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Do "G" Forces Help Create Vapour Trails?  
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Posted (12 years 5 months 15 hours ago) and read 7601 times:

Hi guys.

The other day I was watching a 737 approaching my home while on left downwind for Toronto's runway 24R (YYZ). Then as it started to turn left onto base straight above me (where they usually turn during hub time), it's wings started to create vapour trails off their tips. These vapour trails where not there before the 737 turned onto base, and they dissapeared once the jet's wings leveled out on it's base leg.

There was high humidity in the air.

This isn't something that I normally see happen with airliners. However, I always see this occur with fighter jets.

Have you ever been to an airshow and noticed that as soon as a fighter jet turns very sharp (like a knife edge turn), or pulls up hard while flying really fast, that vapour trails suddenly form at the jet's wingtips & other parts of the aircraft? These vapour trails usually are not there untill a high "G" force manouver is performed.

I've seen video footage of a flight of 4 fighter jets flying in a diamond formation with none of them leaving vapour trails, then one of the jets will suddenly bank sharp and pull away, or pull up hard ......... and it instantly creates vapour trails!

Why does this happen?

Why does high "G" forces create vapour trails? Or do they?

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Chris  Smile

"Just a minute while I re-invent myself"
8 replies: All unread, jump to last
User currently offlineVaporlock From , joined Dec 1969, posts, RR:
Reply 1, posted (12 years 5 months 15 hours ago) and read 7567 times:

Mr Spaceman, the vapour trails that you saw while the aircraft was turning left onto base may have been a direct result of excessive "G's" that the aircraft was pulling while turning through the humid air -- thus causing more friction over the wing.

Just a thought!

Phyllis  Smile/happy/getting dizzy

User currently offlineSlamclick From United States of America, joined Nov 2003, 10062 posts, RR: 66
Reply 2, posted (12 years 5 months 15 hours ago) and read 7568 times:

In a word - sort of.

Warm air will hold more water vapor than cold air. Producing lift lowers air pressure in a very localized way. Lowering the pressure drops the temperature and water converts from vapor to droplets which are visible.

With the A-330 you show, if he slowed down, eventually the trails would be seen coming off the outboard flap ends. At very low speeds on approach that part of the wing is carrying most of the load. The outer ends, with no high-lift devices are not exactly stalled, but certainly not carrying much of the load at that point. You can find pictures in the DB that show this.

A 25,000 lb. fighter pulling two G's "weighs" 50,000 lbs. That is the lift the wings must generate. For the fighter that pulled out of the formation, they are all flying in the same atmospheric conditions, his wings are generating twice as much (or some such number) lift than the rest. More lift, greater pressure drop - greater temperature drop. More visible moisture.

I hope that makes sense. I'll now leave it to someone who is so inclined to shoot my simplified explanation full of holes. (It is still pretty much the truth of it.)

Happiness is not seeing another trite Ste. Maarten photo all week long.
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 3, posted (12 years 5 months 14 hours ago) and read 7511 times:

What you are seeing is the wing tip vortices. The strength of the vortices is related to the lift coefficient, which is higher for a high angle of attack. The lift coefficient is related to lift created, so in one sense the double weight explanation is perfectly valid. However, you'll also need a higher lift coefficient to maintain 1 G flight when slow, which is why you'll often see aircraft begin generating tip vapour trails when slowing down on the approach.

Inside the vortex, the pressure is lower as the rotating air is pushed out by the centrifugal forces (or accelerated inwards by the centripetal force generated by the lower pressure, to be 100% nitpickingly correct).

As the pressure is lowered, the gas expands. As the gas expands, it cools off. With the dewpoint close enough to the initial temperature, the final temperature will be below the dewpoint making the moisture in the air condensate and form vapour - which is what you see.

The vapour trails off the edges of the flaps are created by exactly the same process as those off the wing tips.

On top of the lifting surfaces, and sometimes in other locations, the pressure is lower as well. In the right conditions, the same phenomenon can be seen there. The leading edge root extensions (LERX'es) of high performance jets is a very typical location for this type of water vapour to form, as the jet goes to a high angle of attack and begins generating vortex body lift. The same goes for delta wings, which can exhibit impressive vapour formation on top of the wings at times.


[Edited 2004-01-03 22:30:24]

I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMITaero From United States of America, joined Jul 2003, 497 posts, RR: 7
Reply 4, posted (12 years 5 months 12 hours ago) and read 7448 times:

>to be 100% nitpickingly correct

excellent  Smile

Wingtip vortices are funny.. they look cool, but, sadly, as an engineer I just see drag when I look at them.

User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Reply 5, posted (12 years 4 months 4 weeks 1 day 14 hours ago) and read 7380 times:

Hi guys, Oops, & girls. Big grin

Thank You for your replies.

> Vaporlock, you mentioned friction. I've been wondering for decades about whether or not the vapour trails from fighter jets at air shows were cause by skin friction heat (kinda like steam!), due to the very high airspeeds. Now we know it's all about aerodynamics.

