Vapour trails

Hello,

Question from a complete landlubber.

I work outdoors beneath the flightpath to Edinburgh Airport.

I take it that vapour trails are the condensed water contained in the
air ingested by the engines. My question is, how much air does an engine
ingest at cruise? How would you visualise that amount of air?

Yours in amazement at how you stay aloft,

Michael

In article ,
Michael Calwell wrote:

Hello,

Question from a complete landlubber.

I work outdoors beneath the flightpath to Edinburgh Airport.

I take it that vapour trails are the condensed water contained in the
air ingested by the engines. My question is, how much air does an engine
ingest at cruise? How would you visualise that amount of air?

Yours in amazement at how you stay aloft,

Michael


Vapor trails are also the condensed ice crystals resulting from just the
movement of the plane through the air. On a humid day, I can pull
contrails off my prop tips.

"Michael Calwell" wrote in message
...
Hello,

Question from a complete landlubber.

I work outdoors beneath the flightpath to Edinburgh Airport.

I take it that vapour trails are the condensed water contained in the
air ingested by the engines. My question is, how much air does an engine
ingest at cruise? How would you visualise that amount of air?

Yours in amazement at how you stay aloft,

Michael

It is not the vapor from the air you are seeing, it is the water that is
produced as a byproduct of burning hydrocarbons.

Ever see water dripping from your car tailpipe, especially in the morning
when you start it up? Water is being produced, and being condensed on the
cold sides of the tailpipe, then it drips out. When the tailpipe gets
warm, the water stays invisible, so you don't see clouds unless the outside
temperature gets very very cold, then you get clouds coming from the back of
the car, as the result of the exhaust cooling rapidly, and the vapor in
exhaust condenses to become visible.

This is similar to what is happening when you see contrails. They almost
always are very high, so the water vapor condenses and then freezes into ice
crystals almost instantly, so then it is harder for the water to
re-evaporate into the surrounding air. This is the reason con trails are
visible for a long time after the aircraft passes by.

The vapor that you see off the tips of props, or from the topside of fighter
jet wings when they are pulling high G-forces are from the pressure in the
air being reduced very quickly, which causes the water vapor to become
visible for an instant, then disappearing just as quickly when the pressure
is returned back to normal.

As far as the amount of air being passed through an engine, for a
reciprocating engine, the amount is much smaller than a jet engine. Gallons
per second for a piston engine, and hundreds of gallons for a jet engine?
Something like that.

I hope this has helped.
--
Jim in NC

As far as the amount of air being passed through an engine, for a
reciprocating engine, the amount is much smaller than a jet engine. Gallons
per second for a piston engine, and hundreds of gallons for a jet engine?
Something like that.

Let me see..... The fuel-air mixture for a piston engine is about 14
pounds of air per pound of avgas, is it not? Would it be about the
same for jet fuel?

How "big" a pound of air is depends on pressure and temperature
(Boyles' and Charles' Laws, if I recall correctly), but I can't even
guess how big a box it would take to hold a pound of air at STP.
Perhaps someone else can. I guess any good chemist could do it.

vince norris

"vincent p. norris" wrote in message
The fuel-air mixture for a piston engine is about 14
pounds of air per pound of avgas, is it not? Would it be about the
same for jet fuel?

Jet fuel averages 6.7 pounds per gallon with more BTUs, so the
stoichiometric ratio is slightly different. Much of the air ingested by a
jet engine is used for cooling, not for burning. Do we include this air as
being ingested? Do we include the fan's cold stream as being ingested?

D.

"Capt.Doug" wrote

Jet fuel averages 6.7 pounds per gallon with more BTUs, so the
stoichiometric ratio is slightly different.

More air, to take advantage of the BTU's, right? Plus jets are more
efficient at altitude, so more air again, right?.

Much of the air ingested by a
jet engine is used for cooling, not for burning. Do we include this air as
being ingested?

For what we were talking about, which is how much air is being used to make
con trails, (my take on it) it would seem to me we are talking about the air
that is being used to burn fuel.

Do we include the fan's cold stream as being ingested?

I wouldn't. It's just a big fancy prop, and props are not making any con
trails.

Another thing that is being overlooked, is the HP rating of the engine. In
talking about the air being injested, we have to remember that piston
engines are at most making a couple thousand HP (most lots less than that)
and the turbine engines on large jetliners are making multiples more power,
burning more fuel, using more air, and making more water vapor, and making
bigger contrails.

I'm no expert on this stuff, but I think my thinking (and guestimates) are
about right. After all, the original question was not a highly defined,
quanitative question, and neither is the answer. g
--
Jim in NC

"Morgans" wrote in message Plus jets are more
efficient at altitude, so more air again, right?.

No, less air, because the density of the ambient air is less as altitude
rises. Less air in the front means less air out the back (though the
pressure ratio can be the same). Jet engines produce less thrust at
altitude. There is less cooling air which means that maximum exhaust
temperature is reached at a lower thrust. The efficiency gains come from the
forward speed of the engine (sort of a ram effect) and the lower aerodynamic
drag at altitude (higher true airspeed).

Another thing that is being overlooked, is the HP rating of the engine.

Turbojets have no torque and therefore have no horsepower. There is an
equation for 'equivalent horsepower' which involves an airspeed of around
375 mph.

I'm no expert on this stuff, but I think my thinking (and guestimates) are
about right.

If you are more confused now than before, you get an A+!

D.

Jets use something like 100% "excess air" in their combustion process,
unlike piston engines. Much of this air is used for internal engine cooling
and some is used as "bleed air" for cabin pressurization, de-icing, etc.

Rod
"vincent p. norris" wrote in message
...
As far as the amount of air being passed through an engine, for a
reciprocating engine, the amount is much smaller than a jet engine.
Gallons
per second for a piston engine, and hundreds of gallons for a jet engine?
Something like that.

Let me see..... The fuel-air mixture for a piston engine is about 14
pounds of air per pound of avgas, is it not? Would it be about the
same for jet fuel?

How "big" a pound of air is depends on pressure and temperature
(Boyles' and Charles' Laws, if I recall correctly), but I can't even
guess how big a box it would take to hold a pound of air at STP.
Perhaps someone else can. I guess any good chemist could do it.

vince norris

Air is pretty heavy, which is why we can fly. It weighs .078
lb/cubic foot at standard sea level pressure and temperature. What's
that, about 13 cubic feet for a pound? The air in a room can easily
outweigh the occupants.

Dan

In article .com,
wrote:

Air is pretty heavy, which is why we can fly. It weighs .078
lb/cubic foot at standard sea level pressure and temperature. What's
that, about 13 cubic feet for a pound? The air in a room can easily
outweigh the occupants.

Dan

I seem to recall .002378 #m/ft3 as air density at STP.