Pedal/collective to slow the rotors
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loggerman you are guilty of engaging the keyboard before thinking about your reply, a mistake which I am guilty of too often.
In the scenario discussed here we are shutting down and at the point of introducing collective to aid in reducing RRPM the engine is closed down and will not be a factor.
I was told that using collective is no good in a 300 as it lifts the blades off the droop stops thus enabling a potential boom strike.
In the scenario discussed here we are shutting down and at the point of introducing collective to aid in reducing RRPM the engine is closed down and will not be a factor.
I was told that using collective is no good in a 300 as it lifts the blades off the droop stops thus enabling a potential boom strike.
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I'm not a huge fan of adding a lot of pedal while shutting down only because your putting stress on the tail boom and attachment bolts(yaw thrust being applied at the end of the boom but skids not allowing the turn) i just dont imagine that being too good for the mounting bolts
I always thought that this is exactly what the tailboom was designed for? What do you do if you want to climb - just let the machine yaw right, because left pedal would put too much stress on the tailboom?
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Have made use of 'collective braking' in 204s, 205s and 206/Ls throughout my entire career and have had the collective at around 80% pitch just moments after shut down.
Never troubled myself with t/r application (too little benefit) but collective braking can reduce the wind-down time when necessary.
If you leave the lever up too long you will definately start slamming the stops and if that is left unchecked you can unseat the entire transmission. However, long before this ever occurs you can feel it approaching and its simply matter of smartly lowering the collective.
That's what I've done but - I'm not recommending it for others and increasingly new ships have rotorbrakes making this practice unnecessary.
Never troubled myself with t/r application (too little benefit) but collective braking can reduce the wind-down time when necessary.
If you leave the lever up too long you will definately start slamming the stops and if that is left unchecked you can unseat the entire transmission. However, long before this ever occurs you can feel it approaching and its simply matter of smartly lowering the collective.
That's what I've done but - I'm not recommending it for others and increasingly new ships have rotorbrakes making this practice unnecessary.
i suppose i should also add that my feelings about the added stress on the tailboom are just that... they are my gut feelings. it may not cause any more stress than what is seen in normal flight, i just personally dont feel comfortable applying those inputs on the pedals. i feel i can control the main rotor much more safely. in the end, these techniques are really only used when i need the rotor stopped quickly or when the wind makes them want to spin forever. im generally not in enough hurry to bother with these techniques.
If you're on your own in an R44 and try raising the collective whilst the blades are turning you're at serious risk of dynamic rollover, even if the rotor RPM is well below normal operating range.
Is the risk of a dynamic rollover less likely if you have others with you in an R44 when you raise the collective after shutting down? Please enlighten us.
The main reason I see adding a little collective as being a bad idea is the risk of ground resonance
If you're going to get into ground resonance, it will have already happened a long time before you shut down.
Is the risk of a dynamic rollover less likely if you have others with you in an R44 when you raise the collective after shutting down? Please enlighten us.
The main reason I see adding a little collective as being a bad idea is the risk of ground resonance
If you're going to get into ground resonance, it will have already happened a long time before you shut down.
Last edited by kev_laline; 19th Dec 2010 at 01:42.
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The tail boom is stressed to take a lot more load than could be applied to a tail rotor at idle and below. There is absolutely no chance of doing damage to the bolts or any other structure.
There may be other reasons for not wanting to use tail rotor pitch to slow the rotors, but none would be caused by loads on the tail boom attachment bolts.
It's also impossible to get into dynamic rollover with the rotor blades unpowered and decelerating from idle RPM - you can't generate enough rolling moment or thrust to tilt the helicopter over.
You might have a problem with blade flapping, but not with dynamic rollover.
Same thing with ground resonance - first of all, you can't get ground resonance in a two bladed rotor (if that's what you're flying). And in any other multi-bladed head, with no power going to the rotor blades the rotor blades will find their own place in lead-lag. If one lead-lag damper was going bad, it would have gone bad when there was a large difference in drag between advancing and retreating side wouldn't it?
Raising the collective would increase the drag on all the blades at the same time, and if you were going to get ground resonance, the collective position wouldn't make any difference.
Where do these last two old wives tales come from???
There may be other reasons for not wanting to use tail rotor pitch to slow the rotors, but none would be caused by loads on the tail boom attachment bolts.
It's also impossible to get into dynamic rollover with the rotor blades unpowered and decelerating from idle RPM - you can't generate enough rolling moment or thrust to tilt the helicopter over.
You might have a problem with blade flapping, but not with dynamic rollover.
Same thing with ground resonance - first of all, you can't get ground resonance in a two bladed rotor (if that's what you're flying). And in any other multi-bladed head, with no power going to the rotor blades the rotor blades will find their own place in lead-lag. If one lead-lag damper was going bad, it would have gone bad when there was a large difference in drag between advancing and retreating side wouldn't it?
Raising the collective would increase the drag on all the blades at the same time, and if you were going to get ground resonance, the collective position wouldn't make any difference.
Where do these last two old wives tales come from???
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Personal experience weather it be good or bad it was experience non the less.
206 L and B full left pedal to slow the blades no prob.....but tried to pull a bit of collective once to slow the the blades in a jet box holy sheit will never do that again, main rotors went bizzuek, its not a good thing.
Seen military pilots pull FULL collective in 205s to slow rotors not pretty but they did it every shut down, was PNG defence force but.
206 L and B full left pedal to slow the blades no prob.....but tried to pull a bit of collective once to slow the the blades in a jet box holy sheit will never do that again, main rotors went bizzuek, its not a good thing.
Seen military pilots pull FULL collective in 205s to slow rotors not pretty but they did it every shut down, was PNG defence force but.
