ATPL Performance: General Theory and Flight Forces

The Four Forces

In steady level flight, the four forces acting on the aircraft are in equilibrium:

1. Lift (L) balances Weight (W).
   • $L = W = m \times g$
2. Thrust (T) balances Drag (D).
   • $T = D$

Drag Characteristics

Drag is the resistance the aircraft faces moving through the air. It has two main components:

1. Parasite Drag ($D_p$)

• Caused by friction and form resistance.
• Increases with the square of speed ($V^2$).
• Dominant at high speeds.

2. Induced Drag ($D_i$)

• By-product of lift generation (wingtip vortices).
• Decreases with the square of speed ($1/V^2$).
• Dominant at low speeds.

Total Drag Curve

The sum of Parasite and Induced Drag creates a U-shaped curve when plotted against speed.

• $V_{MD}$ (Minimum Drag Speed): The speed at the bottom of the curve where Total Drag is minimum.
• At $V_{MD}$, Induced Drag equals Parasite Drag.

Thrust vs. Power

It is crucial to distinguish between Thrust and Power, especially for different engine types.

• Thrust ($T$): The force produced by the engine (Newtons or lbs). Important for Jet aircraft.
• Power ($P$): The rate of doing work ($P = T \times V$). Important for Propeller aircraft.
  • Power Available ($P_A$): The power the engine can deliver.
  • Power Required ($P_R$): The power needed to overcome drag ($P_R = D \times V$).

Flight Regimes

Region of Normal Command

• Speeds higher than $V_{MD}$.
• To fly faster, you need more thrust/power. Drag increases as speed increases.
• Stable speed control.

Region of Reversed Command (Back side of the power curve)

• Speeds lower than $V_{MD}$.
• To fly slower, you need more thrust/power because induced drag increases rapidly.
• Unstable speed control (slower speed $\rightarrow$ more drag $\rightarrow$ speed decreases further unless power is added).