, 2019
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