We Can Put A Man On The Moon, But We’re Still Figuring Out How Birds Fly
    By Kimberly Crandell | April 11th 2009 12:18 PM | 14 comments | Print | E-mail | Track Comments
    About Kimberly

    I'm a mother of three, with an aeronautical engineering degree.  Although it's been a while since I've done any aircraft


    View Kimberly's Profile
    Spring has officially arrived. I don’t need the budding trees or the warmer temperatures to tell me – I can tell just by the chatter of birds that has returned, kicking into high gear as soon as the sky begins to lighten each morning. We have a large tree in our back yard, and it appears to be one of the neighborhood meeting places for local birds of all shapes and sizes. It’s not something I mind; in fact I’m sure I’ve encouraged it by hanging a fairly substantial bird feeder on one of the lower branches of this particular “meeting tree.”

    As the weather warms and my family and I spend more time outdoors, I’m cheered every time I look up and see various feathered friends swooping in to congregate among the tree’s branches. Due to the fact that there is food involved, quite often squabbles break out. And then it is just as entertaining to watch the defensive acrobatics that take place as some enterprising sparrow tries to feed, while at the same time preventing any others from doing the same.

    Blue Jay in flight

    As entertaining as all this is, there is some pretty amazing physics taking place as these birds turn, loop, hover, and land – but as much study that has gone into discovering how exactly birds fly, there is still a lot that is still unknown. However, researchers at the University of North Carolina have uncovered at least one of the secrets of flight: when birds, bats or bugs make a turn, all they have to do is start flapping their wings normally again and they straighten right out.

    It may sound like common sense, but the scientists that poured over frames of high-speed film expected it to be a much more intricate affair for creatures-of-flight to pull out of a turn. They were simply anticipating there to be more steps; more of a beginning-middle-end sequence of events to perform a turn, and then a return to normal flight. Not so, says lead researcher Tyson Hedrick. It’s a simpler, one-step process. Launch a turn and then simply flap normally to end it and fly away. (1)

    “We didn’t expect things to fall out this neatly,” Hedrick said, particularly because the process is the same for animals of all sizes from the fruit fly to the bat to the cockatoo. “It’s sort of unusual” to find a general rule to cover six orders of magnitude in size.

    The findings will hopefully help in the development of a robotic flying machine. However, with this study they have tackled only one of the three axes of motion – turning left or right, or yaw. They still have pitching (nose up or down) and rolling (tilting left or right) to deal with. The situation does become more complicated with more complex maneuvers, “and that is clearly the next step,” Hedrick said.

    The physics of flight is all about airflow over the wings. The lift and thrust generated by the bird’s flapping motion, offsets the weight and wind resistance or drag of the bird flying through the air. The bird’s anatomy is uniquely designed to reduce weight as much as possible (more on this later). And the design of the bird’s wings produce lift due to their concave airfoil shape – resulting in a pressure differential between the upper and lower surfaces of the wing as air flows across it.

    Front View of Bird Wing
    This front view of a bird's wing show the leading edge of the wing (red line), the top surface of the wing (green line) and the trailing edge of the wing (blue line).  The air that moves over the top of the wing has further to travel to get across the wing, thus it speeds up. This causes the pressure to drop because the same amount of air is exerting its pressure over a greater area. Meanwhile the air going below the wing experiences the opposite effect. It slows down, generating more pressure. As a result, a bird with air moving over its wings is feels an overall force pushing it upward. The more curved the airfoil, the greater the lift – providing the degree of curve does not impede the flow of air.

    Airflow over bird and aircraft wings

    It is this concept that is applied to modern fixed-wing aircraft. The wings of a plane are designed to mimic this curved airfoil shape. At lower speeds, planes extend flaps behind the wing to extend the airfoil and increase its curve – to maintain lift even as less air flows over the wing. What modern science hasn’t been able to replicate is constant movement and adjustment, twisting and bending, that a bird does with its wings throughout its flight. This dynamic flexibility enables birds to cut and dive in the air, and even hover in place – abilities that are far beyond what can be addressed with a simple fixed-wing aircraft.

