Excellent explanations dcm.
I'll add the panning issues at slow shutter speeds.
When going for prop blur, the aircraft's speed and distance from the camera can produce very different results.
The approach angle is also to be considered.
Typical airshow shooting has a large distance(radius) compared to shooting 500mph planes at the Reno Air Race pylons. When shooting at short distances and panning, different parts of the plane will have a different speed relationship to the center of frame, resulting in motion blur. This gets really crazy when shooting for full prop blur. Picking a spot to shoot from that is about the center of the turn radius provides best results for nose to tail sharpness but lessens the prop arc circle to more of a side view.
My other consideration is DOF. If I want multiple aircraft in focus, I tend to go heavy on the aperture, resulting in a higher ISO to keep the shutter speed up.
My strategy is to start shooting at higher shutter speeds- 320 to 500, depending on prop rpm. Once I have a keeper, I start dropping the speed. When you go through all the time, effort, travel & money, it's a pisser to walk away with nothing.
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dcm said:
Propeller/rotor RPMs drive shutter speed considerations. The maximum RPM depends on length of the blades. The helical (rotational +translational) velocity of the tips is limited by the speed of sound, generally the upper limit is 0.85 Mach. Exceeding this produces noise and loss of efficiency due to the shock wave. The engines typically spin much faster and drive the blades through a reduction gear.
Some examples: A Spitfire with 3.3 meter blades on a Merlin engine with a 2:1 reduction gear has engine RPMs in 2400-3000 range with propeller RPMs in 1200-1500 range. A C130 with a 13.5 foot blades with a 13.54:1 reduction gear has engine RPMs up to 13820 with propeller RPMs of 1021. A Piper Cub with 74 inch blades driven directly spin up to 2300 RPM.
Helicopter rotors are longer so RPMs are lower. A UH-60 BlackHawk gearbox reduces from 20,000 engine RPMs to 258 rotor RPMs for its 16.36m rotor diameter. A UH-1B Huey gearbox reduces from 6,600 engine RPM to 324 rotor RPM for its 13.4m rotor diameter. A Sikorsky S-64 SkyCrane gearbox reduces 9,000 engine RPMs to 185 rotor RPMs for is 22m rotor diameter.
Fast shutter speeds will freeze the blades, slow shutter speeds will blur them. Depends on the effect you want to achieve. What works well for one plane/helo might not work well for another. It might be worthwhile to bracket your shutter speeds.