hevansrv7a
Well Known Member
I have been struggling with the way we estimate BHP. This is just a small piece of the puzzle.
If I understand it correctly, manifold pressure (MP) is a pure pressure measurement just as is the pressure altitude. But, the temperature of the ambient air can vary considerably. Therefore, the density altitude of a given stream of air at a given MP can vary as much. The amount of oxygen in a cubic foot of air is the same for a given density altitude, but not necessarily for a given pressure altitude.
Engines use oxygen. We control the physical mixture of fuel and air (which is roughly 22% oxygen and varies very little). Thus we can use mixture control to get a (for example) best power mixture at any time. My operating assumption is that the ratio of oxygen to fuel (by weight) for best power is pretty constant, especially at high power settings.
The oxygen available thus controls the amount of fuel that can be used beneficially for best power and the density altitude controls/describes the amount of oxygen available. But we keep using MP because we don't have anything better. Or do we?
Well, perhaps not in flight. However, from the comfort of my desktop, I can compensate for the temperature of the incoming ambient air and come to the conclusion that if the temp is lower the oxygen available is higher and put a number to that.
So that means that I can more closely estimate the BHP at a given combination of RPM and MP by compensating for the difference between PA and DA.
A] Is this what the power charts from Superior and Lycoming are talking about? (1% per 15 degrees F or similar)
B] Is this a correct analysis? Is this a correct solution?
C] What if anything is the relationship of this issue to the "fact" that engine power for a given MP x RPM is greater at higher altitude by a factor of roughly 1% per 1,000 feet?
Thanks in advance for your contributions!
If I understand it correctly, manifold pressure (MP) is a pure pressure measurement just as is the pressure altitude. But, the temperature of the ambient air can vary considerably. Therefore, the density altitude of a given stream of air at a given MP can vary as much. The amount of oxygen in a cubic foot of air is the same for a given density altitude, but not necessarily for a given pressure altitude.
Engines use oxygen. We control the physical mixture of fuel and air (which is roughly 22% oxygen and varies very little). Thus we can use mixture control to get a (for example) best power mixture at any time. My operating assumption is that the ratio of oxygen to fuel (by weight) for best power is pretty constant, especially at high power settings.
The oxygen available thus controls the amount of fuel that can be used beneficially for best power and the density altitude controls/describes the amount of oxygen available. But we keep using MP because we don't have anything better. Or do we?
Well, perhaps not in flight. However, from the comfort of my desktop, I can compensate for the temperature of the incoming ambient air and come to the conclusion that if the temp is lower the oxygen available is higher and put a number to that.
So that means that I can more closely estimate the BHP at a given combination of RPM and MP by compensating for the difference between PA and DA.
A] Is this what the power charts from Superior and Lycoming are talking about? (1% per 15 degrees F or similar)
B] Is this a correct analysis? Is this a correct solution?
C] What if anything is the relationship of this issue to the "fact" that engine power for a given MP x RPM is greater at higher altitude by a factor of roughly 1% per 1,000 feet?
Thanks in advance for your contributions!