Question - Stopping Rotors
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Question - Stopping Rotors
When closing down; some pilots say - that the application of power pedal before rotor brake serves to get the Nr to the required limit before application and can prolong the life of the brake as it can be applied at a lower limit.
Question: Are the torsional loads through the T/R shaft greater (ie: beyond limits) when slowing the rotors with pedal bearing in mind the loads are being transmitted in reverse? Is this good practice?
Question: Are the torsional loads through the T/R shaft greater (ie: beyond limits) when slowing the rotors with pedal bearing in mind the loads are being transmitted in reverse? Is this good practice?
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Sounds as daft as pulling collective for aerodynamic braking. You deflect blades below their operating RPM, without knowing the effect on the blades.
Why not wait a little longer before applying the rotor brake if you're that desperate to prolong its service life?
Why not wait a little longer before applying the rotor brake if you're that desperate to prolong its service life?
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I meant to pose the question about pulling pitch as well - but forgot... I can't see any problem with this; however (not that I can see a need to actually do it) because all the forces are working in the correct sense - surely gusty wind conditions flex the blades to the same degree?
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I don't know if it is true but . . . .
I work at a Bell Svc. center and we have a pilot who flies for a small 2 helicopter (B407-B206L4), 1 pilot company, during one of the maintenance acceptance flights I noticed that as soon as he shut down the engine he stepped on the pedal all the way to slow the rotor down and we later figured that this was the reason why he was having constantly excessive axial play in the T/R blade bearings, the Bell people said that stepping on the pedal with lower than normal RPM's makes the want to "bend" more since they don't have the centrifugal force to keep them straight. (something like that)
But then on the other side of the coin, when I went to PHI for training the instructor did it on a 206 because he didn't want the rotor coasting in the high winds, and he said its a common practice, but you should apply smoothly, so now go figure.
But then on the other side of the coin, when I went to PHI for training the instructor did it on a 206 because he didn't want the rotor coasting in the high winds, and he said its a common practice, but you should apply smoothly, so now go figure.
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the 206 with 1 boost pump is great example my instructor who checked me out in the 206 some time agao now, alsways told me to start with one boost pump, for what reason i dont know? i always start with 2 boost pumps in as per flight manual.
we also have a rotor brake fitted to our 206 and its placarded engage brake between 38% and 30% rrpm
we also have a rotor brake fitted to our 206 and its placarded engage brake between 38% and 30% rrpm
"Just a pilot"
Full antitorque pedal application is a common practice at PHI. Offshore, yoou frequently run into situations where the rotor autorotates in the wind, keeping NR above the PHI rotor brake limit. You have a choice- wait for it to coast down (not in full control), and it seems to take forever. Pull rotor brake early and high. Or push a little pedal and add a little drag.
You definitely don't want to pull collective as it exagerates main rotor excursions. I'd guess your tail rotor bangs a round a bit with added pitch, and that components wear more quickly, but it's less flexible and better restrained than the big fan which can hit something or somebody.
On the beach and in ordinary breezes, I don't do it and don't miss it.
You definitely don't want to pull collective as it exagerates main rotor excursions. I'd guess your tail rotor bangs a round a bit with added pitch, and that components wear more quickly, but it's less flexible and better restrained than the big fan which can hit something or somebody.
On the beach and in ordinary breezes, I don't do it and don't miss it.
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Conserving Battery power for the start is the usual (misguided) given reason for the 1 pump start.
Seeing as a pump only draws a few amps it is a pretty poor reason, if you were so worried about battery and starting performance, why use any at all.
Seeing as a pump only draws a few amps it is a pretty poor reason, if you were so worried about battery and starting performance, why use any at all.
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Whoa Nellie or there goes my tail boom.
I flew with the senior Bell Helicopter test pilot in a 214 on several occasions and he used to slow the rotor down by pulling collective pitch. He also did it when we were tracking and/or balancing the blades. That however was in 1976 and maybe they do it a bit different nowadays.
