Why do jets leave a white trail behind them.

"Morgans" wrote


"Jim Macklin" wrote in
message news:kbWzf.66479$QW2.33865@dukeread08...
Most jet aircraft have heated fuel tanks and or use PRIST.
The fuel control units are heated by engine oil or electric
elements so the screens stay ice free.
\\
Yes, I do know all of that, but do smaller jet fueled planes
(Malibu and smaller) have heated tanks?

I don't know about the Malibu, but the large jet transports that I
have flown, B-707, B-727, certainly did not have heated fuel tanks.
Yep, the fuel filters had a heating capability using bleed air from
the engine compressors.....flightcrew controlled. At higher Mach
numbers (.80+), the temperature ram rise at the wing leading edge
kept the fuel from waxing under most circumstances.

Bob Moore

"Bob Moore" wrote\

I don't know about the Malibu, but the large jet transports that I
have flown, B-707, B-727, certainly did not have heated fuel tanks.
Yep, the fuel filters had a heating capability using bleed air from
the engine compressors.....flightcrew controlled. At higher Mach
numbers (.80+), the temperature ram rise at the wing leading edge
kept the fuel from waxing under most circumstances.

Humm, that surprises me. Do they have excess fuel returned to the tank,
which could carry some heat with it?

No worries with the Malibu, since the Mach numbers will keep things nice and
toasty. g
--
Jim in NC

On Thu, 19 Jan 2006 16:44:08 GMT, "AliR" wrote:

Hi everyone,

Me and a friend are wondering why jets leave a white trail behind them. My
guess is that it's moisture being compressed as it passes through the
turbine (because some leave a single trail, while others leave two separate
trails). My friend thinks it's because of moisture being cooled as it
passes over the wing (Bernoulli effect).

While flying a bonanza I have seen a small white trail extending from the
wingtip for a little while right after I break through the clouds
(descending).

Is either one of us right?

While I've never seen condensation with my Warrior, I do have some
observations regarding condensation trails or contrails. There are
different types of contrails.

1. Old WWII newsreels showing B-17s flying over Europe. They're
leaving massive contrails from each of those big round engines. I've
read articles saying that during the war they tried to avoid
altitudes/temperatures where that would occur while over enemy
territory since it would make make the aircraft more visible. Just in
case anyone thought this phenomenon was exclusively a jet
characteristic.
http://www.wpafb.af.mil/museum/resea...s/b17f-10a.jpg

2. TV news coverage of the Space Shuttle landing in Florida (where the
air is often more moist than at Edwards). As the shuttle flew admidst
a few cloud buildups just before touching down, a trail of cloud
appeared at each wingtip. Particularly interesting since the shuttle's
engines aren't operating during the landing phase.

3. Scene from the TV show "Black Sheep Squadron" (AKA "Baa Baa Black
Sheep") showing the Corsairs as they made a tight turn. A whisp of
contrail appeared at each wingtip during the high-g maneuver.

4. At an airshow, the B-1 Bomber come to show off its stuff. As the
last part of the demo, it accelerated to just below mach 1 to thrill
the crowd. As it flew along, the entire amidships of the aircraft was
enshrouded in a white cloud during the time the aircraft was operating
at high speed. Didn't leave much of a trail, but impressive phenomenon
all the same.

5. Films of the moon launch with the Saturn V rocket. I've seen this
footage several times, including I think the movie "Apollo 13." As the
rocket accelerates, a ring of cloud appears around the upper end of
the rocket. I've noticed the same thing on TV coverage of Space
Shuttle launches. Again, no trail.

6. Somewhat unrelated. Once while VFR over a smooth undercast over
Asheville, NC (AVL) I saw that nearby Mt. Mitchell was adorned by a
lovely lens cloud on its downwind side. While the air where I was was
smooth as glass, I can only assume that things wouldn't be so pleasant
over near the mountain. Lens clouds are, of course, formed by the
pressure changes due to high winds over the mountain, similar to some
of the aerodynamic contrails I've noted above. (BTW, in case anyone is
wondering, the undercast cleared out as promised by FSS at my
destination of TRI.)

