Please Critique this strut design

Discussion in 'General Experimental Aviation Questions' started by addicted2climbing, Apr 15, 2013.

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  1. Apr 15, 2013 #1

    addicted2climbing

    addicted2climbing

    addicted2climbing

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    Hello all,

    A group of us have been finishing the build of a Cygent SF-2A that was donated to our eaa chapter. The builder passed away before finishing it. Needless to say we found many parts that had mistakes and we have been spending a fair amount of time fixing them over the last year. We are now to the struts and the ones built by the builder have some serious issues. One is 1.5" longer than the other and it is not adjustable in any way. Also when we replaced all the wing attachment brackets and strut fittings we found that even if the struts were the correct length they still did not fit. This has led me to redesign the struts in a way that allows for some adjustment. Since the wing is swept forward the struts are swept as well which is why I incorporated a knuckle with 2 axis of motion at the fuselage attachment point. At the wing attachment point I have a high load spherical ball joint and 2 spacers to allow the bolt to align itself as needed with the wing fitting. The ball joint gives me some adjustment in 3 axis as well as I can adjust the length of the strut. The ball joint has a 3/8-24 threaded shank that is threaded into the fitting bolted in the end of the strut. My only concern is in tension are the threads strong enough to not pull out of the fitting? I can helicoil the mating thread if needed if that will help. All the bolts attaching the fittings to the strut as well as strut to wing and fuselage are 5/16 AN bolts.. The strut is a Carlson aircraft aluminum large strut and its total length installed is around 88". Without having to go the FEA route and more the TLAR route, how does this look?

    Any suggestions will be greatly appreciated.

    Marc

    Strut Assy.jpg Strut Assy - 2.jpg
     
  2. Apr 16, 2013 #2

    Dana

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    Since the rod end has fine threads, you want to use steel for the fitting it screws into; fine threads in aluminum have a nasty tendency to strip out. I'd use steel at the other end as well, for that matter. I wouldn't use heilcoils, either; a properly tapped class 3 thread is what you want.

    Dana

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  3. Apr 16, 2013 #3

    Pops

    Pops

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    I used the Carlson Struts on the aircraft that I fly. I just propped the wings up to desired dihedral and measured for the length of the struts. Also measured the angle and had 2024-T3 bar stock milled to the correct angle. With the one strut of the Cygent there is no need for an adjustable strut. I also used the geodetic wing construction on my aircraft.

    The late Ernie Carlson and I started flying RC model airplanes together with we were 15 years old. His wife Mary, still runs the business.
     
  4. Apr 16, 2013 #4

    highspeed

    highspeed

    highspeed

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    You should be able to figure out the stresses involved. Here's a link to some formulas for screw threads. I have some of this on my computer at work, but I don't have access to it right now.

    Screw thread Calculations
     
  5. Apr 16, 2013 #5

    PTAirco

    PTAirco

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    That second drawing looks familiar ;

    IMG_0096.jpg

    I used 2024 bar and 3/8" high capacity rod ends (good for something like 8,000 lbs). I did worry about using 2024 at first, but after going through a couple of different formulae to figure out the thread strength, I felt pretty confident. Especially since at 9G this strut only sees about 5,500 lbs. I was extremely careful tapping the threads, I admit. And when rigging the struts, I made sure that way more than 0.5" length of thread was engaged. 0.5" was the minimum the formula gave me to develop the full thread of the rod end and I have something like 0.8" in there.

    Of course, as with every single part on a new design, by the time you've made it, you've already thought of a better way to do it...
     
    Last edited: Apr 16, 2013
  6. Apr 16, 2013 #6

    addicted2climbing

    addicted2climbing

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    Hey PTAirco,

    Ha ha, great minds think alike... Is your strut being used on a plane with a single strut?

    Anyhow, the ball end I chose is a High capacity one with a 10,382 lb radial load rating. I can run the thread calcs to see how much the pullout force is on a 3/8-24 in 2024,7075, 6061 or possibly 400 series stainless and see what happens. I dont have the plans in front of me and can do a free body diagram to figure out what kind of force the strut would see. I would figure worst case and not consider the spar aiding in the support. Just the lever/moment on the strut with the pivot points per plans. For the weight, do I just figure in the eight of the Fuselage, gas and occupants or just for safety sake the gross weight of the plane and not care that the wings are essentially weightless since they are lifting items.

