Merlin negative G cutout too quick?

[IvanK]

A second related document is in the UK National Archives "AVIA 18/1281 Tests of RAE devices for the reduction of "Negative G" engine cutting on merlin engined fighter aircraft". This document details flight test data on 3 devices (Including the Schilling orifice ... though its called the RAE Restrictor .... PC in action back in the 40's).

It compares each of the devices to an unmodified aircraft. In the tables presented the G used to induce cutout are in the order of -0.5G up to -1.5G. Though emphasis of the document is on the time taken to recover from cutout rather than preventing it, despite the document title.

Given the document is not looking at specifically preventing cutout itself but rather minimising the time of the cutout it needs to be put into perspective when using it to decide on initial cutout values. However it is of interest (imo) that reasonable values of Negative G were used (i.e. significantly less than 0G) in all the tests.... i.e. not just smooth nose position changes.

In our discussion here we are only interested in unmodified systems. The jpg below is from the document referring to an unmodified or "Normal Fuel System" aeroplane

[Crumpp]

That document clearly states:

An engine cut out occurred almost immediately negative G was applied.

Pretty clear cut but I am surprised at the amount of time it took for the Merlin to recover, 6-10 seconds and averaging ~8 seconds.

No wonder the Luftwaffe makes note of the effectiveness of bunting.

[WTE_Galway]

Originally Posted by IvanK

A second related document is in the UK National Archives "AVIA 18/1281 Tests of RAE devices for the reduction of "Negative G" engine cutting on merlin engined fighter aircraft". This document details flight test data on 3 devices (Including the Schilling orifice ... though its called the RAE Restrictor .... PC in action back in the 40's).

It compares each of the devices to an unmodified aircraft. In the tables presented the G used to induce cutout are in the order of -0.5G up to -1.5G. Though emphasis of the document is on the time taken to recover from cutout rather than preventing it, despite the document title.

Its worth pointing out that the Shilling modifications (which involved more than just fitting the famous flow constrictor) main effect was to substantially delay the onset of the second stage flooding cut-off. The shilling orifice was a stopgap.

The "Shilling Orifice" did not actually fix the problem, just delayed its onset a few seconds. Sustained inverted flight was still impossible in a Shilling equipped Spitfire, that required a pressure carburetor.

[Crumpp]

which seems a tad excessive.

What you need is a G meter. That does seem kind of excessive. I got the game and can check it out.

[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.