How is the required gear ratio calculated?

Motor RPM equals KV times battery voltage. The gear ratio needed to reach your target head speed is that motor RPM divided by the head speed. For example, a 450 KV motor on 44.4 volts spins about 19,980 RPM, so a 2,200 RPM head needs roughly a 9:1 reduction.

Why is real head speed lower than the calculation?

KV times voltage is the unloaded motor speed. Under the aerodynamic load of the rotor, the motor turns at roughly 80 to 90 percent of that figure, so a governor will hold a slightly lower head speed than the pure geometric ratio suggests. Size the gearing a touch high and let the governor trim it.

What collective pitch range should I run?

Scale and sport flying typically uses about minus 2 to plus 10 degrees, sport 3D runs symmetric minus 10 to plus 10, and hardcore 3D often goes to minus 12 to plus 12. Hover sits around 4 to 6 degrees of positive collective on most electric collective-pitch helis.

Why does blade tip speed matter?

Tip speed equals pi times rotor diameter times head speed divided by 60. As tips approach the speed of sound, drag rises sharply, the rotor gets loud, and blades are stressed. Keeping tip Mach below about 0.65 avoids transonic losses, which is why larger rotors run lower head speeds.

How do I pick the right pinion?

Divide your main gear tooth count by the required gear ratio to get the ideal pinion. Round to the nearest available pinion, then check the resulting head speed the tool reports. Swapping a tooth or two on the pinion is the usual way to fine-tune head speed.

RC Helicopter Blade Pitch & Head Speed Calculator

Dialling in an RC helicopter means matching the motor and gearing to a target head speed, then setting a collective pitch range that suits how you fly. This calculator ties those together: it sizes the gear ratio and pinion from your motor and battery, checks the blade tip speed for transonic trouble, and recommends a pitch range for hover through hardcore 3D.

How it works

The gearing reduces unloaded motor speed to head speed, and tip speed follows from rotor geometry:

motor RPM   = KV × voltage
gear ratio  = motor RPM / target head speed
pinion      = main gear teeth / gear ratio   (round to nearest)
tip speed   = π × diameter(m) × head speed / 60   (m/s)
tip Mach    = tip speed / 343 m/s

Because KV times voltage is the no-load speed, a governed setup actually holds about 80 to 90 percent of that under rotor load, so the geometric ratio is a starting point you trim with the governor and pinion choice.

Example and tips

A 600-class heli (600 mm rotor) on a 450 KV motor and 12S (44.4 V) pack spins the motor near 20,000 RPM unloaded. To hold a 2,200 RPM head it needs roughly a 9:1 reduction, which is about a 12-tooth pinion on a 112-tooth main gear. At that head speed the blade tips travel around 69 m/s — well under Mach 0.65, so there is room to run higher head speeds for 3D. Larger rotors hit the transonic limit sooner, so watch the Mach figure and back off head speed if it climbs past about 0.65.