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Autorrotation - Freewheeling unit


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I have been doing some testing with autorrotation behaviour. As you can see at the video, the freewheeling unit uncoupling is not properly simulated if you just move the throttle to the idle position, (as required for a recovered autorrotation) and the rotor and engine needles (N2) move together up and down except on the final touchdown.

 

At idle, engine needle should show around 4000 rpm (68%-72% N1) and rotor rpm can be controlled with the collective control.

 

 

Probably it is not the most important aspect to review in the simulation model, but maybe sometime it is found the time to fix it.

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  • 5 months later...

This has been a problem for a long time now.

 

https://forums.eagle.ru/showthread.php?t=131945

 

I haven't tried recently, but I believe the N2/NR split is modelled (not sure how accurately) with engine off, but with engine on the N2 seems to 'windmill' too high.

 

- Bear

Pacifism is a shifty doctrine under which a man accepts the benefits of the social group without being willing to pay - and claims a halo for his dishonesty.

 

- Robert A. Heinlein

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I'm not sure what you mean.

 

In the real UH-1H:

 

On the ground with the throttle at idle the N2 will be ~4000.

 

In autorotation with the throttle at idle the N2 will be ~5500 (as the NR is no longer 'resting' on the N2 and slowing it down to ~4000).

 

If you were taking about the N1 then it will be at 68-72 with the throttle at idle.

 

Sorry if I have misunderstood your post.

 

- Bear

Pacifism is a shifty doctrine under which a man accepts the benefits of the social group without being willing to pay - and claims a halo for his dishonesty.

 

- Robert A. Heinlein

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  • 1 year later...

 

For parts of the video the instruments are too dark and the resolution is too low to read the gauge markings, but at times during autorotation you can clearly see the needle positions, particularly the N2.

 

- Bear


Edited by HuggyBear

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@Belsimtek: did this get you what you needed? I should be able to grab a video from a 412 simulator tomorrow if you need something clearer

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Hi guys!

 

We are searching for the real life simulated (with working engine) autorotation video in which we can see cockpit avionics.

 

UH-1H, UH-1D, Bell 212 will do.

 

THANK YOU!

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I'm not sure what you mean.

 

In the real UH-1H:

 

On the ground, with the throttle at idle, the N2 will be ~4000.

 

In autorotation with the throttle at idle, the N2 will be ~5500 (as the NR is no longer 'resting' on the N2 and slowing it down to ~4000).

 

If you were talking about the N1, then it will be at 68-72 with the throttle at idle.

 

Sorry if I have misunderstood your post.

 

- Bear

 

 

It is that simple.

 

It`s not only the freewheeling unit that disconnects the engine. In fact, in a turbine driven helicopter, there is under any condition "NO" mechanical connection between the engine N1 and the driveshaft (N2) and the Rotor. The drive Shaft is driven only by fast-moving hot Air, like a windmill. The freewheeling unit disconnects the Gearbox from the Rotor, to eliminate as much friction as possible that may cause the Rotor to slow down.

 

So any change in Power setting leads to a correlating change in N1

Any sudden change (decrees) in power (Governor OFF) strong enough to “activate” the freewheeling unit, leads to a Needle split. Engine RPM N1 decreases instantly followed by N2, while Rotor RPM starts dropping slowly due to the Energeia stored in the Rotor system.

 

So Engine RPM N1 correlates at any time with the Power setting regardless of Rotor RPM.

 

One example:

During Startup when N1 reached 10 to 12 %. The Airflow within the Turbine should be strong enough to drive the drive shaft and moves the rotor as well.

If there is anything wrong, and for what reason ever, more friction than normal within the system N1 always reacts according to the power setting while N2 and of cause Rotor RPM will stay behind “normal values” during startup.

 

Another example:

If the Gearbox blocks, The freewheeling unit allows the Rotor to turn further on, depending on Energeia stored and propper Pilot Reaction (autorotation initiated). N2 drops suddenly to 0 while N1 will not be effected.

 

On the other Hand. If the engine quits, N1 and N2 drop to 0 while the freewheeling unit allows the Rotor to turn further one, dependent on the Energeia stored.

 

 

What means that for simulating autorotation’s in DCS ?

Basically, N1 correlates to the power setting.

When decreasing power, this is basically true for N2 as well. But not when increasing power, as already very good simulated during Startup. N2 has to overcome the drag in the system, friction in gearboxes or bearings, as well as the drag created by the mass of the system and the airflow on the rotor blades, so it always stays behind N1 when Engine power increases.

 

During normal flight conditions, level off from a turn, or when decreasing forward speed, which is something like a smooth flare, Rotor RPM increases due to the aerodynamic driving forces and N2 may follow slightly and for a short period, because there is too much power provided, and the Governor may need a second to react. But this may irrelevant when it comes to the simulation of Needle split during auto.

 

So as always, there are two ways to handle that problem.

