Allowable Control Surface Hinge Position Error?

Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by wsimpso1, Jul 1, 2018.

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  1. Jul 1, 2018 #1

    wsimpso1

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    I am building in fiberglass, and my control surfaces have hotwired foam cores and vacuum bagged fiberglass skins, then hinges in each are imbedded with flox in high density foam blocks. After much careful fixturing and checking, I have built two ailerons and one flap with the three hinge bearings as perfect as I can tell. But one flap, well less good. The center hinge is about 1/16" (1.5mm) aft of the line from the end bearings... Bearings are sphericals press fit in the control surface hinge pieces.

    I immediately thought about removing the hinge, making a new one, and installing it. Then I told myself not to go off half cocked... There is risk in doing this, of damaging the skin and cores and fairing already completed. There is always some level of error in parts - we can not build "perfect", even if we try to be perfect. So, anybody have a tolerance for allowable misalignment between hinge pieces? How about a basis for coming up with a tolerance? And no, I will not accept the idea that everything must be as perfect as I can make it... that might be nice, but no airplane would ever fly if that were really applied...

    Now, besides the desire to make the parts perfect, we have flight loads due to aerodynamics, and I have a pretty good estimate of the maximum of these loads. I fully recognize that the aero loads will create a simple curve in the wing with g's and the flaps will sag upwards from its hinges under these same lifting forces - I sized the flap skins and gaps to prevent this deflection from making the flaps press on the lip of the upper skins. I am planning to seal the flaps with a foam rubber seal at the nose of each flap, so the actual gap can be a little big without big leakage and drag.

    Just to let you know I am trying to be realistic, I have gone back to my calcs and am working out how much load I would get in the bearings if I build the fixed hinge halves to fit with zero inherent load at zero flaps, and then deflect the flaps to max. I will share that when I am sure of my numbers. Checking tomorrow morning and comparing to the loads from all other sources too.

    So, does anyone have a set of tolerances for position errors in controls surface hinges? Any basis for the tolerances? We know that there has to be some level of tolerance, right?

    Billski
     
  2. Jul 1, 2018 #2

    Pops

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    Don't know about tolerances , but the surfaces should move full travel without any binding or side load on any hinge. I have had to shim hinges in different ways to get them true. I usually run a very small dia wire down the center of the flap/aileron bearings to get hinges lined up without any binding. The flaps and ailerons on the Bearhawk are hinged on rod ends and that makes it easy to adjust to a few thousands. Also have to get the elevators hinges lined up on the Bearhawk by adjusting the tailwires and still get the proper tension on the wires. Slow process.

    A 1/16" out of aliment is a lot and will cause a unwanted side load on the hinges and stresses on the control surfaces. That much will be felt in the stick. Needs to fixed some how, but you are the best judge of how. On the wings of the SSSC and JMR, I used piano hinges and could put aluminum shims behind the hinge to get them correct. Couple has .010 thick shims. Doesn't take much to make a large bind.
     
    Last edited: Jul 1, 2018
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  3. Jul 1, 2018 #3

    BJC

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    Without having made any calculations, I would try for better alignment. I second Pop’s comments about using wire; I like to use alignment wire.


    BJC
     
  4. Jul 1, 2018 #4

    Toobuilder

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    The main question in my mind is the distance between the three hinge points. The further the distance, the less angular deflection the surface will see. Also keep in mind this is a flap, not a primary flight control. A little bind probably won't be felt and is unlikely to show any wear in its lifetime.
     
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  5. Jul 1, 2018 #5

    Hot Wings

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    My instinct and feel is that you have no problem - unless your surfaces are very stiff.

    For a quick test (if the surfaces can be mounted at this point) is to just mount them and articulate by hand. If you feel a significant over center effect then maybe you need to do some rework. If the force needed to deform the surface to align the hinges for mounting is insignificant then you also have some idea of the additional loads imposed due to the non linear bearings.
     
