The great Helo debate

[Jim123]

Maybe I'm misunderstanding something. Rotor Brake Start: Nr=0. Np>0. Explain.

Rotor brake start:

Rotor brake = on, start engines
Ng=otherwise normal idle
Nr=0 (because the rotor brake is holding it)
Np=0* (because the freewheeling unit makes Nr => Np <-- two ways of saying the same thing --> Np =< Nr )

Rotor brake = off/disengage
Ng=still otherwise normal
Np accelerates* and takes Nr with it

Some helicopters (H-34, R-22, others) also feature an additional on/off clutch to completely engage and disengage the engine from the drivetrain, but that works more like the clutch pedal in a manual transmission car (or shifting an automatic between D and N). Note that without the load of the rotors, when Ng is at a steady idle speed then Np will normally seek some equilibrium speed of slightly less than 100%. Also note that this note does not apply to the R-22 and piston-engined variants of the H-34. Also also note that the H-34 and R-22 also have a freewheeling unit (aka overrunning clutch) for the same reason as any helicopter. Also also also note that there are a few unlucky helicopters do not feature a freewheeling unit.

*The main problem with a rotor brake start is it puts a lot of stress on the drivetrain because the engines make a lot of torque when Np is at or near 0. That is a basic characteristic of free power turbine turboshaft engines. (Random related fact, the experimental Chrysler turbine cars in the 1960s had a lot of snap off the line for this reason.) The torque indications in the 60 don't really show this well but only because they are designed to be accurate when the Np is rotating at normal speeds.

 

[RotorHead04]

Maybe I'm misunderstanding something. Rotor Brake Start: Nr=0. Np>0. Explain.

Rotor brake prevents head from turning at the output drive shaft aft of the transmission. Heads are not turning. Transmission is not turning. Input modules are not turning. Freewheeling unit is engaged. Engine output drive shafts are not turning. Power turbine (Np) is stationary. Hence, Np is zero. This is why you aren't supposed to routinely do Rotor Brake Starts -- the Np shaft rub issue referenced in the Start Procedures Warning is the logical result of the Np shaft literally rubbernecking when the brake is released.

Point Pags.

Edit: Jim beat me to it!

 

[Pags]

Maybe I'm misunderstanding something. Rotor Brake Start: Nr=0. Np>0. Explain.

The rotor brake is applying a large enough frictional force that the motors running at idle can't overcome it, thus holding the rotors and therefore the transmission static. Since the rotor brake is between the transmission and the oil cooler, the motors are applying torque to the main transmission via the input mods.

But hitting a tree with the rotor engaged is a different situation. If it was a big enough tree and the rotors somehow didn't disintegrate it would be like throwing the rotor brake on at 100% Nr and the spike in torque would probably spin the fuselage around.

But in reality if you hit a big enough tree or object the blades would most likely just disintegrate. Sure the blades might droop in the second before they splintered, but at that points its kind of academic as you'd be less a couple of blades and probably no longer flying.

Edit: Didn't mean to pile on. My computer died before I could post and others replied before I could find my power cord.

 

[flaps]

river rats

http://www.youtube.com/watch?feature=player_embedded&v=DR2C-gm5GkA#t=5s

 

[81montedriver]

What's the difference between Np and Nr? In the Herc we only have Ng and Np. I was hoping to not contribute to all the nerdery going on but I got more curious as I read on

 

[Jim123]

What's the difference between Np and Nr? In the Herc we only have Ng and Np.

Np (helo) is power turbine (not propellor... obviously) speed. Sometimes called Nf (free turbine). Analogous to Nf (fan) and N2. Nr is for rotor.

I was hoping to not contribute to all the nerdery going on but I got more curious as I read on

 

[pilot_man]

I come here to see cool pictures and instead I get all of this Helo crap. Please make it go away.

