Manifold pressure, carburetor heat and high altitudes

[Tucano_uy]

From the manual, page 31:

 

"The carburetor provides automatic control of the fuel-air mixture passed from the air intake to the supercharger and onto the engine manifold for combustion in the cylinders.

The Packard Merlin engine has an injection-type carburetor and an automatic manifold pressure regulator. The manifold pressure regulator is effective only at pressures in excess of 41 in. The automatic pressure regulator alleviates the pilot from having to jockey the throttle to maintain a constant manifold pressure in the high-speed range during a climb or descent. The pilot is only required to set the desired pressure by setting the throttle lever and the pressure regulator does the rest. It compensates automatically for the difference in air density at different altitudes by gradually opening the carburetor butterfly valve in a climb or smoothly closing it in a descent."

 

A) Please somebody explain me. Is this supposed to happen at all altitudes? I climbed to 35.000 feet and the manifold pressure decreased all the way down to about 30-35. Is this correct?

 

B) I thought that the loss of power might have been due to carburetor icing (however I'm not sure that it would show on the manifold). So I selected unrammed air and carburetor heat, probably lost more power because of getting unramed air, and I noticed...

 

C) Carb temp continued extremely low. Does carb heat is actually simulated on DCS? I've noticed very low carb temps at almost all altitudes, way below the green arc and heating doesn't seem to change anything.

 

D) Is it correct to have such low carb temperatures, below the green arc?

 

E) Is carburetor icing simulated?

 

F) Why is carburetor heat not required when reducing power to land like in smaller GA aircraft like Cessnas? I know the engines are different like day and night but I would appreciate if somebody knows something more technical to explain.

 

Thanks

[sobek]

A) Please somebody explain me. Is this supposed to happen at all altitudes? I climbed to 35.000 feet and the manifold pressure decreased all the way down to about 30-35. Is this correct?

 

Yes, at angels 35 you are well above the critical altitude of the second blower stage, that means that the compressor can no longer supply enough compression to supply boost pressures up to 61inHg, especially not at lower engine RPM.

 

B) I thought that the loss of power might have been due to carburetor icing (however I'm not sure that it would show on the manifold). So I selected unrammed air and carburetor heat, probably lost more power because of getting unramed air, and I noticed...

 

It is extremely unlikely for the carburetor to ice up in the cold dry air up at angels 30 (also carburetor icing is not modelled currently AFAIK). Your loss of power was most likely due to simply running out of boost on the engine.

 

 

C) Carb temp continued extremely low. Does carb heat is actually simulated on DCS? I've noticed very low carb temps at almost all altitudes, way below the green arc and heating doesn't seem to change anything.

 

Carb heat is not modelled currently.

[Tucano_uy]

Thanks Sobek.

 

Pity the carb heat is not modeled.

 

It comes as a surprise to me, I thought that all the engine was simulated.

[Yo-Yo]

Thanks Sobek.

 

Pity the carb heat is not modeled.

 

It comes as a surprise to me, I thought that all the engine was simulated.

 

Carb heat (to be accurate - the incoming air heating) is done. But as the icing is not implemented the heating can only spoil power performance. Heating the air ou pay the cost for nothing.

[dave4002000]

The critical altitude of the high blower is around 26,000ft. Any higher than that, and you will start to notice a drop in manifold pressure. Full fine on the prop at 35,000ft should give you around 43-44". Lower RPM's will, of course, give you lower manifold pressures.

 

On the Merlin, carb heat is not supposed to be used above 12,000ft anyway. Low carb temps are normal at high altitudes.

 

Also, due to the fact that the Merlin uses a pressure carb, it's nowhere near as susceptible to carb ice as your normal GA aircraft with float carbs. The amount of air passing through the carb at anything other than idle throttle makes it almost impossible for ice to form in the venturi.

 

I guess what i'm trying to say, is that even if icing was modeled. You'd hardly ever notice it while flying the Mustang anyway.

[boogie_man]

Not to hijack but, how do you keep the temp gauge down, I was in the escort mission and both the coolant temps were pegged, open air vents, went back to auto, no difference. The plane made it no problems leaving me to wonder if the gauges were accurate. Had rpm at the edge of green 2500? as well as the throttle kept the cruise at 15000 feet at approx 200 knots....hmmmmm

[dave4002000]

200 is a rather low cruising speed for the Mustang. Low speed means less air through the radiator. Try increasing your speed. Normal cruise for these things is between 325-350mph (280-305 knots)

[Yo-Yo]

The critical altitude of the high blower is around 26,000ft. Any higher than that, and you will start to notice a drop in manifold pressure. Full fine on the prop at 35,000ft should give you around 43-44". Lower RPM's will, of course, give you lower manifold pressures.

