Page 36 - Understanding Flight

CH02_Anderson 7/25/01 8:55 AM Page 23

How Airplanes Fly 23

Fig. 2.5. The variation of the speed of a fluid near an
object.

The differences in speed in adjacent layers cause shear forces, The characteristic of fluids to
which cause the flow of the fluid to want to bend in the have zero velocity at the
direction of the slower layer. This causes the fluid to try to surface of an object explains
wrap around the object. The characteristic of fluids to have why one is not able to hose
zero velocity at the surface of an object explains why one is dust off of a car.
not able to hose dust off of a car. At the surface of the car the
water has no velocity and thus puts little or no force on the dust
particles.

Viscosity and Lift
For those with some familiarity with aerodynamics, there
may be some confusion with the connection of viscosity and
lift. Many simulations in aerodynamics are done with zero
viscosity or, more accurately, in the limit of zero viscosity.
Viscosity is introduced implicitly with the Kutta-Joukowski
condition, which requires that the air come smoothly off at
the trailing edge of the wing. So, in reality these “zero
viscosity” calculations reintroduce viscosity via the Kutta-
Joukowski condition. In a fluid without viscosity, such as
superfluid helium, a wing could not fly.
Also, in most mathematical descriptions of lift the boundary
layer is considered so small that it is ignored. Many
erroneously claim that ignoring the boundary layer is
equivalent to having zero viscosity.This is incorrect because
viscosity is implicit in the condition that air follows the
curvature of the wing.