The aerodynamics of revolving wings II. Propeller force coefficients from mayfly to quail

High force coefficients are required to account for hovering flight in animals ranging from small insects (e.g. Ellington, 1984a,b,c,d,e,f)to medium-sized birds (Norberg,1975) and bats (Norberg,1976). Ellington et al.(1996) showed leading-edge vortices to be present over flapping real and model hawkmoth wings. These leading-edge vortices, created by dynamic stall and maintained by spanwise flow, contribute significantly to lift production in slow-flying hawkmoths. The accompanying paper (Usherwood and Ellington, 2002) shows this phenomenon, and high force coefficients, to be a stable aerodynamic characteristic of revolving model moth wings. The present paper aims to determine how robust this characteristic is to variations in wing design and Reynolds number. Model hawkmoth wings with a range of aspect ratio and real and model wings from a number of `key species' are tested.

Insects have wings of AR ranging from 2.8 (butterflies, Dudley and DeVries, 1990) to 10.9 (craneflies, Ellington,1984b). Vertebrates capable of hovering have wings ranging in AR from 4.4 (pied flycatcher, Norberg, 1975) to 8.2(hummingbirds, Wells, 1993). The aspect ratios of the wings in this study range from 4.53 to 15.84, and angles of incidence greater than 90° are tested, so our results are relevant to studies of animals that hover using a vertical stroke plane or swim using drag-based propulsion.

Conventional propellers and wind turbines revolve, but delayed stall and high force coefficients typically exist only at the wing (rotor) bases[Himmelskamp in Schlichting,1968 (propellers); Graham,1992 (wind turbines)]. Otherwise, flow over high-ARpropellers and turbines at high angles of incidence stalls conventionally, and blade-element analyses using coefficients derived from steady, two-dimensional flow conditions are effective. So, it is reasonable to expect that the high-lift mechanisms described by Usherwood and Ellington(2002) for wings of AR=6.34 might gradually or suddenly decline with increasing aspect ratio.

Reynolds number Re has a large impact on the behaviour of fluids flowing past an object; Vogel(1981) presents the concepts clearly in a biological context. It is therefore reasonable to expect Re to have a similar bearing on the flow (and so lift and drag)acting on wings. Indeed, it is frequently supposed that many of the unexpected phenomena associated with insect flight may be accounted for by the low values of Re at which they operate. However, predictions based on Re arguments are not always founded; while it is true that viscous drag forces are higher for smaller animals, it is not true that the very small and `fringe-winged' insects (Re<28) `row' through the air using drag-based mechanisms (Ellington,1984a). Indeed, the vertical stroke plane associated with drag-based weight support is surprisingly seen in larger insects (butterflies)at Re values of approximately 2800(Maxworthy, 1981; Ellington, 1984a; Sunada et al., 1993).

So, at this stage, it is unclear whether insects, small, and even large,vertebrates operate in the same flow regime; it is not known whether there are significant qualitative differences in flow analogous to the transition between laminar and turbulent conditions. Is there a subtle gradient from one regime to another (this does not appear likely given the properties of normal laminar/turbulent transitions)? Or is there a biologically significant threshold above (or below) which certain aerodynamic mechanisms are unable to operate? If so, where are these boundaries?

To gain more information of biological interest, this study investigates several key species for which the appropriate parameters are known. Bees are of particular interest as they show a considerable size range both within a species (between different castes of bumblebee) and among related genera (e.g. the Euglossini or orchid bees). A bumblebee wing was therefore tested to provide information on the aerodynamic properties of wings in revolution for an insect for which there is a great deal of morphological, kinematic and energetic data (Dudley and Ellington, 1990a,b; Cooper, 1993) and which should also be applicable to studies of euglossine bees(Casey and Ellington, 1989; Dudley, 1995; Dudley and Chai, 1996).

To determine the steady aerodynamic performance of wings in revolution at low and high Re, a `mini-spinner' was built covering the range of Re from 1100 to 26000. The model animals chosen for these extremes were the mayfly Ephemera vulgata and blue-breasted quail Coturnix chinensis, for which fresh wings were available. The `mini-spinner', a smaller and simplified version of the more elaborate propeller described in Usherwood and Ellington(2002), proved a robust and effective tool. It also allowed the use of real bird wings over a limited size range, so both real and model quail wings were tested.

Smoke observations for simple model hawkmoth wings by Usherwood and Ellington (2002) supported the finding (Ellington et al.,1996) that the mechanism for high lift is a leading-edge vortex. However, wing speeds and designs in the present study precluded such observations. As shown in Usherwood and Ellington(2002), flow separation can nevertheless be inferred if the resultant force is approximately normal to the wing surface.

