Having a Percent Power computation and display on my EFIS makes for the ability to see some interesting (if subtle) engine operating behaviors. Recently, I have taken some time to really observe what happens as I am setting cruise power, and thought there might be a little interest. What?s really neat is that you can actually see the relationship between Manifold Pressure, RPM, and Mixture settings ? all reacting just as you think they would, based on the simplest understanding of how the engine should work.
There are two basic things that I wanted to ?prove? to myself about the Percent Power calculation. The first was that the horsepower actually does go up with both RPM and Manifold Pressure (as shown by the Lycoming graphs). The second was that the power peaks when you lean for ?best power?. The first is ?Duh!? obvious?.so I really hoped the Percent Power computation reflected it properly. The second is simply a logical conclusion for an engineer. The engine?s combustion should be the best at the ?proper? (stoichiometric) air fuel mixture. Best combustion should be the point where you get the most power out of the fuel. Best power kind of implies ?most efficient? to me?.so that as you get off of the ?best mixture?, you should lose power. (I'm using the term "best" here in a fairly imprecise way - not implying best power or best economy.)
So?.does it work? You bet! Here?s how I set power, and what I see?
After take-off, I usually reduce RPM to about 2550, but leave the throttle all the way in. I climb this way, with the mixture full rich, until 5,000?, when I lean a bit ? not all the way, but just enough to smooth things out (Interestingly enough, the old rule of thumb that you can lean above 5,000? has you leaning at about 82% power?.). I usually target for cruise between 9-12K, or higher if I can get good winds. When I reach a suitable altitude in this range, I am usually already down to 75% power or less with the throttle all the way in, but if I level off lower due to headwinds aloft, I?ll bring the power back to about 77%, then reduce RPM to about 2350, which seems to be my smoothest point (where we dynamically balanced the prop to the accuracy of the instrument). As I bring the prop back, the % Power drops right down to 75% - I found that 2% drop through trial and error.
The next thing is leaning. Now I am not going to start another whole LOP/ROP/Peak argument ? I generally lean until all of the EGT?s match each other on the moving graph shown by the GRT monitor screen. This usually happens right about peak. I can generally lean a little further and still have a smooth running engine, but sometimes not?..I usually like to run where it?s smooth, simply for comfort. This also corresponds to a little less than the fuel flow shown on the Lyc graphs for the particular % power, so I feel like I?m doing well. Now the important thing for this discussion is that as I lean, with the throttle and prop controls fixed, the % power usually climbs by 1 or 2 percent (along with the MAP) ? confirming to me that you get more power as the mixture approaches stoichimetric perfection. Which is exactly what I wanted to prove to myself.
It?s nice to know that the physics works!
Paul
There are two basic things that I wanted to ?prove? to myself about the Percent Power calculation. The first was that the horsepower actually does go up with both RPM and Manifold Pressure (as shown by the Lycoming graphs). The second was that the power peaks when you lean for ?best power?. The first is ?Duh!? obvious?.so I really hoped the Percent Power computation reflected it properly. The second is simply a logical conclusion for an engineer. The engine?s combustion should be the best at the ?proper? (stoichiometric) air fuel mixture. Best combustion should be the point where you get the most power out of the fuel. Best power kind of implies ?most efficient? to me?.so that as you get off of the ?best mixture?, you should lose power. (I'm using the term "best" here in a fairly imprecise way - not implying best power or best economy.)
So?.does it work? You bet! Here?s how I set power, and what I see?
After take-off, I usually reduce RPM to about 2550, but leave the throttle all the way in. I climb this way, with the mixture full rich, until 5,000?, when I lean a bit ? not all the way, but just enough to smooth things out (Interestingly enough, the old rule of thumb that you can lean above 5,000? has you leaning at about 82% power?.). I usually target for cruise between 9-12K, or higher if I can get good winds. When I reach a suitable altitude in this range, I am usually already down to 75% power or less with the throttle all the way in, but if I level off lower due to headwinds aloft, I?ll bring the power back to about 77%, then reduce RPM to about 2350, which seems to be my smoothest point (where we dynamically balanced the prop to the accuracy of the instrument). As I bring the prop back, the % Power drops right down to 75% - I found that 2% drop through trial and error.
The next thing is leaning. Now I am not going to start another whole LOP/ROP/Peak argument ? I generally lean until all of the EGT?s match each other on the moving graph shown by the GRT monitor screen. This usually happens right about peak. I can generally lean a little further and still have a smooth running engine, but sometimes not?..I usually like to run where it?s smooth, simply for comfort. This also corresponds to a little less than the fuel flow shown on the Lyc graphs for the particular % power, so I feel like I?m doing well. Now the important thing for this discussion is that as I lean, with the throttle and prop controls fixed, the % power usually climbs by 1 or 2 percent (along with the MAP) ? confirming to me that you get more power as the mixture approaches stoichimetric perfection. Which is exactly what I wanted to prove to myself.
It?s nice to know that the physics works!
Paul