5.secondary-controls-flaps-trim. Secondary Flight Controls: Flaps and Trim Systems
Secondary flight controls supplement the primary flight controls (ailerons, elevator/stabilator, and rudder) by improving aircraft performance characteristics or relieving the pilot of excessive control forces. The principal secondary controls are wing flaps, trim systems, leading edge devices, and spoilers. This section focuses on the two systems student pilots encounter on virtually every training aircraft: flaps and trim.
Wing Flaps
Flaps are high-lift devices attached to the trailing edge of the wing that, when extended, increase both the lift and drag produced by the wing. They allow a steeper approach angle without an increase in airspeed and permit safe flight at lower airspeeds, particularly during takeoff and landing. The four common flap designs are:
- Plain flap — The simplest type. The trailing edge of the wing hinges down, increasing camber and thus the coefficient of lift (and drag).
- Split flap — Deflected from the lower surface of the wing while the upper surface remains fixed. Produces high drag with a modest lift increase.
- Slotted flap — The most popular flap on modern aircraft (including most Cessna and Piper trainers). A duct between the flap and the wing allows high-energy air from below to flow over the flap's upper surface, delaying airflow separation and producing a large lift increase with comparatively less drag.
- Fowler flap — Extends rearward on tracks before deflecting downward. This increases wing area first (more lift, little drag) and then camber (more lift and drag). Often built as double- or triple-slotted on transport-category aircraft.
Effects of Flap Extension
When flaps are extended, the pilot will observe:
- A nose-down pitching moment in most aircraft, because the center of lift shifts and downwash over the tail increases.
- A reduction in stall speed (V_S0 with full flaps is typically 8–15 knots lower than V_S1 clean).
- A steeper descent path at the same airspeed without adding power.
- Increased drag, allowing precise airspeed and glidepath control on final approach.
Flap operating ranges are marked on the airspeed indicator by a white arc, which extends from V_S0 (power-off stall, full flaps, landing configuration) at the lower end to V_FE (maximum flap extended speed) at the upper end. Exceeding V_FE risks structural damage to the flap and its attach points.
Trim Systems
Trim systems relieve the pilot of the need to maintain constant pressure on the flight controls. They do not change the aircraft's basic stability or controllability — they reposition the control surface (or its neutral point) so the airplane will maintain the desired attitude hands-off. The most common types are:
- Trim tabs — A small, hinged surface on the trailing edge of a primary control (most often the elevator). The pilot moves the tab in the direction opposite the desired control movement; aerodynamic force on the tab then deflects the primary control. For example, rolling the elevator trim wheel nose-down moves the tab up, which forces the elevator down.
- Balance tabs — Linked to the airframe so they move automatically opposite the primary control, reducing the force required to deflect that control.
- Anti-servo tabs — Found on the stabilator (e.g., Piper Cherokee). Move in the same direction as the stabilator to add resistance and provide an artificial feel, preventing the pilot from over-controlling. They also serve as the trim surface.
- Servo tabs — Used on larger aircraft. The pilot's controls move only the tab; aerodynamic force on the tab then moves the primary surface.
- Ground-adjustable tabs — Non-movable metal tabs on the rudder or aileron, bent on the ground to correct minor heading or wing-low tendencies in cruise.
Most light trainers also have a rudder trim (in some models) and many high-performance aircraft add aileron trim to correct for fuel imbalance or asymmetric loading.
Proper Use of Trim
The correct procedure is pitch–power–trim: establish the desired pitch attitude, set the appropriate power, allow the airspeed to stabilize, then trim off any remaining control pressure. Trimming first and then flying the airplane to the trim setting is a common error that produces porpoising and altitude excursions. After every configuration or power change (gear, flaps, climb to cruise, descent, approach), expect to retrim.
Runaway and Mis-set Trim
A mis-set trim — particularly nose-up trim left in for takeoff — can produce a sudden pitch-up at rotation that may exceed the pilot's ability to push forward. The before-takeoff checklist therefore calls for verifying trim is set to the takeoff range (a green band or T/O mark on the trim indicator). In aircraft with electric trim, a trim runaway is countered by overpowering the system manually, disengaging the autopilot, and pulling the electric trim circuit breaker.
Summary
Flaps and trim are the two secondary controls a private pilot manipulates on every flight. Flaps modify the wing's lift and drag for takeoff and landing; trim removes sustained control pressures so the pilot can fly accurately with minimal fatigue. Mastery of both — knowing what each does, when to use it, and the speeds and limits that apply — is fundamental to precise aircraft control.