Force measurements were made using two experimental propellers. The larger design, described by Usherwood and Ellington(2002), allows `early' (from the first half-revolution) and `steady' vertical and horizontal forces to be measured using foil strain gauges. The smaller, much simpler, design could only measure `steady' forces, but could do so over a much larger speed range.

Vertical and horizontal force coefficients were derived from measured vertical forces and torques as described by Usherwood and Ellington(2002). Following Usherwood and Ellington (2002), the term`propeller coefficient' is used to distinguish force coefficients derived from propeller experiments.

Unless otherwise stated, all aspects of the experimental method for the large propeller experiments were identical to those described by Usherwood and Ellington (2002). Methods of wing construction, force measurement and data processing were suitable for a limited Re range, appropriate for hawkmoths and queen bumblebees.

The standard hawkmoth planform was adapted to produce wing pairs with a range of five aspect ratios (Fig. 1A): all wings were thin and flat. The wing length in every case,including the offset due to the method of attachment to the propeller head(see Usherwood and Ellington,2002), was 556 mm, and the relevant second, r̂₂(S), and third, r̂₃(S),non-dimensional wing moments of area remained constant: r̂₂(S)=0.547 and r̂₃(S)=0.588. Wing thickness was constrained by the material used, and the angular velocity was kept constant. The mean wing thickness (relative to mean chord) and Re (defined using the conventions of Ellington, 1984f) were therefore confounding variables (Table 1). The constant angular velocity also resulted in smaller signal-to-noise ratios for higher-AR (narrower) wings, because they experienced smaller forces.

The planform for a bumblebee (Fig. 1B) Bombus terrestris wing design was taken from a previous study and used to produce a wing pair as described for the hawkmoth(Usherwood and Ellington,2002). Bumblebee B27 was selected because its aspect ratio and radii for moments of area were the closest to the population means. Again, the wing shape was kept constant so that the offset due to the attachment of the wings to the propeller head changed the wing moments, as shown in Table 2.

The propeller was driven slightly more slowly than for the hawkmoth tests,at 0.147 Hz, thus reducing the Reynolds number to 5496, a value appropriate for the largest queen bumblebees and large euglossines. Further reduction in speed produced very noisy results because of dominating mechanical oscillations, while reducing the wing length would have confounded the effects of the offset, which otherwise was kept constant for experiments on the large propeller.

Fig. 2 shows the basic construction of the `mini-spinner'. It uses the same principle for the measurement of vertical forces (moments about a knife-blade fulcrum, forming a`see-saw') as used by Usherwood and Ellington(2002) but different principles for torques. Unsteady force measurements and flow visualisations are impossible with the mini-spinner, but the smaller size requires higher frequencies of revolution for Re similarity, with the advantage that low-Re models can be used while minimising the effects of random air movements; random air movements will be negligible compared with the flow generated by the wings. The size and relative stiffness of the mini-spinner also allows the use of real bird wings. The extremes in Re are represented by model mayfly forewings and both model and real quail wings.

Model mayfly forewings were based on those from a 26.4 mg male mayfly Ephemera vulgata (Fig. 3A). The hindwings were not included in the model because they were small and their orientation during flapping flight was unknown. The planform was maintained, so the small shift due to the diameter of the rotor head (of diameter 9 mm, causing an offset of 4.5 mm) influences the wing moments. Table 2 shows the resulting wing parameters.

The model mayfly wings were constructed from stiff, thin (0.15 mm) card glued to 0.57 mm diameter wire running half-way down the ventral surface of the wing. This resulted in a wing thickness at the position of mean chord of 5% of wing chord.

Geometric angles of attack were set by rotating the wire wing-stems within the propeller head and measured using a inclinometer, which achieved an estimated accuracy of ±2°. Angles from 0 to 90° were used, with 10° increments. The angles of incidence were calculated as in Usherwood and Ellington (2002).

