The Perfect Flying Machine

A graduate of Harvard, class of 1929, GUY MURCHIE, JR., was a navigator of transport planes from 1942 to 1945. In the process he acquired an unusually observant knowledge of all things pertaining to the sky, including the wind, the clouds, the birds, and those cumbersome devices with which, over the centuries, man has striven to keep himself aloft. Mr. Murchie’s new book, Song of the Sky, from which the following article has been drawn, is the December selection of the Book-of-the-Month Club and is being published by Houghton Mifflin.



THE birds that we see flying successfully around us every day are only the surviving ten per cent or so of much larger numbers that hatch from eggs, who in turn represent far less than one per cent of all that would be hatched if the extinct birds survived in their former numbers. In other words, our present birds are a very select bunch, being actually only the topflight athletes, the champions who have won the flying tournament of evolution by their prodigious feats of soaring, of diving at terrific speed, plunging deep into the sea, or fighting in the air.

Can you imagine any better example of divine creative accomplishment than the consummate flying machine that is a bird? The skeleton, veryflexible and strong, is also largely pneumatic — especially in the bigger birds. The beak, skull, feet, and all other bones of a 25-pound pelican have been found to weigh but 23 ounces. Yet the flesh too is pneumatic, and in some species there are air sacs around viscera, muscles, and, where balance and streamlining permit, immediately under the skin. The lungs are not just single cavities as with mammals but whole series of chambers around the main breathing tubes, connected also with all the air sacs of the body, including the hollow bones. Thus the air of the sky literally permeates the bird, flesh and bone alike, and aerates it entire. And the circulation of sky through the whole bird acts as a radiator or cooling system of the flying machine, expelling excess humidity and heat as well as exchanging carbon dioxide for oxygen at a feverish rate.

This air-conditioning system is no mere luxury to a bird but vitally necessary to its souped-up vitality. Flight demands greater intensity of effort than does any other means of animal locomotion, and so a bird’s heart beats many times per second, its breathing is correspondingly rapid, and its blood has more red corpuscles per ounce than any other creature. As would be expected of a high-speed engine, the bird’s temperature is high: a heron’s 105.8°, a duck’s 109.1°, and a swift’s 111.2°.

Fuel consumption is so great that most birds have a kind of carburetor called a crop for straining and preparing their food before it is injected into the combustion cylinders of the stomach and intestines, and the speed of peristaltic motion is prodigious. You may have heard of the young robin who ate fourteen feet of earthworm the first day after leaving the nest, or the house wren who was recorded feeding its young 1217 times between dawn and dusk. Young crows have been known to eat more than their own weight in food per day, and an adolescent chickadee was checked eating over 5500 cankerworm eggs daily for a week.

The main flying motors fed by this bird fuel are the pectoral muscles, the greater of which pulls down the wing against the air to drive the bird upward and onward, while the lesser hoists the wing back up again, pulling from below by means of an ingenious block and tackle tendon. This extraordinary halyard which passes through a lubricated pulley hole at the shoulder is necessary because the heaviest muscles must be kept at the bottom of the bird so that it will not fly top-heavy. Just as the motor may weigh half of a small airplane, the powerful wing muscles of a pigeon have been found to weigh half the whole bird. These pectorals, by the way, are the solid white meat attached to the breastbone or keel, a location insuring the lowest possible center of gravity — just forward of such other lowslung ballast as gizzard and liver, and well below the very light lungs and air sacs.

If you want to see the ultimate in vertebrate flexibility, you must examine a bird’s neck. More pliant than a snake, it enables the beak to reach any part of the body with ease and balances the whole bird in flight. Even the stocky little sparrow has twice as many vertebrae in its neck as the tallest giraffe: fourteen for the sparrow, seven for the giraffe.

The most distinctive feature of all in a bird, of course, is its feathers, the lightest things in the world for their size and toughness. The tensile strength of cobwebs is great but feathers are stronger in proportion in many more ways, not to mention being springy and flexible.

The growth of a feather is like the unfoldment of some kinds of flowers and ferns. Tiny moist blades of cells appear on the young bird, splitting lengthwise into hairlike strands which dry apart into silky filaments which in the mass are known as down. At the roots of the down lie other sets of cells which, as the bird grows older, push the down from its sockets. These are the real feathers, and when the down rubs off they appear as little bluegray sheaths which may be likened to rolled-up umbrellas or furled sails.

