A basic overview of the forces associated with straight and level flight, climbs, descents, and turns.
Lift, Weight, Thrust and Drag
STRAIGHT AND LEVEL FLIGHT
In straight and level flight, lift is equal to weight and thrust is equal to drag. Airspeed and altitude do not change.
Note: Whether in straight and level flight, a climb, or a descent, weight always points directly down, toward the center of the Earth, due to gravity.
CLIMBS
If a climb is entered with no change in the power setting, airspeed will diminish.
1. Notice how the weight vector moves in relation to the aircraft between level flight (shown with the light blue arrow), and the climb (dark blue arrow). As the aircraft pitches up, weight continues to point straight down and essentially moves backward from where it was in level flight (shown by the dashed black line).
2. When inclined upward, this rearward movement of weight acts in the same direction as drag, and has the same effect as increased drag. Without a corresponding increase in thrust, there is now more drag than thrust, and the aircraft slows until the forces are balanced again. Therefore, in order to maintain airspeed in a climb, thrust must be increased.
DESCENTS
If a descent is entered with no change in the power setting, airspeed will increase.
1. Notice how the weight vector moves in relation to the aircraft between level flight (shown with the light blue arrow), and the descent (dark blue arrow). As the aircraft pitches down, weight continues to point straight down and essentially moves forward from where it was in level flight (shown by the dashed black line).
2. When the aircraft is pitched down, this forward movement of weight acts in the same direction as thrust, and has the same effect as increased thrust. Without a corresponding decrease in thrust, the thrust vector outweighs the drag vector, and the aircraft accelerates until the forces are balanced again. Therefore, reduced thrust (or increased drag) is needed to maintain a consistent airspeed during a descent.
STRAIGHT AND LEVEL
As shown above, in straight and level flight, lift is equal to weight (or load). When the aircraft is banked, the total lift and total load rolls with the aircraft while weight continues to point toward the center of the Earth (as shown in the next two graphics).
As an aircraft banks, the total lift vector is divided into 2 components, a Vertical Component of Lift (VCL), as well as a Horizontal Component of Lift (HCL). The VCL is opposed by Weight, and the HCL is opposed by Centrifugal Force.
When these forces are balanced, Total Lift is equal to Total Load, and the aircraft maintains a level, coordinated turn. In this section, we'll assume a coordinated turn (HCL = CF), and look at why back pressure is required in a turn (VCL vs Weight).
LEFT BANK, NO BACK PRESSURE
Lets imagine we roll into a 30 degree coordinated left turn without adding any back elevator pressure. As the aircraft rolls, the total lift we saw in the first diagram is divided into a VCL (green) and HCL (yellow). The greater the bank angle, the greater the HCL, and the smaller the VCL. Because the weight of the aircraft is unchanged, and weight now exceeds the VCL, the aircraft will descend.
In order to compensate for the loss in vertical lift, the VCL must be increased with the elevator. Altitude is maintained when the VCL is equal to weight (as shown in the next graphic). When the VCL is equal to Weight and the HCL is equal to the CF, Total Lift becomes equal to the Total Load on the aircraft, and a level, coordinated turn is made.
LEFT BANK WITH BACK PRESSURE
In a level, coordinated turn:
To summarize, back elevator pressure is required to compensate for the portion of vertical lift that has been transferred horizontally, and maintain altitude in a turn.
COORDINATED TURN
As mentioned above, in a turn some VL becomes HL. As you may remember, from Newtons 3rd Law, every force has an opposing force. The force opposite the HCL is called Centrifugal Force (CF).
As shown in the graphic above, a turn is coordinated when the HCL is equal in strength to the CF opposing it. If the HCL is not equal to the CF the turn will not be coordinated.
SLIPPING TURN
In a slipping turn, the HCL is greater than the CF. This is because the rate of turn is too slow for the angle of bank. The aircraft is yawed to the outside of the turning flight path in a slipping turn.
A slipping turn can be corrected by decreasing the angle of bank and/or increasing the rate of turn (increased rudder in the direction of the turn).
SKIDDING TURN
In a skidding turn, the CF is greater than the HCL. This is because the rate of turn is too great for the angle of bank. The aircraft is yawed to the inside of the turning flight path in a skidding turn.
A skidding turn can be corrected by reducing rudder in the direction of the turn, and/or increasing the bank angle.