1 Introduction - Cambridge University Pressassets.cambridge.org/97811070/37267/excerpt/...1 Introduction Bird,bat,andinsectﬂighthasfascinatedhumansformanycenturies.Asenthusiasti-callyobservedbyDial[1],mostspeciesofanimalsﬂy.Basedonhisacuteobservation

1 Introduction

Bird, bat, and insect flight has fascinated humans for many centuries. As enthusiasti-cally observed by Dial [1], most species of animals fly. Based on his acute observationof how birds fly, Leonardo da Vinci conceptualized flying machines, which can beseen in documents such as the Codex on the Flight of Birds, published circa 1505 [2];some illustrations of his work are shown in Figure 1.1. Otto Lilienthal was among themost dedicated and successful creators of flying machines at the dawn of human flight.He designed and demonstrated many hang gliders (see Fig. 1.2). Unfortunately,Lilienthal lacked sufficient knowledge of the science of flight and was killed in a fatalfall. For those who wish to explore in greater detail the history and the technologyof early flight, John Anderson’s Inventing Flight [3] offers interesting and well-documented information. Of course, there are ample records of humankind’s interestin natural flyers from the artistic angle. Figure 1.3 shows four examples: (Figure 1.3a)decorative art done about 2,500 years ago, in China’s Warring Period; (Figure 1.3b) abronze crane model uncovered from the First Emperor’s grave, who died in 210 BC;a pair of bas-reliefs (Figure 1.3c,d) uncovered from the Assyrian palace in today’sIraq, dated back to the 8th century BC; (Figure 1.3e) a stone sculpture of a standingowl from the Shang Dynasty, China, created in the 12th century BC or earlier!

There are nearly a million species of flying insects, and of the non-insects, another13,000 warm-blooded vertebrate species (including mammals, about 9,000 species ofbirds, and 1,000 species of bats) take to the skies. In their ability to maneuver a bodyefficiently through space, birds, bats, and insects represent one of nature’s finestlocomotion experiments. Although aeronautical technology has advanced rapidlyover the past 100 years, nature’s flying machines, which have evolved over 150million years, are still impressive. Considering that humans move at top speeds of3–4 body lengths per second, a race horse runs approximately 7 body lengths persecond, a cheetah accomplishes 18 body lengths per second [4], and a supersonicaircraft such as the SR-71 “Blackbird” traveling near Mach 3 (∼900 m/s) coversabout 32 body lengths per second, it is remarkable that a Common Pigeon (Columbalivia) frequently attains speeds of 22.4 m/s, which converts to 75 body lengths persecond. A European Starling (Sturnus vulgaris) is capable of flying at 120 bodylengths per second, and various species of swifts are even faster, flying more than140 body lengths per second. Whereas the roll rate of highly aerobatic aircraft (e.g.,A-4 Skyhawk) is approximately 720◦/s, a Barn Swallow (Hirundo rustics) has a roll

1

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2 Introduction

Figure 1.1. A drawing of a design for a flying machine by Leonardo da Vinci (c. 1488). Thismachine was an ornithopter, with flapping wings similar to a bird, first presented in his Codexon the Flight of Birds circa 1505 [2].






Figure 1.2. German engineer Otto Lilienthal flies his hang glider some 2,000 times during1891–1896 before a fatal fall [3].

(a) (b)

(c) (d) (e)

Figure 1.3. Birds recorded in early human history: (a) design of a wine vessel, early Warringperiod (475-early fourth century BC), China (Shanghai Museum, Shanghai); (b) a bronzecrane-eating fish, uncovered inside the First Emperor’s grave site, Xian, China (Museum ofEmperor QinShihuang, Xian); (c,d) Assyrian bas-reliefs, circa eighth century BC (BritishMuseum, London); (e) Stone sculpture of a standing owl, Shang Dynasty, China, around the12th century BC or earlier (Academia Sinica, Taipei).






4 Introduction

Figure 1.4. According to the simplified static aerodynamics, bumblebees were proclaimed tobe unfit to fly.

rate in excess of 5,000◦/s. The maximum positive G-force permitted in most generalaviation aircraft is 4–5 G, and select military aircraft withstand 8–10 G. However,many birds routinely experience positive G-forces in excess of 10 G and up to 14G. Such superior maneuvering and flight characteristics are primarily because of the“scaling laws” with respect to a vehicle’s size, as well as intuitive but highly developedsensing, navigation, and control capabilities. As McMasters and Henderson put it,humans fly commercially or recreationally, but animals fly professionally [5].

