GROUND EFFECT

At landing and take-off airplanes fly for very brief (but none the less extremely important) time intervals close to the ground. The presence of the ground modifies the flow past the airplane significantly, so that large changes may take place in the trim and stability. For conventional airplanes, the take-off and landing cases provide some of the governing design criteria.

The presence of the ground imposes a boundary condition which inhibits the downward flow of air normally associated with the lifting action of the

wing and tail. The reduced downwash has three main effects, being in the usual order of importance:

(i) A reduction in e, the downwash angle at the tail.

(ii) An increase iti the wing-body lift slope amb.

(iii) An increase in the tail lift slope at.

The problem of calculating the stability and control near the ground then resolves itself into estimating these three effects. When appropriate values of де/dot, awb, and at have been found, their use in the equations of the foregoing sections will readily yield the required information. The most important items to be determined are the elevator angle and stick force required to maintain <?£max in level flight close to the ground. It will usually be found that the ratio ajawb is decreased by the presence of the ground. Equation (6.3,36) shows that this would tend to move the neutral point forward. However, the reduction in de/dot is usually so great that the net effect is a large rearward shift of the neutral point. Since the value of Cm& is only slightly affected, it turns out that the elevator angle required to trim at Cimax is much larger than in flight remote from the ground. It commonly happens that this is a critical design condition on the elevator, and may govern the ratio SJSt, or the forward C. G. limit (see Sec. 7.6).