#219 from R&D Innovator Volume 5, Number 6          June 1996

Thank Goodness I Owed $100,000
by Paul B. MacCready, Ph.D.

Dr. MacCready is founder and chairman of AeroVironment, Inc. in Monrovia, California, providing products, services, and developments in environment, alternative energy, and efficient vehicles.  In 1956, Dr. MacCready was international sailplane champion.  In 1980 he was awarded the Ingenieur of the Century Gold Medal by the American Society of Mechanical Engineers.   In 1982, he received the annual Lindbergh Award for contributions "to achieving a balance between technology and the environment", and has been honored by many groups for pioneering developments of vehicles for land and air.


I had guaranteed a loan for my friend to start a company, but the company didn't succeed and now I was stuck with a $100,000 obligation.  While daydreaming, mining the subconscious, on a vacation trip in 1976, I mused about a possible connection between this debt and the 1959 Kremer Prize for human-powered flight that I knew had reached 50,000.  When I noted in a newspaper that 1 was now worth exactly $2, the light bulb of invention turned on!  The prize equals the debt!

Designing for Success

Now, continuing the vacation, I had the motivation and the opportunity to think seriously about the Kremer Prize.  With my Ph.D. in aeronautics from Caltech, I knew something about airplane design.  However, all my early design concepts were like those of some British teams that had logically started with the most efficient airplanes, sailplanes, and made streamlined versions lighter and larger.  These elegant, complex machines got off the ground on the initial high power of the pilot, but could not stay aloft long, could not turn effectively, and took forever to repair after a crash.  I dropped the subject from its high priority in my thinking.

A week later, lateral thinking or serendipity unexpectedly intervened.  I had recently written an article comparing hang gliders to birds, and was in the process of writing an article on the speeds and lift coefficients of soaring birds.  While driving along I was making observations on the flight of hawks and vultures.  Note the time to circle 360o, and estimate the bank angle, and you can quickly calculate flight speed and turning radius; look up statistics on wing loading typical of that species, and you can estimate lift-coefficient, power per pound, etc.  Considering the scaling laws interrelating birds, hang gliders, and sailplanes, the new simple solutions to human-powered flight arose.  Another "Aha!" moment.  Keep weight constant and increase size, and power needed goes down.  Triple all dimensions of a 30-foot span hang glider, and the power for level flight is cut to 1/3 -- from 1.2 horsepower to 0.4 horsepower, now within the capability of a strong cyclist.  Success was assured, if a plane so large and light could be constructed, even if streamlining were virtually ignored.

Insights from building indoor model airplanes in the 1930s and hang gliders in the 1970s, both using exterior wire bracing, led the way to the Gossamer Condor wing structure:  exterior wire bracing (0.030-inch piano wire) supporting thin-wall aluminum tubes loaded in compression.

The giant size, while allowing for low power, also presented some disadvantages.  The mass of air with which this huge airplane interacted greatly exceeded the mass of plane and pilot, which can cause control troubles, especially while turning.  Also, because of the speed, vehicle efficiency would be decreased by turbulence, even turbulence so light it would be undetectable in an ordinary airplane.  Unexpected advantages from low speed also presented themselves:  only minimal flight training was needed, flight testing was easy, and the many accidents we experienced were gentle.

Theory defined the huge wing size, and put a high priority on light weight.  The project evolved from there, with an attitude of paying no heed to how airplanes were designed in the past.  A strategy was to design for ease of construction, repair, and alteration since changes and damage would be common.  The team also had the philosophy of assuming problems will be solved by the simplest route.  Sometimes this didn't apply, but in most cases it did, and the time saved then permitted proper priority to be given to the problems that turned out to be tough.  We used standard bicycle pedals, cranks, and sprockets for extracting power from the "engine."  Toy wheels were adequate for takeoff, although they often had to be replaced.  The leading edges of the wing were made from corrugated cardboard. 

But we weren't tied to this simple approach, as sophisticated computer techniques were used when needed such as for designing the airfoil and propeller and working on stability and control challenges.  If a part broke during testing, we'd make it stronger.  If some parts had never broken, we would replace them with weaker but lighter ones. 

During development, we made approximately 400 flights that included four major, and innumerable minor, crashes.  Each crash was informative, and helped us toward material design changes.  (This is not the way to develop airliners.)  We focused doggedly on winning the prize, not even making complete drawings of the plane until after the prize was won.  We kept improving and cleaning up the plane, and on August 23, 1977, Bryan Allen piloted it around the Kremer course we had set up at Shafter Airport near Bakersfield, California.

The Gossamer Condor is hanging next to the Wright brothers' 1903 Flyer and Lindbergh's Spirit of St. Louis at the National Air and Space Museum (one of five pioneering vehicles by AeroVironment acquired by the Smithsonian).  The participants in this adventure certainly felt the kind of excitement that the early flight pioneers must have experienced.  I was surprised at the interest of so many other people in this "impractical" airplane.  Subsequently, as project after project evolved leading toward more obvious societal value, I have become wiser and so better appreciate the non-quantifiable value of pioneering.

More Fun

Henry Kremer then put up a 100,000 prize for human-powered flight over the English Channel.  We realized how readily the Gossamer Condor could be improved, and that a small decrease in required power greatly extends the time a human can provide the power.  We built the Gossamer Albatross, a next-step clone of the Gossamer Condor, using carbon fiber tubes instead of aluminum, many more ribs to improve the accuracy of wing shape, and greater structural integrity to provide safety for a much more demanding task. 

In 1979, Bryan Allen piloted the Gossamer Albatross across the Channel on a fantastic flight lasting about three hours, fighting head winds and turbulence, and somehow coping with only a 2-hour supply of water.  If it had been high tide the flight would have had to be 150 feet longer and would probably not have succeeded.  The engineers, but not Bryan, were delighted with the design that had not wasted resources on being better than required. 

DuPont had supported our Gossamer Albatross program, and then agreed to sponsor our project for solar-powered flight.  Thus our Solar Challenger in 1981 was piloted 163 miles from Paris to an airfield in England at 11,000 feet, powered only by sunbeams.  The pilot had to weigh less than 125 pounds for the plane to fly at all -- solar power will not obsolete jet engines. 

Consequences of the flight included getting the public to appreciate that photovoltaic cells can be a significant part of the world's energy future, and being asked by General Motors in 1987 to develop the AV/GM Sunraycer solar-powered car and later the Impact battery-powered car.  Another consequence has been our 100-foot solar-powered Pathfinder that in 1995 climbed over 50,000 feet.  Now we are working on a more advanced solar craft that will incorporate an energy storage system to remain at 65,000 feet for many months, a practical "non-orbiting satellite" for telecommunications and stratospheric monitoring.

Our Secret Weapon

On the lecture circuit after the Gossamer Condor program, the most-asked question was why we had won the Kremer prize while other teams with far more resources and experience had for years been doing excellent pioneering but had not come close to winning.  I gave this question a lot of thought.  Finally, after I learned more about the mental blinders that characterize all humans, I realized that my secret weapon was being totally inexperienced in aircraft-wing structural design, while at the same time having familiarity with those fragile indoor model airplanes of my youthful hobby and the newly-evolving hang gliders.  The other teams certainly were knowledgeable about hang gliders and indoor models, but they followed their expertise, the techniques and principles involved in modifying "state-of-the-art" aircraft, and did not range out to look at the task unconventionally.

There are many factors helping creativity, such as persistence, a positive attitude, treating barriers just as things you get around somehow, creative associates, opportunity, and good luck.  I highly recommend a $100,000 debt to stimulate motivation. 

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