Each of these sheaths is actually an instrument of almost unimaginable potentiality and at the right moment it suddenly pops open — revealing in a few hours feathers that unfold into smaller feathers that unfold again and again. Each main shaft or quill sprouts forth some 600 dowls or barbs on either side to form the familiar vane of the feather. But each of the 1200 barbs in turn puts out about 800 smaller barbs called barbules, each of which again produces a score or two of tiny hooks known as barbicels. The complete interwoven mesh of one feather thus contains some thirty million barbicels, and a whole bird normally is encased in several hundred billion tiny clinging barbicels.

It is hard for the human mind to take in the intricacy of this microscopic weaving that is a feather. There is nothing chemical about it. It is entirely mechanical. If you pull the feather vane apart in your fingers it offers outraged resistance: you can imagine the hundreds of barbules and thousands of barbicels at that particular spot struggling to remain hooked together. And even after being torn the feather has amazing recuperative power. Just placing the split barbs together again and stroking them lengthwise a few times is sufficient to rehook enough barbicels to restore the feather to working efficiency—by nature’s own zipper action.

The feather webbing is so fine that few air molecules can get through it and it is ideal material for gripping the sky. In a sense the feather is as much kith to sky as kin to bird, for by a paradox the bird does not really live until its feathers are dead. No sooner is a feather full grown than the opening at the base of the quill closes, blood ceases to flow, and it becomes sealed off from life. The bird’s body does not lose track of it, however, for as often as a feather comes loose from a living bird, a new one grows in its place.

Did you ever notice how similar are a feather and a sail? The quill, though it can be bent double without breaking, is stiff enough for a mast. The forward or cutting vane is narrow as a trimmed jib, the aft or lee vane wide like a mainsail. The barbs correspond to the bamboo lugs of a Chinese junk, strengthening the sail, enabling it to withstand the full typhoon of flight. The primary feathers of some sea ducks, however, can be as jibless as catboats, having virtually no vane forward of the quill, which thus itself becomes the leading edge. The cross-section of such a feather is rather like an airplane wing and highly efficient in lift.

Besides its individual qualities a feather is a perfect part of a whole, its shaft curved to blend exactly into the pattern of the wing, its shape to fit the slipstream of the sky. Like roof tiles, feathers are arranged in overlapping rows, the forward part of the bird corresponding to the top of the roof. Which explains why birds do not feel right unless they are facing the weather, why when they turn to leeward the wind blows against their grain letting the rain soak through to the skin just as shingles reversed will funnel the wet into the house instead of shedding it away.

Different feathers naturally have different functions and shapes. The wing-tip feathers are called the primaries, usually ten in number, and serve to propel the bird. These grow out of what corresponds to the bird’s hand. The forearm feathers nearer the body are the secondaries, twelve to fourteen of them —they mainly support the bird — while the upper arm tertiaries or tertial feathers fair in the secondaries with the body and stabilize the bird in the air. The tail feathers average about fourteen, overlapping from the center outwards, but they vary so much in different birds that hardly any generalization can be made about them. They are rooted in a pincushionlike muscle mound, the pope’s nose, which in some species has been found to house more than a thousand individual feather muscles, each capable of moving one feather in one direction.

One might think that as a bird moults, shedding feathers from once to three times a year, the uneven loss would sometimes set the bird off balance in flight. But nature provides that at least the important primary feathers drop off exactly in pairs, one from the right wing with the corresponding feather from the left wing, waiting then for two new feathers to replace them before moulting the next pair.


WHEN Leonardo da Vinci and later Otto Lilienthal and the French glider pioneers studied birds in order to learn the principles of flight, their concentration naturally was focused on the motions of wings and tail. But these movements turned out to be so fast, complex, and subtle that their analysis was extremely difficult. Even today much remains to be learned of them.

One of the first facts revealed by close observation and the high-speed camera was that wings do not simply flap up and down. Nor do they row the bird ahead like oars. The actual motion is more that of sculling a boat or screwing it ahead by propeller action, a kind of figure-eight movement.

A bird’s “hand” (outer wing) is longer than the rest, of its “arm” (wing) but it has almost as complete control over it as a man has. It is true that two of the original “fingers” have fused into one and the others have disappeared, but the big primary feathers have replaced them so completely that it has gained many more digits and muscles than it has lost. It can twist its “ hand ” to any position, spread its ten primaries, waggle or twiddle them, shrug its shoulders, even clap its “hands” together behind its head and in front of its breast. That is why wing motion is so complex, variable, and hard to comprehend.

