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Chairmanofthebored
28th Sep 2006, 00:51
When the aircraft is shut-down the blades slow down to stop directly fore and aft - more often than not...
Why???
...is it cause I am that good. Thought so...

Arm out the window
28th Sep 2006, 01:45
If you land and shut down into wind mostly, then they tend to align themselves that way.
I've definitely noticed that with the Jet Rangers in a strong wind - you end up with a blade pointing into wind.

GLSNightPilot
28th Sep 2006, 01:52
Especially if you're experienced with the rotor brake. :)

B Sousa
28th Sep 2006, 01:59
GLS ....your too good...........

Gordy
28th Sep 2006, 03:37
Especially if you're experienced with the rotor brake. :)


Gotta agree with you there----I can always get mine to stop at "10" o clock and "2" o clock........

Arm out the window
28th Sep 2006, 05:55
Rotor brake ... I dream of having a rotor brake!
Would have been very handy the other day when shutting down on a pad, if you could call it that, right on the edge of a coastal ridge with 30 kt blowing in off the sea - rotor wasn't going to stop, ever.
Eventually with some judicious pitch-pulling, gnashing of teeth and rocking around on the skids as the transmission bounced around, it finally stopped - not very pretty at all.
The takeoff was a bit nicer though, hardly any power needed to hover or climb away.

John Eacott
28th Sep 2006, 06:09
AOTW,

Good luck with the next head overhaul, if it makes it that far :uhoh:

It sounds as if you're lucky that the blade tips didn't hit the tail boom, apart from the no-no of pulling pitch to slow down the blades :eek:

FlightOops
28th Sep 2006, 07:57
John Eacott... for the benefit of the inexperienced, could you explain why pulling picth to stop the rotors is a no-no ?

I have been with a pilot with a squillion hours doing that as well as applying full left pedal in a 206 that had no rotor brake.

As the RPM was well below any value sufficient for lift, and owing to my inexperience I assumed this was a safe practice.

Your comments would be very welcome.

Thanks in advance,
FO

Arm out the window
28th Sep 2006, 08:29
John, I've exaggerated somewhat with the bouncing around part, but certainly did use some pitch to slow the rotor down, as little as I could get away with. If the rocking on the skids had been any worse than it was, I would have just started up again and departed.
Having said that, it was common practice for all on 205s at one place I worked - shut down, and pull a bunch of pitch to slow the blades more quickly than if they ran down themselves.
Everyone did it - not saying it was right, and there was some talk of it being bad for the TT straps, but the maintainers never voiced any concern about it.

Semi Rigid
28th Sep 2006, 08:52
jump out & climb upstairs & grab hold of the mast with ya paws.sheez.pussies.

Ascend Charlie
28th Sep 2006, 10:02
In a Huey in the 70s, the blades didn't want to stop. Groundie says "I'll fix it, sir!" and before I realised what he was doing, he scrambled up the side steps and reached for the blade above him. Well, grabbing it at 1/3 of the blade's length wasn't ever going to give him much of a moment to stop it, and I gasped as I saw his legs disappear past the right door and past the right overhead window. This turned to laughter when i saw his face looking down with horror through the left overhead window.:eek: He was smart enough to let go the blade before it pulled him off the roof.:ooh:

Also watched a new bograt standing out the front with the tie-down, waiting for the right time to reach up and grab the almost-stopped blade - there is a certain amount of momentum in a Huey blade. As the blade spun down, I called out (jokingly - it was way too fast!!) "This one, Marron!" and he leapt off the ground and grabbed it....:sad: Well, I have never laughed so hard, tears running down my face. He grabbed the blade, feet off the ground, and he turned a beautiful cartwheel in mid-air and landed on his @rse in the dust. Wonderful to watch, I think he is a Qantas captain now.:8

GLSNightPilot
28th Sep 2006, 13:44
Once, long ago, we had rotor brakes in the 206 that were part of the aircraft hydraulics. You got one chance to stop the blades, and then there was no brake left. I shut down on an offshore platform with a wooden deck, stopped the blades, got out, got the tiedown stick out of the baggage compartment, and saw that the blades had started turning because of the wind. I walked to the left front quadrant and casually jumped up to catch the blade as it lazily swung around. The next thing I knew I was on the deck near the tailboom, almost on the fence, with splinters in my backside and back, and pain throughout my torso. I wasn't sure I could get up, but I finally did, and now the blades were turning at a pretty good rate. I crawled back into the cockpit, started the engine to regain hydraulics, stopped the blades, got out, and the blades started turning again. I finally kept hitting the leading edges with the tiedown stick as they passed overhead, and eventually succeeded in stopping the blades and getting them tied down. I was in no shape to fly back to the beach right then, and didn't want to lose the aircraft because of the windmilling blades, which can turn at a good rpm with a strong lifting wind. After a few hours, my passenger was ready to go and I sort of felt up to flying, and we got back to the beach, but that's the last time I ever tried to stop any blades manually. A rotor brake is an essential investment. If you don't have one, you will eventually learn that.

