Motor Selection Question

[Gyropilot]

I started a thread over in the Low Head Speed Helicopter subforum and haven't had any response so I thought I post the question again here:

Can someone please help me understand what happens when a motor Kv rating is decreased and everything else stays the same EXCEPT for the gearing to keep headspeed at the same rpm?

For an example, let's assume we start with:

450 size heli with 325mm blades
3S 35C Lipo battery
Scorpion HKII-2221-8 motor with 3595 rpm/volt
Assuming 90% efficiency, a 13.60:1 gear ratio @ 80% throttle = ~2100 rotor rpm.

If we change out the motor to a Scorpion HKII-2221-12 which has only 2580 rpm/volt and also adjust the gearing to 9.71:1, then at 80% throttle we should get roughly the same rotor rpm.

Besides the ability to achieve lower headspeed with the lower Kv motor, what else would I gain or lose by doing this? Should flight time be longer, shorter, or the same?

In case any of this is important, here are complete specs for the two motors:

HKII-2221-8

Stator Diameter: 22.0 mm (0.87 in)
Stator Thickness: 21.0 mm (0.83 in)
No. of Stator Arms: 9
Magnet Poles: 6
Motor Wind: 8 Turn Delta
Motor Wire: 16-Strand 0.21mm
Motor Kv: 3595 RPM per Volt
No-Load Current (Io): 2.31 Amps @ 10 volts
Motor Resistance (Rm): 0.024 Ohms
Max Continuous Current: 45 Amps
Max Continuous Power: 475 Watts
Weight: 81 Grams (2.86 oz)
Outside Diameter: 27.9 mm (1.098 in)
Shaft Diameter: 3.17 mm (0.125 in)
Body Length: 38.3 mm (1.508 in)
Overall Shaft Length: 58.7 mm (2.311 in)
Max LiPo Cell: 3s
Motor Timing: 5deg
Drive Frequency: 8kHz

HKII-2221-12

Stator Diameter: 22.0 mm (0.87 in)
Stator Thickness: 21.0 mm (0.83 in)
No. of Stator Arms: 9
Magnet Poles: 6
Motor Wind: 12 Turn Delta
Motor Wire: 12-Strand 0.21mm
Motor Kv: 2580 RPM / Volt
No-Load Current (Io): 1.50 Amps @ 10 volts
Motor Resistance (Rm): 0.038 Ohms
Max Continuous Current: 38 Amps
Max Continuous Power: 530 Watts
Weight: 81 Grams (2.86oz)
Outside Diameter: 27.9 mm (1.098 in)
Shaft Diameter: 3.17 mm (0.125 in)
Body Length: 38.3 mm (1.508 in)
Overall Shaft Length: 58.7 mm (2.311 in)
Max Lipo Cell: 4s
Motor Timing : 5deg
Drive Frequency: 8kHz

Thanks in advance,

John

[RasmusDK]

Hi.

See the max continuous watt rate, - you will get more power out of it. But you will lose flight time :-)

If its flown on the same voltage

[Gyropilot]

Originally Posted by RasmusDK

...you will lose flight time...

Thanks!