SSTjumbo From , joined Dec 1969, posts, RR: Posted (9 years 7 months 4 days 14 hours ago) and read 1272 times:
I was watching an ATC special on video in one of my classes some time back, and it occured to me while the video rollers were filming aircraft on takeoff at JFK on a particularily moist day that not only were vortices coming off the wingtips, but additional vortices were coming off the outboard edges of the flaps. Could anyone explain how the airflow works in the scenario of flaps down that causes the four vortices? Thanks.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 34 Reply 2, posted (9 years 7 months 4 days 13 hours ago) and read 1237 times:
Yep. Anywhere there's a pressure differential without wing or other structure in the way, there will be a vortex. High pressure to low, high to low... Obviously, the little edge that extended flaps create is no exception, thus adding a 'second' vortex to each wing.
Mr Spaceman From Canada, joined Mar 2001, 2786 posts, RR: 9 Reply 3, posted (9 years 7 months 4 days 12 hours ago) and read 1227 times:
As was already explained, the outboard edge of the flaps act like a wingtip when they're extended and provide an area for high pressure air under the wing/flap to try to wrap around the flap's edge to fill the low pressure air above the wing/flap .... thus the swirling vortices off the flap tips.
I find it interesting to see that in many of these photos, there's only a visible vortex flowing off the flap's outboard tips, and none flowing off the wingtip itself.
Does this mean that enough high pressure air is escaping from under the flap tips to interfere with the developement of a vortex out at the wingtip?
The number of vortices depends to some extent on how aerodynamically "clean" the wing is. If the flaps extend in a way which produces a more or less straight, unbroken profile, then the wingtip vortices will be strongest (eg 757). If the flaps are only covering part of the wing, and have straight edges (i.e. the airflow sees them as little wings of their own with little direct connection to the main wing), then they shed vortices at the edge of each flap (eg 747, MD11)
(sorry for the poor explanation, it's 2am here...)
Vzlet From United States of America, joined Mar 2004, 827 posts, RR: 0 Reply 6, posted (9 years 7 months 4 days 10 hours ago) and read 1182 times:
Each wingtip in the above pictures is producing a vortex, but the temperature/humidity conditions aren't suitable for producing visible condensation within those vortices. An extended flap is at a higher angle of attack than the wing, increasing the pressure differential and, in the above cases, producing vortices strong enough to create enough of a pressure drop within the vortices that the moisture in the air condenses.
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid
Mr Spaceman From Canada, joined Mar 2001, 2786 posts, RR: 9 Reply 7, posted (9 years 7 months 3 days 12 hours ago) and read 1081 times:
Thanks for your expanations.
OK, I understand that even though you may not be able to actually see the vortices flowing off the wingtips, they are still there and are always there from the moment the wings start producing lift during take-off untill they stop producing lift after landing. Also, these vortices will only be seen when the conditions are right between the amount of humidity in the air (how saturated the air is), and the air pressure differential at the wingtips or flap edges.
> Vzlet, I believe your expanation about how the flaps have a higher angle of attack than the rest of the wing, and thus a higher air pressure differential between their upper & lower surfaces which causes visible condensation in the core of the vortices off their ouboard tips is an excellent way to understand what's causing only flaptip votices (and not wingtip vortices) to be visible.
Vzlet From United States of America, joined Mar 2004, 827 posts, RR: 0 Reply 9, posted (9 years 7 months 3 days 9 hours ago) and read 1050 times:
A couple of other factors:
1. Miguel's stunning DC-10 shot clearly shows the lift distribution across the span of the wing: The thicker the layer of condensation, the stronger the lift at that point. Most of the lift is produced by the inboard half of each wing. (Which makes sense, because that's where the majority of each wing's area is.) As a result, the pressure differential at the outboard corners of the inboard flaps is stronger than further out towards the wingtips, making it more likely to generate stronger (and thus potentially more visible) vortices.
2. The benefit of winglets (besides looking spiffy, of course) is that they reduce drag by reducing the energy lost in producing vortices at the wingtips. Weaker vortex = less pressure drop within the vortex and less chance of condensation.
3. (Man, that DC-10 shot is a beauty!)