A 61.6 g blue-breasted quail Coturnix chinensis was killed by decapitation as part of another study(Askew et al., 2001). The right wing (fresh mass 2.29 g) was removed at the base of the humerus and pinned to dry using hypodermic needles. The pinned position mimicked a typical mid-downstroke position determined from the video recordings of ascending flight used by Askew et al.(2001). Once stiff, the wing was connected using four sutures to a rod bent to follow the humerus and radius/ulna. It was only possible to use a single wing because a second right wing accurately matching the first was not available and the dorsal/ventral asymmetry of bird wings makes use of the left wing inappropriate. To balance the propeller, the stem of the rod attached to the wing was allowed to protrude through the propeller head. The wing was only slightly twisted(maximally 3°) but was strongly cambered, particularly at the base. At the`elbow' joint between the humerus and ulna/radius, the wing depth (including camber and thickness) was 28.8% of the chord; at the `wrist', over the alula base, this value was 24.1%; half-way between the alula and wing tip, it was 10%.

The wing, once attached to the rod, was scanned(Fig. 3B), and the appropriate moments were calculated. A print-out of the scanned image was used as a template for a wing model. The model wing was constructed from stiff, thin(0.3 mm) card glued to 1.4 mm diameter wire running half-way down the ventral surface of the wing. This resulted in a wing thickness at the position of mean chord of 4% of the chord. The single model wing was counterbalanced in the same way as the real wing. Again, the propeller head was considered when calculating wing moments (Table 2).

Angles of attack α were set by rotating the wire wing stems within the propeller head and measured using an inclinometer. The arbitrary`representative' α was taken across the wing chord from the base of the alula to the tip of the innermost primary. The angle of incidenceα′ was calculated as in Usherwood and Ellington(2002).

A variable power supply was used to drive the propeller head, using a 22 mm diameter, 12 V motor (RS) connected to a 24 mm diameter 7.2:1 gearhead. The rotational frequency was varied using the power supply until it reached 3.3 Hz for the model mayfly wing pairs, as judged with the use of a Drelloscop Strob 2009S07 stroboscope. Rotational frequency was set before and checked after each test. A rotational frequency of 3.3 Hz resulted in an Re based on the mean chord (Ellington,1984f) of 1100, close to values estimated from video recordings of mayflies in ascending flight taken in the field and reasonable for the parameters described by Brodsky(1973) for the same species.

The rotational frequency for the quail wing and model was 12.5 Hz,resulting in an Re of 26000 based on the mean chord. Askew et al.(2001) have observed a maximum downstroke angular flapping velocity ω of 190° s^-1 for a quail with a wing length of 95 mm. This corresponds to a maximum Reof 48000, so 26000 for the propeller implies that it is operating in a similar flow regime to the flapping wing for most of the downstroke. The upstroke has little aerodynamic effect.

The mini-spinner as shown in Fig. 2A has the motor, gearbox and propeller head oriented vertically. During steady revolution, a moment is created about the fulcrum due to the vertical force and the arm length to the right of the fulcrum. This is equal and opposite to the moment created by the tension force applied from a wire connected to the under-hook of a Mettler BasBal BB240 balance situated directly above, and the appropriate arm length to the left, of the fulcrum. This arrangement was calibrated with the repeated application of a 1 g mass to the centre of the propeller head, which resulted in an imperceptible deflection and produced values consistent with the geometry of the arrangement and the accuracy of the balance. The inherent linearity of the `see-saw'arrangement was confirmed during set-up and testing. Thereafter, a single point calibration was sufficient. Five (or 10 at values of α of particular interest) vertical force measurements were made at each angle of attack.

Aerodynamic torques were measured by rotating the motor, gearbox and propeller head unit to a horizontal orientation as shown in Fig. 2B. During steady revolution, the moment about the fulcrum is equal to the aerodynamic torque from the revolving propeller head and wings. This torque can thus be calculated given the distance from the fulcrum to the wire attachment (140 mm)directly below the balance. The same number of measurements was made as for the vertical forces, and the aerodynamic effects of the motor head and stings were determined from tests without wings and removed.

Each vertical force and torque value was the mean of a pair of runs,starting with the wings in opposite positions. The measurements taken for each run consisted of a `zero' and a 9s average after steady revolution had been achieved. This takes into account any error due to an imbalance between the wings.

Vertical forces were measured exactly as for the mayfly wings except that the moments were opposed by a stiff steel shim on which was glued a pair of strain gauges instead of the vertical wire leading to the balance: forces were too large and variable for the balance to provide accurate results. Signals from the strain gauges were amplified electronically before being sampled at 50 Hz using a Macintosh Quadra 650. Vertical force signals were averaged over 50s. Five values from 10 paired runs, taking imbalance into account as above,were found for each angle of attack.

