While the retreating main rotor blade was nearing the tail boom the top skin of the blade suddenly delaminated from the blade structure and curled up destroying. Is there any part of a helicopter that's more important than the rotor at working on tail rotor blades from Bell's , and models. that their stainless steel main rotor blades were delaminating due to Quick Links. with many composite components such as main and tail rotor main and tail rotor blades. .. rithmic linear relationship between the delamination growth.
It took awhile for us to process what had just happened but between the NTSB, the FAA, and ourselves, we pretty soon discovered the sequence of events that led to this crash.
While the retreating main rotor blade was nearing the tail boom the top skin of the blade suddenly delaminated from the blade structure and curled up destroying all lift on that blade. The advancing blade was nearly in the forward position of the rotor disc when the retreating blade delaminated.
- Blades and Composites: Staying Sharp
The forward blade was now the only blade that was producing lift and it immediately slammed up so hard that it bent the main rotor shaft and snapped the bottom skin and spar of the retreating blade.
Due to gyroscopic precession, the tremendous upward force that the forward blade exerted on the rotor mast was "felt" 90 degrees in the direction of rotation- to the right. This tremendous force rotated the helicopter about its longitudinal axis nearly degrees where the skids never touched the ground and the main rotor swashplate hit the ground first. The delaminated blade with the broken bottom skin and spar continued only degrees to the forward position held onto the rotor hub only by the small brass counter-balance rod on its leading edge.
The following sequence of photos pretty well tells the story. The above photo shows the dirt inbeded in the main rotor hub from hitting the ground when the heliciopter inverted.
The flat plate on top of the rotor hub is a device used for fine balancing of the rotor system.
Note that these blades are not Rotorway blades but were made by the Safari company. Richard's helicopter came to rest on its side and the upper skin of the rotor blade in the foreground shows where it delaminated from the blade spar. The above photo shows the bottom skin and spar of the delaminated blade completely separated from the main rotor and held on only by the brass balance rod.
When a blade first arrives at a composite repair facility, technicians review the accompanying paperwork, checking to see what kind of work is needed.
If the blade is in for a routine, scheduled inspection, the technician will send it through a battery of tests prescribed by the manufacturer. If the blade is in because it needs repair, the technician will review the notes made by the mechanic who sent the blade, then set about confirming the suspected problem.
Whether an inspection or a repair is the task at hand, composite blade technicians begin by looking closely at every inch of the blade, checking it for obvious deformities.
They then perform a tap test across the surface using a small metal hammer to detect any unusual reverberation, a tip-off that debonding — a condition where something is no longer in proper contact with something else — has occurred. One non-destructive testing NDT technique is ultrasonic inspection, or ultrasound, which he was eager to describe.
A properly calibrated machine "tests the thickness of the skin to ensure that is has not been sanded too thin," said Kerrick. The results of the ultrasound test appear on an oscilloscope-like screen for interpretation.
The Tail Rotor
Eddy-current testing is a common diagnostic procedure RBI uses on any number of the 2, blades it inspects annually at its headquarters in Broussard, La. It plans to open a facility in Calgary, Alberta later this year. That current fluctuates or eddies as it passes over a crack, causing an analog meter to jump and an audible alarm to sound.
Another diagnostic procedure popular among blade repair stations is fluorescent penetrant testing FPT. Rotor-Tech International in Stockton, Calif.Loss of Tail Rotor in Forward Flight
Then you spray the developer on. The reacting chemicals will glow under fluorescent light, clearly identifying the crack. As with most blade repair companies, the people at Composite Technology, Inc uses thermal imaging at its headquarters in Grand Prairie, Texas, as well as its locations in Singapore, Brazil, England, Canada and its sister operation, Helitech Industries, in Brisbane, Australia.
Safari blade delamination Rotorway helicopter
Composite Technology is a Sikorsky Aerospace Services subsidiary. The thermal imager reads heat signatures the same way forward-looking infrared — a type of thermal imager — does when used by a soldier on a battlefield at night. With moisture being a serious enemy of composite rotor blades, one company decided to be proactive in preventing water seepage into one particular brand of rotor blades: But while other manufacturers allow debonding to be repaired, Robinson requires blades damaged in that way to be replaced.
Airwolf came up with a supplemental type certificate STC to prevent delamination. It uses a kit that consists of specially developed polymer tape that seals blades from the elements. The FAA also recognizes the tape installation as an alternative means of compliance with Airworthiness Directivewhich targets the Robinson blade delamination problem. Use of the tape eliminates the need for daily preflight visual checks of the blades that are otherwise required by the AD. Quest added that the Army uses the same system on its helicopters stationed in the Middle East, which resulted in blade lives jumping from 20 to flight hours.
Vendors are pushing the state of the art in composites inspection and repair.
Thermal Wave Imaging of Ferndale, Mich. Navy to develop a portable thermal inspection system for detecting subsurface flaws in metal and composite structures. According to the company, its ThermoScope system uses a unique approach to processing infrared image data to detect delamination, corrosion and small amounts of fluid infiltration "that were previously undetectable by thermal inspection methods.