 

On the Merlin, carb heat is not supposed to be used above 12,000ft anyway. Low carb temps are normal at high altitudes.

 

Also, due to the fact that the Merlin uses a pressure carb, it's nowhere near as susceptible to carb ice as your normal GA aircraft with float carbs. The amount of air passing through the carb at anything other than idle throttle makes it almost impossible for ice to form in the venturi.

 

I guess what i'm trying to say, is that even if icing was modeled. You'd hardly ever notice it while flying the Mustang anyway.

 

Icing is described in manuals for P-51 but MP regulator makes it not noticable till the ice clogging begins severe. THe second note is that icing itself is possible inside the temperature range where free water presents in the air. High and extremely low temperatures are relatively safe regarding icing.

 

By the way, in DCS model as in real P-51 using of air heating causes erratic mixture leaning and it can lead even to engine overheat.

Edited May 21, 2013 by Yo-Yo

[Tucano_uy]

Thank you all for the explanations.

 

Now other questions arise:

 

Page 66 of the manual, regarding the carburetor air temperature indicator "

 

Carburetor Air Temperature Indicator

The AN5790-6 Carburetor Air Temperature Indicator shows the temperature of the air running through the carburetor air scoop. The gauge indicates temperature in degrees Celsius (°C) and is graduated from - 70° to 150°C. The face is scaled to 10°C. The green range indicates normal operating temperature of 10° - 30°C. The Red Line indicates maximum temperature of 40°C. "

 

1) Normal in what circumstances? If lower temps are OK then what's the purpose of the green arc?

 

Before asking and reading your answers, I was (wrongly and unsuccessfully) trying to keep the needle within the green arc with heated air.

 

2)Maybe not in DCS, but in real life, shouldn't the needle go up when using carburetor heat?

 

3) In real life, what would make the temperature go above the red line? I see that the instrument is graded up to 150 degrees C.

 

Thanks again guys, and sorry if the questions are about minute details that nobody cares about hahahah.

 

I've been surprised by DCS P51, finding fun and joy in things that I've never cared about before.

Edited May 21, 2013 by Tucano_uy

[Yo-Yo]

Thank you all for the explanations.

 

Now other questions arise:

 

Page 66 of the manual, regarding the carburetor air temperature indicator "

 

Carburetor Air Temperature Indicator

The AN5790-6 Carburetor Air Temperature Indicator shows the temperature of the air running through the carburetor air scoop. The gauge indicates temperature in degrees Celsius (°C) and is graduated from - 70° to 150°C. The face is scaled to 10°C. The green range indicates normal operating temperature of 10° - 30°C. The Red Line indicates maximum temperature of 40°C. "

 

1) Normal in what circumstances? If lower temps are OK then what's the purpose of the green arc?

 

Before asking and reading your answers, I was (wrongly and unsuccessfully) trying to keep the needle within the green arc with heated air.

 

2)Maybe not in DCS, but in real life, shouldn't the needle go up when using carburetor heat?

 

3) In real life, what would make the temperature go above the red line? I see that the instrument is graded up to 150 degrees C.

 

Thanks again guys, and sorry if the questions are about minute details that nobody cares about hahahah.

 

I've been surprised by DCS P51, finding fun and joy in things that I've never cared about before.

 

The green arc means the area where icing is not possible. Higher temperatures spoil blower ability and make the engine prone to detonation. In lower temperature range (usually +5...-15C) icing becomes possible. The range lower than -25C is relatively safe because of low free water percentage in the air and low relative humidity. So, you have to use hot air intake in two cases: as the temperature is in - 15...+5C and if there are obvious signs of intake clogging.

The air passes inside engine cowl so the engine recieves unrammed and hot air that reduces its power. That's why you must use heating only if there is a danger of icing or actual icing.

[dave4002000]

2)Maybe not in DCS, but in real life, shouldn't the needle go up when using carburetor heat?

 

 

I, honestly, have never used the carb heat in DCS. However, in the real world, you need to switch from ram to sheltered air before turning the heat on for it to have any effect.

 

If you are running sheltered with heat and the carb temp still doesn't increase, then that is a programming issue within the DCS P51.

[KaspeR32]

Sorry to revive this thread, but I've just been flying the P-51 as of late, and had a question.

 

I know the high blower doesn't kick on until a certain altitude, but I find myself almost pegging the throttle around 11,000 feet just to get 40" manifold pressure before it actually kicks in, then when it does, my manifold spikes to 61" briefly until I lower it back down to 46". Is that normal?

 

Also, at about 23,000 ft. Running the engine at 2400 rpm has my throttle almost pegged again just to keep it at 35"-40" manifold pressure. Also normal?