Trim tab types and direction
A trim tab is a small aerofoil hinged to a control surface. It relieves control pressure after an attitude change, so the pilot need not hold pressure by hand.
Tab types:
| Type | Set by | Use |
|---|---|---|
| Fixed tab | Ground, preset | Balanced control loading at normal cruise speed. Not adjustable in flight |
| Hinged tab | Pilot, in flight | Adjustable in flight |
Tabs by aircraft:
| Aircraft | Elevator | Ailerons | Rudder |
|---|---|---|---|
| Most small single-engine trainers | Controllable trim | Fixed tab, if fitted | Fixed tab, if fitted |
| Larger aircraft | Hinged tab | Hinged tab | Hinged tab |
- Most small single-engine trainers:
- Only elevator trim pilot-controlled: longitudinal stability keeps changing with attitude.
- Aileron and rudder loads shift less: fixed tabs adequate for lateral and directional stability.
- Larger aircraft: hinged tabs compensate for lateral load shifts. Multi-engined types also get extra rudder authority after engine failure.
- Variable incidence tailplane: some aircraft tilt the whole horizontal stabilizer to trim longitudinally, instead of an elevator tab.
Tab direction:
| Tab | Fitted to | Moves | Effect on control |
|---|---|---|---|
| Normal elevator trim tab | Fixed stabilizer with elevator | OPPOSITE the elevator | Relieves pressure |
| Antiservo tab | Stabilator (one-piece pivoting stabilizer, no separate elevator), in place of a normal trim tab | WITH the stabilator trailing edge | Stiffens control, so the stabilator's high sensitivity cannot cause overcontrol. Still does the trimming job |
| Servo tab (contrast) | Lightens control force; antiservo the reverse |
Elevator trim tab direction: tab moves opposite the direction the elevator needs to move.
| Trim | Tab | Airflow effect | Elevator trailing edge | Result |
|---|---|---|---|---|
| Full nose-down | Full UP | Up tab catches airflow | Forced DOWN | Tail raised, nose lowered |
| Full nose-up | Full DOWN | Reverse | Forced UP | Reverse |
Trap: applying one tab's direction rule to the other. A normal elevator trim tab moves OPPOSITE the elevator; an antiservo tab moves WITH the stabilator trailing edge.
References
- TP 1102 Ex.2 Components (Trim Tabs); TP 1102 ch.1 Basic Principles of Flight (Trim Tabs, Variable Incidence Tail Planes); FAA-H-8083-25 PHAK ch.6 (Trim Tabs, Stabilator, Antiservo Tabs)