Re: Sighted a shock wave above airliner wing

Date:         12 Oct 98 00:02:09 
From:         Tom Turton <tturton@cowboys.anet-dfw.com>
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Phil Dunn wrote:
> Do you think this could be condensation instead of shock wave?

Wouldn't have any condensation up around cruise altitudes - too cold and
dry.

> I have flown lots of supersonic formation and you don't see shock waves.
> You can feel them though.

No doubt true enough if you are blowing through Mach 1; what the earlier
poster was probably referring to was 'local' Mach greater than 1 over
the wing, but the aircraft was probably still at cruise Mach (between
0.8-0.9 I'd guess).

> But as the pressure gets very low, you can see transient condensation of
> water and this appears like shock wave.
>
> Much like the stuff you see coming off the wing with high lift
> configuration entering high humidity area.

Yes, but again, you'd need high humidity and you have to really 'work'
the air to get the low pressure, either by dropping flaps or pulling a
high angle of attack.  Normal cruise shouldn't produce either of these.

> And shock wave formation will kill lift aft of it pretty suddenly, no?

Don't think that's true, but alas, I've forgotten too much of my high
speed aero!  Drag will certainly increase, but again I think you are
talking about fully developed shock waves at and above cruise Mach =
1.0.   The small Mach waves shouldn't cause any drastic effects.  I
never saw them myself, but I've heard others remark on this same effect
that the original poster mentioned - a 'wave' sort of dancing around
mid-wing and best seen when the sun is on the opposite side of the
viewing window.

---Tom Turton