Making Your Own Cloud
by Don Mercer
Title
Making Your Own Cloud
Artist
Don Mercer
Medium
Photograph - Photograph
Description
The phenomenon depicted is called a "vapor cone" by some and a "shock collar" or "shock egg" by others. It is seldom seen from the ground because it is not that usual an occurrence for aircraft to pull high G maneuvers at a low altitude, especially in view of the public.
When an aircraft is in flight, low pressure is created above the wing due to the airflow's higher speed, and high pressure beneath it due to a lower speed, thus providing the wing's airfoil with lift (Bernoulli's Principle). When air pressure drops, which pulling high G forces exacerbates, the temperature also drops. This brings the temperature ever closer to the dew point, that point at which dew forms, also known as fog when on or near the ground.
Add transonic flight to this "equation" of high humidity and high Gs, and a cloud of varying proportions and shapes will form around part of an aircraft. Transonic flight is that regime when some parts of an aircraft are actually supersonic, while airflow passing over other sections is not supersonic and remains subsonic, below the speed of sound.
The end result of all the above is that small vapor clouds may be seen, as but one example, at times around an engine's cowling, even when commercial flights. These phenomena may also be seen around a large part of an aircraft, such as with this Navy FA-18 "Hornet" seen at the Langley Air Show held on April 23, 2016, at Langley Air Force Base, Virginia.
As was the case in this situation, the aircraft was at high speed, pulling high Gs, low to the ground, and all was taking place on a high humidity day. Add to that the sheer good fortune of shooting a burst of images; and, as luck would have it, an image of a vapor cone resulted. Actually, three images were taken in less than 2/100ths of a second. Only the middle image of the burst of three images depicted a vapor cone, indicative of just how rapidly the airflow over an airfoil changes its characteristics. Thus, the vapor cone lasted for less that 1/100th of a second!
Lastly, the image may appear slightly out of focus even though shot at 1/2500th of a second, as there is vapor trailing from the top of the canopy and other sections of the aircraft.
Uploaded
April 25th, 2016
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