Manifold Pressure

[HoHun]

Hi Hitech,

>GScholz: Auto gear change. Or infinatly varible?

The device used in the Daimler-Benz engines actually was a hydraulic torque converter (infinitly variable, as you point out), with the term "clutch" being somewhat of a mis-translation.

Regards,

Henning (HoHun)

[sb1086]

Originally posted by hitech
No use in talking generalities.

Pick one plane lets take the p51d?.

If you have the critical alts you can do some simple calcs to know the RPM when the manifold will start droping off for each stage of the supercharger.

Basicly take ambient Pressure at critical alt, and max manifold pressure full RPM at critical alt. This gives you a base compresion ratio of the supercharger. Now the compression ratio will vary linarly with RPM and you can then calc the pressure out of the super charger at any alt & RPM.

If This pressure is greater than waist gait pressure , no drop will be seen with RPM change, if it is less, a drop will be seen.

HiTech

Hitech as a retired Tomcat driver I have to ask, which do you know better Aces High, or real A/C, I ask because I Flew I c1 cod before I got into Tomcats
And reading this thread you seem to know more about piston A/C engines than most of the "old" guys that I know LOL

[hitech]

Sb1086: Thanks for the compliment.

What it comes down to is I have been makeing flight sims for 15 years. And to make sims you have to understand all items on a plane from a system level. Some you choose to detail in the sim, some you don't.

Look at our resent discusion on the torques of jet engines and where it comes from. This is one i had not event thought about before,hence sparcs my interest greatly. Why things work as always been a fancination of mine.

Always looking for new knowledge on how things work. Like why I asked how the varible supercharge worked.

Pyro: Is the knowledge guy on any paticulare plane, I.E. what specific items any plane had.  

HiTech

[sb1086]

Try Jet engine thrust:)

[GScholz]

Thanks for the clarification HoHun.

Yes HiTech, infinitely variable. Which is why the DB powered aircraft are the only supercharged planes in AH that has smooth speed and climb curves (almost like the turbo powered rides), and not the typical jagged curves of multi-stage/gear supercharged engines.

[Widewing]

Originally posted by hitech
Normal aviation nominclature.

Turbocharge Gas driven.
Supercharge Mechanicly driven.

Capt. But I do agree on you technical definition.

And turbochargers I belive can also effected by rpm.  

Basicly the turbo will spin based on the volumn of gas (not fuel) exiting the engine. More Throttle, more expansion, more gas more turbo rpm.

Like you state.

But also, Same gas per stroke, higher RPM,more gas leaving cylinder hence more turbo rpm.

Now some  of the planes we have also had a turbo rpm control to not over speed the turbo. Basicly a type of waste gate before the turbo.

HiTech

P-38s had wastegates AND turbo regulators. Maximum turbo speed was governed by the regulator(s). Wastegates dumped excess boost. Different functions. IIRC, both aircraft had sufficient exhaust flow to spin the turbos to max speed at just 1,800 rpm for the P-38L, and 1,650 rpm for the P-47D (typically). You should not see a drop-off of available boost until engine speeds go below these levels. This is why it was so critical to P-38 engine reliability to increase rpm before increasing throttle, else the engines be severely overboosted and detonate. At least that's what the pilots have told me.

My regards,

Widewing

One of the reservations expressed by pilots being introduced to Lindbergh's low rpm/high MAP cruise settings was concerns about the long term effect of running high boost at low rpm. Most pilots cruised at higher rpm to MAP ratios than Lindbergh advocated.

[Widewing]

Originally posted by sb1086
Hitech as a retired Tomcat driver I have to ask, which do you know better Aces High, or real A/C, I ask because I Flew I c1 cod before I got into Tomcats
And reading this thread you seem to know more about piston A/C engines than most of the "old" guys that I know LOL

Geez, I logged about 1,600 hours in the C-1A as crewchief (CV-60) and it's rare that I run across anyone else who flew or crewed CODs. It's been 26 years since I last flew a hop in the C-1A, but I can still recite the checklists from memory... Must some form of Idiot Savant....  

My regards,

Widewing

[Captain Virgil Hilts]

Originally posted by Widewing
P-38s had wastegates AND turbo regulators. Maximum turbo speed was governed by the regulator(s). Wastegates dumped excess boost. Different functions. IIRC, both aircraft had sufficient exhaust flow to spin the turbos to max speed at just 1,800 rpm for the P-38L, and 1,650 rpm for the P-47D (typically). You should not see a drop-off of available boost until engine speeds go below these levels. This is why it was so critical to P-38 engine reliability to increase rpm before increasing throttle, else the engines be severely overboosted and detonate. At least that's what the pilots have told me.

