Oil pressure on startup?

[hazzer]

On the p51 and all piston modules on startup the oil pressure shoots up to over the red line even at low power settings (1000-1200rpm). Usually in aircraft the oil pressure starts low and slowly rises to normal and if it doesn't rise in a about 30 seconds you shut down and abort the flight. 

Unsure if all the ww2 aircraft are different from usual aero engines but dcs does not exhibit this. 

[grafspee]

DCS do not model this initial behavior, and skips to point where engine generate max oil pressure for given temp and engine rpm.

If you notice that oil pressure start rising as soon engine is cranked which impossible to get any oil pressure at starter rpm.

Looks like oil system in engines in DCS are air bleeded out before start up.

Another thing is that DCS do not model is higher engine rotation resistance while cold, which often require a little bit of more throttle to maintain same idle speed compere when engine is warmed up.

Edited December 25, 2021 by grafspee

[Art-J]

On 12/24/2021 at 11:10 PM, hazzer said:

On the p51 and all piston modules on startup the oil pressure shoots up to over the red line even at low power settings (1000-1200rpm). Usually in aircraft the oil pressure starts low and slowly rises to normal and if it doesn't rise in a about 30 seconds you shut down and abort the flight. 

Unsure if all the ww2 aircraft are different from usual aero engines but dcs does not exhibit this. 

Well, the "going over the red line" thing is in my opinion not wrong in itself from the principle point of view. Rotax 912 and 915 engines we install in the company I work for have normal upper oil pressure limit of 5 bar when warm and above 3500 rpm, but 7 bar at really cold-start with the oil still being just too thick. The pressure builds up pretty fast once we start them up, but that's also because we always bleed air from the system before each cold start, as per operator's manual. Never measured the time though, 'cause it's irrelevant from practical point of view (as long as the pressure goes up at all, of course). I might check it early next week as we plan to do some ground tests of a new engine and it's getting pretty damn cold over here ;).

Haven't had hands-on experience with really big aero engines, however, so can't judge if the pressure in DCS goes up too fast or not. Maybe it does and should be tweaked, but I'm not bothered by the rising part in itself.

[grafspee]

Second thing P-51 engine in dcs gain rpm very fast when you starting it up, if you puch throttle a bit and star the engine rpm goes up immediately, when in RL engine it take some time to establish idle rpm, this definitely impact how fast oil pressure rising. In RL P-51 you need to crack throttle otherwise you wont start cold engine.

[hazzer]

One of the common examples online of warbit flying is kermit weeks. In this video here you can see him starting up the p51c (from cold). Just like in other piston engined aircraft the oil pressure slowly rises and does not rocket up above the red line. 

 

 

1 hour ago, Art-J said:

Well, the "going over the red line" thing is in my opinion not wrong in itself from the principle point of view. Rotax 912 and 915 engines we install in the company I work for have normal upper oil pressure limit of 5 bar when warm and above 3500 rpm, but 7 bar at really cold-start with the oil still being just too thick. The pressure builds up pretty fast once we start them up, but that's also because we always bleed air from the system before each cold start, as per operator's manual. Never measured the time though, 'cause it's irrelevant from practical point of view (as long as the pressure goes up at all, of course). I might check it early next week as we plan to do some ground tests of a new engine and it's getting pretty damn cold over here ;).

Haven't had hands-on experience with really big aero engines, however, so can't judge if the pressure in DCS goes up too fast or not. Maybe it does and should be tweaked, but I'm not bothered by the rising part in itself.

The only thing is rotax engines are a bit different than the usual aero engines as standard so maybe that's an outlier. The only engines I have flown with are lycoming, continental and austro and they all start low and build up. 

[grafspee]

@hazzer look at his oil gauge it is 30C oil temp.

Second thing look how engine building revs, in DCS it immediately jumps in to idle rpm here it took couple good seconds to do it most of time revving just below 500rpm.

Any way oil pressure looks extremally high in P-51 in DCS even at temp 40C pressure is going above red line, and i can remember that some time ago this was not an issue, before ED did some changes or tweaks in to oil system.

At cold temp oil pressure would go up very high, so there is a certain critical point when engine can not be started unless prewarmed by other means.

When P-51 was operated at cold temp, pilot had to dilute oil for 2 minutes before shut down,  so he could start engine next day with no danger of very high oil pressure.

Edited December 26, 2021 by grafspee

[hazzer]

Yeah agreed. Really needs looking at, all the piston aircraft really. 

[=475FG= Dawger]

It depends on how cold the oil is. When I was flying big pistons, the oil pressure rocketed up past red line when the oil was cold and you stayed at idle until it warmed up enough for the pressure to drop.

Some of the aircraft I flew had a manual oil radiator bypass valve. Start a cold motor with it closed and the oil radiator cracked from the over pressure. Takeoff with it open and bad things happened in a hurry.

Start the P-51 with the outside temperature really hot and see if it behaves the same way.