Troy Schank and Dr. Paul Eastman, Mechanical Engineering
Introduction
Ground-effect is a well-known phenomenon that occurs as an aircraft approaches the surface of the earth. Flight in ground-effect changes the characteristics of the airflow around the wing causing both positive and negative effects. Though ground-effect is a well-known phenomenon, the details of lift and drag are less understood. I was able to gain an understanding of the details of ground effect from existing studies and learn from my own experiments in this research.
Lift
A certain amount of lift and drag are induced upon an airfoil (wing) as it moves through an air stream at a given angle of attack. The same airfoil in close proximity to the ground (between 0 and 1 chord length from the ground) will experience a slight increase in induced lift, and up to 60 percent reduction in induced drag (1). “The ground-effect decreases the drag coefficients for all cases with different Re and a. This result is because of the change of pressure drag of the airfoil with ground-effect. The negative pressure on the upper surface of the airfoil decreases with the decreasing ground clearance and the pressure drag is reduced naturally” (1). This change in performance can be measured from experimentation or numerically computed using complex models and computational fluid dynamics techniques (1). An increase in lift and a decrease in drag will have a large impact on the lift to drag ratio (L/D), which is the measure of aircraft efficiency.
In an attempt to measure the increased ground-effect performance for myself, I conducted an experiment varying three factors. The three factors were airspeed, angle of attack, and height above the ground. Using statistical sampling techniques I took several measurements at all combinations. Unfortunately, I was unable to measure any real effect due to random noise in the data. Usually, these types of experiments are conducted with very expensive wind tunnels and instrumentation.
Stability
The tendency of the vehicle to return to an equilibrium position after a disturbance, stability, is of utmost importance in any aircraft. Two criterions must be met for an aircraft out of ground effect to be stable.
I. Cm,0 must be positive.
II. CM, cg / a must be negative.
As stated above in I, the moment coefficients at zero lift must be positive. Most cambered wings naturally have a negative moment coefficient, caused by the sum of the lift and drag components (2). In order to counteract this natural negative moment, a tail is added behind the wing and angled downward. This negative lift on the tail will create an overall positive moment coefficient for the whole aircraft. As stated in II, the moment coefficient with respect to angle of attack must be negative. In this manner, disturbances that tend to push the nose of the craft up or down will be counteracted by moments causing the aircraft to return to equilibrium.
Both conditions that give stability in an aircraft are a combination of many factors: wing profile, angle of attack, center of gravity, tail area, tail distance from main wing, etc. Static stability for ground-effect flight must meet these same criterions with the addition of one more requirement: height stability. The pitching moment of an airfoil is a function of the lift and drag components. Lift and drag vary radically with small changes in height in ground-effect. This condition presents a major variance in stability for minor changes in height. The relation that gives static height stability is expressed below in equation (1) (3).
vo 2cLh(1 – cmh cL / cm cLh) (1)
“Therefore, to provide a WIG (Wing In Ground-effect) with a sufficient margin of static height stability, (it must have) high values of both (-cLh) [change in lift with respect to height] and of static pitching stability (-cm). The former term is mainly influenced by the airfoil characteristics of the wing. The latter derivative can be increased, without increasing cmh [pitching moment with respect to height] as well, by a high horizontal tail working out of ground-effect” (3).
One object of this research was to find a wing planform with the best cLh characteristics. My plan was to build several wing planforms and test them in a wind tunnel. However, as noted above, the small change in performance characteristics could not be measured with the instrumentation we had access to.
References
- Hsiun, Chih-Min and Chen, Cha’o. “Aerodynamic Characteristics of a Two- Dimensional Airfoil with Ground-Effect.” Journal of Aircraft. April 1996. pp. 386-92.
- Anderson, John D. Introduction to flight. 1985 McGraw-Hill, Inc.
- Staufenbiel, Rolf. “Some Nonlinear Effects in Stability and Control of Win-In-Ground Effect Vehicles.” Journal of Aircraft. August 1978. pp. 541- 44.