Helicopter rotor brake question
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Helicopter rotor brake question
I hope that this is the correct forum for this question, which arises after watching the BBC ten o'clock news last night about the sad accident to the Super Puma. I have no experience in helicopters, but being in engineering/design and R & D for many years I have a keen interest in all things mechanical and I believe that test pilots are most likely to share my feelings for the machine and manufacturer's intentions as opposed to day to day operations.
There was a film-clip in this news item showing a Super Puma from the same fleet as the lost aircraft, taxying in and shutting down. My attention was drawn to the apparent speed at which the rotor stopped, I am used to seeing helicopter rotors slow quite sedately and finally creep to a halt, but this was noticably quicker, I realise that the clip could have been edited, but my impression of a quick stop was reinforced by the fact that immediately after rotation stopped, the whole head "kicked back" a few degrees as if the remaining momentum in the blades had wound-up the head/gearbox mountings, and this was released as the flexed blades rebounded in the disk-plane.
I was under the impression that rotor brakes were fitted to prevent windmilling when parked and not to save a little time in the shutting down process. As an engineer, this treatment seemed to me to be a rather harsh way to treat a highly stressed mechanism. Is this correct, or am I being over sensitive? I realise that these reversal loads are probably miniscule compared to the centrifugal loads the blades put on their fixings but it all just seemed rather brutal, like selecting "Park" on an auto gearbox before actually stopping!
Thanks for any opinions.
There was a film-clip in this news item showing a Super Puma from the same fleet as the lost aircraft, taxying in and shutting down. My attention was drawn to the apparent speed at which the rotor stopped, I am used to seeing helicopter rotors slow quite sedately and finally creep to a halt, but this was noticably quicker, I realise that the clip could have been edited, but my impression of a quick stop was reinforced by the fact that immediately after rotation stopped, the whole head "kicked back" a few degrees as if the remaining momentum in the blades had wound-up the head/gearbox mountings, and this was released as the flexed blades rebounded in the disk-plane.
I was under the impression that rotor brakes were fitted to prevent windmilling when parked and not to save a little time in the shutting down process. As an engineer, this treatment seemed to me to be a rather harsh way to treat a highly stressed mechanism. Is this correct, or am I being over sensitive? I realise that these reversal loads are probably miniscule compared to the centrifugal loads the blades put on their fixings but it all just seemed rather brutal, like selecting "Park" on an auto gearbox before actually stopping!
Thanks for any opinions.
Hi there.
Rotor brakes are fitted to slow the rotors down. I simple tie down is all that is required to keep the rotors still when not moving. The Blades rely heavyly on Centrifugal force to keep them up and away from the tail boom and above passengers heads when under the disk. As the blades slow down there is very little control lift to keep them where you want them so the idea is to take them from the lower control speed to stationary as quickly as possible.
I saw that clip as well and it did look quite harsh but if you really knew what is happening to those blades during flight you would understand that it is not actually that bad.
I fly aircraft with and without rotor brakes and i know which one i would prefer to be in when shutting down on a windy day.
Hope this helps
HB 999
Rotor brakes are fitted to slow the rotors down. I simple tie down is all that is required to keep the rotors still when not moving. The Blades rely heavyly on Centrifugal force to keep them up and away from the tail boom and above passengers heads when under the disk. As the blades slow down there is very little control lift to keep them where you want them so the idea is to take them from the lower control speed to stationary as quickly as possible.
I saw that clip as well and it did look quite harsh but if you really knew what is happening to those blades during flight you would understand that it is not actually that bad.
I fly aircraft with and without rotor brakes and i know which one i would prefer to be in when shutting down on a windy day.
Hope this helps
HB 999
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Thanks HB 999
I thought that I was probably being over protective towards the machinery. It's just that I'd never noticed it before during fifty plus years of observation, but I guess that rotor brakes are a fairly recent addition and I don't get to see as many helicopters as I used to (on the ground that is)!
I have always been in awe of rotor head design and I appreciate the tremendous loads they have to handle. I assume, now that I know that they are indeed intended to stop the rotor, that they are suitably sized so that at optimum condition they cannot overstress anything. Are they inhibited above certain power levels or in flight and of a fail-safe design?
I thought that I was probably being over protective towards the machinery. It's just that I'd never noticed it before during fifty plus years of observation, but I guess that rotor brakes are a fairly recent addition and I don't get to see as many helicopters as I used to (on the ground that is)!
I have always been in awe of rotor head design and I appreciate the tremendous loads they have to handle. I assume, now that I know that they are indeed intended to stop the rotor, that they are suitably sized so that at optimum condition they cannot overstress anything. Are they inhibited above certain power levels or in flight and of a fail-safe design?
