(September 2011) In 1920 it was already common knowledge among pilots that, as airplanes got very close to the ground, they seemed to slide along on a slippery cushion of air. A decade later, the phenomenon — “ground effect” — had been investigated in wind tunnels and flight tests and was well documented, even if the precise mechanisms involved were still imperfectly understood. A 1934 summary of existing research by a French investigator, Maurice Le Sueur, was translated by the National Advisory Committee for Aeronautics, NASA’s wonderfully productive and helpful predecessor, and published as Technical Memorandum 771 under the title “Ground Effect on the Takeoff and Landing of Airplanes.”
“Observations on airplanes in free flight,” Le Sueur wrote, “have enabled us to observe certain systematic phenomena such as: the greater facility of low-wing airplanes for taking off; the impossibility of certain heavily loaded airplanes to gain altitude; the prolonged gliding power of low-wing airplanes at landing, etc.”
During the 1920s several long-range attempts had ended in disaster when airplanes crammed with fuel rose from the ground, sometimes even after a shorter run than expected, but then would not climb out of ground effect. It still happens today, especially at high density altitudes, but most of us are more likely to encounter ground effect on landing, when we find certain airplanes floating effortlessly past the point where we intended to touch down.
All these phenomena are manifestations of three basic properties of ground effect. The strongest and most readily apparent is the reduction of induced drag, or drag due to lift, which, at minimum speed, is the bulk of any airplane’s drag. Entering ground effect — typically defined as one wingspan above the ground, but really noticeable at less than half a span — has the same effect as increasing the aspect ratio would: The airplane glides better.
The influence of ground effect on lift is less obvious. Contrary to the impression created by the phrase “cushion of air,” ground effect does not increase maximum lift or reduce stalling speed; but lift increases more rapidly with angle of attack in ground effect than out of it, and so the airplane stalls at a lower angle of attack. When the trailing edge of the wing gets quite close to the surface, however, as happens when a short-legged low-wing airplane — a Piper Comanche, for instance — with flaps down flares just above the ground, the maximum lift may in fact diminish and the stalling speed go up.
At speeds well above the stall, dropping closer to the surface increases the lift for a given angle of attack, and so if the angle of attack is held steady, a wing close to the surface — less than a chord length above it — resists getting any closer.
