Ammeter - possible bug or not?

[6S.Duke]

Hello everybody,

Me and my friend have noticed that the ammeter works strange.

The P-51D flight manual tells:

 

AMMETER. The Ammeter gauge indicates the amount of current being supplied by the generator.

The gauge is graduated from 0 to 150 amperes and scaled to 10 amperes. Normal maximum current

is 100 amperes and should only be used for a short period of time. Takeoff should not be performed

if a reading of over 50 amperes is present.

 

When we do the startup procedure, the ammeter remaing at 0 AMPS until 1300 RPM of engine. From about 1350 RPM, the indicator moves a little bit and reaches around 5 AMPS and only for 2000 RPM we have 90 AMPS on the gauge.

I think the generator should recharging the battery and give a positive indication on the ammeter gauge at low RPM (about 1000 RPM). Is this wrong or not? Please, give me an explanation... :)

[sobek]

Is this wrong or not? Please, give me an explanation... :)

 

Yes it is wrong, the voltage that a generator supplies is proportional to its RPM. If it does not supply more voltage than the battery, then it will not charge it. The more voltage it supplies, the more voltage difference between generator and battery, the more charging current it will supply.

[6S.Duke]

Good! Thank you for the explanation! So it isn't a bug, but I was wrong... :)

But another question: during the take off the AMPS indication exceed the 50 AMPS ever... I don't understand why happens that, whereas the flight manual tells that the take off should not be performed if happens that the ammeter shows over 50 AMPS...

Edited May 9, 2012 by 6S.Duke

[flightace37]

To that end, there's also a cutoff to prevent the generator from hooking into the electrical system until it's generating sufficient power. Until then, the ammeter will read zero.

 

Page 43 of the DCS P-51D Flight Manual:

The ammeter indicates how much current is flowing from the generator and also shows whether or not the generator has cut in at 1500-1700 RPM as it should.

[sobek]

Good! Thank you for the explanation! So it isn't a bug, but I was wrong... :)

But another question: during the take off the AMPS indication exceed the 50 AMPS ever... I don't understand why happens that, whereas the flight manual tells that the take off should not be performed if happens that the ammeter shows over 50 AMPS...

 

Yes, the starter motor drains the battery (a lot, it's a big motor), as does running other things off of it. The generator needs to resupply this charge that was previously drained. Also the battery charge may suffer from low temperatures, e.g.

 

To that end, there's also a cutoff to prevent the generator from hooking into the electrical system until it's generating sufficient power. Until then, the ammeter will read zero.

 

Indeed, if it were not for that, it would work as a big resistor that drained the battery.

[6S.Duke]

It's all clear now! Thank you very much! :)

[xxJohnxx]

OT: What is the voltage of the P-51's electrical systems? 24V?

[sobek]

Btw. once the motor temps and failures are implemented, you will have to do a warmup procedure anyway, that should charge the battery back to an acceptable level before T/O.

[Yo-Yo]

OT: What is the voltage of the P-51's electrical systems? 24V?

 

conventional 28V but it presumes 12 cells acid-lead accu or twice to car electrical system.

[xxJohnxx]

conventional 28V but it presumes 12 cells acid-lead accu or twice to car electrical system.

Thank you for that info!