Shockwave?

[G-DALE]

The photo below depicts a B742F descending into New York with a 'shockwave' looking effect around the nose area and flaps on the port side wing.

I've seen shockwaves formed by military a/c and Concorde whilst breaking the sound barrier etc, I understand that. However I've never come across anything such as the below. I would be confident in saying that with a 200-400mm lens the a/c is below 10,000ft and therefore speed is not in excess of 250KIAS. Is it a high rate of descent that's causing this?

[photoid=6472254]

Thanks in advance.

[Bok269]

It couldn't be a wake vortex could it? I can kind of make out a spiral formation.

[BoeingKing77]

What happened to my post?Anywhy,my guess is as good as urs.hot dogging the desent

[pilotgolfer]

Not sure exactly about that one, but all in all it's a pretty sweet shot.

[brianw999]

...and there's a couple of elliptical marks in front of the starboard outer. Could they actually be higher than the aircraft ?, maybe a fighter contrail ?

[E-Diddy!]

That can't be a shockwave. It doesn't look like any kind of sonic shockwave I've seen before, a sonic shockwave wouldn't form condensation in the manner seen behind the flaps, and a 747 travelling fast enough to create a visible sonic shockwave would probably be experiencing Mach tuck.

[G-DALE]

I see someone has also asked the same question at PPrune. The most popular answer also seem to be that they're trails from military/fighter aircraft.

Of course, the real Mr. Knowitall members on there (and there's hundreds of them) are certain that the whole thing is photoshopped

[Gabriel]

Photoshop, con(chem)trails, clouds, UFOs, whatever... I don't know.

But one thing I know: Those ain't NO shock waves.

[Dmmoore]

It's not a sonic shock wave. A sonic shock wave may or may not be visible depending on atmospheric condition. At sea level, an aircraft may have airflow exceeding MACH 1 somewhere on the airframe when IAS >650 knots. A 747 has a VNE <400 knots.

There are conditions where the pressure and moisture content of the air is such that the aircrafts motion causes the mosture to condense and become somewhat viaible. The effect would only be seen when the sun and viewer are at optimum viewing angles.

On damp days (temperature and due point very close to rach other) a layer several feet thick vapor may condense on the wings upper surface. The pressure drop above the wing is the cause. The effect seen in the photo appear to be similar in that the vapor visible aft of the L/H (port) wing appears to be vapor trailing off the upper wing surface.

[ATFS_Crash]

I’m going to disagree with all of you. I don’t think it is supersonic but I think it is a shockwave.

Some of you seem to be implying that all shockwaves are supersonic. I’m going to disagree. I feel this is a subsonic bow shock wave. Though the term shock is commonly to reference supersonic; it also can mean a wave that has a change of pressure, temperature, etc….

It could be optic glare in the lens; but the location seems to suggest it is a bow shock; the type of aircraft and the speed that is suggested in the thread suggests that it is a subsonic bow shock wave.

Once again this is a difference of opinion of terminology. Though shockwave is generally used in the context of supersonic; the word shockwave can also apply to subsonic waves.

The shock is a sudden change of pressure that is apparently causing the water vapor in the air to be visible.

Though “shock” is more commonly used in the context of at or in excess of the speed of sound it can also apply subsonically.

Grab your saws; I’m out on a limb. Let the ridicule begin.

It’d be less controversial to drop the word “shock“; and just call it a “bow wave“. That would be more difficult to argue with as it would be more common usage.
----
Of course there is a tinfoil hat crowd that might suggest the subsonic bow wave is evidence of some sinister government electromagnetic weapons testing. Some people may consider it a plasma artifact; suggesting drag reduction and/or stealth and/or weapon.

[Gabriel]

Ok, so let me rephrease it:

Those ain't no waves.

[E-Diddy!]

Ok, so let me rephrease it:

Those ain't no waves.

I second that.

[ATFS_Crash]

IMAO I disagree. I think the doughnut is the trailing edge decompression shock that following the high pressure bow wave. The bow wave forms a bowl or umbrella; this doughnut is at the rim of the bowl/umbrella bow wave. Vapor trails are often generated at the rim of the bowl/umbrella of the bow shock.

http://en.wikipedia.org/wiki/Bow_shock_(aerodynamics)

[AJ]

I think the main area of confusion is the usual amazing symmetry of a shockwave or bow wave:
[photoid=364692]

[E-Diddy!]

That can't be any kind of a wave, because the wave will start at the areas where the airflow is accelerated the most: over the wings. As the aircraft goes faster, the wave moves aft. The waves don't start forming (bow, shock, whatever you want to call it) ahead of the aircraft until you get very near the speed of sound (thinking like the M.97ish area).

Here are a few simple reasons why a 747 can't go that fast:
1. The parasite drag force at that airspeed would most likely be enough to cause the aircraft to break up. If for some reason (like made in USA quality) it holds together, then:

2. As the aircraft accelerates, a shockwave begins to form over the thickest part of the wing (usually) which is where the airflow is the fastest. This is also at the wings center of pressure. As the mach increases, the shockwave moves aft, and with it, the center of pressure. Because the center of pressure (acting up) moves further away from the center of gravity (acting down, forward of the center of pressure), the lever arm gets longer and the aircraft will violently pitch nose down, called "Mach Tuck." The airspeed at which this occurs is M-crit. At this point, the aircraft is flying well above Va and is pointed straight nose-down. If the pilot attempts to pull out of the dive with the stick (if they are able to overcome the extreme control pressures needed) then the aircraft will break up, if the aircraft is allowed to continue accelerating, it will go through M-limit at which point the dive will be unrecoverable. This region of flight is known as "coffin corner" and very few have gone there and come back.

3. A 747 skin is made of aluminum. The extreme heat encountered at very high mach would likely melt it to the point that it would fatigue the skin enough to cause structural failure.

Notes:
- A 707 once encountered mach tuck accidentally. The pilot recognized what had caused his aircraft to go from level flight to straight down, and was able to recover by deploying full flaps, full spoilers, and full thrust reversers while in the dive. The extreme forces caused two of the engines to depart the aircraft and extensive damage was caused, but the aircraft made it back.

- WTF is a subsonic bow wave? Is that the phenomenon that occurs when a body moves through a fluid where the wave transmits that object's kinetic energy through the fluid, like what occurs in water? Because that occurs in water, not so much air.