The last few weeks have seen some progress on instrumenting the #Drone Test Stand. You can check out that project for more details, but so far I've made a prototype of electronics to measure the thrust while Goliath is being tested and put together the hardware to incorporate the load cells into the test stand.
Previously the support lines were directly attached from Goliath to the test stand. The new setup has Goliath attached to two support bars, one for the front and one for the back. Each support stand rests on (2) load cells, one under each end. The load cells are mounted on 2x4 blocks that have a recess to keep the load cells in place. To ensure that the load cells receive a purely axial load, two brackets that swivel on a piece of all-thread hold the support bar in place. The all thread also helps to keep the 2x10s spread apart at the correct distance to keep the support bar in place on the load cells.
I attached one of support bars to the test stand and decided that it'd be good to run a test with just the one support bar to make sure the setup works before adding the second support.
The plan for the test was to start up with the choke on, make sure the support bar was okay, and then keep running the engine on idle to warm up the engine. After that I would shut it off and turn off the choke (there's no servo setup for it yet) and then try another run at full power. Here's the video for the test:
As you can see from the video one of the propellers broke free after idling for a while. The propeller broke free after all three of the bolts that hold it onto the pulley failed. Replaying the video after test it was clear that there was a increase in oscillations before it failed. It's likely that one bolt failed and then the others failed under the increase in load. Fortunately having the shaft supported at both ends keeps the prop captive instead of the prop sailing off in the sky. There was no damage to the vehicle other than the three bolts. Here's a closeup of where the bolts broke.
The bolts were just generic zinc 1/4" cap screws 3 1/2" long. The bolt passes through the pulley and are threaded into the QD bushings and hold the two parts together. The bushings come with shorter 1" long bolts for this purpose, but I swapped them out with the longer bolts so that they also served the purpose of the prop bolts. The prop then goes over the bolts and nylon lock nuts go on above the prop to hold it in place. The bolts broke where the prop and bushing interface.
The easiest fix for this is to get higher strength bolts (grade 5 or grade 8). Unfortunately it looks like I'll have to special order them as I need the full length threaded and all the local hardware stores only carry partially threaded. I did find some stainless one today that I can use in the meantime, but stainless is a little weaker than a Grade 5 bolt so I'll just use those temporarily until I receive the others. It should be relatively quick to swap them out at all the propellers, add the second support bar and do the next test.