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UH-60 with "bad" rotor blade

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UH-60 with "bad" rotor blade

Old 12th Dec 2014, 12:41
  #21 (permalink)  
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Being an old pre-AWC blade (Sans anhedral or taper) , the main pickled titanium spar was left intact (doesn't even look that the BIM blanket was exposed or that they popped the schrader valve) and the leading edge sheath assembly (nose taco shell, heater blanket, and abrasion strip) looks to be fully intact as well.

Once the skin+core afterbody departed, theres really not much left to come apart, other than the sheath-spar bondline, which is sized pretty stoutly.

Had they flown for much longer I am quite sure they would have lost the Sikorsky (patented) removable blade tip assembly as those fasteners are already a fatigue issue.
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Old 12th Dec 2014, 15:00
  #22 (permalink)  
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" BERP Plate "

Peter, you made me " burp " with that one! The S-67 swept tip first flew in 1970, long before the BERP blade flew. The UTTAS tip was pretty much a copy of that idea, and applied to the UTTAS design for generally the same considerations, and also in consideration of a bit higher tip speed as well.

Sans, cannot concur with the assumption that had they flown much longer the tip assembly would have departed. That assembly stands on its own structurally and has been flight tested to 3.53 G and a free stream mach number of 1.0. Auto Nr to 125%. The structure was initially designed to be field replaceable ( rather than run a spar all the way to the tip as in the UH-1 ) as it was accepted that the Army tactical environment would inevitably lead to tree strikes. ( Subject of an intense internal trade-off during UTTAS design phase in-house ). Never say never in aviation discussions, but the background indicates the tip cap wasn't going anywhere in the immediate future.

Last edited by JohnDixson; 12th Dec 2014 at 15:15. Reason: Added info, words.
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Old 12th Dec 2014, 15:11
  #23 (permalink)  
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John, I am digging into memory and cannot remember if the aircraft remains controllable if the tip cap departs from one of the blades.
I am sure the balance is shot, and vibes would go up significantly. Would the blade go divergent in that case? (My brain thinks is would flap too much and hit something, but I don't recall why I think that.)
Has anyone had one leave in flight?
I should have some data points on this in my memory, but it's been enough years since I was operating that class of helicopter that it's been pushed out of long term memory by other less important details.
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Old 12th Dec 2014, 16:46
  #24 (permalink)  
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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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Old 12th Dec 2014, 18:28
  #25 (permalink)  
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With the missing afterbody, would you not expect in-plane chordwise oscillatory forces at the cap to increase dramatically?

With the TE wedge long gone, the skins are no longer contributing to a "torque box" at the attachment location for the cap.

I don't doubt in testing the cap would survive a plethora of overspeed and other conditions...but that is with, at worst, and intentionally placed flaw within the laminate on a fatigue test - not missing an entire afterbody!
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Old 12th Dec 2014, 20:05
  #26 (permalink)  
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Sorry, Sans. When you refer to afterbody, what are you referring to?
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Old 12th Dec 2014, 20:18
  #27 (permalink)  
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Basically everything aft of the aft spar wall.

Upper skin, lower skin, core, and trailing edge wedge insert.
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Old 12th Dec 2014, 21:19
  #28 (permalink)  
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I looked in vain for a drawing that shows how the tip cap is attached, but its pretty simple, thus easily replaced. Anyhow, in the two incidents to date, the tip cap has survived ( I think the Ft campbell tip cap had some tree dings ). The tip cap on the South Carolina Guard blade will get a thorough inspection, and that will answer your question. Be happy to post the findings. I am betting that the tip cap is good for much more than the max duration of fuel incl the outboard 230's if one has the ESSS.
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Old 13th Dec 2014, 06:22
  #29 (permalink)  
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Tip cap attachment

Rotor Blade Balancing - Rotor & Wing Aviation Services - Rotor Track - Static and Dynamic Balance

John, See the attached link for a pictorial view of the tip cap attachment screw holes and underlying span tip weights. It can be seen that the tip cap could be conceivably lost in a tree strike and the tip weights not be affected….another great design feature…. obviously a lateral would develop in such an incident but certainly remain quite flyable.

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Old 13th Dec 2014, 10:37
  #30 (permalink)  
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My mistake, I thought the angled tip and the Berp were a universal sort of design with the Berp being a more modern piece of kit having seen it up close on the Speed record Lynx, but the question was answered in reading on through the next posts.
But still one hell of a strong helicopter to take that sort of breakdown/vibration and still recover from such height.

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Old 13th Dec 2014, 14:23
  #31 (permalink)  
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Peter, no apology necessary: I was just kidding anyway.

Ring Gear: thanks for the drawing link. The point you noted re the tip weight attachment being separate from the tip cap is important. The tip cap itself is extremely light. Might be kevlar now (?), but it started out as Al sheet and even that was really light. Point is that from a balance point of view, if the tip ( or tips, more likely ) get smashed on some tree limbs the resultant mass/aero imbalance won't be such as to create a more serious difficulty.
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Old 13th Dec 2014, 14:40
  #32 (permalink)  
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Different failure mode, I'm sure, but I can't help but think of this blade failure. You'll probably have to scroll down a bit to see the photo.
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