Tuesday, May 13, 2014

Birds in Flight, or Why Did Dad Fly That Way?

This video shows Dad bringing a freshly killed rabbit to the nest. After pausing in a tree, he circles over a field before landing in the nest with his catch. What is going on? Why did Dad pause in a tree, and why didn’t he fly directly to the nest after taking a break? I’m going to write a little bit about the basics of flight and why Dad might have flown the way he did.

There are four basic forces that govern flight: lift, thrust, weight, and drag.
  • Lift pushes birds upwards. Lift is produced primarily by wing curvature, which induces low air pressure on the top of the wing and high air pressure on the bottom. Primary and secondary feathers both help create lift. 
  • Thrust propels birds forward. When a bird flaps, its primary feathers twist at an angle relative to the rest of its wing and the bird’s line of flight. This twisting or spinning motion induces low pressure ahead of the bird’s wing and high pressure behind it, aiding forward motion. Depending on the strength of the flap, a bird’s entire outer wing might twist in response. 
  • Weight equals mass times the acceleration of gravity, so weight can be defined as the force of gravity on an object. Weight pulls birds down and puts an upper limit on flighted bird size.
  • Drag counteracts forward motion. Air passing over the bird’s wing produces friction and creates wingtip vortices that direct the air behind the wing downward, inducing downwash and causing drag. We think birds have a number of ways to counteract drag. Wing slots increase the span factor of bird wings and spread vortices horizontally and vertically, dissipating their kinetic energy and reducing their sucking power. As the bird’s wing is tilted upwards, its angle eventually becomes so steep that air cannot flow smoothly over the surface of the wing and lift decreases. The alula, a small projection on the front wing of birds, can be manipulated to create a temporary slot that allows birds to steeply angle their wings without stalling out. 
Catching prey in flight requires less force, since acceleration, lift, and thrust are already part of the equation. Unfortunately, Dad is on the ground. The rabbit could be adding two or more pounds of weight and has no aerodynamic qualities whatsoever. Dad needs powerful wing flaps and a lot of force to counteract gravity, gain lift, thrust forward, and hoist his rabbit into the air.

Dad takes a breather on a tree limb before bringing his prey to the nest. But if carrying prey is so hard, why doesn’t he fly directly to the nest? Although the angle makes it hard to tell, the tree limb is below the nest and the approach may not have been good. Dad puts gravity to work by dropping from the branch, gaining a little no-cost acceleration. He spreads his wings wide and pitches them slightly back as he launches, catching the air for a little free lift. This reduces the amount of work required to lift and carry prey, since force = mass x acceleration and Dad is using gravity to assist lift - something he can't do from the ground. His spread alula can be seen at 1:11, allowing Dad to steeply angle his wings for a bigger downstroke without loosing his hard earned lift. Note that we don't see the alula in regular flight later in the video.

In short, it’s a lot easier to drop off a high place than fly up from the ground, especially when prey is involved. With a little assist from the tree branch, Dad was able to spiral around at the far end of the field and gain enough lift to drop easily into the nest. Eagles fly so wonderfully that it’s easy to forget how much work and experience goes into it.

Thanks to Rick Black for the video! Check out the Canadian Museum of Nature to see a brief animation of birds in take off, flapping, and gliding flight: http://nature.ca/discover/exb/hwdbrdsfly/index_e.cfm