6.3 Basic Principles of Flight, Stability & Control

Forces as a Decision Tool

A practical model uses four forces:

• Weight acts downward through the centre of gravity.
• Lift is the aerodynamic force supporting the aircraft.
• Thrust is propulsive force from motors and propellers.
• Drag opposes motion through the air.

In steady hover, total upward thrust balances weight. In straight level fixed-wing flight at constant speed, vertical lift balances weight and forward thrust balances drag. An imbalance produces acceleration.

These are operational ideas. A heavier aircraft needs more supporting force. More speed generally raises drag. A gust changes airflow and required response. Near people, each change affects path, stopping distance, or failure energy.

Multirotor Control

A multirotor varies individual rotor speed.

• Climb or descent: total thrust changes.
• Pitch: thrust is redistributed front to rear.
• Roll: thrust is redistributed left to right.
• Yaw: the controller changes the balance of opposing rotor torque.

To move horizontally, the aircraft tilts and directs some thrust sideways. Total thrust may need to increase to maintain height. Heavy payload, weak battery, hot conditions, or strong wind can leave too little reserve for both position and altitude.

During braking, the controller tilts against the motion. The aircraft still travels while velocity changes. Higher speed and mass, tailwind, reduced mode authority, and poor battery performance can lengthen ground stopping distance.

If a rotor, motor, or controller degrades, the remaining system may lack authority to balance forces and torque. Some aircraft have redundancy; many small multirotors do not. Use the model-specific procedure.

Fixed-Wing Lift and Stall

A fixed wing develops lift from airflow and wing geometry. The angle of attack is the angle between the wing reference and relative airflow. Increasing it normally increases lift only up to a point. Beyond the critical angle of attack, separated airflow causes a stall.

• A stall is caused by exceeding critical angle of attack, not one universal speed.
• Stall speed changes with mass, bank, manoeuvre, gust, and configuration.
• Groundspeed can be low in a headwind while airspeed remains safe.
• Groundspeed can be high in a tailwind at the same airspeed.

A steep turn raises lift demand and can reduce stall margin. A slow tight turn near a boundary needs more speed and space than a map view suggests.

Recovery normally requires reducing angle of attack and following the manufacturer procedure. Close to the ground there may be insufficient height, so prevention is the main A2 strategy.

Stability and Centre of Gravity

Stability is the tendency after a disturbance. A stable aircraft tends to return toward trim; a poorly balanced one may diverge or require constant correction.

The centre of gravity (CG) is the effective balance point of the complete configured aircraft. Payload position changes it.

A lateral or fore-aft multirotor CG shift makes some motors work harder. The aircraft may hover tilted, lose symmetrical thrust reserve, consume more energy, and respond differently in an emergency. A swinging load creates a moving CG and oscillation.

On a fixed wing, excessively aft CG can reduce stability and stall-recovery authority. Excessively forward CG can raise control force, speed, or power needs and make flare difficult. Exact limits are model-specific.

A gimbal can move and alter drag or balance. A payload secure but outside the approved CG range remains unsafe.

Airspeed, Groundspeed, and Wind

Airspeed is motion relative to air. Groundspeed is motion relative to earth. For a fixed wing, lift and stall depend strongly on air-relative flow, while people separation and ground footprint depend on ground track. For a multirotor, a headwind can demand high tilt and power to hold position; a tailwind can create rapid drift and longer recovery.

A forecast wind value is not simply added to brochure speed. Gusts, turbulence, angle limits, battery, and the need to return upwind matter.

Energy and Control Margin

Kinetic energy grows with mass and the square of speed. Doubling speed has a much larger energy effect than a small mass increase. Low-speed mode can reduce energy, but a label does not guarantee control in gusts.

Control margin is unused capability available to reject a disturbance. A multirotor tilted hard into wind has little horizontal margin; a fixed wing near stall has little angle-of-attack margin; a battery near cutoff has little power margin. Good planning avoids several boundaries at once.

Examples

Tailwind approach. A fixed wing keeps safe airspeed but crosses the landing area faster over the ground. It needs a longer footprint or different approach.

Off-centre camera. A multirotor is below MTOM but hovers tilted with uneven motor load. Reposition within the approved CG range.

Rapid stop. A fast multirotor does not stop at the waypoint line. Plan tested braking distance and wind margin.

Slow fixed-wing turn. Pulling harder in a slow bank raises angle of attack and can stall the wing. Use adequate speed and space.

Ask: what must the aircraft do next, what margin remains, and where will it go if an expected force is lost?