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Generally there is a mechanical interlock so that if the engine power levers, ie engines running, are forward, it is impossible to pull the rotorbrake lever aft and apply the brake.
The Rotor head in question also has "Lead/lag" hinges and dampers so there is some fore and aft movement on the blades. That probably made it look worse to the casual observor.
HB999
HB999
Not entirely sure of the Puma rotor brake, but on the S76 type, it's basically a hydraulic system, similar to a caliper and brake disc on a car, whereby the disc is a part of the tail rotor drive shaft. Now they do generate a lot of heat, hence the reason why there is a limited time between applications.
One of the issues in flight, if it starts to tighten, is the massive amounts of heat generated and /or failure of the drive shaft itself.
Some people (those without mechanical sympathy for the machines they fly) can be seen applying the brake quite harshly, and as an ex engineer, it always makes me wonder why people continue to do this.
The movement of the blades backwards is probably just from the blade dampers (again, a hydraulic piston and cylinder) being compressed with the forward inertia of the blades, and once the blades come to a standstill, the hydraulic pressure in the dampers moves the blaeds backwards. Nothing more than that and nothing to do with winding the transmission up.
People could be a bit more gentle with the brake tho, IMHO.
One of the issues in flight, if it starts to tighten, is the massive amounts of heat generated and /or failure of the drive shaft itself.
Some people (those without mechanical sympathy for the machines they fly) can be seen applying the brake quite harshly, and as an ex engineer, it always makes me wonder why people continue to do this.
The movement of the blades backwards is probably just from the blade dampers (again, a hydraulic piston and cylinder) being compressed with the forward inertia of the blades, and once the blades come to a standstill, the hydraulic pressure in the dampers moves the blaeds backwards. Nothing more than that and nothing to do with winding the transmission up.
People could be a bit more gentle with the brake tho, IMHO.
Bell Jet ranger rotor brake to be applied below 38% NR (Rotor speed)
Tend to find the smooth application of the brake to full on at this speed is enough to warm the disks and allows the rotor to stop nice and smooth without judder or vibration. If the brake is applied at around 30% and below the cold disks catch and bind causing a horrible judder and shudder from the MRGB. Then its a case of small applications to bring the speed down.
Bell 206 does not have Lead/lag dampers so no for and aft movement of the blades.
Rotor brake on 206 is Hydralic and just like a car brake. Sometimes Hydralic pressure is low/fluid is low so you pump the lever to build the pressure.
Im not an engineer but i treat the aircraft like its made of glass, wish others would too!
Tend to find the smooth application of the brake to full on at this speed is enough to warm the disks and allows the rotor to stop nice and smooth without judder or vibration. If the brake is applied at around 30% and below the cold disks catch and bind causing a horrible judder and shudder from the MRGB. Then its a case of small applications to bring the speed down.
Bell 206 does not have Lead/lag dampers so no for and aft movement of the blades.
Rotor brake on 206 is Hydralic and just like a car brake. Sometimes Hydralic pressure is low/fluid is low so you pump the lever to build the pressure.
Im not an engineer but i treat the aircraft like its made of glass, wish others would too!
Chief Bottle Washer
All rotor brakes that I have come across are essentially similar in design to a car disc brake, with 2 or 4 pistons activating calipers either side of the brake disc. Application is limited to below a specific Nr, most Bells are 38-40%Nr, Sikorsky varies more by type, and so on. Robinson is the only non hydraulic system I've used (pull a chain in the roof of the cockpit which activates the pucks via a cable!), all others are usually via a large overcentre hydraulic piston in the cockpit. S76 has an electrically operated brake in some models: now that's fierce, no control at all 
The disc can be in a variety of locations, from the short shaft between the engine and MGB of the 206, to a specific shaft out of the side of the MGB, or even on the output shaft from the MGB. It depends on the designer, and who knows what influences him......
Bell are quite specific in their operating instructions, requiring the rotor brake to be fully applied until nearly stopped, then eased off to allow a 'soft' stop. Many pilots 'milk' the brake, which is detrimental to it's operation. the 206 wind-up/judder is usually caused by this type of application, and is the tail rotor drive shaft winding up the thompson couplings, not the MGB.
As mentioned, some helicopters (such as the Sea King) have the ability to apply the rotor brake with the engines still running at idle. The SK has an aux drive which bypasses the MGB, and allows blade folding: the rotor brake is one of the interlock requirements to allow folding.
