tip vortices *do* exist!

From:         jonathan@hermes.chpc.utexas.edu (Jonathan Thornburg)
Date:         23 Mar 93 00:24:47 PST
Organization: U of Texas at Austin / Physics Dept / Center for Relativity
Followups:    1 2
Next article
View raw article
  or MIME structure

On the first leg of a recent (still-in-progress!) trip I saw a
really beautiful triplet of aerodynamics/physics in action:

B-727, I was in a window seat just ahead of the wing leading edge,
we were departing San Antonio, Texas.  Air temp was ~12-15 C 55-60 F.
The relative humidity on takeoff must have been almost 100%, because
just as we were passing over the end of the runway on climbout
a really textbook-perfect pair of condensation clouds appeared,
one over the aft part of the wing surface (the pressure on top of
the wing really *is* lower than elsewhere!), and the other a beautiful
spiral tip vortex trailing aft from the wingtip.  (Yes, the
direction of spiraling was correct :-) .)  The diameter of
the vortex cloud was around ~ 25-40 cm (10-16 in), it oscillated slightly
as we hit light turbulence, and the spiral rotation period looked
like ~ 5 m (15ft).  Very pretty!  Alas, I didn't have a camera with
me, so no GIFs...

Both clouds disappeared in ~15 seconds, as would be expected as
$C_L$ came down enough at higher airspeed for the pressure differentials
to drop below the water-vapor-condensation point.

[A question for people who know more aerodynamics than I do:
Since the pressure drop is strongest
over the *leading* 1/2 of the wing, why did "my" main-span cloud form over
the *trailing* 1/2?  That is, in terms of an xy coordinate system with
the relative wind being in the +x direction, "up" being in the +y direction,
the leading edge at (0,0), and the trailing edge at (1,0), I was
under the impression that the strongest pressure drop was in the
general vicinity of (0.25,0.50), whereas "my" cloud extended roughly
from (0.50,0.10) to (1.00,0.10) to (0.50,0.50) to (1.00,0.50).
Perhaps there's enough of a time delay after peak pressure drop
before a visible cloud forms to account for the rearward shift?
At (say) 300 knots = 150 m/s, a 3 m (10 ft) shift would be ~ 2 milliseconds
time delay.  Is this reasonable?  Perhaps some expert could comment?]

Just too add sauce to the "Jearl Walker" cake [JW is the author
of "The Flying Circus of Physics"], a few minutes later, just after we
climbed above a cloud deck at ~12,000 ft (we had a "travelogue" copilot),
I saw a really beautiful 180-degree "glory" rainbow completely surrounding
our shadow on the cloud deck.  (180 degrees is the sun-observer-rainbow
angle, i.e. I was looking directly away from the sun, right at the
aircraft's shadow.)  I could see it for ~ a minute before I lost it
as we gained altitude.

A final point:  We had some discussion in this newsgroup a few
months ago on in-flight use of spoilers.  As I recall, the consensus
seemed to be that they're used for roll control, but not usually for lift
dumping as such.  On this same trip, coming in to Salt Lake City over
the Wasatch mountains, I could clearly see the spoilers being deployed
several times, *without* their being any perceptable roll.  I believe
the Salt Lake City approach path is somewhat notorious for requiring
rapid altitude loss.  (Indeed, one of the early B-727 deep stall crashes
was on such an approach, I think.)

- Jonathan Thornburg
  <jonathan@hermes.chpc.utexas.edu> or <jonathan@einstein.ph.utexas.edu>
  [until 31/Aug/93] U of Texas at Austin / Physics Dept / Center for Relativity
  and [until ~Apr/93] U of British Columbia / {Astronomy,Physics}