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Thread: I have a question. The clutches in helicopters.

  1. #1
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    Default I have a question. The clutches in helicopters.

    They have clutches, don't they? They must. How can the human mind devise a clutch that can synchronize something spinning so fast, with something starting so low, with such torque transmitted?

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    Default Re: I have a question. The clutches in helicopters.


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    Default Re: I have a question. The clutches in helicopters.

    Mike hit on it, they are not clutches in the automotive sense. I'll find some better info in a bit.

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    Default Re: I have a question. The clutches in helicopters.

    I think it's basically a complex, expensive, aviation-quality, version of the simple over-running clutch that is used on a myriad of devices. Sort of like a ratchet that works with friction instead of stepped teeth.



    Now, synchronization and speed differentials between the main rotor and the tail rotor, that's a bit more complex, I believe. Unless they are always synched and adjustments are made via the blade angles and such. My memory fails me, I used to know this stuff. By the way, all heli-o-whirly-copters do NOT work the same, there are very significant fundamental differences in rotor systems to accomplish the same ends.
    Last edited by Bob (oh, THAT Bob); 03-24-2009 at 11:52 PM.
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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Bob (oh, THAT Bob) View Post
    I think it's basically a complex, expensive, aviation-quality, version of the simple over-running clutch that is used on a myriad of devices. Sort of like a ratchet that works with friction instead of stepped teeth.
    Yep, think sprag clutch.

    Quote Originally Posted by Bob (oh, THAT Bob) View Post
    Now, synchronization and speed differentials between the main rotor and the tail rotor, that's a bit more complex, I believe.
    Of the 3 major airframes I've wrench on, H-60 Jayhawk/Blackhawk, H3 Pelican/Jolly Green, and of course the Huey, all fed engine power (single or dual) into the main gearbox where various gears transferred the power and increased the rpms to a tail rotor drive shaft take off flange. So the tail rotor drive and main rotor shaft (mast) are driven off the main gear box. Both are mechanically geared together in the gearbox and rotated at the same ratio. I can't remember, or seem to Google the ratio though. The main rotor turns at 253rpms on the H60 (324 on the Huey) and the tail rotors much faster.

    Much of the noise you hear from a hovering helicopter is tail rotor generated.

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    Default Re: I have a question. The clutches in helicopters.

    There is one situation with the Huey main transmission during an autorotation that can cause the sprag clutch to lock up. But I can't recall it exactly. Has something to do with rolling the power back in during recovery. Perhaps John Teetsel can explain it, as he ran a similar engine/transmission combo in the Cobras.

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    Default Re: I have a question. The clutches in helicopters.

    I THINK I understand the sprag clutch: I believe it's intended to permit the thing to autorotate if engine power is lost (there was a series on cable, a few years back, showing a guy building a 'kit' helicopter, and I saw the episode about how the sprag clutch worked).

    What I don't understand is how the engine can be started, and applied to the rotors. If they were direct geared, the engine would have to be able to apply power at VERY low RPM's to get the rotors turning.... is there some sort of friction clutch used to apply power to the rotors after the engine is started?
    "The rights and interests of the laboring man will be protected and cared for -- not by the labor agitators, but by the Christian men of property to whom God has given control of the property rights of the country, and upon the successful management of which so much depends."
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    Default Re: I have a question. The clutches in helicopters.

    Thats what I was talking about, Norman.

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    Default Re: I have a question. The clutches in helicopters.

    Aha, good question. With a multi-spool gas turbine, I think it is pretty easy because you kind of have a built in transmission. With a single-spool turbine, or reciprocating engine, you would need a separate transmission. And I forgot how they do it.
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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Bob (oh, THAT Bob) View Post
    Aha, good question. With a multi-spool gas turbine, I think it is pretty easy because you kind of have a built in transmission. With a single-spool turbine, or reciprocating engine, you would need a separate transmission. And I forgot how they do it.
    With a recip there is usually a belt drive connecting the engine and transmission.