If this is done on a fully articulated rotor system (Sikorsky) you had better be sure that the droop stops have retracted. If not you have wasted a perfectly good tail boom and a set of blades as well as the rest of the dynamic system and drive line.
If this is done on a fully articulated rotor system (Sikorsky) you had better be sure that the droop stops have retracted. If not you have wasted a perfectly good tail boom and a set of blades as well as the rest of the dynamic system and drive line.
You guys have to remember something: Traditions die hard in aviation. Some of us are old enough to remember when things were done a certain way - like pulling collective to slow the main rotor. Like...remember when the Bell flight manuals used to allow pulling up to 1/3 of the collective pitch travel on shutdown? I do. And hey- if something worked in the past, why wouldn't it still be okay today? Because we learn from experience, that's why.
I once flew for awhile a 47J-2 with no rotor brake. Sometimes, getting that thing stopped meant full left pedal and <ahem> as much collective as was necessary. Certainly slowing the rotor with the collective is no worse than doing a hovering auto. No?
In the good old early days of the 206A, with those weak old NiCad batteries and maybe a Ceco fuel control that wasn't "tuned" too well, you'd want as much juice as possible to get from the battery (way up in the nose) to the starter/generator (way back there in the engine compartment). So I would start with *no* boost pumps - no electrical consumers on at all in fact. Then Bell decided that in such a situation the fuel system is under suction (from the engine-driven fuel pump) and not pressure and could possibly suck a molecule or two of air in through a hole too small for a molecule of fuel to pass through. Voila! Air in the system. Now *both* boost pumps have to be operating anytime the engine is started and flight is intended.
Batteries are better now, and Bendix F.C.'s start better than the old ones, but there are still a lot of dinosaurs out there who just refuse to keep up and change with the times (like maybe some Bell Helicopter 214 test pilots). Is there any real danger to staring a RR/Allison engine with *no* boost pressure? Probably not. But do I start a 206 with both boost pumps operating now? You betcha!
And sure, I used to slow the MR down with left pedal. A slow, gradual application...what could it hurt? Well I guess it does - to those feathering bearings Blender Pilot referred to. I believe that PHI even recently changed their policy to prohibit using left pedal to slow the rotor, despite "Devil 49's" admission of still doing it.
But isn't it ironic that on the old H-500C I used to fly, the left pedal was spring-loaded forward! Seems that Hughes didn't mind using left pedal to slow the rotor.
But to address Letby Avenue's actual original question - about torsional loads - I'm not an engineer (I must defer to Lu here), but I doubt it's an issue. The loads are not transmitted in reverse. The load against the Thomas couplings and t/r drive gear in the engine is always the same. The t/r is always providing resistance/drag whether the rotor system is under power or not. Let us consider that the drivetrain is capable of absorbing the application of full left pedal at 100% of operating rpm - in other words, however much h.p. that consumes. As the rpm is reduced, then the total loads are being reduced as well.
The bending moments on the blades however - well, that is another story.
I once flew for awhile a 47J-2 with no rotor brake. Sometimes, getting that thing stopped meant full left pedal and <ahem> as much collective as was necessary. Certainly slowing the rotor with the collective is no worse than doing a hovering auto. No?
In the good old early days of the 206A, with those weak old NiCad batteries and maybe a Ceco fuel control that wasn't "tuned" too well, you'd want as much juice as possible to get from the battery (way up in the nose) to the starter/generator (way back there in the engine compartment). So I would start with *no* boost pumps - no electrical consumers on at all in fact. Then Bell decided that in such a situation the fuel system is under suction (from the engine-driven fuel pump) and not pressure and could possibly suck a molecule or two of air in through a hole too small for a molecule of fuel to pass through. Voila! Air in the system. Now *both* boost pumps have to be operating anytime the engine is started and flight is intended.
