Engine start and fuel pressure

[Piper_EEWL]

Hi guys,

After the 4-7-16 update I took my civilian Mustang out for a flight to test the new landing sound effects. Pretty nice I have to say. Thanks for that.

I just noticed two things:

1. The engine seems to require a lot less priming for start up. So I tend to flood it now. I guess I'll have to get used to the new dynamic there.

2. On rollout after the landing (throttle at idle) the fuel pressure dropped very rapidly to zero but the engine kept running. Once I advanced the throttle a bit the fuel pressure came back up into the green. Retard throttle to idle and fuel pressure dropped again to zero. Is that correct? Shouldn't the engine stall with the fuel pressure at zero? Also the shift from normal pressure to zero and back seems very abrupt to me. Maybe there's a bug?

Thanks for the update!

[LZ-WIL]

Hi Piper_EEWL,

Yes on both questions, I confirm they have been that way since the T6 came out with it's first update. I find the new features of the P51 - amazing feels much more real now. Since 4-7-16 (I just made one touch and go and a full stop landing) I find it bit more difficult to handle on take off and landing, it started to resemble it's sibling taildragger T6. (I almost wasted myself on initial take off roll after today's update) After several hours in T6, I started to feel P51, very easy to land (very stable once on the ground), but now s.o.b. wants to ground loop, if not handled properly like the T6.

On second question, have you tried to land with your fuel booster "ON"? I always take off and land with the fuel booster "ON" and have also noticed, that the fuel pressure drops more than before. I don't know, it might be a bug, Scott and team will say and Mr. Henriques may give us a heads up as well.

P.S. I find it very exciting with it's new features and mind split which one I want to fly more - the Mustang or the Texan


Cheerz,
Will

[Piper_EEWL]

Hi Will,

Thanks for the reply. And yes you're right the Mustang is a lot more unstable on roll out now. I noticed when I did my landings and had problems keeping it in a straight line It's very nice that the ground physics and sounds of the T-6 made it into the other taildraggers. Thank you Scott and everyone at A2A for that.

Regarding the fuel pressure issue. No I haven't tried to takeoff and land with the fuel boost pump on. I've never done that before in the 65 or so hours I have on the Mustang. But I've also never noticed the fuel pressure drop to zero before the update (unless I shut the engine down or there was no fuel in the tank of course). For me it doesn't make sense that the fuel pressure drops to zero but the engine continues to run fine. And once I add a bit of throttle the fuel pressure "jumps" back up to its normal value (about 18psi if I'm not mistaken?).

I did try to replicate the issue after I was parked on the ramp but there the fuel pressure was very stable and didn't crop even after idling the engine for a prolonged amount of time.

Take care

[Piper_EEWL]

Ok guys. Got some more P-51 time in today. And I can reproduce the issue. On rollout with the throttle idle the fuel pressure suddenly drops to zero. The engine continues running. If I turn the fuel boost pump on the fuel pressure comes back up. The engine itself doesn't seem to be bothered by the fuel pressure (or by what the gauge is shwoing). The interesting thing is that the issue only seems to occur when the aircraft is moving. When I'm parked it doesn't happen!?

Everything fine in the maintenance hangar. Does anyone else have this?

[Lewis - A2A]

Hey Piper i'll pass it along to Scott

thanks,
Lewis

[Jacques]

Sebastian, I think you're supposed to have the booster pump on from before startup to just before shut down in this model. I may stand to be corrected, but I think that is how the manual is written! Fuel pressure showing zero does seem a bit dodgy, though, since there may be an engine driven pump as well?

[Piper_EEWL]

Hey Piper i'll pass it along to Scott

Thanks Lewis I appreciate you guys looking into this! It's not a big deal since it doesn't really affect the performance of the Mustang. I thought I should report it because we just got so sensitive to the instruments with Accusim that it is disturbing to see zero fuel pressure

Sebastian, I think you're supposed to have the booster pump on from before startup to just before shut down in this model. I may stand to be corrected, but I think that is how the manual is written! Fuel pressure showing zero does seem a bit dodgy, though, since there may be an engine driven pump as well?

Hi Jacques. You may be right that the fuel boost pump is supposed to stay on all the time. If I remember correctly the check lists are not specific about this. And if the engine would stall without the fuel boost pump on and the fuel pressure dropping I would think its Accusim at work. But the fuel pressure dropping to zero and the engine still running smoothly I feel like it's a bug!?

We'll see what Scott has to say hopefully!

Thanks guys!

[Tomas Linnet]

This is the way I do it:-) not saying this is the correct way

Engine start
Left tank, booster on.
Need to draw fuel from left tank if it's full, booster on as it's needed to prime the mighty Merlin

Warm up
Left tank, booster off.
To save power from the battery

Taxi
Right tank, booster off
To save power from the battery and to check if the engine will draw from right tank.

Run up
Fuselage tank(if used), booster off
To check if the engine driven pump will deliver fuel at a higher rate

Take off
Left tank, booster on
To help if one pump fails at a low altitude.

Climb
After reaching a safe altitude I switch the booster off, should the engine driven pump fail, I'll know right away and I'll divert immediately. if the booster is always on, there no way of knowing if the engine driven pump is operational.

