I have some really stunning load test pictures of our aluminum and carbon fiber spars. But before we get going on them, here is some necessary preamble, given our recent fantastic showing on MythBusters:
If you are looking for Belite Aircraft's production photos from MythBusters, click here. We were thrilled to be part of the "Duct Tape Plane" episode!
If you would like to follow James' tweets, @jamespwiebe is the handle to find on twitter.
If you would like to go to the main Belite Aircraft website, here is the link to follow.
Now, on to the spar loading.
Extremely long time blog readers may recall my tests of carbon fiber spars a couple of years ago. I wanted to repeat that testing, but this time I wanted to test both our current aluminum spars and our carbon fiber spars. And I was pushed to increase the load test from a 4G test up to a 6G test, which I did. (Thanks, Mike!) Also, the length of our spars had increased from 11.5 to 12 feet, as we've increased wing span (and also the wing chord.) And we had switched vendors for both the aluminum spars and the carbon fiber spars. So it was time for another test, and time to push to new limits in our testing.
This is a simple spar loading test, not a wing loading test. Of course, the airplane has two wings and each wing has two spars. We make an assumption that each one of these four spars is 'carrying' one fourth of the total weight of the airplane. Since our gross weight of our Belite is 550 pounds, the 6G loading of the entire wing structure is 3,300 pounds -- more than 1.5 tons. Therefore, the per spar loading is 825 pounds. That's a lot of weight for a single spar to hold. (The aluminum spars weigh 7 pounds; while the carbon fiber spars weigh 4 pounds, so they are capable of supporting 100 to 200+ times their own weight. Wow!)
The distribution of weight on the spar loading test spread the load over the length of the spar, with weight added at each location where a rib would be attached. Also, the spar is supported at the root (where it attaches to the fuselage) and at the strut attach point (where a strut would transfer load to the fuselage).
So here's a photo of the outcome with the aluminum spar test:
 |
| 6G Spar Loading Test on Ultralight Airplane (Belite Aircraft) |
 |
| End Section of the aluminum spar -- one of these rib/weight locations is overloaded, even for 6Gs. |
 |
| Mid Section of the aluminum spar |
 | ||||
| Root Section of the aluminum spar |
We also did the same test with a carbon fiber spar. Here's a photo of the outcome:
 |
| 6G Load Test on Carbon Fiber Spar |
I accidently overloaded the aluminum spar on the second to last outboard rib position, so I labeled the aluminum spar photos 6G+ :-)
The aluminum spar showed absolutely no bend after the weights were removed -- it returned to a perfectly straight attitude. In other words, this wasn't close to being an ultimate load test. There's lots more strength hiding in Belite's spars. Likewise with the carbon fiber spars -- the bend is very mild in the above photo.
Who knows how much more it could hold before it would snap? (This test does not cover other elements of the wing design, such as the lift struts, ribs, etc --- that work is saved for another day.)
IF I WAS CHOOSING, I would prefer the carbon fiber spars in my own airplane, (they are a $2200 option) but the aluminum spars prove to be an economical and beefy answer as well, albeit at a weight penalty of about 11 pounds per airplane.
