Merlin negative G cutout too quick?

[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??

[TomcatViP]

Originally Posted by cheesehawk

LOL!! So do German kids make "vrrrrrrrp brattt bratttbratttttt" when they play planes?

"Meeeep Mep MeeepMeeep " here in the country of thundering Renault engines (excluding F1 obviously)

Regarding the cut out and turbulences : more consumption -> more flow -> higher capacity fuel pumps -> more fuel momentum -> less probability of a cutout from turbulences only

It seems as if some of us wld hve to compute the exact flow rates of the eng pump to end this debate

But definitively Crumpp is right for the Lynco.

[IvanK]

Originally Posted by TomcatViP

"Meeeep Mep MeeepMeeep " here in the country of thundering Renault engines (excluding F1 obviously)

Regarding the cut out and turbulences : more consumption -> more flow -> higher capacity fuel pumps -> more fuel momentum -> less probability of a cutout from turbulences only

It seems as if some of us wld hve to compute the exact flow rates of the eng pump to end this debate

But definitively Crumpp is right for the Lynco.

Ok some Maths for the Boffins. Knock yourselves out guys Here are the Maths used in the design of the first attempt at the Negative G solution. The Source is the First document I referred to in previous posts. Note Its Merlin XX data.

[TomcatViP]

Thx IvanK for providing such a valuable source.

I understand there that CoD devs has alrdy really worked the point.

Considering that if the 0.2g was the design limit to sustain for the CutOut on the converted engine, the CutOut began much earlier on standard Merlin's (although obviously bellow 1G).

Doing a quick calculation (to be refined) I have a 0.017G as the min value for the cutout to begin in a non-modified eng with an assumed similar geometry (you need then to add the time that the 2nd carb chamber emptied it self of its remaining fuel - Vip as done that before - negligeable).

So am fully converting myself to Crumpp idea now. As a culprit of false assumption I condemn myself to run around my neighborhood both arms raised like wings and making loudly sputtering "WAAAA waaaaaaaa WWWAAAAAAA" engine noise.

[Crumpp]

i condemn myself to run around my neighborhood both arms raised like wings and making loudly sputtering "waaaa waaaaaaaa wwwaaaaaaa" engine noise.

lol!!

[klem]

Originally Posted by TomcatViP

Thx IvanK for providing such a valuable source.

I understand there that CoD devs has alrdy really worked the point.

Considering that if the 0.2g was the design limit to sustain for the CutOut on the converted engine, the CutOut began much earlier on standard Merlin's (although obviously bellow 1G).

Doing a quick calculation (to be refined) I have a 0.017G as the min value for the cutout to begin in a non-modified eng with an assumed similar geometry (you need then to add the time that the 2nd carb chamber emptied it self of its remaining fuel - Vip as done that before - negligeable).

So am fully converting myself to Crumpp idea now. As a culprit of false assumption I condemn myself to run around my neighborhood both arms raised like wings and making loudly sputtering "WAAAA waaaaaaaa WWWAAAAAAA" engine noise.

I don't think the devs have re-worked the G cutout in his patch, it 'feels' the same to me but if it has been reworked then that's that.

Not sure I follow what you mean regarding the 0.2G design threshold and the 0.017G cutout on unmodified engines but if the cutout on early engines was 0.1G it would make sense to have a design threshold (valve operation to prevent cutout) at a higher level so that it is already active before the 0.1G level is reached.

[TomcatViP]

Sry the 0.017G theoretical value is the actual neg G you wld hve to push to experience a cutout on a non-modified Merlin. I mean that actual accel value wld be (1-0.017)G.

The doc is related to a redesign of the carb to allow at near zero G the engine to function properly. The 0G value is approximated as 0.2G in this computation as the way they ran the calculation, 0G was not permitted (see on top of pg 2 the ratio for the plumber calculation (counter-weight) - if G wld hve been 0 the ratio can't be calculated that way)

A new design had to be introduced latter for full neg G aerobatic which was not done on Uk produced Merlin as I can understand reading the extract of "A Merlin History" that was provided to us earlier.

Note also that the redesign involve an anti-vibration induced cutout device (a spring) that lead me to think that the Merlin encountered some vibration problem once fitted in fighter planes. My assumption goes for the Spitfire but only on the grounds of assumptions made on Sidney Cam's robust design.

[winny]

This is probably a little bit irrelevant as it's a MkV, but it clearly shows what happens when the negative G cut outs occur.

If you watch and listen closley you can hear the engine misfire and see the smoke (especially when he's inverted). It happens quite a few times. Just thought it was interesting.

Alex Henshaw flight testing a MkV at Castle Bromwich 1941. (ignore the terrible acting bits!)

http://www.youtube.com/watch?v=nCmzYccyBYM