>Slamclick & FredT, Thank You guys for your excellent explanations  Big thumbs up about why a fighter jet will suddenly start creating visible wingtip vortices as well as vapour trails from other parts of the jet as soon as the AOA is rapidly increased.

Question: I clearly understand now why these vapour trails suddenly form when a fighter jet pulls up hard, but why do they also form during a high G force turn?

My mind is telling me that when a fighter pilot banks 90 degrees into a knife edge manouver & then pulls back on the stick to fly a 360 degrees turn over the ground, that the act of pulling back on the stick makes the wings create more lift, but, then i think of how lift is always oposite of weight/gravity. So I'm a little confused over this. Can you help me out with this mental block?  Nuts

I was taught that any aircraft flying at any airspeed while in a 60 degrees bank will always pull exactly +2 G's. Is this true?


Chris  Smile

"Just a minute while I re-invent myself"
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 6, posted (12 years 4 months 4 weeks 1 day 10 hours ago) and read 7358 times:

Mr Spaceman,
any aircraft flying level and coordinated in a 60 degree bank will be pulling 2Gs of vertical acceleration (nz = 2*g). If there is a vertical component of acceleration or if it is in a spiral up or downwards, things change.

But that is the nitpicky engineer in me talking. It is bound to make MIT happy.  Big grin

As for lift always being opposed to weight, think of it this way: To make an aircraft accelerate in any direction, a force is needed. Gravity is always creating a force accelerating the aircraft downwards. If you want to keep your vertical velocity, you have to counter gravity with a force equal to this force - weight.

That being done, we can be in a steady climb or descent along a straight line. We can even fly level. If the lift becomes larger than the weight (such as if you increase the AoA by pulling on the stick or if you accelerate), the aircraft will accelerate upwards. As it accelerates upwards, oncoming air will appear to be coming from above, reducing the angle of attack and the lift generated until it again equals the weight of the aircraft. There are special cases, but that's the general idea.

But, we want to control our path over the ground as well, right? To do this, we need to change the direction we are moving horizontally. If you have a ball rolling along a flat surface, you need to push it from the side to make it change direction. The same holds true for an aircraft in flight. To turn, we need to create a lateral (that's engineerish for sideways) force. To do this, we can yaw the aircraft. Skidding the fuselage sideways through the air will create a slight lateral force, but it is very inefficient. That is what is called a flat turn, which largely went out of style early in WWI. Today, we strive to keep the fuselage aligned with the direction of travel as it generates a lot of drag otherways (which has it's uses, but that's another discussion).

A more effective approach is to tilt the lift sideways. We achieve that by banking the aircraft, as the lift is always perpendicular to the wings. This will divert some of the lift to the side, accelerating the aircraft sideways and making it turn. To keep the part of the lifting force that is keeping the aircraft out of the grip of gravity equal to the weight, you need to increase the total lift - i e pull the stick backwards and increase. This all adds up nicely and geometrically to produce the 2Gs at 60 degrees of bank.


[Edited 2004-01-05 02:00:40]

I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineMr Spaceman From Canada, joined Mar 2001, 2787 posts, RR: 8
Reply 7, posted (12 years 4 months 4 weeks 12 hours ago) and read 7303 times:

Hello FredT.

Once again, Thank You very much for your very thorough explanations of aerodynamic forces. Your efforts are much appreciated!  Big thumbs up

I Love learning about aerodynamics, although I must admit, there's so many areas of the subject that I can't get my mind around!  Nuts I don't know how guys like you can handle having so many Engineerish thoughts running around in your mind all the time. I'm sure you've had your share of mental blocks to punch through!

You stated .......

Today, we strive to keep the fuselage aligned with the direction of travel as it generates a lot of drag otherwise (which has it's uses, but that's another discussion).

I suspect the "uses" that are for another discussion that you're refering to are sideslips & forward sideslips.

I have 54.3 hours of flight time so far towards my PPL. During my airwork, I've done many steep turns. It's a pretty weird feeling to pull only +2 G's in a little Cessna 152. I can only imagine what a +9 G pull up must feel like in a fight jet.

The best I've done is maintain altitude in a 70 degrees bank. That required full throttle and more than average back pressure. Big grin

Chris  Smile

"Just a minute while I re-invent myself"
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 8, posted (12 years 4 months 4 weeks 11 hours ago) and read 7320 times:

affirm on the slips.  Smile

Go gliding sometime, and get sbdy to show you how to thermal in a tight strong thermal. Especially with other aircraft in the thermal, you get a sore neck from keeping your head on a swivel with a brain weighing twice as much as you're used to (in my case, that brings it close to the normal average)!

BTW, still learning. Part of that learning is done through explaining it again. Another hugely important part of that learning is learning to explain things in a way that makes sense to people who aren't already involved in engineering!

Grateful for that myself. Cheers,

I thought I was doing good trying to avoid those airport hotels... and look at me now.
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