4.3 Aerodynamics: Lift, Drag, Thrust, Weight and Energy

The Four Forces

Every aircraft, crewed or remotely piloted, is acted on by four forces:

• Lift — the force that opposes weight and holds the aircraft up.
• Weight — the force of gravity acting through the centre of gravity.
• Thrust — the force that drives the aircraft forward (or, on a multirotor, upward).
• Drag — the force that resists motion through the air.

In steady, unaccelerated flight lift equals weight and thrust equals drag. A multirotor collapses two of these into one: the rotors produce a thrust vector, and in a hover that vector points straight up and is the lift. Tilt the airframe and the same thrust splits into a vertical component that still has to equal weight and a horizontal component that produces forward motion — which is why a multirotor must add power when it accelerates, and why a strongly tilted multirotor in a high wind is close to its performance ceiling.

Aerofoil Terminology

Schedule 4 topic 5(a) asks for a specific vocabulary.

• Aerofoil — a shaped surface, such as a wing or rotor blade, designed to produce lift when air flows over it.
• Chord line — the straight line from the aerofoil's leading edge to its trailing edge.
• Relative airflow — the airflow as experienced by the aerofoil, equal and opposite to the direction of the aerofoil's motion through the air.
• Angle of attack — the angle between the chord line and the relative airflow. This is not the same as pitch attitude, which is measured against the horizon.
• Centre of pressure — the single point through which the total lift force is considered to act.
• Centre of gravity — the single point through which the total weight is considered to act.

As angle of attack increases, lift increases — but only up to the critical (stalling) angle, typically around 15 to 16 degrees for a conventional aerofoil. Beyond it the airflow separates from the upper surface, lift drops sharply and drag rises. A fixed-wing RPA stalls at the same angle of attack every time regardless of speed or weight; what changes is the speed at which that angle is reached.

Two Explanations of Lift, Both Correct

Schedule 4 names three principles by name, so know all three:

• Bernoulli's principle — where a fluid speeds up, its static pressure falls. Air accelerating over the curved upper surface of an aerofoil is at lower pressure than the slower air beneath, and the pressure difference produces lift.
• Newton's third law — every action has an equal and opposite reaction. The aerofoil deflects a mass of air downwards; the reaction pushes the aerofoil upwards. On a multirotor this is the more intuitive description: the rotors throw air down, and the aircraft goes up.
• The Coandă effect — a fluid jet tends to stay attached to a curved surface. It explains why the airflow follows the aerofoil's upper camber rather than separating immediately.

These are not competing theories to choose between. They are three descriptions of the same physical event, and an exam answer that says lift comes from both the pressure difference and the reaction to deflected air is the correct one.

Drag: Two Families

Total drag is the sum of two components that behave in opposite ways with airspeed.

Parasite drag is everything that is not a by-product of producing lift. Schedule 4 breaks it into:

• Form drag — resistance caused by the shape of the body pushing through the air (a landing-gear leg, a camera gimbal, a squared-off battery pack).
• Skin friction — resistance from air rubbing along the surface.
• Interference drag — extra drag generated where two components meet, such as an arm joining the centre body, because their airflows interact.

Parasite drag rises with the square of airspeed: double the speed and parasite drag quadruples.

Induced drag is the unavoidable by-product of producing lift. Generating lift leaves the air behind the aerofoil with downward momentum and trailing vortices, and the energy that goes into that wake appears as drag. Induced drag is greatest at low speed and high angle of attack, and it falls as speed rises.

Because the two curves move in opposite directions, total drag is a U-shape with a minimum somewhere in the middle. That minimum is why a fixed-wing RPA has a specific best-endurance airspeed rather than simply flying as slowly as possible, and why a mapping mission flown too slowly can burn more battery than the same mission flown at the recommended cruise.

Energy and Inertia

Schedule 4 topic 4 asks for three terms:

• Potential energy — energy stored by virtue of height. An RPA at 120 m AGL holds more potential energy than the same aircraft at 20 m.
• Kinetic energy — energy of motion, given by $KE = \tfrac{1}{2}mv^{2}$ where m is mass and v is velocity.
• Inertia — the tendency of a body to resist a change in its state of motion. Mass is the measure of inertia.

The kinetic-energy formula is the reason CASA's whole regulatory structure is built on weight categories. Because velocity is squared, doubling the speed quadruples the energy; because mass is linear, doubling the mass only doubles it. But mass sets the floor. A 250 g micro RPA striking a person at 15 m/s carries about 28 joules. A 2 kg drone at the same speed carries about 225 joules — eight times the energy for eight times the mass. A 25 kg small RPA carries about 2,800 joules. That progression, not an arbitrary line, is why micro RPA get concessions the small category does not.

Inertia has a handling consequence too. A heavier RPA accelerates more slowly, decelerates more slowly, and needs more room to stop. A pilot flying a 7 kg aircraft toward an obstacle at the same closure rate they are used to on a 900 g aircraft will run out of stopping distance. Plan the deceleration point by aircraft mass, not by habit.