Glossary

Helicopter words used around these docs.

Term	What it means
Anti-torque rotor	The tail rotor. It pushes the tail sideways so the fuselage doesn't spin the opposite way to the main rotor.
Articulated hub	Each blade has hinges so it can flap up and down and lead and lag in the rotor plane. Normal for 3 or more blades.
Autorotation	Coming down with no power, where the air flowing up through the rotor keeps it turning so you can still land.
Bell bar	The stabiliser bar on older Bell two-blade rotors. It damps how fast the rotor responds.
CG (centre of gravity)	The balance point of the loaded aircraft.
Chord	Blade width, from the leading edge to the trailing edge.
Collective	The lever that changes every blade's pitch at once, for more or less lift.
Combining gearbox	Where the power from several engines comes together before it goes to the rotors.
Cyclic	The stick. It tilts the rotor disc by changing blade pitch once per revolution, which tilts the aircraft.
Disc	The circle the blades sweep.
EGT	Exhaust gas temperature, the engine's main temperature limit.
Flap / lead-lag	A blade moving up and down / forward and back on its hinges.
Flat-plate area	How big a flat plate would have to be to make the same drag as the fuselage.
Freewheel (sprag clutch)	A one-way clutch that lets the rotor keep turning if the engine stops.
Governor	The engine control that keeps rotor RPM steady as you move the collective.
Ground effect (IGE / OGE)	Extra lift close to the ground. IGE = in ground effect, OGE = out of it (higher up, needs more power).
Hinge offset	How far out from the shaft the flap hinge sits, as a fraction of the radius. More offset = more control power.
Inertia (rotational)	How hard something is to spin up or slow down, in kg·m².
Mast tilt	The rotor shaft is leaned forward so the fuselage sits level in cruise.
Ng / N1	Gas generator (compressor) speed, %.
N2 / Nf	Power turbine speed. This is what drives the rotor through the gearbox.
Nr	Rotor speed, as a % of 100 % RPM.
N/rev	Vibration that happens N times per rotor revolution. Rotors mostly shake at blade count × rotor speed.
Phase (swashplate)	The rotor responds about 90° after a pitch input, so the swashplate leads by that much.
Pitch-flap coupling (δ3)	Blade pitch drops as the blade flaps up. It's there for stability.
Precone	The built-in upward angle of the blades on a teetering hub.
Referred inertia	A part spinning N times faster than the rotor counts as N² times its inertia at rotor speed.
Root cutout	The inner part of the blade that doesn't make lift (hub, grips).
Swashplate	The mechanism that turns stick and lever movement into blade pitch.
Teetering hub	Two blades on a seesaw hinge (Bell, UH-1).
Tip speed	How fast the blade tips are moving. Limited by noise and compressibility.
Twist	Blade pitch changing from root to tip. Usually less pitch at the tip (washout).
VRS (vortex ring state)	A dangerous state in a steep, slow descent where the rotor sinks into its own downwash and loses lift.
Wake / downwash	The air a rotor pushes down.