Birds

In the Everglades in December I had a lot of chances to look at birds. They were everywhere, ancient and amazing.

At the same time, I was taking flying lessons.

So it was impossible not to notice birds exploiting the same aerodynamic effects I learned from flying.

Here are a few, for your reading pleasure.

Ground Effect

Wings fly because they are supported by the air. The wing pushes down and the air pushes back up.

When a wing is very close to the ground, the air that’s pushed down is trapped between the wing and the ground and forms a higher-pressure cushion of air, giving the wing more lift, so that it can fly at a lower speed. This is called ground effect.

One of the things you figure out pretty quickly when you’re learning to land is that in ground effect, the airplane just wants to keep floating and floating. And so if you have a limited amount of runway to work with, you want to approach the landing without carrying too much extra speed.

In Florida I noticed a lot of birds skimming the water and it was amazing how far they could glide just above the surface without having to flap their wings once.

Dihedral

Stand in front of an airplane and look at its wings and you’ll notice that they are not completely parallel to the ground – they are angled up. This angle is called the dihedral.

The purpose of the dihedral angle is to make an airplane self-stabilizing. If a gust of wind causes one wing to drop, the airplane will slip sideways toward the lower wing. This causes the lower wing to generate more lift, to rise, and to restore the airplane to wings-level, without the pilot having to do anything.

And that’s why in a little Cessna, even in slightly rough air, you can often let go of the yoke and let the plane fly itself (unless the guy sitting next to you is a big fatty and unbalances the airplane).

Swarms of turkey vultures dot the sky over Southern Florida, making the location of every road kill.

In this picture you can see that when turkey vultures are soaring, their wings are angled up, like a Cessna (or an Airbus).

I’m not sure why some birds have dihedral and others don’t, but I suspect turkey vultures benefit from it because they do so much gliding. Some airplanes actually have negative dihedral – fighter jets, for example – to make them less stable and more maneuverable.

Landing Flare

When you’re landing an airplane, at the very end, you pull back on the yoke as the airplane sinks, to stay in the air as long as possible so that you touch down with the slowest possible airspeed.

The landing flare also angles the lift vector backwards and helps to slow the airplane down.

Unfortunately I don’t have a cool picture of this but I noticed a lot of birds would flare at the last minute before perching on a tree branch or landing on the ground. They would also flap their wings as they flared, sort of like a thrust reverser on a jet.

Gyroscopes

Of course, birds are ornithopters and fly differently from airplanes. They don’t have propellers or jets creating a longitudinal thrust.

They also don’t have spinning gyroscopes and an artificial horizon to tell them which way is up when they’re flying inside clouds, like instrument-equipped airplanes do. Which is why it has long been believed that birds cannot fly through clouds.

Or can they? Pilots have reported bird strikes in instrument conditions. And in 1972, an ornithologist in New York bought a military surplus radar and tracked birds flying through clouds for several miles – and they were going straight.

It shouldn’t be possible for birds to fly through clouds, but it is. How do they do it? Do birds have some kind of gyroscopic organ, or a magnetic sense that tells them which way is the ground?

I’ve done some googling but haven’t found a definitive answer. The best article is this 1993 classic, The Turn in The Atlantic.

Happy reading.