AS350 B3e course · Rotors and flight controls
The Starflex head
Three hinges' worth of motion, and no hinges
The essentials
A rotor blade needs to flap, lead and lag, and feather. Most older heads give it a metal hinge for each. The Astar's Starflex head has none.
A three-armed composite star bends to let the blades flap. One laminated spherical bearing per blade lets it pitch, flap and lead-lag. An elastomeric frequency adapter per blade damps lead-lag.
Fewer parts and no lubrication — but the elastomers are now the working parts, and they are what you inspect.
A hub with no hinges
A blade needs three freedoms: to flap up and down, to lead and lag in the plane of rotation, and to feather in pitch. Classic heads give it a metal hinge for each.
The Starflex replaces them all. The hub is a three-armed star of glass-fibre/epoxy composite whose arms flex, and it has flown on the Écureuil and Dauphin families since 1974.
Explore the head
The 3D model needs JavaScript and WebGL.
Drag to orbit. Tap a part to light it up, or explode the head to see how it stacks. Move the controls — everything above the servos moves with them.
- Starflex star
- Three composite arms, one per blade. They bend to let the blades flap — the star is the flapping hinge and its spring in one.
- Spherical thrust bearings
- Laminated steel and elastomer. Flexible in pitch, flap and lead-lag; rigid in compression, so each carries its blade's centrifugal pull into the hub.
- Sleeves
- A rigid sleeve holds each blade and carries the horn the pitch link pulls. Between the sleeve and the star-arm tip sits an elastomeric frequency adapter — the drag damper.
- Blades
- Three composite blades. Cut short here so the head stays visible — the orange tip lets you follow one blade round.
- Pitch links
- Rigid rods from the rotating swashplate to each blade's pitch horn. As the swashplate rises or tilts, they set each blade's pitch.
- Rotating swashplate
- Turns with the rotor, carrying the pitch links, while tilting and rising with the plate beneath it.
- Stationary swashplate
- Doesn't turn. The servos raise it for collective and tilt it for cyclic.
- Main servos
- Three hydraulic servos — two lateral, one fore-and-aft — move the stationary swashplate. Their positions here are illustrative.
- Mast
- Carries the rotor's lift down into the gearbox, and the gearbox's torque up to the rotor.
- Main gearbox
- Slows engine speed to rotor speed. Only its top is drawn.
One bearing, three motions
One Starflex arm and the three motions it allows. This diagram needs JavaScript.
Each blade pivots on a single laminated spherical bearing of steel and elastomer. Pick a motion and watch what flexes.
From Alouette to Starflex
Aérospatiale's earlier hubs, like the Alouette II's, used metal hinges and bearings that needed lubrication and overhaul. Heli Archive puts the Alouette II's rotor head at roughly 1,000 parts against about 300 for the Écureuil's.
The Starflex, developed in the 1970s under René Mouille, replaced that hardware with flexing composite and elastomer: no hinges, no lubrication, on-condition parts with no required overhaul. Not everything is on-condition, though — Airbus substantiates the star's thick centre as safe-life, with a service life, and inspects the arms at intervals.
The parts, one by one
- Star — the glass-fibre/epoxy hub, thick and rigid at the centre, flexible in flap towards each arm's tip.
- Sleeve — a rigid fork per blade. The blade bolts to its outer end; its inner end meets the star through the bearing.
- Spherical thrust bearing — laminated steel and elastomer, carrying the sleeve's centrifugal pull into the star centre.
- Frequency adapter — elastomer blocks between the sleeve and the star-arm tip, giving lead-lag stiffness and damping.
- Pitch horn — on the sleeve; the pitch link pulls it to feather the blade.
Flap: the arm bends
Each arm of the star is thin enough towards its tip to bend up and down. The bend is the flapping hinge, and the arm's stiffness is its spring.
That stiffness means the hub transmits a moment to the mast when the disc tilts. The aircraft responds to cyclic directly and keeps doing so at low g — a theme picked up in the low-g chapter.
Lead-lag and feathering
The spherical bearing — alternating thin layers of steel and elastomer — is stiff in compression, so it carries the blade's huge centrifugal pull into the star. It is soft in shear, so the blade can twist for pitch and swing for lead-lag around it.
The frequency adapter sits between the sleeve and the arm tip. It sets how stiff the blade is in lead-lag and damps the motion, which matters for ground resonance.
Pitch horns and pitch links
Feathering is driven from below. Each sleeve carries a pitch horn; a rigid pitch link runs from the horn down to the rotating swashplate. Raise the plate and every horn rises together; tilt it and each horn rises and falls once a revolution.
Because the blade twists about the spherical bearing, the pitch link is working against the bearing's elastomer as well as the blade's aerodynamic load. Pitch-link length is also one of maintenance's track adjustments — which is why a disturbed link is a vibration question.
The droop-stop ring
At rest and at low rotor speed there is too little centrifugal force to hold the blades out, and they would droop on the flexible star arms. A droop-stop ring around the mast, common to all three blades, catches them: a shoe under each blade's root bears on it.
