RBAK - Basic Aviation Knowledge
Aerodynamics, Weight and Balance
Connect aerofoil terminology, forces, centre of gravity and loading limits to safe RPA operation.
Lesson references
- Last reviewed
- 2026-05-18
- Lesson references
- RePL Study Guide pp. 199-206; Part 101 MOS C10 pp. 92-93.
Four forces, one aircraft
Lift acts upward, weight acts downward, thrust moves the aircraft forward or upward depending on the design, and drag resists motion through the air.
Angle of attack is the angle between the aerofoil chord line and the relative airflow. Centre of pressure is where the aerodynamic force acts. Centre of gravity is where the aircraft balances.
Three steps. Step 1, aerofoil: air flows past an aerofoil section from left to right. This is the relative airflow, opposite to the direction the aerofoil moves. A dashed chord line runs from the leading edge to the trailing edge. The angle of attack (AoA) is the angle between the chord line and the relative airflow, and it opens and closes slightly. Step 2, hover: a multirotor seen from the side. Lift acts up and weight acts down through the centre of gravity, and they are equal (L = W). Step 3, forward flight: the multirotor tilts forward, so the rotor thrust tilts and gets longer. Its upward part is still equal to weight and its forward part is thrust. Drag acts rearward, opposite to the motion, and in steady level flight thrust equals drag (L = W, T = D). The Climb button shows lift greater than weight (L > W). The Descend button shows weight greater than lift (W > L). Arrow lengths are schematic. The still frame shows step 3.
Lift has more than one explanation
The MOS expects remote pilots to understand Bernoulli, Coanda and Newton's third law at a basic level. In practical terms, an aerofoil or rotor creates lift by shaping and accelerating air so that a useful force is produced.
The exam point is useful, but the operational point is better: damaged propellers, wrong payloads, high density altitude or poor loading can reduce the lift and control margin you thought you had.
Air flows from left to right past an aerofoil. Three steps. Step 1, Bernoulli: air over the curved upper surface moves faster than air under the flatter lower surface, so the dots above pull ahead of the dots below. Faster air has lower pressure: a low pressure region above and a high pressure region below give an upward force, lift. Step 2, Coanda: the air next to the upper surface follows the curve instead of going straight, and leaves the trailing edge turned downward. Step 3, Newton's third law: the aerofoil pushes air down (action), and the air pushes the aerofoil up (reaction), which is lift. All three work together; no single principle explains lift on its own. The flow is schematic, not measured. The still frame shows step 3.
Loading changes the aircraft
Empty weight, operating weight, maximum gross weight, arm, moment, datum and centre of gravity limits are all ways of describing whether the aircraft is loaded within its approved envelope.
For RPA operations, payloads are the everyday trap. A camera, sprayer tank, LiDAR unit or extra battery can change gross weight, balance, endurance and emergency handling.
A 12 second loop showing a multirotor hovering, seen from the side with its nose to the left. A dashed datum line stands at the nose, and an arm dimension runs from the datum to the centre of gravity (CG). A payload hangs under the body. When the payload is centred, the CG is at the centre of the rotors and the front and rear thrust arrows are equal. When the payload slides forward, the CG moves forward, the arm gets shorter, the nose briefly dips, and the front motor, the one nearest the CG, makes more thrust and is marked as working harder while the rear motor makes less. When the payload slides aft, the rear motor works harder. The payload then returns to the centre. Thrust arrow lengths are schematic. The still frame shows the payload forward with the front motor working harder.
Practice Questions
Why can adding a payload change flight safety?
- It can change weight, balance, endurance and control margin.
- It only changes the colour of the aircraft.
- It always improves climb performance.
- It removes the need for battery planning.
Answer: It can change weight, balance, endurance and control margin.
Payload affects aircraft loading and can push the RPA outside expected performance or balance limits.
Next step after study
Complete your Remote Pilot Licence training
The free study guide is a strong theory foundation. To actually be issued with a RePL, students still complete approved training, practical flying and assessment with a certified provider.