The powerful downstroke that obviously lifts and propels the bird also is a forward stroke, so much so that the wings often touch each other in front of the breast and almost always come close at take-off and climb. Many people have trouble understanding this fact proven by the camera until they reflect that it is likewise forward motion of the airplane wing that generates lift. Just as a sculling oar or a propeller drives a boat ahead by moving at right angles to the boat’s motion, so does the force of the bird’s wing resolve itself into a nearly perpendicular component.

Even the wing’s upstroke plays its part in driving the bird upward and onward — for the same reason. This quick flip of recovery takes half the time of the downstroke and has much less power, but is still part of the sculling motion that is almost peristaltic

— like a fish in the sea or a snake in the grass — probably closest of all to the rotor screw action of a moving helicopter: forward and down, backward and up and around.

A fuller understanding of this marvel can be gained by careful study of photographs taken at very high speed, flashed in slow motion on a screen or strangely frozen in stills. The pliable feathers at the wing’s tip and trailing edge are then seen to bend according to the changing pressures, revealing how the air is moving.

The downstroke plainly compresses them tightly upon each other the whole length of the outstretched wing, each feather grabbing its full hold of air, while the upstroke, lifting first, the “wrist,” then the halffolded wing, swivels the feathers apart like slats in a Venetian blind to let the air slip by. It is an automatic, selective process, probably nature’s most graceful and intricate valve action, the different movements overlapping and blending smoothly, the “wrists” half up before the wing tips stop descending, the “forearms” pressing down while the tips are yet rising. The convexity of the wing’s upper surface and the concavity of its lower aid this alternate gripping and slipping of the air — this compression of sky into a buoyant cushion below the wing while an intermittent vacuum sucks from above — this consummate reciprocal flapping that pelicans accomplish twice a second, quail twenty times, and hummingbirds two hundred times!

Birds are clearly way ahead of the airplane in aileron or roll control; and some, like ravens and roller pigeons, close their wings to make snap rolls just for fun or courting. And the same bird superiority holds in the case of flaps which brake the air to reduce speed in landing, the birds fanning out their tails for this purpose as well as their wings. Web-footed birds such as geese usually steer and brake with their feet also, and inflect their long necks like the bow paddle of a canoe to aid in steering and balancing.

The tail is of course intended primarily for steering — steering up and down as well as to right and left. Some birds with efficient tails can loop the loop, fly upside down, or do backward somersaults like the tumbler pigeon. Small-tailed birds such as ducks are handicapped by not being able to make any kind of sharp turns in the air, though their tails steer well enough in water and in slapping the waves on take-off from water. The male whidah bird has such a long tail that on a dewy morning he actually cannot get off the ground until the sun has evaporated the extra weight from his trailer. The variety of bird tails never ends, nor does the multiplicity of functions. Furled to a mere stick or fanned out 180° and skewed to any angle, tails serve for everything from a stabilizing fin to a parachute, from a flag to a crutch.


MAN cannot hope to match the bird in sensitivity of flying control, mainly because he usually has to think air or read it off instruments, while the bird just feels air everywhere on his feathers and skin. This is not to say, however, that the bird is a faultless flyer. Birds make plenty of mistakes — even forced landings! Not a few are killed in crashes. Usually bird slip-ups happen so fast they go unnoticed. But when you get a chance to watch a flight of birds coming in to land in slow-motion movies you can see them correcting their errors by last-moment flips of tail or by dragging a foot like a boy on a sled. If a landing bird discovers he lowered his “flaps” too soon he still can ease off these air brakes by raising his wings so that his secondaries spill wind, his primary feathers remaining in position for lateral or aileron control. Buzzards do something almost similar called a “double dip” to correct a stall. But lots of times excited birds do not notice their mistakes soon enough and lose flying speed while trying to climb too steeply, or fall into a spin from tight turns or from simply misjudging the wind. Once they have ceased making headway they tumble downward just as surely as a stalled airplane.

I saw a heron one day muff a landing in a tree, stall, and fall to the ground, breaking his leg. This is particularly apt to happen to heavy birds like ducks when they are tired. Sometimes ducks lose half a pound on a long migratory flight and are so exhausted on letting down that they splash into the water and cannot take off again for hours.

