A quick how-to on using an
eAnSys gearbox with a 24"-26" prop, based on my experiments, other people's posted data and some calculations.
The optimal rpm range for a light CF 24-25"13" pitch prop seems to be 6200-7000 rpm, 6kwt or less. 6200-6500rpm for a 26" prop, about 7.5kwt. Props differ, of course, so look up data for your prop. This post mostly deals with Falcon Electric CF.
- Use the Single-Gear-L gearbox. If you want to use the 26" prop, get the power expander. That ensures more of the belt in contact with the motor pulley, so the belt can take more load.
- Pick a 5-6kwt heli motor if you are looking to use a 24" or 25" prop, 7-8kwt motor for the 26"
- Target rpms, the expected voltage and your motor's kV will let you estimate the GB ratio. Alternatively, you can pick a GB ratio and adjust for motor kV.
- Get a couple of extra pulleys, to adjust based on your measurements/flying preferences. eAnSys gearboxes can be tweaked for lower rpm/more flight time or more power - takes about ten minutes to remove the spinner/prop/cowl, replace the pulley and put it all back together.
Example:
- I'd like to turn a Falcon CF electric 24x13 at 6500 rpm. That should take 130-140A, so 6kwt max, so probably no need for the power expander.
- I'll use the 2.57 to 4.0 version of the gearbox. Motor rpm under full load are (prop rpm)*(gb ratio). We get 16700 to 26000 rpm.
- I need the unloaded motor rpms to compute the motor kV range, so divide by 0.9 (assumes the rpm drop of 10% under load). So 18300 to 29000rpm.
- kV = (estimated unloaded motor rpm)/(estimated voltage). We expect to have ~43V under load, so 420 to 675kV
- Most heli motors in the 5-6kw range seem to be 500 or 550kV. Use 550kV, assume 43V under load, 10% rpm drop under load. Motor rpm under load V*kV*0.9 = 43*550 rpm/V *.09 = 21300 rpm.
- I want 6500 rpm at the prop, so 213/65=3.28. The closest available is 3.27 - good enough. The effective kV of the whole system is about 170 rpm/V.
- You should probably get pulleys for 3.0 and 3.47, to experiment/adjust. The prop drilling template is also useful.