Compared to vehicles with flapping wings, conventional airplanes with fixedwings are relatively simple; the forward motion relative to the air causes the wingsto generate lift, with the thrust being produced by the engine via either propellers orexhaust gas. However, in biological flight the wings not only move forward relative tothe air but they also flap up and down, plunge, and sweep [1] [4] [6]–[8], so that bothlift and thrust can be generated and balanced in accordance with the instantaneousflight task. Aymar [9] and Storer [10] provide early photographs and some generalobservations. Although in the early days of flight studies, much of the analysis offlapping wing aerodynamics was based on an analogy to the fixed-wing counterpart,it was known that this approach encountered qualitative difficulties, especially whenthe size of a flyer became smaller, as in small birds, small bats, and insect regimes.In 1934, Antoine Magnan, an entomologist, discussed an analysis by Andre Sainte-Lague, an engineer, that whereas the lift generated by wings can adequately supportan aircraft to stay aloft, the same is not true at equivalent speeds of a bee [11]. Inother words, an airplane the size of a bee, moving as slowly as a bee, should not beable to fly [12]. Yet, of course, bumblebees, shown in Figure 1.4, can fly.

This example illustrates in simple fashion the implied conclusion – thatthe theory of fixed-wing aerodynamics cannot explain certain critical aspects of






Introduction 5

Figure 1.5. The instantaneous flapping wing patterns can sometimes look reasonable to beanalogous to stationary wings under a series of quasi-statically defined conditions.

flapping wing aerodynamics. The aforementioned framework of fixed wings essen-tially considers flapping wing dynamics as a series of “snapshots” (as illustrated byFig. 1.5), neglecting the influence of the aerodynamics and wing motion at an earliermoment on the aerodynamics at a later time, based on the so-called quasi-steadyapproach. In reality, a small flyer can often benefit from manipulating unsteady fluidflows using flapping wing aerodynamics. A rich variety of natural flyers, as depictedin Figure 1.6, can be observed to characterize the instantaneous flapping wingmotions. Depending on the real-time flight requirements, these complex motionsand wing shapes generate the desirable lift and thrust in different flight environ-ments.

As another example, Franco et al. [13] investigated fluid-structure interactions(FSIs) seen in jellyfish. They reported aperiodic flow despite the relative simplicityof a jellyfish’s body shape and motion, as shown in Figure 1.7. Muscle contrac-tion reduces the volume of the subumbrellar cavity (i.e., the region underneath itsumbrella-shaped body), resulting in a net downward flux of fluid. The motion of thelower margin of the bell generates vortex rings of opposite rotational sense during thecontraction and relaxation phases of the swimming cycle. Franco et al. [13] observedthat these vortices act to entrain fluid from above the animal into the subumbrel-lar cavity, where the feeding and sensory apparatuses of the animal are located.Furthermore, despite the approximate periodicity of the swimming motion, inspec-tion of the flow created by the animal indicates that it is indeed aperiodic in time.Because the animal does not swim at a constant velocity, a periodic flow cannot beconstructed by any Galilean transformation of frame. Instantaneous streamlines ofthe flow field, measured by Franco et al. [13] using digital particle image velocimetry,






Figure 1.6. In reality, the instantaneous flapping wing patterns are very complicated. Depend-ing on the real-time flight requirements, necessary lift and thrust are generated by dynamicmechanisms resulting from unsteady wing movement and shape changes.

0.0 s

1.5 s

3.7 s 4.4 s

2.5 s

0.6 s

Figure 1.7. Dye visualization of jellyfish vortex wake. Time series shows vortices of clockwiseand counterclockwise rotation sense generated during the contraction and relaxation phasesof the swimming cycle, respectively. Bell diameter is 10 cm. Images from Franco et al. [13].






Introduction 7

Figure 1.8. Instantaneous streamlines of flow around a jellyfish as it swims vertically. Left:End of relaxation phase of swimming cycle. Right: End of contraction phase of swimmingcycle. Bell diameter is 10 cm. Images from Franco et al. [13].

indicate local entrainment of fluid from above the animal into the subumbrellarcavity during the entire swimming cycle. Simultaneously, as shown in Figure 1.8,a net downward momentum flux propels the animal forward. This study illustratesthat (i) the flow field and force generation processes are highly time dependent andcannot be described based on a simple quasi-steady framework without substantialcorrections, and that (ii) the movement and shape deformation of the body influenceanimal locomotion in major ways.

In addition to generating the aerodynamic forces, flapping wings can also signif-icantly enhance the maneuverability of a flyer. Figure 1.9 illustrates several maneu-vering characteristics of biological flyers; these capabilities are difficult to mimicby human-made machines. By combining flapping motion, wing deformation, bodycontour, and tail adjustment, natural flyers can track targets precisely at amazingspeeds. Another issue of interest is the weight of the wings relative to the total ani-mal weight. As summarized in Table 1.1, bat wings, depending on the species, areclose to 20 percent of the wing-to-total bat weight. The wings of other natural flyers,as shown in Table 1.2, including large birds such as osprey and vultures, account for20 percent or more of their body weight. Many butterflies (such as Scarce Swallow-tail, Large White) also have relatively heavy wings, around 15 to 25 percent of bodyweight. The wings of a Small Heath, a small, yellow-orange butterfly that flies closeto the ground, are about 5 percent of its body weight. Bees, wasps, flies, and the likeall have very light wings, typically less than 1 percent of their body weight.

