Ba757gla From United Kingdom (Scotland), joined Mar 2006, 716 posts, RR: 2 Posted (3 years 5 months 2 weeks 4 days 18 hours ago) and read 2278 times:
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?
Pilotaydin From Turkey, joined Sep 2004, 2237 posts, RR: 51 Reply 3, posted (3 years 5 months 2 weeks 4 days 14 hours ago) and read 2185 times:
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
The only time there is too much fuel onboard, is when you're on fire!
Starlionblue From Greenland, joined Feb 2004, 13588 posts, RR: 68 Reply 4, posted (3 years 5 months 2 weeks 4 days 13 hours ago) and read 2166 times:
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.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
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?
FredT From United Kingdom, joined Feb 2002, 2101 posts, RR: 30 Reply 13, posted (3 years 5 months 2 weeks 4 days 9 hours ago) and read 2031 times:
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,
Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
SlamClick From United States, joined Nov 2003, 9996 posts, RR: 79 Reply 15, posted (3 years 5 months 2 weeks 4 days 7 hours ago) and read 1978 times:
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.
Happiness is not seeing another trite Ste. Maarten photo all week long.
SlamClick From United States, joined Nov 2003, 9996 posts, RR: 79 Reply 16, posted (3 years 5 months 2 weeks 4 days 7 hours ago) and read 1971 times:
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.
Happiness is not seeing another trite Ste. Maarten photo all week long.
Newark777 From United States, joined exactly 5 years ago today! , 8796 posts, RR: 41 Reply 17, posted (3 years 5 months 2 weeks 4 days 7 hours ago) and read 1961 times:
Quoting SlamClick (Reply 5): I believe that the drop in air pressure over the top of the wings can produce the temperature drop necessary
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?
Bobster2 From , joined Dec 1969, posts, RR: Reply 18, posted (3 years 5 months 2 weeks 4 days 7 hours ago) and read 1943 times:
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.
Newark777 From United States, joined exactly 5 years ago today! , 8796 posts, RR: 41 Reply 19, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1933 times:
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.
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:
Starlionblue From Greenland, joined Feb 2004, 13588 posts, RR: 68 Reply 21, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1911 times:
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.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
Newark777 From United States, joined exactly 5 years ago today! , 8796 posts, RR: 41 Reply 22, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1911 times:
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:
Bobster2 From , joined Dec 1969, posts, RR: Reply 23, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1907 times:
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. 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.
Bobster2 From , joined Dec 1969, posts, RR: Reply 24, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1904 times:
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.
Ba757gla From United Kingdom (Scotland), joined Mar 2006, 716 posts, RR: 2 Reply 25, posted (3 years 5 months 2 weeks 4 days 6 hours ago) and read 1902 times:
thanks for the feed back
26 B2707SST: 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
27 Newark777: 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, whi
28 Bobster2: 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
29 Bobster2: OK. You're correct. Thanks to B2707SST in Reply 26 for clearing up the issue that was confusing both of us.
31 Newark777: Thanks a lot, that is exactly what I was looking for. Thanks to Bobster also. Harry
32 Bobster2: 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 warm
33 2H4: Good point. It could be the condensation of water on the surface of the wing. 2H4
34 CosmicCruiser: 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
35 XFSUgimpLB41X: Rumor mill: I heard they both went outboard for increased instability and better combat manueverability.
36 SuperDan: 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
37 L-188: 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 o
38 FredT: The water isn't in the fuel. It is created through combustion. http://en.wikipedia.org/wiki/Combustion
39 L-188: Wikpedia is only half right. Water is a byproduct, but there is also water suspended in all fuels, that water doesn't burn.
40 FredT: ...but is a very marginal contribution to the forming of contrails.
41 Bongo: That only happens on the Flight Simulator...always at FL30 or higher....only.
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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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