Gliding like a frigate bird

July/August 2020

Q. A magnificent frigate bird is gliding in calm air with a ground speed of just a few knots. Normally, such a low ground speed would be impossible for a U-2 spy plane. But the pilot, looking at the tropical forecast, predicts that later in the day he can match the frigate bird’s low ground speed. How might that be possible?

Draft a response of no more than 250 words and email it by midnight Eastern Aug. 4 to aeropuzzler@aiaa.org for a chance to have it published in the September issue.

JUNE’S AEROPUZZLER

FLAPPING FLYER: We asked you what pocket-sized aircraft you might bring back in time to help convince Otto Lilienthal that his apparatus won’t work. There was no winner this month, so we asked Haithem Tata of the University of California, Irvine, to answer.

RESPONSE: I would take with me either the Aerovironment Nano Hummingbird or the Harvard Robofly, and also point him to the work of his contemporary, Charles Renard. In “Nouvelles experiences sur la resistance de l’air” (1889), Renard showed that power loading is proportional to the square root of the wing loading, W/S, where W is the weight of the aircraft and pilot and S is the wing area. So, in order to reduce the power requirement to a value that could conceivably be generated by human muscles, the area of his wings, S, would have to be large — so large, in fact, that while he could glide, he could not flap or oscillate his wings. I would then let him marvel at how the flapping wings of the Nano Hummingbird and Robofly easily support their weight.