Merlin negative G cutout too quick?

[klem]

Originally Posted by IvanK

The documented value in RAE documentation specifically investigating this problem is cutout onset at 0.1G ...."i.e. at accelerometer readings of less than 0.1g"





The evolution of the cutout and time taken for recovery is also well documented in AVIA 18/1281 Tests of RAE devices for the reduction of "Negative G" engine cutting on merlin engined fighter aircraft" Though specifically looking at various cutout reduction methods some good info on cut duration and recovery in there with various amounts of negative G application.

Both these documents are available at the UK National Archives. The devs have copies of both these documents.

Excellent IvanK,

now perhaps we can all stop guessing.

[Ze-Jamz]

Lol they will stop guessing Klem, but they won't agree :p

[Crumpp]

now perhaps we can all stop guessing.

Exactly. There is no need to guess. Pilots who fly float carburetor equipped aircraft have been telling your community from the beginning the effect is instantaneous.

The physics and science of a float carburetor fuel metering system supports their experience.

There is a reason why allied pilots complained about it and why German pilots equipped with direct injection fuel metered engines could bunt to escape. It speaks volumes for the realism of your game that players complain as well.

The effect is instantaneous upon the application of negative accelerations. The instrumentation used in the report backs that up very nicely within the accuracy of a mechanical dial gauge accelerometer.

The problem is when people try to interpret things they don't understand and push it as fact.

You can see that in many of the "home-made" graphs pushed around the flight sim community where the author of the graph did not understand such things as TAS, EAS, CAS, or IAS or density altitude effects.

[klem]

Originally Posted by Crumpp

Exactly. There is no need to guess. Pilots who fly float carburetor equipped aircraft have been telling your community from the beginning the effect is instantaneous.

The physics and science of a float carburetor fuel metering system supports their experience.

There is a reason why allied pilots complained about it and why German pilots equipped with direct injection fuel metered engines could bunt to escape. It speaks volumes for the realism of your game that players complain as well.

The effect is instantaneous upon the application of negative accelerations. The instrumentation used in the report backs that up very nicely within the accuracy of a mechanical dial gauge accelerometer.

The problem is when people try to interpret things they don't understand and push it as fact.

You can see that in many of the "home-made" graphs pushed around the flight sim community where the author of the graph did not understand such things as TAS, EAS, CAS, or IAS or density altitude effects.

Yep, its probably almost instantaneous when it hits 0.1G. A pilot that currently fies a float-carburreted MkI Hurricane has told us that he estimates a cutout at 0.3G after a second or two (some people might call that 'instantaneous'), possibly instantaneous at negative G.

It would be a brave or arrogant person that was prepared to argue with the Royal Aircraft Establishmnent (RAE) who had the aircraft/engines to make tests with ( we don't ) and the skills and instrumentation to determine the problem. Beatrice Shilling was working for the RAE when she came up with her 'orifice'.

I just wonder how much factual documentary evidence will be needed before people stop thinking the early Merlins farted every time the pilot hiccupped.

[Crumpp]

Yep, its probably almost instantaneous

No probably to it, it is instantaneous. A Merlin engine sucks ~40-130 gallons per hour....there is not enough gas in the float bowl to fire the cylinders through one complete cycle.

Read the document again. Cut out occurs when it hits .9G on a mechanical dial gauge accelerometer. An acelerometer reads 1G at wings level or on the ground.

It only takes .1G of negative acceleration as measured on a mechanical dial gauge accelerometer to induce cut out.


That is 1/10th of a G...

By all means read that small amount of accelerations accurately on a dial gauge please.....

http://www.aircraftspruce.com/menus/...rs_falcon.html

The correct answer is "when the needle moves, cut out occurs...." That is what you see in the air with a float carburetor.

Crump, read again. It says at 0.9g negative acceleration and 0.1g instrument reading. Which means it will cut out fully when the measured acceleration reaches 0.1g (which may be different from the actual acceleration level experienced by plane and pilot).

The question is will the cut out appear in an on-off manner as we have now or will it be more a stepwise cut out as we had initially. My belief is that it will be rather a stepwise. With less g than level flight but with acceleration superiour to 0.1g the hydrostatic pressure in the lines and in the tank bottom will be less and my guess is that the engine will cough a little because of this.

[IvanK]

Stormcrow is correct we have been down this very argument before. The cut commences at 0.1G.

A mechanical G meter/accelerometer used in Aviation uses 1G as its static reference. Sitting in your chair holding a G meter it would read 1G.

Here is a typical G meter as fitted in a YAK52 sitting on the ground the needle showing 1G.



In cruising flight the aircraft is at 1G as the pilot progressively pushes forward the G decreases towards 0 G. The document states that a reduction of 0.9G. So the G meter would be reading 0.1G that's the needle just above the 0G mark as indicated in the graphic. As the document clearly states ..."i.e. at an accelerometer reading of less than 0.1g" ... or 9/10ths of G worth of Push ! or mathematically 1.0 - 0.9 = 0.1

I would though be a bit carefull with the number 0.1g. It may have read like that in the planes that were used to test this (nowhere is it mentioned to be Spitfires or Hurricanes so it could be any plane that had (which?) Merlin). It does not mean that at the location of the carburator it was 0.1g. And also the acceleration at the carburator in plane x will be different to the acceleration at the carburator in plane y even if the cockpit instruments reads the same acceleration for both planes. This is due to different location of carburator with respect to centre of gravity of the plane.

[IvanK]

Yep aware of that. The same thing also applies inside the FM as to where the coders are taking their "G" from. The presumption is its at the c of G.... which is of course in a differrent location to the carbys themselves.

Unfortunately the RAE document refers to instrumented aircraft but doesn't state exactly the set up.

My guess is that a Russian 0.1g is the same as a British 0.1g (same unit and there is only one definition of the g-unit which is 9.81 m/s²).

On the gauge: The most straight forward thing would be that the gauge shows the acceleration at the CoG.