From Flight Safety, August 14, 2002
The basic idea behind a gurney flap is to increase the lift of an airfoil at higher angles of attack. When the flap is on one side only (most applications) it is located on the high pressure side of the airfoil. As Nick said, it's positioned at the trailing edge, and stands up 90 degrees to the airfoil surface.
The flap creates a small vacuum behind it, and since the flap in on the high pressure side of the wing, it forms a dam to the airflow on that side, so that the only way to fill the vacuum is from the low pressure side of the wing. The benefit to high angles of attack, is that the vacuum pulls the airflow over the low pressure side of the wing back down unto the wing surface at higher angles of attack. This helps to control the boundary layer separation that occurres on the low pressure side at high angles of attack, and the loss of lift associated with it. The price for this however is more drag.
Up to a point, taller gurney flaps further enhance the high angle of attack performance of a wing. but always at increasing drag penalties. The size of the flap is usually referred to as a percentage of wing cord, the same way that Nick referred it.
Last edited by Flight Safety; 19th Aug 2002 at 11:26.
From AIAA
A. Effect of Gurney Flaps and T-Strips on Baseline Wing Lift Curve
The effect of Gurney flaps on the baseline wing lift curve is shown below in Figure 4. Figure 5 shows the effect due to trailing edge T-strips. The coefficient data presented in Figures 4 and 5 was taken at a Reynolds number of 1.95x106. The coefficient data shows that Gurney flaps produced a positive increment in lift coefficient, a negative shift in the zero-lift angle of attack, and an increase in the wing maximum lift coefficient. Larger Gurney flaps produced larger lift increments. T-strips produced an increase in the slope of the lift curve and an increase in maximum lift coefficient. However, T-strips produced no shift in the wing zero-lift angle of attack.
Cannot post the graph....
Most Rudders have a symmetrical airfoil, and need equivalent lift in both directions of deflection. So one Gurney flap is counter intuitive. The "T-Strip" makes sense in that it is effective in both sweeps. So why does Cessna add a "t-Strip"? Do they?
Last edited by Lyman; 28th May 2012 at 14:01.