Heavier wings require more energy to flap; nevertheless their larger inertiaenables the flyers to make turns within one or two flapping periods. Those with ahigher wing-to-body mass ratio and moment of inertia, such as bats and butterflies,are more maneuverable, capable of making abrupt changes of trajectories within atime comparable to that of a flapping cycle. However, they pay a penalty for thisability because a heavier wing consumes more energy while flapping. Many small






8 Introduction

(a) (b)

(d)(c)

(e)

Figure 1.9. Maneuvering capabilities of natural flyers: (a) Canadian Geese’s response to windgust; (b) speed control and target tracking of a seagull; (c) precision touchdown of a finch;(d) a hummingbird defending itself against a bee; (e) the asymmetric movement of wings andtail of a Black Kite while hunting.

flyers such as hummingbirds and insects (with many butterflies as a noticeable excep-tion) tend to have much faster flapping time scales than their bodies’ response timescale. For the higher wing-to-body mass ratio group, with the flapping and bodyresponse time scales being comparable, the flyer’s flight dynamics and control needto be closely linked to the instantaneous aerodynamics, because the time history ofthe flapping wing aerodynamics directly affects a flyer’s performance characteristics.For the lower wing-to-body mass ratio group, whose flapping time scales are signifi-cantly shorter than their bodies’ response time scale, the lift, drag, and thrust varia-tions during the flapping cycle tend to be smoothed out over the entire flight flappingcycles.






Introduction 9

Table 1.1. Mass of wing and body, and wing dimensions for eight species of bats. With relativelyheavier wings, bats can maneuver and make a turn within a stroke or two. On the other hand, they haveto work harder to flap

Species mb (kg) mw (kg) mw /mb (%) b (m) S (m2) AR (−)W/S(N/m2)

Egyptian Fruit Bat(Rousettusaegyptiacus)

8.34×10−2 2.06×10−2 24.70 5.30×10−1 4.65×10−2 6.4 22

Minor EpaulettedFruit Bat(Epomophorusanurus)

4.16×10−2 8.76×10−3 21.03 4.00×10−1 2.90×10−2 5.8 18

Common Pipistrelle(Pipistrelluspipistrellus)

4.57×10−3 7.34×10−4 16.08 2.09×10−1 6.50×10−3 6.7 8.0

Common Noctule(Nyctalus noctula)

2.35×10−2 3.00×10−3 12.76 3.44×10−1 1.61×10−2 7.4 16

Northem Bat(Eptesicusnilssonii)

8.20×10−3 1.70×10−3 20.73 2.77×10−1 1.15×10−2 6.7 8.4

Particoloured Bat(Vespertiliomurinus)

1.24×10−2 1.72×10−3 13.87 2.98×10−1 1.22×10−2 7.3 11

Brown Long-EaredBat (Plecotusauritus)

7.83×10−3 1.17×10−3 14.94 2.70×10−1 1.23×10−2 5.9 7.2

Large-EaredFree-Tailed Bat(Otomopsmartiensseni)

3.01×10−2 5.48×10−3 18.19 4.49×10−1 2.17×10−2 9.3 16

Notes: mb is body mass, mw is wing mass (total), b is wingspan, S is wing area (total), and AR is aspect ratio. Notethe body mass is computed by the subtraction of the total wing mass from the total mass.Source: [47].

However, this feature does not mean that the flapping wing aerodynamics ofsmall flyers can simply be considered as quasi-steady. As is discussed in detail inChapter 3, the time history of the wing motion is often important to flapping wingaerodynamics. Figure 1.10 shows hummingbirds conducting highly difficult and pre-cise flight control. As illustrated in Figure 1.11, in which several photos highlightthe flapping pattern along with a flow field illustration from Warrick et al. [14],hummingbird wing motion exhibits a figure-eight pattern and is highly adaptive toaccommodate the challenges posed by wind gust, target tracking, and mitigation ofpotential interference and threat.

Natural flyers synchronize their wings, body, legs, and tail to perform manytasks. As shown in Figure 1.12, they can take off on water, from land, and off a tree,exhibiting varied and sophisticated patterns. While gliding, as shown in Figure 1.13,they flex their wings to control their speed and direction. On landing, as depicted inFigure 1.14, birds use wing-tail combinations to correct flight trajectory and to adjustfor the location of the available landing area. If they need to slow down and adjust thedetailed flight trajectory, they fully expand their wings to increase drag and reducespeed; otherwise, they simply fold their wings to reduce lift without slowing down.






