Merlin negative G cutout too quick?

[Crumpp]

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

Yes cut out is a cut out.

The onset is depending on the Onset Rate...

At typical ROR acceleration onset rate of 6G/sec the Merlin as tested in the report cuts out in ~1/6 of a second.

In the cockpit, that would be instantaneous.

[klem]

You know, all we are really concerned about is that:-

1. in a 'combat bunt' the engine cuts = LW happy
2. in a 'descent' that doesn't hit an extremely low or negative G the engine doesn't cut and we'd like it to be as close to reality as possible, which seems to be somewhere between 0.3G and 0.1G depending on who you believe = Allies happy because it isn't a pain in the a$$ to fly - because it wasn't. It was a dream to fly (see many Spitfire veteran pilot's interviews).

I'm not looking for a way around the 'combat bunt' problem. That was a reality and one I actually want to have to live with. As it is now the 'combat bunt' problem is there. I just think the normal 'descent' is porked because the cutout seems too sensitive. I have done a quick and dirty in FSX and by comparison it seems to cut in the region of 0.7G.

Now, if Luthier will just tell us what parameters he is using for the Merlin III/XII cutout and if they are close to the figures given by the RAE and a current MkI Hurricane pilot I will be happy and accept that CoD's representation is how it was. Even if the cutout parameters are too sensitive and are adjusted so the cutout isn't so sensitive, the LW guys still lose nothing in the combat sense. A severe bunt will still cut the engine.

[TomcatViP]

Don't forget that in flight nobody likes to pull some neg G. It's more natural to roll and pull than simply push. It does not impact the way you can enjoy flying a spitfire

The G cut out being well documented by IvanK I don't see how we can disagree. With a bit of practice it comes naturally to avoid G in a fight (but obviously that nasty push over done by 109 still is difficult to match .. but wait that sounds historic )

@Crumps : A G is a G what ever is the rate of sampling. or is there something that i didn't "Catch"

G meters in cars are often used to test crash conditions and are way out of a G meter you can find in a plane (electric/mechanical device). I know that's not what Cheese is talking abt but I thought it had to be said.

[IvanK]

"Does the document state the onset rate? Without that the information is interesting but useless.
Huge difference between .9 G at an onset rate of 25G a hour and 25G a second.... "

Neither of the two RAE documents have any data on G onset rates even in the Tabular results presented.

[klem]

Originally Posted by IvanK

"Does the document state the onset rate? Without that the information is interesting but useless.
Huge difference between .9 G at an onset rate of 25G a hour and 25G a second.... "

Neither of the two RAE documents have any data on G onset rates even in the Tabular results presented.

Well, unless I misunderstand my long-ago mechanics/physics classes, G is acceleration which is all about change of state versus time so 0.1G is 0.1G however fast you get there. So if a cutout occurs at 0.1G, that is where it cuts out. The 'onset' seems to me to be about how quickly you get to 0.1G and the only two things I can think of that 'onset' would affect is the perception of how 'sensitive' the cutout is - "blimey, if I really shove the stick it cuts out a lot quicker" - but still at the same G level and I suppose a slow onset might enable the carburettor to stay with it a little longer but are we splitting hairs?

[Crumpp]

I suppose a slow onset might enable the carburettor to stay with it a little longer but are we splitting hairs?

Yes the onset rate is extremely important. Very rarely do you experience gradual onset rates in an airplane depending on the definition. That is why I asked IvanK about the onset rate information in the report.

Even wind gust will accelerate an airplane at Rapid Onset Rates:

MISCONCEPTION

Pilots notice that a heavily loaded airplane rides smoother in turbulent air. They perceive this as an indication that the airplane should be loaded to its maximum whenever turbulence is expected. This is a bad assumption.

Consider an airplane that has a maximum allowable gross weight of 3,000 pounds. If it encounters a +30 fps gust that results in an additional 2-g load factor, the airplane experiences a total of 3 Gs load factor. Multiply the 3-g load factor by 3,000 pounds and the wings are supporting 9,000 pounds.

Assume the airplane is loaded to 1,500 pounds and that it is subjected to the same gust. With half the inertia, the gust acceleration is doubled, causing the airplane to experience a 5-g load factor (4-g force plus 1-g level flight). Multiply 1,500 pounds by 5 gs and the wings are supporting 7,500 pounds.

The lightly loaded airplane is subjected to 1,500 pounds less load when encountering the same gust. Even though the heavy airplane realizes less load factor, it incurs more strain. The pilot recognizes load factor; the airplane recognizes load.

http://law.justia.com/cfr/title14/14...1.3.164.7.html

That being said, GOR has a very broad definition and is generally defined in any study. It is the onset rate that tells us how much time it takes to reach a specified load factor.

