Page 98 - Understanding Flight

CH03_Anderson 7/25/01 8:56 AM Page 85

Wings 85

In discussing the stall of a wing, we have focused on the reduction
in lift. Almost as great a problem is the wake caused by a stalling
wing producing significant form drag. Just when the airplane needs
all the power it can get to recover from the stall, there is a large
additional power requirement put on the airplane, making the
recovery even harder.

THE GOLF BALL

Turbulence can have a beneficial effect on reducing the
wake of an object. If the surface air in the boundary layer
becomes turbulent, the higher energy in the turbulent flow
will help the air stick to the surface longer before
separating.The result is lower form drag.
A smooth ball traveling through the air will have high form
drag, as shown in Figure 3.24.The laminar flow soon
separates, producing a wake behind the ball that far
outweighs the skin drag. One solution to improve the
range of a ball is to energize the air around the ball by
churning up the air.This will allow the air to remain
attached to the surface longer and will reduce the size of
the wake.
A golf ball’s size would normally result in laminar flow and
a sizable wake.That is why the surface is covered with
dimples to encourage turbulence.As seen in the figure, the
dimples reduce the size of the wake and thus reduce the
form drag.The result is that golf balls with dimples travel
much farther than if they were smooth.

Vortex Generators

A boundary layer with more energy will be less prone to separate. Vor-
tex generators are a means for adding energy to the boundary layer and
thus increasing the stall angle of attack. Vortex generators are simple
devices that can be retrofitted to any airplane. They are simply small
wings set at an angle of attack to the local air, as shown in Figure 3.25.