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Tailboom strike by main rotor

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Tailboom strike by main rotor

Old 11th Oct 2022, 11:44
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The old BK-117 was equipped with a small foam insert at the vertical stabilizers to minimize the damage in case of a rotorblade strike ( SB-MBB-BK-30-103 from 1990 ) which could occure in heavy gusts during startup.

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Old 11th Oct 2022, 15:34
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Thanks gents, all good info.
FH
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Old 12th Oct 2022, 00:00
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Originally Posted by FlyingHead View Post
Hi guys, I have a question?
How a 4 blades system (like the S76,Bell 429, Agusta 109) can it the tailboom, what are the conditions?

Thanks
FH
Another possibility...

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Old 12th Oct 2022, 13:16
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Originally Posted by 212man View Post
Inflight it would be rare but probably not impossible with extreme handling.
Here's an example of that.
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Old 17th Oct 2022, 20:20
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Slight creep off topic, ie this is a three bladed machine, (Cabri G2) but nonetheless it's a powerful image.

I believe anecdotally that this was the result of a night EOL, but I am quite happy to be corrected.



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Old 17th Oct 2022, 21:18
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Nice clean cut!
Kudos to whomever sharpened the blades.

Originally Posted by Dai Whirlybird View Post
Slight creep off topic, ie this is a three bladed machine, (Cabri G2) but nonetheless it's a powerful image.

I believe anecdotally that this was the result of a night EOL, but I am quite happy to be corrected.

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Old 17th Oct 2022, 21:20
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Judging by the registration, I think it probably did!
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Old 18th Oct 2022, 05:48
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UH-60M has a problem with tail boom strikes doing tactical landings the UH-60A/L didn't have , the A/L had metal blades with the sweep at the tip of the blade flat , when the Black Hawk was thrown on the ground the stiffer metal blade still clears the tail, the newer UH-60M with new wide cord composite blades has a swept tip that angles down and are a softer blade , if you throw a UH-60M onto the ground with the same vigour as a UH-60A/L you will have a blade strike on the tail , pilots coming from older metal blade versions have to learn a new less vigorous tactical landing in the UH-60M.
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Old 18th Oct 2022, 13:53
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BH9.....perhaps John Dixson can offer some perspective on that as he has provided very useful information re the Blackhawk in the past and is certainly quite knowledgeable about the Aircraft and its design and flight characteristics.

I will send him a Jingle and tell him of your post.
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Old 18th Oct 2022, 15:41
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The new composite rotor with anhedral tip was tested in the 1990’s, and yes the word “ anhedral “ means that an outboard tip was tilted downward a few inches. The hover performance improvement was fairly substantial. So sure, if you hamfisted the landing, and have the stick near or on the aft stick stop while dropping the collective to hit the ground with some vertical rate, you’d have less clearance and are asking for trouble. Not to mention that if you are hot coming in, the right way to handle it is to put the tail wheel on the ground and use the rotor to kill your speed*. I should note here that for those reading that have no UH-60 background, there is a mechanical control mixing unit installed. One of several functions relates to limiting aft cyclic range with increasingly lower collective position. Can one defeat the built in protection? There were a couple of instances with the original blades in the US Army Usually related to something else going on ( my personal but limited knowledge ), but the normal ( incl.maybe a bit hotter than normal ) tactical landing isn’t a problem.
*in fact, this is the reason why there is a tail wheel on the ship in the first place.

Last edited by JohnDixson; 18th Oct 2022 at 16:14. Reason: Typo
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Old 18th Oct 2022, 15:52
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John:
Any truth to the old tale that. the engineer who designed the mixing unit for the 76 ended up in deep therapy?
US after looking at the mixing unit on display at FSI WPB: “Can you explain how this all works?”
FSI Instructor of great knowledge and experience: “NO!”
We all laughed at that one.

Originally Posted by JohnDixson View Post
The new composite rotor with anhedral tip was tested in the 1990’s, and yes the word “ anhedral “ means that an outboard tip was tilted downward a few inches. The hover performance improvement was fairly substantial. So sure, if you hamfisted the landing, and have the stick near or on the aft stick stop while dropping the collective to hit the ground with some vertical rate, you’d have less clearance and are asking for trouble. Not to mention that if you are hot coming in, the right way to handle it is to put the tail wheel on the ground and use the rotor to kill your speed*. I should note here that for those reading that have no UH-60 background, there is a mechanical control mixing unit installed. One of several functions relates to limiting aft cyclic range with increasingly lower collective position. Can one defeat the built in protection? There were a couple of instances with the original blades in the US Army Usually related to something else going on ( my personal but limited knowledge ), but the normal ( incl.maybe a bit hotter than normal ) tactical landing isn’t a problem.
*in fact, this is the reason why there is a tail wheel on the ship in the first place.
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Old 18th Oct 2022, 16:28
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Hah-I haven’t heard that one.
A related anecdote from the UH-60 basic controls design: the lead designer for the UTTAS ( original name for that competition ) was a relatively new, quite intelligent young engineer, who forced thru a design that had fixed length control rods throughout. Thats the way to save on cost and maintenance man-hours said he. Chief Engineer on the program bought in ( although the old head chief of that area in engineering was dead against ).
Scene shifts to the experimental build-up area, where the initial three prototypes were in the final stages of assembly and pre-flight test procedures. BUT…..hold on,…..we couldn’t get any of the three ships controls rigged to specification and all those fixed rods were chucked in place of adjustable rods.
As to mechanical mixing units in general: necessary in the “ old days “ in order to get decent control harmony and the top deck of the 92 is but one example of why fly by wire brings both flying qualities and safety improvements thru getting rid of all that clap-trap and the opportunities for human beings to make rigging and/or maintenance errors.

Last edited by JohnDixson; 18th Oct 2022 at 16:46.
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Old 18th Oct 2022, 16:59
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Any truth to the old tale that. the engineer who designed the mixing unit for the 76 ended up in deep therapy?
That’s what I was told when I did my conversion in 1994, so I guess it has some basis!
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Old 19th Oct 2022, 12:52
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Getting old and had to ask for a reminder from Nick, 212. The main rotor servos on the 76 were placed in a not normal position for other reasons, and the solution to obtain control orthoganality ( choice of words questionable ) was to add another bar in the mixer to account for that. So yes, the 76 mixer has one more function than the usual. Messrs Bursteiner and Irons, the designers, did not require a shrink, but gained some new respect for the accomplishment.
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Old 19th Oct 2022, 13:24
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The static stiffness of the blades is likely irrelevant. Once centrifugal force is applied due to rotor rpm the blades become really stiff.
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Old 19th Oct 2022, 15:14
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Rotor Tip Behavior

Originally Posted by Al M View Post
The static stiffness of the blades is likely irrelevant. Once centrifugal force is applied due to rotor rpm the blades become really stiff.
Yes to that. Soon after signing in at SA in 1966, one of the senior handling qualities engineers, Dick McCutcheon was talking about this subject and related some findings in a test done on an S-58, to wit, that the tip response to a collective input would result in a transient overshoot of the tip path, relative to the new tip path trajectory, whereas a cyclic input did not. That sounds counter-intuitive doesn't it and when I remonstrated with Dick, he gave me the legendary Igor Sikorsky advice about what to do when the facts and theory disagree!
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