Are high lift wings bad for faster speeds

Question

I notice a trend in many aircraft designs. If the aircraft was designed to go slowly but have shorter takeoffs/landings they use airfoils with a high lift.

However for long endurance flights they use a flatter airfoil that produces less lift.

To my understanding the reason for this is because you never get anything for free. When a wing produces lift, it is trading the forward movement of the plane for vertical movement.

I understand that at higher speeds the high lift wing would fight horizontal movement too much and so they chose the flatter airfoil.

However my question is what if you change the angle of attack for the high lift wing? Pointing the front of the wing downward, thus making its lift more of a forward direction.

Would this somehow still be less efficient than a flatter airfoil or do they just chose the flatter foils for design purposes and structural reasons?

Answer 1

Every airfoil has a range of lift coefficients where the flow around the wing is most favorable. Airfoils with high camber shift this range to higher lift coefficients. Now the question comes down to: At which lift coefficient will the aircraft operate most efficiently?

The efficiency of transport aircraft is maximized when none of the three parameters payload, range and speed can be changed without reducing the product of the three. As it turns out, wing weight has a big influence on this, especially for large aircraft: A heavier wing will reduce the possible payload. It is better to limit wing span such that the ratio of wing span to chord is between 7 and 10: This will limit the bending moment at the wing root and allow to build the wing lighter. At this aspect ratio, the optimum lift coefficient for best transport performance is around 0.4 to 0.6, a number at which the low camber airfoil will feel right at home.

At such a low lift coefficient, however, the high camber airfoil, on the other hand, will experience a strong suction peak on the lower side of its nose. At high flight Mach numbers, this will lead to locally supersonic flow and a big increase in drag as well as cyclic loads on the wing (buffeting).

Now you could correctly argue that the wing could be much smaller with a high camber airfoil for the same lift. Yes, but the wing needs to adapt to take-off and landing conditions, too, and needs to have enough volume to contain the trip fuel. And for this landing condition the wing of an airliner will even have more camber than your example airfoil:

Typical landing configuration of an airliner wing, from an article by A. M. O. Smith, McDonnell-Douglas, in Journal of Aircraft, Vol 12 No 6, 1975.

Answer 2

what if you change the angle of attack for the high lift wing? Pointing the front of the wing downward, thus making it's lift more of a forward direction.

The engine of the aircraft creates forward movement.

The wings (and body) of the aircraft absorb some of this forward movement energy to create lift and drag.

If you simply change the angle of attack to have the lift vector pointing into a more forward direction (instead of upward direction only) you still create the drag in proportion to the lift. Therefore if you want to fly high speed it is better to have a wing with less lift resulting in less drag.

On the other hand the lift to drag ratio depends on the angle of attack too. So if you change the angle of attack away from the best ratio angle (in either direction) you get less lift and more drag. Unfortunately the ratio gets worse faster for negative changes (nose down) than for positive changes (nose up).