Engine HP given RPM, MP and PA

[Bruce Fenstermacher]

I have a manifold pressure gauge on my fix pitch 0-300. I enjoy using it to set power because it is quicker and easier to set more precisely to a power setting than the RPM.

RPM will vary depending on PA. At higher altitudes RPM is nearly 2700 at much over 22 HG and doesn't drop much until MP is 20 HG or below. This makes my partner nervous as he has always flown RPM and rarely over 2400 and most cases 2300 no matter the altitude. He does understand the engine at altitude is not making the same HP at the same RPM as at sea level and it doesn't matter the RPM so long as it is at or under red line. It is psychological, he sees the RPM high and he wants to pull the throttle back. I'd hoped the MP would show him we were not taxing the engine even though the RPM is near 2700.

So I'm looking for a graph or table that shows me HP for a given MP and RPM.

From x30015 C-125, 145, 0-300 Owners Manual, pg 16

I found this graph in x30015, C-125, 145, 0-300 Owners Manual which I think will give the the info I seek. Following the instructions in the top left, it just doesn't make sense to me so I don't understand how to find the result. Yes I know I could use the power cruise chart found in the later (55) 170B owners manuals to look up the PA and the RPM and get BHP but I think with the above chart I can get more info at more altitudes and correlate that to MP at a PA rather than RPM.

So has anyone unlocked the secret Figure 6, above?

[blueldr]

It is kind of a pita to figure ot horsepower when using a fixed pitch prop.

[DaveF]

The plot shows the gain in horsepower with altitude for fixed MP/RPM combinations. In the example on the chart, 23"/2300 is 95hp at sea level and 101hp at 6500. The gain is due to the lower temperature higher density air, and lower air friction operating with a more open throttle.

[Bruce Fenstermacher]

OK guys Richard Pulley called to tell me how to use this cart. I couldn't figure it out because I was looking for it to do something else which was a list of BHP for a given MP at a given RPM at an altitude. It will do that but only one at a time. Richard also pointed out that on this graph they left of the C point but they did include it on the identical graph for the C-145-2H.

I've added a few color coded lines to explain how to use this graph. First I placed a red C at that data point.

From x30015 C-125, 145, 0-300 Owners Manual, pg 16

First step is to pick a MP and a RPM and in this example it is 23" MP and 2300 RPM. You plot data point A following the yellow arrows on the right graph.
Second step is to plot data point B on the left graph for 23" MP and 2300 RPM as I did with the vertical green line. From B plot data point C following the horizontal green line. C is the BHP at sea level for 23" MP and 2300 RPM.
Third step is to draw a line from C to A on the right graph which I did in red.
Now if you plot a line up from the bottom as I did in dark blue on the 3000 PA line to the red line then go left you can see that by going from sea level to 3000 PA and opening the throttle to maintain 23" MP the BHP goes up to about 97 BHP. If we went to about 6400 PA outlined in light blue and maintaining 23" MP our RPM would be 2300 RPM and BHP would be about 102.

This is how Richard says the chart works and it seems to work. Does anyone think it works different?

So to get the info I want I need to fly at my usual altitudes of 1500 MSL and maybe 2500 MSL and see what RPM I get at various MP settings.

[DaveF]

That's how I understand it. The red line is the horsepower vs. altitude at a particular MP/RPM combination. Horsepower goes up because of decreasing OAT and pumping losses.

[hilltop170]

Bruce-
Don't forget to calculate the temperature adjustment for non-standard temps.

[Bruce Fenstermacher]

Richard, I just have to figure out how that part of the chart works.

[VFR52170B]

Interesting read folks!

However, I'm a bit confused by the premise of this discussion!

Maybe I'm way off base here, but to my understanding 'horsepower' is a figure that is directly related to engine RPM, which in this case, is the same as the propeller RPM. Anything that contributes to more RPM, will, for lack of a better word, contribute to more "horsepower".

Let's just assume you're in a common cruise, at maybe 7k. We'll presume that the atmospheric conditions (temp/PA = DA) is constant, for the sake of the study. Also, we'll just guess that you are producing 22" inHG , and in level cruise, and producing an airspeed of say 105kts. Maybe this is giving you 2500 RPM. Your horsepower will be a reflection of several things, yet it will always be directly related to RPM.

Now dive.. Not a lot, just enough to gain 15-20kts, and maintain that 22" on the MP gauge. What happens? Now you're going 125kts, and you're still at 22", but the engine is at 2750rpm. Also, your HP (if you could read it) would have gone up something like 15-20!! HOWEVER, your thrust (again, if you could read it) would have significantly lowered (the part you feel, pulling you). Of course how much would depend on the propeller's design. So as Bruce mentions in the initial post, it would be more correctly stated that the engine/prop combo is producing less thrust at altitude & RPM as compared to sea level at the same RPM.

More RPM at the prop/hence at the engine produces more friction, which in turn = more wear. More doesn't = better. In some instances/depending on propeller installation, 2500 might actually cruise faster (and with better economy) than 2700.

Just the humble thoughts of a Sim programmer ..

- Joseph

[VFR52170B]

Reciprocating engines are air pumps. The RPM is just one factor that determines the horsepower output of the shaft turned by the air pump. Air density & temperature are just as important.

2700 RPM does not always mean 145 HP of output on an O-300. Atmospheric pressure gets lower as you go higher and there is less available energy for your engine/air pump.

I agree 100% !

Only pointing out that RPM is one of those factors.

*smile

[Bruce Fenstermacher]

OH NOoo my head is hurting reading and thinking about what Joseph and others have said.

It is simple or at least I thought it would be if I could figure out how to use the chart. Tachometers are harder to read and less stable that manifold pressure gauges. So I use mine to set power. It is simple. I'm at full power and I want to go to my economy cruise setting I pull the throttle back to 21.5" MP and level off. Eventually the RPM will settle down to about the same RPM because I always cruise at about the same altitude. Thats it, I don't chase the tach.

I just wanted to know what my power setting where based on the MP rather than the tach.

But like many things it is just not that simple.

[cessna170bdriver]

OH NOoo my head is hurting reading and thinking about what Joseph and others have said.

It is simple or at least I thought it would be if I could figure out how to use the chart. Tachometers are harder to read and less stable that manifold pressure gauges. So I use mine to set power. It is simple. I'm at full power and I want to go to my economy cruise setting I pull the throttle back to 21.5" MP and level off. Eventually the RPM will settle down to about the same RPM because I always cruise at about the same altitude. Thats it, I don't chase the tach.
K
I just wanted to know what my power setting where based on the MP rather than the tach.

But like many things it is just not that simple.

Bruce,
The new mechanical tach I installed at overhaul time in 2006 is rock steady. I can tell whether the needle is pointing to the low side or high side of a line. The old one had about 50 rpm or so waver, the main reason I replaced it. That said, I do prefer using my manifold pressure gauge to set the throttle, then verify with the tach.