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Question on how things work - blades don't spin when turbine first starts up?


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Completely non-PC related but it's got me curious. When I start the UH-1, it seems as though the starter has to get to a certain RPM before I see the rotor blades start spinning - the blades don't immediately start spinning even the tiniest bit until a while after the starter has been held down.

 

Is there a clutch that engages at a certain RPM speed? Or is someone able to advise a clueless noob as to how the starter's speed is not directly proportional to the main blades?

 

I know this isn't strictly a DCS question - but just really curious as to how the turbine, starter and rotor blades work with each other.

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In a turboshaft helicopter engine like we have in DCS, clutch is not needed. The power output shaft is not physically connected to the rest of engine. Once the engine spools up to high RPM and starts producing enough exhaust gases, the power turbine will start to move too.

 

https://commons.wikimedia.org/wiki/File:Turboshaft_operation_(multilanguage).svg#/media/File:Turboshaft_operation_(multilanguage).svg

 

There's a clutch too somewhere, but it's there only in case of engine failure.

 

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Using the diagram above from some1.

The starter is the backup electrical generator/starter connected to the compressor turbine shaft. Instead of generating electricity as the shaft spins, you use electricity to turn the generator into an electric motor that turns the compressors and starts gas generation (Air flow and compression). Fuel is added and burned.

15% gas pressure is when the power turbine starts to spin (main rotor starts to turn) and you need to hold the starter till 40% for the process to be self sustaining (it will increase to 100% on its own).

 

After the engine is started you set the starter to standby generator and it becomes a backup electrical generator again. The main generator is attached to the transmission gearbox and generates from there. (the power turbine shaft)

 

In the huey the power turbine shaft comes out of the front of the engine and not the back. So the transmission is at the cool end of the engine.

But the turbine shaft passes through the compressor shaft and is not directly connected to it. (a shaft within a shaft)

Why the engine is to the rear of the rotor. with the exhaust to the rear.

The mi-8 has the power turbine shafts coming out the rear and the engines are mounted in front of the rotor. With the exhausts to either side of the rotor and transmission.

 

The free wheeling clutch only operates when the main rotor is turning and the power turbine shaft has stopped. so it only disengages in emergencies and engine shutdown.

during start up and normal operation it is always engaged.

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Thanks all. I didn't realise that the rotor wasn't directly linked to the turbine - so we basically have two different mechanisms spinning independently of each other, and it's the compressed gas that's generated by one that causes the other to spin. Until there's enough pressure - it won't spin - kinda like a windmill in the wind to use a crude example.

 

Sweet - one less thing for me to be scratching my head over now as I start it. Appreciate the feedback to a real noobish question! :thumbup:

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Hi,

if you're interested, there's this channel on Youtube run by a Canadian guy "AgentJayZ" who owns (works at?) a jet engine repair shop. I like his videos a lot, you can actually see the guts of such engines, in detail, and learn quite a bit about these crazy engines that blow air instead of (properly) pushing pistons :) In some episodes he talks about turboshaft engines with free turbine, i.e. the same design (basically) as in helicopters, for he sometimes gets "ground" engines of such design for repair/overhaul. Good stuff, if you're geeky enough :)

 

 

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15% gas pressure is when the power turbine starts to spin (main rotor starts to turn) and you need to hold the starter till 40% for the process to be self sustaining

You are supposed to release the starter button at 30% to prevent overheating the starter. Its actually self sustaining at around 25% but technically you are supposed to depress it until at the very latest 30%.

 

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The procedure for starting in Huey's manual would it be wrong?

 

 Main rotor – Check that the main rotor is turning as Gas Producer speed reaches 15%. If the rotor is not turning, abort the start.

 Start switch – Release at 40% Gas Producer speed or after 40 seconds, whichever occurs first.

 Throttle — Slowly increase to set normal RPM.

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Well, If Im wrong, Im wrong. Ive been going by the Army manual thats a bit outdated and its been a while since I last looked at it. But I recall the checklist being cut starter at 30% or 30 seconds to prevent overheating the starter. The DCS Huey's engine is self sustaining at += 25% per my experiences. Again, if Im wrong, Im wrong.

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Well, If Im wrong, Im wrong. Ive been going by the Army manual thats a bit outdated and its been a while since I last looked at it. But I recall the checklist being cut starter at 30% or 30 seconds to prevent overheating the starter. The DCS Huey's engine is self sustaining at += 25% per my experiences. Again, if Im wrong, Im wrong.

 

The last time I released the starter too early at 38-9% she started to spin down. And I had to jump back on the starter.

 

The manual says 40% or 40 seconds to prevent overheating.

 

edit: Just tested on the acrobatics server with normal day conditions and she spins down at anything below 40%.

