APU DC generator power bug

[Strekaya]

There seems to be a situation regarding the APU failing which seems to be tied to whether it is being used as a standby DC power generator.

 

In 1.2.11 and prior when running the APU, power could be seen on the voltmeter when the DC selector knob was set to the standby DC generator section, even before flipping the standby DC gen switch.

 

In the most recent 1.2.12.34848 beta, there is a different behavior where sometimes flipping the standby gen switch once the APU is on causes the voltimeter to come up and show power, and likewise on the accessory (AKK) bus if the equipment test switch is on. Also, the battery voltage sometimes doesn't drop after turning on the APU (it stays at 24 VDC instead of dropping to 20-21 in prior versions).

 

What seems to happen for reasons I can't figure out is that at some point, the APU DC generator suddenly 'gives out'. The newly functional AI-9U ammeter will drop to zero, along with the DC generator voltage. If it happens during engine startup, the APU still functions to crank the engines. (Of course, I do disengage the DC generator before cranking the engines, but this has happened with using only the APU DC generator by itself, no attempt to start engines.)

 

However, after this event happens where the power drops out, the APU can't be restarted once stopped. The APU EGT never seems to completely zero out. In prior versions, I can start and stop the APU on the ground or in the air, as needed, certainly useful maintaining system power in engine out scenarios.

 

It is nice to see the AI-9V ammeter working; the only thing I noticed regarding that was that engaging the minimal circuit breakers to get the APU running and switching the DC generator, it was showing a roughly 50 amp pull, but once I turned on the minimum CBs to get the engines started, it was close to 100 amps. After a certain amount of time at that level, the power just cuts out. When I get a chance, I'll try other things like turning on spotlights and see if it cuts out quicker.

 

There was a reply in the main forum regarding this being fixed soon, but thought I'd mention it.

Edited December 4, 2014 by Strekaya

[Mr. White]

See here as well ...

http://forums.eagle.ru/showthread.php?t=135337

 

Is Belsimtek generally taking notes of those bug reports here anyway? Or is there a bugtracker somewhere else?

[Volator]

On 12/3/2014 at 6:53 AM, Strekaya said:

In 1.2.11 and prior when running the APU, power could be seen on the voltmeter when the DC selector knob was set to the standby DC generator section, even before flipping the standby DC gen switch.

This is still the current behaviour of the APU generator, and I was wondering if this is correct like that? I assumed that the voltage indicator for the stand-by generator should only show voltage if the stand-by generator is activated?

@AeriaGloria @admiki You are sort of an authority on Soviet helicopters for me, do you know anything about this? 

[admiki]

Thanks for the accolades.

Unfortunately, I was never able to get my hands on electrics schematic for the Hip/Hind, so anything I say would be a guess at best. Manual/s I have don't delve into systems themselves, apart from operational standpoint.

Maybe AG can help you out.

 

Edit: look in my next post about what I think is correct behavior.

Edited May 28, 2024 by admiki

[K-dot-B]

@Volator @admiki I stumbled upon some technical document that has a section on STG-3, the generator used in the APU, that says it is a "DC generator with shunt excitation". It also has a cross section of it showing that it is essentially a simple brushed DC motor.

Since it is permanently attached to the APU turbine shaft (with "gear ratio 4" apparently) it is generating voltage as long as the APU is spinning, and so the voltmeter should be able to measure it. In game you can even measure its voltage while the APU is spinning up, which I found pretty cool.

This was in chapter 13. of this document (I just translated it with google): GTD-350 Описание двигателя

Edited May 28, 2024 by K-dot-B

[admiki]

It makes sense that generator is constantly producing power with shunt excitation.

It's important to note that 30 min generator limit is defined by APU itself (reasons are not stated), not the generator assembly.

According to Mi-35 manual, which should be the same for Mi-8/24 in this regard, DC voltmeter should indicate voltage only with APU GEN switch in ON position.

Problem is that I can't get any voltage to show, but I do get ammeter to show load with switch ON.

Edited May 28, 2024 by admiki

[AeriaGloria]

On 5/28/2024 at 1:40 PM, K-dot-B said:

@Volator @admiki I stumbled upon some technical document that has a section on STG-3, the generator used in the APU, that says it is a "DC generator with shunt excitation". It also has a cross section of it showing that it is essentially a simple brushed DC motor.

Since it is permanently attached to the APU turbine shaft (with "gear ratio 4" apparently) it is generating voltage as long as the APU is spinning, and so the voltmeter should be able to measure it. In game you can even measure its voltage while the APU is spinning up, which I found pretty cool.

This was in chapter 13. of this document (I just translated it with google): GTD-350 Описание двигателя

 

You just saved me doing exactly what you did.  Bravo! 

[Flappie]

Correct as-is then?

[AlphaOneSix]

Once I actually get my setup working I should probably check on this.

[Raffi75]

I played around with the voltages a bit.

Mi-8: nothing is on, battery knob I and II, indicate 24V. I start AI-9W, voltages checked, momentary jump on the ammeter to 300A. AI-9W is working, I turn on STG-3, voltage increases to 27(28)V, STG-3 -100A, batteries - 0A. STG-3 off. procedure for starting engines, switching on generators and rectifiers. After switching on rectifiers, the STG-3 ammeter reading drops to 0A and from that moment on there are no indications. AI-9W off, STG-3 generator does not work, does not supply voltage. Checked.

Mi-24: battery knob, no indications. I turn on L. battery, R. battery, battery knob - 26V. This is the first difference between Mi-8 and Mi-24. Then everything looks similar, until the rectifiers are switched on, where the STG-3 readings show 80A and only after switching off the generator do they drop to 0A. The rest is similar to MI-8. Checked.

To my knowledge, this generally looks like correct operation.

Edited July 28, 2024 by Raffi75

[Raffi75]

The instructions for the AI-9W state:

„The uninterrupted operating time of the engine in the generator mode /power supply of the on-board electrical installation with direct current of no more than 3kW/ should be ...... no more than 30 min, after which the engine must be switched off and cooled down in no less than 15 min."
   It follows from this that the engine's operating limit is the temperature it reaches during operation.

I also found an interesting entry that will have to be checked in the DCS.

„Maximum operating altitude of the engine in the air and on the ground /starting the AI-9W engine, receiving air needed to start the helicopter's main engines or work in the generator mode/...... 4000 m."

 

 

[Raffi75]

I am pleasantly surprised. 6500m AI-9W cannot be started, 5000m the same. Only at 4200m I could start the engine.