Airbus Helicopters Canada has warned that the screws carrying the ring's retainers must sit between the star arms, not in line with the sleeves. Fitted wrongly, the ring can't pass one blade's load to the other two, and the retainers and the mast can be damaged. That's a maintenance detail — the pilot's part is noticing anything that looks different from last time.
Fail-safe by design
The star and blades are composite. Damage in them tends to grow slowly and visibly rather than letting go without warning, and the Starflex was designed around that idea. The blade's centrifugal load runs through the sleeve and bearing to the star's centre, not along the arm — so a cracked arm is not, by itself, a lost blade.
It is only fail-safe if someone looks. Cracks in the star, unbonding or extrusion of the elastomers, and damage to the sleeves are all things a walkaround can catch.
What inspection is looking for
In principle, the head is inspected for three things:
- the star — cracks, delamination at the arm ends, and the bond around the bushes
- the elastomers — cracking ("checking"), bulging or blistering, separation of layers, and extrusion of rubber from between the plates
- the metal — sleeves, bolts and the droop-stop parts, for cracks, corrosion and fretting
The pass/fail criteria live in the maintenance documentation, and they are specific. A pilot who sees a change reports it; an engineer decides.
The head in numbers
Why it matters to you
Fewer parts, no lubricated hinges, and a composite star designed to be fail-safe.
The elastomers are the working parts now — the bearings and the frequency adapters — which is why the head deserves a careful look on every walkaround.
Source 1
A blistered bearing, 1994
An AS350 BA was on a ferry flight in Canada when a main-rotor vibration began and grew. The pilot slowed for a precautionary landing; in the flare, control was lost. The pilot had minor injuries and the helicopter was substantially damaged.
One of the three spherical thrust bearings had failed in fatigue. A pilot had earlier noted checking of the bearings' rubber, and an engineer had found one checked and blistered — but it was left in service until the next scheduled inspection without the manufacturer's rejection criteria being applied.
The elastomers tell you they are failing, and the rejection criteria exist for that moment. A growing vibration from the head is a reason to land now.
Source 8
A star past its life, 2008
During a run-up in Georgia in May 2008, an AS350 B2 shook violently as power was brought up to flight rpm. After an emergency shutdown the pilot found the tail boom buckled and a Starflex arm broken.
According to the operator's director of maintenance, the Starflex had been overlooked during a conformity inspection and had flown about 586 hours beyond its life. The NTSB attributed the failure to inadequate maintenance inspection.
On-condition doesn't mean unlimited. Some head parts — the star among them — carry life limits that depend on part number and service bulletins. The records are part of the aircraft.
The rotor brake
Where one is fitted, the rotor brake stops the rotor after shutdown so it isn't left windmilling — and so it can be held still in wind.
It is a ground device with a speed limit. Brakes are applied only once rotor speed has fallen well below normal: above that, the energy to absorb is high enough to overheat the disc and damage the brake. The fitment, the rotor-speed limit and the technique are in your RFM.
Source 11
On a Starflex head, what lets a blade flap?
The composite star arm flexes and the blade pivots on its spherical bearing — no flapping hinge at all.
What are the frequency adapters for?
They're the drag dampers: elastomeric blocks between sleeve and star arm. The spherical bearing carries the centrifugal load.
What carries each blade's centrifugal load into the hub?
The spherical bearing. Stiff in compression, it carries the centrifugal pull; soft in shear, it lets the blade pitch, flap and lead-lag.
On the walkaround you see one spherical bearing bulging, unlike the other two. What now?
Report it before flight. A blistered bearing left in service until the next inspection failed in fatigue in the 1994 Canadian accident.
What does the droop-stop ring do?
It catches the blades when there isn't enough centrifugal force to hold them out — at rest and at low rotor speed.
Sources
- Starflex rotor hub — European Rotorcraft Forum paper · European Rotorcraft Forum
- Aérospatiale AS 350B Écureuil — in depth · Heli Archive
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 4, Helicopter Components, Sections, and Systems
- Starflex rotor · AOPA · 25 May 2011
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 3, Helicopter Flight Controls
- Flap stops device with retractable upper stops ring, and rotor head including it (US 5,588,801) · U.S. Patent and Trademark Office (Eurocopter France) · Background (droop restrainer ring)
- Tech Tip 2015-005 (main rotor attach screws and droop-stop retainers, AS350/AS355/EC130) · Airbus Helicopters Canada · 2015 — page now redirects to airbus.com; no archived copy found
- Aviation Investigation Report A94W0144 (AS350 BA, main rotor spherical thrust bearing) · Transportation Safety Board of Canada · 14 August 1994
- Helicopter Flying Handbook (FAA-H-8083-21B) · FAA · Chapter 8, Ground Procedures and Flight Preparations
- Final report: Starflex arm failed (NTSB MIA08LA106, AS350 B2, Hiram, Georgia, 21 May 2008) · Aviation International News · 1 December 2008
- Rotor brake as well as rotor and rotary-wing aircraft with such a rotor brake (US 2005/0058536) · U.S. Patent and Trademark Office · Background (Eurocopter Deutschland, 2005)
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.