Remember, this is a Belite ProCub ultralight aircraft with a tiny 36.5 HP engine. It should not perform this well, but it does! And the airplane has massive 21″ tundra tires slowing it down…
Starting from a field elevation of 1400 feet, an immediate full power climb was established to 4000 feet AGL. Temperature was approximately 90 degrees for a calculated density altitude of 3900 feet. (Final density altitude for this demonstation would be about 6500 feet!). The blue marks are 640 feet apart in altitude and 110 seconds apart in time — this calculates to 350 fpm.
Based on the standard rule of thumb that Density Altitude causes a loss of 3.5% of power per 1000 feet; the initial climb was started at a real power setting of (100% – (3.9K x 3.5%)) = (100% -13.65%) = 86.35% of full power, which is equivalent to (86.35% of 36.5HP) = 31.5HP.
Stay with me here: we’ve just lost 5 HP due to density altitude issues. If we had that 5 HP back, it would contribute to climb rate according to the following formula:
(Excess HP x 33000) / gross weight = rate of climb
With a gross weight of 550 pounds for our Belite ProCub, the math looks like this:
(5 HP x 33000) / 550 = 300 FPM increase in rate of climb
This means that the sea level performance of the same engine / ProCub combination would have been 650 FPM, given standard altitude conditions. I’m thinking that’s a little optimistic, but without argument, the performance would be a lot better at sea level with the little 36.5 HP engine.
Conversely, we can use the same type of math to deduce when the climb rate would degrade to 100 FPM, which is the standard definition of service ceiling. I’ll spare you the math and just tell you the result:
The calculated service ceiling of the ProCub with the 36.5HP Polini Thor engine is 7100 feet with 21″ tundra tires. Changing to smaller tires will improve this number considerably.