  6. Jul 1, 2018 #6

    Toobuilder

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    Agree with HotWings. I once saw a C-170 under restoration that had the lower rudder hinge replaced many years prior. It was out of position to one side by a good half inch IIRC. It was so far out that the rudder itself would pop as it was articulated. Couldn't feel it with your feet, but you knew it swinging it by hand. Even then it was a very subtle feel. There was no appreciable effect on the flying qualities, nor any outward signs of structural distress.

    Yes, it was repaired during the restoration.
     
  7. Jul 1, 2018 #7

    wsimpso1

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    Hmm, no clear consensus. Folks whose opinions I trust are voting both ways. Thanks for convincing me I have to figure this one out.

    Several things are working against my jumping in right in, pulling the current hinge piece and installing a new one...

    This is the subject hinge piece:

    032.jpg

    There are more photos at the bottom of this page https://www.homebuiltairplanes.com/forums/showthread.php?t=8344&page=2

    Working against me is that removing the current one will have some risks that could require substantial rebuild or a new flap. The hinge piece is 1/4" thick by about 1-3/4" wide with roughly 2" buried in a block of high density foam, and glued in with flox. I can slide in a vibratory saw on the long sides, and use a long 1/16 drill to try to free the short sides, then, wiggle the hinge piece and attempt to break the remaining flox near the flap upper skin. The risk comes in several ways: I can cut into the flap skin; I can damage the high density foamand/or the regular foam and/or the upper skin of the flap. Might be tricky, might require a lot of work to rebuild it, and might require that I build a new flap completely...

    Then, being the uber engineer, I have done some calculations. This is a flap, not an aileron, so if I were to feel it, it would have to be through all of the other loads on the flaps and through the flap Johnson bar. The flap is 84 inches long, with hinges at 1", 42", and 83". The bearings are sphericals. The flap is long and kind of flexible, I expect that it will deform the small amounts we are talking about rather easily, so let's see if that works out...

    I have a long history of making calcs and finding them close, so I trust them here too. The first calc I did was figure out how much load we would develop if I left them alone... Assumed the wing and wing side hinge halves are fixed, all of the deflection is in the flap, that the fixed side halves fit perfect (no preloads) to the flap at flaps zero, then rotate the flap to max angle, and calculate the load to deflect the flap back to the center hinge. For a simply supported beam with mid span load, d = P*L^3/(48EI). L is 82", EI is 0.93M the easy way and 15.5M the hard way. d the easy way is 0.020", and P is about 4 pounds. d the hard way is 0.044" and P is 30 pounds. So, it will generate about 30 pounds to bend the flap when it goes to full deflection.

    Well, how much other load is generated by flight? 1 G will curve that part of the wing to about a 7500" radius. The center hinge point deviates from a straight line between the inner and outer hinges by 0.111" and would put 8 pounds extra on the pivot. 6 G takes that to 48 pounds. Max lift load on the flaps at 6 g is about 500 pounds, with half of that on the center hinge for about 250. At 1 g, this about 40 pounds.

    None of the out-of-shape loads is huge compared to what the hinges develop in flight, and the loads are small compared to limit loads. I do not expect that 1/16" position error is huge, but it might be...

    The very next construction step is to fit the flaps to the wing, and make the fixed hinge pieces. This is fortuitous - I can check if the deflection loads are offensive and/or huge relatively easily. This is my plan - I will build the pieces and try them for little effort that might end up wasted. If it fits and rotates without forces that look damaging or offensive, I continue. If it fights me the whole way, I dismount that flap, remove the hinge halves, do whatever repairs I must to fiberglass and fairing compound, and make new fixed halves for the offending hinge.

    Billski
    .
     
    Last edited: Jul 1, 2018
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  8. Jul 1, 2018 #8

    Hot Wings

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    Rather than pull the hinge block, if you decide the alignment needs to be fixed, would boring the hole for the bearing oversized and inserting an offset bushing be an option? With a small lathe both the tool and bushing wouldn't be a huge chore.
     
