H.Evan'sRV7A
Well-Known Member
I was just curious, so I started with the size of my engine, the characteristics of fuel and air, the way a 4-stroke, gasoline IC engine works and the numbers from Superior. I eventually concluded that the volumetric efficiency of the engine is 97.83% at full power, WOT, specified conditions (by Superior). Since this is a high but seemingly reasonable finding, I tentatively conclude that the way I am doing the calculations is valid. I did use a simple, 6 pounds/gallon figure for the fuel, so right there you can find fault, circumstances depending.
By itself, this is pretty useless, but, to start with, I don't recall ever seeing this number. Also, I was told by someone who has done extensive, independent engine testing that the Superior is more volumetrically efficient than the equivalent Lycoming. This means, if I understand it right, that the engine can pump more fuel-air mix for a given displacement and RPM. That suggests it is also more powerful, but that is not shown in the charts. Of course, some folks at Superior told me that they just copied the Lycoming charts, but I'm not sure if they meant they copied every number or just the way the chart is done.
You can view my crude spreadsheet at: https://docs.google.com/spreadsheet/ccc?key=0AnQBLSO8JYPJdFU4bXd4cm5sdExrRFlLQXZkdEdXTVE
My next little project will be to use Jan Carlsson's design for a test prop. Thanks, Jan! The prop is being built by Lonnie Prince for the experiment. I will use it with my GRT's tachometer and scales under the airplane to measure torque. That should, I hope, allow me to calibrate the prop's power absorption. I have already checked with others that Jan's basic math is correct, but when you build it, you introduce deviations from theory, right?
But I will also, at the same time, calibrate the power indications which are derived from manifold pressure, density altitude and RPM. My GRT EFIS will then indicate power correctly. At least, that's the goal.
And hopefully, I will also be able to build and calibrate an instrument to use the twist of the FW forward area to tell me torque when in flight. That should agree with the instrument calibration, of course. The real purpose of that instrument will be for use in aero-diesels where MP is useless.
Why bother? Well, engine manufacturers don't really lie, but they often tell the truth with different test conditions and usually don't say what those conditions were. So if we compute propeller efficiency with a BHP number, we can be off by a big amount. The power we get from an installed engine is often not what the book says it is.
Aside from being interesting and fun, this stuff is what has been missing from a lot of discussions about propeller efficiency.
More to come..
By itself, this is pretty useless, but, to start with, I don't recall ever seeing this number. Also, I was told by someone who has done extensive, independent engine testing that the Superior is more volumetrically efficient than the equivalent Lycoming. This means, if I understand it right, that the engine can pump more fuel-air mix for a given displacement and RPM. That suggests it is also more powerful, but that is not shown in the charts. Of course, some folks at Superior told me that they just copied the Lycoming charts, but I'm not sure if they meant they copied every number or just the way the chart is done.
You can view my crude spreadsheet at: https://docs.google.com/spreadsheet/ccc?key=0AnQBLSO8JYPJdFU4bXd4cm5sdExrRFlLQXZkdEdXTVE
My next little project will be to use Jan Carlsson's design for a test prop. Thanks, Jan! The prop is being built by Lonnie Prince for the experiment. I will use it with my GRT's tachometer and scales under the airplane to measure torque. That should, I hope, allow me to calibrate the prop's power absorption. I have already checked with others that Jan's basic math is correct, but when you build it, you introduce deviations from theory, right?
But I will also, at the same time, calibrate the power indications which are derived from manifold pressure, density altitude and RPM. My GRT EFIS will then indicate power correctly. At least, that's the goal.
And hopefully, I will also be able to build and calibrate an instrument to use the twist of the FW forward area to tell me torque when in flight. That should agree with the instrument calibration, of course. The real purpose of that instrument will be for use in aero-diesels where MP is useless.
Why bother? Well, engine manufacturers don't really lie, but they often tell the truth with different test conditions and usually don't say what those conditions were. So if we compute propeller efficiency with a BHP number, we can be off by a big amount. The power we get from an installed engine is often not what the book says it is.
Aside from being interesting and fun, this stuff is what has been missing from a lot of discussions about propeller efficiency.
More to come..
