The Prandtl Wing: Maximum Efficiency

21 Apr 2017
09:35 - 10:05

The Prandtl Wing: Maximum Efficiency

Understanding current wing technology ­gives us insight for possible improvement. In Applications of Modern Hydrodynamics to Aeronautics [Ludwig Prandtl, NACA Report 116, 1921] “…the wing system having the least drag …” and this is the elliptical load requirement of all wings for a given span. These wings result in very definite wing tip vortices.

Why is “a given span” the ultimate goal? We believe this is misdirection, it is a bad requirement. What happens if we use a different definition as a constraint. Instead let’s use structural weight. What does that result teach us?

Prandtl writes another paper [Ludwig Prandtl, Uber tragflugel kleinsten induzierten widerstandes, 1933]. Prandtl removes the elliptical load, uses the same structure with a different lift profile and lower drag. This is 11% more efficient, the load tapers to zero at the tip, and the wing exhibits thrust at the tip instead of drag. Why is all of this of any interest?

Using the new tapered load on a wing, the lift vector is rotated across the wing. This rotation of the lift vector comes from a distorted flow field, and this distortion allows the aircraft designer to eliminate the tail completely. The adverse yaw is eliminated. Tails constitute about 30% of total aircraft drag. This is very different to the traditional approach.

Additionally, the same rethinking of the wing can be applied to propulsion systems (no more “minimum induced loss”). We believe this results in a gain of 15.4% thrust for the same input torque/power.

We discuss this technology and its current state [Bowers, Albion H, NASA TP-2016-219072].