Slow Flight
When practicing slow flight, a pilot learns to divide attention between aircraft control and other demands.
How the airplane feels at the slower airspeeds aids the pilot in learning that as airspeed decreases, control effectiveness decreases.
For instance, reducing airspeed from 30 knots to 20 knots above the stalling speed will result in a certain loss of effectiveness of flight control inputs because of less airflow over the control surfaces.
As airspeed is further reduced, the control effectiveness is further reduced and the reduced airflow over the control surfaces results in larger control movements being required to create the same response.
Pilots sometimes refer to the feel of this reduced effectiveness as “sloppy” or “mushy” controls.
When flying above minimum drag speed (L/DMAX), even a small increase in power will increase the speed of the airplane.
When flying at speeds below L/DMAX, also referred to as flying on the back side of the power curve, larger inputs in power or reducing the AOA will be required for the airplane to be able to accelerate.
Since slow flight will be performed well below L/DMAX, the pilot must be aware that large power inputs or a reduction in AOA will be required to prevent the aircraft from decelerating.
It is important to note that when flying on the backside of the power curve, as the AOA increases toward the critical AOA and the airplane’s speed continues to decrease, small changes in the pitch control result in disproportionally large changes in induced drag and therefore changes in airspeed.
As a result, pitch becomes a more effective control of airspeed when flying below L/DMAX and power is an effective control of the altitude profile (i.e., climbs, descents, or level flight)
It is also important to note that an airplane flying below L/DMAX, exhibits a characteristic known as “speed instability” and the airspeed will continue to decay without appropriate pilot action.
For example, if the airplane is disturbed by turbulence and the airspeed decreases, the airspeed may continue to decrease without the appropriate pilot action of reducing the AOA or adding power.
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