Demo
STRAIGHT AND LEVEL FLIGHT
We have already seen that in straight and level flight, the four forces acting on the microlight are balanced with their opposite, (assuming a constant airspeed). Level flight is possible at any speed above the stall speed, right up to the maximum attainable in level flight. Let us look at a microlight in level flight at full power. The four forces are balanced, with thrust equalling drag, and lift equalling weight. If we were to reduce power to, say 75%, what happens? As the thrust is reduced, drag slows the aircraft down until thrust equals drag again.
If we had kept the angle of attack the same, the aircraft would now begin to descend because at the same angle of attack the lower speed will produce less lift. So, in order to maintain level flight at this lower speed, we have to increase the angle of attack to produce the lift needed.
This process continues as we slow down further. The slower the airspeed, the greater the angle of attack needed to produce the lift needed for level flight. However by increasing the angle of attack remember we are also increasing the drag created by the wing.
We can counter this by applying more power, but if the aircraft is in slow flight with a nose high attitude then there comes a point where there will be insufficient extra power available. This situation is called "flying on the backside of the power curve" or "Deadmans Curve!" and occurs when the power available is insufficient to overcome the drag when flying at a high angle of attack.
The situation is most likely to develop if the pilot throttles back while climbing and then at a slower airspeed, and in a nose high attitude, opens the throttle again and the aircraft does not respond as expected. The recovery is simply to lower the nose allowing the airspeed to return to a higher figure. The danger of the above situation is much greater if near the ground as there may be insufficient height for recovery. Recovery will mean a loss of height.
A similar situation could develop if the pilot attempts to initiate a climb immediately after take-off and before the aircraft has accelerated to an airspeed that will allow it to climb safely. The recovery is to lower the nose, reducing the angle of attack, and waiting for the airspeed to return to a higher figure.
The purpose of this section is to point out that a high power setting does not necessarily mean flight at high speed. It is the elevators and thus angle of attack which is primarily responsible for speed control. Remember, flight is possible at low speed and high angle of attack, high speed and low angle of attack, and an infinite number of combinations of angle of attack to obtain the same value of lift.