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First published online June 12, 2009
Journal of Experimental Biology 212, iv (2009)
Copyright © 2009 The Company of Biologists Limited
doi: 10.1242/jeb.021394

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Outside JEB

STABILITY: JUST KEEP ON FLAPPING

James Usherwood

Royal Veterinary College

jusherwood{at}rvc.ac.uk

If you have ever played with remote controlled helicopters, you will have a grasp of the issues of stability and manoeuvrability in slow flight. Part of the success of the recent, cheap toy helicopters is that they have been made so stable that even a child can fly them. However, this stability can make them somewhat boring to fly – they cannot dart around with great agility. So is there an inevitable conflict between stability and manoeuvrability in slow flight? Ty Hedrick, from the University of North Carolina, teaming up with Bo Cheng and Xinyan Deng, from the University of Delaware, has demonstrated that this may not be the case for flapping animals. Instead, a remarkably simple aerodynamic mechanism accounts for the time taken to recover and regain a steady heading after a perturbation (such as a turn) in slow-flying animals ranging from fruit flies to cockatoos; and changes that would increase this `stability' would also increase the potential for `manoeuvrability'.

The mechanism they propose is termed `flapping counter-torque' (FCT), and is merely what would be expected to happen if the flapping animal just kept on flapping in its normal manner after its perturbation. Consider a turn to the right: as the animal turns to the right, both wings flap forward and back as usual, but because the animal is turning, the left wing moves through the air relatively quickly as it flaps forwards while the right wing moves relatively slowly, and vice versa (left wing slow, right wing fast with respect to air) as they flap back again. All else being equal, this results in an aerodynamic `counter-torque' that acts counter to the direction of turn to stabilise the animal as it continues on its way. Hedrick, Cheng and Deng calculate the way in which this mechanism for recovering from a turn should scale with animal size and wingbeat frequency, and compared this with observations for a range of insects, birds and bats during slow flight involving turns of 60 deg. or greater. As predicted, with similarly shaped animals, the time taken (in terms of number of flaps) to lose half the turning speed is broadly constant, despite vastly different body sizes and wingbeat frequencies. For instance, fruit flies, bluebottles and hummingbirds take around two wingbeats to halve their rate of turning. In addition, this timing changes as predicted with different shapes: hawkmoths', bats' and cockatoos' wings are around double the size of those of the previous group (relative to body weight) and, agreeing with the exceedingly simple FCT model, take less than one wingbeat to lose half the turning rate.

Interestingly, the factors that increase stability due to the passive FCT mechanism, especially higher wingbeat frequency, also increase manoeuvrability. Hummingbirds may provide an example of this: males typically have higher wingbeat frequencies, potentially conferring benefits for both manoeuvrability during display flights and stability when recovering from a perturbation. The cost? Probably energetic: higher flapping frequencies might be expected to require more power, though this has yet to be demonstrated conclusively.

So, Hedrick and colleagues have shown a simple, passive mechanism available to slow, flapping-winged fliers conferring both stability and, potentially, agility. Sadly, this mechanism is unavailable to helicopters; I will have to wait for advances in flapping micro-air vehicles before finding the dream stable-and-yet-also-manoeuvrable toy in my (nephew's) Christmas stocking.

References

Hedrick, T. L., Cheng, B. and Deng, X. (2009). Wingbeat time and the scaling of passive, rotational damping in flapping flight. Science 324,252 -255.[Abstract/Free Full Text]

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This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Email this article to a friend Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Usherwood, J. Search for Related Content PubMed Articles by Usherwood, J. Social Bookmarking                         What's this?