Fig. 4 shows `early' and`steady' results for the hawkmoth wings over a range of aspect ratios. In each case, the `pooled' data for the flat hawkmoth wings shown in Usherwood and Ellington (2002) are presented(both `early' and `steady' values) for comparison. The shift between `early'and `steady' values seen at intermediate angles of incidence for the standard hawkmoth wings is visible for all aspect ratios. The relationship between both C_h,early and C_h,steady(Fig. 4A,C) and α′at low angles is very consistent for wings of every AR tested. However, under both conditions, AR has a progressively greater effect at higher α′. Low-AR wings achieve considerably higher maximum horizontal force coefficients, peaking at C_h,early=3.4 and C_h,steady=3.5 nearα′=90°, while the highest-AR wings achieve maximum horizontal force coefficients of only 2.5(Fig. 5).

The relationship between both C_v,early and C_v,steady (Fig. 4B,D) and α′ is dependent on AR While the maximum values reached, approximately 1.7 for C_v,early and 1.3 for C_v,steady, are very similar for the entire range of aspect ratios and occur at similar values of α′, between 40 and 60°, the initial gradients differ significantly. The relationships are approximately linear between α′=-20 and +20°. The gradients dC_v/dα′, with their 95 % confidence intervals over this range, are given in Fig. 6. Lower-AR wings, and wings in `steady' revolution, have lower gradients.

Fig. 7 shows the results for the Bombus wings. C_h and C_v,both `early' and `steady', show remarkably few differences compared with the`pooled' hawkmoth results from Usherwood and Ellington(2002).

Fig. 8 shows the `steady'force coefficients for the model mayfly and model and real quail wings derived from force measurements using the `mini-spinner'. These are plotted with the`steady' coefficients for Bombus and pooled hawkmoth wings. Slight differences are visible in the horizontal force coefficients, with the mayfly showing lower coefficients (although with high standard errors) and the quail higher coefficients. The relationship between C_v,steadyand α′ was remarkably consistent over the whole range of wings tested. All wings achieved maximum vertical force coefficients well above 1 at values of α′ between 40 and 60°.

Deflections were visible in the revolving quail wings, with the tips of both real and model wings bending backwards, especially at higher values ofα. The values of α′ shown for the quail wings in Fig. 8 must therefore be considered approximate and lower than the true values.

These results suggest that blade-element analyses of revolving,perpendicular `wings' may be in serious error if conventional, steady,two-dimensional force coefficients are used. In particular, older analyses of pectoral-fin swimming in fish (Blake,1978) may have to be re-assessed.

The measurements made on the bumblebee wings are near the lower limits of the large propeller rig. However, all propeller coefficients(C_h,early, C_h,steady, C_v,early and C_v,steady) agree so well with the values found for hawkmoth wings that little comment is possible,other than to observe that similar aerodynamic mechanisms are almost certainly available to bumblebees and hawkmoths.

Remarkably consistent, high force coefficients are achieved for simple,thin, flat model wings in steady revolution at Re from 1100 to 26000;the real quail wing, with thickness and camber, not to mention feathers,produces very similar force coefficients. Drovetski(1996) gives polar diagrams from 0 to 25° for simple model galliform (game bird) wings. The video recordings of Askew et al.(2001) (and, consequently, the wing and wing model used in this study) do not show the trailing-edge notch described by Drovetski (1996);it appears that such a notch is present only in gliding flight or is an artefact of pinning the wings in a fully extended position. The maximum lift coefficients cited by Drovetski(1996) for wing models ranging from California quail Callipepla californica to turkey Meleagris gallopavo were between 0.61 and 0.80; it seems that some aspect of revolution may as much as double the vertical force coefficients. Values for blackbird Turdus merula, house sparrow Passer domesticus and mallard Anas platyrhynchus(Nachtigall and Kempf, 1971)range from 0.9 to 1.1, higher than for the galliforms of Drovetski(1996) but still considerably lower than those for revolving quail wings.

The aerodynamic phenomenon resulting in high force coefficients, presumably associated with the creation and maintenance of leading-edge vortices, appears remarkably robust. Some of the force measurements on the flapping Drosophila model of Dickinson et al.(1999) are equivalent to the`early' measurements described here, and their simple harmonic relationships are shown in Fig. 9 together with the `early' results for the hawkmoth AR range. The Drosophila model shows a higher minimum horizontal force coefficient at low values of α because of relatively larger viscous forces. However,at higher values of α′, there is very good agreement in both C_h and C_v with the values shown for hawkmoth planforms. If it is reasonable to suppose that shifts from `early' to`steady' conditions are relatively constant throughout the Re range,then it appears that similar force coefficients are possible from Drosophila (Re≈200) to quail (Re≈26 000). If the mechanism for these high force coefficients is indeed the leading-edge vortex, then the insensitivity to Re is not as surprising as it may appear. Leading-edge vortices over sharp, thin delta wings are effective lift-producers for slow paper aeroplanes, Concorde and the space shuttle; a vast range of Re.

