Merlin negative G cutout too quick?

[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.

[TomcatViP]

yes alrdy discussed with even some kind of calculation.

Merlin flyer hve to keep in mind that flying was done a different way at the time (and nobody likes neg G).

Did the German had there an advantage ? Certainly. And this is why the allied took the efforts to cancel this by their own design. It's all abt BoB and post BoB.

This debate will be only legitimate if we were discussing a 1943 simulation.

Take it as it goes : as soon as your pushing the stick frwd your eng start starving -> RPM decrease slightly then rapidly -> eng sputter (single cylinder miss firing) then the carb is empty the eng stop -> traction vector is null, you feel instantly the drag that was compensated by the eng power (your pilot head shld lean frwd-> then the carb is full and cylinder flooded -> the eng rev is maintained by the prop and the air stream -> then eng restart -> Sputtering (rich condition) -> then back to normal.

Remember that at a time when fuel cells were not wallowed, pilots were used to experienced eng cut out while manoeuvring.

I really don't understand this debate. Change your habits pull and don't push... Easy. Damn nobody ask you to turn gay !

[IvanK]

Great work Klem 0.5g was the original FM setting. As suspected the value has never been changed just the cut effects.

I really don't understand this debate. Change your habits pull and don't push... Easy. Damn nobody ask you to turn gay !

I think you are missing the point Tomcat. No one is disputing the phenomena just the onset. The fact is normal every day control inputs like setting up a cruise descent can result in the cutout they didn't in real life for such everyday piloting tasks .... it was referred even back in the day as "Negative G cutout or effect". All we are suggesting is getting the onset values correct.

"Take it as it goes : as soon as your pushing the stick frwd your eng start starving".... No only if you exceed a given threshold G. Stay below that threshold and you should have any 'negative" effects.

[TomcatViP]

Originally Posted by IvanK

Great work Klem 0.5g was the original FM setting. As suspected the value has never been changed just the cut effects.

I really don't understand this debate. Change your habits pull and don't push... Easy. Damn nobody ask you to turn gay !

I think you are missing the point Tomcat. No one is disputing the phenomena just the onset. The fact is normal every day control inputs like setting up a cruise descent can result in the cutout they didn't in real life for such everyday piloting tasks .... it was referred even back in the day as "Negative G cutout or effect". All we are suggesting is getting the onset values correct.

Thx to the doc you have provided, we hve alrdy computed the inner pressure in the carb and the fuel flow and see that the flow is impacted with even a very minor value of neg accel.

That is why the cut out effect start as soon as you are in condition where the load is < 1G. There is no physical delay unless it is very low (the 0.1G value). Of course the effects are marginal (slight rpm decrease). But here they are.

It's wrong to search for a magical number where you wld experienced a transition. It's simply progressive all around the 1G down to 0G load in an asymptotic way (1/x² as it's is a pressure value).