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HeliTorque :: View topic - Rotor blades vs. Cockpit
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HeliTorque Forum Index » Flight Dynamics

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Rotor blades vs. Cockpit
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Logan
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PostPosted: Wed Jan 07, 2009 3:55 am    Post subject: Rotor blades vs. Cockpit Reply with quote

why (as i read in "Fatal Traps") it seems whenever a helo goes down the blades like to dissect the canopy? is it because of them slowing down so much that centrifugal forces are decreased and they are then free to move where they please? Actually while on that note, it seems they also like to slice through the tail boom and or the canopy???




- just checked, its 0400 in the UK, sweet dreams from 2100 in Calgary Very Happy
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PostPosted: Wed Jan 07, 2009 6:29 am    Post subject: Fatal Traps Reply with quote

logan wrote:
is it because of them slowing down so much that centrifugal forces are decreased and they are then free to move where they please?


Well, actually I think it is the rotors breaking off that causes them to move where they please Laughing

Seriously though, most of the time when a copter goes in, it tends to roll if the blades strike. The blades strike the ground, "dig in" and bend in toward the body. Once they shatter, all hell breaks loose and the first thing they hit since they are short stubs is the canopy, or the blades that have not sheered off yet, can continue to rotate and sheer off the tail. The blades become nothing less than uncontrollable, flapping pieces of metal.

As for the tail itself....any time you severely unload the rotors, they are free to move about and wobble on the rotor head (low G condition). This can lead to mast bumping. The rotors may then exceed their flapping limits and strike the airframe, usually the tail (since there is little up front ahead of the nose). When a copter hits the ground hard, the main rotors will flex downward, striking the droop stop plate (or equivalent)... "overflex" and as you have read, strike the tail in many cases.
By HARD, I don't mean a hard landing but a bang, bong, kidney crushing, super hard splash on the ground. Get the actual picture?

If the copter is for some reason still powering the blades when they hit, the copter will spin violently in the opposite direction. Look at it this way. When the blades strike under power, the engine torque is suddenly removed but the tail rotor is still producing thrust. The copter spins because there is no longer engine torque to "balance" tail rotor thrust.

Freebie: If Low G/mast bumping occurs in flight due to "pushover", basically the same thing happens. Without the main rotor thrust component, and the tail rotor above the CG, the TR thrust causes the copter to rapidly roll right. (counter rotating system) Same principle!

There is nothing good about a hard landing. It can rapidly become an uncontrolled crash!

I am sure other members will have their own take on the subject.

Very good question though. It so boggled my mind at 1:30am that I need some sleep. Such a short question, but with a long-winded explanation. Comes from being bored at this early hour. Rolling Eyes

Logan, your really trying to wear me down. Laughing

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PostPosted: Wed Jan 07, 2009 1:43 pm    Post subject: Reply with quote

Good answer that's what i was going to say Very Happy

not nice but heres an example where you can see the tail rotor torque effect AB describes

http://uk.youtube.com/watch?v=G9DAZ6YdpVY
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PostPosted: Wed Jan 07, 2009 2:26 pm    Post subject: Reply with quote

animalsticks wrote:
Good answer that's what i was going to say Very Happy


As long as a member receives a proper answer, does it matter from whom it comes? Sometimes we help collectively (pun) and sometimes individually.

Your contributions have always been "right on". Laughing

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PostPosted: Wed Jan 07, 2009 5:01 pm    Post subject: Reply with quote

perfect AB thanks, that makes perfect sense!
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PostPosted: Wed Jan 07, 2009 6:33 pm    Post subject: Reply with quote

animalsticks wrote:
Good answer that's what i was going to say Very Happy

not nice but heres an example where you can see the tail rotor torque effect AB describes

http://uk.youtube.com/watch?v=G9DAZ6YdpVY


Jeez! Look at the coning angle on the blades just before it hit the water!
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PostPosted: Wed Jan 07, 2009 8:48 pm    Post subject: Reply with quote

fendersim wrote:
not nice but heres an example where you can see the tail rotor torque effect AB describes
http://uk.youtube.com/watch?v=G9DAZ6YdpVY


It appears the pilot attempted a takeoff instead of bringing the craft up into a hover first. The rotors seem to be spinning up, conning increases, the nose then moves forward and buries itself into the water. I guess it was the same effect as Dynamic Rollover, except it became Nose Rollover.

