Free Wheeling unit question that I can't figure out

[wisper7]

I've been trying to figure this out for an hour and so I finally had to just make an account here and hope someone knows.

 

I understand the purpose of a freewheeling unit, in that it allows the engine to run slower than the rotor head.

 

How does a helicopter run engines without turning the rotor head though? For instance, when doing engine runs, or engine washes, the engine is turned on. I don't know if all are like this, but when this is done on the H60 the rotor brake is applied and the engines run without the head turning.

 

It seems to me that if the output shaft is turning, and that goes into the freewheeling unit, this would mean the engine is turning faster than the head (...obviously). Does this mean that the engine is just putting out so little power since it would be in the idle setting that the freewheeling unit just slips? Or is something else going on that I don't understand. As far as I know, there is no 'idle' for the transmission?

 

Hope that makes sense!

[helipilotm]

First off your correct somewhat in your definition of the freewheeling unit but... The only purpose it has is to disengage the rotor system from the engine if the engine quits or is not running at 100% so the rotor rpm doesn't get dragged down with it. Backwards from what you said but the same thing if that makes sense.

Not very many helicopters can have an engine running at idle and the rotors parked. I can't speak about the H60 but other helicopters that can run without turning the blades have a "clutch" that's seperate from the freewheeling unit. I don't know what the correct term is or would be. It simply disengages the engine from the transmission. It's usefull in twin engine aircraft to use the first engine to power the hydros, provide electrical power to start the second engine etc. The downside is weight and another part to maintain.

Edited February 1, 2015 by helipilotm

[Astro]

 

I don't know what an H60 is, but we piston engine flyers have a clutch system that allows us to start the engine without turning the rotor. It works off of a belt and pulley system which slowly engages the rotor as the engine speeds up. For some its just the flick of a switch and an actuator does all the work. For others you have to engage and disengage it while getting the engine rpm up. Then there's one where you just pull up on a lever. This clutch system is essentially so the heavy load of the rotor doesn't stall the engine at low rpm on startup.

 

The freewheeling unit, or sprag clutch, is a different clutch all together which just seperates the engine from the rotor allowing them to keep spinning after an engine failure.

[Eric Hunt]

In a turbine engine, there is no direct connection between the power-producing section and the output shaft - it is just the movement of the gases that causes the output turbine to rotate, so the engine can be running, but if the rotor brake is on, the blades aren't turning.

 

Same as in a car with auto transmission - you can be in Drive, but with your foot on the brake, the car doesn't move from the traffic light. In a manual car, you can't be in gear and stationary, you must have the clutch depressed.

 

In some turbine helicopters, you are permitted to start one engine with the rotor brake ON. The brake is strong enough to overcome the force from the gases wanting to turn the rotors. Usually the limit is one engine only, and only idle RPM. Any more than that causes damage to the rotor brake, and hot spots on the turbine wheel because it can't rotate and the same spots are exposed to the hot gases from the nozzles.

 

I regularly started one engine of the S-76B with the rotor brake on, to get electrics warmed up and aircon cooled down for the arrival of the Boss. Once he was inside the cabin, release the rotor brake, first engine up to engage speed, and start the second engine - allowed for faster getaways.

[wisper7]

Ahhh, ok! I did not think about the power turbine being held still so that gasses would just pass around it. That makes perfect sense (just like an auto-transmission).

 

The Sikorsky H60 is essentially a Blackhawk helicopter. I know that both engines can be run at the same time without the rotor turning, but I am not sure what their ouput powers would be, so I am assuming they must be quire low as that would indeed be a lot of stress on the rotor break.

 

Thank you all for the help! I should have stated it was a turbine engine from the start, didn't think how important that little fact would be

[Cheese]

In the AH-1W and UH-1N we engage starters but never introduce fuel so the engines can be washed without creating combustion (Being turned by electrical power). The new models use an APU instead of batteries to accomplish the same thing.

[Wally]

helipilotm

First off your correct somewhat in your definition of the freewheeling unit but... The only purpose it has is to disengage the rotor system from the engine if the engine quits or is not running at 100% so the rotor rpm doesn't get dragged down with it. Backwards from what you said but the same thing if that makes sense.

Not very many helicopters can have an engine running at idle and the rotors parked.

 

I believe that a free turbine could start and run with a stationary drive system. As long as nothing is turning in the drive, there's little force delivered by the stationary engine NF. As soon as the NF and drive systems turn, the torque rises rapidly as the turbine becomes more and more efficient. The issue is that most single engine aircraft are not designed to handle those forces, neither the drive train or the airframe.

The mantra was if you started with blades tied down and/or rotor brake locked, stay in the seat until the shutdown was complete, then...

[Nearly Retired]

Wisper7 said:

It seems to me that if the output shaft is turning, and that goes into the freewheeling unit...

 

 

Aha! And here is the root of your misunderstanding. The "output shaft" (that part of the drivetrain going to the transmission) is not turning.

