Page 34 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 21

How Airplanes Fly 21

look at another picture of air flowing around a wing (Figure If the wing has no net effect on
2.3). First the air comes from below the wing. This is the the air, the air cannot have any
upwash, which will be explained later. The air splits around net effect on the wing and there
the wing and leaves the wing at a slight downward angle. This is no lift.
downward-traveling air is the downwash and as we will see is
the source of lift on a wing. In this figure there has been a net change
in the air after passing over the wing. Thus there has been a force
acting on the air and a reaction force on the wing. There is lift.
The final question to be answered before we can go on to really
understand lift is why does the air bend around the wing? The answer
is in an interesting phenomenon called the Coanda effect.

The Coanda Effect

The Coanda effect has to do with the bending of fluids around an
object. For the forces and pressures associated with low-speed flight
air is considered not only a fluid but an incompressible fluid. This

Fig. 2.2. Based on Newton’s laws, this airfoil has no lift.

Upwash Downwash

Fig. 2.3. The airflow around a real airfoil looks like this.