reported Incorrect N2 indication in autorotation

[HuggyBear]

At the moment in autorotation the N2 maintains around the same position as the NR.

 

In autorotation with the N1 at idle, the N2 should decay to around 5500.

 

Not sure what the N2 will decay to with N1 near zero as I've never shut the engine completely off for an autorotation. But I am assuming it will continue falling towards zero.

 

NR will be equal to or greater than N2 depending on whether the blades are in autorotation.

 

The NR should never be less than N2.

 

- Bear

Edited October 6, 2014 by HuggyBear
further info

[HuggyBear]

The UH-1H is constantly being improved and updated, but I'd still love to see this resolved.

 

Bumping to keep it in view.

 

- Bear

[ruprecht]

deleted, turns out I was doing it wrong.

 

Hi Bear!

Edited February 21, 2015 by ruprecht

[HuggyBear]

Still broken in .15

 

You win BST, I give up, I won't bump this again.

 

I don't care any more.

 

- Bear

[Ramsay]

I still care, so here is a bump for what I consider is an important unaddressed issue.

 

Any word on this, in DCS we pretty much fly to instruments to get a "feel" for what the engine/aircraft is doing.

 

When following real life proceedures the split between the N2 and NR needles is an important indication of successfully entering autorotation.

This and EGT engine failure are the main things I'd like to see on a BST "to do list" for the Huey and would have thought simulating a "sprague clutch" between the power turbine (N2) and rotors (NR) would have been one of the easier things.

 

Go on BST, find the time to make one of the best modules - into a "as close to the 'real thing' as you can get" module.

 

Originally Posted by Bearfoot

 

I love the Huey. I am neither a former Huey pilot nor even been in a helicopter in RL. But I love flying (or rather learning to try to fly and then crashing) the DCS Huey.

 

Out of curiosity, what are the glaring errors that drive your crazy?

The 'worst' one off the top of my head is the N2 response in autorotation.

 

IRL The N1 (Gas Producer) section of the engine drives the N2 (Power Turbine) section of the engine, this in turn drives the NR (Rotors).

 

When the N1 is reduced to idle or off as in an engine failure, there is nothing driving the N2, therefore nothing driving the NR.

 

When you enter autorotation the air coming up through the main rotor as you descend drives the NR and should keep it somewhere around 6600-6900 depending on your collective position and how well the engineers and maintenance pilots set the rotor head.

 

With the N1 at idle the N2 should settle somewhere around 5400 IIRC due to idle airflow passing through the engine slowly turning the N2 turbine blades.

 

In the DCS UH-1H the N2 incorrectly stays 'pinned' to the NR, rising and falling as you raise or lower the collective in autorotation as if there was some connection between NR and N2. IRL a 'sprague' clutch allows the N2 to drive the NR but not the other way around. Kind of like a bicycle wheel, you can pedal to drive the wheels, but if you stop the wheels will not drive the pedals (except for a really crappy bike I had when I was a kid).

 

In real life a primary indication of successfully entering autorotation is the split between the N2 and NR needles, indicating that the airflow is now driving the NR faster than the N2 (engine).

 

The DCS UH-1H still autorotates fairly well, certainly much better than any other sim I've tried, with X-Plane a close second, FSX/P3D is just horrible.

 

This is why I say a normal user probably wouldn't mind. If you keep your eyes outside looking for a landing site and just keep your NR nice and high, an autorotation is going to be pretty close to the real thing. My problem is that I was trained to also check the engine instruments to maybe identify the problem and possibly recover the engine or confirm that no recovery is possible and then secure the engine.

 

- Bear

 

Source: http://www.bmsforum.org/forum/showthread.php?21876-DCS&p=368161&viewfull=1#post368161

Edited March 7, 2016 by Ramsay

[msalama]

+1

 

I care too.

[Aginor]

Me too.

[flying rabbit]

I have checked it on 2.01 and the needles do split.

[Flamin_Squirrel]

Strange, I tried 1.5 and 2.0 at the weekend (both up to date) and the engine/rotor RPMs still seem tied together in both.