 

1. When reducing Power (N1) Reduce N2 as well, and refer with the Rotor RPM in respect to the aerodynamic conditions (autorotation initiated yes or no).

2. Make it very complicated and try to simulate all the effects and combinations of effects described above and we may never get a Needle split in a reasonable time.

 

There is one thing I have to menschen in the end.

Under some circumstances, there is a needle split in the DCS Huey. But I was not able to reproduce that. So I am quite sure, the basics are already set but do not work for any kind of reason.

Always happy landings ;)

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Hi guys!

 

We are searching for the real life simulated (with working engine) autorotation video in which we can see cockpit avionics.

 

UH-1H, UH-1D, Bell 212 will do.

 

The german army posted a video of their autorotation training on youtube in 2013. Unfortunately it's in german, but maybe it still helps:

 

 

The german variant of the UH-1 is named UH-1D, but it actually is a UH-1H, which has been licensed and produced by former german aircraft manufacturer Dornier, hence they got relabeled as UH-1D in german service, although technically they are still UH-1H.

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DCS Panavia Tornado (IDS) really needs to be a thing!

 

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OK, so here is a video I took especially for you guys. It is from a Bell 412 simulator (very similar to a Twin Huey, or B212). Unfortunately I don’t have access to a Huey proper, so this was the best I could do.

 

A couple of things to note here:

 

1. The instrument you are looking for is the triple tach, at the bottom. The 2 long needles are N2 for Engines 1 and 2 (labelled with a 1 and 2), and the little needle is the Rotor RPM (labelled with the R). Obviously the UH-1H only has 1 N2 needle, as it only has one engine. Same concept, times two.

 

2. This is a practice auto, with the throttles rolled to idle. What you’ll see is the rotor moves freely above the N2 needles, which go no higher than around 90% as the engines are at idle. However, because our most efficient rotor RPM is 91%, the pilot is targeting this RRPM, which is very close to the idle N2 and therefore the RRPM is close to the N2. A few times you can see the rotor go higher and the N2 stays lower, and if the RRPM goes below 90%, it “drags” the N2 down with it, as there’s not enough power being produced for the N2 to maintain RPM against the drag of the rotor

 

3. As this was a practice auto, it’s being done to overshoot. On the overshoot, the pilot is rolling the throttles up until the N2 starts to drive the rotor again, then flying it away. The sim instructor introduced a faulty “sprag clutch” (freewheeling unit) on engine 1, so on the overshoot you clearly see the N2 for engine 1 exceed the RRPM. This isn’t a normal indication and was due to another simulated failure.

 

Although it’s on a different airframe, I think it’s the clearest video of the instrument itself and how the N2 and Nr respond to each other. I hope it helps! If you have any more questions about what’s going on in the video, please let me know.

 

 

Edit: for some reason, embedding the video won’t work. Let me know if this link is OK.


Edited by Sandman1330

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Nice Video.

But for me, it makes no sense. There is no real needle split at all.

I would expect N2 drop far more because there is nothing that supplies the driving Part with energy. Engine N1 is at idle, and the rotor with its aerodynamic forces is disconnected du too the freewheeling unit. So from where comes the energy, that keeps N2 that high

 

It's for sure at our real-life UH-1 pilots in the forum to comment on the simulator behavior.

Always happy landings ;)

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It does make a bit of sense.

 

Even at idle, the gas producer section (N1) has to produce quite a bit of power just to remain self sustaining and keep itself spinning (idle at 62% N1). This will also drive the N2 section, which when the sprag clutch is disconnected during an auto, takes next to nothing to turn because it has zero load on it. Remember the N1 and N2 is this type of engine have no physical connection to each other.

 

Unfortunately I don’t have Huey experience, but this is how the 412 behaves and the engines are similar in design (though of course not the same)

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It's quite a while ago. And I flue a lot of pistons powered aircraft like the Bell 47, Robinson Models and Schweizer 300 in which I practice lots of autos, and there is the Needle split huge. But of cause not to compare with a turbine engine.

Can`t remember how the N 2 needle in the Bell 206 or the Hughes 500 behaved during the auto, :( because it was not essential and all the focus lays on the Rotor RPM during autorotation.

 

But these zero load thing of cause makes sense and your absolute right. The same technical principle like in the 412, same behavior.:thumbup:

Always happy landings ;)

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Next time I do an auto in the aircraft, I will pay closer attention to the N2 needles. Like CHPL says, one doesn’t look too closely at them as the focus is on Nr. I do think in the aircraft they may be a bit lower, but I don’t recall ever noticing TBH. This was a certified full motion sim, so it should be very accurate. It might be awhile before I can report back from the actual aircraft though.

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The two videos of the D/H pretty clearly show the N2 stabilising at around 5500, with the NR recovered up around the mid-high 6000s.

 

IK8HWR3.png

 

xovZ35s.png

 

The first video even shows the needle split when the throttle is retarded to idle.

