Controls: Primary and Secondary Flight Controls, Flaps, Slats, Spoilers, Trim and Fly-by-Wire
This domain tests what each control does, what it costs, and what its secondary effects are. Items mix identification (which surface moves for a given input), handling (trim, flaps, spoilers) and modern transport concepts such as fly-by-wire laws.
Primary controls
- Elevator controls pitch, ailerons control roll, rudder controls yaw. Each has secondary effects: ailerons produce adverse yaw, rudder produces roll, and elevator changes alpha and hence speed.
- Differential and frise ailerons reduce adverse yaw, but rudder is still needed in a turn entry.
Secondary controls
- Flaps increase CL and CLmax, add drag, and produce a pitching moment, usually nose-down, that must be trimmed out.
- Slats delay the stall to a higher alpha and raise CLmax; they are used with flaps in the approach configuration.
- Spoilers reduce lift and increase drag; they are used for descent, roll control and ground lift dumping.
- Trim tabs (trim, balance, servo and antiservo types) hold a control force. Trimming changes the hands-off equilibrium, not the static stability.
Fly-by-wire
- Normal law provides protections such as pitch attitude limits, bank limits, high-alpha protection and alpha floor. Alternate and direct laws degrade or remove them.
- Sidesticks give no natural feedback, so artificial feel and centre detents are provided.
- The Mach trim system counteracts the pitch-down at high Mach; it is not a stall protection.
Worked example
Flap extension produces a nose-down moment coefficient of 0.05 about the CG at q = 5000 Pa, with S = 30 m² and mean chord 1.5 m. The moment is 0.05 × 5000 × 30 × 1.5 = 11,250 N·m. If the elevator acts 8 m behind the CG, an extra downforce of 11,250/8 ≈ 1400 N is needed, and the aircraft must be trimmed nose-up to supply it.
Traps
- Spoilers reduce lift and increase drag; they are not flaps.
- Trimming does not improve static stability; it only removes the steady force.
- In direct law, full stick authority is available and the normal-law protections are gone.
- Flap extension can cause a balloon or a sink depending on speed and configuration.
- Mach trim is not a stall or overspeed protection.