Originally Posted by
The Sultan
I once witnessed (from extremely close range) an AH-1W running on the ground at 100% rpm release an internal main rotor blade weight due to a decade old manufacturing error. When the weight departed it:
1. Blew the end of the blade off,
2. Snapped the main transmission horizontally in half,
3. As the rotor/top case exited the still attached controls put in full pitch,
4. The aircraft was lifted 6’ or so by the control tubes which then failed dropping the fuselage back to the ground,
5. The now unattached rotor climbed 80’ before loosing energy and crashing down in front of the airframe.
The magnitude of the unbalance was so large the xmsn was snapped within one revolution of the rotor so no large 1/rev forces were transmitted to the fuselage. The only fuselage damage was to the cross tubes on one side. The pilot commented that he heard a bang, the aircraft smoothly lifted before falling back to the ground. The airframe was refurbished and returned to service.
if the weight had been less and not immediately snapped the xmsn case, the excessive 1/rev would probably have broken the pilot’s neck and damaged at least the tailboom.
So back to this thread. It is plausible a failure in a blade caused excessive 1/rev which buckled the tailboom and put in full tail rotor pitch through the still connected controls tubes causing the observed yaw. Then a combination of 1/rev loads and aero forces caused the rotor/xmsn/nodal beam to depart with the rotor appearing to fly away intact.
Won’t know until we see both blades.
Vibration also generates nodal points. that is an area where the vibration becomes concentrated away from the source.
This was explained to me following a mid air collision between a Bell 47 and a Hughes 500.
The 47 suffered a buckled blade, the 500 lost about 7 inches off one blade including all the blade weights.
The grim reaper must have been busy elsewhere that day as everybody walked away.
However I couldn't understand the damage pattern on the 500 so I spoke to the AAIB (AIB).
It was possible to lift the tail gearbox and boom up and down about 3 feet.
I assumed this had happened in the forced landing..
What I didn't understand was why the instrument console had sheared off completely from the structure with no impact damage..
I was told that the area had become a nodal point strong enough to shear the structure, same applied to the tail boom support structure.
It certainly paid testament to the inherent strength of the 500 and Bell 47 main rotor blades..