"That's so stupid! If they're so secret, why are they out where everyone can see them?" - my kid
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 34 Reply 10, posted (9 years 7 months 3 days 4 hours ago) and read 1032 times:
The benefit of winglets (besides looking spiffy, of course) is that they reduce drag by reducing the energy lost in producing vortices at the wingtips.
Keep in mind that winglets do more than just that; the belief that reducing energy lost to wingtip vortices is all that they do is a common misconception. What distinguishes winglets from older-fashioned endplates is that rather than just being flat plates to stop vortex flow, they are of course little airfoils. The little bit of vortex that still remains interacts with the free airstream and creates a resultant flow which the winglet utilizes to create forward lift - by definition, thrust. This obviously counteracts drag, thus increasing range and improving other performance parameters further. In other words, they don't just weaken the vortices (thus weakening downwash and decreasing induced drag), they utilize them to increase thrust/decrease drag further...
Alright, I sense myself climbing up onto the soapbox, so I'll stop. However, if you'd like a more in-depth explanation, feel free to ask!
FredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26 Reply 13, posted (9 years 7 months 3 days ago) and read 1003 times:
They're creating lift which is, as always, perpendicular to the local airstream. As the vortices have the air going inwards, part of that lift will be directed forward. They're also generating drag parallell to the local airflow. The trick is to make sure that the component of the added drag which is parallell to fuselage is smaller than the component of drag parallell to the fuselage.
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Vzlet From United States of America, joined Mar 2004, 827 posts, RR: 0 Reply 14, posted (9 years 7 months 2 days 22 hours ago) and read 994 times:
No fears of "soapboxing"! More detail and/or reference to a diagram that depicts the interaction would be appreciated. I understand the principles, but can't picture how everything ties in. (Much like my remaining unclear, despite reading several detailed descriptions, on exactly how the SR-71 inlet system produces the majority of that plane's thrust.)
In the course of searching around, I was amused to read this about one of the factors that shaped Boeing's 737 winglet decision:
Mr Spaceman From Canada, joined Mar 2001, 2786 posts, RR: 9 Reply 15, posted (9 years 7 months 2 days 11 hours ago) and read 927 times:
It's always interesting to learn about what the invisible forces that act apon a wing are doing ....... and what forces cause those forces to become visible.
"May the force be with you!" Sorry, I couldn't resist.
>> Yikes!, you asked me ............
Did you used to go by the handle uwochris, or something like that?
No Sir, I never did.
In fact, I was quite surprised to find out that the username I requested was already taken by another member when I joined A.net back in March of 2001.
I wanted to use just "Spaceman" because good friends of mine nick-named me that when I was finished school and working with them. They decided to call me Spaceman because I was taking flying lessons and my locker at work was covered with pictures of all kinds of aircraft both inside and out.
Because "Spaceman" was taken, I chose to use "Mr Spaceman." I like it.
Why do you ask? Does my wording or something remind you of another member? By the way, I don't remember ever seeing a username like uwochris.
QantasA332 From Australia, joined Dec 2003, 1500 posts, RR: 34 Reply 16, posted (9 years 7 months 2 days 9 hours ago) and read 914 times:
To visualize how exactly the forward lift (thrust) is generated by winglets, just set out the orientation of all the involved flows and alignments clearly. First, as I said before, there is whatever remains of the wingtip vortices, which flow inwards and approximately perpendicular to the free airstream. That flow interacts with the free stream, and creates a resultant flow moving diagonally both aft and inwards. A basic, approximately-parallel-to-the-free-stream winglet will obviously be at an AoA to that resultant flow, and because the lift produced has to be pretty much perpedicular to that same flow, there's forward lift. Below is a rough little sketch (made in paint) to illustrate, not at all to scale or with accurate other measures. The blue lines represent the vortex flow, the red ones the free stream, the green the resultant flow, and the yellow the lift produced...
On another note yes, as Fred said, winglets also produce drag. It mainly comprises skin friction drag and induced drag (quite ironic, but to be expected). You never get anything for free, that's for sure...
Hopefully all of that clears things up!
(Sorry I hadn't replied sooner - I've been away from my computer for a while).