Bryan Becker
Topic Author
Posts: 327
Joined: Fri Mar 02, 2001 12:38 pm

Airflow Over Wing

Mon Apr 09, 2001 12:40 am

Bryan here,I just have a quick question.Iwas looking throught some photos of sky harbor,An I have seen many more of these pictures.I just don't under stand how the air flow is going over these wings or eny other wing with these flaps deployed,those flaps in the front(i don't know what they are called) how does the air go smoothly over them.They are streight down from what it looks like.here is a photo to show what i'm talking about. Smile


Click for large version
Click here for full size photo!

Photo © Saul Loeb




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737LAME
Posts: 75
Joined: Sat Apr 07, 2001 3:14 pm

RE: Airflow Over Wing

Mon Apr 09, 2001 1:47 am

Hi Bryan.

The wing Leading edge devices you ask about are called. Slats and Krueger flaps.

Slats are movable part of the wing Leading edge (LE). They are placed from the engine nacelle and outboard on B737,747. DC9 have the whole wing LE with Slats.

When slats are extended they create a slot that will assist the airflow over the wing at high angles of attack. Like landing and take off. With slats you will get a laminar airflow on the wing upper side much longer than without. Your stall speed will get lower before the turbulence wins and you will stall out the wing.

Small aircraft can also have slats, but they are usualy a fixed slot in the wing LE and are therby called slots.

Inboard of the engine nacelles on B737,747 or the B727 you have in the photo (from the landing light and inbd). There are a lift augmenting device called Krueger flap. This LE flap will also assist the airflow over the wing at high angles of attack. But this Krueger flap makes a longer wing upper surface curve.

I suggest that you next time you are in a library, loan a book about aerodynamic basics. You will then understand it better than I can explain here.

Atle
 
Bryan Becker
Topic Author
Posts: 327
Joined: Fri Mar 02, 2001 12:38 pm

RE: Airflow Over Wing

Mon Apr 09, 2001 2:04 am

THANKS!!!!!!!!!!!!!!!!!!!!!!.......................... Smile



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HeavyCapt
Posts: 61
Joined: Mon Jan 15, 2001 5:00 pm

RE: Airflow Over Wing

Mon Apr 09, 2001 2:06 am

Bryan,
What you are looking at is a picture of a combination of leading edge slats and flaps (the a/c being a 727). Some aircraft have all leading edge flaps (also called Krueger Flaps) such as the 747 family. Most other transport category aircraft have slats. At certain settings these devices working together produce more lift than drag at lower speeds. That is why they are used for take-off. They enhance the lift of the wing by changing the airfoil. At other settings (out in the breeze more) The devices used together produce quite a bit of drag as well. This is when they are used for landing. They augment the lift but also help maintain controllability in the slow speed regime.
As for how they work....if you took a cross section of a wing (airfoil) with all devices retracted, just a clean wing. Then select a point at the leading edge of the wing and label it A. Then select a point at the trailing edge and label it B. Now draw the shortest line possible from A to B over and under the wing. (Not a straight line....this is fluid dynamics not mathematics the air cannot pass through solid mass). You can see that the path from A to B under the wing is shorter than the path over the wing. This produces lift because the shorter path under has higher pressure than the longer path over. The wing will want to fill that space with lower pressure.
Now that you have done this with a clean wing. Extend the points A and B out from where you have them to allow for the extension of flaps and slats. You will see that the under path is still shorter than the over path. This is how they work. Remember that although these devices augment lift, they also produce drag. They are only efficient during the low speed regime. I hope this helps. Please remember that this is a HIGHLY oversimplified explanation, but you should get the picture.

Brock
 
Bryan Becker
Topic Author
Posts: 327
Joined: Fri Mar 02, 2001 12:38 pm

RE: HeavyCapt

Mon Apr 09, 2001 3:02 am

How do know when you need to use them,do you lookat the performance charts? SmileTHANKS!!!!!!!!!



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prebennorholm
Posts: 6447
Joined: Tue Mar 21, 2000 6:25 am

RE: Airflow Over Wing

Mon Apr 09, 2001 8:37 am

Dear Brian,
They are always used for take off and landing.
The trailing edge flaps can be set at different angles, and they are normally deflected much more for landing than at take off.
In an emergency you may be able to land without extending them, but it would be quite tricky with a heavily loaded plane, and it would mean a lot more wear on wheels and brakes. And a much longer roll on the eunway.
If the runway is long enough it might be possible to take off without them, at least with a lightly loaded plane. But I would bet that it is illegal and a crew doing such a stunt would never fly again.
The common name for these things is "high lift devices". Remember that the wing is designed for efficient flight at 500 mph at thinner air at cruising altitude. That corresponds very roughly to 250 mph at sea level air density. This is far too fast for take off and landing. Therefore these high lift devices assist with greatly improved lift with a huge drag penalty. The drag doesn't mean very much because they are extended for only a short time - few minutes at most - and in the dense air at low altitude the engines can produce far more power that at high cruising altitude.
Remember what 737LAME wrote - go and borrow books about aerodynamics. Those high lift devices are much more complecated than described here, and their function - as well as aerodynamics in genereal - is facinating.
When studying high lift devices you will soon learn that they are nothing invented by mankind. Birds used them millions of years before the first man stood upright on his two legs.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs

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