My regards,

Widewing

One of the reservations expressed by pilots being introduced to Lindbergh's low rpm/high MAP cruise settings was concerns about the long term effect of running high boost at low rpm. Most pilots cruised at higher rpm to MAP ratios than Lindbergh advocated.

THANK YOU! I was almost certain I was seeing something missing here. I can't seem to get it to work that way in AHII. I also noticed that Levier found the 8th AF doing it wrong when he got to Britain on his trip.

Levier's report, and instructions on P-38 engine management:


Report on P-38s in the ETO, by Tony Levier.

“Having just returned from a four month mission to England on 29 May 1944 where I had been testing and demonstrating some of the new improvements on our P-38J’s, I filed the following report:”

Immediately upon arriving in England, I proceeded to the 55th FG HQ at Nuthamstead, an airbase in East Anglia, north of London, where conditions were pretty grim as far as their P-38’s were concerned. They had just received their first P-38J’s, and had no operational information on them. Their lack of information concerning correct power combinations was appalling.

For combat missions some pilots were using anywhere from 2000RPM to 3000RPM with whatever manifold pressure that would giver them their desired air speed. Some of these pilots were blowing up their engines with high manifold pressure and critically low RPM, while others were running out of gas and failing to complete missions because of such power combinations for continual cruise at 2800RPM with 24” of manifold pressure. Many returned with hardly more than a cupful of gas remaining in their tanks, while others were forced to bail out over enemy territory.

The reason they were cruising at 2600, 2800 and even 3000RPM was that somewhere along the line they had been taught to use high RPM and low manifold pressure. They were under the impression that should they get jumped by a Jerry they could get their power faster if they already had their engines running at high RPM.

The fact is you can get your power quicker if you have low RPM and high boost which gives you a high er turbosupercharger speed. With turbochargers putting out high boost you only have to increase your engine RPM to get your desired power.

Rather than add to their confusion with power curves and range charts we devised the following rule of thumb for their long range missions. It is simple and easy to remember and insures maximum engine efficiency/fuel economy: USE 2300RPM AND 36” MANIFOLD PRESSURE AS THE MAXIMUM FOR AUTO LEAN AND CRUISE CONDITIONS. IN REDUCING POWER FROM THIS SETTING, REDUCE ˝” TO 1” FOR EACH 100RPM; FOR GOING ABOVE THIS SETTING PUT YOUR MIXTURE IN AUTO RICH AND INCREASE THE MANIFOLD PRESSURE 2” FOR EACH 100RPM.   

After using this rule the boys marveled at their increased range. Some were returning from 4 hour missions with as much as 150 to 200 gallons of fuel left. Quite a bit more than the cupfuls they had been returning with.

The day after I arrived at another base in England some P-38 pilots who had been escorting Forts over mainland Europe reported a “sort of engine trouble”.

When pinned down they said their engines had been surging and momentarily cutting out while they were flying at altitude under reduced power. The passed it off saying, “it’s probably caused by some extra low octane gas.”

But after further discussion with the boys I suddenly remembered a series of tests we had run back home during flight test operations for proper turbosupercharger settings. As I recalled, the symptoms were very similar.

So I obtained permission from the 55th FG CO to test a P-38 at altitude for proper turbosupercharger operation, and sure enough, the turbosupercharger on the left engine was so rigged that the resulting backpressure and high turbosupercharger wheel speed caused the airflow to the engine to surge, resulting in erratic operation and inability to pull power.

Upon landing, I reported the trouble and recommended that each Lightning driver be given the following procedure for checking his ’38 at altitude for proper turbosupercharger operation: At 30K feet set your RPM at 2600RPM and 37” manifold pressure and back off slowly on the power down to 10 to 15” manifold pressure. While doing this, fix your eyes on the manifold pressure and note if there is the slightest engine failure or surging. Record the exact manifold pressure at which this surging occurs.

Now repeat the process beginning with 2300RPM and 37” manifold pressure , and again record the boost at which surging occurs. (At this lower RPM it should occur 2 or 3” higher.)

If, after you’ve completed this procedure, either of the recorded manifold pressures are above 22”, it indicated the turbosupercharger regulator is set improperly. Tell your mechanic the boost at which the roughness occurred and he will make the necessary adjustments.[/I]

[sb1086]

Originally posted by Widewing
Geez, I logged about 1,600 hours in the C-1A as crewchief (CV-60) and it's rare that I run across anyone else who flew or crewed CODs. It's been 26 years since I last flew a hop in the C-1A, but I can still recite the checklists from memory... Must some form of Idiot Savant....  

My regards,

Widewing

Some things you never forget man, I can still go through the whole thing from startup to shutdown at another field. I bet ya I can even stil deck run one on a carrier too.