Ground Effect
At landing and takeoff 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 can take place in the trim and stability. For conventional airplanes, the takeoff and landing cases provide some of the governing design criteria.
The presence of the ground imposes a boundary condition that 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:
1. A reduction in e, the downwash angle at the tail.
2. An increase in the wing-body lift slope awb.
3. An increase in the tail lift slope a,.
The problem of calculating the stability and control near the ground then resolves itself into estimating these three effects. When appropriate values of дє/да, awb, and a, 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 control force required to maintain in level flight close to the ground. It will usually be found that the ratio aja is decreased by the presence of the ground. (2.3,23) shows that this would tend to move the neutral point forward. However, the reduction in дє/да is usually so great that the net effect is a large rearward shift of the neutral point. Since the value of Cmo 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 the critical design condition on the elevator, and it will govern the ratio SJSt, or the forward CG limit (see Sec. 3.7).