Understanding manifold pressure...

[Nic727]

Hi,

 

I’m having some difficulties understanding manifold pressure. I know it’s a mix of RPM and throttles, but other than that, I’m having difficulties keeping my plan at a good speed.

 

P40: To get a good cruise speed I need 70% RPM and 70% throttles. But it’s still slow and not good in combat. If I go higher RPM or throttles, I’m in combat mode...

 

P39: The worst... even at 40% RPM and 40% throttles I’m in combat mode and very slow...

 

For both planes it say to keep oil mixture at 66% to be optimal. It also say to put mixture at 90% for takeoff mode... which I didn’t find yet. Always in emergency mode...

 

Can you help me understanding how to be fast without blowing your engine?

[Ehret]

The P-40 doesn't have manifold pressure governor so you have to track MP and manage it manually. MP will be changing with altitude, engine RPM (higher RPM higher boost) and your speed (ram air effect).

The P-39, at least as she is in the sim, has the MP governor so it's a bit less workload.

 

In both planes to cruise you have to keep the MP under 40" and RPM no higher than 2600. In the P-40 you can keep the mixture 66% (auto-rich setting) whole time and to the contrary what is stated in manual you can do the same with the P-39. For the latter higher mix is useful for extra engine cooling in situations such as continuous turning and step climbing.

 

Now, important and often overlooked stuff for the P-39 - to be quick in the Airacobra you have to set radiators to so called "flush" setting - it's 60% for water radiator and 45% for the oil radiator. You can keep it as long you have some speed; engine shouldn't overheat except when the weather is very hot.

Edited October 28, 2018 by Ehret

[Nic727]

For the P39 how to figure which manifold pressure do we need?

 

i found a good mix of 70% RPM and 77% throttles for flight for ~4000 MP, but at takeoff and a bit after I am in emergency mode at 2000 MP with 50% RPM and 80% throttles.

 

edit: One thing I learned. Don’t go above 15k ft with the P39. You need RPM at 80-90% and max throttles and can’t keep up FW190 at high speed. I guess P39 is better between 5k and 12k ft.

Edited October 31, 2018 by Nic727

[F/JG300_Gruber]

If you want to understand Manifold Pressure, just forget about those stupid percentages.

70% throttle indicates you that your throttle lever is at 70% of it's total course. Absolutely not helpful. 

 

Look at the technical specification tab for the plane your are interested in (on the map, it's the second tab of the briefing page)

You will have engine modes (continuous, combat, emergency) and an associated MP and RPM (and the time limits for each mode)

You have to memorize these parameters for each plane you fly, or at least for each engine model. 

 

To keep it simple :

Manifold pressure is the air pressure in the engine air intake (the manifold) 

It is affected by altitude (the higher you fly, the less dense the air, so the pressure drops), by engine RPM (since engine displacement is fixed, higher RPM means higher volume of air sucked in for the same amount of time, and a higher possible M.P.) and by the supercharger, that compresses the air before feeding the engine. 

 

Since Manifold pressure is affected by RPM, always set your RPM first according to your needs and then adjust the MP by looking at the needle. And forget forever about the percentages. 

 

Some engines like the Daimler Benz features a complex governor that will constantly adjust the manifold valve and the supercharger gear ratio to keep a certain MP for a given throttle position, so it is possible to fly on percentages on these engines. But on planes like the P40 where there is no such thing, always fly with the needles if you want to get the most out of your engine. 

 

 

 

 

Edited October 31, 2018 by F/JG300_Gruber