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  9. Jul 1, 2018 #9

    wanttobuild

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    No ANSI standard here. Is the hinge in the center?
    Really it is a question of what you can tolerate.
    But it really needs 2B right on the money
    Ben
     
  10. Jul 1, 2018 #10

    Pops

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    Yep, going to be judgement call on you. Its your build. Like you said, it would be worst if it was the aileron and could feel the bind in the stick, but not likely with the Johnson Bar Flap Handle. I trust your judgement on finding the load on the flap and deciding the best way to deal with it. We all want our builds to be as perfect as possible. That is the challenge.
     
    Last edited: Jul 1, 2018
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  11. Jul 1, 2018 #11

    wsimpso1

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    A good idea. The wife (also a problem killing machine) suggested the same thing. If I had left more material on the part, we could have done that. Sigh.

    Billski
     
  12. Jul 1, 2018 #12

    TFF

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    If you can feel it over- center you will want to fix it. you could go with a solid material bearing and a sleeve so the bolt is not the wear surface. It will take a long time for wear to matter if it's all straight. You could always put an eccentric on the other end.
     
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  13. Jul 1, 2018 #13

    Pops

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    I like that idea.

    Dan
     
  14. Jul 1, 2018 #14

    Toobuilder

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    Yep, my first thought was to bore and offset the bearing; the second was to do a much smaller bearing if there is not enough meat left. No real need to be spherical on all 3 points.

    The outer bearing bracket on my Rocket aileron was damaged and repaired prior to my ownership. Unfortunately, when reinstalled it was about .250 low. No binding as there are only two hinges, but the trim drag was unacceptable (as was the quality of the repair itself). Anyway, I used a laser to pinpoint the hole location in situ while clecko'd to the ship. Seems like it would be pretty simple to construct a custom center bushing using a similar method.
     
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  15. Jul 1, 2018 #15

    Topaz

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    Bill,

    Since your next step is to mount them anyway, mount that one first and try it. I'm with the others that are saying that if you can feel an over-center or binding, go ahead and rework it. I'll put it in a different perspective, though: If you can feel it in operation, you'll feel it forever when you're flying and it's always going to bug you, knowing that it's not right. Even if it's not a mechanical issue that affects the safety of flight, you'll still know it's wrong, and eventually you'll do the rework anyway. You're not the sort of person, in my experience of you here on HBA, that could simply blow that mis-function off and never think about it again. I know I'm not that sort of person either. Given the length of flap and the distance between the bearings, I'd lay even odds as to whether you'll be able to feel it at all but, if you can, you're going to notice it every time you operate the flaps and it's going to be a constant thorn in your side when you try to enjoy the project into which you've poured so much thought, time, and energy.

    Fixing it now means a short delay in completion of the aircraft. Fixing it later means pulling a completed airplane out of service to do the fix. IMHO, test it now and fix it now, if you can feel any forces through the mechanism that result from the misalignment.
     
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  16. Jul 2, 2018 #16

    Tiger Tim

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    I assume you’re keeping the plane when it’s done so take a good look inside yourself and see if any of the suggestions above will get you to stop thinking about that hinge. I get the impression that just sticking it on as-is may be good enough for the plane but won’t be good enough for you, how about other solutions offered up?
     