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The last time I released the starter too early at 38-9% she started to spin down. And I had to jump back on the starter.

 

The manual says 40% or 40 seconds to prevent overheating.

 

edit: Just tested on the acrobatics server with normal day conditions and she spins down at anything below 40%.

 

I managed to get it to start after it hit 25%.

 

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Hammer1-1 is hot starting her.

If you watch the engine temp below the gas producer then he goes much hotter than I do on a similar start.

He is above 625c but I cannot tell if he is above 675. If he is then he is there for too long (10 seconds when max is 5)

So he is using more throttle.

 

The real start position is actually below the idle release position, in the page up and page down region of the throttle.

But even starting at the idle release position I don't get this hot. And she does not go till 40%

And she will get nowhere near 625c

 

So the difference is the amount of throttle used.

In external view you may see flames coming out the engine on a hot start.

The manual says avoid hot starts.

check your throttle is going to zero.

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So the difference is the amount of throttle used.

 

Exactly, if the throttle is on Idle, check the curve of axis, the mini must be at 0, even at 2% the phenomenon occurs and the temperature goes on red.

 

With the curve starting at 0, the temperature at start-up remains at the minimum of green (~400°)

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Hammer1-1 is hot starting her.

 

(...)

The manual says avoid hot starts.

check your throttle is going to zero.

 

Yes, hot starts are pretty bad for the engine, they can really ruin it. I remember from the Bell 206 start-up procedure you are supposed to bring turbine RPM to something around 20% with the starter before even moving the throttle / injecting fuel at all. I was wondering why this isn't the case with the 205/UH-1.

 

 

 

 

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Hmm.. I wonder if that's what I'm doing.

 

I normally use the mouse to 'crack' the throttle so that it's responsive to my hotas knob, and then turn it all the way 'off', then press and hold the starter, and once the engine is self-sustaining I then increase the knob.

 

I wonder if by 'cracking' it to start with is putting too much fuel in the system?

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So when I start a real huey I open the throttle all the way, take it down to the idle stop, push the stop release and close the throttle, open it back to full, back to idle stop, push the stop release and turn it just enough towards closed to get it off the cam so I cam close it if I lose power. Fuel on, start fuel on, hold the starter, start fuel off at 25% n1 or 400 egt (thats for a 205 with the -17A) off the starter at 40-45% n1 depending on who you work for. The dcs huey doesn't have start fuel and its a different engine so limitations are a little different. But what you described above should start it fine. Are you waiting to increase throttle until the everything has stabilized at idle?

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So when I start a real huey I open the throttle all the way, take it down to the idle stop, push the stop release and close the throttle, open it back to full, back to idle stop, push the stop release and turn it just enough towards closed to get it off the cam so I cam close it if I lose power. Fuel on, start fuel on, hold the starter, start fuel off at 25% n1 or 400 egt (thats for a 205 with the -17A) off the starter at 40-45% n1 depending on who you work for. The dcs huey doesn't have start fuel and its a different engine so limitations are a little different. But what you described above should start it fine. Are you waiting to increase throttle until the everything has stabilized at idle?

 

What I'm doing is opening the throttle until it's back to idle stop (at the stop release point) - I'm guessing that it's too much getting in there then, and I might be hot starting it still. I'll try leaving it completely off and see if it's any different the next time I fly as it's most likely me doing the wrong start-up procedure. Thanks for the tip. :thumbup:

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So from my understanding the helicopter was made to be started at the idle stop. They have you start it just on the other side of idle because the idle stop release is electricly actuated, so if you lost electricity you wouldn't be able to shut off fuel and you'd burn up the engine. And I'm pretty sure I've done lazy starts in dcs with the throttle sitting on the idle release so that shouldn't be the problem.

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So from my understanding the helicopter was made to be started at the idle stop. They have you start it just on the other side of idle because the idle stop release is electricly actuated, so if you lost electricity you wouldn't be able to shut off fuel and you'd burn up the engine. And I'm pretty sure I've done lazy starts in dcs with the throttle sitting on the idle release so that shouldn't be the problem.

 

Hmm.. will give me something to play around with a little more then. Interesing.

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So from my understanding the helicopter was made to be started at the idle stop. They have you start it just on the other side of idle because the idle stop release is electricly actuated, so if you lost electricity you wouldn't be able to shut off fuel and you'd burn up the engine. And I'm pretty sure I've done lazy starts in dcs with the throttle sitting on the idle release so that shouldn't be the problem.

 

I always wondered why starts were done with the throttle just on the safe side of the stop. I assumed it was just so it could be quickly shut off on reflex without worrying about the stop button if the start had a problem. I had no idea that the stop button was so mechanically complex that it wasn't just a physical catch that prevented throttle rotation and could prevent a shutdown entirely.

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