    Pops, As for my reason to use the ball joint is that due to some misalignment issues and the mounting holes for the struts not being perpendicular I figure I might need a bit of roll adjustment as my bolt may not in up parallel to the mounting flats inside the struts. I think I can get by with a hard mount at a slight angle for the swept forward strut omitting the swing knuckle and then just the ball end at the wing attachment. However, we may just try and make a fixed strut as you say and omit the ball end if we can and if I run into the roll issue, I use a new end fitting with the Ball joint.

    Best regards,

    Marc
     
  7. Apr 16, 2013 #7

    PTAirco

    PTAirco

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    Yes, it's a single strut wing. Same problem as you have - the axis of the attachment fittings at the wing and at the fuselage are not quite in the same plane. I could put a solid end at one fitting or the other and just use the rod end at one, but another point to consider is getting the strut to lie at 0 deg. incidence at cruise. That also complicates things a little; the rod ends would allow me to fine tune that position. Remember, these struts have flats on the inside and that determines the angle my fittings have to lie at. Unless I want to get into really messy drilling problems; setting up tilted tables etc. Rod ends seemed easier.
     
  8. Apr 16, 2013 #8

    Hot Wings

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    Gives one a real appreciation for the skills of the old school paper and pencil draftsman! Solidworks is for slackers like me.
     
  9. Apr 17, 2013 #9

    addicted2climbing

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    Hello PTAirco,

    YOU hit the nail on the head with what I was worried about in regards to th eorientation being forced by the internal flats on the struts and it not being exactly parallel with my bolt hole. I also wanted to use 2 ball ends as you mention to be able to pivot the strut into a 0 incidence condition, but I was afraid I may be pushing it strength wise and stability wise by using 2 ball ends. So you have 2 on each strut? That would be my preffered choice and the easiest option. One thing a fellow EAA member was worried about was that if I use 2 ball ends or even one and my pivot knuckle is that as the plane lands hard or is jarred for any reason all the swing forward momentum is transferred to the wing attach fittings on the fuselage. By having 2 ball ends the strut is essentially free to move and only the jam nuts on the ball end are holding its orientation to the wind. All other axis are non constrained. I mentioned to him that the Kitfox has a strut that pivots back when the wing is folded and the wing is only held in place with the pin and the pin does not seem to be damaged by the wing wanting to swing for and aft. I see your in Corona, do you have these struts mounted on a plane your cirrently flying down there? If so I may want to visit you and have a look or your welcome to head to whiteman if you like.

    Best regards,

    Marc
     
  10. Apr 17, 2013 #10

    Dan Thomas

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    3/8-24 thread is pretty minimal for that application. Airplanes like the Piper ragwings and Citabrias have larger threads on their adjustable struts. Half-inch or better, and that's on the adjustable rear strut, not the front, main load-carrying strut. And those rod-end threads are rolled, not cut. Piper had an AD on many of their older arplanes after several struts failed in flight due to cracking though the cut thread roots.

    We've been making some aircraft bits for mounting waterbombing tanks on large aircraft. This particular application uses several different sizes of UNJF threads as opposed to the more common UNF, or fine thread. The UNJF has a radiused thread root instead of the flat or sharp root typical of most fine threads. Much less likely to fail. The rod ends have rolled threads.

    Dan
     
  11. Apr 17, 2013 #11

    Pops

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    I still think that you are over thinking the problem and making it harder than it really is. Picture of my rear strut with the 2024 -T3 machined to the correct angle and the front strut end. My rear strut would be the same as your main strut, except you need to do the same on both ends.

    Measure and machine, you just have 2 angles to worry about. Not hard. I would rather do it this way than have to rely on aluminum threads. But, that is just me.
    Dan
     

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    Last edited: Apr 17, 2013
  12. Apr 17, 2013 #12

    BBerson

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    A two strut wing can use rigid strut end fittings because two struts are torsionally rigid and the fitting wont bend in flight.

    In this case, the one strut upper end fitting ( if rigid) could be fatigued from normal wing torsional flexing.
    I think the rod end bearing is needed, in this case.
    But I might go with a 1/2" rod end.
     