 

Surely this is enough to amend the DCS UH-1H.

 

 

Sandman - to embed, click the 'go advanced' button below the reply box and there should be a 'YouTube' button above the reply box.

 

When you have the YouTube tags just paste the video ID (9tChBUf0YyM for yours) between the tags.

 

 

Is that a Griffon cockpit? Triple Tach and Torque look like a normal 412EP but our old EPs had the IAS and RADALT to the left of the AI and the Triple Tach and Torque were over on the right.

 

Your layout looks much nicer for the left seat.

 

Cheers,

 

- Bear

Pacifism is a shifty doctrine under which a man accepts the benefits of the social group without being willing to pay - and claims a halo for his dishonesty.

 

- Robert A. Heinlein

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Is that a Griffon cockpit? Triple Tach and Torque look like a normal 412EP but our old EPs had the IAS and RADALT to the left of the AI and the Triple Tach and Torque were over on the right.

 

Your layout looks much nicer for the left seat.

 

Cheers,

 

- Bear

 

Thanks! I think the problem I had was I put in the whole link, not just the YouTube ID.

 

This is indeed a Griffon cockpit. The video is taken from the left seat, but the layout is identical for both sides. Here’s an image:

 

1024px-Bell_CH-146_Griffon_%28412CF%29%2C_Canada_-_Air_Force_AN1761958.jpg

 

Good screencaps of those videos, I watched them but couldn’t get a good look at the tach, your pictures show it well. The N2 does appear to be a bit lower than my video, and on reflection as I mentioned above I do think 90% is a bit high for the N2.

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That's a nice layout for the CP there. We're single-pilot so I guess no one cares what the Crewman is looking at. :)

 

Very different cyclic head too, we just had the old-school B8 style.

 

I also wouldn't have a clue where the N2 settled in the 412. As long as the N1 was still running and the NR recovered I was eyes out looking for a place to land. :)

 

Although conceptually similar, the very 'sleek' PT6T-3DF (is this the same for the Griffon?) was a fairly different engine to the big old T53-L13B so I guess it's not too strange that the resting N2 is so different.

 

Cheers,

 

- Bear

Pacifism is a shifty doctrine under which a man accepts the benefits of the social group without being willing to pay - and claims a halo for his dishonesty.

 

- Robert A. Heinlein

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I'm going to ask for a video record of the maneouveer in a 212, since we have no longer the UH-1H in service in our Army. It may take a few days to have it, till this emergency drill is scheduled in the trainning program.

 

If engine fails, motor rpm will drop as it will rotor rpm meanwhile yoy keep collective in high positions, because rotor in high pitch will keep of trying to maintain the chopper in flight, loosing energy.

 

As soon as you pull down the collective, needles must split unless there is a freewheel malfunction. Rotor rpm can be controlled then with collective to keep rotor needle in the green arc. Airspeed affects to this too as it does the weight.

 

In a practice autorrotation one thing that you must check is that the engine didn´t go to a complete stop before trying to recover the maneouver with power.

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That's a nice layout for the CP there. We're single-pilot so I guess no one cares what the Crewman is looking at. :)

 

Yeah, we put our aircraft commander on the left, and the first officer on the right. Reason being, the FO flies the aircraft and the AC runs and coordinates the mission and the rest of the crew (flight engineers and door gunners). With the FO doing most of the flying, it makes sense for us.

 

Very different cyclic head too, we just had the old-school B8 style.

 

We updated them recently, we can now control the searchlight and a bunch of other stuff right from the cyclic. It works nice.

 

I also wouldn't have a clue where the N2 settled in the 412. As long as the N1 was still running and the NR recovered I was eyes out looking for a place to land. :)

 

Agreed! Though our sim shows it being that high, I have a gut feel it's lower in the actual aircraft. I think I would have noticed the N2 sitting right up on the Nr at 91% rotor before if it were that high - but then, I never noticed it in the sim until I was looking for it... Next time I do an auto in the A/C I'll have to look.

 

Although conceptually similar, the very 'sleek' PT6T-3DF (is this the same for the Griffon?) was a fairly different engine to the big old T53-L13B so I guess it's not too strange that the resting N2 is so different.

 

Yeah, we still use the PT6T twin-pak (I think it's a -3D but I'm not sure). It would make sense the actual RPM the N2 sits at would be slightly different, but the behaviour with respect to the Nr (sitting slightly below or matched but never exceeding) would be the same. Indeed, the N2 RPM could and would vary a fair bit depending on density altitude and other environmental factors, as the gas producer section is going to be producing more or less power to remain self sustaining depending on the DA.

 

Also note that we intentionally hold the rotor at 91% during the auto, as it's the most efficient RPM for our aircraft. Does the UH-1H also droop it that low, or hold it closer to 100%? That could also account for the lesser split, if the old Huey SOP is to hold closer to 100% in the auto.


Edited by Sandman1330

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