Once the blades dug in, they bent, twisted and took off the tail as the copter started to spin.

Terrible tragedy in that the pilot was lost.

Was it an engine failure, or some sort of test to show the copter can float? Seems like they had camera crew on hand to film the entire event.

Peculiar.

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PostPosted: Wed Jan 07, 2009 10:39 pm    Post subject: Reply with quote

from articles i've read the pilot didn't die just the youtube poster wrote that. apparently they were demonstrating the helicopter on marine exercises and a bow wave took the pilot by surprise. of course this too could be speculation as this info is from another you tuber.

Someone else said that both pilots died as they drowned as they couldn't get themselves free as their necks snapped with the whipping motion.
Now theres a sobering thought...
can anybody find the accident report and translate?
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PostPosted: Thu Jan 08, 2009 1:57 am    Post subject: Reply with quote

They missed the first part of the video.(this had lots of threads on another forum)

The helo is hovering, and then loses one engine. Blades cone up, machine splashes onto water and sits there for a while.

Normal practice is to lower the wheels to bring the CG lower and reduce the chance of toppling over.

But then he decides to try a single-engine takeoff - not a stupid thing in itself, but he forgot to raise the gear first, so as soon as he started to move, the wheels caused lots of drag and made the nose go down.

You would think he would stop the attempt when his window goes under water the first time, but he continues and then this video starts - you saw the rest.
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PostPosted: Thu Jan 08, 2009 2:08 am    Post subject: Reply with quote

Ascend_Charlie wrote:
They missed the first part of the video.(this had lots of threads on another forum)

A-C,
Appreciate the followup. It seemed something was missing. I too saw that he attempted the takeoff with the wheels down. It would be foolish to try a takeoff wheels down with both engines, but with one out!

I imagine that once the copter started to rock nose down, pulling back on the cyclic would have the same effect as in Dynamic Rollover. The horizontal lift component had already exceeded the vertical one, and the vertical lift that was created would have little or no effect at stopping the nose from going under.

Aborting and lowering collective may have been the pilot's only option.
Would you agree?

Sad event considering it started off as a demonstration.
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PostPosted: Thu Jan 08, 2009 5:22 pm    Post subject: Reply with quote

Now i know, thanks Very Happy
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PostPosted: Sun Jan 18, 2009 1:26 pm    Post subject: Re: Rotor blades vs. Cockpit Reply with quote

Logan wrote:
why (as i read in "Fatal Traps") it seems whenever a helo goes down the blades like to dissect the canopy? is it because of them slowing down so much that centrifugal forces are decreased and they are then free to move where they please? Actually while on that note, it seems they also like to slice through the tail boom and or the canopy???




- just checked, its 0400 in the UK, sweet dreams from 2100 in Calgary Very Happy


saw the wreck of a 300C years ago after a heavy landing, no doubt with low rotor rpm - a MR blade had flexed down through cabin, picked up instrument panel, taken the left pedal out from under the pilot's foot. There was a nasty scrape on side of fuel tank from the instrument panel as the blade carried it out....all blades were still attached, though a bit mangled!! The pilot walked out without a scratch...i guess the only reason he didn't loose the top of his head was he was only about 5' 5" tall.
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PostPosted: Mon Jan 19, 2009 12:00 am    Post subject: Reply with quote

how fortunate was he. imagine how quick it went from being before his eyes... think how fast the blades turn... don't blink you'll miss it!
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