Biology and physics of locust flight. III. The aerodynamics of locust flight

Abstract A proper understanding of how locusts fly must be based upon knowledge of how the wings are moved. A desert locust was suspended from a balance and placed in an air stream so that it flew under nearly the same conditions as during natural forward flight. Four stroboscopic slow-motion films were selected for measurement. The movements of the wings, i.e. their positions, velocities and accelerations, were then calculated in sufficient detail to show how these quantities vary with time during one complete wing stroke. The aerodynamic lift and drag of the entire natural wing were measured in a wind tunnel with the wing arranged in different positions relative to the flow. By placing it in the boundary layer of the tunnel, the wind speed was graded from tip to base in approximately the same way as during the actual flight. There is therefore no error due to scale effect or to the induced drag. In most respects the wings resemble ordinary, slightly cambered airfoils. Their characteristics are given as polar diagrams. The kinematic and aerodynamic analyses make it possible to calculate the forces which act upon the locust at any instant of time. It is here necessary to presuppose that the non-stationary flight situations are essentially similar to a sequence of stationary situations. For locusts, this presupposition is justified: (i) from theoretical estimates of the quantitative effect of non-stationary flow; and (ii) from control measurements of the average thrust and lift produced during flight. It was found that the calculated vertical force, when averaged over an entire wing stroke, equalled the average reduction in body weight, as measured directly on the flight balance. Similarly, the average thrust of the wings corresponded to the drag of the body. The analysis shows how the aerodynamic forces vary during the wing stroke. The hindwings are responsible for about 70 % of the total lift and thrust. About 80 % of the lift is produced during the downstroke. During flight at normal lift the angles of attack (middle part of wing) are small during the upstroke and vary between 10 and 15° during the downstroke. When the lift was larger or smaller than the body weight these figures increased or decreased respectively. The forewings are peculiar in two ways: (i) during the middle part of the downstroke a true flap (the vannus) is put into action; (ii) during the upstroke the proximal part has a Z-shaped cross-section and gives but little lift and drag. The hindwings are characteristic in that the posterior part (vannus) is flexible and becomes moulded by the wind, increasing the angle of attack at which stalling occurs to about 25°. Since both the movements of the wings relative to the body and the aerodynamic forces are known at any instant, the exchange of power with the surrounding air can be calculated. The moments of inertia of the wing mass being known, the power for accelerating the wings can also be estimated. The sum of these contributions is the power which passes the wing fulcrum; this estimate is used in a later paper (part IX) where the energetics of flight is discussed in detail. The diagrams are correct to scale. The restriction of freedom caused by the suspension is discussed, together with the possible errors of a stationary analysis.