Team 665 Fan/s and More

[martin417]

Based on some preliminary, very unscientific testing* with bare props, directly mounted to Fisher Price motors, in pusher configuration, we were getting ~2.0 - 2.5 Lb.s of thrust from APC slow fly 10 x 3.75 props. They were running at about 9,000 RPM. If you look at this paper (yes it is old, but the science is still good) you will see that it is possible to gain about 50% to 75% in thrust, as well as improved efficiency by building a shroud. Since you have to protect the props in something for safety anyway, why not make it an asset? Here is a picture of the fiberglass shroud we made for the prop:



Link:

On another note, due to the internal resistance of the battery and other components, if we ran two of these at the same time, the voltage dropped enough that the RPMs fell off to about 8,000, and the thrust went down to less than 2 lb. per prop.

We are building these modules, but still don't know whether or not we will add them to the finished bot. We still have to do testing to see whether the benefits outweigh the hassle and if they fit within the weight budget.

*Unscientific test: stand on digital scale holding motor/prop in hand and record displayed weight. Point the prop straight down and engage motor (use caution here, very dangerous, do not attempt this at home! these are trained professionals very stupid people!). record displayed weight again. Calculate difference, This is your thrust.