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Thursday, August 1, 2013

Powered by the Pilot

By Frank Lombardi

In 1980, the American Helicopter Society (AHS International) established a human powered helicopter competition in the name of Igor Sikorsky. Over the years the competition has gone from a $10,000 to a $250,000 prize. After more than 30 years, recent strides seemed to bring the prize within reach of two dueling design teams. On June 13, 2013 at 12:43 EDT, AeroVelo, a small team of University of Toronto students, alumni, and volunteers, officially accomplished the feat. They had been going head to head with a team from the University of Maryland, whose design was also proving to be a worthy contender. I thought the science behind this accomplishment would be an interesting topic of discussion for this month’s column. For a more in-depth look at the teams and their designs, visit www.aerovelo.com and www.agrc.umd.edu.

The contest required flight duration of 60 seconds, within which time the craft had to reach a height of 3 meters, and remain within a 10-meter box. To tackle the challenge, the teams had to become intimately acquainted with the concept of power required vs. power available, as this is what achieving flight always comes down to.

Power is defined as the amount of work done, or energy transferred, over a certain time. To stay aloft, the pilot must transfer his stored energy to the air by accelerating the air downward. Therefore, the power required to turn the rotors must be kept as low as possible, putting it within the ability of a human pilot to produce for the required time.

Why was it so hard to do? For starters, a human power plant has a very low power output for its weight, so the pilot had to be strong and fit, yet light. The key factor was time. Very fit athletes have been known to be able to put out more than 1,000 watts (1.3 horsepower), but for short bursts. The pilot of the winning team was able to produce an average of about 772 watts for a minute, using approximately 900 watts during a 15 second burst to reach the 3-meter mark, then pedal at a reduced rate of around 600 watts for the next 45 seconds.

Helicopters require much more power than airplanes, so structurally it had to be light yet strong, with strength in key locations to minimize bending and withstand breakage. It was imperative that power be efficiently transferred to the transmission to minimize losses. Bobbing or surging would waste energy and require more power. The challenge was to build it just strong enough to withstand the stresses produced. Anything stronger than that would add unnecessary weight. Both teams employed a very lightweight carbon fiber truss system in an X-shape, with pilot in the middle, to support four two-blade rotors constructed largely of foam and mylar, and both used ingenious transmission methods of a one-way spool of cord that when wound would turn the rotors. The winning structure alone weighed in at just over 120 pounds, and 282 pounds with pilot included. Controlling such a design proved very challenging for both teams. The winners were able to do so by warping the fragile structure with pilot “body english.”

Low disk loading (rotor thrust divided by disk area) is key for maximum efficiency. It is important to work with as much air as possible. A big rotor handles a large amount of air, which means it can develop the required thrust with a lower downwash velocity, i.e., using less energy. The winning design’s rotors are each approximately 67 feet in diameter, and rotate at about 12 rpm. The aircraft is large, with a total span of 154 feet.

As expected, blade aerodynamics for this project were critical. The power required to hover depended on overcoming the induced drag (due to creating lift) and profile drag (due to pushing the airfoil through the air). Using the benefits of ground effect proved to be a key factor in keeping power requirements low. Being under-slung, the rotors operated in deep ground effect, less than one-half a rotor diameter from the surface. This minimized induced drag. In deep ground effect rotor performance is highly sensitive to changes in height, so coning had to be structurally designed out to keep the rotors virtually horizontal to the ground. Customizing airfoil shape along the rotor span, and tapering the chord from root to tip further decreased drag and optimized lift distribution.

In no way does this article sum up the monumental task that was accomplished. Besides the advances in engineering over the years, trial and error surely played a large role. But the biggest contributor to the accomplishment of this challenge is without a doubt the ingenuity and dedication of all persons involved. The long-standing contest has sparked new innovations in engineering, furthered understanding of human physiology, and fueled further projects in the area of human powered flight. Well done by all.

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