Vapor on surfaces of an Aircraft

DeltaASA16

DeltaASA16

Well-Known Member
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!
 
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
 
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.
 
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
 
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
 
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.
 
AV8R N8 said:
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.
Click to expand...

:confused:
If volume increases (divergence) wouldn't pressure drop? Or do I have Boyle's Law (or is it Charles' Law) completely messed up?
 
Chris_Ford said:
:confused:
If volume increases (divergence) wouldn't pressure drop? Or do I have Boyle's Law (or is it Charles' Law) completely messed up?
Click to expand...



as volume increases, velocity will decrease and pressure will increase.
 
Foxcow said:
as volume increases, velocity will decrease and pressure will increase.
Click to expand...

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.
 
cime_sp said:
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.
Click to expand...



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
 
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