Topic Author
Posts: 2051
Joined: Sat Apr 23, 2005 2:38 am

Cruise Economy And Climb Performance

Mon Jun 13, 2005 11:58 pm

Obviously the wing has to be different to deliver both, as most obviously demonstrated in the two different wings for the 783 and 788. But it is already possible to make an adjustable wing through the use of flaps and slats. Flaps and slats give better takeoff performance but they're retracted fairly soon after takeoff and most of the climb is down with a clean wing. Isn't it possible to device a flap and slat mechanism that can be used throughout the climb to give good climb performance on a wing, which when clean gets good cruise economy?
Posts: 36
Joined: Wed Jun 01, 2005 4:19 am

RE: Cruise Economy And Climb Performance

Tue Jun 14, 2005 2:26 am

Flaps and slats will reduce the rwy length for take off. Disadvantage of flaps at climbout is the increased drag. Clean config will give you BETTER climb performance, so no need for a different flap and slat system.
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Joined: Wed Mar 09, 2005 3:22 am

RE: Cruise Economy And Climb Performance

Tue Jun 14, 2005 3:26 am

Really the flaps are like the low gears in an automobile. As they are deployed, lift increases but so does drag, the result is the airplane can fly at slower speeds. Flaps can actually hinder climb performance. Unfortunately I don't know about airliners, but if I fly a C172 with full flaps I simply can not climb unless I have a good headwind.

The secret to improving climb performance lies in a blend of higher power and wings that produce less drag (other ways of reducing drag are good too). If you can get the same airspeed with a higher angle of attack and a similar amount of releative drag you should get better performance.
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RE: Cruise Economy And Climb Performance

Tue Jun 14, 2005 8:08 am


As 727EMflyer has stated, flaps are for low speed maneuvers only and they are not used above a ball park speed figure of 230 kts.
That's the way they are designed.

As we are speaking of take-off performance, the crew has to chose between reducing the runway length necessary for a given weight and in this case,they will use high flaps settings, or if runway length is not the limiting factor, use the lowest setting to improve climb performance.

In order to climb, one has to consider two factors :
1-The engine thrust (the more thrust, the better)
2-The wing characteristics.The best angle of climb will be achieved at the best lift-to-drag ratio (aka Cl/Cd).
If one considers this ratio, a clean wing will always have a better one than with flaps extended as the high lift devices generate an absolute hell of drag rise when extended.

Taking Glom's initial question a bit further, probably all aerodynamicists and all manufacturers dream of a perfect "mission adaptative wing" which could change its aerodynamic characteristics, like thickness and curvature during each phase of flight, thus adapting to weight or maneuvering requirements as need arises. Some of this concept is already apparent on modern combat airplanes through the use of the leading edge flaps.

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