It’s a beautiful day in San Francisco. I’ll bet I could see the Sierras on climb out, but all my club’s planes are taken, and I’ve got more than a year to wait for my own. Meanwhile, I puzzle over nerdy questions like this:
The SR22 cruise performance numbers show the fuel flow falling off as the temperature increases. (Actually I’m looking at Paul Traina’s spreadsheet – thanks Paul.) But the SR20 POH shows the fuel flow remaining constant as the temperature increases. Here are comparable excerpts from both:
SR22: 8000ft pressure altitude, 2500rpm, 21.7map:
ISA-30: 79%pwr, 19.1gph, 178ktas
ISA-00: 75%pwr, 18.4gph, 181ktas
ISA+30: 71%pwr, 17.9gph, 184ktas
SR20: 8000ft pressure altitude, 2500rpm, 22.2map:
ISA-30: 73%pwr, 10.1gph, 152ktas
ISA-00: 69%pwr, 10.1gph, 154ktas
ISA+30: 66%pwr, 10.1gph, 156ktas
The 22’s behavior makes sense to me. As you hold everything else constant but increase the air temperature, the air density decreases. So the air mass that both engines are pumping decreases. The 22 appears to be keeping its fuel/oxygen mixture constant, while the 20 is keeping its fuel flow constant (and thus making the mixture progressively richer).
Is this the “altitude compensation” that’s vaguely referenced in the SR20 manuals? And does the SR22 lack that device? What are the benefits? Do you spend more time fiddling with the mixture control on one plane than the other?
Curiously, the SR20 figures show this constant fuel flow phenomenon at all altitudes and temperatures except at 6000 feet. That table shows the same GPH numbers for ISA-30 and ISA, but drastically lower numbers for ISA+30. This only happens at 6000 feet – at 4000 and 8000 (and all other altitudes) the flow is constant. POH misprint? Or is everything else a misprint?
Of course, the correct Zen answer is to stop rationalizing and fly…