Batteries are better now, and Bendix F.C.'s start better than the old ones, but there are still a lot of dinosaurs out there who just refuse to keep up and change with the times (like maybe some Bell Helicopter 214 test pilots). Is there any real danger to staring a RR/Allison engine with *no* boost pressure? Probably not. But do I start a 206 with both boost pumps operating now? You betcha!
And sure, I used to slow the MR down with left pedal. A slow, gradual application...what could it hurt? Well I guess it does - to those feathering bearings Blender Pilot referred to. I believe that PHI even recently changed their policy to prohibit using left pedal to slow the rotor, despite "Devil 49's" admission of still doing it.
But isn't it ironic that on the old H-500C I used to fly, the left pedal was spring-loaded forward! Seems that Hughes didn't mind using left pedal to slow the rotor.
But to address Letby Avenue's actual original question - about torsional loads - I'm not an engineer (I must defer to Lu here), but I doubt it's an issue. The loads are not transmitted in reverse. The load against the Thomas couplings and t/r drive gear in the engine is always the same. The t/r is always providing resistance/drag whether the rotor system is under power or not. Let us consider that the drivetrain is capable of absorbing the application of full left pedal at 100% of operating rpm - in other words, however much h.p. that consumes. As the rpm is reduced, then the total loads are being reduced as well.
The bending moments on the blades however - well, that is another story.
Spot on, PPF1.
I have been watching this thread to see when somebody would point out the bleeding obvious - the rotor brake is usually mounted on the tail rotor drive shaft, so using the tail rotor to slow down has similar effects on the main transmission as does using the rotor brake.
I personally have never used the left pedal to slow RRPM - the thought of it thrashing itself to death on its stops at maximum flap and little centrifugal force was enough to prevent me from ever doing it.
If the blade doesn't want to slow down to the rotor brake range, then I reckon that a gentle early application of the brake is far better than a thrash of the tail rotor bearings.
Our Huey and B47 both lack a rotor brake, and I just have to put up with a wait at the controls to make sure it doesn't sail around while winding down. The S76 can have the rotor brake on while the engine is running, so there is no problem with that one.
I have been watching this thread to see when somebody would point out the bleeding obvious - the rotor brake is usually mounted on the tail rotor drive shaft, so using the tail rotor to slow down has similar effects on the main transmission as does using the rotor brake.
I personally have never used the left pedal to slow RRPM - the thought of it thrashing itself to death on its stops at maximum flap and little centrifugal force was enough to prevent me from ever doing it.
If the blade doesn't want to slow down to the rotor brake range, then I reckon that a gentle early application of the brake is far better than a thrash of the tail rotor bearings.
Our Huey and B47 both lack a rotor brake, and I just have to put up with a wait at the controls to make sure it doesn't sail around while winding down. The S76 can have the rotor brake on while the engine is running, so there is no problem with that one.
Like AC, I have never used left pedal in the Huey to slow the rotor, but I have had to feed in a bit of pedal to reduce tail rotor flapping when parked out of wind. This could be either left or right pedal to find a comfortable position to stop flapping, never was it used to slow the rotor.
I had always thought that introducing pedal to slow the rotor was linked to excessive flapping loads, leading to eventual tail rotor failures and tears. I even thought that the reason PHI had banned the practice was linked to thier 407 tail rotor failures. Same as stopping the rotor with collective: the rotor was not designed for the flapping and coning possible without the centipetal force.
Are these misconceptions?
Is there a difinitive answer to this?
I had always thought that introducing pedal to slow the rotor was linked to excessive flapping loads, leading to eventual tail rotor failures and tears. I even thought that the reason PHI had banned the practice was linked to thier 407 tail rotor failures. Same as stopping the rotor with collective: the rotor was not designed for the flapping and coning possible without the centipetal force.
Are these misconceptions?
Is there a difinitive answer to this?
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I believe the 407 FM has something about not using the Tail rotor to slow down the main rotor.
And why not put in a rotor brake that works at a good RPM? Does PHI have more restrictive RPM than the company FM? If so, why?
