Airframe Structures and Flight Controls
What this domain tests
This domain examines airframe construction, load paths, flight control surfaces, and the aerodynamic and mass-balance problems that affect control.
Core rules
- A semi-monocoque structure uses skin to carry shear and some bending, stringers to stiffen the skin and carry axial load, and frames or ribs to maintain shape and transfer concentrated loads. Spars carry bending and torsion.
- The wing torsion box lies between the front and rear spars; ribs distribute load and may form integral fuel tanks.
- Primary controls are ailerons for roll, elevator for pitch, and rudder for yaw. Secondary controls include flaps, slats, spoilers, and trim.
- Trim tabs move opposite to the control surface; balance tabs assist the pilot; servo tabs aerodynamically move the surface; anti-balance tabs increase feel.
- Mass balance ahead of the hinge prevents flutter. Aerodynamic balance reduces hinge moment. Irreversible actuation with artificial feel prevents aileron reversal at high speed.
Worked example
An aircraft of mass 1,500 kg is certificated to +4 g and has a wing span of 11 m. Total lift = 4 × 1,500 × 9.81 = 58,860 N. Each wing carries half, 29,430 N. If that lift acts 2.5 m from the root, root bending moment = 29,430 × 2.5 = 73,575 N m. For ultimate, multiply by 1.5: about 110,360 N m. The reasoning shows why half-span and load factor both matter.
Traps
Using total lift on one wing; forgetting to halve the lift; mixing span with semi-span; omitting the ultimate factor when asked; confusing balance and anti-balance tabs; thinking mass balance reduces hinge moment rather than preventing flutter.