ba757gla
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Why Do I Never See Turbprops Making Contrails?

Fri Jun 16, 2006 9:37 pm

i live under flight path to INV and some to ABZ. I have seen flybe dash8s high up but i have never seen them pull contrails despite people on a neyt saying that they pull contrails! so why dont they ? is it you have to be over FL30 TO PULL contrails?
 
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Starlionblue
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RE: Why Do I Never See Turbprops Making Contrails?

Fri Jun 16, 2006 10:00 pm

Quoting Ba757gla (Thread starter):
is it you have to be over FL30 TO PULL contrails?

You do not. Contrails depend on temperature. When it's very cold (say, at the South Pole), contrails are quite possible on the runway.

I'm pretty sure I've seen pics of Tu-95s with contrails.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
keta
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RE: Why Do I Never See Turbprops Making Contrails?

Fri Jun 16, 2006 10:16 pm

Quoting Ba757gla (Thread starter):
Why Do I Never See Turbprops Making Contrails?

Because they don't bother puting chemicals on turboprops, it's enough having chemtrails on jets  Wink

Seriously, though...

Quoting Starlionblue (Reply 1):
I'm pretty sure I've seen pics of Tu-95s with contrails

Googled a little bit, here you go:

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pilotaydin
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 1:35 am

there may be contrails but you might not see them, i guess it also has to do with how much thrust comes from the fan and how much from the core of the engine....i think the more thrust from the core, makes it a more favorable environment for contrail formation
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Starlionblue
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 1:57 am

Quoting Pilotaydin (Reply 3):
there may be contrails but you might not see them, i guess it also has to do with how much thrust comes from the fan and how much from the core of the engine....i think the more thrust from the core, makes it a more favorable environment for contrail formation

Sounds right. Since contrails are formed by hot air from the core being cooled to a point where water droplets form, a turbofan with cool air that envelops the core air more completely will have more contact area than a turboprop.
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SlamClick
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 2:04 am

Quoting Pilotaydin (Reply 3):
i guess it also has to do with how much thrust comes from the fan and how much from the core of the engine..

Not exactly...

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Photo © Bailey - AirTeamImages



I believe that the drop in air pressure over the top of the wings can produce the temperature drop necessary. (The Bernoulli Coulli)
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miamiair
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 2:16 am

What about those pictures of B-17's during WW2? It looked like a road map of New Jersey. Piston powered engines will cause contrails as well.
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KDTWflyer
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 3:44 am

B-17 contrails..

http://www.af.mil/shared/media/photodb/photos/020903-F-9999b-006.jpg
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David L
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 3:58 am

Quoting Kdtwflyer (Reply 7):

From the propeller tips, too. Cool!
 
411A
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 4:29 am

Those who are old enough to remember the many overflights of B-36 bombers during the 1950's will surely recall the contrails they left behind...a LOT.

Also, you could hear these aircraft before you could see 'em, way up high above 30,000 feet.

No, not engine noise...propellor noise.
MUCH lower frequency.
 
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 5:05 am

Quoting Kdtwflyer (Reply 7):
B-17 contrails..

All the contrails shows all the props are turning in the same direction. Not the opposite on the other wing.
I thought they'd have opposite rotating props on the opposite wing?
 
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 5:29 am

Quoting 777WT (Reply 10):
I thought they'd have opposite rotating props on the opposite wing

I think that's pretty rare. Why complicate things by not keeping all the engines and prop gear the same?
 
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 5:32 am

Aha! I thought there was something about this recently...
Do Turboprop Multis Counter Rotate? (by KELPkid May 18 2006 in Tech Ops)
 
FredT
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 6:16 am

Above a certain altitude, contrails will form. Below it, it will not.

Turboprops fly at lower altitudes than jets.

Hence, it is rare to see contrails behind turboprops. Above the contrail altitude, they will generate contrails. As will recips, for that matter.

Pressure drops, such as those found in vortices coming off wings, propeller tips, flaps, and in the low pressure region above the wing, generate a drop in temperature (recall the formula for an ideal gas, pV = nRT). If the dewpoint is close to the ambient temperature, this temperature drop will put the temperature below the dewpoint and mist will form. It will typically dissipate fast though, normally forming just a small region of mist in the affected region.

Cheers,
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2H4
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 6:41 am



There was an interesting thread about contrails awhile back:

http://www.airliners.net/discussions/tech_ops/read.main/149810




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SlamClick
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 8:04 am

Quoting 411A (Reply 9):
old enough to remember the many overflights of B-36 bombers during the 1950's

Remember them well.