  13. Apr 17, 2013 #13

    Pops

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    This is the only time that I will be commenting on ANY of your post. The Cygent has Geodetic wing construction and has a very, very little torsional flexing. Not a factor. But I do agree on using 1/2" threads. Dan R.
     
  14. Apr 17, 2013 #14

    PTAirco

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    Machining a fitting like that is certainly a way to do it, but few of us have access to milling machines etc. I debated with myself a long time about using aluminium threads, but the numbers won out ; I just used a much higher safety factor than I would have with steel. It would take something like 13G to pull out these threads.
     
  15. Apr 17, 2013 #15

    PTAirco

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    Yes, two rod ends at each strut. They are held at the correct angle by a small clip. I suppose you could let float freely, but they might need some kind of vane at the trailing edge to make them line up effectively with the airflow.

    The airplane is currently on hold, got sidetracked by an Aeronca. But anyone is welcome to drop by my shop and talk airplanes, just send me a PM.
     
  16. Apr 17, 2013 #16

    Dana

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    If the strut is only held in orientation by the jam nuts, remember it still can pivot on the balls.

    Dan mentioned the larger thread sizes on factory planes, note that those are also steel as well.

    Re alignment as the wing shifts fore and aft, if you have enough motion to cause problems there you've got problems, period. Even with a simple pinned end there is some play (oversized hole)if you want to be able to assemble it; that play is more than enough to accommodate any reasonable motion. Kolb aircraft use a single strut with a flat tab at each end (vertical at the lower end and horizontal at the upper) and a single pin at each end, with no trouble. At each end the tab on the strut goes between two plates attached to the fuselage or wing, so the pins are in double shear.

    The only time ball joints should be used is if you're expecting significant relative motion during operation... something that should not be happening with a wing strut!

    Dana

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  17. Apr 17, 2013 #17

    PTAirco

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    I would point out that rod ends have been used on many biplane interplane struts. If you can use any other method, and you absolutely don't have to, I would use a simple single pin joint too, but I don't think there is anything inherently wrong with it in principle, as long as adequate safety factors are used.
     
  18. Apr 17, 2013 #18

    Hot Wings

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    I've been following this thread and have refrained from directly commenting so far because I don't have a string of impressive letters behind my name, but I've got to side with Pops and Dana on this one. There is nothing inherently wrong with this application but it makes me very uncomfortable. Maybe it's just my TLAR paranoia due to my seeing so many similar bolted aluminum structures "magically" turn the aluminum threads into little flat sided helicoils.

    I'm aware of one non aviation situation where an 11mm stud in an aluminum casting, with similar loads, has proven inadequate and the 12mm upgrade still fails on occasion. Fortunately the only real hazard is to the pocketbook. Granted the alloy involved isn't as "strong" as 2024 and the cyclic loading is on the order of thousands per minute but the math says it shouldn't fail.

    A couple of other observations:
    Is the jam nut going to be able to pre-load the threads enough to isolate them from cyclic flight loads?

    Regarding the OP's use of spacers at the rod end; How long are they? What is their diameter? From what material are they made? Will the bolt be able to provide enough clamping force to keep the assembly from failing by bending?

    Pop's strut end may take more planning and time/cost to build but it's far easier to analyze with certainty. Inspection is going to be easier as well. The peace of mind from this alone would be reason enough for me to use Pop's method.
     
  19. Apr 18, 2013 #19

    addicted2climbing

    addicted2climbing

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    Hello Pops,

    I have decided to try and make it non adjustable as you have it shown in your pics. At least for our first attempt. I am still worried about the missalignemnt in the bolt being off axis, but I think we can get it close. I plan to get some measurements from the plane tomorrow and then draw up a new version. Also I was aware after i typed my post that with dual ball joints I would end up with a strut that could pivot in incidence. I would then need to add a small bracket to constrain it. Anyhow, I hope I can go the fixed route and be OK.

    Marc
     
  20. Apr 18, 2013 #20

    BBerson

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    I thought the ball joint was from the original plans. But after re-reading post #1, I see that was not the case.

    So, I recommend trying to find the original strut detail design, before considering changes.
     

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