Tab

le1.

2.M

ass

ofw

ing

and

body

,and

win

gdi

men

sion

sfo

rso

me

inse

cts

[48]

and

bird

s[4

9].W

ithlig

hter

win

gs,w

hich

resu

ltsin

time

scal

edi

ffer

ence

sbe

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nth

ew

ing

mov

emen

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the

body

mov

emen

t,fly

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such

asbu

mbl

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sne

edto

flap

anu

mbe

rof

times

befo

rebe

ing

able

tom

ake

turn

s

Fly

ers/

Spec

ies

mb

(kg)

mw

(kg)

mw

/mb

(%)

R(m

)S(

m2 )

AR

W/S

(N/m

2 )

Bir

dsG

riff

onV

ultu

re(G

yps

fulv

us)

7.27

×10

01.

60×

10−3

22.0

06.

98×

10−1

1.05

×10

01.

8567

.59

Osp

rey

(Pan

dion

halia

etus

)1.

11×

100

3.10

×10

−128

.05

4.96

×10

−12.

92×

10−1

3.37

37.0

7P

allid

Har

rier

(Cir

cus

mac

rour

us)

3.86

×10

−17.

51×

10−2

19.4

63.

57×

10−1

1.41

×10

−13.

6126

.77

Hum

min

gbri

d(L

ampo

rnis

clem

enci

ae)

8.4×

10−3

6.00

×10

−47.

148.

5×

10−2

3.50

×10

−38.

2625

.2

Inse

cts

Gia

ntP

eaco

ckM

oth

(Sau

rnia

pyri

)1.

89×

10−3

3.00

×10

−415

.87

7.00

×10

−21.

20×

10−2

1.85

1.54

But

terfl

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Scar

ceSw

allo

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apili

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dalir

ius)

3.00

×10

−48.

00×

10−5

26.6

73.

70×

10−2

3.60

×10

−31.

520.

82L

arge

Whi

te(P

ieri

sbr

assi

cae)

1.27

×10

−42.

10×

10−5

16.5

43.

10×

10−2

1.84

×10

−32.

090.

68Sm

allH

eath

(Coe

nony

mph

apa

mph

ilus)

4.60

×10

−53.

50×

10−6

7.61

1.61

×10

−24.

80×

10−4

2.13

0.94

Mot

hs,B

ees,

and

Oth

erIn

sect

sD

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’s-H

ead

Haw

kmot

h(A

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onia

atro

pos)

1.60

×10

−36.

70×

10−5

4.19

5.10

×10

−22.

05×

10−3

5.08

7.65

Hum

min

gbir

dH

awkm

oth

(Mac

rogl

ossu

mst

ella

taru

mL

.)2.

82×

10−4

9.18

×10

−63.

262.

13×

10−2

3.79

×10

−44.

797.

29H

awkm

oth

(Man

duca

sext

a)1.

60×

10−3

9.00

×10

−55.

634.

85×

10−2

1.80

×10

−35.

239.

20G

erm

anW

asp

(Ves

pula

germ

anic

aF

.)2.

40×

10−4

1.39

×10

−60.

581.

62×

10−2

1.33

×10

−47.

8917

.68

Eur

opea

nH

orne

t(V

espa

crab

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.)5.

97×

10−4

5.68

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952.

43×

10−2

3.04

×10

−46.

0819

.25

Eur

opea

nH

over

fly(E

rist

alis

tena

x)1.

29×

10−4

1.13

×10

−60.

881.

27×

10−2

8.26

×10

−57.

7815

.31

Hon

eybe

e(A

pis

mel

lifica

L.)

9.75

×10

−54.

25×

10−7

0.44

9.95

×10

−35.

98×

10−5

6.62

15.9

8R

ed-T

aile

dB

umbl

ebee

(Bom

bus

lapi

dari

es)

4.95

×10

−43.

10×

10−6

0.63

1.65

×10

−21.

65×

10−4

6.60

29.4

0B

uff-

Tai

led

Bum

bleb

ee(B

ombu

ste

rres

tris

)3.

88×

10−4

2.50

×10

−60.

641.

60×

10−2

1.42

×10

−47.

2126

.78

Blu

eF

ly(C

alip

hora

eryt

horo

ceph

ala)

6.10

×10

−55.

27×

10−7

0.86

1.04

×10

−26.

33×

10−5

6.83

9.44

Sour

ce:I

nsec

ts[4

8]an

dbi

rds

[49]

.

10






Documents

1 Introduction - Cambridge University Pressassets.cambridge.org/97811070/37267/excerpt/...1 Introduction Bird,bat,andinsectﬂighthasfascinatedhumansformanycenturies.Asenthusiasti-callyobservedbyDial[1],mostspeciesofanimalsﬂy.Basedonhisacuteobservation