When taking photos of aircraft that are propeller driven it is very important to not “stop the prop”. If your camera’s shutter speed is too fast it will freeze the motion of the propeller blades. Usually freezing the action is a good thing, but with airplanes it will look too static and lack energy. If the airplane is in flight, then a frozen propeller will also look very wrong. The problem for casual photographers is the fact that during bright daylight conditions the camera’s auto settings will usually make the shutter very fast to create a correct exposure. The key in this case is to set your camera to shutter priority and then pick a shutter speed that will give the aircraft at least a little prop blur. In my example photo you will see a small amount of prop blur, but it is enough to convey motion to the viewer. A slower shutter speed will show the propellers completely blurred. What is best depends on your artistic taste. I usually start around 1/250 second and then look at the preview to see how it looks. I will slow down the shutter if needed to get enough prop blur.
Helicopters are a special case that poses more problems because the rotors spin much slower than an airplane’s propeller. I found when taking photos of a CH-47 that I had to slow the shutter down to 1/60 of a second to get enough rotor blur. The problem with a shutter speed that slow is two fold: camera motion will bur the entire photo, and if the ambient light is to strong then the aperture setting my not be able to compensate for the correct exposure. If you find it necessary to shoot at a very slow shutter speed it helps to have a steady hand – practice makes perfect with this. For shooting in bright day light with slow shutter speeds it can help to have a neutral density filter on hand to keep the exposure correct.
In short – use shutter priority to control the degree of prop or rotor blur.