RPAS Airframes: Structures and Configurations
An airframe is the physical structure that carries the payload, powerplant and control surfaces. Candidates should understand the main configurations and the structural trade-offs.
Fixed-wing airframes generate lift from wings and are efficient for endurance and range, but require launch and recovery space and cannot hover. Multirotor airframes use multiple rotors for lift and control; they hover and manoeuvre precisely but have lower endurance. Helicopter (single main rotor) configurations offer efficient hovering with mechanical complexity. Hybrid/VTOL designs combine vertical take-off with wing-borne cruise.
Know the structural elements: the fuselage or body, booms and arms, landing gear, control surfaces (ailerons, elevators, rudders) on fixed-wing types, and motor mounts and propeller guards on multirotors. Understand centre of gravity (CG): the aircraft must balance within a specified range; moving the battery or payload shifts the CG and can degrade stability or control authority. Also understand payload integration - gimbal mounts, cameras and sensors affect weight, drag and balance.
Traps:
- Confusing stability with manoeuvrability; a design trade-off, not a quality judgement.
- Ignoring how adding a payload changes CG and endurance.
- Assuming all multirotors are equally stable regardless of configuration (quadcopter vs hexacopter redundancy differs).
Worked example: A candidate is asked why a hexacopter may be preferred over a quadcopter for a critical inspection. The answer relates to redundancy: with six rotors, loss of one motor may still permit controlled flight, whereas a quadcopter loses control authority. Rule of thumb: more rotors means more redundancy, not more efficiency.