Thirty six of them in six cells of six, came over our house at maybe two thousand feet once. Staggering! But at altitude they, and the B-50s and other big recips would draw a con from one horizon to the other.

Quoting 411A (Reply 9):
you could hear these aircraft before you could see 'em, way up high above 30,000 feet.

No, not engine noise...propellor noise.

That was a wonderful sound. You get just a taste of it in Strategic Air Command with Jimmy Stewart.

Last time I saw a B-36 in flight must have been about 1958 or so at Hamilton AFB, California (KSRF) A pair of them did a low flyby during open house. Announcer said they were at 1500' and they blotted out the sun.
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SlamClick
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 8:12 am

Quoting 777WT (Reply 10):
All the contrails shows all the props are turning in the same direction. Not the opposite on the other wing.
I thought they'd have opposite rotating props on the opposite wing?

If I'm not mistaken they'd have been the first airplane ever to have such an innovation if they'd had. They predate the P-38 by quite a bit.

It is just not so much a problem with four engines. If you lose two on one side you've in for a bad day, but just losing one - well, you've still got three. You can even pull the symmetric engine back a little and still have half your thrust.

Any gains in lower VMC would have been offset by lack of commonality in parts. And consider that when that plane was in its heyday it was flying out of England at the far end of a very long supply train that reached all the way back to Seattle. Spare parts were rarely put on airplanes for the trip because they mainly needed to carry gasoline to cross the Atlantic. Most spares got put on ships and the German U-boats were in the business of preventing their delivery. So the engine and prop on a B-17 would also hang on a number of other airplanes. Commonality was your friend.
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Newark777
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 8:27 am

Quoting SlamClick (Reply 5):
I believe that the drop in air pressure over the top of the wings can produce the temperature drop necessary

Happens right on the ground:


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Photo © Stefan Sjögren - Stockholm Arlanda Photography



Although that is a different phenomenon than the contrails forming behind engines from freezing moisture being expelled by the engines.

I have a question about the aerodynamic contrails in the picture before. On the ground what is happening is that the air rushing over the wing looses pressure, therefore cooling, and dropping below the dew point, causing the condensation. Is this simply the same thing happening at a higher altitude, or is there something more complicated going on?

Harry
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 8:55 am

Quoting Newark777 (Reply 17):
Is this simply the same thing happening at a higher altitude, or is there something more complicated going on?

At higher altitude you're seeing moisture from the burning fuel being added to the cold, moisture saturated air, resulting in ice formation. That's why you get contrails from any airplane that burns fuel that has water as a product of burning when the atmosphere is saturated with water. It doesn't have to be a jet, it depends on the temperature and water content of the air.

On the ground you're seeing moisture saturated air cooled until mist precipitates out. So the ground contrails are water, high altitude contrails are ice particles.

[Edited 2006-06-17 01:58:21]
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Newark777
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:04 am

Quoting Bobster2 (Reply 18):

At higher altitude you're seeing moisture from the burning fuel being added to the cold, moisture saturated air, resulting in ice formation. That's why you get contrails from any airplane that burns fuel that has water as a product of burning when the atmosphere is saturated with water. It doesn't have to be a jet, it depends on the temperature and water content of the air.

Yes, but this doesn't explain the "contrails" coming off the wings, not the engines, linked above, which I was referring to.

Harry
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2H4
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:14 am




Quoting Newark777 (Reply 19):
Yes, but this doesn't explain the "contrails" coming off the wings

Most contrails at altitude are produced by the engines, but then affected by the wake vorticies from the wingtips:


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Photo © Josef P. Willems






Quoting Bobster2 (Reply 18):
So the ground contrails are water, high altitude contrails are ice particles.

Contrails don't care about their proximity to the ground. They only care about the ambient atmospheric conditions. If the proper atmospheric conditions exist on the ground, you'll get contrails:

http://polar4.home.att.net/feb-04-pics/contrails-take-off-Baker-cr.jpg




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Starlionblue
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:39 am

Quoting Bobster2 (Reply 18):
At higher altitude you're seeing moisture from the burning fuel being added to the cold, moisture saturated air, resulting in ice formation. That's why you get contrails from any airplane that burns fuel that has water as a product of burning when the atmosphere is saturated with water. It doesn't have to be a jet, it depends on the temperature and water content of the air.

On the ground you're seeing moisture saturated air cooled until mist precipitates out. So the ground contrails are water, high altitude contrails are ice particles.

Good point. In cold weather, cars produce contrails.
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Newark777
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:41 am

Quoting 2H4 (Reply 20):

Most contrails at altitude are produced by the engines, but then affected by the wake vorticies from the wingtips:

Yes, but I was specifically referring to the photo linked already by SlamClick:


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It is clear much of that is NOT coming from the engines.

