Merlin negative G cutout too quick?

[TomcatViP]

Originally Posted by CWMV

Thought I saw it posted somewhere around here that it was documented to happen at .9G?

Yes it is.

[WTE_Galway]

Am I missing something here (I haven't bothered reading the whole thread everyone in here is too cantankerous) or are people expecting the Merlin flooding problem to turn on and off at a particular G setting like a switch ?

As far as i recall the flooding was not instant and neither was the recovery, anecdotally the flooded Merlin engines would eventually clear and self restart if still spinning, but certainly not instantly.

[41Sqn_Banks]

Originally Posted by TomcatViP

Yes it is.

IIRC the document stated "reduction of 0.9g" so it meant 0.1g. But I don't have the document, I think it was posted from IvanK.

[IvanK]

0.1G is the documented value. Though changes were made I don't believe it is at this value in the FM yet. All I believe that was changed was the actual "effects" side of the cutout. A 0.1G pushover is a LOT more than one would use in day to day type general flying .... like entering a cruise descent.

The source paragraph comes from the reference document in the UK national archives AVIA 13/234. The Devs have a complete copy of this document. The "G number line" is mine to illustrate how I think things should happen in game. Reference G in Straight and level flight is 1.0G.

[WTE_Galway]

Originally Posted by IvanK

The source paragraph comes from the reference document in the UK national archives AVIA 13/234. The Devs have a complete copy of this document. The "G number line" is mine to illustrate how I think things should happen in game. Reference G in Straight and level flight is 1.0G.

Interesting document. The G value given is likely correct but the suggested reason for cut out whilst not totally wrong only accounts for the initial momentary "splutter" the full cut out was for a totally different reason.

From Technical Order T. O. No. 02-55AC-2

"An idiosyncracy of the original SU carburetor was a condition known as "rich cutout" caused by negative g. In fact, the negative g cutout was a two-stage event. At the onset of negative g, fuel was forced to the top of the float chamber, which exposed the main jets to air. This caused the first, momentary lean cutout (Fig. 4.31). If a negative g condition continued, the floats reacted to the reverse of normal conditions and floated the wrong way, that is, they floated to the bottom of the float chamber. The needle valve opened wide, allowing full fuel pressure from the engine-driven pump to flood the carburetor. And excessively rich mixture was then admitted to the supercharger, causing the more serious rich cutout."


Note the two stage process over a period of time... which is why I was commenting that having the engine suddenly cut at a particular G figure is highly unrealistic.

[IvanK]

I agree the 0.1g value is the ONSET only and we should be seeing a more complex event. The graphic is only a small excerpt from the original document. The original "style" of the cutout modelled in CLOD was better imo though starting way to early, and still does imo. A reasonable fix would be to take the original model which had depth (splutter to cutout etc) but have it starting at the 0.1g value.

[klem]

I did some tests using a modified version of a script called BlackBox which I found elsewhere on the forums to capture the RPM cut and G value.

Chart is attached. It seems the cutout is set to 0.5G and seems instantaneous. You can see the 'highest' G-value cut, the third red line from the left. Others are just more examples. 0.1G it definitely ain't.

The chart shows RPM vs G-force and the G-cut is easily seen just before the characteristic RPM overshoot that occurs after it recovers.

You Merlin flyers need to vote on this unless its already in hand. Many very minor bugs and features have more votes than this.
http://www.il2bugtracker.com/issues/230

From an earlier discussion there was the conclusion that there is a two stage cut out, one arriving quite early the second later IIRC.

Up to know only one is simulated while it should be both.

[Sutts]

Originally Posted by klem

I did some tests using a modified version of a script called BlackBox which I found elsewhere on the forums to capture the RPM cut and G value.

Chart is attached. It seems the cutout is set to 0.5G and seems instantaneous. You can see the 'highest' G-value cut, the third red line from the left. Others are just more examples. 0.1G it definitely ain't.

The chart shows RPM vs G-force and the G-cut is easily seen just before the characteristic RPM overshoot that occurs after it recovers.

You Merlin flyers need to vote on this unless its already in hand. Many very minor bugs and features have more votes than this.
http://www.il2bugtracker.com/issues/230

Great work klem. Are you going to add this to the top section of the bug tracker report?...had a quick look but couldn't see it. Cheers.