I was wandering about the birds which were flying over my head recently, why do they actually fly like this? I started to think it over and I amazed myself that the answer is just that obvious. To reduce drag!
Okay, don't get there too fast, so let's consider the air first. It's invisible, therefore making our life's harder. In the case of a ship, when it passes through water we can see the bow wave, wash astern and all the turbulence it creates. When an aeroplane makes its way through air we don't see any difference in the formation of air but we know about the drag. In fact, drag is a defined resistance produces to its motion. Drag determines the efficiency of flight therefore all aircraft are well designed to achieve the least possible resistance with air. Before moving to birds, there is still another concept I would like to explain. Wakes, which in the case of airplanes are very similar to the wakes made by boats or ships. It's a displacement, as the vehicle passes through this fluid, in one case it's water, in one case it's air. The wingtips are creating wake vortices. It's a very large cone, coming back of the tip of the airplane. If there is a trailing plane which intersects that cone, you'll feel choppiness.
Birds are all smart. They probably have felt the same effect if they fly behind another bird, therefore they are avoiding it. In fact, they are even smarter that they form a V shape, fly in the upwash of the wingtip vortices of the leading bird. The upwash helps them to support their weight, so achieving a reduction in the induced drag. The one flying on the front have the largest possible drag from all, but they all change places in time to have equally distributed flight fatigue among the other birds.
In military aviation it is a very common practice to fly in a V shape, also due to the same reason. In such flights they are doing a so called 'vortex surfing' to reduce drag.
|An 26 (photo: István Horváth)|
- A. C. Kermode - Mechanics of Flight