AS350 B3e course · Astar hazards
Low g, and why mast bumping isn't an Astar hazard
The right lesson from the wrong rotor
The essentials
Mast bumping — the hub striking the mast in low-g flight — is a hazard of teetering rotors, like those on Robinson and Bell two-bladed helicopters. Without a stiff hub they lose their control moment in low g; the fuselage rolls right, and a reflex left cyclic can drive the hub into the mast.
The Astar's Starflex has no teetering hinge. Like other rotors with three or more blades it keeps some control moment at low g, so mast bumping as the handbook describes it doesn't apply.
Low g is still not a manoeuvre: abrupt push-overs load the rotor and controls in ways the aircraft isn't approved for.
What mast bumping is
A teetering rotor tilts as one piece on a single hinge at the top of the mast. In normal flight, thrust acting through the tilted disc gives the pilot control. In low g, thrust — and so control — almost vanishes.
The tail rotor's thrust then rolls the aircraft, and an instinctive cyclic correction can flap the disc so far that the hub strikes the mast. The handbook describes it as often fatal.
Source 1
Two ways to steer a fuselage
A rotor can move the fuselage in two ways. One is thrust tilt: the disc tilts, thrust leans with it, and the fuselage follows. That needs thrust. The other is hub moment: a stiff hub bends or twists the top of the mast directly when the disc tilts. That doesn't.
A teetering rotor has only the first. A hub with flapping stiffness has both — and the second is the one that survives at low g.
Why the Starflex is different
A hub with flapping stiffness — like the Starflex, whose arms bend like springs — transmits a moment to the mast whenever the disc tilts relative to it. That moment exists whether or not the rotor is producing thrust.
So at low g the Astar still answers its cyclic — less crisply — and it has no teetering hinge for the hub to bump. The handbook notes that multi-bladed rotors keep some control at low g.
Teetering rotor and Starflex, side by side
| Teetering — hinge | One teeter hinge; the two blades tilt as a unit1 |
|---|---|
| Teetering — control at low g | Largely lost: control comes from thrust tilt1 |
| Teetering — low-g hazard | Mast bumping1 |
| Starflex — hinge | No teetering hinge; composite hub carries mast bending moment2 |
| Starflex — control at low g | Partly retained through hub moment1 |
| Starflex — low-g hazard | Not mast bumping; droop-stop pounding if flapping is excessive1 |
Where low g comes from
Low g isn't only a deliberate push-over. It comes from pushing the nose down sharply at the top of a climb or after a pull-up, cresting a ridge and pushing over to follow the terrain down, and from strong turbulence.
Passengers feel it as floating in their seats. In the Astar it's a sign you're flying more abruptly than the aircraft needs — and a dive that starts with low g often ends with the high-g pull-out that servo transparency feeds on.
Why low g still matters
The Astar isn't an aerobatic aircraft. Abrupt push-overs and large control inputs at low g load the hub, blades and controls in ways outside the aircraft's approval. The handbook warns that multi-bladed rotors can suffer droop-stop pounding, a cousin of mast bumping, if flapping clearances are exceeded.
Flight manuals list prohibited manoeuvres such as aerobatics — check yours. Fly smoothly, and keep the g positive.
Why is mast bumping not a hazard on the AS350?
Mast bumping belongs to teetering rotors. The Starflex keeps some control at low g, and there's no teeter hinge to bump.
Does that make low-g push-overs acceptable in the Astar?
The Astar isn't an aerobatic helicopter. Abrupt low-g manoeuvres are outside its approval.
Why does a teetering rotor lose control at low g?
With no hub moment, a teetering rotor steers the fuselage only through thrust. No thrust, no control.
Which of these can produce low g without meaning to?
Pushing over abruptly to follow terrain down unloads the rotor. Smooth inputs avoid it.
Sources
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 11, Helicopter Emergencies and Hazards
- Starflex rotor hub — European Rotorcraft Forum paper · European Rotorcraft Forum
- Jack be nimble · Flight Safety Australia · August 2024
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 5, Rotorcraft Flight Manual
Educational only. Not approved training material, and never a substitute for the Rotorcraft Flight Manual, an instructor, or your operator's procedures. Figures marked as manufacturer data or test results are not limitations. This content is a draft and has not yet been reviewed by an instructor.