FighterPilot
Topic Author
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Judging Speed By Contrail Length?

Mon Dec 25, 2006 3:40 am

While watching some planes fly overhead the other day, I began to wonder to myself. Could you theoretically approximate an aircrafts speed by the length of it's contrail?

Lets say there is a very low temperature, say around -50 to -60, and about 50% relative humidity. This producing a contrail that dissipates some distance behind the plane. Now, lets say there are two different planes flying in the same "perfect world" atmosphere, remember this is all theoretical. One flying at X speed and the other at Y.

Would one be able to guess how much faster one is traveling then another? Also if you knew the actual values for the temperature and humidity, could you create a formula to approximate the planes speed?

Cal  airplane 
*Insert Sound Of GE90 Spooling Up Here*
 
SlamClick
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 4:31 am

Theoretically - maybe for sure!

There are 1.5 kiloplethoras or .275 megasurfeits of factors here.

Flying in identical atmospheres.

Airfoil causing identical pressure (& therefore temperature) drop which is unlikely at two different mach numbers.

Engines producing identical heat plumes.

Engines emitting identical amounts of particulates as condensation nucleii.

Et cetera.

Once all things are truly equal (yeah right! Like that is going to happen!) then the rest is easy. Length of time from ice condensing on particulates. Fixed length of time later, ice sublimates and is no longer visible. Trail of visible ice of greater length laid down in the same amount of time = higher speed.

Might be easier to lay an angle across your field of view then time the passage and do the trig. Pretty easy compared with manipulating an entire atmosphere.  Smile
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
bond007
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 5:20 am

Quoting FighterPilot (Thread starter):
Now, lets say there are two different planes flying in the same "perfect world" atmosphere, remember this is all theoretical.



Quoting SlamClick (Reply 1):
Engines producing identical heat plumes.

Engines emitting identical amounts of particulates as condensation nucleii.

Right, but when you mean "two different planes" do you mean "two different planes but identical types" ?? ...if you see what I mean  Smile

But I guess as long as you the formulas for how long the contrails were for each aircraft/engine type/speed etc. etc. etc., it wouldn't matter, right?

Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
 
SlamClick
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 5:25 am

Quoting Bond007 (Reply 2):
do you mean "two different planes but identical types

I don't think that would be any easier than achieving this condition with two different types. For one thing, two "identical" airplanes doing different speeds in the same atmosphere would have two different mach numbers and that means that the faster would produce more ram rise - therefore the temperature changes would be different. "Identical" doesn't really help. Also, with identical airframes the same engine would have to be run at higher power - with more heat - to give a faster speed.

This one is like nailing jelly to the wall, but at least it doesn't require conveyors or dividing infinity by zero.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
bond007
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Joined: Mon Mar 14, 2005 2:07 am

RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 6:01 am

Quoting SlamClick (Reply 3):
but at least it doesn't require conveyors

LOL - Happy Holidays  Smile

I'm sure there are some new a.netters saying "what does he mean, conveyors ??"


Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
 
N231YE
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 7:21 am

I thought the length of contrail had to do with the humidity at higher altitudes. As a matter of fact, I have heard you can predict the weather by the contrail size. I.E:

Short contrails > good weather, because the atmosphere is dry and the contrail evaporates quickly,

Long Contrails > poor weather, because the atmosphere is already humid (oncoming system), and the contrail takes time to evaporate
 
SlamClick
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Joined: Sun Nov 23, 2003 7:09 am

RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 8:41 am

Quoting N231YE (Reply 5):
I thought the length of contrail had to do with the humidity at higher altitudes.

You thought right. We are talking about conditions like that, where an observer on the ground can see the entire contrail at once and discern the speed, or at least the relative speed by the difference in length.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
meister808
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 3:17 pm

Of course, one must take into effect that the aircraft going faster is going to have a higher power setting, meaning it will burn more fuel, which (assuming equal efficiency across the power curve (which is a bold-ass lie)) means it will put off more particulate emmissions, which will act as condensation nuclei, meaning that moisture will be more likely to condense in that region.

Not to mention that the wheels aren't attached to the conveyor, which of course isn't attached to the cage of farting hamsters next to the caged birds, which hasn't been corrected for the Coriolis Effect.

Merry Christmas, all.

-Meister
Twin Cessna 812 Victor, Minneapolis Center, we observe your operation in the immediate vicinity of extreme precipitation
 
ThirtyEcho
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RE: Judging Speed By Contrail Length?

Mon Dec 25, 2006 8:02 pm

A very easy question.

Put both thumbs to the top of your ears and both first fingers to the inside edge of your eyebrows; point your nose directly at the airplane and point your tongue as far down your chin as you can as you count, out loud, eighteen seconds as in, "one chimpanzee, two chimpazee," et seq.

The time that it takes for the contrail of the nearest outboard engine to equal the distance between your two fingers, multiplied by the airplane's Vmu, will give you the current airspeed, plus or minus two knots.

If the airplane stops making a contrail, it means that has stopped at a Roy Rogers restaurant.

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