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Essentially my question is, in general is it better to have a lower drag or a larger lift. I know lift to drag ratios are used as an indicator but I think these don't give the best picture. For example, I'm running a 2D airfoil analysis and for 2 cases the CL/CD changes varies between 80 and 160 (obviously doesn't account for 3D drag). So you'd expect the best option as CL/CD as 160.

However the CL/CD only looks at the % change. So if the % increase in drag is say 70% and for lift it's 10% CL/CD will be larger for the increased drag but the amount of increase in lift force greatly outweights that of the drag (an increase of 175N of lift vs reduction of 14N of drag).

I guess it would depend on the application as well.

Any help would be greatly appreciated, thanks in advance!

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Your lift will be prescribed and cannot be freely chosen. So what you need to do is to minimize the drag that goes with this given lift.

In straight flight lift needs to equal weight. In turns, the centripetal force is added to this. At high speed a small angle of attack will produce that lift while at low speed the angle of attack needs to be high. In most cases, takeoff and landing requirements will determine minimum wing size and the maximum lift coefficient of the airplane. In order to not drag around massively oversized wings, most airplanes use flaps to have a decent maximum lift coefficient for takeoff and landing while minimizing their wetted surface at high speed in order to keep drag down.

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  • $\begingroup$ Thank you, this makes sense. I had forgotten that the lift is pre-determined by the straight and level flight condition so drag is the factor to change. Thanks! $\endgroup$ – Barnaby Apr 2 at 14:17

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