Tuesday, October 2, 2012

Pic(k) of the week 40: Propeller math

Being in the middle of my Airbus A380 type conversion training as a pilot, I unfortunately have little time to work on new images...

Therefore an image I shot at the Zoersel PhotoFlying days last August.

Crew Chief climbing a P-51 Mustang at the 2012 Zoersel PhotoFlying days

The image above of a World War II P-51D Mustang, highlights the difficulty of shooting prop-aircraft with a running engine. When running the RPM (Rotations Per Minute) of their propellers varies between 500 RPM at IDLE to above 2500 RPM during take-off, climb and cruise. 

If one shoots at relative fast shutter speeds like 1/500s or more, the prop freezes and it looks like the engine just stopped. Obviously not looking natural for an aircraft, especially in flight!

In a perfect world one wants to have a full prop-disk displayed. In order to achieve this, we first need to do a little bit of math;

First of all, we need to know how many prop blades the aircraft has. Some are equipped with 2 bladed propellers while others have 3, 4 or even more. The P-51 above has a 4 bladed prop.

Next lets suppose a low RPM on the ground of 1200 RPM, which equals to 20 rotations per second. A full prop disk will be displayed if the shutter speed is 1/80s (RMS x number of blades) or less. The image was shot at 1/60s and a little bit of the full prop circle is missing. This proofs that the RPM was more than likely lower than 1200RPM. End of maths lesson!

Image details:
Nikon D800 with 70-200 VR2 2.8
ISO 100, 165mm, f11, 1/60s
RAW development in Lightroom 4.1" 
Black and White conversion in SilverEfex Pro 2

Remember; "A photograph is normally looked at, seldom looked into" - Ansel Adams

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