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Explain that to a Mechanic that was killed in Orlando
He was ground running a 300 to warm up the engine oil (yes they did it with the rotors engaged, at flying RPM (don't ask me why, I guess they wanted to reduce the risk of shaft overspeed [when rotors are not engaged], or maybe just wanted it to warm up quicker) The practice has since been stopped.
He applied collective to slow the rotors, just after engine shut down. The helicopter got into ground resonance (I will not go into the details of what happened next) He was killed, may he rest in peace. There were a bunch of un-willing witnesses, as it was right in front of the hangar.
As you can imagine the rotors would of been at a high RPM for this to have happen. Just be careful.
My question, what's is the rush??? OK in windy conditions I can understand some assistance in stopping the rotors. But otherwise, you can not wait the extra couple of minutes??
DT
He was ground running a 300 to warm up the engine oil (yes they did it with the rotors engaged, at flying RPM (don't ask me why, I guess they wanted to reduce the risk of shaft overspeed [when rotors are not engaged], or maybe just wanted it to warm up quicker) The practice has since been stopped.
He applied collective to slow the rotors, just after engine shut down. The helicopter got into ground resonance (I will not go into the details of what happened next) He was killed, may he rest in peace. There were a bunch of un-willing witnesses, as it was right in front of the hangar.
As you can imagine the rotors would of been at a high RPM for this to have happen. Just be careful.
My question, what's is the rush??? OK in windy conditions I can understand some assistance in stopping the rotors. But otherwise, you can not wait the extra couple of minutes??
DT
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simondh:
You can't generate enough lift with the rotor at idle and no engine powering it to get the helicopter light on the skids, let alone roll it over. Most machines stop producing any useful lift at about 65% rotor RPM, even with the engine powering the blades around.
You can't generate enough lift with the rotor at idle and no engine powering it to get the helicopter light on the skids, let alone roll it over. Most machines stop producing any useful lift at about 65% rotor RPM, even with the engine powering the blades around.
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I'm with shawn on this one. after full down auto in UH1, you have enough inertia in MR blades to just get off the ground do a 360ish turn and sit back down again, you are going down as you are turning so better be good at it. Once blades get down below about 80-90% maybe more or less, not really looking inside, you aren't going pretty much anywhere, and that is in a light utility UH1. no way in hell a 44 will do much, except smack a tailboom if you are silly about, once engine has been wound off and rpm started to decay.
Also if you park your 44 just right, shut the engine off, no rotor brake, about 15-20 kts, blades will free wheel all day, now what do you do? Good for all those lowtime pilots in the UK, as they log flight time until the blades stop turning.....
What a lot of us here have probably not said, as it was assumed, is shut down, wait for rpm to decay, 60 sec or so, then when about 50% pull collective. As previously said, we only tend to do it in the wind to slow the damn things down, otherwise they do it fine themselves.
Also if you park your 44 just right, shut the engine off, no rotor brake, about 15-20 kts, blades will free wheel all day, now what do you do? Good for all those lowtime pilots in the UK, as they log flight time until the blades stop turning.....
What a lot of us here have probably not said, as it was assumed, is shut down, wait for rpm to decay, 60 sec or so, then when about 50% pull collective. As previously said, we only tend to do it in the wind to slow the damn things down, otherwise they do it fine themselves.
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I think it prudent to add that any pilot using these techniques should consider the type of rotor system on the particular aircraft they are flying. For example a standard semi-rigid rotor like most of the Bells aren't going to give you any problem with a collective raise until the rpm gets fairly low and there is the possibility of hitting the teeter stops too hard but, as another already pointed out, you can see that one coming and once the rotor is slowed way down you don't gain much braking from collective anyway.
Though I have never personally seen or heard of damage from this I would think one should be a little more careful in a Robinson or most articulated rotor systems simply because these aircraft have droop stops that actually support the weight of the blade when there is no lift or CF, unlike a true semi-rigid rotor. And these droop stops are very short coupled to a very long blade with lots of leverage. So the blade is capable of generating fairly high forces on these droop stops if allowed to jump up and pound the stops. But again, this can be seen relatively easy by the pilot and avoided.
Though I have never personally seen or heard of damage from this I would think one should be a little more careful in a Robinson or most articulated rotor systems simply because these aircraft have droop stops that actually support the weight of the blade when there is no lift or CF, unlike a true semi-rigid rotor. And these droop stops are very short coupled to a very long blade with lots of leverage. So the blade is capable of generating fairly high forces on these droop stops if allowed to jump up and pound the stops. But again, this can be seen relatively easy by the pilot and avoided.
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John, in relation to the icy pads I believe you are referring to the friction of the transmission. That is where the nose of the helicopter tries to follow the direction of the disk. Just like the bottom of an auto with the engine actually failed as opposed to idle. You will see it there also. There is no torque per se.
fadecdegraded, I agree about not doing it in a 206 (pulling collective after shutdown to slow the blades), or at least being very cautious about it, as the transmission mounts (spring and cup type) can really get some nasty-sounding and feeling bounce going - I had to do it to stop the blades once when parked in a hillside pad with a very strong updraft, and it wasn't much fun.
The 205, on the other hand, with the 5-mount transmission, is a much better prospect, and I've certainly used collective many times to slow the blades after shutdown in those. I think the worst you can do in that case is make the blade grip reservoirs leak if you have them.
Comes down to the type you fly, naturally enough, and I certainly wouldn't rule it out as something never to be done if the need was there.
The 205, on the other hand, with the 5-mount transmission, is a much better prospect, and I've certainly used collective many times to slow the blades after shutdown in those. I think the worst you can do in that case is make the blade grip reservoirs leak if you have them.
Comes down to the type you fly, naturally enough, and I certainly wouldn't rule it out as something never to be done if the need was there.
