Quite often a question pops up, asking after oxygen add-on to our fleet of high performance GA airplanes. This is sometimes in reference to their seemingly high ceilings, which are well above 10000 ft. I for one find such heights redundant in our current GA fleet, while others want to bring the thing high up there. It is simply a matter of our simming preferences. But this brings another question: what are the benefits and downsides of getting up there? What is the tradeoff?
The idea is very well demonstrated in the A2A Comanche's manual, in particular on page 66 from where the following image is adopted. Look it carefully and you get the idea. Note that this is clearly a calculated chart, not raw flight test results.
A2A Comanche:
You do find a similar graph in Bonanza's manual, it only does not plot the full throttle graphs down to the sea level. I picked this from Comanche's book because it illustrates the principle better.
How would an airplane with a turbo do? Well, if we take a turbo-normalizer as an example (it only takes a little imagination to figure out how one with some boost would do), it would tilt the full power graph to have a similar linear segment from its current sea level point all the way up to turbo's critical altitude. Thereafter it would plot a similar full throttle curve the naturally aspirated engine does. It would only happen at much higher altitude - and with massive increase in TAS with even relatively low critical altitude. Now, this is not strictly what happens because we are not usually holding precisely constant 100 % power up until critical altitude, but we shall discuss why if it sometimes comes relevant to our A2A GA fleet.
If, for any reason, we wanted to keep our cruise power setting at some given percent of maximum continuous power, our optimum altitude in terms of speed performance would be the altitude where the 'knee' occurs in the graph. It is not coincidence that this happens to be the full throttle altitude for that percent of power. For instance, using the graph above, doing 65 % of MCP, we would find our optimal airspeed at about 9000 ft. If we wanted to do faster, we'd come lower while keeping wide open throttle.
The speed is one thing, but what about economy? Let us pick one image from the Bonanzas manual, because in this case, it illustrates the principle better! (Please don't mind the red arrows illustrating the manual's example problem.)
A2A Bonanza:
It takes a little bit of imagination to extrapolate the full throttle lines at various RPMs. They won't keep on rising eternally, but at some point fold together and reach their maximum at fairly close to the absolute ceiling of the airplane but not quite.
This means that if you want to reach your absolute maximum range, or reach your destination with very minimum amount of fuel used in cruise flight, flying high is preferred.
I prefer flying my A2A GAs at around altitudes where the 'knees' of the cruise power settings of around 75...65 % of MCP are, because this is the altitude range that gives me the most flexibility in going fast with wide open throttle, high RPM and proper mixture if I prefer, or doing slightly lower RPM and lean mixture if I want reasonable economy. I never fly such long sessions so that absolute maximum range would become a significant factor. Of course, other factors such as weather probably play the biggest part. I enjoy going under the clouds in between the peaks if those are around, smashing bugs and treetops. I get more from the scenery and it allows me more activity than following some straight lines.
So, I prefer relatively Low, that is, mostly under 10000 ft. I feel the naturally aspirated GAs are most home at there.
What is your preference, and why?
-Esa