Fig. 6 shows relationships between aspect ratio and the rate of change of vertical force coefficient with angle of incidence, dC_v/dα′. The relationships for both `early' and `steady' conditions are very similar: the gradients for regression lines through each plot on Fig. 6 are not significantly different. This phenomenon is well known for translating wings and is due to the larger downwash of lower-ARwings, which produce greater forces for the same wing length. This results in a greater downwash angle ϵ, and so a smaller increase in `effective angle of incidence' (α<sub>r</sub>′=α′-ϵ) for a given increase in α′. The non-zero slope of the dC<sub>v</sub>/dα′ relationship for `early' conditions shows that the `early' induced downwash, while small, is not negligible; even before development of the propeller wake, the tip vortex appears to produce a downwash analogous to that for wings in translation. C_h,early and C_v,early therefore provide slight underestimates for C_D,pro and C_L (see Usherwood and Ellington, 2002). However, the significance of this effect is minor compared with the surprisingly similar magnitudes of force coefficients for the AR range discussed above.

Clearly, many other aspects influence wing design in insects: aspect ratios may be determined by inertial power or weight considerations or by the energetics of unsteady or forward-flight aerodynamics. Also, manoeuvrability,visibility, protection when folded and developmental cost may all push wing design towards non-energetically adaptive optima. However, the above relationships do suggest a possible pressure towards broader wings in insects for which efficient hovering with a horizontal stroke plane is of selective significance.

The energetic advantage to butterflies of low-AR wings is clearer. The large cabbage white Pieris brassicae hovers with a vertical stroke plane (Ellington 1984a), which means that horizontal force coefficients as defined here act in the vertical plane. While use is made of unsteady mechanisms such as the `clap and fling', the benefits due to a low-AR wing can be seen by considering steady propeller coefficients. The lowest-AR wing tested had a maximum horizontal force coefficient of 3.52, 1.4 times that of the highest-AR wing. Thus, lower-AR wings produce larger forces because of their larger areas and because of their higher force coefficients. This should allow the butterfly to flap disproportionately slowly, lowering the power requirements for hovering.

Fig. 10 shows the results for the AR range of the three transformations described in Usherwood and Ellington (2002) that convert C_h,steady and C_v,steady into C_D,pro and C_L, respectively. At values of α′ greater than 50°, the models progressively underestimate C_L with increasing AR. However, both large-angle models give good fits to C_h,early and C_v,early for values of α′ below 50°, which are more realistic for hovering insects.

In conclusion, the aerodynamics of revolving wings appears quite insensitive to variations in both wing morphology and kinematics: force coefficients for a range of model insect wings and for the wing of one small bird closely match those previously found for Drosophila wings. In addition, aspect ratio has remarkably little influence on aerodynamic force coefficients, at least at low-to-moderate angles of attack.

List of symbols
 
• AR

  aspect ratio

 
• C

  sum of speed-sensitive components of electrical power loss

 
• C_D,FP

  drag coefficient for a flat plate in perpendicular flow

 
• C_D,pro

  profile drag coefficient

 
• C_h

  horizontal force coefficient

 
• C_L

  lift coefficient

 
• C_R

  resultant force coefficient

 
• C_v

  vertical force coefficient

 
• F_v

  vertical force

 
• I

  electrical current

 
• n

  wingbeat frequency

 
• P_aero

  aerodynamic power

 
• P_winding

  power due to winding in electric motor

 
• Q

  torque

 
• r̂₂(S)

  non-dimensional second moment of area

 
• r̂₃(S)

  non-dimensional third moment of area

 
• r_e

  electrical resistance

 
• R

  wing length

 
• Re

  Reynolds number

 
• S

  area of a pair of wings

 
• V

  voltage

 
• α

  geometric angle of attack

 
• α′

  angle of incidence

 
• αr′

  effective angle of incidence

 
• ϵ

  downwash angle

 
• ω

  downwash angular flapping velocity

 
• Ω

  angular velocity of the propeller

Subscripts
 
• early

  before propeller wake has developed (e.g. C_v,early)

 
• steady

  after propeller wake has developed (e.g. C_v,steady)

The help of Ian Goldstone and Steve Ellis and the support of members of the Flight Group, both past and present, are gratefully acknowledged.