 

In a piston-engine helicopter with a centrifugal clutch (Bell 47, Hiller 12) there will be no provision to run the engine without the main rotor turning. As soon as you start the engine the main rotor will turn. In piston helicopters equipped with a "regular" clutch system (Robinson, Enstrom, etc.) the engine can be completely disconnected, like pushing the clutch pedal in on your manual transmission car.

 

In modern turbine helicopters it gets a little more complicated. There are two sections of the engine: 1) the compressor/gas producer; and 2) the power turbine. It's interesting to note that there is no mechanical connection between these two sections. It is only the flow of hot combustion gas created by the compressor and ignited in the burner can that causes the power turbine wheels to spin. If you were to "hold" the turbine wheels stationary the engine would still start and run just fine.

 

The "complication" is that downstream of the burner can are some additional "turbine" wheels - but we call them "gas generator" wheels because these are connected by a shaft to the compressor, which "closes the loop" in terms of keeping the engine running. In a pure jet (like an airplane) that's all you really need. Thrust goes out the back and pushes the plane along. In our case, we put another set of turbine wheels "behind" the gas generator wheels. We use that exhaust gas to turn these wheels which are connected by shaft to: 1) the freewheeling unit, and 2) the transmission.

 

But! The gas generator and power turbine wheels are housed in the "turbine section" and they look as though they are one unit. They're not, they spin independantly of each other (and at different rpm!). They appear to be on the same shaft but they're not. It is through one of those shaft-within-a-shaft deals that the whole thing works. and it is in here that this crazy "gas coupling" stuff takes place.

 

As others have mentioned, helicopters like the S-76 can start an engine and, with the rotor brake on, keep the rotors from turning. But all they're really doing is holding the power turbine wheels of that engine. You could do the same in your basic Bell 206 if the rotor brake was strong enough, which it isn't (or more correctly isn't designed for). In this case, the output shaft of the engine is not turning, nor is the freewheeling unit.

 

I know all the previous posters have basically said the same things, so I'm only rehashing what's been written. But these modern turbine engines really are marvelous devices when we understand how they work.

[iChris]

I don't know if all are like this, but when this is done on the H60 the rotor brake is applied and the engines run without the head turning.

 

 

This type of engine start were the rotor brake is engaged to hold the drivetrain and rotors stationary, while the engine runs at idle, is more common with large twin-engine helicopters were there is an increased demand for more powerful hydraulic and electrical systems.

 

As an example, the S-64E’s fully articulated main rotor head is around 7 feet in diameter and weighs approximately 1,900 pounds. This is in addition to its 6 main rotor blades totaling 2,100 pounds (350lbs./blade). Therefore, you can’t engage and control a rotor system with that amount of turning mass without hydraulic assist.

 

With the S-64E (E-Model) you get the ball rolling by starting the 72 HP turbine auxiliary power plant (APP, APU, or aka the "P"), that brings the Utility, Hoist, and Make-up hydraulics on line along with electrical; thereby, allowing you to start the 4,500 SHP Pratt & Whitney engines via the hoist pump that supplies the engine hydraulic starter motors.

 

In order to safely engage the rotors and accomplish the main rotor servo and flight control checks, you first need to power-up one of two stages of the main rotor tandem servos. In the S-64E the #1 engine drives the 2nd stage servo pump; therefore, the rotor brake is set to hold and delay rotor engagement until the #1 engine is started and the 2nd stage hydraulic pressure is up at its normal (2,000psi) allowing the preflight servo and flight control checks to be completed. Thereafter, the rotor brake is released and the drivetrain and rotors start turning and the 1^st stage hydraulic system (3,000psi) and remaining systems come online.

 

You have to be specific to the helicopter type because systems vary. With the S-64F (F-Model), there’s no need to set the rotor brake, the APP drives the 1^st stage hydraulic pump from the accessory section of the main rotor transmission without the engine running or the rotors turning. In any case, you need hydraulic assist prior to rotor engagement.

 

One of the older Sikorsky models, the S-58T, has the same normal procedure for start-ups with the rotor brake set and the electrical ground hydraulic pump on prior to rotor engagement.

Edited February 2, 2015 by iChris

[Joe_P148]

The H60 has a rotor brake and or a gust lock. The engine runs but doesn't turn the rotors.

 

It can be run with the engines at IDLE only.

[apacheguy]

The AH64 Apache has the rotor brake start capability as well; supposedly it allows us to do shipboard operations (where you might want to crank engines but not spin rotors due to the close quarters) or when operating in dusty conditions (minimizes the dust being blown around prior to takeoff). It's not commonly done but it is possible if needed.

[superstallion6113]

In the CH-53E, a normal start is done with the rotor brake engaged, and is not released until all three engines are online at ground idle. Once that happens, the rotor brake gets released. The rotor brake disk on the 53 is quite huge though. Around 2 feet in diameter with a massive brake caliper. Like someone mentioned earlier, seems to be a large helicopter thing. More controlled starts.

Edited February 4, 2015 by superstallion6113