Best Defense

Who bent that prop?: The camera did it!

The bent prop is an optical illusion. I used to work in a company that designed propellers and they don’t bend that much.

seafury_launch-2

 

By Mark Mangelsdorf
Best Defense guest respondent

The bent prop is an optical illusion. I used to work in a company that designed propellers and they don’t bend that much.

I’m pretty sure the optical illusion is due to the way the camera works. (Going off of my memory of an Air and Space magazine article from the late ’80s early ’90s about aerial photography that I read as a kid. It had the same problem.)

Normal cameras (yesterday’s film SLRs included) use a shutter that has a window that only exposes part of the film at a time. It travels across the film from one side of the frame to the other. This means that the each side of the frame gets data from a different time. If the shutter moves from left to right, the right side is taking a picture from a slightly later time than the left side. I think this is what is going on in the picture on your blog. It doesn’t matter with the airplane moving at only 50 to 80 mph, but with the tip of the propeller moving at about Mach 1 (761 mph), it makes a difference.

The Air and Space magazine article talked about how Sherman Fairchild (if memory serves correct) developed a new type of shutter for aerial photography to solve that same type of problem that cropped up when taking photos from fast moving airplanes.

Mark Mangelsdorf  is an aeronautical engineer who reads this blog to keep track of what the groundpounders are up to. If he did indeed work for the U.S. government, this wouldn’t represent any official views, but only his personal ones.  

Photo credit: U.S. Navy/Wikimedia Commons

Thomas E. Ricks covered the U.S. military from 1991 to 2008 for the Wall Street Journal and then the Washington Post. He can be reached at ricksblogcomment@gmail.com. Twitter: @tomricks1

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