    Even if scientists can figure out the physics behind all of these various elements of flight, they’re going to have a hard time out-engineering the unique design and structure of birds which enable them to put all of this physics into motion. The entire anatomy of a bird is designed for flight. It’s quite fascinating how these living creatures are uniquely designed to minimize the pull of gravity, and maintain the essential “load balancing” required for sustained and controlled flight.

    Duck landing on water

    Birds have adapted to be as light as possible. They have hollow bones, and light hollow feathers instead of fur. As everyone knows, birds lay eggs – but this makes sense so that babies are developed outside of their mothers’ bodies, instead of carried inside while she tries to maintain normal flight. (I can only imagine what the flight of a heavily pregnant bird would look like.)

    Birds eat foods that are very high in usable calories so they get as many calories as possible from a small amount of food. Seeds, fruits, and meat (from prey) are the main food items for birds. Virtually no birds eat leaves, which take a long time to digest. Their efficient digestion allows birds to get rid of useless weight very quickly. In addition, birds don’t have bladders. A bird urinates as soon as it has to, getting rid of useless weight. Which is why of course we are wary any time we find ourselves beneath a bird, and why you can never housebreak a pet bird – even the really smart ones.

    Birds don’t have teeth or a nose, which are heavy and would be too far forward. To grind their food, their stomachs have a gizzard near their center of gravity. They use their mouth and nostrils located on the top of their lightweight beak to breathe. But bird lungs don’t fill up with a lot of air like ours do. To maintain their center of gravity, all of a bird’s heaviest organs must reside in their chest. Therefore, that precious real estate cannot be taking up by expansive, lightweight lungs filled with air. Instead, a bird’s lungs, which can hold very little air, are flat and sit against their back ribs. The air birds breathe in flows through the lungs into big balloon-like air sacs that fill much of their lower abdomen, behind their center of gravity. When they breathe out, the air flows back through the lungs through different passages. (2)

    I completely understand the desire of scientists to discover, and mechanically replicate, the elements that make it possible for birds to fly. It’s a mystery after all, and we all love a good mystery. But in addition to trying to develop a robotic flying machine, I’m glad we’re also still uncovering modes of flight that don’t resemble things that birds can do at all. This baby, for example. I can’t wait to buy a ticket as soon as one of these little beauties is available.

    In the meantime, this summer I’ll continue to enjoy observing the spectacle of birds in my back yard that have already perfected all the elements of flight. That is, until the morning comes months from now, when instead of hearing the chattering of birds I wake to the sound of honking Canadian geese – a sure sign of the onset of autumn, just as the songbirds mark the arrival of spring.



    Wow that's fascinating thanks!

    Fossil Huntress
    Great article Kimberly!
    I thought the theory of flight that's based on pressure differentials in laminar flow was largely debunked -- not enough lift for either a jumbo jet or a dove, so that in reality flight apparently involves turbulence effects or some such. Is that not so?

    Quote: The air that moves over the top of the wing has further to travel to get across the wing, thus it speeds up.

    I am sorry but this is just plain WRONG! Just consider the flat plate, which also creates lift when it is set under an angle of attack (both sides have the same length).

    It is just Newton's third law which governs the generation of lift. Air (or fluid) is accelerated downwards by the shape of the airfoil, an equivalent force is exerted upwards resulting in "lift".

    Bit sad to see this mistake is still made, even by people who should know better...

    I'm not sure capitalizing 'wrong' makes your case.   Planes fly because they accelerate some of the air that flows past them downwards, which causes a reaction force upwards.   The debate is not whether that is Bernoulli or Newton but how much of each and that hasn't been settled the way you seem to think it is.   Yes, Bernoulli was dismissed in a pop science flight book but that doesn't make it so.