 

Very annoying; this was working so I hope they can fix it soon.

[Ramsay]

The needles split in 1.5.3 if you shut down the engine during autorotation - i.e. Main Fuel Switch

 

So it seems the "Sprague clutch" is modelled but engine idle airflow turns the N2 turbine blades to quickly to use the needle split during powered practice landings.

[Flamin_Squirrel]

Ah good catch.

[Ramsay]

Oh and P.S. Are we getting a clutch?

 

If by clutch you mean needle split within autorotation, I think we should address it in a separate thread. There is some nuance in clutch modeling which our devs can't find, extented documentation on that matter would help as well.

 

I can create a new thread if it's absolutely neccessary, but we already have several on the subject of the missing N2 / Rotor split.

 

I don't really understand the nuance in the clutch modelling that would cause an issue, perhaps you meant the 'Power Turbine' ?

 

This is my understanding of the UH-1H transmission

 

 

It seems the gas flow from the 17,000 rpm gas producer causes the power turbine to spin @ 22,500 rpm in DCS, rather than 17,300 rpm (N2 pilot feedback of 5400rpm * 3.2 =17,300 rpm) that RL pilot feedback suggests.

 

Note: The N1 Compressor Turbine and N2 Power Turbine blades/stators are in series, so it is not surprising they have similar rpm.

 

Refs:

http://www.aviation-history.com/engines/t53.html

TM 55-1520-220-23-1, Aviation_Unit_and_Intermediate_Maintenance Volume 1

...

etc.

 

If you need a particular ref. for something I've generalised on I'll try to give it but I doubt I've anything BST don't already have access to.

Edited December 6, 2017 by Ramsay
Update diagram with better RL figures, principle remains the same.

[Ramsay]

Not sure if needed, but I'll add this for completness -

 

The T53-L-13B appears to produce very little power below N1 = 80%, which probably explains why the N2 power turbine is more or less turning in sync with the N1 compressor turbine at flight idle.

 

 

Ref: FUEL CONSERVATION EVALUATION, UH-1H Flight Tests, ADA125667

 

Edit:

 

Maximum Idle Power Output, TM 55-2840-229-23-1

• Flight Idle, (N1=63-68%) = 150 shp

• Ground Idle (N1=48-52%) = 40 shp

 

Overspeed governor is driven at a speed proportional to N2 speed. It biases main metering valve opening to maintain a constant selected power output shaft rpm.

Edited December 12, 2017 by Ramsay
Add max idle power from engine ground test

[Chic]

The Sprag clutch (Free Wheel in diagram) disengages the Main Rotor from turbine power drive allowing the Rotor to maintain RPM as the turbine drive input deteriorates.

That's what allows the "needle split".

Edited December 6, 2017 by Chic

[Ramsay]

The Sprag clutch (Free Wheel in diagram) disengages the Main Rotor from turbine power drive allowing the Rotor to maintain RPM as the turbine drive input deteriorates.

That's what allows the "needle split".

I'm aware of that, however in DCS, the Sprag clutch only disengages when the engine is shutdown, either by cutting the fuel supply or through engine damage.

 

Apparently there's some nuance to modelling the Sprag clutch and BST have asked for any extended documentation, unfortunately I can only guess as to what info BST might be missing, as it's been covered pretty extensively already.

 

... There is some nuance in clutch modeling which our devs can't find, extented documentation on that matter would help as well.

Edited December 6, 2017 by Ramsay

[Chic]

Yah, didn't see your post#10. Thanks.

[Ramsay]

The best I've been able to come up with, as to why N2 would turn at 5400 rpm at flight idle (N1=68%) when off loaded and NOT overspeed to match the main rotor rpm is - the fuel control, overspeed governor and boost pump maintain a set idle N2 rpm in the same way they do for higher N2 speeds, i.e. a pilot can adjust N2=6600 rpm to 6300 or 6000 rpm and the fuel controller will increase or decrease fuel supply to maintain the desired rpm (avoiding overspeed, etc).

