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Old 12th Dec 2014, 16:46
  #24 (permalink)  
JohnDixson
 
Join Date: Mar 2007
Location: Hobe Sound, Florida
Posts: 647
Tip Cap

LW, I cannot recall a specific design requirement spec there, but one assumes that making the tip cap replaceable due to expected tree strikes indicates an aero assessment of the impact upon rotor stability was made to support the design feature. In any case the loss of that outer 10-12 ft of blade at 145 KIAS at Ft Campbell answers the question, as that impact on blade pitching moment had to exceed the tip cap contribution.

Now that I think of it, there was one time during the pre fly-off testing when we found a longish crack in one tip cap skin, like 10-12 inches. Engr evaluated it and ok'd it for flight. I asked about the impact of the failure progressing to failure, they said not to worry, but in the end decided to replace it prior to flight anyway.* Torsional stiffness of the UTTAS Ti spar blades was higher than the previous Al spar machines. ( not the only factor of course, but an important one, in getting to the free stream Mach 1.0 point with not a hint of a rotor stability issue ).

*In reading how this sounds, it seems to indicate a rather snap decision making process. Actually we had a few serious discussions before proceeding.

Last edited by JohnDixson; 12th Dec 2014 at 17:04. Reason: typos
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