# Thread: sailplane spar joining methods

1. ## Re: sailplane spar joining methods

Thanks Norman, I see what your saying.

Now that I think about it, using no pins in the spar results in the box taking the stress whereas if overlapped and fastened, the loads are more concentrated into the spars themselves.

Hmmm.... a lot to think about here.

Mike

2. ## Re: sailplane spar joining methods

Originally Posted by George Sychrovsky
That is not true
Thanks for the link. Another page on that site shows more pictures of the spar layup.

Notice that I said "or nearly so". Ir27's drawing looked like it had too much taper to me. Structurally you don't really gain much, if anything, by tapering the root end of the spar but it does complicate the mold so if you don't have to why bother. Now bending the bottom cap would allow you to set a dihedral angle so that would be a good reason for taper but just a few degrees.

3. ## Re: sailplane spar joining methods

That picture was just conceptual. Th taper merely saves you some weight, I think, and some expensive material, perhaps.

Just for yucks, I ran the crippled FEA that goes with the CAD, and it showed the max. stress down near the base of the stub. So if the taper is excessive, it's not by much.

The FEA had some simplifying assumptions that make it less than definitive.
Originally Posted by Norman
Thanks for the link. Another page on that site shows more pictures of the spar layup.

Notice that I said "or nearly so". Ir27's drawing looked like it had too much taper to me. Structurally you don't really gain much, if anything, by tapering the root end of the spar but it does complicate the mold so if you don't have to why bother. Now bending the bottom cap would allow you to set a dihedral angle so that would be a good reason for taper but just a few degrees.

4. ## Re: sailplane spar joining methods

Here is a G109 motorglider spar stub photo.
The Schweizer 1-26 or 1-23 is worth looking at for a metal spar stub end.
BB

5. ## Re: sailplane spar joining methods

The photos and links you guys posted are interesting. Here are two photos of a glider spar and pocket I found so far. Thanks for the tip BBerson. Mike

6. ## Re: sailplane spar joining methods

That looks like a late model metal skinned Schweizer 1-26 wing photo. To get a good look, just help somebody assemble one sometime at the airport and see how it works. The Pilatus B4 metal glider has a real neat wing spar attach also.
BB

7. ## Re: sailplane spar joining methods

Thanks BB, I'm going to try and look up the Pilatus tonight and see what I come up with. There are some guys I know with gliders so I should be able to get a first hand look.

Mike

8. ## Re: sailplane spar joining methods

Pilatus B-4 wing yoint looks pretty similar as you show on your two photos above. Maybe a bit different look but the same principe.

Mitja

9. ## Re: sailplane spar joining methods

This is what I’m talking about. Contrary to your statement “They should be full depth (or nearly so) all the way to the end” they taper down almost to one third, from 6 inches to 21/4 to be exact.

10. ## Re: sailplane spar joining methods

OK, George, it can be done either way. I would still rather keep the extreme fibers as far away from each other as possible.

11. ## Re: sailplane spar joining methods

Well, I find the wing root atachment to the fuselage of Pilatus B4.

Mitja

12. ## Re: sailplane spar joining methods

Originally Posted by Norman
OK, George, it can be done either way. I would still rather keep the extreme fibers as far away from each other as possible.
You really don't need to nor, in some instances, is it even advised. If you draw yourself a free-body diagram of the loads and the resolution of same into the mounting pins, what you'll find is that the pin at the end is really only in shear. In other words, the only load on the inner-most pin is a vertical in-plane shear (no bending). As such, the material surrounding the pin must be able to take the localized vertical load and of course the resultant bearing stress caused by the pin and its bushing.

Moving outboard from that "base" mounting point you then have the vertical shear load acting further and further away from the area of interest - as such, the vertical shear load times the distance then equals a moment and it is for that moment that you're trying to increase the distance between the spar caps. For that reason can the caps can be tapered down at the end (there's no moment there).

