Main Rotor Blade Analysis inHelicopter Accident Investigation

by Sam Webb

➔Establish the last flight path, heading, andattitude and of the helicopter

➔Establish the RPM of the main rotor and enginepower levels

➔Establish drive train continuity➔Establish sequence of events of the accident

➔Investigator must havea limited knowledge ofthe design of the rotorsystem and blade

➔Wooden symmetricalairfoil

➔Nickel leading edge,wooden filler, steelspar

➔Aluminium spar andskin, honeycomb core

➔Compositeasymmetrical airfoil

➔Uni-directional S-2glass/ epoxy fabric &E glass/ epoxy tape,fibreglass spar

➔Superior fatiguetolerance, notch andcorrosion resistance

➔Normally wider cord /longer length blades

➔Higher main rotorRPM/ higher bladeinertia

Two Bladed Systems,(teetering, semi-rigid)

➔Blades usually lighterconstruction, lesscord, less length

➔Main rotor RPMvalues marginallylower to compensatefor cord areadifferences

Fully articulated, multi-blade systems

➔Blades sameconstruction as multi-bladed system

➔Main rotor RPMvalues same as multi-bladed system

Coaxial- counterrotating, multi-bladesystems

➔Blade bending, notshattering

➔Blades generally intact➔Blade bending inboard

and downward➔High angle of attack=

blade tearing aft ofspar

Tip weights

Low Main Rotor RPM

➔Blade tip weightsintact

Low Main Rotor RPM

Weights

➔Spar fractures, trailingedge separation

➔Honeycomb fracturedand separated

➔Damage mainly tooutboard sections ofblade Fractured spar

High Main Rotor RPM

➔Tip weights �ejected�➔May travels for

kilometres from theimpact site (fartherdistances for multi-bladed systems

High Main Rotor RPM

➔First blade to impactsurface mostdamaged

➔Following bladesexhibit lesser damagedue to main rotorinertia bleed off

High Main Rotor RPM

First impact

➔Damage is in plane

High Main Rotor RPM

Shattering

➔Massive bladedistortion on multi-bladed systems

➔Blades sustain moredamage due to lighterconstruction

➔Articulated systems,dynamic stops fail

High Main Rotor RPM

➔Blade spindling/distortion

➔Spar fracturesdepending on mainrotor RPM

➔Bending of spardepending on angle ofattack

Spar bend

Damage From Water Impact

➔Inertia of the blade- with high inertia will loseRPM slowly with increased angles of attack

➔Higher the helicopter gross weight and/or densityaltitude= more the blade wants to overspeed

➔Manoeuvring- tends to increase RPM due toenergy enhancement of the rotor system

➔Will collaborate otherevidence

➔Distorted or broken➔Direction of force will

collaborate power onor off and autorotation

Main Rotor Controls

P/C rod ends

➔Damage to pitch horn,blade grip, mastassembly on semi-rigid and rigidsystems= indication ofhigh RPM

Main Rotor System

➔Torque-tension strapdamage

Main Rotor System

➔MR Spindle sheardamage

Spindle sheared

Main Rotor System

➔Swashplate duplexbearing damage=indication of high RPM

Main Rotor Swashplate

➔Pitch change rod endattachment damage

➔Static overloadindications= high RPM

➔�Necking� indications=low RPM

Main Rotor Controls

Sometimes you justhave a bad day!

Sometimes you neverunderstand!?!?!?!?!