Quick Estimates of Flight Fitness in Hovering Animals, Including Novel Mechanisms for Lift Production

7. Exp. Biol. (1973). 59. 169-230 l6g With 23 text-figures Printed in Great Britain QUICK ESTIMATES OF FLIGHT FITNESS IN HOVERING ANIMALS, INCLUDING NOVEL MECHANISMS FOR LIFT PRODUCTION BY TORKEL WEIS-FOGH Department of Zoology, Cambridge CBz ^EJ, England (Received 11 January 1973) INTRODUCTION In a recent paper I have analysed the aerodynamics and energetics of hovering hummingbirds and DrosophUa and have found that, in spite of non-steady periods, the main flight performance of these types is consistent with steady-state aerodynamics (Weis-Fogh, 1972). The same may or may not apply to other flapping animals which practise hovering or slow forward flight at similar Reynolds numbers (Re), ioa to io*. As discussed in that paper, there are of course non-steady flow situations at the start and stop of each half-stroke of the wings. Moreover, it does not follow that all hovering animals make use mainly of steady-state principles. It is therefore desirable to obtain as simple and as easily analytical expressions as possible which should make it feasible to estimate the forces on the wings and the work and power produced. In this way one may make use of the large number of observations on freely flying animals to be found in the scattered literature. It may then be possible to identify the deviating groups and to approach the problems in a new way. This is the main purpose of the present studies, which both include new material and provide novel solutions. Major emphasis must be placed on simplicity. This involves approximations since the true flight system is so complicated as to be unmanageable. However, when we confine ourselves to free flight and make use of the most reliable flight data available, the task is neither as difficult nor the conclusions as unrealistic as one would expect, since it is possible to introduce simple corrections. Although entitled ' Quick estimates', this does not mean that the approach is super- ficial, but rather that a procedure has been devised whereby the flight performance of a given animal can be evaluated quantitatively as well as qualitatively on the basis of only a few accurate observational data and a minimum of computation. In other words, given reliable information about bodily dimensions and wing-stroke parameters, the method enables one quickly to arrive at a first-order approximation so as to assess whether the animal makes use of well-established mechanisms or employs unusual or novel principles. The following procedure is, in essence, the strategy of the present investigation. From measurements of the size and shape of the wings, the geometry of the wing stroke, the frequency of the wing beat and the weight of the animal which is sustained in hovering flight one can calculate, on the basis of steady-state aerodynamics, the minimum coefficient of lift which must be ascribed to the wings. From the same measurements one can also calculate the Reynolds number under which the wings operate, 170 TORKEL WEIS-FOGH Vertical w Horizontal Chord, c(r) Fig. 1. Simplified diagram of normal hovering flight. (A) The instantaneous forces. (B) The animal seen horizontally from the side, and (C) vertically from above. and from this figure one can obtain (using published data on wings) the maximum coefficient of lift which may be expected under these conditions. If the minimum coefficient of lift for steady-state aerodynamics does not exceed the maximum coefficient of lift obtainable at the Reynolds number in question one assumes that steady- state aerodynamics are adequate to explain hovering flight, and from then one can go on to calculate the power requirement and other parameters of the flight mechanism. If, on the other hand, the minimum coefficient of lift for steady-state aerodynamics is greater than the maximum obtainable, it is clear that hovering flight cannot be explained by steady-state aerodynamics and that a new approach must be made. AERODYNAMIC RELATIONSHIPS The main simplifications are that the animal is assumed to make use of steady-state flow patterns only, that lift is produced at right angles to the direction of the relative wind, that the stroke plane is horizontal with no tilt, that the induced wind can be disregarded because it is relatively small and, finally, that the wing movements are sinusoidal and of similar shape and the same duration for both the morphological Flight fitness in hovering animals 171 dr Fig. 2. Diagram of the wing parameters (A) and the movements during hovering (B). upstroke and downstroke. As