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Norman Bernstein View Post
    What I don't understand is how the engine can be started, and applied to the rotors. If they were direct geared, the engine would have to be able to apply power at VERY low RPM's to get the rotors turning.... is there some sort of friction clutch used to apply power to the rotors after the engine is started?
    Not a clutch, it's the basic design of a turboshaft engine. Once the air enters the intake it runs through the compressor section, then through the combustion section, then it passes over the 'gas generator' turbine wheels, causing them to spin. (For whatever reason, the air is called 'gas' after it hits the compressor section) So the gas generator turbine wheel(s) are connected by a shaft forward to the compressor section, and spin it to makes sure the engine always runs.

    So at that point, you have an engine that will sustain itself till the fuel is shut off.

    Behind the gas generator turbine wheels, the gas (air) flows over the power turbine wheels. They spin a shaft which runs forward (usually, depends on the application) inside the shaft that turns the compressor and connects to the transmission.

    Here's two pictures, the first is the front half of the GE-T700...



    and the rear half...



    So when the engine is started, the compressor and gas generator turbine wheels can spool right on up, while the power turbine wheels can take their sweet time, as they are not directly linked to the rest of the engine.

    On some helicopters equipped with rotor brakes, the power turbines stay completely still until the brake is released. It doesn't take long for them to catch up, and in fact advancing the throttles above ground idle can cause the brake to slip.

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    Default Re: I have a question. The clutches in helicopters.

    Ah, I see.

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by BrianW View Post
    Not a clutch, it's the basic design of a turboshaft engine. Once the air enters the intake it runs through the compressor section, then through the combustion section, then it passes over the 'gas generator' turbine wheels, causing them to spin. (For whatever reason, the air is called 'gas' after it hits the compressor section) So the gas generator turbine wheel(s) are connected by a shaft forward to the compressor section, and spin it to makes sure the engine always runs.

    So at that point, you have an engine that will sustain itself till the fuel is shut off.

    Behind the gas generator turbine wheels, the gas (air) flows over the power turbine wheels. They spin a shaft which runs forward (usually, depends on the application) inside the shaft that turns the compressor and connects to the transmission.

    So when the engine is started, the compressor and gas generator turbine wheels can spool right on up, while the power turbine wheels can take their sweet time, as they are not directly linked to the rest of the engine.

    On some helicopters equipped with rotor brakes, the power turbines stay completely still until the brake is released. It doesn't take long for them to catch up, and in fact advancing the throttles above ground idle can cause the brake to slip.

    OK, that completely explains the situation with turbine-powered helicopters.... thanks!

    HOWEVER... what about piston-powered helicopters?
    "The rights and interests of the laboring man will be protected and cared for -- not by the labor agitators, but by the Christian men of property to whom God has given control of the property rights of the country, and upon the successful management of which so much depends."
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  14. #14
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    Default Re: I have a question. The clutches in helicopters.

    Well, Norman, in a piston-powered helicopter, the difference in RPMs is not nearly so great, so I suspect that it wouldn't take anything very exotic.

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by PatCox View Post
    Well, Norman, in a piston-powered helicopter, the difference in RPMs is not nearly so great, so I suspect that it wouldn't take anything very exotic.
    It's gotta take SOMETHING.... you mean to tell me that the rotor load is on the engine, in a piston-powered helicopter.... and the starter motor has to overcome the inertia of the rotor blades to get started? Doesn't seem likely......
    "The rights and interests of the laboring man will be protected and cared for -- not by the labor agitators, but by the Christian men of property to whom God has given control of the property rights of the country, and upon the successful management of which so much depends."
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    Default Re: I have a question. The clutches in helicopters.

    Turbine engines start slowly enough that there is no need for an engagement mechanism. The engine's drive shaft is connected directly to the transmission and the blades start turning at the same time the engine does -it's all slow and smooth.

    Recip engines on the other hand go from 0 to idle rpm in a second or two. That's not going to work well with the rotor system so there is an engagement clutch that isolates the engine and transmission. After the engine is started, the clutch is gradually engaged to smoothly bring the rotor rpm up to match the engine rpm.

