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An interesting article accompanying the new supersonic F-22 photo

T

tigisfat

Guest
An interesting article accompanying the new supersonic F-22 photo

http://news.yahoo.com/s/livescience/20090630/sc_livescience/whatsupersoniclookslike


Please note that there is only one sentence that says "This effect does not necessarily coincide with the breaking of the sound barrier, although it can" even though the headline screams "WHAT SUPERSONIC LOOKS LIKE". It does offer up the correct explanation, but it seems to me that the media is now fueling and breeding into the myth that any jet producing a vapor cone must be supersonic. Such is the source of arguments for many youtube and aircraft know-it-alls who've played one to many video games where this effect has been prominently featured as the aircraft goes supersonic.
 
You have to remember to look at ANY "news article" as being full of tripe, they are no longer in the business of reporting the news, instead they are selling the "news."
 
I've argued this many times with people who will scream and shout to all concerned that the Prandtl-Glauert singularity always means the aircraft is supersonic but they will never listen.
 
Alain_F355 said:
Humidity is the main factor. Look at these photos from an airshow. Even planes that cant go supersonic like the C-130 and A-10 produced the same effect that day.

http://www.richard-seaman.com/Aircraft/AirShows/PointMugu2007/Highlights/index.html
Click to expand...

And, loading up the aircraft. I remember from my flying days whenever I saw vapor coming off the wings like that it was while pulling Gs in humid air. If you look at the pictures in almost ever case it is obvious, the aircraft is loaded with Gs.

Trying to recall information from fluid dynamics classes taken too many years ago, I think it is due to instability of the air flow when the aircraft is loaded.

When pulling the aircraft into a loaded condition, you increase the angle of attack relative to the surrounding airflow and you cause of compression of the air flowing over the airframe as the aircraft pulls into the airflow. The higher AOA disturbs the laminater flow of the air causeing turboulation ((sp) I hate it when I misspell a word so badly, even spell checkers are lost).

It is this tumbling of the air and the compression of the Gs that I believe causes the water droplets to condense and come out of the air forming the vapor cloud.

Now, a practicing engineer can correct me. :)
 
I always thought of the wingtip vortices a lot simpler. There's always air spilling from bottom to top around the tip, but higher AOA simply increases the spillage. Spin the vortices hard enough under the increased load and the pressure inside them drops to the point where the vapor condenses. It's similar to the Venturi effect - the air's moving faster, so the pressure drops.
 
Around here I usually see the effect on 747's heavy with cargo. The wings (usually flaps still down) do this as they struggle to gain altitude. As such I have never supposed it had anything to do with speed.

Despite the bad title (remember journalists are experts on nothing who report on everything), the picture of the F-22 is nice. I just wish the Stennis were closer. All I got to see was the F-22's and a few F-18's flying over my house as they pendled between Elmendorf AFB and the Stennis.
 
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