we shall see (p. 192), these assumptions lead to values for the average lift coefficients CL needed to remain airborne which are slightly higher than in the more complete treatment (Weis-Fogh, 1972). As to the specific aerodynamic power PJ, the estimates are too small by a factor of about 2 but, as we shall also see (p. 195), it is possible in a simple manner to make appropriate corrections. The instantaneous aerodynamic force on a wing element F(t, r) of chord length dr) and its vertical (lift) and horizontal (drag) components are shown in Fig. 1 A. In Fig. 1 and in the text, any quantity X which is a function of time t and distance r from the wing hinge, or fulcrum, is written as X(t, r). Appendix 1 gives a list of symbols and constants used. The justification for the simplifications is that the majority of insects, birds and bats have been found to tilt the long axis of the body towards the vertical (Fig. 1B) when hovering, so that the wings beat in an almost horizontal plane, usually symmetrically about the average positional angle y of 900 (Fig. 1C). (a) Coefficient of lift Fig. 2 A shows how the outline of a wing of total length R, and in particular the wing chord dj), varies with the distance r. In practice I have found that most wing 172 TORKEL WEIS-FOGH contours can be described quite faithfully by means of a simple mathematical functionl whether the moving wing is morphologically two-winged as in Lepidoptera and Hymenoptera or consists of only one part as in Diptera. It should be stressed here that there are hovering insects which make use of two pairs of wings which beat out of phase, as is the case in Odonata and Neuroptera, and these insects are not included in the present analysis. The angular movement of the long axis of the wing in the horizontal stroke plane is 7(0 = y + W>sin(2nnt), (1) where y is the average angle, <f> the stroke angle, n the wing-stroke frequency (number of complete stroke cycles per unit time) and t is time. The angular velocity and acceleration are then dyjdt = 77710 cos {zrnit), (2) d*yldt2 = -2nW<f)sin(z7mt). (3) The instantaneous force on a wing section at distance r from the fulcrum depends on the square of the instantaneous velocity of the section (v(t, r))2, on the force coefficient, and on the area of the segment of width dr (see Fig. 2 B). In the case of the instantaneous lift produced by the section, we then have 2 dL(t, r) = \px CL(t, r) x area x velocity . (4) Thus dUt, r) = \pxCL(t, r)xc{r)drxr\dyjdtf, (5) where p is the mass density of air and GL(t, r) the instantaneous coefficient of lift at distance r. This means that 2 x i i dUt, T) = i/OT^V * CL(t, ') c(r)r dr x cos (2nnt). (6) The four terms will be treated in turn. The first is a constant. The second (CL(t, r)) will be taken as constant in this treatment for the following reasons. In large insects and hummingbirds the wing is twisted linearly with respect to r so that succeeding sections have the same geometrical angle of attack (Jensen, 1956; Hertel, 1966). In small insects like DrosopMla the wing is rotated as a whole plate but the lift coefficient hardly varies at all with the angles of attack actually applied by that insect (Vogel, 1967a). In both cases the angle of attack is set almost instantaneously at the beginning of each half-stroke and remains constant so that it is justifiable to consider the average coefficient of lift CL as a constant, as previously discussed (Weis-Fogh, 1972). Of course this need not always be the case, but it is a reasonable approximation to begin with. It also has the advantage that one can use steady-state lift/drag diagrams for fixed wings of known shape and aspect ratio later in the analysis. The third term in equation (6) is the derivative with respect to r of the second moment S of the total wing area of all wings about the fulcra; it depends only on how the area varies with the distance r from the base, i.e. it is a function of the contour of the wing. Since many biologists are unfamiliar with this expression it is treated in Appendix 2. The general expression is S = <rcR\ (7) where a is a shape factor characteristic for the particular wing shape. It can be found by graphical integration based on actual wings or from the mathematical function by Flight fitness in hovering animals 173 Table 1. Shape factor a for the second moment of the area of two whole functional wings (or two pairs of wings) (eq. 7), shape factor rfor the third moment of two whole wings (eq. 50), together with the radius of gyration rg, the chord length ca at distance r^ the factor in eq.

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