why do maximum L/D and max. lift not occur at the same angle of attack?

v_arsha

Hello,
I did a wind tunnel test with a NACA 0012 airfoil. And my results showed that the highest lift-to-drag ratio occurred at an angle of 12degrees where as my maximum lift is at 15degrees. I even read somewhere that these values do differ but there was no such explanation as in why is that so..

Can someone please help me out..
Thanks

JaredJames

Can't really word it better than the wiki entry so here it is:

The peak L/D ratio doesn't necessarily occur at the point of least total drag, as the lift produced at that speed is not high, hence a bad L/D ratio. Similarly, the speed at which the highest lift occurs does not have a good L/D ratio, as the drag produced at that speed is too high. The best L/D ratio occurs at a speed somewhere in between (usually slightly above the point of lowest drag). Designers will typically select a wing design which produces an L/D peak at the chosen cruising speed for a powered fixed-wing aircraft, thereby maximizing economy. Like all things in aeronautical engineering, the lift-to-drag ratio is not the only consideration for wing design. Performance at high angle of attack and a gentle stall are also important.

http://en.wikipedia.org/wiki/Lift-to-drag_ratio

physixlover

That is a very good question,
We actually try to calculate maximum lift of the aircraft or aerofoil to know the maximum lift is how far and the next point of the increase in the Angle of attack (AOA) causes downfall of the lift and increase in drag and tends to stall.
And when coming to L/D ratio or gliding ratio we take into account of the lift how many times greater than and this ratio is important for a stable flight and ideal performance. For example: in emergency landing the aircraft has to land quickly and we have a problem of insufficient fuel if L/D ratio is 12 at for ‘X’ AOA then the pilot tries to get the aircraft to that point or at particular AOA so that he can save some fuel for emergency landing and he knows that lift is 12 times greater than drag and pilot knows what to do.

JaredJames

Quote by physixlover

That is a very good question

.................

OK, but that doesn't answer why the highest L/D ratio isn't necessarily the maximum lift point. Again, for that I refer you to the wikipedia entry for the best description.

Here goes for my attempt at an explanation:

The optimum L/D ratio is the point where you get the most lift possible with as little drag as possible.

Above this point, you may get more lift but you get significantly more drag.

Below this point, you may have significantly less drag but you don't have much lift.

In your example, your optimum L/D ratio may be 10 (Lift=20/Drag=2) but your maximum lift L/D ratio could be 5 (Lift=25/Drag=5)*. Although in the latter case you are getting more lift than the previous, you are also getting significantly more drag. Making the AoA less efficient to use.

* Numbers purely for demonstration.

v_arsha

Thanks, it really helped :)

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