Chairmanofthebored
29th Sep 2006, 00:56
So apart from all the other useless info here, nobody can supply a decent reason why they would orientate to fore and aft? I was considering that it might be something like a worn G/Box bearing, sun gear tooth...
??

imabell
29th Sep 2006, 01:11
cotb,

just coincidence, no secret, happens a lot.

pulling pitch to slow the main rotor, especially at low rpm can and does damage the straps in the head. ask an engineer. (didn't know there where straps in the head???) i think that's what john is referring to.

that is why you should never raise the collective and at the same time move the cyclic through full travel when the blades are static.

it can be very expensive.

kjw57
29th Sep 2006, 01:21
Having flown a whack of hours in the Long Dog I can only remember the blades alining themselves fore and aft when finally stopped just a couple of times. As for pulling pitch to slow them down a good question for the Bell Techies I suspect.

Mind you all that bothersome inertia comes in handy at the bottom end of a silent descent...

Arm out the window
29th Sep 2006, 03:48
Does anyone know what kind of damage is done to the TT straps by pulling pitch on shutdown, or the kind of movements imabell described, and why it isn't a factor when you pull pitch with power on?
The greater coning angle of the blades with pitch at low rpm would be one thing that comes to mind.

This is one of those things where some people say it's a no-no, others don't, so some tech info, an AD or a maintenance manual note, something along those lines would be good if anyone knows.

John Eacott
29th Sep 2006, 04:29
AOTW,

Since I can only find the reference by omission, I called Nelson H (Bell Customer Support Rep) this morning who agrees, and confirmed what I thought: but agrees that it is only referred to in the FM by what is not said, rather than what is said! There is a reference to the flight controls being locked prior to shut down, and an allowance for the cyclic to be used to minimise static stop contact, but not to allow collective movement.

He is asking Bell for a firm reference, and I will post the reply when it arrives.

Sandy Toad
29th Sep 2006, 07:15
In the mists of time I seem to recall it was to do with Grip Seals and extra flexing allowed oil to seep out; then when oil changed to grease the restriction didn't apply. But could be wrong.
Do remember flying BO105 with the old and learned who designed the magnificent rotor system and being told not to use full pedal to slow Nr as they had never done the stress sums for it.
Guess we'll always find something to do the designers never thought of!

Arm out the window
29th Sep 2006, 07:20
Thanks, John, much appreciated.

Sandy, the reference to grip oil leaks brings back memories from the 205, I think that's one of the things that was bandied around as to the cons of pulling pitch to slow the blade, when we still had the blade grip reservoirs - was said to make them leak, or maybe my brain cells are dieing out quicker than I thought.

Phnix
29th Sep 2006, 13:28
that is why you should never raise the collective and at the same time move the cyclic through full travel when the blades are static.
it can be very expensive.
so how do you check for full and free movement on your pre start check?
or should the movement be only limited to a few inches,
I have put some hours on the same Jet Rangers in the last few years, and my pre start check has always been full and free, and I know that the straps have been replaced, but only when there time was up.

thecontroller
29th Sep 2006, 17:01
i thought pulling pitch to slow the rotors resulted in blade flapping and possible tail boom strike?

