Manifold Pressure and OAT

Hello to everyone,

I am a quite inexperienced Pilot. I only have about 200 hours at the age of 19. And I just startet my initial training in the SR22, its a great aircraft. After I got all the manuals read, I stumbled again and again over there performance chart. As we all now the SR22 has a combined Prop and power lever. Till a certain point she is increasing the RPM, thereafter holding 2500 RPM and at a specific position (always the same).
When we now look at the Performance Chart, you can see that the manifold Pressure is always the same at full power for different OAT´s. As I understood that actually can’t be possible, because with rising temperatures the density altitude increases. Therefor “less” air is going trough the manifold. So, the pressure (MP) has to be lower. In addition the engine is producing less power in higher altitudes (pressure and density), therefore it’s “pulling” less. I’m very interested in your answer and ideas. And very grateful for any help.

Hi Paul.

great question. Here’s a good way to think about it:

The throttle (black knob) controls manifold pressure.

The prop governor (blue knob in other planes) controls rpm by adjusting the pitch of the prop.

With the Cirrus linked controls, peak throttle occurs when the prop reaches 2500 rpm, pushing the “throttle “ up further, only increases rpm after that.

You are correct about decreasing air density impacting engine and aircraft performance negatively.

Note that the performance chart you posted is for a constant pressure altitude of 2000ft.

Air density decreases with increasing pressure altitude and increasing temperature.

The chart is indicating that at a pressure altitude of 2000ft (29.92 in @ sea level) a normally aspirated Cirrus SR 22 (not turbocharged) can reach a maximum manifold pressure of 27.4, which is about the atmospheric air pressure at 2000ft). With higher temperatures, notice that the resulting airspeed diminishes. Climb performance will also diminish.

Have a look at the performance charts for higher pressure altitudes, and you will see that the Max obtainable MP is lower, just as you expected.

I think you’re confusing pressure and density. Pressure, temperature and density are all related, so the mass of air entering the engine is determined by both pressure and temperature. So the chart is correct, simply because it starts with pressure as the invariant, and shows the effect of temperature on density and thus performance.

You could easily construct a chart with temperature as the invariant and variable pressure, or you could have a chart correlating air density with performance, but since the two things we can easily measure are pressure and temperature, the chart you posted is the easiest to use.

Hey,

Tanks for your answer. I know the effects of density and pressure altitude. Maybe I was a bit unclear in my explanation. Let’s say for now we are flying in 4000ft at standard pressure (1013 hPa). So we have a constant volume of air. When we have this pressure at an OAT of for eg 10 degrees Celsius the amount of air going through the engine (MP) is a lot bigger than the amount of air when the OAT is for eg 30 degrees Celsius. (Always think for full Throttle) Don’t forget, in this example the pressure altitude is always 4000ft. Of course, this leads to a loss of power. Temperature does not effect pressure altitude. But both of them combined is the altitude the airplane and engine “sees“.
I really appreciate this discussion.

Standard pressure at 4000’ is not 1013, it is 875 hPa.

Probably better to use the terms “mass” or “volume” when talking about gases - “amount” is ambiguous. As I’m sure you’re aware it’s the mass of air that matters when it comes to engine operation.

Harking back to your original post:

Has this discussion resolved that for you?

It’s semantics, and I have little doubt you meant otherwise, but “peak throttle” or in local parlance, WOT, does not begin when reaching 2500RPM. That is simply the beginning of the governed range.

WOT happens somewhere near the “top” of the governed range (around 2500RPM give or take) when no more Manifold Pressure can be squeezed out (my own highly technical term).

Okay Guys,

The green line in the picture represents the point at wich the RPM starts advancing towards 2700 RPM. The throttle has a spastic position at this point (fully oben). When we fly at a certain pressure altitude at a cold temperature (for eg 10 degrees celsius), the volume of air going through the manifold is more than the volume at a higher temperature (eg 30) with the same pressure altitude. Actually the air is less dense at higher temperatures for the same pressure altitude. this results of course in a loss of power, the chart is showing that. But how does the engine mange it, to have still 27,4 in hg at all different temperatures for the same throttle position (the point at wich the RPM starts advancing form 2500 RPM). Because theoretically the volume of air going through the manifold should be less for higher temperatures and constant pressure.

No, the volume of air is constant for a given RPM. It is simply determined by the displacement of the engine and the number of intake strokes in a given time. The mass of the air going in is determined by the volume, the pressure and the temperature. The pressure and temperature determine the density. The mass is the product of volume and density. So at a constant volume and pressure the mass, and consequently the power, varies with temperature. Hence the table showing variation in power with temperature at constant pressure.

Is the MP equal to the mass? Or volume?

Neither - it is Manifold Pressure. The mass is a function of three variables:

  • Volume - measured by RPM
  • Pressure - measured by MP
  • Temperature - measured by OAT (for a normally aspirated engine)

Those three things determine the mass of air entering the engine. That’s why those are the variables in the performance tables in the POH. For our purposes In this context they are independent variables, i.e. each can vary without affecting the others.

Footnote for the technically minded - I have deliberately omitted any mention of humidity because its effect is small and it would only complicate the discussion.

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