About Me

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I take pictures of things. Mostly with film. Mostly with cameras. I am an engineer by trade. What really makes me happy, though, is to find an old camera at a swap meet or antique store and bring it back into working order with a few hours at the workbench. I then like to take them out and shoot some film. This blog is in large part the result of that activity.

Tuesday, August 30, 2011

This is not a photo of an airplane.

part of a Navy F/A-18E Super Hornet, going pretty darn fast
This is not a photo of an airplane.  More precisely, it is a photo of part of an airplane.  It is the back half of an F/A-18E Super Hornet doing the high speed pass portion of his flight demonstration at the Hillsboro airshow in Oregon earlier this month.  As you know if you are a regular reader, I do love me the airplanes. This time, though, I'm not really going to write about the airplane or about the photography. I'm going to write about some other things...


The first thing is that I'm not very good at tracking airplanes with at 300mm lens when they are going by very very quickly.  Even if I had been able to center him in the frame I'm zoomed in too much for him to fit.  No amount of skeet shooting would have helped.   I'd have done much better to zoom out a bit. Lesson learned.  


However, this shot is useful for another reason.  That is to ask:  What are those crazy looking wedge shaped clouds just aft of the wings?   That brings us to the second thing.  Those are condensation in the shape of  the shock waves coming off the flaperon actuator humps on the wings.


What?


When these pilots are doing flight demonstrations, they are told NOT to exceed the speed of sound during their high speed pass.  They can get close, but they'd best not go over if they want to keep doing this for a living.  In all the airshows I've gone to I've only seen a pilot go supersonic once in a low altitude, high speed pass, and that only for just a second.  (I have seen them intentionally go supersonic twice, but they were quite high up, and that's a story for another entry)  The reason for this, of course, is the sonic boom.  What we call the sonic boom is a shock wave in the air caused by the aircraft as it exceeds the speed of sound, and it is a very powerful wave, quite well able to break all the windows on buildings near the runway, and to severely startle the people who are watching.


What this young, steely eyed Lieutenant is doing, then, is holding it -just- shy of Mach 1.  Mach 1 is the speed of sound.  You notice I didn't say how many miles an hour that is.  The speed of sound in air varies, depending on the density of the air.  Air density is a function of altitude and temperature.  In this case, we were are a field elevation of 208 feet, and he was probably 100 feet above that, and the air temperature was 97 degrees.  Yes, 97 degrees in Hillsboro Oregon.  Hot.  Really hot.  Anyway, at 300 feet and 97 degrees, Mach 1 is around 760 miles per hour.   So our stalwart Lieutenant is gingerly goosing the throttle, keeping the speed JUST under that... BUT... some parts of the airplane are actually going supersonic.


How is that!?  I'll 'splain.  While the airplane is traveling over the ground at something close to Mach 1, the air does not flow past the airframe surfaces at the same speed everywhere.  The airplane is being pushed through the air and the air is moving out of the way, and past it.  Where there is more airplane, the air moves past the surface faster.  As just one example, the top surface of the wing is convex, the bottom less so, even possible concave, so the air travels across the top and bottom of the wing at different speeds.  What this means is that right there at the place where the flaperon actuator pokes out of the wing, the air IS going supersonic even if the airplane as a whole is not.  


When an object moves through air at supersonic speeds a shock wave is formed.  This is an area where the air undergoes some rather startling changes of pressure, density and temperature, essentially immediately.   One of the side effects of that is that moisture in the air condenses into actual water droplets.  What you get is a cloud in the shape of the shock wave.  That shock wave shaped cloud is clearly visible from both wings and from the vertical stabilizers in the photo above.


See?  No camera trivia whatsoever this time.

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