Tiedown bar strength calculations
Hearing all the discussions here and on some of the Oshkosh threads about their favorite anchor system or this rope versus straps, etc. - I wonder what the anchor point in the wing of an RV is good for? Marginal threads in an aluminum block in the wing?? I dang sure wouldn't stand under a 3-4,000 lb block hanging from the tiedown under the wing of an RV! Intuitively, I'm thinking 1,000 lbs is about it.
In the RV-4, the 1/2" bar is held on with three 3/16" bolts. 40,000 psi tensile strength for 6061 alum. 1/2 x 3 x 3/16 x 40,000 ~ 11,000 lbs.
However, looking at the structure the bar bolts to:
1 bolt through (1/8 + 1/4 + 0.04) x 3/16 x 40,000 ~ 3,112 lbs
1 bolt through 1/16 x 3/16 x 40,000 ~ 468 lbs
1 bolt through 0.040 x 3/16 x 40,000 ~ 300 lbs
About 3,880 lbs
As for the strength of the 3/8-16 threads into the bar, assuming a conservative 1/32" thread width or contact area: 3/8 x Pi x 1/32 x 40,000 ~ 1,472 lbs per thread. With 5 thread engagement ~ 7,300 lbs. With 10 thread engagement ~ 14,700 lbs. Drawing specifies 1" of thread (16 threads). So close to 23,000 lbs. Someone please verify those thread strength numbers.
For the RV-4 it appears that the structure that the bar is bolted to is the limiting factor.
Currently have my wheels off the ground to fit gear leg fairings and wheel pants, hanging on engine lift to engine mount near firewall. I guess my worries about using 3/8" bolts screwed into the tie-down holes as sole support are unfounded.
Finn