Lift and drag

Bird flight fascinates me.  The wing is both an instrument of power and a specialized surface that generates the lift that carries them aloft.

Long wings with a huge surface area -- Brown Pelicans are superb at dynamic soaring.

Long wings with a huge surface area — Brown Pelicans are superb at dynamic soaring.

The feathered surface of the elongated forearms is central to flight, and humans have tried to copy the principles of avian flight in designing aircraft.   Two forces govern the operation of the wing in generating flight:  lift provided by the differential air masses moving over the upper and lower surface of the wing and frictional drag induced by the bird’s body (and wing) moving through the air.  To optimize flight, birds need to maximize lift, while minimizing drag.

Some examples of how this is accomplished.  Birds can reduce drag by presenting as little body surface to the airstream as possible, as illustrated by the flattened profile of an Anhinga, with its outstretched neck in perfect alignment with its body and tail.  Airflow is directed rapidly over the relatively flat upper surface of the bird, reducing air pressure there, and causing the body to rise.

cormorant in flight

Ducks, Geese, Swans, and Cranes adopt the same strategy (flying with outstretched neck) to minimize frontal drag, but long-necked wading birds (Herons and Egrets) double-up their necks while flying.  This doesn’t seem as aerodynamically efficient, but these species don’t tend to fly long distances, anyway.

Great Egret or American Egret

Great Egret or American Egret.  Photo taken at Bayou Segnette Park in New Orleans.

Louisiana Heron at take-off.  Photo taken at Bayou Segnette Park in New Orleans.

Louisiana Heron at take-off. Photo taken at Bayou Segnette Park in New Orleans.

Lift is directly related to wing surface area — the more area, the greater the lifting power.  Long, wide wings make the best lifters, as seen in hawks, eagles, vultures, pelicans, albatross, etc.

Long, wide wings of this juvenile Red-tailed Hawk are ideal for riding thermal air currents.

Long, wide wings of this juvenile Red-tailed Hawk are ideal for riding thermal air currents.

Adaptable wings serve a variety of purposes in osprey:  they can soar in circles over water while hunting, make rapid course changes, even hover briefly as they put down their feet to grab a fish.

Adaptable wings serve a variety of purposes in osprey: they can soar in circles over water while hunting, make rapid course changes, even hover briefly as they put down their feet to grab a fish.  Photo by co-blogger Alison.

But big, long wings dictate slow, coursing flight, and some birds need to get there faster or make quick changes in direction as they chase prey or evade predators.  Most song birds, pigeons and doves, parrots, and a few raptorial birds like Accipter hawks and Falcons have short, round (elliptical) wings that enable high maneuverability and agility while flying.  Lift is generated by the wing as it forces the air mass under it down. Pectoral muscle contractions generate the power both to move forward and to move up in the air column.

Elliptical wing shape of the Monk Parakeet (seen in downtown New Orleans)

Elliptical wing shape of the Monk Parakeet (seen in downtown New Orleans).

The king of speed (Peregrine Falcon) has the short, swept-back wings of a fighter plane.  Its flat flight profile reduces drag, but the key to its success is its short but powerful wings that allow the bird to change direction rapidly as it chases its prey.

Powerful down strokes of the wings propel the bird through air horizontally, like a oars on a rowboat.  But when the bird folds its wings next to its body in head-downward flight, it can reach speeds up to 200 mph.

Powerful down-strokes of the wings propel the bird through air horizontally, like oars on a rowboat. But when the bird folds its wings next to its body in head-downward flight, it can reach speeds up to 200 mph.  Photo by Mike Baird from Wikipedia

And how about the ability remain stationary in mid-air or to even fly backward?

Male Anna's Hummingbird from Berkeley Botanic Garden, November 2012

Male Anna’s Hummingbird taken at the Berkeley Botanic Garden, November 2012.

Hummingbird wings move in a figure-eight pattern back and forth above the plane of its body, and the mobile shoulder allows the wing to rotate so that it presses down on the air mass both on the down-stroke and on the up-stroke of the wing beat.  So, unlike other birds that develop almost 100% of their lift during a downward wing flap, hummingbirds can generate up to 25% of their lift during the up-stroke was well.

For some really spectacular photos for birds in flight, please check out Phil Lanoue’s Photography (e.g., October flights).  At the end of each month, Phil showcases some of the dramatic photos of airborne birds he has shot that month.

12 thoughts on “Lift and drag

  1. Fantastic post, Sue! I love the photos and your excellent explanations for how it all works. The picture of the peregrine is gorgeous! What a bird! I think it’s funny that herons and egrets fly with their necks pulled back in that “S” shape, but cranes don’t. I wonder why? Any ideas?

      • Ha! I bow to your superior investigative skills! It would be fun to know why the difference in birds built mostly the same, though cranes are a bit bigger.

  2. What a great post, Sue. Normally I focus on only a single bird at a time and don’t think more broadly as you did here about the differences in wings and flight patterns. Have you ever considered doing a post on takeoff techniques? I noticed that some ducks, for example, seem able to take off really quickly and others need to gather a head of steam before rising into the air.

    • I imagine that the take-off effort is related to the wing structure as well, and more specifically wing loading (weight per unit of wing area). If the bird is relatively heavy for its given wing area, it should build up more speed before “lift off”. Long wings with lots of surface area for a given body weight would provide more lift and require less effort on take off. That’s my best guess anyway.

  3. Pingback: What to do with your neck when you fly… | Back Yard Biology

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