Landing
Fullest tank, booster on.

After landing/ Taxi
Booster off

I agree the check list is not very specific on whether the booster pump should be on or off during flight, but in the before landing checklist the booster pump should be switched to "on". no word about switching it off though....

If I go to 500 RPM, the pressure will drop to 0 for a few seconds, and as I hit 600 it picks up and return to normal. Maybe it's related to the slower fuel pressure? still seem wrong that the pressure drops to 0, but maybe the pump needs a certain amount of revs before is delivering the pressure?

[Piper_EEWL]

If I go to 500 RPM, the pressure will drop to 0 for a few seconds, and as I hit 600 it picks up and return to normal. Maybe it's related to the slower fuel pressure? still seem wrong that the pressure drops to 0, but maybe the pump needs a certain amount of revs before is delivering the pressure?

Hi Tomas,
First I had the same thought. Bit if it would be an Accusim failure or behavior I would expect that the engine show a reaction. Like a stutter or that it stalls. Because no fuel pressure means no power!? Like the T-6 did with the fuel filter clogged. See here.

Maybe we'll hear back from Scott since Lewis passed it on to him.

Happy flying

[Tomas Linnet]

i read that and forgot it you gotta respect what accu-sim bring armchair pilots like myself

[Jacques]

Hi Tomas and Sebastian,

Curiousity got the better of me this afternoon. First, let me say I don't thnk the manual is ambiguous at all: turn on the booster pump before starting and turn it off while shutting down.

I flew around a bit to get the engine warm. I had turned off the booster pump after I got the engine started. Once I landed, I retarded the throttle until the engine was at 500 RPM and as you two noted the fuel pressure dropped but the engine continued to run. At 500 RPM the engine doesn't sound too happy anyway. I moved the RPM up to 600 and the pressure slowly climbed back up to normal. Finally, I brought it back down to 500 RPM and left it there while I listened to the prop. After about a minute, the engine finally died.

So perhaps the pressure drop to zero is really a drop to zero and you're just burning what the engine can scavange from the lines?

Regardless, I felt really bad subjecting the Mustang so such a low idle!

[Tomas Linnet]

Before Landing(page 60)
• Mixture RUN.
• Oil and coolant shutters AUTOMATIC.
• Fuel selector to fullest tank. NOTE: Never land on droppable wing tanks. Good idea, I use the landing gear, the drop tanks tends to leak after very few landings.. .
• Booster pump switch to ON. I read this as it was off during flight

I didn't like the low idle either

didn't DH comment on this somewhere???

[Piper_EEWL]

• Booster pump switch to ON. I read this as it was off during flight

I'd have to re-read the manual to make a statement on the use of fuel boost pump. And I don't really see that happening. I still haven't read the complete T-6 manual

I flew around a bit to get the engine warm. I had turned off the booster pump after I got the engine started. Once I landed, I retarded the throttle until the engine was at 500 RPM and as you two noted the fuel pressure dropped but the engine continued to run. At 500 RPM the engine doesn't sound too happy anyway. I moved the RPM up to 600 and the pressure slowly climbed back up to normal. Finally, I brought it back down to 500 RPM and left it there while I listened to the prop. After about a minute, the engine finally died.

Well good that you can replicate the issue too Jacques. So it seems like it's the general behavior of the simulation rather than an issue with my installation/system. An yes you're right. The engine doesn't sound happy at that rpm. But since the latest update it doesn't seem to skip as much as it used to when idling low. Or am I just imagining that?

So perhaps the pressure drop to zero is really a drop to zero and you're just burning what the engine can scavange from the lines?

I think it's more likely that the spark plugs fouled after idling that low for a while. At least that's what happened to me yesterday during warm up. If it would be due to fuel starvation I would suspect it to die as quickly as if I cut the mixture or the fuel shutoff valve?!!

NOTE: Never land on droppable wing tanks. Good idea, I use the landing gear, the drop tanks tends to leak after very few landings.. .

[Jacques]

Yes Tomas, I see your point...I read it as just a check to make sure it is still on!

A couple of manuals I found on line for the P51D mention two variations of switch: an earlier one with three settings OFF, NORMAL and EMERGENCY and a later version that was simply ON or OFF.
When an aircraft was fitted with the later version, the switch was to be kept ON during flight. Which is why I interpret the checklist as I do above!

I don't think in our case as just simmers, there are any consequences for leaving the switch on, or turning it off during flight...but that is pretty interesting behaviour you guys observed at those low RPMs!

[Piper_EEWL]

Hey guys,

did a flight in the Mustang yesterday after installing the 4-16-16 update. The no fuel pressure at idle issue still persists.

Also after the latest updates I find that the engine seems to run a lot smoother. It seems impossible to stall it due to fouled spark plugs. It can idle forever and ever without running rough. And then when advancing the throttle it just speeds up fine. No coughing no sputtering, no hesitating like it used to do before the updates that were published after the T-6 release. I have to say I miss that in the P-51. I don't have any real world experience with the Mustang (unfortunately) but it feels a little to sterile, to smooth now. Does anyone else experience that too?

Thanks