Dayaks

Good afternoon,

I'm new to prop selection & motor selection, but I've been fiddling with an online calculator.

Please correct me if I'm wrong, but it looks like if I want to go fast I want a smaller prop at high RPMs.

If I want efficiency I go large and slow. I'm a direct drive kind of guy, so I'm looking into the 4410 Neumotor (low RPM) or the 2230 (high, fast).

For example, with a 7.5 kW load I could do a 36x16 prop, 3750 RPMs and have 73 lbs of thrust @ 57 MPH (so I'll assume my max speed is around 50 MPH?)
or with 7.5 kW I could choose a 18x10 prop at 11,100 RPMs and have 46 pounds of thurst @ 106 MPH (half way to a turbine, right? .78 Mach.. so max speed of around 95 MPH I'd guess?).

Is this as simple as it seems? Are there any huge nuaunces to watch out for? I realize over .65 mach gets noisier, .85 mach bad things begin to happen.

MTK

You might want to post the question in the Aerodynamics Forum. Your Mach numbers are right regarding tip speed and your math looks sound, except there was no concern for plane drag.

Large scale models won't perform on an 18x10 anywhere near 100 mph even if the prop wants to go that fast. A 35 pound model may have trouble taking off even.

7 1/2 KW (10 HP) may not be adequate power for the larger models to get them to perform (aerobatics)

Dayaks

Thanks for the reply. I can see what you mean about prop size. I'll check out that thread and keep looking around.