What causes it? Most HD cameras use rolling image acquisition. Unlike the old family camcorder that takes numerous snap shots in series making a video, HD Sports cameras use CMOS technology. CMOS cameras scan across the image similar to a copy machine scanning across the glass. This technology has given us the small affordable high definition cameras we love. The down side is rolling shutter distorts fast moving objects such as propellers. The propeller moving during the scan or acquisition of the image causes what appears to be a bending or detachment of your propeller.
The great news is there is something you can do about it. By simply adding a propeller filter to the camera lens you can reduce the effects of rolling shutter. The goal is to increase the exposure value of the camera which essentially softens the image helping to blur the fast turning propeller.
You can think of a prop filter as a pair of sunglasses for your camera. The brighter the sun, the darker the filter you will need. If you are operating in lower light conditions, then you may not want a filter at all. There is a trade off though. Using the filter to trick the camera into fixing the prop issue does degrade video quality.
The higher the frame rate, the less of an effect the rolling shutter will have. The more frames taken per rotation of the propeller, the less prop strobe effect there will be. At times it can be more beneficial to sacrifice definition for frame rate if you do not want a distracting prop effect in your video. A lighter prop filter in combination with a higher frame rate you may find to be the best combo.