Straight and Level Flight

The Four Forces

Four forces act on an airplane in flight:

Weight

Acts vertically downward through the center of gravity (CG). It is the force of gravity on the airplane and everything in it.

Lift

Acts perpendicular to the relative wind, generated primarily by the wings. In level flight, lift acts upward to oppose weight.

Thrust

The forward force produced by the engine and propeller. It accelerates the airplane and opposes drag.

Drag

The rearward resistance of the airplane moving through the air. It opposes thrust and acts parallel to the relative wind.

Equilibrium in Straight and Level Flight

For an airplane to maintain straight and level unaccelerated flight, the four forces must be in equilibrium:

• Lift = Weight — the airplane neither climbs nor descends.
• Thrust = Drag — the airplane neither accelerates nor decelerates.

If any force changes without a compensating adjustment, the airplane will depart from its straight and level condition. Your job as a pilot is to detect and correct these deviations promptly.

Lift and Factors Affecting Lift

Lift is generated when air flows over the wing, creating lower pressure on the upper surface and higher pressure on the lower surface. Two primary factors under the pilot's control affect the amount of lift produced:

Angle of Attack (AoA)

The angle of attack is the angle between the wing's chord line and the relative wind. Increasing the angle of attack increases lift — up to a point.

• In normal cruise flight, the angle of attack is approximately 4 degrees.
• As AoA increases, lift increases — until the critical angle of attack is reached (approximately 14-16 degrees for most light airplanes).
• Beyond the critical angle, airflow separates from the upper wing surface and lift decreases rapidly — this is a stall.

Airspeed

Lift increases with the square of airspeed. Double the airspeed and lift increases four times (all else being equal). This is why at higher speeds, only a small angle of attack is needed to support the airplane's weight, and at lower speeds, a greater angle of attack is required.

Drag

Drag is the total aerodynamic resistance opposing the airplane's motion through the air. It has two main components:

Parasite Drag

Parasite drag is caused by the airplane's shape, surface friction, and any protrusions (antennas, landing gear, etc.) moving through the air. It increases with the square of airspeed — double the speed, four times the parasite drag.

Induced Drag

Induced drag is a byproduct of lift production. The wing tip vortices and downwash create this rearward component of the lift force. Induced drag is greatest at slow speeds and high angles of attack — exactly when the wing is working hardest to produce lift.

Total Drag Curve

When you plot parasite drag and induced drag together against airspeed, you get the total drag curve:

• At low speeds: induced drag dominates (high AoA needed for lift).
• At high speeds: parasite drag dominates (airframe resistance).
• The minimum point on the total drag curve represents the speed at which total drag is lowest — this is significant for range and endurance.

Stability in Pitch

An airplane is designed to be longitudinally stable — meaning that if disturbed in pitch, it tends to return to its original attitude without pilot input.

How It Works

The center of gravity (CG) is positioned ahead of the center of lift (also called the center of pressure). This creates a natural nose-down pitching tendency. The horizontal stabilizer (tailplane) produces a downward force to balance this couple and maintain the desired pitch attitude.

Effect of CG Position

• Forward CG: More stable, but requires more tail-down force and more back pressure/trim. Harder to maneuver and slightly less efficient (the tail down-force means the wing must produce more lift than the airplane's weight).
• Aft CG: Less stable, lighter control forces, potentially unstable. If the CG moves behind the aft limit, the airplane may become uncontrollable in pitch.

Stability in Roll

Most training airplanes have positive lateral stability — if a wing drops, the airplane tends to return to wings-level without pilot input. The primary design feature providing this stability is wing dihedral.

How Dihedral Works

Dihedral is the upward angle of the wings as viewed from the front of the airplane. When a wing drops and the airplane begins to sideslip:

1. The lower wing meets the relative airflow at a higher angle of attack.
2. The higher wing meets it at a lower angle of attack.
3. The lower wing produces more lift, rolling the airplane back toward level.

This self-correcting tendency is why you can momentarily release the controls and the airplane will tend to return to wings-level (assuming it is properly trimmed and in smooth air).

Stability in Yaw

The vertical stabilizer (fin) provides directional stability. If the airplane yaws — for example, due to a gust — the fin is presented to the airflow at an angle. This creates a sideways aerodynamic force on the fin that weathercocks the airplane back into alignment with the relative wind, like a weather vane.

The larger the fin area and the farther it is behind the CG, the stronger this stabilizing effect.

Power + Attitude = Performance

This is one of the most important concepts in early flight training:

In straight and level flight, the sequence is always Power — Attitude — Trim (P-A-T):

1. Power: Set the desired power for the airspeed/configuration you want.
2. Attitude: Adjust the pitch attitude to maintain altitude at the new power setting.
3. Trim: Trim away the control pressure so you can fly hands-off in the new configuration.

This sequence applies to every transition between flight configurations — it is worth committing to memory immediately.

Slow Safe Cruise

Slow safe cruise is a reduced-speed configuration useful when you need more time to assess your situation — for example, when temporarily lost or encountering deteriorating weather.

• Reduce power to a lower cruise setting.
• Once airspeed is below V_FE (maximum flap extended speed), deploy the first stage of flap.
• Adjust pitch attitude (nose slightly lower than clean cruise) to maintain altitude.
• Trim for the new configuration.

The result is a lower groundspeed, giving you more time to navigate, check charts, and make decisions — while remaining in a safe, stable flight condition.

Maximum Range Airspeed

The maximum range airspeed gives you the greatest distance per unit of fuel burned. It occurs at the speed where the ratio of speed to drag is highest — slightly above the minimum drag speed (L/D max).

• Useful for long cross-country flights or when fuel is a concern.
• Found in the POH performance tables for your airplane.
• Typically close to the best glide speed (since both relate to minimum drag).

Maximum Endurance Airspeed

The maximum endurance airspeed allows you to remain airborne for the longest time on a given fuel load. It occurs at the speed requiring minimum power — slightly below the minimum drag speed.

• Useful when you need to hold or orbit (e.g., waiting for weather to clear at your destination).
• Lower than maximum range speed.
• The airplane is flying "on the back side of the power curve" — be aware that at speeds below this, maintaining altitude requires more power, not less.

Flight at Critically High Airspeed

The airspeed indicator has colored arcs that define operating limits:

Arc/Mark	Speed	Meaning
Green Arc	VS1 to VNO	Normal operating range
Yellow Arc	VNO to VNE	Caution range — smooth air only
Red Line	VNE	Never exceed speed

Maneuvering Speed (V_A)

V_A (maneuvering speed) is the maximum speed at which you may apply full, abrupt control deflection without risking structural damage. Above V_A, aggressive control inputs can overstress the airframe.

• V_A decreases with lighter weight (lighter airplane stalls sooner, limiting the load factor).
• Above V_NO (top of green arc), fly in smooth air only and avoid abrupt control inputs.
• Never intentionally exceed V_NE (red line) under any circumstance.