And the maximum strength needs to be at the point of the other pin since that immediate area sees the maximum root bending moment of the wing.

You can keep the caps apart as you say, but then you also need to design in the proper shear mechanism into the web since that too is a potential failure source.

There's a few design variables to balance here so the end configuration will depend on a few specifics of the actual layout. For instance, if you have a very high bending moment it is unlikely that you want to put a kink in the spar cap that's usually under compression. But in an aerobatic application it may be beneficial to avoid the kink in both caps. But if you have a kink, then you also need to stabilize the cap there for the resultant out of plane load. This means more external structure capable of handling the proper amount of shear.

Choices, choices.....

13. ## Re: sailplane spar joining methods

So the important thing to remember here is there must be a complete understanding of all the wing reactions. The design of this sailplane wing root is similar to the ASW 28. Notice the steel pin at the nose of the root rib? There is also a similar pin near the trailing edge. This wing joint is designed to react wing bending internally by pinning both spars together. This joint is made independant of the fuselage, in other words there is not a common bulkhead that this pin would pass through. The other wing load reactions that need to be considered are Vert and Fore/Aft shear, drag and accelerated stall condition. All of these latter conditions are reacted through the two wing root pins. Each pin has a receptor socket in the fuselage root rib, these sockets each have a compression tube that attaches to its opposite mate on the other side of the fuselage. All of the sockets are capable of reacting vert and fwd/aft shear. The drag and the accelerated lift loads are reacted as a couple between these two sockets. Look at the zero g dive condition, there is a drag load that is trying to bent the wing aft. This load is reacted by placing the rear sockets in compression. The aft shear, is again reacted by both pins and sockets. The much heaveir load that needs to be accounted for and reacted is due to the acceperated stall condition. During a sharp pull up at V Max, the wing lift vector changes direction and a componant of this vector is reacted by the wing trying to bend FWD, this load and shear again is reacted by placing the FWD pin in compression and again, both FWD and AFT pins react the shear. When you add self connecting controls, the brilliance of this design is really apparent. I really like this design because it only requires two pins to connect both wings. I am in the process of redesigning my Robin Robin Ultralight wing to this configuration. I am using a blade/fork design rather than two tapered spars. The redesign also allows me to correct an oversight on the prototype wing and that is the lack of a wing walk. I have to reinforce the root ribs to the point where a pilot walking on them is not a design load condition

14. ## Re: sailplane spar joining methods

I am considering a wing attach method similar to the above diagram. Except the spar and caps will be one continuous piece (laminated wood) and the pins going through the upper and lower caps would 1) be in the same vertical plane and 2) would be located at the side of the fuselage. I had planned on a, as yet to be determined, welded and or bolted steel truss of sorts to pin the wing to.

1) Am I in LA LA land? IOW would this be a generally accepted/satisfactory method?
2) Is there a reason/benefit to having the upper and lower pins in different vertical planes, as in the diagram?
3) I'm assuming this arrangement would be considered a "Pinned" wing?

15. ## Re: sailplane spar joining methods

Originally Posted by Lucrum
I am considering a wing attach method similar to the above diagram. Except the spar and caps will be one continuous piece (laminated wood) and the pins going through the upper and lower caps would 1) be in the same vertical plane and 2) would be located at the side of the fuselage. I had planned on a, as yet to be determined, welded and or bolted steel truss of sorts to pin the wing to.

1) Am I in LA LA land? IOW would this be a generally accepted/satisfactory method?
Everything can be made to work.
2) Is there a reason/benefit to having the upper and lower pins in different vertical planes, as in the diagram?
Not that I know, but there're a couple of good reasons not to do this. The bending moments are reacted to the fuselage in an almost scary way. Search for the Ka6 and Ka2, ASK 13 gliders. If I understand you correctly they have exactly the same method which works fine (same materials too).

Why the asymetrical idea (pins on the side of the fuselage)?

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