Airliner sharp wing tip vs elleptical

Question

Now that most airliner manufacturers are using composite materials for wings, making compound curves is much easier to produce, why aren't we seeing elliptical wings, outer panels or wing tips?

Aviation textbooks say the elliptical wing is the most aerodynamically efficient. If so, why aren't manufacturers using this to their advantage for better fuel efficiency, speed or range?

Also, what would be the approximate speed, fuel or range benefits of using an elliptical wing or wing tip?

Answer 1

Airliners have some boundary conditions which make elliptical wings less preferable.

1. Wing sweep. A swept, elliptical wing suffers lift loss at its center and will easily see its tips stall first at low speed. Both can be avoided by de-sweeping the wing at the center and giving it a nonzero chord at the tip.
2. Structural mass. Scaling laws tell us that the structure will become proportionally heavier as airplane size goes up. In order to built lighter wings, a non-elliptical planform with more lift and chord at the center and less towards the tips will built lighter, thus avoiding lift and, consequently, induced drag.
3. Transsonic aerodynamics. In order to keep wing isobars aligned with local chord lines, camber, incidence and the relative chord of maximum thickness must be finely tailored over span. The result is a rather unintuitive chord and incidence distribution but gives an overall optimum of maximum lift at minimal surface area and structural mass for flight in the troposphere at Mach 0.78 to 0.85.

Sorry, but there are no range benefits or fuel savings to expect from an elliptical wing. Speed is limited by Mach effects and planform will not affect this. For an airliner with an elliptical wing you need to go back to designs like De Havilland's Dragon Rapide. The next design, the Dove, changed to a trapezoidal wing.

Answer 2

An elliptical lift distribution is optimal in terms of induced drag. It can be achieved by an elliptical planform (with a straight span and un-cambered airfoils and no twist), or by twisting a more complex wing.

An elliptical lift distribution is not optimal in terms of wing weight. It may also not be optimal in terms of handling qualities, tip stall, flutter, aileron reversal, etc.

Modern composite wings also flex a tremendous amount in flight some of this can be planned for, but you still need to design a wing that operates well at off-design conditions, not just at a narrow design point.

The theoretical elliptical wing is un-swept. To fly at transonic speeds of an airliner, you must have a swept wing.

So, with all that, a traditional elliptical wing is not optimal for an airliner. There are many good reasons not to use one.

That said, composites would allow more swoopy curves on a wing -- while wings still appear to be mostly straight. Even if they aren't elliptical, why not use more compound curves?

There are still good reasons to prefer straight lines. Airliner wings have control surfaces and high lift devices all over them -- slats, flaps, ailerons, and spoilers. All of these are mechanisms that need to operate without binding. To do so, it is best to make the hinge line straight -- or to design with aligned tracks and linkages.

Even with a straight hinge line, the 787 breaks the control surfaces into many segments -- to accommodate the flex of the wing.