And why not put in a rotor brake that works at a good RPM? Does PHI have more restrictive RPM than the company FM? If so, why?
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Another opinion. Personal that is.
Here is my personal opinion, some of which is based on fact. If the POH states that you should not move the main rotor using the tail rotor then you should never use the tail rotor to slow the main rotor. It has to do with torsional windup of the tail rotor drive shaft. The R-22 has this requirement.
There should be minimal flapping of the tail rotor when on the ground and the main rotor is winding down. The flapping and attendant pitch flap coupling (Delta-3) is to equalize the lift across the tail rotor when in forward flight. During the main rotor slowdown, there is minimal if no downwash and no forward speed component. So, when you add left pitch to the tail rotor it will act like a propeller and the loads on the pitch bearings (Centrifugal and thrust) are less than those that would be encountered in flight.
But as indicated above the rotor brake on some helicopters is on the tail rotor drive shaft. In cases like this application of the rotor brake will cause reverse torsional windup of the tail rotor drive shaft and the designers will have calculated these loads taking them into consideration during the design of the tail rotor drive shaft and support bearings.
The pendulum swings both ways in this situation and as pilots it is better to err on the side of conservatism.
There should be minimal flapping of the tail rotor when on the ground and the main rotor is winding down. The flapping and attendant pitch flap coupling (Delta-3) is to equalize the lift across the tail rotor when in forward flight. During the main rotor slowdown, there is minimal if no downwash and no forward speed component. So, when you add left pitch to the tail rotor it will act like a propeller and the loads on the pitch bearings (Centrifugal and thrust) are less than those that would be encountered in flight.
But as indicated above the rotor brake on some helicopters is on the tail rotor drive shaft. In cases like this application of the rotor brake will cause reverse torsional windup of the tail rotor drive shaft and the designers will have calculated these loads taking them into consideration during the design of the tail rotor drive shaft and support bearings.
The pendulum swings both ways in this situation and as pilots it is better to err on the side of conservatism.
"Just a pilot"
Dinosaur, eh? Youngsters today got no respect for their elders!
Nobody else has emphasised a tangential caution here, so I will say it again- Don't add collective coasting down in high winds!
Nobody else has emphasised a tangential caution here, so I will say it again- Don't add collective coasting down in high winds!
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Guys spare a thought for some of the EH101 crews... a couple of incidents occurred when the electronic rotor brake came on when it felt like doing so, the pilots were left out of that particular decision making process.. needless to say..... crash, bang, splash.
At least you have a choice when to apply it.
At least you have a choice when to apply it.
Devil 49:
No offense intended, Devil. At my advanced age (high side of 50 now), I myself sometimes get referred to as a dinosaur by the younger bucks who seem to know so much more than I do. That comment was kind of a tongue-in-cheek dig at my generation. We do kind of get set in our ways.
But I know myself better now. For a long time I had these "Truths" - these absolutes - these things that I just *KNEW* were correct and so never questioned them. Until one or two of them were proven wrong. And then it made me start to question and doubt everything. And so I've had to re-learn stuff that I was either taught incorrectly or that I learned incorrectly or that I remembered incorrectly. It's a little unnerving. Because when you start thinking about it, you go, "Hmm, I wonder what else I *know* that is wrong? It is why we can never, ever rest on our laurels and think we know everything just because we've been around forever.
Dinosaur, eh? Youngsters today got no respect for their elders!
But I know myself better now. For a long time I had these "Truths" - these absolutes - these things that I just *KNEW* were correct and so never questioned them. Until one or two of them were proven wrong. And then it made me start to question and doubt everything. And so I've had to re-learn stuff that I was either taught incorrectly or that I learned incorrectly or that I remembered incorrectly. It's a little unnerving. Because when you start thinking about it, you go, "Hmm, I wonder what else I *know* that is wrong? It is why we can never, ever rest on our laurels and think we know everything just because we've been around forever.
