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View Full Version : Angle of Attack


skypimp92
10-13-2011, 08:11 PM
Hey Guys,

So maybe someone can shed some light on this for me. If you were to maintain an airspeed from a lower altitude to a higher altitude. Would the wing require a higher angle of attack to maintain this altitude due to the decrease in air density ?Would this affect stall speed?

Best Regards,
Ken


UAL T38 Phlyer
10-13-2011, 08:24 PM
skypimp:

I need to know if "airspeed" means Indicated, or True.

As long as we are not talking compressible theory (ie, light airplanes and low Mach numbers), then for a given IAS, the AOA will essentially stay constant.

For constant True Airspeed, as you climb, the Indicated would go down...and you would need more AOA for the same amount of lift.

Even for what I fly (T-38), at altitudes up to the low 30s, and speeds usually up to 0.85-0.90, there is no discernable difference in AOA if the Indicated speed stays the same, regardless of altitude.

The noticable effect is from aircraft weight. Heavy with fuel at the beginning of the flight, more AOA; at the end, less.

Cubdriver
10-13-2011, 08:39 PM
The basic lift formula is L = q S CL,
where q or "dynamic pressure" = 1/2 x density x velocity^2
S= wing area,
CL = normalized wing lift (normalized = reduced to the coefficient)

The only variable with altitude is q (dynamic pressure).

If L is constant, which it must be to keep the airplane at constant altitude, and the air density goes down with altitude as we know that it does, then to maintain constant lift, the airplane must go faster which it does. And it goes a LOT faster which is why high altitudes are preferred for best speed.

A jet going 260 knots "indicated" will exhibit a true airspeed in excess of 420 knots at high altitude. Higher speeds are common, transport jets often go over 500 knots true.

So is the AoA higher? No, the true airspeed is much higher.