    Before start, the tach looks like this. (There are actually two needles - engine & rotor.) After start, the engine would be set at 1500 or so - ready to engage the clutch - and the rotor would still be at 0. After gradually engaging the clutch the rotor would slowly catch up to the engine and the needles would be rejoined.



    Now about the clutch. It's a just series of V-belts on two pullies, one fixed, one moveable via an electric motor. This is a preflight checklist item to make sure it's slack for start. Take out just enough slack so the belts slip over the pullies and the rotors will slowly start to turn. Take out a little more and they turn faster until they are up to speed with the engine and the clutch is fully engaged. Ready to go.

    This is called "Checking the rubber bands". No kidding. Hey, let's rent a Robinson and go for whirl.



    Here's a diagram of the same setup.



    Autorotation during an engine malfunction is accomplished via the sprag clutch on both turbine and recip.
    Last edited by John of Phoenix; 03-25-2009 at 02:53 PM.

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    Default Re: I have a question. The clutches in helicopters.

    Norman, I didn't say it wouldn't need a clutch, I was saying it wouldn't be anything exotic, and it turns out its not, its the same clutch I have seen on home-made Briggs and Stratton powered go-karts.

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by John Teetsel View Post
    ...so there is an engagement clutch that isolates the engine and transmission. After the engine is started, the clutch is gradually engaged to smoothly bring the rotor rpm up to match the engine rpm.

    Now about the clutch. It's a just series of V-belts on two pullies, one fixed, one moveable via an electric motor. This is a preflight checklist item to make sure it's slack for start. Take out just enough slack so the belts slip over the pullies and the rotors will slowly start to turn. Take out a little more and they turn faster until they are up to speed with the engine and the clutch is fully engaged. Ready to go.

    This is called "Checking the rubber bands". No kidding. Hey, let's rent a Robinson and go for whirl.
    OK, that answers the question. It DOES seem a bit 'mickey mouse', and I wonder about wear on those belts, which must slip and heat up during gradual engagement.... does the pilot have some sort of lever, or is the engagement rate automatic?
    "The rights and interests of the laboring man will be protected and cared for -- not by the labor agitators, but by the Christian men of property to whom God has given control of the property rights of the country, and upon the successful management of which so much depends."
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    Default Re: I have a question. The clutches in helicopters.

    This site: http://www.pilotoutlook.com/helicopt...rifugal_clutch explains that some helicopters use a centrifugal clutch and some use a tensionor system to tighten the drive belts on their pullies. From the brief article it sounds as though the centrifugal system is automatic, controlled by engine speed, but it doesn't explain whether the belt tension is controlled automatically or through a manual control. From Mr. Teetsel's post it sounds like a manual control, but maybe it can be either depending on the designer's choice. Maybe Mr. Teetsel can elucidate.
    Wherever you come near the human race, there's layers and layers of nonsense. --Thornton Wilder, Our Town

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    Default Re: I have a question. The clutches in helicopters.

    As to wear and heat, yes it's a factor so the belts are checked before every flight and replaced on a regular basis. I've not flow a Robinson but the Hughes 300 that I learned on the engagement is manual and is accomplished by an electric motor that moves an idler pully. The engine is held at 1600 rpm and the clutch switch is moved to the engage position until the engine drops 200 rpm then the switch is moved to hold while the rotor catches up. Once the needles join after a few seconds, the switch is moved to engage and left there until shutdown.

    The engagement clutch is pretty simple. The sprag clutch, OTOH, had to be devised with the use of some kind of hallucinogenic drug.

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    Default Re: I have a question. The clutches in helicopters.

    John, just curious, did you do any civilian flying after you got out of the military. Did you also get a fixed wing license in addition to whirlibird?
    Simpler is better, except when complicated looks really cool.

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    Default Re: I have a question. The clutches in helicopters.

    Did the above answer your question, Pat? If not, maybe the context of why you're wondering would help?

