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ryan88
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Homework Statement
I need to find an expression for the minimum drag coefficient, [tex]C_D[/tex], of an aircraft during straight and level flight.
Homework Equations
[tex]C_D=C_{D_0}+kC_L^2[/tex]
Where:
[tex]C_{D_0}[/tex] is the profile drag coefficent
[tex]C_L[/tex] is the lift coefficient
[tex]k[/tex] is just a constant
The Attempt at a Solution
I started off by plotting the equation for [tex]C_D[/tex] shown above and got the following:
http://g.imagehost.org/view/0218/drag
So I thought that the minimum drag would be when:
[tex]\frac{dC_D}{dC_L}=0[/tex]
However, I remember my lecturer saying something about I would have to do:
[tex]\frac{d}{dC_L}\left(\frac{C_D}{C_L}\right)=0[/tex]
But I am unsure why, I was wondering if someone could shed some light on this please?
Thanks,
Ryan
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