Procub demonstrates 350 FPM climb at 3900 ft density altitude.

Belite ProCub Rate Of Climb

Belite ProCub Rate Of Climb

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.

4 thoughts on “Procub demonstrates 350 FPM climb at 3900 ft density altitude.

  1. Nice. I am putting a Rotax 447 (40 hp) on my Ultracub which I am building as an E/AB. The airfields around here are pretty close to sea level with temps rarely hitting 80. Should be able to cruise around the Cascades a little. Any idea what the service ceiling would be with the F23? regular tires?

  2. James, your formula for the estimate of the increase in rate of climb on descending to sea level is incomplete. It should read: EHP * 33000 * prop efficiency / gross weight = ROC. This results in a smaller (and less “optimistic”) estimate of the sea level rate of climb. For example, you cite 5hp as “lost” due to density altitude, but with a prop efficiency of 80% this is equivalent to having “lost” only 4 hp (using your formula), or, put another way, the increase in rate of climb on return to sea level would only be expected to be 300 * 80% = 240 FPM.

    However, correctly including prop efficiency has the reverse effect on the estimated service ceiling. The amount of rate of climb lost to density altitude is smaller, therefore the residual rate of climb is larger for any given altitude above the datum altitude. If your rate of climb at sea level is 590fpm (350fpm @ 3900ft + 240fpm “restored” by returning to sea level as calculated above), then the change of rate of climb with altitude is 240fpm / 3.9Kft = 61.5 fpm lost per thousand feet of altitude gained. Then the service ceiling is (590fpm – 100 fpm) / (61.5 fpm / 1000ft) = 7965 ft (rounded) as opposed to your estimate of 7100 ft.

    And, yes, you’re right, changing to smaller tires will have a disproportionate positive impact on rate of climb. Every bit of parasite drag you can shed pays double dividends in the increase of rate of climb. That’s because a reduction in parasite drag is matched by a reduction in induced drag (where the curves cross – remember?) which happens at a slightly higher airspeed where the propeller just happens to be more efficient, too…..

    Keep up the good work!

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