Today there was much rejoicing as we hit a very significant milestone: under a load of roughly 2000 lbs we pulled our first test spar off it's aluminum mandrel
The carbon fibre tubes that make up the primary structural members are constructed by wrapping sheets of unidirectional carbon fibre around a tapered aluminum tube. The carbon fibre is cured in an extremely long oven, set to cool, and then pulled off the tube. At least in theory. If the carbon is wrapped at an angle greater than about 35 degrees then, when it cools back to room temperature, aluminum will shrink more than the carbon and the tubes pull apart easily. If the wrap angle is less than 35 degrees then the carbon shrink more than the aluminum and tubes end up clamped together. Unfortunately, to get a structure that can resist large bending forces, the angle needs to be quite shallow. To maximize the structural strength and stiffness for a given amount of material we designed our tubes for an extremely ambitious wrap angle of 20 degrees. We tapered the aluminum mandrel to make this whole process easier, but to even get the tube to budge we first have to overcome the enormous static friction.
To make a long story short, this is one of several milestones that we'll have to overcome to make a helicopter that is truly as light as possible. In the end it took a winch system with about 2000 lbs of force, but now that we know that it works we are full go to start the spar production line! This Friday we'll be celebrating our new ability to pull things off of sticks by having shish kabob's at the weekly BBQ 😉
Check back later early next week for a video showing how the spars are made.