GOR's definitions that I have seen range from .1G/sec to 4G/sec. That would range from ~ 9 seconds to .225 seconds before cut out.

Well, unless I misunderstand my long-ago mechanics/physics classes, G is acceleration which is all about change of state versus time so 0.1G is 0.1G however fast you get there.

No klem, the aceleration in this case is a fixed point but does not define the rate at which we reach that point. I think you should understand that airplanes arrive at that small a load factor very very quickly under the vast majority of flight conditions. Even transitioning from a climb to level flight will result in reaching that .9 load factor in less than a second unless a pilot makes a careful effort.

I would take you flying and you could watch the G-meter in my aircraft. When I reset it at the end of flight, it is generally ranging from ~(-)1.5G to (+)2.5 just in normal operations on a cross country trip.

Cut out will occur in a fraction of a second and is instantaneous from the pilots point of view.

I am sure you can see the distinct tactical disadvantage of a fighter trying to dogfight with a float carburetor fuel metering system.

[Crumpp]

"Does the document state the onset rate? Without that the information is interesting but useless.
Huge difference between .9 G at an onset rate of 25G a hour and 25G a second.... "

Neither of the two RAE documents have any data on G onset rates even in the Tabular results presented.

Ok. They were probably just looking for the threshold on the test carburetor. Not all carbs will meet that same threshold, some will be better and others worse but it is a very good data point.

Combine it with typical onset rates and the tactical disadvantage becomes clearer.

Does the game model the longitudinal instability of the Spitfire, pre-stall, and post stall behaviors?

Do you guys have copy of the actual Spitfire Mk I POH?

[TomcatViP]

I hve never thought abt that but if there is diff onset for G-meters it might be linked with what you want to measure.

For ex if wind gust is what you are seeking then the onset will be very small (less period -> high frequency). On the contrary if you are trying to evaluate pilot action on the plane, what you absolutely want not is to see some histerisys on your nice curves from the interferences of the wind gusts.

That said, I don't think it's a mater of discussion here. We have simply to assume that RR test-engineers knew what they were talking abt. And as they were certainly among the best in the world at the time well....


SO 0.9G wld be 0.9G corrected of any parasite accel ... If you add the fact that there is not atmospheric model in CoD until now (?) then ...

By the way yes it was a disadvantage but pls remind that ALL the Fighters in the world at that time might HAVE HAD THE SAME PROB except those that had Fuel Injection in 1940. Let me guess they were German

If you run for the war earlier that's for sure you'd get some (unfair ?) advantages.

Si vs pacem etc... etc...

[Viper2000]

Originally Posted by Crumpp

Yes the onset rate is extremely important. Very rarely do you experience gradual onset rates in an airplane depending on the definition. That is why I asked IvanK about the onset rate information in the report.

Onset rate doesn't make much if any practical difference to the carburettor's behaviour because the determining factors are the geometry of the float chamber, the position of the float and the fuel flow rates into and out of the chamber, none of which are going to be a strong function of dg/dt.

As soon as the g level falls below about +0.1 indicated, the float stops floating properly, and the carburettor therefore stops metering. Exactly what reduced positive g will cause misbehaviour will depend upon the friction in the system and any slosh in the float chamber, leading to slight variation on a case by case basis; but this sort of detail is way beyond the scope of a simulation of this nature.

Much earlier in this thread I calculated the approximate subsequent chain of events for both the reduced positive and negative g cases.

In both cases, I would expect a lag between departure from 1 g flight and cut behaviour due to the volume of the float chamber, engine demand, and fuel pump delivery rate.

(For this reason, normal turbulence would seem quite unlikely to produce cut behaviour.)

[Crumpp]

Onset rate doesn't make much if any practical difference

Only if you want to the answer to the question "WHEN does cut out occur?"

(For this reason, normal turbulence would seem quite unlikely to produce cut behaviour.)

Yes, it certainly will produce a cut out if the acceleration reaches the threshold.

As soon as the g level falls below about +0.1 indicated, the float stops floating properly, and the carburettor therefore stops metering. Exactly what reduced positive g will cause misbehaviour will depend upon the friction in the system and any slosh in the float chamber, leading to slight variation on a case by case basis; but this sort of detail is way beyond the scope of a simulation of this nature.

Once again, in an engine consuming 100 gallons per hour, the tiny bit in the float bowl will not last a cycle....

Even in a lycoming consuming 9 gallons per hour, a cut out and rpm change can be heard in turbulence or any negative acceleration.

Where do think kids get the airplane engine noises, "WAAAA waaaaaaaa WWWAAAAAAA" when playing from??