ASPECT RATIO AND THE GENERAL EFFECTS OF IT.
As air flows over and under the wing of an aircraft in flight the pressure above the wing becomes lower than that beneath it. This pressure difference causes air to migrate around the wing tips from the underside to the upper side. The overall effect of this migration is the creation or rotating vortices at the wingtips.
These vortices create downwash behind the trailing edge, thereby reducing the effective angle of attack of the wings. This reduced effective angle of attack increases the critical angle of attack at which the wing stalls. The vortices also create additional drag in the form of Lift Induced Drag. The stronger the wingtip vortices, the greater will be the increase in critical angle and the drag. Any factor that decreases wingtip vortex strength will decrease critical angle and drag.
The strength of the wingtip vortices is determined by the magnitude of the pressure difference between the upper and lower wing surfaces, and by the time during which this pressure difference acts to drive the air into the vortices. The greater the pressure difference or the longer the time available, the stronger the vortices will be.
The time during which the pressure difference acts upon the air is proportional to the time it takes the air to pass from the leading edge to the trailing edge of the wing. So for any given airspeed, the length of the wing chord determines the time taken. This means that decreasing the chord length will decrease the vortex strength, which will decrease the stalling angle and the drag.
The term “Aspect ratio” refers to the ratio of the chord length of a wing divided by its span. For any given wing area, increasing aspect ratio requires a decrease in chord length and an increase in wingspan as illustrated below. This reduced chord length will reduce the vortex strength, the critical angle and the induced drag.
So a high aspect ratio compared to a low aspect ratio will:
a. Decrease the induced drag.
b. Decrease the critical (stalling) angle.
c. Increase the effective angle of attack.
d. Increase the aerodynamic efficiency of the wing.
EFFECT OF ASPECT RATIO ON GUST RESPONSE
When a wing experiences vertical gusts in turbulence, the angle of attack of the wing is randomly increased and decreased by the changes in airflow direction. These random changes in angle of attack cause changes in CL and Lift. The overall effect is that the aircraft rises and sinks as the lift force changes.
The amount by which the angle of attack (alpha) changes is determined by the intensity of the turbulence. But the amount, by which the lift changes, is determined by the amount by which the CL changes. This in turn is determined by the gradient of the CL:alpha curve. The greater the gradient of the CL:alpha curve, the greater will be the change in CL for any given change in alpha.
The principal factor that determines the gradient of the CL:alpha curve is the aspect ratio. The greater the aspect ratio, the greater will be the gradient of the CL:alpha curve and the greater will be the sensitivity to turbulence. So any factor that decreases the aspect ratio will decrease the sensitivity to turbulence.
Aspect ratio is equal to wingspan divided by chord length. If the wings are gradually swept backwards the wingtips will move closer together. This will decrease the wingspan and hence decrease the aspect ratio. This in turn will decrease the sensitivity of the wing to turbulence. So the wing shape that is least sensitive to turbulence is the swept wing.