Contrail Formation
 

I know there's been many threads about contrails. Heck, even I posted a circle contrail picture back in the dark ages. ;)

This question came to me when I was on the bus on I-5 this afternoon. Flights were landing to the south and they fly sort of parallel to I-5 for a while before turning west a little bit towards SEA. By the time the bus was on the north end of BFI, I happen to look up and saw a NWA plane coming in. I couldn't tell the kind of aircraft, but know for sure it was NW. Well, the thing had two contrails from the wings, but the contrails did not form at the tips of the wings.

In every picture I've seen of contrails, they were coming out of the tips of wings, not further in the wings. Is this normal? Does it happen on certain kinds of aircraft, but not others? What causes it? I suppose it is not illogical for contrails to come out of everywhere. I know I was not seeing things, although having just concluded an hours long meeting with 10 other lawyers can play tricks on the eyes and mind. :)

You are talking about condensation trails(contrails are frozen exhaust and ambient humidity), they form when the low pressure causes the surrounding humid air to condense. They are usually seen on humid days and can form anywhere on the aircraft, but tend to be most prominent over the areas of highest lift/ lowest pressure.

The can also form in the intakes as the fan blades act much like small wings.

http://www.airliners.net/open.file?i...1279594&size=L

Weathermaking
 

Watch the intake on a humid day as power is advanced for takeoff, or even during taxi. You may see a small condensation "tornado" form and reach down toward the runway.

The trails that you saw probably were formed off of the edge of the flaps...

Contrails aren't formed at low altitudes, only high..so if they were landing..what you were probably seeing is the accelerated air over the wing (since it's cooled when accelerated) condense (since as air cools, it can hold less water)....very common when the temp/dewpoint spread is right.

Contrails do not form over the wing tips at altitude, FYI,,they're produced by the engine exhaust heat interacting with the air.

If you fly a prop job with a decent bit of power, the prop tips can generate these same condensation trails - again, it comes from enough of a pressure differential to pull the moisture out of the air. I've seen it plenty flying behind a Russian M-14P, I think the lowest HP engine I've seen pulling a "genie," as we call them, is 200 HP. Might be possible on a smaller engine in extreme moisture? I donno.. but it looks cool!

http://home.att.net/~polar/130contrail.jpg

I never had the horsepower to make trails like that - it'd only make about one complete spiral on 375 HP.

Really? 200 HP? Would it have to be a 2 blade prop (Higher prop loading than a 3 blader)? I fly a 310 HP SR-22 on some pretty humid days, but I've never seen it.

Gosh, I donno.. The ones on 200 HP aren't super huge- but they were on a two blade prop. On 375 HP, it was much more noticable, but was on a 3 blade MT propeller. :-/ Maybe it's a byproduct of blade design (Aerobatic props, lots of static thrust)...?

I have experienced a vapor trail during run-up in a Howard DGA-15 powered by a P&W R-985 (450HP). Really cool looking with the vapor coming off the prop tips.