Wing joiner strength?

[kc8qvo]

I am having some trouble with determining the proper strength of a wing joiner. The way I see it is the joiner tube/rod/stick, or whatever, is going to be carrying the whole weight of the airplane. My original plan was to use a 3/4" square hardwood spar as the joiner in my giant trainer (10 foot span, weight should be about 20lbs.. but just to be safe Ill say it will be under 25). I thought I would be smart about this and try a little test to see how much weight the spar will take. I took one of the pieces I had cut for one of the wing panels and stressed it at the halfway point of the wing panel, in this case 2 feet (each wing panel is 4 feet, plus a 24" center section.. so I guess 2 feet isnt EXACTLY mid-span, but half of the wing panel). I first started at 10lbs and it really started bowing. It held the weight fine, but it didnt look too sturdy. I tried to go for 15lbs but I could just tell it was too much weight. I didnt allow all the weight to rest on the spar otherwise I would have snapped it. This has me a little bit worried. 10lbs is about half of the weight of the airplane, multiply that by 2 and you have the entire weight of the airplane.. and thats just 1G. I think I should have something strong enough to hold up to 5G's, in this case that would be 100lbs total, or 50lbs per wing panel. If I remember correctly, aerobatic airplanes are spec'd at around 7G's. Since my airplane is not meant for aerobatics I figure I can cut back a little bit and still be alright. What does everyone think here?

Can anyone give me some comments/advice on testing the strength of a joiner? I dont know if my method was a very good one or not. Perhapps this 3/4" hardwood spar will be enough once its built into the wing? The wing is foam core and there will be 3 plywood rib sections that the spars attach to (mainly the joiner) spaced 1 at every 18" and one at the root (3 feet into the panel, the panels are 4 feet total).

Is there any other kind of joiner I should look into getting? Preferably something I can build myself (as opposed to a manufactured aluminum set)?

Steve

[markdrela]

http://groups.yahoo.com/group/Allegro-Lite/files/
"joiner_sizing.txt"

[Jimmbbo]

With a 10 foot span, a 3/4 inch single spar is too small... The spar will also have to support the weight of the airplane during maneuvers... So if your airplane weighs 20 lb, in a 5g turn (easily done with any RC model) the spar will experience 100 lb - If you use a safety factor of 50%, then the spar will have to support 150 lb... Am trying to imagine a 150 lb man standing on a 3/4 inch 10 foot single spar - unlikely to succeed ;)

A thicker (top to bottom) spar resists bending better. In flight, most of the stress in a wing is at the top and bottom of the spar, dropping to zero near the center (with positive g's, the top of the spar is being compressed (in compression), while the bottom is being pulled (in tension)), so one common design is to use relatively large upper and lower spar caps joined by a thin vertical shear web to distribute the load.

The size of the spar components depends on the weight of the model, wing thickness, intended use (extreme aerobatics or just bumpin' around the pattern)... Other "nice to know" information would be the wing's chord length and area.

[BMatthews]

Steve, this is why a joiner needs to be VERY strong or made to be as tall as possible within the wing. If you find the formula for calculating bending stresses you'll find that it's FAR more important to make the item taller than it is to make it wider.

Take your same joiner and make it from maple and make it 1 1/4 tall by 1/2 wide and you'll find that it takes far more load to bend it and to finally break it than your 3/4 sq piece.

Also us modelers tend to think of anything stronger than balsa as a "hardwood" but that is certainly not the case. Maple or Oak will offer far more load bearing than spruce. Fir, if it has a tight growth ring count and is cut correctly, can also offer excellent load bearing strength.

But mostly you're finding out why so many models use large diameter aircraft aluminium tubes as the joiners.

ANother way you could go about this is to use carbon fiber strip caps top and bottom glued very well to the hardwood. The wood would be acting as a web and the carbon would be carrying the load. And to do this well the carbon caps would need to be a good 50 to 60 thou thick. Serious carbon usage in other words.