The last time we flew the plane we pretty much smashed up everything but there was just enough left to test a new set of propellers that we got.
There's no photo of the DJI q-tip prop unfortunately but if you imagine the DJI prop with about 5mm of the tips turned 90 degrees into the flow of the air then this is what it looks like!
We tested each propeller for the amount of ultrasound that it gave off and here are the results. The axis along the bottom is the frequency and up the side is the volume, so we want the blade with the lowest peak.
Top left - clockwise: 3 blade, DJI normal, DJI with q-tip, Graupner eprop, below: Graupner slow fly
The Graupner slow fly propeller, above, (which is wider than the other propellers) was much quieter than the other props. In fact, since the volume is measured on a log scale (decibels) this propeller was 10 times quieter than the loudest prop - the three-bladed propeller.
This test demonstrates that the propeller in use does have a significant affect on the amount of ultrasound produced and that is is possible to reduce the ultrasound interference from this source.