vorticey
1st Oct 2006, 06:18
after working around busy airports with a b47 id have to stop the blades as quick as posible so i could leave the machine to get pax from the terminal many times in the day. i got quite good at slowing the blades with collective. it slows them realy fast at high rpm but as the blade slows the wind effect causes more sailing so you move the cyclic slowly toward the wind direction to keep the disc level. then when the cyclic runs out you start to put the collective back down to keep the disc level. once it gets to the point that the effect is gone, put the collective right down and cyclic in the centre and climb up and stop it with my hands. only did this when i had to, and not realy good in gusty conditions as the disc will tilt verry quickly with the collective up and a puff from a different angle. seems to work just the same with the jet ranger??
those straps are what take the weight of the blades, they twist when pitch is applied, why would it be bad for them to twist without being under strain?? because another thing i do is leave the collective up and the cyclic away from wind slightly which holds the blades over without turning in windy conditions so they dont bounce (if you cant tie it down or going again shortly)

krobar
1st Oct 2006, 16:27
Lowest point on the blade's rotation should be 11o'clock if I'm not mistaken. I'de venture a guess that in some wind the blade will come to rest in it highest/lowest flapping points in the disc rotation. All irrelevent if you use the rotor brake. Then the main goal is preventing buying beer for the crew when u stop it off-centre.
I've recently had a engineer tell me that the factory teaches pumping the brake from 50% and applying full rotor brake from 40%. This apparently heats the brake disc enough to prevent the shudder some machines have.
Any opinions on this? I know it works, but since I got the idea from word of mouth, its kinda not enough.

John Eacott
2nd Oct 2006, 01:27
This is the reply from Bell Product Support Engineering:


Bell Helicopter does not publish what not to do in the manuals, we do not publish any scenarios of things or procedures that are not authorized or approved. We only publish approved procedures.

Failing of carrying the approved procedures (sic) may lead to damage of the helicopter. To answer your question on shut down the cyclic has to stay in the center and collective down, if you pull on the collective it will change the pitch of the blades, create excessive flapping which will result in mast bumping.

finetune
2nd Oct 2006, 01:57
From my understanding, Lifting the collective on both the 205 and 206 after shutdown of the engine increases coning up on both the rotor blades and can overstress the inboard doublers which can become dissbonded. At normal rpm not as much coning occurs. At very low rpm, more coning occurs inboard.

NickLappos
2nd Oct 2006, 02:04
Undoubtedly, the loads that stress the blades and head when you raise the collective are induced from the bouncing and thumping the rotors can make under some wind conditions, and not due to the tiny lift forces generated by the blades are they sail aound the mast.

At low rpm, when the pitch is raised, both blades (upwind and downwind) go to the new increased angle, but only the upwind blade sees a lift increase because the other blade is at or beyond stall in its down-wind state, so it is non-aerodynamic. The upwind sweeping blade lifts up more than it would have at flat pitch, and then crashes down as it swings down-wind and stalls and its mate sees the lift. This big up/down swing (we have all seen it, it occurs often enough when the pitch is flat) is vastly increased if you raise the collective, and the shock loads on blades and head are not nice. Additionally, the chance to bump the mast is also increased, as the Bell blurb says.

This crash-bang cycle can be induced if you tilt the swashplate very much with cyclic, too.

vorticey
8th Oct 2006, 05:07
looks like you got a cut and pasted answer there mate. i dont know what he means by it, but there's definantly published information in the 206 manual about what not to do.
warning: could cause injury or death
caution : could cause damage to machinery
note: important information

eg. caution : vne 69kts with front doors off
caution : do not try an airstart above 12000ft as the TOT will rise too fast to control
caution : do not light off below 12% N1
caution : do not try to start above 150deg TOT

these are just the ones i can remember, if they realy thought pulling the collective up during shutdown would be a problem it would have a caution in the book. eg. caution : do not raise collective to slow blades on shut down.
also at the begining of the general section it says (from memory) :this section is for standardization purposes only and may not be suitable in all aplications!!

of course you can mast bump if you dont do it properly , you can mast bump just by putting the cyclic forward while the engines running, you can watch the bump stops touch. you can cause lots of damage in a helicopter if something is not done right but if it IS done right it works quite well.

someplace
8th Oct 2006, 07:44
Well chairman we are still none the wiser but I have learnt one thing - the lowest point in the disc is 11 o'clock. Maybe?

Chairmanofthebored
9th Oct 2006, 06:32
Yeah - none the wiser and it is still aligning itself more frequently than not with the boom.
Hmmm...

9th Oct 2006, 09:14
Surely it's not rocket science to conclude that at low rpm your cyclic control is very ineffective and so you have no way of controlling the disc flapping if you choose to raise the lever to slow down the rotors - therefore as Bell says, mast bumping may occur which I think is universally accepted as a bad thing.

Interested as to a detailed explanation of why the 11 o'clock is the lowest point for the rotors........maybe it is a local time v GMT answer:)