Discussion of Structural Strength of Wing Spars


Steve Craigle
In a posting to the mailing list, 06/13/2001

Here are the results of my look at the wing spar stresses for the BD-4. I assumed the Schrenk method of airload distribution, a tail download of 5% adding to the weight and 130 lb (~22 gal) of fuel in each wing tank. I looked at weights from 2000 to 2500 lb and a 24 and 29 ft span. The wing spar is critical in local crippling just outside the center spar. The numbers below incorporate a safety factor of 1.50 against the crippling load in accordance with Part 23. They may be compared with the minimum allowable loads for the normal (3.8), utility (4.4) and aerobatic (6.0) categories of certified aircraft. It should be understood that th load factors below apply only to the wing. Other parts of the aircraft could fail by breaking or deforming before reaching some of the load factors listed. Format: (weight, load factor (n1))

Short (Standard) Wings: (2000, 6.01) (2100, 5.64) (2200, 5.31) (2300, 5.01) (2400, 4.75) (2500, 4.51) Reduce these factors by about 0.25 if you put the fuel in the fuselage. Also keep in mind that a 2500 lb short wing plane will have a flaps up stall speed near 74 mph, which could use up a good deal of real estate on takeoff.

Long (29 ft) Wings: (2000, 4,69) (2100, 4,38) (2200, 4.12) (2300, 3.88) (2400, 3.67) (2500, 3.48) Reduce these values about 0.15 if the fuel is carried in the fuselage. The flaps up stall speed at 2500 lb is about 69 mph.

Summary: Long wings look like a reasonable way to spend some extra building time if the weight is kept below 2300 and maneuvers are restricted to the normal category range. They will improve takeoff, climb, and landing performance and give a higher ceiling at a given gross weight. This seems to be confirmed by Fred Hinsch's experience with his plane. Any of the above situations will be improved by more fuel in the wings (at the same gross weight).