Presentation discussing out-of-the-box thinking about new methods of flight: towards more bird-like flight

Bowers, Albion H.

NASA Technical Reports Server

TEDxNASA: Towards More Bird-like Flight: Thinking Outside the Box 
 
Assumptions are a fact of life, but when they couple to ideas, the ideas become 
concrete and the assumptions fade into the background. The problem is that the 
assumptions become solid unseen walls that we use to build the boxes we think 
inside of in our minds. Until the assumptions are rethought, the solutions cannot 
change. These limitations are artificial; they are self-imposed.  
[image: Al flying hang glider]  
 
I was captivated by flight when I was very young. When I was 12 years old I began 
messing around with hang gliders. I think about the things I didn’t know then, and 
what I assume I know now.  
[image: albatross in flight] 
 
The model of flight for time immemorial had been the flight of birds. This is a photo 
of a Wandering Albatross. The bird model was used until Wright brothers. Along the 
way they ran into the problem of adverse yaw. Adverse yaw is a product of induced 
drag. The high pressure is under the wing and the low pressure over the top of the 
wing. The air tries to “escape” around the wingtip, and this leaves energy imbedded 
in the air we can see as tip vortices. So to bank the aircraft, the wing with the greater 
lift will induce more drag, and so the wing you lift will end up going BACKWARDS. 
The aircraft will bank LEFT but yaw RIGHT. This is adverse yaw. 
[image: 1902 Wright Glider in coordinated banked flight] 
 
The Wrights kept crashing until they added a rudder to control the yaw of the 
aircraft. We still solve the problem of adverse yaw this way today. We had departed 
from the bird flight model. [black] 
 
At the same time as the Wrights, Ludwig Prandtl discovered a method to calculate 
the induced drag of wings, the Lifting Line. He calculated the Elliptical spanload has 
the minimum induced drag for wings of a given span [assumption]. The elliptical 
spanload solution is taught in all aerodynamics textbooks. All of this makes adverse 
yaw a fact of life and direct yaw control, usually vertical surfaces, a necessity.  
 
The elliptical solution was not good enough for Prandtl. He questioned his own 
assumptions. If we construct a wing with an elliptical spanload, and calculate the lift 
along the span, there is a wing root bending moment. This wing root bending 
moment determines the structure to carry the load. Prandtl asked the fundamental 
question: is there another spanload with the same lift and the same structure that 
produces LESS induced drag than the elliptical spanload? In 1932 he found the 
answer, a new spanload with 22% more span and11% less drag (top graphic).  
[image: spanload graphic] 
 
Prandtl never carried the thought experiment through to its logical conclusion. 
Reimar Horten used the new Prandtl spanload (which he called bell shaped), built a 
series of aircraft and made a fundamental discovery. Horten had to twist the wing to 
https://ntrs.nasa.gov/search.jsp?R=20110023706 2019-08-30T18:22:24+00:00Z
get Prandtl’s new spanload. And he found that the wing tips of a bell shaped spanload 
have induced thrust (top graphic). Induced drag becomes negative at the wingtips 
and pushes forward. Now when lift is increased on the wingtip of a bell shaped 
spanload, the induced thrust at the wingtip increases. So as the aircraft banks left 
and it will yaw left as well. There is no adverse yaw, and vertical tails are not 
necessary. [black] 
 
It took me 11 years to reverse engineer that puzzle. Horten did not write out his 
technique before he died. And when I calculated his solution, I could not believe that it 
could be correct. The box in my mind said it could not work.  
[image: Mike Allen & model sailplane] 
 
Fortuitously, I had a student working with me at that time, Mike Allen. Mike was not 
bound by my conventions. He built a small model using Prandtl and Horten’s 
principles. The spanload was achieved with twist. He allowed me to fly it. It worked 
perfectly. Mike was unencumbered by years of schooling, by the box of the 
conventional wisdom. He was able to see what I could not, assumptions had clouded 
my vision. 
[image: albatross] 
 
The tip feathers of a bird are very soft at the ends, and only if the load at the tip goes 
to zero can the feathers remain straight. If birds carried elliptical spanloads, the tips 
of their wings would bend straight up. Birds must minimize their chest muscles to 
carry their wing root bending moment. Birds minimize their inefficiency when 
maneuvering. Birds need to maximize their flight performance. For birds to fit into 
their ecological niche perfectly, they must solve these problems optimally. To carry 
their bodies, reproduce, and thrive. Birds have known this all along.  
 
Prandtl created the Lifting Line and the bell shaped spanload to optimize structure 
and performance at the same time. Horten created the twist for that spanload, and 
in addition found it solved control too. I recreated the solutions of Prandtl and 
Horten and Mike Allen built the model to reprove it worked. This is the solution that 
birds have known about all along. In design, when we reduce a solution to a 
minimum the result is clean, simple, elegant, and beautiful. It is the natural solution. 
 
If we follow this solution through to its conclusion, this idea not only reduces impact 
by saving fuel, but also in reduced manufacture energy (minimum structure), 
maintenance and repair (things don’t break if they aren’t there and elimination of 
the unnecessary), and this is true throughout the entire lifecycle of the aircraft. This 
compounds the minimization of the carbon footprint of the aircraft. We must 
reexamine the bird flight model.  
 
Prandtl rethought his assumptions to find a superior solution. To solve today’s 
problems we need to reexamine our assumptions. 
 
And I believe this is an idea worth spreading. Thank you. 

Photo by Phil Barnes 



Photo by Phil Barnes 


Towards More Bird-Like Flight: Thinking Outside the Box

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