Observation on 737 in the rain 3

[franzh]

Over the last summer, I was sitting in the window seat of a Boeing 737, on the right side, just in front of the wing root, directly inline with the inlet of the turbofan. It was raining and there was quite a bit of water standing on the tarmac.
After pushback, the engines spooled up. I watched the vortices generated by the rapidly inrushing air turn into a “Snake” of water and air, pulling forward from the engine about 3 meters, then turning 90 degrees downward to the ground. The snake was about 30 inches in diameter and as long as the engines were at taxiing speed, the snake was consistent, becoming more visible as we taxied through water puddles.
When we reached the runway, the engines spooled to takeoff power and the snake stretched out almost out of sight of the window, but still staying attached to the ground.
As we approached takeoff speed, the snake disappeared but it stayed visible to just before rotate velocity.
In all of my flights over 40 years, I have never seen such a sight. I never realized the drop in air pressure would vaporize the water so visibly. Opinions?


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[MikeHalloran]

Being able to see the air can be educational.

One time we took off from Albany, NY ~30 min late, and arrived at Atlanta, GA ~10 min early. Along the way, at altitude, I noticed a very odd thing; a step change in the index of refraction of the air, in a vertical spanwise plane, above the thickest part of the wing. I had to look it up when I got home, right after changing my shorts; it was a shock front.

I think the plane was a 727, still my favorite, and fuel was much cheaper than time in those days. You know how the pilot throttles back when the plane reaches altitude? He never did, until it was time to land.






Mike Halloran
Pembroke Pines, FL, USA

 

[scotty7]

Can't find any piccies on the net, but this happens on a regular basis, especialy when the aircraft is stationary or moving slowly. The strength of the vortex depends on engine speed (i.e. mass flow and velocity in the inlet) and the ratio of intake diameter to height from the ground. This is why a 737 has funny-shaped intakes, to minimise this effect given other considerations (such as using existing undercarriage from the smaller-engined 737-200 series). The vortex can easily be strong enough to pick up debris from the pan and runway and suck them through the engine - which can be a bit of a problem... Hence in turn the effort thast goes in to keeping aircraft hardstandings free of rubbish, bolts, bits of wire etc.

 

[rb1957]

a pic of a shockwave on a B1 ...

http://tinypic.com/471s47c.jpg

 

[BigBadJet]

I have never seen the refraction myself but have looked for it several times. The shock forms when the wing reaches critical mach (airflow over the wing becomes transonic). I've seen it on video from Boeing flight test engineers but I guess the light has to be just right (?).

 

[MikeHalloran]

It was not opaque like in the B1 pic; completely transparent, but easy to see. Nowadays, I assume going that fast would be a career decision for an airline pilot.



Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

a careerlimiting decision ;-)

TTFN

 

[franzh]

This inlet vortex was unique in that is was fully visible, even the ground-standing water was vaporizing. I average about a dozen flights a year in all kinds of weather and this is the first vacuum snake I have seen. I have seen the contrails extending from the wingtips in heavy humid air, and even from the flaps when extending.
The combination of the 737 with the big air inlet (I suppose the Airbus is the same), my seat, me observing the engine inlet, and Saturn rising allowed me to see the anomoly.
Truly unique, will remember that one.
Franz

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[Dan320]

There is a NACA report nbr? on visible refraction changes at a shock wave a'la Mike Halloran. I have seen it myself in a B767 in descent. Real cool.

 

[scotty7]

The links sow a nice inlet vortex, and the second link shows what can happen if the vortex goes into the core of the engine (as opposed to the bypass duct...).
I've been about 20 feet from an engine that took a vortex into the core - makes quite a loud bang... (fortunately it was on a ground test stand and I was in a control room at teh time).

http://www.airliners.net/open.file?...v_id=1132368&next_id=1132131&tbl=type2&size=L

and the slight oops...

http://www.airliners.net/open.file?...r=5&prev_id=1132263&next_id=1132045&tbl=type2

 

[notnats]

I have seen a shadow on a wing which I assumed was refraction caused by the low pressure in the air above the wing. It moved about a bit with changes of attitude. This was about 15 years ago flying between Sydney and Melbourne in the afternoon and looking over the left wing. The sun would have been on the right of the aircraft. I don't think we were flying particularly fast, and I assumed it was a normal condition.

Jeff