 

[HokiePilot]

Between the power turbine and the transmission is a freewheeling unit (aka one-way cluch or sprag clutch). It is anagalous to the clutch in a bicycle. At a fixed gear ratio, you can't move the pedals faster than the rear wheel no matter how hard you try. But when you stop pedeling (or shut of an engine) the rear wheel (rotor) can still spin.

When stopped at a light, you are putting a little pressure down on one pedal, but it isn't spinning because you are holding the brake on the wheel (rotor brake). Once you release the brake, the pedals and rear wheel can start spinning.

 

[bert]

I see what you're saying, and we're getting WAAAY into weeds, but now I'm curious. I know... Ng has nothing to do with the freewheeling unit, so at what point does it allow the engine to spin freely with no regard to Nr?

Others have replied, but I will add this explanation in case it didn't get through.

Ng and Np have no physical connection - thus the "gas coupled" blurb they used to make RP's memorize. Np and Nr are only connected one way: Nr can drag down Np but Np cannot drag down Nr. The only ways to have a power turbine spin and not drive the rotor system is to have Nr > Np (quick stop or autorotation, for instance) in which case the free wheeling units are engaged and the DECUs will have dropped the engines to minimum speed because they don't care about Nr, they only want to keep Np at the desired value, or to have the high speed shaft fail (power out shaft no longer physically connected to the XMSN), in which case you will most likely have a quick overspeed and a shutdown.

A lot of new guys get the basics in the 60 confused but think of it this way for normal situations: the HMUs ensure the engines can provide maximum power in the governing range. The DECUs dial down that power as required to keep Np set at the value you input on the INCR/DECR switch. Nr either stays with Np or spins up above it if driven by other inputs (like airflow in an auto).

 

[Alpha_Echo_606]

I come here to see cool pictures and instead I get all of this Helo crap. Please make it go away.

If I had the powers that be I'd take this entire rotor brake talk and give it its own thread.

 

[helolumpy]

Maybe I'm misunderstanding something. Rotor Brake Start: Nr=0. Np>0. Explain.

Aren't you the same Otto that is always complaining about things like silly decisions by O-4's? Yet you fail to understand something that I would expect a Cat 1 student to be able to explain... Interesting.

 

[RotorHead04]

Aren't you the same Otto that is always complaining about thinga like silly decisions by O-4's? Yet you fail to understand something that I would expect a Cat 1 student to be able to explain... Interesting.

He's always talking about VERTREP and being a HAC, so I was surprised what the standard indications for a rotor brake start would be ... You have been to the boat, right Otto?

 

[RotorHead04]

Where's Peanut? Felker's getting kinda old...

N 37° 7.704' W 76° 36.500'

Day only for Navy folks per Wing Guidance.

 

[phrogdriver]

I love that ugly, leaky, smelly, underpowered battle ax because she brought me home.

That's like a beaten wife loving her husband because he always makes sure to not bruise her on the face.

It almost killed you a minute ago. You should be pissing on that machine, not trying to make sweet, sweet love to it.

 

[Gatordev]

Looks like I missed the big discussion, but...

This is why you aren't supposed to routinely do Rotor Brake Starts -- the Np shaft rub issue referenced in the Start Procedures Warning is the logical result of the Np shaft literally rubbernecking when the brake is released.

This isn't completely true. I know that's what gets put out at Natops conferences, but if you actually talk to GE guys, they'll tell you there is absolutely nothing wrong with a rotor brake start, you just need to let it sit at ground idle for (I forget the exact number) ~15 seconds. By that time, the shaft has spun slowly enough that it will not rub but "fast" enough that it spins out the sag of the shaft so it's back to a symmetrical shape so that when you then go to fly, it can't rub.

This was a huge issue at a Natops conference back in '05-ish (or thereabouts). The result was the abortion of a start-up sequence we now have when all that was needed was a warning about leaving the head at idle whenever you can. I worked with one of the guys at that conference who was trying to get the simple solution in the checklist, and he was pretty frustrated with the whole thing.

Fun fact: some GE guys actually think what I've described above is in our Natops and part of the checklist. Sadly, it is not.