Just a few examples I remember seeing. Hopefully, someone else can
expand on this interesting subject.

RK Henry

"Skywise" wrote in message
...
Others have already explained how there is water diluted in the
fuel, but I think you forgot something. What do you burn that
hydrocarbon with? That's right! O2. When the fuel bruns, some of
that H combines with some of that O2 and you end up with some
H2O.


Nearly all of the H should combine with O2 to make H2O, nearly all of the C
should combine with O2 to make CO2. 95% to 99% would be ball park numbers.
CO goes up as you go rich a lot faster than unburned HC in a piston engine.

Contrails are no different than the white water vapor you see behind cars on
a cold day.

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

Jim Macklin wrote:

It is hydroscopic and absorbs water. The water in the
"spaces" between the hydrocarbon is where the fungus grows.

I still believe that the water contained in the fuel is a small part of
the water contained in the exhaust that is formed when the hydrocarbon
is combusted.


Matt

Morgans wrote:


"Matt Whiting" wrote

If jet fuel is full of water, how do you get it to burn? When I took
chemistry, hydrocarbon fuels contained .... drum roll please ...
carbon and hydrogen. No water of any level of significance.

\\
Combustion leaves some hydrogen unclaimed, and the oxygen is really
happy to claim it, and when it combines two H's, and one O, you get water.

Weren't awake during that day of chemistry, were you, Matt? g

That was my point precisely. The water isn't IN the fuel. It is formed
by the fuel and the oxygen in the air via the combustion process. That
isn't at all the same as saying that the water is IN the fuel.


Matt

"Matt Whiting" wrote

I still believe that the water contained in the fuel is a small part of
the water contained in the exhaust that is formed when the hydrocarbon is
combusted.

Sure, some, but remember, the piston heavy bombers left a real nice
contrail, at somewhat lower altitudes, and there was no water in that fuel
being burned, no doubt.
--
Jim in NC

"Matt Whiting" wrote

That was my point precisely. The water isn't IN the fuel. It is formed
by the fuel and the oxygen in the air via the combustion process. That
isn't at all the same as saying that the water is IN the fuel.

My misinterpretation. Sorry.

As far as water in the fuel as a contaminate, not a problem getting it to
burn, especially at the temps that occur inside a jet engine combustion
chamber. Any water in the fuel is very quickly evaporated, then there is no
problem getting the combustion to take place. The biggest problem (I'm
guessing) would be at startup, when temps are lower, initially. That all
changes in a hurry, as hot starts are a big thing to avoid.

A couple of things come to mind, when talking about water vapor in the
combustion chamber, which is what the water will be at these temperatures.
Air has a lot of water vapor in it, before it is compressed, and we all know
that works OK. Also, some engines, jet engines included (the Harrier is the
best know example) use water injection to control temperatures and add
efficiency. The Harrier uses a LOT of water, and although most people don't
know it, the water capacity is what determines how long the Harrier can
hover on each flight, because without water injection, the engine will
overheat.

To the other person that said there is not enough left over H to make water
with the O in the air, sorry, incorrect. Water and heat are two byproducts
of combustion. I wish I remembered enough chemistry to balance the
equation. No, I don't! g
--
Jim in NC

Morgans wrote:


"Matt Whiting" wrote

I still believe that the water contained in the fuel is a small part
of the water contained in the exhaust that is formed when the
hydrocarbon is combusted.


Sure, some, but remember, the piston heavy bombers left a real nice
contrail, at somewhat lower altitudes, and there was no water in that
fuel being burned, no doubt.

Precisely my point. The contrail is formed not from water IN the fuel,
but by water FORMED when the fuel and air are combusted.

Matt

Morgans wrote:

To the other person that said there is not enough left over H to make
water with the O in the air, sorry, incorrect. Water and heat are two
byproducts of combustion. I wish I remembered enough chemistry to
balance the equation. No, I don't! g

I do remember enough to balance a simple reaction like this. All you
need to do is tell me which hydrocarbon you want to combust! I don't
know the formula for avgas. :-)


Matt