Harry
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Bobster2
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:44 am

Quoting Newark777 (Reply 19):
but this doesn't explain the "contrails" coming off the wings, not the engines

I tried to explain in my second paragraph, but I omitted some details. The air flow over the wings causes the temperature to drop above the wings, because a drop in pressure results in a lower temperature. If the air is already saturated with water, the temperature drop will force some of the water to precipitate out as mist. So in this type of contrail, you're just dealing with water that was already there, it is not necessary to add water; the steam coming from the engine is hot and the air isn't cold enough to freeze it.

Quoting 2H4 (Reply 20):
Contrails don't care about their proximity to the ground.

Well, I oversimplified.  Smile The air temperature on the ground is usually much warmer than the air temperature at the South Pole. In most parts of the world, you have to get to a high altitude in order for the air to be cold enough to turn heated steam into ice in a few seconds. This type of contrail requires water to be added to the air, the water comes from the burning fuel.
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:47 am

Quoting Newark777 (Reply 22):
It is clear much of that is NOT coming from the engines.

In that case, the caption on the photo answers your question. The aerodynamics (in other words, the pressure drop and temperature drop above the wings) causes the part of the contrail that isn't coming from the engines.
"I tell you this, no eternal reward will forgive us now for wasting the dawn." Jim Morrison
 
ba757gla
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:49 am

thanks for the feed back
 
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:50 am

Quoting Newark777 (Reply 19):
Yes, but this doesn't explain the "contrails" coming off the wings, not the engines, linked above, which I was referring to.

Harry

I can think of (at least) three possible ways to create contrails:

1. Water vapor in engine exhaust condenses and freezes into a contrail. These are usually only found at high altitudes.


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Photo © TriplET
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Photo © Sigurdur Benediktsson - AirTeamImages




2. Low pressure created over the wing, in engine inlets, or in vortices spun off from wingtips, the edges of flaps, propellor tips, etc. causes water vapor to condense into visible trails. These are usually found only at takeoff and landing and are probably more commonly called vortices, although condensation is involved.


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Photo © Steve Morris
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Photo © Robert Budde



The first two are by far the most common, but I can think of a third way:

3. In order to condense into liquid water or solid ice crystals, water vapor needs a solid surface to condense around, such as a particle of dust or smoke, called a nucleation site. These can be rare in the upper atmosphere, and if humidity is high, the temperature is low, and nucleation sites are not available, the result is a "supersaturation" of water vapor in the atmosphere without clouds forming.

When an aircraft encounters supersaturated air, the airframe will provide a nucleation surface for condensation and water vapor will instantly condense or freeze around it. This appears to be happening in the photos below, where the contrail is clearly not just the result of engine exhaust. Condensation due to low pressure over the wing may also help in the process, but note that in the left photo, condensation appears to take place around the nose as well.


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Photo © Jeffwell
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Photo © Jeffwell



As mentioned, this type of contrail would be much rarer because it depends on very particular meteorological conditions. http://en.wikipedia.org/wiki/Cloud_condensation_nuclei and related pages have more on the mechanics of water vapor supersaturation and cloud condensation.

--B2707SST
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Newark777
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:53 am

Quoting Bobster2 (Reply 23):
So in this type of contrail, you're just dealing with water that was already there, it is not necessary to add water; the steam coming from the engine is hot and the air isn't cold enough to freeze it.

Right, I understand the whole concept of condensation forming close to the ground. I'm just trying to figure out what is going on in the picture, which appears to be at or close to cruising altitude. Since it is already probably really cold, I doubt that the lower temperature caused by the aerodynamics of the aircraft would cause the same effect that is happening near the ground, where the air is dropping below the dew point. So there are two types of contrails:

1) Those that form in the warm air, caused by air pressure being lowered, and
2) From the engines in cold air, which is water vapor being frozen.

The above example doesn't seem to fit in either of these cases, and here is another example:


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Photo © Bailey - AirTeamImages



Harry
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Bobster2
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:54 am

Quoting Starlionblue (Reply 21):
Good point. In cold weather, cars produce contrails

Yes, but only when the engine is cold. The cold engine has excess water that gets blown out the exhaust for a few minutes. A hot car doesn't usually make contrails because the steam is too hot to condense quickly, it gets dispersed into the air before it has a chance to freeze.
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Bobster2
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 9:59 am

Quoting Newark777 (Reply 27):
Since it is already probably really cold, I doubt that the lower temperature caused by the aerodynamics of the aircraft would cause the same effect that is happening near the ground, where the air is dropping below the dew point.

OK. You're correct.

