Thanks, now it's clear.

Just to show what "quick check" means a few diagrams. I know it's not really an explanation, at least for non-engineers, but anyway (because it's fun for me).

The NEU-F3A-1 (1300 kv, 6.7:1 gear) was assumed with the data published on the website. Impedance of ESC, battery (8s 12 Ah LiPo), and so on is only estimated. The data for the Graupner CFK 20x12 prop are calculated with Martin Hepperle's program and quite reliable, except near static. A rather simple airframe was assumed with 20 lbs weight, 2200 sqin wing area and 5m (197") span, giving 17.6 aspect ratio. (Quite ambitious, and seems no case for a composite construction, at least for wing and tail.) Horizontal stab is 20% of wing area and vertical stab 10%. Tail moment arm 51". Airfoil is Anderson SPICA flat bottom (sorry BMatthews) because it's quite cambered (4.62%) and I have data.

Now all is put together in spreadsheets. I made these 10 years ago and unfortunately didn't automate them so far because I dislike Excel programming. So they are virtually useable only for me. I will share them anyway if requested.

First a comparison of drive rpm and efficiencies at ground level (standard atmosphere), full power and cruise power. Yellow is rpm, dark blue motor-gear efficiency, medium blue prop efficiency, light blue total drive efficiency, that is the power effective on the airplane. All dependent on airspeed. The airplane's minimum power demand is at 12 m/s (27 mph) airspeed.

At full power the drive's maximum efficiency would be at 23 m/s (52 mph) so in this case the prop's pitch is too big. Second diagram the same except at partial power (0.465 of full power). That's just enough to cruise at 12 m/s which is not the absolute minimum speed/power but still gives a small amount of speed stability. Now the drive's maximum efficieny is at 10 m/s (22 mph) so the prop's pitch is a bit too small. That's what I meant in my previous post, but it's merely a "theoretical" comparison. What really matters you'll see now.

Yellow is drag of the airframe, pink is drive thrust at full power, blue at cruise power. It's not easy to imagine at which speed the height will be maximized. Next diagram: yellow is airframe power consumption, pink is power delivered to the airframe at full power, dark blue at cruise power setting. Light blue is difference between full drive power and airframe power demand. This difference is climb power. Dotted pink is reachable altitude/height with maximum 6700 m (22k ft) at 16 m/s (36 mph). Climb rate is 5.6 m/s (1100 ft/min). Of course that's ground air density.

But next two diagrams show the case of about half ground density, that is 18k ft. Because the air is thinner the airplane flies faster (airspeed 17.5 m/s, 39 mph) for maximum reachable height. This reachable height is slightly reduced (6300 m, 20.6k ft) because the power delivered is lesser and a bigger part of it is needed to overcome drag. Here you see that the airframe's aerodynamic quality is still of minor importance (maybe except aspect ratio). But cruise flight requires a higher power setting (0.605). Climb rate is 3.1 m/s (610 ft/min). So even though some parameters change with altitude and climb rate gets lower and lower, target altitude could be reached with even a bit charge in the battery for some loitering and landing approach.

There are no reserves which means the accuracy (not to mention correctness) of this calculation would be crucial. So no responsibility is taken for the correctness of this information.