The energy required of a heavy bird at take-off of course is very great. It has been estimated at five times normal cruising energy. Many birds, like the swan, need a runway in addition to the most furious beating of wings to get up enough speed to leave the ground. Others like the coot are lighter but low-powered and take off like a 1915 scout plane missing on three cylinders. All birds naturally take off against the wind for the same reason that an airplane does: to gain air speed, which is obviously more significant than ground speed at take-off. You have surely noticed that birds feeding by the lee roadside will often take off across the path of an approaching car, actually tempting death to gain the wind’s help.

Heavy birds that dwell on cliffs of course have the advantage of being able to make a catapult take-off, dropping into a steep glide until they build up flying speed — but again they must beware of landing at a place where this needed gravitational asset is not available. The penalty for lack of such foresight can well be death.

I heard of a loon that made the mistake of alighting on a small pond set amid a forest of tall pines. When he wanted to take off an hour later he found himself stymied. He could not climb steeply enough to clear the trees or turn sharply enough to spiral out. He was seen thrashing along over the water, whipping the waves with his wings to get under way, oven pedaling at the water desperately with his webbed feet before getting into the air. He almost made it a couple of times but also nearly got killed crashing into the big trees, then plowing back through the underbrush on his sprained wishbone. Finally he had to give up. But this particular loon was lucky. After four frustrated days in his pond jail a very strong wind came up and enabled him to take off and climb so steeply against it that he just brushed between the treetops and was free!

A very different and special capacity is required in bird formation flight or mass maneuvering. Did you ever see a puff of smoke blowing against the wind? I did — but it turned out to be a tight flock of thousands of small birds. When you get close enough to watch a large flight of starlings feeding on a field you wonder how each bird can so completely lose its individuality as to become part of that smooth, flowing mass. Sometimes the flock moves like a great wheel, individual birds alighting and rising progressively as parts of the rhythm of the rim. Now it rolls as a coach on the highroad, now with uneven grace like a tongue of sea fog folding over and over.

Sandpipers, plover, turnstones, sanderlings, and other small shore birds are all expert in this sort of flying, which seems to depend on extreme quickness of eye and a speed of selfless response not equaled elsewhere in nature — not among its counterparts in the hoofed animals of the plains, not even among the mysterious schools of the deep sea where whales and the lesser fish have been seen to dive in wonderful co-ordination miles apart. No one knows exactly how this amazing unity of action is accomplished or why. It may depend on much more than visual contact. It may be a part of one of the seams of life where the individual is granted a pretaste of absorption into a greater order of consciousness, far above and beyond his own little being.

When birds migrate they often fly in V formation and for the same reason that the Air Force does. It is the simplest way to follow a leader in the sky while keeping out of his wash and retaining good vision. Birds instinctively do it, peeling off from one heading to another and sometimes chasing after man-made gliders. They even have been seen pursuing power planes until they were unable to keep up with them. I know of a glider pilot who was followed for half an hour by a young sea gull who copied his every maneuver: figure-eights, vertical turns, spins, loops. It was only after the glider led the gull into a vertical dive for three thousand feet which blew most of the bird’s feathers off that it realized it had been a little too gullible and left off the chase.

Lots of birds, far from feeling jealous of human trespassing in their ancient territory, seem to get such a kick out of airplanes that they hang around airports just like human kids watching the big ones take off and land. Many a time I’ve seen sea gulls at the big Travis Air Base near San Francisco flapping nonchalantly among the huge ten-engined B-36 bombers while their motors were being run up. The smoke whipping from the jets in four straight lines past the tail accompanied by that soul-shaking roar would have been enough to stampede a herd of elephants but the sea gulls often flew right into the tornado just for fun. When the full blast struck them they would simply disappear, only to turn up a few seconds later a quarter mile downwind, apparently having enjoyed the experience as much as a boy running through a hose — even coming around eager-eyed for more.


OF all birds the hawks have probably contributed most toward teaching man to fly — through their example of soaring over the zones of the earth where most men live. But how they accomplish their miracle has been discovered only a little at a time over long periods.

Sir George Cayley around 1810 concluded that a rook, whose weight is about a pound for each square foot, of its wing area, would be able to glide horizontally as long as it could maintain a speed of at least twenty-five miles an hour. What could enable it to keep up that speed, however, he did not pretend to know.

