System failure w/ replay

[Echo38]

Okay, so, being terribly out-of-practice, I decided to do some touch&goes in the P-51D. Random system failures was on in the mission options, but I didn't specifically set any up, so the probability for everything was, I believe, at the default 0% for all systems.

 

I thought I'd done everything right; warm-up at (sim) manual-recommended RPM to the recommended temps, levers in the right places, kept it from over-cooling on landing, etc. However, after my ~second touch&go, I noticed that my amp meter was showing nearly zero. The manual says 50 is normal, but I can never seem to get it above 30 or so (why is that, anyway?), and this time it was somewhere around 10. I returned to the airfield & landed with no issues beyond the low voltage figure.

 

The only thing I can think of that I did wrong (well, other than wobbling all over the place) was that I forgot to retract my gear on one of the takeoffs, so I was flying around with my gear down for a while, but I never went over 200 MPH while doing so (IIRC), and besides that, I can't imagine that stressing the gear would affect the electrical system.

 

It's a long and relatively boring replay, ~40 min. of warmup and touch&goes, but if anyone's bored/curious enough, here it is. Any ideas on what caused the amp meter to display such a low figure?

amps too low I think.trk

Edited May 4, 2015 by Echo38

[Echo38]

Mmm--just remembered--while following the (sim) manual's instructions to the letter, the oil pressure exceeds the redline for a significant amount of time. I'm in a new mission, doing another cold-start; this time, I'm gradually increasing the RPM up to the 1200 RPM instead of going there immediately after startup (the manual doesn't say gradually, but only by doing it gradually am I able to prevent oil pressure from going higher than the redline).

 

I don't know enough about engines to know if overdoing the oil pressure can screw with the generator (or whatever it is that the amp meter is keeping tabs on), but I'm going to keep a close eye on my amps this flight.

[Echo38]

Hum. Just happened again. This time, I was keeping an eye on it; it happened gradually, about 10 amps lost per touch&go on avg. Perhaps my relatively-long periods of idle power, on my approaches, are causing this? I'm trying to not over-cool the engine, but perhaps the generator doesn't like the engine being at idle for long periods of time, even when the temp's good. (Makes sense that a generator "cares" more about engine RPM than engine temp.)

 

The other idea is that my fairly-rapid temperature changes caused by the touch&goes (cooling on approach, heating upon takeoff) may be screwing something up. I don't know how quick the temp change has to be in order to cause damage to anything, but this does seem awfully rapid to me.

 

Edit: adding the track for this flight, too. Oh, the mystery! The excitement! ... Okay, not really. This is pretty much the same as the last track, except that the takeoffs aren't quite as bad and I'm trying more "interesting" airfield approaches. (Still not comfortable enough with it to try my old "flat spin approach," though.) The amp-meter goes down notably during (or around) the middle two out of the four landings. IIRC, the second landing (the first one on which the meter went down) was the one with the engine at idle for the longest period of time. This idle time is my current suspect.

amps again grr.trk

Edited May 4, 2015 by Echo38

[Echo38]

Well, I think I've narrowed it down to two variables. Eliminating both has caused the issue to stop presenting itself. The two variables are exceeding oil pressure redline immediately after startup, and long periods of idle power upon approach. So far, so good, although I never have seen my amp meter above 40.

[Echo38]

I flew some more today. Same deal--whenever doing touch&goes, I run into this problem if I throttle back for more than the shortest of times--that is, if I throttle back a few seconds before I cross the runway threshold, I'm okay, but if I throttle back any earlier than those few seconds, I run into this problem within a few such landings.

 

It sounds like the bug described here (http://forums.eagle.ru/showthread.php?t=129159), except that in this case, the electrical instruments aren't affected, and the bug (if bug it is, in this case) is still present in the current version of the sim.

 

I'm really thinking that it's a bug with the battery not being charged properly, and I suspected this before I saw that thread on the FW 190's similar bug. The battery seems to get drained at low power settings, but not recharged at high power settings.

[Cpt.Rookie]

I did few tests and I can confirm that it seems that letting the RPM drop under 1500 (inflight) for some time causes that ammeter is later showing incorrect value after RPMs are again high enough to start generator working (>1500 RPM).

 

However, battery is definitely re-charging once RPMs are high enough. I turned on all the power-draining systems (position lights, landing light, gun heating, pitot heating, radio), disconnected the generator and waited several minutes until battery was drained (green gear light is quite nice indicator of battery level as it becomes more dim if the electricity is being taken from the battery) and then re-connected it. After tens of minutes, battery was charged again. It is question whether rate of charge is somehow affected by this (i.e. whether it is too low), but I don't have any info about how much time it takes battery to be fully charged.

 

So even if there is probably something fishy between generator, ammeter and battery, it doesn't seem to have any significant effect.

 

Ad your question about the initial ammeter values - I was watching some of your tracks and while not sure about lights and radio, you're definitely not turning on pitot and gun heaters. These two systems are eating quite a lot of power (just watch the ammeter jump when turning them on).

So if you turn on all the systems mentioned above, the initial value should be somewhat close to 50 A.

[Echo38]

So, it looks like the problem is that I don't understand what the ammeter is or how it works. : D

 

During one of my cross-country flights, I turned on the pitot heat & gun heaters (I generally don't need 'em for touch & goes, but if I'm going high and/or fast for long periods of time, I figure they could use some heat). The ammeter immediately shot up to the value it's supposed to be at. (You were right, Rookie.)

 

So, apparently, the ammeter measures electrical power draw rather than generator output or battery charge. Is that it? Maybe I should RTFM again. (I've read it at least twice in completion, but I am a Bear of Very Little Brain and so my eyes tend to glaze when it comes to things like electrical currents.)

Edited May 27, 2015 by Echo38

[Echo38]

Hmm, no, wrong again. The DCS: P-51D manual says:

 

"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."

 

Trying to reconcile this explanation with what I'm seeing in the sim--so, then, it measures generator output, but this is dependent on the draw from the rest of the electrical system. Did I finally get that right?

[msalama]

the amount of current being supplied by the generator

But the generator only supplies a residual amount of current if there're no major consumers, right? It will of course continually supply said tiny amount when the circuit is live, because there's always something drawing current there, i.e. the resistance of the circuit is never infinite. Still, no current flows if there's nothing drawing it ;)

 

EDIT: Ach, sorry, kind of overlooked the rest of your post. Yes, you got that right!

[iantron]

Keep in mind that your battery will be a bit drained from the engine startup procedure. While the generator is charging it up again the current meter will read positive. If there are no other electrical systems using power (turned on) then it may be normal for the generator supply current to approach zero as the battery finishes charging. I'm not sure if there are any electrical systems that are always on, but the engine shouldn't need electrical power after startup.