Inflate aluminium tube to increase it’s strength?

swerdna

I’m designing and hope to build a human powered helicopter (HPH) and am wondering if pressurising a thin walled aluminium tube by filling it with compressed air would strengthen it’s structural properties.

Hope this is the right place to post this.

dlgoff

Good luck with the HPH. Here's a place to start when considering pressure.

e.g. consider the tension on the tubing's wall.

swerdna

Thanks.

AppleBite

Also, it will depend on the loading on the beam. Pressurizing the thin walled tube induces axial and hoop stresses. Thus you increase it's compression strength in the axial direction, but be very careful that the hoop stresses are not increased so much that you exceed the yield strength causing rupture.

From a stress analysis and fracture mechanics perspective, I would be much more inclined to increase the thickness of the tube walls, even if it increases the weight. Ideally, you can fit several thin tubes inside each other, in order to prevent brittle fracture (which would be veeery bad for you application)

swerdna

Thanks. I agree that pressurising aluminium tubes probably isn’t the best method. I’m working on other methods that include pre-stressing styrene foam, cable-staying and creating an inverted conical shape with the wing spars and tying them together with multiple lines like a spider‘s web. Overall I’m trying to reduce the weight of the wing spars I use.