Manifold Pressure

[gwshaw]

Decreasing engine rpm in a turbo-supercharged US plane should lower manifold pressure just like any other plane.

The turbo-supercharger was setup to provide a certain mass flow of air at SL+ pressure up to the rated altitude. The B series on the P-38 were designed to provide approx 31 in Hg at the diffuser outlet. You will lose some of that pressure in the ducting, so there would be about 29.92 in Hg at the carb inlet on the engine. The integral blower in the engine provide boost above that. The P-38H/J/L engines with their 8.1:1 x 9.5 in blowers did slightly over 2:1, managing just over 60 in Hg at 3000 rpm. If you decrease engine rpm to say 2600 rpm you will no longer be able to get 60 in Hg from them. I'm too lazy to do the math right now, but you would likely only get about 50 in Hg at 2600 rpm.

The turbos could be tweaked to provide higher outlet pressure by closing the waste gate, but that appears to have been much more common on the P-47 than any others. But by design the system was supposed to provide SL pressure to the engine up to the turbo-superchargers rated altitude.

Greg Shaw

[HoHun]

Hi Greg,

>Decreasing engine rpm in a turbo-supercharged US plane should lower manifold pressure just like any other plane.

Wouldn't the exact outcome depend on the exact altitude both for turbo-superchargers and mechanical superchargers alike?

For mechanical superchargers, there's a specific full throttle height for each boost and speed combination. The same should be true for turbo-superchargers.

So reducing rpm above the target rpm full throttle height would reduce boost, while reducing it below target rpm full throttle height would leave it as it is because below full throttle height, boost is limited by the regulator (mechanical) or waste gate (turbo) and not by the supercharger capability.

Does that make sense? :-)

Regards,

Henning (HoHun)

[Casca]

MAP changes with RPM reduction.

Naturally aspirated:
   Full thottle - No change.
   Partial throttle - Increase

Turbocharged Only, Below critical altitude.  
  Full throttle - No change.
  Partial throttle - Increase

Turbocharged Only, Above critical altitude.
  Both conditions - Decrease
 
Supercharged Only - Decrease
 
Supercharged with additional Turbocharger - Decrease

Note:  The turbochargers on the class of engines we are discussing, to split hairs, are not actually turbosuperchargers but are turbonormalizers.  That is to say they are, as Shaw said, merely providing roughly Sea Level pressure to the induction system at altitude.

[Straiga]

This all very interesting but in AHII at about 5,000 ft take any airplane lower the Prop RPM and watch what happens to the manifold pressure. Also we have a lot of planes in here with two stage superchargers Low boost for low altitude and high boost for high altitude. At 5,000 feet with full power we are not tasking any turbo or supercharger enough to have this notable of a change manifold pressure when prop RPM is decreased.

Also at about 5,000 feet lower prop RPM all the way on any airplane and lower the nose from 100 KTS to about 200 KTS with power at idle and watch the prop RPM climb. This small change in airspeed will not effect the prop in high pitch enough to make the RPM increase at this rate. If this is the case then we would all overspeed abd blow up engines when we desend out of 15,000 with half power at the point of compression to go after and shoot someone down.

Straiga

[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

[Straiga]

But were not talking high altitudes to have a critical altitude for turbos or superchargers in AHII were lucky if we get to 15 k before we have to come down to fight or drop bombs in a fighter.

 If we were at these altitudes we would have to look at the loss of propeller efficience at those altitudes. This also has implacations to the manifold pressure, less mass airflow from the prop because of air density.

Well lets talk about the P-51D at 26,000 you should be able to pull 61Hg 3000RPM and supercharger in high. In auto climb this works, but when I hand flew it I was only able to get 57Hg at 3000 RPM.

Cruise power 15,000, 35 Hg, 2050 RPM In AHII at 35 Hg I could not select 2050 RPM it would not go below 2175 RPM.

Max cruise 15,000, 29Hg, 1700 RPM In AHII I could not get it below 2000 RPM.

But for any flight at 15,000 I should be able to pull 61Hg and have a RPM of 2000.