  17. Jul 2, 2018 #17

    Rockiedog2

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    I use dental floss for ticky alignment like what you're doing. Gotta rig it in the exact center of the o/b holes of course. Creativity. Did you say which way the misalignment?...up down, in out? Seems it would matter more if it's in out that's the stiffest direction, right? or is it stiff as an anvil both ways?
    I wonder how you got the 1/16"...we got tolerances in everything maybe the tolerances ganged up on you and showed up as 1/16" at the hinge? Maybe it's a 32nd at the hinge and a 32nd somewhere else? Naw, it's Billski no way. Dental floss will tell for sure if you can get it centered.
    I doubt you'll be able to feel it at the lever(and maybe not much at the TE by hand but you may be able to see the hinges try to flex. not pretty) but like already said you'll think about it everytime you pull it. I think before you can make a judgement you gotta mount it on something that exactly matches the hinges on the wing and move it around and look at it a while; not easy. If you leave it like it is you know you gonna have some amount of preload in the hinges...I'm thinking that preload as compared to the airloads is gonna be not much(they 4 feet apart) but yeah it's a different kinda load. Or is it? I can't evaluate that but you can Billski.
    The danger I see here is that the fix may be worse than what you got now. Highly possible around here. My last resort would be to cut up what you got now. I would look more at pressing the bearing out and machine a plug with an off center over size hole and keep making those til i got it right then press a bushing in it and on the 10th or so annual maybe replace it. You got lotsa chances that way Bill, w/o really messing it up.
    It won't be "perfect" but it would pass around here...you maybe gotta loosen your tolerances a little for this one.

    edit: Maybe I missed it...is the misalignment on the fitup to the already built wing? if so then much of the above may not apply.

    You could find an old uniball with slop in it and replace the new one with the old one. Just kidding. Sort of. LOL.
     
    Last edited: Jul 2, 2018
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  18. Jul 2, 2018 #18

    wsimpso1

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    IF I were to install a eccentric bushing with a sleeve that the bolt would clamp to the fixed hinge half, what materials would you use for the eccentric bushing and sleeve? Remember that the fixed and moving hinges are 6061-T6. And how much clearance do you think I would need to get it to run well? I just guessed at 5 degrees of possible angular error between the fixed and moving hinge halves and the true rotation axis of the flap, and 1/4" long through the bushing. I would need 0.022" clearance to let it run without binding, which is one-third the error I am trying to deal with already. It begins to sound like I am dealing with decimal dust here...

    Understand that this will not give me an over-center feel - I am planning to build the wing side hinges to where the flap side bearings are when the surface is correctly positioned at zero flaps. It will then have zero load as it sits on the ground and add nothing in flight while at zero flaps. I am anticipating that it will feel like it is spring loaded to return toward the zero flap position. Move the flap trough its travel (on the ground) and it will develop load as the flap will have to bend over that travel. 30 pounds times 1/16" means the moment will be about 2 inch pounds... That is what I will check. In flight, I am assuming the worst, that the loads will be additive to the flight loads, but the loads due to the error look like they will be small compared to the max in-flight loads. Which is what I will check.

    Billski
     
  19. Jul 2, 2018 #19

    wsimpso1

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    Agreed!

    Topaz, you think too highly of me, but yes, if I can feel much of anything, I will likely do what I have to.

    Agreed!

    Billski
     
  20. Jul 2, 2018 #20

    Jay Kempf

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    The 1/16" is just a strain that you are introducing in the part. 1/16" over whatever your flap hinge spacing is is probably a pretty small number that will convert to a pretty small stress in the part. This is no different than the stresses that the bending of the wing is going to introduce. Fiberglass will take a set over time and compensate is my guess. I don't think you'll notice it in the flying bird. Alternatively instead of trying to cut around your existing part with a saw why not cut it off flush then put it up on a milling machine and machine out the current part. Better control, won't wreck the surrounding material. Third, is there any other bearing you could use in that one position that is smaller OD like a single sleeve bearing or a ball bearing that would allow you to make an eccentric patch to correct the offset? One non-spherical bearing in that sort of application actually is fine. Last, what is the other side of the assembly like? Could you make a holder and bond it on the side of the current bearing holder and move the hinge half that is on the wing? Only so many ways to skin this cat.

    Just dawned on me that you could maybe do the calcs and use the next smaller bolt for that hinge and make an eccentric sleeve assuming the strength has enough margin.
     
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