Need some tried and true baseline specs for aluminum main spar tubes through (hot-wire-cut-Rutan-style) foam wings. What gauge and diameter aluminum tube are folks using for that main spar?
And then the heavily-loaded lower rear wing that mounts to your attenuator or gearbox. What size here are people using to max out their stiffness/strength/weight ratio here?
I'm sure expensive carbon fiber tubes have been employed as well. Any info on perhaps using carbon tubes? (Also, forgot... is "Uni" the stiff one?)
And then, just because I'm curious... does anyone know what kind of load is that lower wing seeing as it carries the top wing's downforce as well as the small amount of its own?
Carbon spars would be a no-no since the rules do not allow carbon wings.
Lower wing - maybe up around 400 pounds - been a long time since I knew the real numbers. In any case, you will want to design for whatever the aero load is and the vibrational loads - use 50 G's as a realistic number that may be a bit on the high side.
1.125 "or 1.25" inch .050 thick seems to be a standard for front. I've tried both steel and aluminum and am going back to steel this time around. The aluminum will bend too easy and cannot be easily straightened while at the track. My main rear wing spar seems to be about 7/8", but this wing as more than one.
Ditto what Rob said.
1.125 or 1.250 - 4130 - in my case .65" wall (probably a bit overkill)
I ve seen aluminum ones bend on the back straight at Road Atlanta in another guy's car. So i skipped aluminum.
If a wing element weighs 6 lb, and produces 150# of downforce at 150 mph... isn't that 25 Gs? Then like all good engineers, double it for safety factor, and presto... you have a 50G number, just like Richard said.
Many cars employ a design where two front wing main elements are attached to the side of the nose cone, one on each side. In some of those designs the wings are supported by a tube that passes all the way through the wings and nose cone. e.g Citation
I would recommend using the largest tube the wing profile will allow. 1.25" dia by .049 is a good place to start. Use 4130 steel, it has good spring properties. I have several profiles with some wings only using .75" OD.
At the rear, vibration is as much of a problem as load. The mounting of the rear lower/mounting element needs a lot of thought to work well and last.
This area is serious buisiness and didn't want to make an error. The vibrational- harmonic events going on in a rear wing assembly must be just wild. And, of course, the loading isn't a small deal either.
Special thanks on the mount heads-up, Steve. I'll be careful there.
Thanks Froggy, now I understand. But is that part licked by the air stream? If not, isn't carbon ok to use?
Unless there is something in the rules that I'm not remembering correctly without looking it up. the wing tube would be considered part of the wing assembly, so, since the wings are not allowed to be made from CF, neither is the tube.
Chris: Yes - 50 G's ! Vibration loads can seem very scary when you look at the G-loading numbers, and 50, while a guess, I think would be a good number to look at and may actually be too low - many car components are designed around 100 G vibrational loading.
Think about the load that a wing will see in a frontal crash when the whole car is decelerating at sometimes upward of 100 G's, and the wing survives - it saw the same decel rates as the rest of the car.
Best also to design the wing so that it does not rely solely on the spar for its bending strength - the skins should actually be taking the majority of the loading.
It's funny how threads like this come up when I'm right in the middle of doing it. These new water jet cut ribs are more difficult to work with than the old style sheet ribs. Somehow, my right wing came out perfect first time, but both attempts on the left wing resulted in the spar not going through the holes in the ribs due to misalignment. Next attempt later today, I'll put the spar through the rib holes while riveting.
If you use the Rutan method with a foam core and fiberglass skins, you don't need a metal tube. Instead you can construct spar caps, which are strips of composite that run the length of the wing at the top and bottom. It's a good application for unidirectional material. Do a Google search on "homebuilt aircraft spar caps" and you'll find lots of information.
The loads at the mounting points can be high, so you'll need to reinforce with lots of glass or an aluminum insert. We use solid aluminum blocks with threaded inserts for mounting inside the core on our lower rear wing.
If you have more than 250 lbs of downforce (at 140 mph) from the upper rear wing, you have made a big mistake and your aero efficiency will suffer!
Chris
I've been using the same technique Nathan described for years. Done properly there's no need for a metal spar tube unless you want to just add more weight to it.
I recommend end caps for the foam which would resemble the wing profile. We machine these either out of aluminum or phenolic. This gives you a hard point for needed threaded fasteners.
__________________
Gary Hickman
Edge Engineering Inc
FB #76
If a carbon fiber spar is INSIDE a wing... and it isn't being licked by the airstream, wouldn't it then become an "internal panel"... and thereby legal? Or is there some overarching regulation concerning cf in wings...
Aluminum wing spart tube rough specs
Cheers, Richard.
I have two spares one is .065, one is .049. 
