Aerodynamics guys: what are those circles in the air from props?

Ive been wondering this for a long time...specifically on newer prop powered aircraft, at full throttle, these wierd spiral circles come out the back of each engine. You can see it at 0:14 of this video:

www.youtube.com...

What are these and what creates them?

It's the prop tips. That particular picture was taken with extremely slow speed film so what you are seeing is the travel of the propeller trips in a helical path (they are making a circle, but the plane is moving forward which will create a helix).

As you watch that compilation of pictures you will see some that the propellers appear to "stopped" even when the plane is in air. Those pics were taken with high speed film. Others you will see the propellers making a circle - those are with slow speed film.

The particular pictures at 00:14 you reference is probably the slowest speed film shot in the whole group. It's the same technique used as when you see the footage of cars travelling down a highway and you get a continuous streak of light caused by their headlights/tail lights.

The tips of aircraft propellors are travelling very fast. In some aircraft they can actually exceed the speed of sound.
The high tip speed and the tip shape generates turbulence and shock waves. These air effects (under the right conditions) can cause atmospheric moisture to condense into visible water vapour. They are, in effect, mini clouds that follow the propellor tips.
They are most likely to happen when the dew point temperature is very close to the actual temperature on days when there is a high degree of relative humidity.

It's called Propwash! Mosture in the air conenses on the spinning prop because of the low pressure area created by the spinning motion. It's very common, especially in area where the humidty is high and the dew point is low.

Tim