Content excerpt from FAA-H-8083-25A Pilot’s Handbook

When an aircraft is just leaving the ground or water, compared to a state of level flight at a certain altitude, it can obtain sufficient Lift at a slightly slower speed. This phenomenon is Ground Effect.

When the aircraft is flying at an altitude of several feet from the ground, the airflow patterns in all three axes of the aircraft change because the vertical airflow near the wings is restricted by the ground. Consequently, the upwash, downwash, and wingtip vortices all change. The ground surface obstructs and alters the airflow pattern during flight, which creates Ground Effect.

After the aircraft approaches the ground, the aerodynamic characteristics of the wings change. When the coefficient of Lift remains constant, the upwash, downwash, and wingtip vortices all weaken.

We all know that the principle of Lift in flight is mainly due to the pressure difference above and below the wings, and the wings generate a continuous downward mass of air to support the aircraft. However, the stronger the downwash, the harder it is for the wings to push the air downward. At the high angles of attack during takeoff and landing, induced Drag is greater and flight speed is slower. During low-speed flight, induced Drag becomes a significant factor affecting power characteristics. (However, parasitic Drag remains basically unchanged.)

Due to the effect of Ground Effect, wingtip vortices weaken, which alters the Lift distribution along the wingspan; consequently, both the induced angle of attack and induced Drag decrease. As mentioned above, since induced Drag is the decisive factor at low speeds and high angles of attack (i.e., during takeoff and landing), once induced Drag is reduced: The wings can obtain sufficient Lift with a smaller angle of attack; If the angle of attack is constant, the aircraft’s Lift coefficient will increase. Clearly, the Thrust should also be reduced accordingly, otherwise the altitude will increase.

Another point to note is that due to changes in upwash, downwash, and wingtip vortices, there is an impact on the speed instruments (change in position error of the airspeed system). Ground Effect causes the data from the static pressure source to become higher, resulting in the Airspeed Indicator and Altimeter showing values lower than the actual figures. Therefore, the pilot will perceive that the speed display upon liftoff is often lower than the normally required speed.

Significant Ground Effect effects only occur when the wings are quite close to the ground. When the wing’s height equals the wingspan, induced Drag is reduced by only 1.4%; When the wing height is 1/4 of the wingspan, induced Drag is reduced by 23.5%; When the wing height is 1/10 of the wingspan, induced Drag is reduced by 47.6%. Therefore, one only feels the influence of Ground Effect just after the aircraft leaves the ground and before landing. The wingspan of a Cessna 172N is 36 feet, approximately 11 meters. Based on the data above, Ground Effect can only be felt when the altitude drops to 2-3 meters.

After the aircraft takes off and lifts off, due to the disappearance of Ground Effect, attention must be paid to the following items: To maintain the same Lift, the system requires an increased angle of attack; The increase in induced Drag and the corresponding need to increase Thrust; Decreased stability and a momentary nose-up pitch; The increase in indicated speed resulting from the drop in static pressure.

For example, if a pilot does not correctly understand the effects of Ground Effect, they might believe that a normal takeoff is possible even when the speed is below the reference speed. However, once the aircraft leaves the altitude of Ground Effect, it may fall into the trouble of insufficient takeoff speed, failing to meet the requirements for aircraft initial Climb performance. Especially under conditions of heavy load, high density altitude, and high temperature, the aircraft may very well fail to obtain sufficient Lift and fall back to the Runway. The pilot also needs to ensure that the aircraft has achieved a certain positive rate of Climb before retracting the landing gear and Flaps.

Similarly, during the Descent (landing) process, if a certain angle of attack is maintained, upon entering Ground Effect the improvement in the Lift coefficient requires a reduction in Thrust, and a floating effect may occur simultaneously. If the pull-up speed at this moment is too high, a floating distance may occur.