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Haigotron
03-25-2006, 01:32 PM
Hey everyone,

just wondering what are the physics behind those air trail that appear at the wing tips right before that dreaded (in my case - habitual) stall?

Air pressure?

tigertalon
03-25-2006, 02:15 PM
Originally posted by Haigotron:
Air pressure?

Yes. The air is pressurised, then expanded in a very short time. When expands, water in it (air humidity is never 0%) condenses.

It is like pumping the air out of bottle to create low pressure in it, and then opening bottle. Inside bottle a fog will appear.

SnapdLikeAMutha
03-25-2006, 02:39 PM
Would I be correct in saying that is a similar process (albeit in a different medium of course) to cavitation of ships and submarines propellors?

NonWonderDog
03-25-2006, 04:50 PM
I'm not very familiar with cavitation, but it's not the same thing.

Contrails can be explained by just the equation of state:

PV=RT

As the air pressure above the wing is decreased, the temperature of the air also decreases. If the pressure is reduced enough, the water in the air reaches condensation temperature. (Contrail is short for Condensation trail.) The contrails show up at the wingtips due to lots of aerodynamic complexity.

With jetliners, the biggest contrails come from moisture in the fuel being exhausted behind the plane and shock-cooled. This only happens if there's already enough moisture in the air, but there usually is.


First person to say "chemtrails" gets a free ***-beating. http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Haigotron
03-25-2006, 06:34 PM
I thank you all for this enlightenment, next time, ill be watching my wingtip in awe, before i start spiralling downward... http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

Haigotron
03-25-2006, 06:55 PM
s the air pressure above the wing is decreased, the temperature of the air also decreases. If the pressure is reduced enough, the water in the air reaches condensation temperature.

oops, follow up question -

I though stall happened because there was less air under the wing, so wouldnt the pressure under the wing be less than at the top of the wing?

VW-IceFire
03-25-2006, 07:20 PM
Originally posted by Haigotron:
<BLOCKQUOTE class="ip-ubbcode-quote"><div class="ip-ubbcode-quote-title">quote:</div><div class="ip-ubbcode-quote-content">s the air pressure above the wing is decreased, the temperature of the air also decreases. If the pressure is reduced enough, the water in the air reaches condensation temperature.

oops, follow up question -

I though stall happened because there was less air under the wing, so wouldnt the pressure under the wing be less than at the top of the wing? </div></BLOCKQUOTE>
Good question...I'm not quite sure. The wing stalls because the AoA has exceeded a certain point and the boundary layer that is normally smoothly flowing over the wing has separated from the wing entirely. The wing ends up producing virtually no lift and is infact causing drag.

ATLAS_DEATH
03-25-2006, 07:37 PM
The wing stalls because the AoA has exceeded a certain point and the boundary layer that is normally smoothly flowing over the wing has separated from the wing entirely. The wing ends up producing virtually no lift and is infact causing drag.


Yes

NonWonderDog
03-25-2006, 10:01 PM
Originally posted by Haigotron:
oops, follow up question -

I though stall happened because there was less air under the wing, so wouldnt the pressure under the wing be less than at the top of the wing?

Stall happens when the air above the wing separates and becomes turbulent. When this happens the airfoil loses effectiveness and you lose lift.

You don't lose all lift at once, you just get into a regime where increasing AoA will decrease lift, instead of the other way around. The wing will still produce lift, and the wing will still create an area of low pressure above it, but the flight controls will exhibit negative stability or, in the case of the ailerons, be completely reversed.

As lift is proportional to AoA up until stall and is also proportional to the pressure differential, you'll get the most noticable contrails at exactly the stall angle of attack.

Haigotron
03-25-2006, 11:00 PM
Thank you all again http://forums.ubi.com/groupee_common/emoticons/icon_biggrin.gif

FI-Skipper
03-26-2006, 11:25 AM
Originally posted by NonWonderDog:
I'm not very familiar with cavitation, but it's not the same thing.

Contrails can be explained by just the equation of state:

PV=RT


I could be wrong but if you are stating the "Ideal gas equation" you mean

PV=nRT

and remember to measure the temperature in Kelvin http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Skipper

Haigotron
03-26-2006, 05:56 PM
how would you go about calculating the number of moles of air under the wing anyways?

