Merlin negative G cutout too quick?

[IvanK]

Yep every G meter I have used for the last 30 years uses 1G as a datum.

I have the entire document (lots of maths), there are is no list in the document as to the equipment being used. The term "G" is standard aviation terminology though. The snippet below comes from a comment in the covering letter to the document discussing testing methods used versus the nature of the original problem of G cut out in unmodified aeroplanes. Again the G used is unambiguous to me. The phenomenon occurring at "0.1 to 0g"



I will look through the second RAE doc that deals with the devices being tested to overcome the cutout (not very well either according to that document) to see if their are any equipment details in there.

Edit. The second document equipment list makes no mention of the Type of G meter used either. It simply lists the specific fuel system in each of the aircraft tested.

[TomcatViP]

Originally Posted by 41Sqn_Stormcrow

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.

Of course you're right but don't forget that in flight the Pilot is the reference : he fly likes he feels and act in concordance.

So if it might be interesting for the engineer to get the true acceleration on the carb float, as a flight safety rule, it seems logical that they hve measured what a pilot would feel.

[IvanK]

Yes but common car accelerometers are primarily used to determine lateral and longitudinal accelerations.
The Bob weight or whatever device is used sits at rest with 1G vertical but )g lateral and Longitudinal. Typically
these are used to determine braking effectiveness



Motor transport investigators use both including vertical accelerations referenced to 1G.

[VO101_Tom]

Originally Posted by cheesehawk

I've only used car accelerometers (which is why I questioned your docs originally) and they always use 0.0g as the base point. Wondered if that came from the English system or German, as they were the original hard-core car racers (ok, someone flame me here, cuz I'm just guessing!) People using different systems would have different scales for the same thing, take the intake manifold pressure gauges in British, German, and American aircraft for example.

http://forum.1cpublishing.eu/showpos...8&postcount=33

[TomcatViP]

Originally Posted by IvanK

Yep every G meter I have used for the last 30 years uses 1G as a datum.

I have the entire document (lots of maths), there are is no list in the document as to the equipment being used. The term "G" is standard aviation terminology though. The snippet below comes from a comment in the covering letter to the document discussing testing methods used versus the nature of the original problem of G cut out in unmodified aeroplanes. Again the G used is unambiguous to me. The phenomenon occurring at "0.1 to 0g"



I will look through the second RAE doc that deals with the devices being tested to overcome the cutout (not very well either according to that document) to see if their are any equipment details in there.

Edit. The second document equipment list makes no mention of the Type of G meter used either. It simply lists the specific fuel system in each of the aircraft tested.

In your document it is said "were reported" and "feb 41"

If the date is of paramount importance for the time frame the fact that they are discussing "reported" fact and not "instrumented" let me think that the G-cut out were measured by a direct reading of a G meter or reading of a graph after the flight. The graphometer would hve been most presumably fixed inside the rear fuselage or otherwise just behind the pilot in place of the radio what makes it doubtful. As the rear fuselage option it less precise than a direct reading by the pilot (aft position) we can speculate that this were read or filmed (a Technic used by the Germans as I know).

Anyway the law of distribution of acceleration states that the Accel a point A (aA) equate Accel at a point b + ABxf(dAlpha/dt²) + AB f(d(Aplha²)dt)

where AB is the distance btw point A and B and alpha is the angle of rotation of the line AB in the plane of travel. D/dt is the time derivation and d/dt² is twice the time of derivation (d/dt(d/dt))

So let's say that
A is the carb float
B is the pilot

when the plane is traveling at cruise speed (300kph+) the radius of turn due to a 1 G push over is huge. Hence Alpha is really small. We can then neglate the third term in Alpha²

Regarding the second term, it translate the influence of the inertia gained wth the rotating mvmt around the CG during the push over. As the mass of the Carb float itself is supposedly far less than that of the entire plane well we can say that if yes this term play it's part when studying the float, it won't have any influence in the value of the G read.

[VO101_Tom]

Originally Posted by cheesehawk

Again, I'm not here to teach physics... Done with explaining basic concepts to the uninformed.

?
It is better if you do not want to teach me anything, especially if you do not understand what is the difference between the aircraft and automobiles accelerometer (vertical (1G) and horizontal (0G) installation). Should not be pompously, especially when talking about stupid things ...

Originally Posted by TomcatViP

Of course you're right but don't forget that in flight the Pilot is the reference : he fly likes he feels and act in concordance.