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Norman Bernstein View Post
    OK, that answers the question. It DOES seem a bit 'mickey mouse', and I wonder about wear on those belts, which must slip and heat up during gradual engagement.... does the pilot have some sort of lever, or is the engagement rate automatic?

    Mickey Mouse, eh? Lol I vacillate between being amazed at Igor Sikorsky's intellect and his audacity.

    Helicopter: 60,000 parts flying in loose formation.

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    Default Re: I have a question. The clutches in helicopters.

    So how a bout a pre-rotator for autogyros? How is that done?

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    Default Re: I have a question. The clutches in helicopters.

    John, I am truly impressed. I could not have remembered how to engage the rotor on a TH-55 if my life depended on it. I have talked to several pilots who have flown the Robinson and think it is a good aircraft. It looks a little to home made for me.

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    Default Re: I have a question. The clutches in helicopters.

    Belts, I'll be damned. In other words, same setup as my old rota-tiller

    I'll bet those belts are just a bit different from average V-belts in that they might have a little lower coefficient of friction and less spread between the static and kinetic friction, to allow smooth takeup. The nice thing about V-belts as a clutch (versus two friction wheels with point contact) is the large (180 degree) circumferential contact area, to spread the wear and heat at engagement. Then the beveled contact surfaces not only add more surface area, but magnify the belt tension forces so takeup is solid. Simple, effective.
    When you can take the pebble from my hand, it will be time for you to leave.

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Bob (oh, THAT Bob) View Post
    Belts, I'll be damned. In other words, same setup as my old rota-tiller

    I'll bet those belts are just a bit different from average V-belts in that they might have a little lower coefficient of friction and less spread between the static and kinetic friction, to allow smooth takeup. The nice thing about V-belts as a clutch (versus two friction wheels with point contact) is the large (180 degree) circumferential contact area, to spread the wear and heat at engagement. Then the beveled contact surfaces not only add more surface area, but magnify the belt tension forces so takeup is solid. Simple, effective.
    I don't know Bob....I remember thinking the first time I looked at the belts on a TH-55 that they looked and felt remarkably like the fan belt on an old Pontiac.

    Taylor, as for remembering to engage the belts.....to forget is very self-evident and self-critiquing!

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    Talking Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Ethan View Post
    I don't know Bob....I remember thinking the first time I looked at the belts on a TH-55 that they looked and felt remarkably like the fan belt on an old Pontiac.

    Taylor, as for remembering to engage the belts.....to forget is very self-evident and self-critiquing!
    Ethan, it wasn't so much remembering to engage the clutch but how it was done. Since it's been about 38 years since I flew the TH-55 maybe you can cut me a little slack (or not).

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    Default Re: I have a question. The clutches in helicopters.

    Lol...no sweat! I had forgotten for a second that you're an old rotorhead....(and by "old", I mean "former"....)

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    Default Re: I have a question. The clutches in helicopters.

    Quote Originally Posted by Ethan View Post
    Lol...no sweat! I had forgotten for a second that you're an old rotorhead....(and by "old", I mean "former"....)
    Former, nice recovery.

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    Default Re: I have a question. The clutches in helicopters.

    Taylor, don't forget, I was an IP at Wolters for a couple of years after Vietnam. After doing it several thousand times, I can't forget it. (At least not yet.)

    John, just curious, did you do any civilian flying after you got out of the military. Did you also get a fixed wing license in addition to whirlibird?
    Paul, I flew the oil patch in the Gulf of Mexico for three years after my military career. It was a good chance to finish my education on the GI bill and transition to a regular civilian job. After a couple years of getting established in a regular job, I went back to flying in the AZ National Guard as a night vision goggle instructor in Cobras. Flying so low I’d have to climb so the rotor blades could clear a tall saguaro was a thrill a minute but not something one can do safely on a part time basis. I decided it wasn't a healthy hobby and hung up my wings. I never had much interest in fixed wing aircraft, especially after these things.


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    Default Re: I have a question. The clutches in helicopters.

    Waats?

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