I’m getting thru DV20 AFM and the page with cruising performance chart is really confusing me.
1) What did they mean by saying increase/decrease “pressure altitude”? What if I fly below Transition altitude and use regional QNH instead of “pressure altitude” (aka QNE aka standard pressure of 1013mb)? How do I calculate how many feet do I need to add or subtract?
2) I also can’t clearly understand what they wanted to say in the NOTE section. What is “economical cruise”? Why do I need to pick something between 2300-2200 RPM and a MP of 1-2 in.Hg below max possible at chosen altitude when I already have the lowest Liters/Hour number from the table?
What is “fast cruise” ? Why do they say to select 2400 RPM and a MP that will be 0.7 in.Hg less than maximum possible MP to reduce fuel consumption? The chart is already showing that max possible MP on all altitudes (except 0 ft) will stick to 2400 RPM. I don’t get it
Please, explain it to me!
1. The performance numbers are given at ISA at pressure altitudes. Then they tell you how to adjust the table for other temperature conditions.
2. Economy cruise is presumably what gives the greatest nautical miles per gallon (or liter)
Fast cruise is the highest recommended power setting for cruise.
1. Let’s say I fly at 4000 ft, -3C (10C colder than ISA) 55% power. MP of 23.3 will be corrected to 23 and what about pressure altitude? Do I have to climb 500ft higher in order to keep that 55% setting or what?
2. Isn’t “economy cruise” is what this table represent?
“Economy cruise” is normally peak EGT. This is the best-SFC (HP per fuel flow) point for an internal combustion engine, or very close to it.
1) I have no idea what they are talking about. The correction of 0,3" for each 10°C deviation from ISA is universally true for non-turbo engines and you will find it in many manuals for constant speed prop aeroplanes. But I cannot find any way to make the pressure altitude correction bit make any sense at all.
2) I believe that the note says something like this:
“We have put all these numbers here because we were told to. We know you cannot be bothered to understand them so we want to provide you with two simple engine setting instructions:
To go fast, select 2400 RPM and almost full throttle. Do not use full throttle because the engine is set to provide a very rich mixture at that setting and so will use much fuel; do reduce manifold pressure a little as that will save a significant amount of fuel while you will hardly notice the reduction in airspeed. We call that the “fast cruise” setting.
To go a little less fast, with an even better fuel economy, use 22-2300 RPM and a throttle setting to give 1-2" below full throttle indication. With that setting the relation between RPM and manifold pressure will be good, the engine will be happy and you will enjoy comparably good economy."
(@Peter: While you are right, I believe the engine is a Rotax with no manual mixture control, so the pilot need not worry about mixture. “Economy” is here about power selection, not mixture.)