Thanks to B2707SST in Reply 26 for clearing up the issue that was confusing both of us.  Smile
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ba757gla
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 10:06 am

CONTRAILS very interesting!1
 
Newark777
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 10:18 am

Quoting B2707SST (Reply 26):

3. In order to condense into liquid water or solid ice crystals, water vapor needs a solid surface to condense around, such as a particle of dust or smoke, called a nucleation site. These can be rare in the upper atmosphere, and if humidity is high, the temperature is low, and nucleation sites are not available, the result is a "supersaturation" of water vapor in the atmosphere without clouds forming.

When an aircraft encounters supersaturated air, the airframe will provide a nucleation surface for condensation and water vapor will instantly condense or freeze around it. This appears to be happening in the photos below, where the contrail is clearly not just the result of engine exhaust. Condensation due to low pressure over the wing may also help in the process, but note that in the left photo, condensation appears to take place around the nose as well.

Thanks a lot, that is exactly what I was looking for. Thanks to Bobster also.

Harry
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Bobster2
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 11:01 am

It's possible for the wings to be much colder than the surrounding air after the fuel gets cold soaked at high altitude and the plane descends to warmer air. Maybe that's another factor in forming contails from the wings.
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2H4
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 11:08 am




Quoting Bobster2 (Reply 32):
It's possible for the wings to be much colder than the surrounding air after the fuel gets cold soaked at high altitude and the plane descends to warmer air. Maybe that's another factor in forming contails from the wings.

Good point. It could be the condensation of water on the surface of the wing.




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CosmicCruiser
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 17, 2006 11:25 pm

Quoting SlamClick (Reply 16):
So the engine and prop on a B-17 would also hang on a number of other airplanes. Commonality was your friend.

Absolutely, I had the pleasure of seeing the Memphis Belle during it's renovation and not only did it have 8 engines during it's tour but they found a piece of wing skin with the original Boeing writing on it that indicated that it came from another B-17.

Quoting SlamClick (Reply 16):
They predate the P-38 by quite a bit.

If the old P-38 pilot that I used to work with was correct the P-38 had counter rotating props that BOTH went outboard making each eng. crtical! more than likely they didn't know or think about it then.
 
XFSUgimpLB41X
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RE: Why Do I Never See Turbprops Making Contrails?

Sun Jun 18, 2006 2:11 am

Rumor mill: I heard they both went outboard for increased instability and better combat manueverability.
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SuperDan
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RE: Why Do I Never See Turbprops Making Contrails?

Sun Jun 18, 2006 2:56 am

Quoting XFSUgimpLB41X (Reply 35):
I heard they both went outboard for increased instability and better combat manueverability.

In this thread about counter rotation RE: Do Turboprop Multis Counter Rotate? (by 2H4 Jun 2 2006 in Tech Ops)#ID155788 , they mentioned that the ones with inboard counter rotation created too much vibration on the fuselage.
 
L-188
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RE: Why Do I Never See Turbprops Making Contrails?

Sun Jun 18, 2006 10:13 am

Quoting Bobster2 (Reply 18):
At higher altitude you're seeing moisture from the burning fuel being added to the cold, moisture saturated air, resulting in ice formation. That's why you get contrails from any airplane that burns fuel that has water as a product of burning when the atmosphere is saturated with water. It doesn't have to be a jet, it depends on the temperature and water content of the air.

Exactly.

All fuel has a % of water. Through filteration we trying and keep that amount as small as possible, but all engines are capable of putting out contrails.

If my truck was at FL300 doing 400 knots it would be a contrail. At 55 MPH on the ground the ice particals form ice fog at a warm -5F Alaskan winter.
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FredT
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RE: Why Do I Never See Turbprops Making Contrails?

Tue Jun 20, 2006 4:46 am

The water isn't in the fuel. It is created through combustion.

http://en.wikipedia.org/wiki/Combustion
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L-188
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RE: Why Do I Never See Turbprops Making Contrails?

Tue Jun 20, 2006 10:04 am

Quoting FredT (Reply 38):
The water isn't in the fuel. It is created through combustion.

Wikpedia is only half right.

Water is a byproduct, but there is also water suspended in all fuels, that water doesn't burn.
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FredT
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RE: Why Do I Never See Turbprops Making Contrails?

Tue Jun 20, 2006 4:25 pm

...but is a very marginal contribution to the forming of contrails.
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bongo
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RE: Why Do I Never See Turbprops Making Contrails?

Sat Jun 24, 2006 4:40 am

Quoting Ba757gla (Thread starter):
is it you have to be over FL30 TO PULL contrails?

That only happens on the Flight Simulator...always at FL30 or higher....only.
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