Drag Formula

[David Luther]

Does anyone know the drag formula?

[John Farley]

Sorry about the lack of traditional symbols but

D = CD1/2 ĉSV2

Where CD = drag coefficient

and ĉ = density of the air

V2 = Vsquared

[machonepointone]

John Farley was almost correct, but the full formula is Drag = Cd½ñV²S where:

Cd = Coefficient of Drag
ñ = Density of the Air
V² = Velocity (TAS) squared
S = Surface Area

[The Greaser]

look exactly the same to me!

[machonepointone]

Sorry, computer problem. The ñ should be p

Not quite, he left out the "S" part

[Techman]

Look again, machonepointone.

[hawk37]

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

[BEagle]

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.

[machonepointone]

Techman,

You're right - I did miss the "S" bit in John Farley's formula. Apologies to John.

[hawk37]

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

[Oktas8]

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... :confused: :uhoh: :ooh:

[David Luther]

thanks guys appreciate it.