Slow Flight

Definition of Slow Flight

Slow flight is defined as flight at an airspeed approximately 5 knots above the stalling speed. At this speed, the airplane is fully controllable but operating near the lower boundary of its performance envelope.

To understand slow flight, you need to know two key stalling speeds:

V_S0 — Stall Speed (Landing Configuration)

The bottom of the white arc on the airspeed indicator. This is the stalling speed at maximum weight, full flap, power off. It represents the slowest speed at which the airplane can maintain controlled flight in landing configuration.

V_S1 — Stall Speed (Clean Configuration)

The bottom of the green arc on the airspeed indicator. This is the stalling speed at maximum weight, no flap, power off. It represents the slowest speed in clean (cruise) configuration.

Forces in Slow Flight

At slow speed, the airplane must fly at a significantly higher angle of attack to generate sufficient lift. In cruise, the angle of attack is approximately 4 degrees. In slow flight, it increases to approximately 10 degrees or more.

This high angle of attack has several consequences:

• Increased induced drag: Induced drag is inversely proportional to speed squared — at half the cruise speed, induced drag is four times greater
• More power required: The airplane needs significantly more power to maintain altitude at slow speed than at cruise
• Speed-unstable region: Below the minimum power required speed, the airplane enters a region where slowing down requires MORE power, not less

Typical Angle of Attack vs. Flight Condition

Flight Condition	Approximate AoA	Relative Drag
High-speed cruise	2°	Low
Normal cruise	4°	Moderate
Slow flight	10°	High
Approaching stall	12–15°	Very high

Effect of Controls

At slow speed, the reduced airflow over the control surfaces makes all controls less effective. You will notice:

• Sloppy feel: The controls feel mushy and unresponsive compared to cruise. Larger control movements are needed to achieve the same result.
• Adverse yaw more pronounced: Because the ailerons are less effective but still creating differential drag, adverse yaw becomes much more noticeable. Coordinated rudder use is essential.
• Slipstream effect more noticeable: With high power settings needed for slow flight, the propeller slipstream creates a stronger asymmetric effect, requiring more right rudder to maintain coordinated flight.

Maneuvering at Slow Speed

The fundamental relationship Power + Attitude = Performance still applies in slow flight, but the roles are reversed from normal cruise:

• Attitude controls airspeed: Lowering the nose increases speed; raising it decreases speed
• Power controls altitude: Adding power arrests descent; reducing power causes descent

When turning in slow flight:

• Limit bank angle to 15 degrees maximum — steeper banks increase load factor, which increases stalling speed
• Keep the airplane in balance — use the ball indicator and stay coordinated
• Add a small amount of power in the turn to compensate for the increased load factor and maintain altitude

Distractions

Inadvertent slow flight is one of the most dangerous situations in general aviation. It typically occurs when the pilot becomes distracted and allows airspeed to decay without noticing. Common distractions include:

• Radio calls: Listening for or making transmissions while in the traffic pattern
• Passengers: Conversation or passenger emergencies diverting attention
• Map reading / GPS programming: Looking inside the cockpit for extended periods
• Looking for traffic: Head-down time searching for reported traffic

Principles of Flight Supplement

To fully understand slow flight, you need to understand the aerodynamic concepts behind it.

Angle of Attack

The angle of attack (AoA) is the angle between the chord line of the wing and the relative airflow. The chord line is an imaginary straight line drawn from the leading edge to the trailing edge of the wing.

Increasing the angle of attack increases the coefficient of lift — up to a point. The airplane does not know its pitch attitude relative to the horizon; it only responds to the angle of attack relative to the air flowing over the wing.

Critical Angle of Attack

The critical angle of attack is approximately 12 to 15 degrees for most light aircraft wings. Beyond this angle, the smooth airflow over the upper surface of the wing can no longer follow the surface contour. It separates from the wing, destroying lift and causing the stall.

Coefficient of Lift (C_L) Curve

The relationship between angle of attack and lift coefficient is shown on the C_L curve:

• From 0° to approximately 12–15°, C_L increases roughly linearly with AoA
• At the critical angle, C_L reaches its maximum value (C_{L max})
• Beyond the critical angle, C_L drops sharply — the wing is stalled

Airflow Separation

As the angle of attack increases toward the critical angle, the airflow begins separating from the upper surface of the wing starting at the trailing edge. This separation point moves progressively forward as AoA increases. When separation reaches approximately the quarter-chord point, the wing stalls.

Center of Pressure Movement

The center of pressure is the point on the wing where the total aerodynamic force effectively acts. As the angle of attack increases, the center of pressure moves forward along the chord. At the stall, it moves rapidly rearward, causing a nose-down pitching moment — this is actually a built-in safety feature that helps the airplane recover from the stall naturally.