    The viscosity of air allows your plate to generate asymetric airflow - and then Bernoulli lifts it.   The camber absolutely does require a greater distance and a faster speed for the air to flow.    
    Ummmm, they say "So both "Bernoulli" and "Newton" are correct" so continuing to use the word 'wrong' isn't actually helping.    You might say 'this is oversimplified' but then pointing to a high school-level NASA document isn't helping much either.

    If you're right then I can just fly my barn door to Paris - and I can not do that - but all they are saying is that lift is created by accelerating mass downwards, which I said too.   The deflection of the air downward is due to the difference in pressure between the upper and lower surfaces of the wing - Bernoulli's Theorem.
    Sorry I will elaborate a bit more.

    I do not say Newton or Bernoulli are wrong, but your picture above is wrong. It still shows the 'equal transit time and that is why on top the velocity is higher to keep up with the air below' theory. Yes, the air at the top does travel much faster than the air below. But even when the top and bottom length are equal. (why flat plates and symmetric airfoils do generate lift at an angle of attack, check this vid of a wind tunnel experiment showing that:

    Consider a hovering helicopter, this is not staying in the air because the Bernoulli effect is sucking it upwards (which is implied by you). Instead it is the air pushed downwards by the small tilted wings on the rotor and the opposite reaction force keeping your helicopter in the air.

    The "Bernoulli effect" is still true. It explains how the top of the wing is able to "pull downwards" on the air flowing over it. And the Bernoulli Effect proves extremely useful in calculations of the lifting force during classes in airplane physics and during experimental work in aerodynamics. But airplanes also obey Newton's laws: accelerate some air downwards, and you'll experience an upwards force.

    I think the NASA documents explain this matter excellent.

    Some more in depth info on this from NASA.

    When an airfoil having zero angle of attack interacts with an airstream, air molecules are deflected away from the wing surface.  The vertical component of this momentum produces a reduction of pressure which can be measured by any instrument which might be placed in the airstream above the airfoil.

    Molecules passing under the airfoil from ahead of the airfoil's boundary layer are deflected much less than those passing over the airfoil.  This difference in deflection causes the force perceived as lift.  It is a matter of mathematical preference whether the lift is seen as being derived from a lower pressure or a higher molecular momentum.  In either case, from the perspective of an observer riding on an air molecule:

    an air molecule travelling over the airfoil, due to its deflection in 3-dimensional space from a straight path to a curved one, travels further than one travelling under the airfoil.  q.e.d.
    Kimberly Crandell
    The fact of the matter is, the principles of both Bernoulli and Newton play a part - but one is the cause of lift, while the other is the effect of lift.

    The Newton explanation is that the wing deflects the airflow so it is canted slightly downward, resulting in the wing imparting downward momentum to the air.  The wing exerts a force on the air, pushing the flow downward -- and from Newton's third law, an equal-and-opposite reaction produces an upward force against the wing, lift.  However, this explanation really involves the effect of lift, and not the cause.

    In reality, the distribution of air pressure across the surface of the wing results in an overall force pushing upward on the wing, creating lift.  As a result of the equal-and-opposite principle, the airfoil surfaces pushes on the air, imparting a force on the airflow which deflects the velocity downward.  Hence, the downward momentum created in the airflow because of the presence of the wing can be thought of as an effect due to the surface pressure distribution; the pressure distribution itself is the fundamental cause of lift.
    Like the article guys...the comments only make the entire read more interesting! :)

    This is a gem of an article.  I'm still mulling over the aerodynamics, but I've learnt more about birds from this than I had before in a Month of Sundays.

    A sea eagle swoops, and brings up birds and Bernoulli in one beakful.
    Robert H. Olley / Quondam Physics Department / University of Reading / England
    Beautiful informative with excellent pictures, Thank You. Nature is curious. Many a time it shows us we don't know much. That feeling makes us modest at the same time drive us more to reveal the secreat. That urge to understand makes the advancement of Science. Some one said "We have counted number of stars in Galxy almost precisely, but we don't have the idea howmany plant species on earth it self" The comment may be a bit harsh, but interesting one!