Vapor on surfaces of an Aircraft

DeltaASA16 · October 11th, 2005, 00:57

I've always had a general understanding of the reason for Vapor.. but lately, I've really wanted to know more. The scientific reasons behind it.

1. The Vapor sometimes seen inside jet engine cowlings

2. Vapor over the wings

3. Vapor trails from wing/flap surfaces.

Any info would be appreciated.
THanks!

AV8R N8 · October 11th, 2005, 03:09

As the air moves over the upper side of the wing the air pressure drops. When air pressure drops it also cools the air. When the air is cooled to its dewpoint it will condense into visible moisture. Same thing with air going into a turbine inlet. The jet inlets area gets progressivly larger so that will drop the air pressure. This is all more likely to happen with high humidity. Hope I got this correct.

-Nate

CAVOK · October 11th, 2005, 03:37

yep...thats right.

You won't see the engine condense air at the inlet unless you are in a high power, low airspeed situation (t/o, go-arounds). The motors are sucking a lot of air through them (low pressure) in those flight regimes.

CAVOK · October 11th, 2005, 03:59

Thought I would also point out that the inlet of the engine does not really get larger as you move inward, but the pressure of the air moving through it initially drops during its induction into the engine at low airspeeds, before reaching the start of the compressor section of the engine. The same principle happens for powerplants with a propeller on them as well, watching a C-130 take off on a humid morning is pretty sweet.

This lower pressure at the inlet of a jet engine is also the reason that inlet anti-ice equipment is always needed at relatively warmer temps than that of airframe anti-ice measures.

....ok....bedtime

AV8R N8 · October 11th, 2005, 04:12

This is what I was getting at with the engine inlet. The inlet diverges, in other words the diameter gets larger from front to back. Here is a picture

DeltaASA16 · October 11th, 2005, 18:13

I thought divergence causes an increase in pressure ...

AV8R N8 · October 11th, 2005, 18:17

Yes it does. But at the edge of the inlet its narrow so its going to drop the pressure. Then as the air continues to go inside the inlet the pressure will start to increase.

Chris_Ford · October 11th, 2005, 21:53

Quote:

Originally Posted by AV8R N8

Yes it does. But at the edge of the inlet its narrow so its going to drop the pressure. Then as the air continues to go inside the inlet the pressure will start to increase.

If volume increases (divergence) wouldn't pressure drop? Or do I have Boyle's Law (or is it Charles' Law) completely messed up?

AV8R N8 · October 12th, 2005, 00:29

This picture should help

Foxcow · October 16th, 2005, 14:57

Quote:

Originally Posted by Chris_Ford

If volume increases (divergence) wouldn't pressure drop? Or do I have Boyle's Law (or is it Charles' Law) completely messed up?

as volume increases, velocity will decrease and pressure will increase.

cime_sp · October 18th, 2005, 12:01

Quote:

Originally Posted by Foxcow

as volume increases, velocity will decrease and pressure will increase.

That's great and all...but not exactly correct. It works for hydraulics and rivers/streams, etc... but in this case you have a very large fan and turbine SUCKING from the other side. So, due to this a vacuum will be created in the space ahead of the turbine/fan. It doesn't really matter so much if the space is converging or diverging....all of that air will be a lower pressure than the air outside/ahead of the engine cowling.

Foxcow · October 18th, 2005, 13:36

Quote:

Originally Posted by cime_sp

That's great and all...but not exactly correct. It works for hydraulics and rivers/streams, etc... but in this case you have a very large fan and turbine SUCKING from the other side. So, due to this a vacuum will be created in the space ahead of the turbine/fan. It doesn't really matter so much if the space is converging or diverging....all of that air will be a lower pressure than the air outside/ahead of the engine cowling.

You are right, that explanation works BEST with a fixed/closed system. However, I do not ever remember from my classes or reading that your explanation is correct. From what I understand, the pressure drops in an engine inlet because of the increase in velocity. Pressure and speed is also regulated with convergence and divergence intakes. A supersonic inlet is going to have a diverging intake to slow the air down before it hits the compressor

October 11th, 2005, 00:57	#1
DeltaASA16 Junior Member Join Date: May 2003 Location: Atlanta, GA Posts: 82	Vapor on surfaces of an Aircraft I've always had a general understanding of the reason for Vapor.. but lately, I've really wanted to know more. The scientific reasons behind it. 1. The Vapor sometimes seen inside jet engine cowlings 2. Vapor over the wings 3. Vapor trails from wing/flap surfaces. Any info would be appreciated. THanks! __________________ My Aviation Photos

October 11th, 2005, 03:37	#3
CAVOK Junior Member Join Date: Jan 2004 Location: Los Angeles Posts: 123	yep...thats right. You won't see the engine condense air at the inlet unless you are in a high power, low airspeed situation (t/o, go-arounds). The motors are sucking a lot of air through them (low pressure) in those flight regimes. __________________ The most beautiful dream that has haunted the heart of man since Icarus is today reality. — Louis Bleriot At least they don't put their feminine ointments next to the mustard, Lois. That was the worst hot dog I ever ate. --Peter Griffin, The Family Guy

October 11th, 2005, 03:59	#4
CAVOK Junior Member Join Date: Jan 2004 Location: Los Angeles Posts: 123	Thought I would also point out that the inlet of the engine does not really get larger as you move inward, but the pressure of the air moving through it initially drops during its induction into the engine at low airspeeds, before reaching the start of the compressor section of the engine. The same principle happens for powerplants with a propeller on them as well, watching a C-130 take off on a humid morning is pretty sweet. This lower pressure at the inlet of a jet engine is also the reason that inlet anti-ice equipment is always needed at relatively warmer temps than that of airframe anti-ice measures. ....ok....bedtime __________________ The most beautiful dream that has haunted the heart of man since Icarus is today reality. — Louis Bleriot At least they don't put their feminine ointments next to the mustard, Lois. That was the worst hot dog I ever ate. --Peter Griffin, The Family Guy

October 11th, 2005, 18:13	#6
DeltaASA16 Junior Member Join Date: May 2003 Location: Atlanta, GA Posts: 82	I thought divergence causes an increase in pressure ... __________________ My Aviation Photos

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October 11th, 2005, 03:09	#2
AV8R N8 Newbie Join Date: Oct 2005 Location: Ventura County, CA Posts: 5	As the air moves over the upper side of the wing the air pressure drops. When air pressure drops it also cools the air. When the air is cooled to its dewpoint it will condense into visible moisture. Same thing with air going into a turbine inlet. The jet inlets area gets progressivly larger so that will drop the air pressure. This is all more likely to happen with high humidity. Hope I got this correct. -Nate

October 11th, 2005, 04:12	#5
AV8R N8 Newbie Join Date: Oct 2005 Location: Ventura County, CA Posts: 5	This is what I was getting at with the engine inlet. The inlet diverges, in other words the diameter gets larger from front to back. Here is a picture

October 11th, 2005, 18:17	#7
AV8R N8 Newbie Join Date: Oct 2005 Location: Ventura County, CA Posts: 5	Yes it does. But at the edge of the inlet its narrow so its going to drop the pressure. Then as the air continues to go inside the inlet the pressure will start to increase.

October 12th, 2005, 00:29	#9
AV8R N8 Newbie Join Date: Oct 2005 Location: Ventura County, CA Posts: 5	This picture should help