Re: wake turbulence

From: (Keith Barr)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         26 Jun 94 21:42:04 
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In article <airliners.1994.1348@ohare.Chicago.COM> (Ed Hahn) writes:
> The wingtip vortex strength is proportional to the amount of lift the wing 
> must generate to stay in the air (to the first order).

The strength of the vortices are proportional (can't remember what order)
to the angle of attack, not the lift generated.  Lift is fairly constant 
through an entire flight regime, but the strength of the vortices changes
throughout the flight envelope.  I know this sounds odd, but it is 
actually excess thrust that makes an airplane climb, not excess lift.  

Knowing that a wing always needs to create lift roughly equal to the weight,
the pilot has three methods to balance this...changing angle of attack, 
changing airspeed, and changing wing camber (flaps & slats).  Since the
lift must remain constant, if you increase the lift by increasing angle
of attack, airspeed, or camber, you must make a corresponding decrease
to the lift by decreasing one of the aforementioned parameters.

If you have one of those great computer flight simulators, they demonstrate
this quite well.  Set you power at some medium power level, and pitch up. 
You will climb momentarily, as you convert kinetic energy during the slow
down, but after that, you won't climb very well until you add more power.

Normal flight Lift balancing:
                        Airspeed    Angle of Attack    Camber
                        --------    ---------------    ------
Initial climb out:       Low           Medium           High
Cruise Climb:            Medium        Medium           Low
Cruise:                  High          Low              Low
Descent:                 Medium        Medium           Low
Landing Flare:           Low           High             High

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