Blakmax... A discussion about Robinson helo main rotor blades was held ~2004 Thread
Main rotor blade failures. At that time fractures appeared to be the main concern.
A few off-the wall comments, this thread.
I've been accomplishing adhesive bonded repair for years. My primary resources for repairs were DAC PABST [Primary Adhesive Bonded Structures Technology] testing/design documents. Are You familiar with the series of documents related to the PABST program? They have MANY rules for design/fabrication/repair/bondline-protection You have not mentioned.
They developed/enforced rules for bondline design; cleanlieness; PAA surface prep [aluminum]; adhesive bond primer application [exceptionally thin]; film adhesive tack-application on sheet metal with minimal voids; installation of similarly prepared skins/doublers/fittings/honeycomb; proper cure parameters [heat, vacuum debulk pressure application, etc... based on adhesive variables, etc]; Post bond physical, dimensional and non-destructive tests... and hot-water immersion tests [for leaks]; over-coating EVERY exposed bondline with epoxy primer and sealant [improved environmental durability], etc. NOTE. Many bond design elements included 50-to-100xT(min) over-laps and highly tapered material thicknesses or scalloped [wavy] edges to reduce edge-peel and improve stiffness parallel to the lap/joint... and increasing edge length.
Bond quality must be rigorously enforced. One miss-step in "the process" and WHAM!!!... failure prevails. One time I was tasked to re-qualify out-of-date very expensive film adhesive. The first time around the lap-shear failures were all-over the map and usually mixed-mode +/15% or more: so I shot down that batch of adhesive. A couple of weeks later the shop chief begged me to try requal that same adhesive again... he was desparate no adhesive stock was available. The second time, the lap-shear failures were within +/-2% and consistent cohesion failure modes... but still fell fell ~5% below minimum allowed. At that time I noticed something exceptionally important: The second technician was very precise and careful in his handlling and approach to the adhesive bonding processes [even for simple lap-shear coupons]. I realized that the first technician was NO WHERE NEARLY as thorough in is approach to the process as the second tech. At that point, I still had to DQ the film adhesive; however I certified the second Technician as
'the only one' to-do [or to lead] all critical follow-on adhesive bond work... and DQ'ed the first tech from ever touching the bond-process without adult supervision.
When I worked with a foreign entity doing USAF Depot maingenance, I paid attention to their commercial and their govt military parts fabrication. They made many sets of rotor blades for OH-6 local manufacture helos. Blade disintegration during heavy-weight lift-offs on hot days destroyed 3-acft before they were all grounded. Found-out that the blades were being assembled with 180F service-temp adhesives, in-lieu-of 250F+ service temp adhesives mandated by Hughes. The blades were painted all-black [top/bottom] for low Vis... hence blade skin temperatures ran close to 170F just sitting in the sun on the ramp on a summers-day... at high ambient humidity. Stresses/vibration during lift-off combined with heat-reduced strength of the adhesives caused the blades to fail in a rapid creep mode during run-up and initial pitch angle lift change. IF I recall correctly they disbonded along the trailing edge which caused a massive assymetrical drag and vibration load as one or more blades peeled/rolled... tumbling the helo.
Aluminum Honeycomb is beatifully strong/light... when assembled correctly. However the least contamination of foil edges will result in no attached adhesive fillet... but thats how it will look on X-ray/US images due to the non-wetting-thickening of the fluid adhesive along the foil edgs!!!! Weird how bad adhesion can look like good adhesion edge-on. DESTRUCTIVE TESTING of random samples/parts/coupons during fabrication is a MUST!
I noted limited adhesion-failure detection success using the Fokker Ultrasonic Adhesive Bond-Test methods per MIL-STD-860... probably why it is no-longer used. Metal-metal seemed OK, metal honeycomb was never satisfactory.
Technicians performing tap-test of homeycomb-aluminum structure have to be specifically trained and equipped to hear the fine difference between a solid hit... and a tinny semi-solid hit. In the 1980s and 1990s my hearing was up-to the challange... not so now since I have significant hearing-range loss. Also, a great tap-test should also include a firm touch-feel test for softness or skin loosening.
Regards, Wil Taylor
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