05 NOV 90
160438, 2K2, 2.3, FCF, Mooch
Spent all day testing 213 with Mooch, it's propellers are out of alignment with the afterburner. Actually, rotor blades won't track and balance. Spent all day on it Saturday and said there was no more that could be done with it, there's something weird going on with it. For a track and balance on a main rotor system, we might have to add weight to the hubs for spanwise balance, sweep blades for chord alignment, adjust pitch for track and rpm, and fine tune with trim tabs to compensate for a diving or climbing blade. We measure vibration with vertical and lateral accelerometers with a test set to narrow down balance issues. Track is measured by bolting reflective targets onto the blade tips and firing a strobe at them. One target is a horizontal strip of reflective tape, the other is vertical. If the blades track together, the targets will make a plus sign. Targets are checked in a hover and at various in flight speeds to see if the blades fly "together" or diverge. If one blade climbs or descends because of its individual aerodynamic characteristics, the strobe targets will split and vertical vibration (beat) increases. Then you have to figure out did one blade climb or the other blade dive? We pick one blade to be the target blade and try to make it fly like the other blade. SO then you may end up with 2 crummy blades and eventually go back to square one. Problems crop up when one blade is relatively new and it gets matched with an older blade with more flight hours. The newer blade is more elastic, older blade rigid. The maintenance books can't cover every scenario, as there is no way of knowing how Mr Molecule will interact with every blade. Add 2 pieces of 20 foot protective tape to the leading edge of every blade, plus a patch job here or there into the equation and you're ready to go ground turn all day in 120 degree heat. Are we having fun yet? Sure we are!
Flew 2.3 today experimenting with numerous combinations and corrections. Minimum test standard is having a vertical vibration level of less than 2/10 of an inch per second at 110 knots. Saturday it was 3.5/10, which means a 100 hour component would last about 60 hours and give a bumpier ride. If I can get this hub and blade assembly to track and balance, I will have rewritten the procedures. Following the maintenance pub sequence, it can't be done. Sometimes you have to accept more deviation in the hover to get the in flight track to fall within tolerances. So after 2-3 days of following the book we'll look at some other things tomorrow that aren't in the book. We're trying to avoid removing the entire assembly and swapping out a blade. I have 18 years of crew chief experience out there working on the problem and they aren't too happy about it. We spent all day on it and have an idea of what might fix it, which will be great if it works and gets put in the maintenance manual someday. Then some old salt can say, "Oh yeah, we knew that. You mean you didn't check that first?" Well, it aint in the book. SO I don't know which will be better, figuring out the bird is AFU or that the book is AFU. As long as it's not me that was AFU.