Straight and Level Flight
In straight and level flight, the four forces are equal. Thrust is balanced against drag, and lift is balanced against the airplane's weight. The airplane is not accelerating in any direction.
Speeding Up
If engine power is increased, the propeller produces more thrust. The forces are now out of balance, thrust being greater than drag. This causes the airplane to accelerate forward. As the airspeed increases, drag increases. Eventually, the increased drag equals the higher thrust. The four forces are once again in balance, and the airplane no longer accelerates. The end result is that the airplane is flying at a faster speed.
Slowing Down
If thrust is reduced, the forces are again out of balance. Since drag is now greater than thrust, the airplane accelerates rearward, slowing down. As it slows, less drag is produced. The airplane will continue to slow until the drag equals the lesser thrust. The airplane will then be established at a slower airspeed.
This example refers to flight at normal cruise airspeeds, faster than the minimum drag airspeed. If the airplane continues to slow beyond the point of minimum total drag, total drag will begin to increase as the airplane slows. This situation will be explored in more detail below, and again in the aircraft performance lesson.
Climbing
Gravity always acts directly toward the earth. As a result, an airplane in a climb is pulled downward and rearward by gravity. This rearward acting gravity is sometimes referred to as the drag of weight. When at a constant rate of climb, all forces are in balance. The wings support the weight of the airplane, while thrust counters drag and the drag of weight.
When transitioning from straight and level flight, the angle of attack is increased by pitching the airplane up to the climb attitude. This increases lift, causing an upward acceleration.
The angle of attack is defined by the relative wind, which is opposite the direction of motion. Because the airplane's direction of motion becomes more upward, while the pitch is maintained, the angle of attack again becomes the same as it was originally.
At this point, lift again equals weight, so the airplane is no longer accelerating upward. The transition from level flight to climbing flight is complete, and the airplane is now established in a climb.
The pilot also increases to climb power when initiating the climb. Thrust must be increased or else the airplane will lose airspeed.
Descending
In a descent, gravity acts forward, resulting in the thrust of weight. In a constant rate descent, all forces are in balance, since the airplane is not accelerating. Lift counters weight, while thrust and the thrust of weight are equal to drag.
When transitioning to a descent from straight and level flight, the airplane is pitched down to the descent attitude. This reduces angle of attack, reducing lift, and resulting in a downward acceleration. As the airplane's direction of motion becomes more downward, the original angle of attack is restored, since the pilot is maintaining the descent pitch angle. Lift again equals weight. Less thrust from the propeller is required to counter the airplane's drag, due to the assistance of the thrust of weight. Thrust must be reduced or else the airplane will begin to gain airspeed.
To transition from straight and level to a descent, decrease the pitch to the descent attitude, while reducing the engine power to the descent power setting.
The Difference Between Pointing Up and Going Up
Drag is a minimum at a certain airspeed. At speeds faster than this airspeed, parasite drag is dominant, resulting in higher total drag the faster the airplane is flown. As speeds slower than this airspeed, induced drag is dominant, resulting in higher total drag the slower the airplane is flown.
It is easy for a pilot to become accustomed to flying at speeds near or above the point of minimum drag. At these speeds, the pilot is used to the idea of pitch up, go up.
This is not always true, however. For example, a pilot on approach to landing might allow the airplane to get too slow. The pilot increases the engine power, but the airspeed stays the same. The airplane begins to get low on the approach. Even at full engine power, the airspeed is not increasing, and the airplane continues to stay too low to make the runway. Drag is so great that the engine and propeller are unable to produce enough thrust to cause the airplane to accelerate.
This situation can be confusing, because under normal circumstances, the airplane climbs when the nose is up and the engine is at full power. The pilot must recognize that the slow airspeed and high induced drag are causing the airplane to behave this way. The answer is to decrease induced drag by decreasing lift, to pitch down. The natural desire is to continue to maintain a nose up orientation, since pitching down feels wrong when close to the ground. Once the pilot pitches down, airspeed rapidly increases and the airplane behaves as the pilot is accustomed. Then the pilot returns the airplane to the climb attitude, the engine power is now sufficient to climb the airplane onto the proper descent path for landing while speeding up.
A similar situation can arise during takeoff, when the pilot rotates early. At the right speed, the airplane will be going fast enough to rotate, but not fast enough to takeoff. Since the airplane is pointing upward, the pilot expects the airplane to takeoff and climb as it normally does. However, the slow speed and upward pitch or the airplane is resulting in the wings digging into the air. Drag becomes very high, and the airplane stops accelerating. The pilot sees the end of the runway approaching, and has a strong desire to continue pointing the airplane upward, becoming confused about its strange behavior. Under these circumstances, the pilot must recognize the excessive takeoff distance and abort the takeoff.
High drag can also overcome engine power during initial climbout. The pilot might maintain a skyward pitch attitude, which results in poor aircraft performance. The airplane might even start to descend on climbout with full engine power. Again, the pilot must recognize that the proper climb speed is not being maintained , resulting in excess drag and reduced lift. Lowering the pitch angle rapidly results normal airplane performance.