The P-51 has a automatic maniflod pressure regulator that maintains a .1 inch Hg between .42 and .61 Hg below critical altitude, regardless of the RPM setting is.


Straiga

[hitech]

Like I said straiga get out a P51d flight manual, compair the numbers. Btw we do also acount for raise in manifold pressure do to ram air.

HiTech

[joeblogs]

For nearly all planes in AH2 this is not true, thanks to the constant  speed unit.

What is true is that changing the propeller speed will determine the engine RPM (holding the throttle constant) as the load on the engine will change.

What is also true is that one should be operating the engine by controlling both RPM and manifold pressure. There is little point in manipulating just one of the controls.

In particular, lower RPM and moderate manifold pressure should maximize endurance by minimizing friction losses while avoiding detonation.

-blogs

Originally posted by hitech
... Props are connected directly to the engine. Hence change Prop rpm, changes engine rpm, which changes the super charger RPM....

HiTech

[TequilaChaser]

ok now that you all have described how manifold pressure works. I would like to know at what alt does the E6B chart ratings derrive from, is there a standard alt used for every plane or does the speed , rpm and manifold pressure ratings come from a different alt per plane on an individual bases.

maybe this is already posted in another thread.  but in regards to manifold pressure I have seen it fluctuate over or under the E6B charts in the game from sea level up to high alt 30K or higher......

the E6B does not say best cruise alt, or what not. So one might be at a given alt and never be able to attain the desired manifold, rpm settings the E6B chart calls for.....

if one knew the alt for which the chart is referencing it might would be more beneficial to people that use the E6B chart settings..anyway to include these alts on the E6B chart?

[joeblogs]

Every engine in a US aircraft at least has an engine calibration curve that maps RPM and manifold pressure into HP for a given altitude. These curves are hard to come by now, but I imagine there were miles of them in cabinets once.

You can still find these things in aircraft manuals, but the best are found in manuals produced by the engine manufacturers.

-blogs

Originally posted by TequilaChaser
ok now that you all have described how manifold pressure works. I would like to know at what alt does the E6B chart ratings derrive from, is there a standard alt used for every plane or does the speed , rpm and manifold pressure ratings come from a different alt per plane on an individual bases.

maybe this is already posted in another thread.  but in regards to manifold pressure I have seen it fluctuate over or under the E6B charts in the game from sea level up to high alt 30K or higher......

the E6B does not say best cruise alt, or what not. So one might be at a given alt and never be able to attain the desired manifold, rpm settings the E6B chart calls for.....

if one knew the alt for which the chart is referencing it might would be more beneficial to people that use the E6B chart settings..anyway to include these alts on the E6B chart?

[gwshaw]

Forget about the turbo-supercharger for now, just think of the engine as being SL rated. The integral blower is sized to provide 60 in Hg @ 3000 rpm & SL inlet pressure. If you lower rpm, it lowers the tip speed and hence the pressure ratio, so at 2600 rpm vice 3000 rpm the pressure ratio would likely be something like 2600/3000 * 2.0 = 1.73 * 29.92 = 51.85 in Hg max, up to the altitude the turbo-supercharger could provide SL inlet pressure. That is assuming that efficiency stays the same at 2600 rpm as 3000, which is most likely incorrect. It is probably a little better, so call it 52-53 in Hg @ 2600 rpm max.

We could also get into the turbo-supercharger rated altitude dropping off with lower rpm and map due to the lower mass flow through the engine. But lets try to keep it somewhat simple, and just think of it as a SL rated engine, with the turbo-supercharger providing that SL pressure.

(supposed to be a response to HoHun's post, but I took too long and screwed up the quoting)

Greg Shaw

[GScholz]

There is one notable exception HiTech; the DB's. They had automatic variable speed superchargers. As for geared SC sets and turbo's I agree with you; pressure would drop with rpm.

[Angus]

Hey Scholzie:
How was the transmission. Hydraulic, right?

[GScholz]

I'm not sure exactly how it worked. From what I've been told it was basically just a big clutch.

[hitech]

GScholz: Auto gear change. Or infinatly varible?