Weis-Fogh (1973) analysed the ‘Clap and Fling’ mode of hovering flight in the small chalcid wasp Encarsia formosa. He argued that the difficulty of generating wing lift sufficient to support the insect’s weight at the very low Reynolds numbers involved (about Re = 20) is overcome by a substantial enhancement of lift associated with this special mode of flight. Recently, the ‘Clap and Fling’ mechanism has been studied in model experiments by Bennett (1977) at Re = 83000.

There is, however, an apparent non sequitur at an important stage in the argument presented by Bennett (1977). After concluding that at a Reynolds number Re = 83 000 ‘Clap and Fling’ lift benefits are real, though modest (about 15% improvement in overall flight), the author discusses whether or not much larger lift benefits might be obtained at the values Re < 100 characteristic of small insects observed to use the mechanism in hovering flight.

He arrives at a negative answer on the basis of a comment that ‘all known inviscid circulation schemes perform rather poorly as Re becomes small’. This statement refers, however, to the conventional schemes, including the fixed-incidence scheme which he uses for comparison. In those schemes, circulation is generated by separation of thin boundary layers and there are difficulties at low values of Re when the boundary layers are thick and less ready to separate.

By contrast, the ‘Clap and Fling’ mechanism simply uses conservation of mass to generate circulation: as the wings are flung open about their common trailing edge, mass conservation demands that air must rush into the opening gap. This makes the mechanism very robust as a function of Re and indicates that its advantages over conventional mechanisms should be enhanced for small values of Re.

Large insects in forward flight, on the other hand, can make considerable use of a 15 % lift gain, as in the case of Locusta migratoria recently observed (Cooter & Baker, 1977) to make a transition to ‘Clap and Fling’ aerodynamics at a transition between forward and climbing flight.

Bennett
,
L.
(
1977
).
Clap and fling aerodynamics - an experimental evaluation
.
J. exp. Biol
.
69
,
261
272
.
Cooter
,
R. J.
&
Baker
,
P. S.
(
1977
).
Weis-Fogh clap and fling mechanism in Locusta
.
Nature
269
,
53
54
.
Weis-Fogh
,
T.
(
1973
).
Quick estimation of flight fitness in hovering animals, including novel mechanism for lift production
.
J. exp. Biol
.
59
,
169
230
.

The main result of my effort (Bennett, 1977) was to demonstrate experimentally that ‘Clap and Fling’, at a large Re, is indeed capable of offering a modest gain in overall lift. The material to which Sir James Lighthill takes exception consists of a clearly labelled conjecture dealing with the effectiveness of ‘Clap and Fling’ at low Re. Here we are dealing with large order unsteady flow in a highly viscous regime. The strength and stability of circulation growth is at issue. To eliminate conjecture from this area it is necessary either to seek an approach to the full Navier-Stokes equations or to perform suitable model experiments.

As an experimentalist, I have no comment on the analytic approach, but I believe that the necessary experiments can be performed with a scaled-articulated wing operating in an oil bath. Hot film sensors offering appropriate sensitivity are reputed to be available. I would be pleased to correspond with anyone attempting this task.