I saw something interesting on takeoff at BCN on Monday morning... as the 757 was increasing thrust, fog suddenly appeared in the nacelle in front of the compressor. It would surge up, disappear, surge up again. Once it was brakes off, it all disappeared (for obvious reasons). It was an interesting phenomenon. Anyone?

S.

Quite often seen, just as you often see the vortices from wing tips, end of flaps and even from the tips of the horizontal stabiliser at times. Caused by the low pressure which makes moisture dissolved in the air condense out to form a "cloud". Also often seen as corkscrew of vapour from the tips of propellers.
Called a Prandtl-Glauert condensation cloud.

Thanks for the correction Brian. As I said, it's been few years since I worried about anything actually transitioning to supersonic speeds. Somehow I still have a distinct memory of a single shock at the leading edge, but it's quite possible that I was severely hung over during that lesson.

This is a fun thread. Reminds me of why I got into this line of work to begin with.

gr8shandini - you and me both. I've attached a photo below to show a visible shock, you will notice no condensation, its purely because of the change in refractive index of the air brought about by changes in the pressure field. Notice too, from the disturbed water that this must be the first area on a F-18 to produce a strong shock. As an aside, 747 crews tell of hearing a "rumble" just behind the top of the cockpit when the air flow over the hump reaches its Mcrit and a shock forms. Further, the design speed of a super sonic aircraft can be ball parked some times by measuring the angle made by a line drawn from the nose to the wing tip and using the sin formula mentioned earlier to calculate the Mach. Thats because you dont want the wing operating ahead of the shock cone formed by the nose.
http://i101.photobucket.com/albums/m.../DSC_7901a.jpg

What a superb photograph ...

A visible shock indeed to all those yachties lucky enough to be looking the right way :), but quite another type of shock for those tipping gin down their throats at the critical moment :E

I think I've found another shockwave shadow on the sunny side of the wing - anyone?
 

I've found another shockwave shadow on the other side :8

Nah, it isn't what you think it is :p, ' cos this one is on the sunny side of the aircraft, and its evidence of a different shockwave I think :)

It is linear, and in high speed cruise appears to run just 2.5m from centre wingroot adjacent to the rearmost 737NG wing emergency exit, diagonally forward to where the engine pod and leading edge meet.

I reckon that means there are actually two roughly parallel shockwaves about 1.5m apart on the broad part of the wing inboard of the engine - can anyone confirm? :ok:

Brian Abraham, thanks for the explanation. I found the phenomenon quite fascinating. :)

I neglected to thank you before. So thank you :)

S.

VAFFPAX - Tis a pleasure to help. I learn a lot myself in finding the answers, so I have to thank you for asking the question. :ok:

I have been puzzling again as to whether my surmise that there must be two roughly parallel diagonal standing shockwaves on the 737NG inboard section can be true.

So I have been revisiting those links that Brian so very kindly provided earlier in the thread, including that brilliant 'fish in the river' illustration.

There is the possibility that what I have seen is some kind of break in the standing shockwave that occurs exactly at the chord where the power unit is mounted i.e. that the somehow the presence of the engine stuck right out in front at that point somehow accelerates the outboard air more than the inboard air that goes around it. That doesn't sit too well in my mind.

I can however imagine that there might be a contact lens type shockwave around the front of power unit, out ahead of the wing, on the basis that the engine cannot possibly 'eat' all the air it confronts even at 39000 where I was sat watching it.

And further to that idea I can then imagine that near the engine, there might be two 'funnels' of fast air resulting which the wing then encounters and causes to go supersonic over the top:
1. inboard between power unit and fuselage
2. outboard but quickly tucking in diagonally towards the fuselage.

Maybe I should try to draw it, scan it and photobucket what I mean, or am I letting my imagination run too far already?

PS Brian, have you seen that Blue Angels video on YouTube? http://www.youtube.com/watch?v=K7rAUu8djZ4