I use the RB nearly all the time. Safer for operations, and also helps to know that it is working: those who leave it until it's really needed can have a nasty surprise waiting when it turns out to need a bleed and fails to work as advertised
The disc can be in a variety of locations, from the short shaft between the engine and MGB of the 206, to a specific shaft out of the side of the MGB, or even on the output shaft from the MGB. It depends on the designer, and who knows what influences him......
Bell are quite specific in their operating instructions, requiring the rotor brake to be fully applied until nearly stopped, then eased off to allow a 'soft' stop. Many pilots 'milk' the brake, which is detrimental to it's operation. the 206 wind-up/judder is usually caused by this type of application, and is the tail rotor drive shaft winding up the thompson couplings, not the MGB.
As mentioned, some helicopters (such as the Sea King) have the ability to apply the rotor brake with the engines still running at idle. The SK has an aux drive which bypasses the MGB, and allows blade folding: the rotor brake is one of the interlock requirements to allow folding.
I use the RB nearly all the time. Safer for operations, and also helps to know that it is working: those who leave it until it's really needed can have a nasty surprise waiting when it turns out to need a bleed and fails to work as advertised
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Chinook one if memory serves me right was made from ceramic, the same stuff as used on the Space Shuttle Tiles and would glow white hot on shutdown, you can watch it from the aft cabin... you were supposed to be able to touch it more or less straight away as it dissapated the heat so well, but no one tried it......
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but I guess that rotor brakes are a fairly recent addition and I don't get to see as many helicopters as I used to (on the ground that is)!
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Thanks for all the answers guys!
I'm glad to see that the consensus seems to be to treat the machinery with respect and not to overdo the brake.
I'm glad to see that the consensus seems to be to treat the machinery with respect and not to overdo the brake.
Chief Bottle Washer
Originally Posted by SincoTC
I guess that rotor brakes are a fairly recent addition
helimutt: Shhhh, don't let anyone else know
Chief Bottle Washer
Originally Posted by Chopper_doctor
Some sort of interlock would be a good idea, but i have yet to see one on any of the types I am familiar with ( Bell, Sikorsky and MBB )
The problem is compounded for "old" 109 drivers, where the start required the speed select to be manipulated alongside the rotor brake lever, unlike the current FADEC switch turned to "Start" down between the pilot's seats 
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The 332/225 rotor brake is certainly very efficient.
SOP is to apply the brake to the "stop"position when 45% RPM is reached after engines shutdown. When it is left in that position the rotors do stop pretty quickly and can "kick back" slightly after stopping.
SOP is to apply the brake to the "stop"position when 45% RPM is reached after engines shutdown. When it is left in that position the rotors do stop pretty quickly and can "kick back" slightly after stopping.
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cyclic gal and SP
Thank you for correcting my rather rash assumption that these brakes were a recent thing, this was based on the fact that I'd only noticed their mention in technical reviews of fairly new types and because I'd never noticed this kick-back behaviour before, possibly the earlier generation brakes weren't so powerful or the pilots released them just before the stop, which would seem like a sensible move.
There were some intersesting comments in this thread from last year (Note to self: I must use the search facilities before posting
)
http://www.pprune.org/rotorheads/314...plication.html
The one from NickLappos summed up my initial thoughts on seeing the news clip.
It does seem like some kind of interlock would be a good thing, especially if it can be as simple as a line of code
Thank you for correcting my rather rash assumption that these brakes were a recent thing, this was based on the fact that I'd only noticed their mention in technical reviews of fairly new types and because I'd never noticed this kick-back behaviour before, possibly the earlier generation brakes weren't so powerful or the pilots released them just before the stop, which would seem like a sensible move.
There were some intersesting comments in this thread from last year (Note to self: I must use the search facilities before posting
)http://www.pprune.org/rotorheads/314...plication.html
The one from NickLappos summed up my initial thoughts on seeing the news clip.
It does seem like some kind of interlock would be a good thing, especially if it can be as simple as a line of code
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I'm sure that we had them in the Whirlwind, and that's going back even further
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The S-76 has a very good, powerful rotor brake. It can be used to stop the rotors with both engines running at idle or stopped (one stopped and the other at idle in the case of the B model only). Both engines can be started with the rotor brake applied. This makes for a very rapid rotor start, useful in the case of high winds. It's applied by one pump of the handle to prime the system then more pumps to apply increasing pressure; a relief valve acts a limiter.
The original electric rotor brake can only be described as brutal!
The RFM states that the max operating rpm for rotor brake application, both engines stopped, in an emergency, is 107% Nr!