So if it might be interesting for the engineer to get the true acceleration on the carb float, as a flight safety rule, it seems logical that they hve measured what a pilot would feel.

For the document you are right, but unless I am wrong we discuss the carburator and its behaviour here so I think the acceleration at carburator location is interesting. That is why I said the cockpit reading cannot be transposed 1:1 to the acceleration experienced by the carburator.

My thesis is that the acceleration experienced by the carburator can be slightly different for different plane types even if pilot acceleration is equal due to distance differences between plane cog and carburator. I cannot tell however how big this difference could be (basically I do not want to make the calculation ).

[klem]

Originally Posted by 41Sqn_Stormcrow

For the document you are right, but unless I am wrong we discuss the carburator and its behaviour here so I think the acceleration at carburator location is interesting. That is why I said the cockpit reading cannot be transposed 1:1 to the acceleration experienced by the carburator.

My thesis is that the acceleration experienced by the carburator can be slightly different for different plane types even if pilot acceleration is equal due to distance differences between plane cog and carburator. I cannot tell however how big this difference could be (basically I do not want to make the calculation ).

I understand the point you are making but I don't think the carburettor location is important. Well, except perhaps in my first point below.

It is unlikely that the accelerometer would be placed at the carburettor location unless they specifically wanted to look only at that issue. The accelerometer would have been used for other tests too. However if it were at the carburettor then, having accepted at what G level the cutout occurs, MG would have to decide how to simulate that in the FM design or calculate a reasonable estimate for their accelerometer wherever it may be located. Just following on from that, a 0.1G level at the carburrettor would be lower than in the cockpit or at the CoG due to the 'moment arm' point you are making so a cockpit or CoG G level could be say 0.15, 0.2, 0.3 -I'm not going to try to calculate it

More realistically, the Accelerometer would be at the pilot position or CoG. Now, I don't know where it is placed by convention but it is highly probable that the same convention was used back then as now and I would guess its the CoG. These were not the 'kite and string' guys of 1912, they were highly qualified experts who, among other things, had won the Schneider trophy and were no doubt sharing knowledge and conventions across the aviation world until a certain Mr Hitler turned nasty.

So, if we can accept say the CoG as the convention (I'm sure an aviation design expert will put me right if not) I will argue that the carburettor location does not matter because:-

It is the cutout itself that is being recorded and it is being recorded using the available instrument, the accelerometer at its conventional location. When the accelerometer is reading 0.1G the carburrettor may be at 0g or 0.05G or -0.5G but that in itself does not matter, the cutout is recorded when it happens and noted using the available accelerometer reference and that reference can be repated in the next aircraft etc.

There is one point to note and that does come back to the 'moment arm' issue. The moment arm (distance from carburettor to accelerometer) may be different in a Spitfire and a Hurricane so there could be minor differences in the cutout reading in the different aircraft but I think that would be quite small.

I understand what you want to say and I agree to it when it boils down to simulate ingame the thing.

I would like to know now if the cut out was an either on or off event with nothing in between.

Here is the drawing:


Is this pre- or post-modification?

As far as I understand the floater shall restrain fuel from flowing into to chamber when negative g's occur. That's above all in terms of function I can see.

Depending on the damping due to hinge friction my guess is that initially the floater will go up when one pushes the stick forward due to inertia that forces all objects to remain in the state they are until the forces get the better of them. So when it shortly gets up it will reduce the pressure loss at the reservoir entry by opening it. Hence more fuel will flow into the reservoir for a short time.

[klem]

Originally Posted by 41Sqn_Stormcrow

I understand what you want to say and I agree to it when it boils down to simulate ingame the thing.

I would like to know now if the cut out was an either on or off event with nothing in between.

Here is the drawing:


Is this pre- or post-modification?

As far as I understand the floater shall restrain fuel from flowing into to chamber when negative g's occur. That's above all in terms of function I can see.

Depending on the damping due to hinge friction my guess is that initially the floater will go up when one pushes the stick forward due to inertia that forces all objects to remain in the state they are until the forces get the better of them. So when it shortly gets up it will reduce the pressure loss at the reservoir entry by opening it. Hence more fuel will flow into the reservoir for a short time.

That design is post-production and covered in Viper2000's excellent post here:
http://forum.1cpublishing.eu/showpos...0&postcount=94
Especially read the pasted in section headed 'Appendix VI'