Source: 2002-Q4 Beartracks, Bob Barrows

Wing was loaded with sand bags at 30% chord line average. Sand load approximating air load distribution from the root to the tip. Wing deflection and observed skin distortion was recorded for loads from 900 to 6000 pounds. Tip deflection at 6006 pounds was 3.378″ at main spar and 4.521″ at rear spar. Load was removed and wing showed no signs of bending or twist. Some small distortion was observed at the center butt rib near the rear spar. Also a slight distortion on wing top skin in gas tank bay area. The 6006 pounds represents a loading of 5.7g at 2500 pounds gross weight, as adjusted for the weight of the wing and 30 gallons of fuel. The test was continued to 6774 pounds, at which point the main spar failed at the top cap strip in a buckling mode, 30 inches in from the 3/8 inch wing attach to fuselage bolt. The 3/32 inch rivets failed in shear that attach the top .032 root skin to the rear spar flange.

Note of interest: up to the 3003 pound test loading the fuel tank had not been installed. This left the top skin in the fuel tank area supported only by the .025 hat sections to control skin deflection. Skin deflected inward about 1/2″ and caused wing twist. After loading was removed, no wing distortion was observed. Top skin basically went back to its original shape. Wing failure occurred at 6774 pounds, or 6.4g. No permanent wing deflection or twist was observed up to 5.7g. The rated aircraft max allowed load is 4.5g at max gross weight.

Recommended Inspection of Completed Wings:
Check relationship of fuel tanks to top skin at stiffeners. Push on top skin at stiffeners, and skin should not deflect more than 1/16″ before contacting fuel tank. Readjust tank straps or shim between tanks and stiffeners as needed with silicone rubber.

Running changes to be made to the wing as a result of tests, for plans serial number 583 and up:
1. .032 Center ribs rear lightening hole to be changed from oval to round (drawing #6).
2. Hat-shaped top skin stiffener to be 5/16-3/8 inches high .050 5052H32 (or 6061T6) aluminum. These materials form easier and will be stiffer (drawing #2).
3. Main spar front top cap strip (1/8×1-1/4) will be made longer so as to butt up against front spar plate at root end (drawing #3).
4. Center root end rib will be doubled up with opposing flanges, ribs riveted together at .025 support angles, and .032 spar attach angles (drawing #2 and #6).
4. Rivets attaching .032 root skin to rear spar flange to be AN426-4 (drawing #2).

It is important to note that none of the listed running changes are mandatory, but seemed to be worthwhile, considering the knowledge we gained from the test and the small extra weight, cost, and effort required when constructing the wing.