 

Overspeed governor is driven at a speed proportional to N2 speed. It biases main metering valve opening to maintain a constant selected power output shaft rpm.

 

Unfortunately the only governor settings I've found so far, relate to high N2 overspeed or physical wear, not flight idle.

 

T53-L-13B Engine Limits

Edited December 12, 2017 by Ramsay

[Ramsay]

There is a small (10 rpm) NR/NP needle split recorded in A182369, however it is not the large split I'd expect with throttle idle. It is in the N2=6000-6700 (NR=300-335) range the overspeed governor has authority in. No information on the throttle position is shown.

 

It looks like the UH-1H was being tested with rotor NR=300rpm and the power turbine NP (aka N2) in step at the time of the compressor stall.

 

When the compressor stall occurred:

 

• the collective was dropped (1)

• this allowed NR/NP to increase (2) i.e. N2

• in response (to maintain the set N2 rpm) the overspeed governor reduced the gas producer NG (aka N1) rpm (3)

• when NP reached it's set rpm, it stabilised at 314 rpm (N2=6280 rpm?)

• NR continued to increase to 324 rpm

• and so the needles split.

 

Although it's possible the throttle setting was altered during the incident, it's more likely it wasn't included because it took no part.

 

 

I hope this helps explain how the overspeed governor maintains N2 when offloaded (even if I can't show the overspeed governor is working to maintain 5400 rpm at idle).

Edited December 13, 2017 by Ramsay

[Gunnars Driver]

There is a small (10 rpm) NR/NP needle split recorded in A182369, however it is not the large split I'd expect with throttle idle. It is in the N2=6000-6700 (NR=300-335) range the overspeed governor has authority in. No information on the throttle position is shown.

 

It looks like the UH-1H was being tested with rotor NR=300rpm and the power turbine NP (aka N2) in step at the time of the compressor stall.

 

When the compressor stall occurred:

 

• the collective was dropped (1)

• this allowed NR/NP to increase (2) i.e. N2

• in response (to maintain the set N2 rpm) the overspeed governor reduced the gas producer NG (aka N1) rpm (3)

• when NP reached it's set rpm, it stabilised at 314 rpm (N2=6280 rpm?)

• NR continued to increase to 324 rpm

• and so the needles split.

 

Although it's possible the throttle setting was altered during the incident, it's more likely it wasn't included because it took no part.

 

 

I hope this helps explain how the overspeed governor maintains N2 when offloaded (even if I can't show the overspeed governor is working to maintain 5400 rpm at idle).

 

 

In this chart the throttle was in flight( called 'flight idle' in some systems, dont know for the UH-1). Im pretty sure the throttle was'nt touched during this event. First thing is to lower collective, and not close the throttle. In this case it might be possible to regain normal flight by increasing the collective again. As soon as the collective was lovered the surge disappeared.

 

 

The small needle split is due to the overspeed governor is keeping the N2 at the beeped N2 speed, when this is reached after the compressor stall, the N2 stays on spot as it should. The needle split difference will depend on the collective( = rotor speed) and the governor commanded rpm by beeping the gov increase or decrease.

 

 

At idle( not flight idle) the governor is not active. The idle on all turbine engines I know is by regulating the N1/ gas generator. The N2/NR isnt governored and can vary a bit depending on transmission temp and air pressure/temperature. In most cases, it would be around the 60-70% mark or somewhere around 200rpm.

 

 

The reason not to regulate the rotor or N2 rpm is that it is very important to keep the N1 above the minimum limit where it can self sustain. If N1 comes under a limit, the engine cannot continue to run without the starter engine. The idle is therefore set by N1 speed.

 

 

In a training full down autorotation the throttle would be put to idle and in this case the N2 meter would fall back to the 200rpm mark or something like that, maybe a bit higher because the idle is regulated on N1 and the rotor is not driven by the engine, causing the free turbine to get a bit higher speed because no drag from transmission and rotor system.

 

 

I havent tried to reduce the throttle in the DCS Huey(yet) , all autos was performed by shutting the fuel valve of, killing the engine.

[theinmigrant]

Still an issue as of today