A partial answer was revealed long afterward by a study of gulls circling close to the sea in autumn and winter, times of year when the relative warmth of the water often produces updrafts in which birds can soar indefinitely. Even in a fresh breeze when these warm columns of air are blown over to leeward until they lie almost flat upon the waves, the gulls have been observed soaring buoyantly along the invisible wind seams — gliding magically upwind upon a continuous fountain of air where two counterrotating columns adjoin.

But when they cannot thus coast “downhill” in air flowing “uphill,” neither on thermal nor deflected updrafts, soaring birds somehow still manage to stay aloft on almost motionless wings — traveling at high speed as often against the wind as with it. A mission of the French scientist, Idrac, to the South Seas during the last century to solve the mystery of the albatross determined that this largest of soaring birds flies at the high average speed of forty-nine miles an hour. When soaring close to the waves and losing altitude, reported Idrac, the albatross uses his remaining speed to gain height. If he can rise only four or five times his wingspread of about eleven feet he usually gets into an air stratum fifty feet up where the wind is blowing three times as strongly as at the surface, thus giving him an extra boost just as a kite will be sent upward by a gust of wind.

The theory of this eventually expanded into one of the first real explanations of why birds soar in circles: since surface friction reduces wind speed at lower altitudes, the bird soars against the wind aiming slightly uphill to take advantage of higher wind velocity as he goes up, giving him the kite boost by horizontal shearing (by differences of wind speed at different horizontal levels). But as forward speed eventually falls off because of the climb, he turns away from the wind and coasts slightly downhill to leeward, again getting a boost from his increase in air speed as the tail wind decreases— then into the wind once more, and so on round and round.

The principles of “static soaring” — soaring on rising air currents — have been worked out in detail mainly during the last twenty-five years as sailplane pilots have experimented with thermal currents over sun-baked fields or up the windward slopes of hills or cold fronts. But only more recently has “dynamic soaring” come into use by man: this more difficult since it depends on the sudden variations of wind speed in gusty air to impart the kite boost by vertical shearing (by differences of wind speed in different vertical planes), relying of course on the pilot to be heading to windward at each increase of wind and to leeward at each decrease, a rhythm that can be irregular to the point of inscrutability.

All of these discoveries have helped explain how birds can glide on motionless wings against the wind, for it became clear that gravity is to the bird as the keel is to the sailboat or the kite string to the kite. All three hold the moving object firm against being blown to leeward, each in its own way.

The form of the wing is obviously another basic factor in flying effectiveness, and birds have adopted a great variety of special shapes just as have the airplane designers after them. There are the narrow, pointed wings of the fast and strong flyers: the falcons and swallows, the swifts and hummingbirds. There are the bent-wrist wings of the fast gliders like the nighthawk; the broad, fingered wings of the slow soarers, the red-shouldered and red-tailed hawks; the short, rounded wings of the woodland darters: grouse, quail, the small sparrows and finches.

Gulls and albatrosses also have narrow, pointed wings, theirs however adapted specially to longrange gliding and soaring over the open ocean. The albatross, in fact, so perfectly geared to the air that he cannot fold his lengthy wing restfully inside his flank feathers when on ground or sea, is thought to stretch out in sleep while actually on the wing — dozing aloft, even as some of us, but literally in his own feather bed upon the sky.

Little by little the factors of wing efficiency have resolved themselves into the separate relationships or dimensions of the wing: specifically, its aspect ratio or the proportion between its length and breadth, its degree of bluntness or pointedness, its camber or curvature fore and aft, its horizontalness or dihedral angle in relation to the other wing, its slotting or sparing of primaries (if any), its degree of sweepback, its fairing or smoothness of surface, its thickness, its flexibility, and innumerable minor points.

Aspect ratio in birds averages around 3:1. That is, their wings are generally about three times as long as they are wide. The albatross exceeds 5:l. Airplanes sometimes reach 7:1, and sailplanes as high as 18:1. Theoretically, the higher the ratio the greater the lift, but practically a limit comes when the wing gets so long and narrow that it may break in gusty air. And of course soaring birds have many considerations besides flying that affect their shape: things like catching food, preening their feathers, folding their wings, laying eggs, raising a family. Thus sailplanes, built, solely for soaring, have a distinct advantage over the bird who must also be somebody’s uncle or grandmother.