I believe he just wanted to show the relationship between T and V (charle's law i believe)

AKA_TAGERT
03-26-2006, 06:07 PM
I thought it was the wing tip fairies jumping ship in lue of a stall? Kind of like rats before the ship sinks

tigertalon
03-26-2006, 06:17 PM
Originally posted by FI-Skipper:
I could be wrong but if you are stating the "Ideal gas equation" you mean

PV=nRT

and remember to measure the temperature in Kelvin http://forums.ubi.com/groupee_common/emoticons/icon_wink.gif

Skipper

Of course he was right stating PV=RT, he just took generalised (or normalised if you like) volume, V'=V/n (it is not really bright taking number of liters and number of moles in such circumstances -any of them is impossible to measure-, number of liters per mole being much more appropriate.)

KrashanTopolova
03-26-2006, 08:51 PM
IMHO stall starts at the wingtips because it is there that most of the power in the lift available is expressed (while the residue of available lift is retained near the wing roots which stall after the wing tips). And it is there at the wingtips that the lift surfaces are at their thinnest.

If the wingtip stall condition progresses it moves inboard to the wing roots...which can lead to flat spin.

condensation is temp/pressure dependent and appears at the wingtips first because it is there that most lift power/temp/pressure resides...

Hence the tendency toward stall (in a theoretical sense) increases with height (pressure dependency) and with lower air temperature (temperature dependent).

NonWonderDog
03-26-2006, 09:14 PM
Other way around; wings are designed with enough twist ("washout") so that the inboard portion of the wing stalls first. This allows one to maintain at least partial aileron control until deep into the stall. Even without the twist the inboard portion of the wing tends to stall first because it's connected to the fuselage and doesn't have tip vortices.

Even within a portion of the wing, the onset of stall is a smooth gradient. It's not an either/or thing.

Contrails show up at the wingtips because of vortices and the direction of airflow over the wings. Nothing more.


I stated PV=RT because R is a distict value for each gas. R/n, or script R is a constant. It's just different notation, is all.

Z4K
03-27-2006, 04:12 AM
What a surprise, KrashanTopolova found his way into this thread http://forums.ubi.com/groupee_common/emoticons/icon_rolleyes.gif. What does "...power in the lift available in the lift is expressed" actually mean? Do you make all this stuff up on the fly, or have you run an aerodynamics book through a shredder, and used handfuls of the confetti as a reference? Aircraft designers try and achieve elliptical lift distributions with the lift at the wingtips dropping away to zero - high lift at the wingtips is a guaranteed bucket of induced drag.

As NonWonderDog says, most aircraft have washout sufficient to start the stall at the root where it interferes less with the ailerons.

The vapour trails at the wingtips are a result of the very low pressure inside the wingtip vortices causing condensation. They occur at high wing loading because that's when the vortices are strongest. You can see them phenomenally clearly at the outboard edge of 737 trailing edge flaps when they're landing on cool, humid days. The vortices can be almost 100m long, and you can easily see their structure and watch as they slowly decay.

The stall depends entirely on local angle of attack, reynolds number, surface roughness, and airfoil geometry.



NonWonderDog:

It's refreshing to hear someone describe the stall properly, particularly dispelling the misconception that at the stall lift drops to zero.

Capt.LoneRanger
03-27-2006, 04:36 AM
If caviation is compared to anything in flightsims, a stall is, what comes closest to it.

It happens when the constant stream over the blades is interrupted because the blades turn to fast or in a way that the water-stream is cut and turbulences occur.
Very much like a stall in the water. Accordingly the effectivity decreases dramatically when cavitating.

Z4K
03-27-2006, 04:45 AM
Cavitation is where the pressure in a fluid falls below the vapour pressure of that fluid and becomes a vapour. It's exactly what happens when you boil a liquid. It's bad because: it can cause severe damage when the bubbles collapse and the energy is focused on a tiny point; and because it makes a lot of noise which is bad for naval vessels.

It is not at all like a stall.

A wing doesn't stall because bubbles of some other state of air (plasma? dark matter? Whatever it was Gumby was made of?) form above the wing. The flow just separates. That's all.

Capt.LoneRanger
03-27-2006, 06:14 AM
Okay. But didn't you just explain what cavitation is and I when it happens? I'm not a submarine, but what causes this rapid drop in pressure if my explanation is plain wrong? http://forums.ubi.com/images/smilies/blink.gif