FighterPilot
Topic Author
Posts: 1296
Joined: Sat Jun 25, 2005 2:27 am

Judging Speed By Contrail Length?

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
*Insert Sound Of GE90 Spooling Up Here*

SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Judging Speed By Contrail Length?

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.
Happiness is not seeing another trite Ste. Maarten photo all week long.

bond007
Posts: 4425
Joined: Mon Mar 14, 2005 2:07 am

RE: Judging Speed By Contrail Length?

 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

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
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Judging Speed By Contrail Length?

 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
Posts: 4425
Joined: Mon Mar 14, 2005 2:07 am

RE: Judging Speed By Contrail Length?

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

LOL - Happy Holidays

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
Posts: 2620
Joined: Tue Jul 25, 2006 9:24 am

RE: Judging Speed By Contrail Length?

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
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Judging Speed By Contrail Length?

 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
Posts: 924
Joined: Sat Jan 01, 2000 11:45 am

RE: Judging Speed By Contrail Length?

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
Posts: 1409
Joined: Tue Jan 01, 2002 1:21 am

RE: Judging Speed By Contrail Length?

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.

Who is online

Users browsing this forum: BravoOne and 3 guests

Popular Searches On Airliners.net

Top Photos of Last:   24 Hours  •  48 Hours  •  7 Days  •  30 Days  •  180 Days  •  365 Days  •  All Time

Military Aircraft Every type from fighters to helicopters from air forces around the globe

Classic Airliners Props and jets from the good old days

Flight Decks Views from inside the cockpit

Aircraft Cabins Passenger cabin shots showing seat arrangements as well as cargo aircraft interior

Cargo Aircraft Pictures of great freighter aircraft

Government Aircraft Aircraft flying government officials

Helicopters Our large helicopter section. Both military and civil versions

Blimps / Airships Everything from the Goodyear blimp to the Zeppelin

Night Photos Beautiful shots taken while the sun is below the horizon

Accidents Accident, incident and crash related photos

Air to Air Photos taken by airborne photographers of airborne aircraft

Special Paint Schemes Aircraft painted in beautiful and original liveries

Airport Overviews Airport overviews from the air or ground

Tails and Winglets Tail and Winglet closeups with beautiful airline logos