Ok, a little background first. The delta in the pictures below has been flown many times with the Norvel Start Up .061. Non-throttled, cheapest engine on the shelf engine. I've been flying it with a Master Airscrew 6x3. It flew very well. Slow and stable like a trainer. I had my black box in it for a while and it showed 40mph in a straight line. (no dive for speed first) Wednesday I decided to experiment with the prop. I had an old cox thimble drone 6x3 prop sitting around, so I took the scissors to it and cut it down to 5x3. Of course, the black box was not in and it performed spectacular. Just eyeballing it I would claim 75+mph without a dive for speed. The engine was spinning over 20,000rpm after it unloaded in the air. (again, just guessing.) That doesn't seem safe to do with the old plastic props.

Q. Does the increased tip pitch on a cut down prop increase speed and thrust?

Q. Are there safer props to use for this?