Drag Formula
Joined: May 2003
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Not trying to hijack a thread, but it always seemed odd to me that as an aircraft speeds up, the coefficient of drag of an aircraft or airfoil increases. I'm assuming level flight, and I'm not considering any compressibility factor.
OTOH, I believe a sphere though has a constant CD regardless of speed, not considering compressibility.
Hawk
OTOH, I believe a sphere though has a constant CD regardless of speed, not considering compressibility.
Hawk
Joined: May 1999
Aviation Qualifications: ATP+Mil
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From: Quite near 'An aerodrome somewhere in England'
Not actually true.
That part of Cd representing lift dependent drag increases with the square of Cl. If you're in level flight and slow down, then up must go AoA and hence Cl if you wish to maintain level flight.
Cdi = k Cl**2/pi * AR (k is a constant, Cl**2 means Cl squared, pi means 3.14159 and AR is the aeroplane's aspect ratio.)
Which is why there's a 'min drag speed' for any aeroplane.
That part of Cd representing lift dependent drag increases with the square of Cl. If you're in level flight and slow down, then up must go AoA and hence Cl if you wish to maintain level flight.
Cdi = k Cl**2/pi * AR (k is a constant, Cl**2 means Cl squared, pi means 3.14159 and AR is the aeroplane's aspect ratio.)
Which is why there's a 'min drag speed' for any aeroplane.
Joined: May 2003
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Sorry, I worded that backwards. I should have said that as an aircraft speeds up, the coefficient of drag decreases. Yet a sphere, or racecar for another example, will not change their Coefficient of drag as speed changes.
However, I can see this is not an apples to apples comparison, since an aircraft in level flight will change it’s profile to the air (as Beagle pointed out via a Coefficient of lift change), while a sphere, or racecar won’t.
Hawk
However, I can see this is not an apples to apples comparison, since an aircraft in level flight will change it’s profile to the air (as Beagle pointed out via a Coefficient of lift change), while a sphere, or racecar won’t.
Hawk
Joined: Jun 2001
Posts: 889
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From: Australia
Shouldn't that be
"As an aircraft speeds up, the coefficient of lift-dependent drag decreases, and the coefficient of lift-independent drag doesn't change, except for small changes in aircraft attitude"?
Helps explain why spheres don't change Cd (lift independent).
But what about the drag of a soccer ball at increasing mach numbers...
"As an aircraft speeds up, the coefficient of lift-dependent drag decreases, and the coefficient of lift-independent drag doesn't change, except for small changes in aircraft attitude"?
Helps explain why spheres don't change Cd (lift independent).
But what about the drag of a soccer ball at increasing mach numbers...
