Cyclic and Collective Response in Relation to Rotor Speed

Phil Noel (copyright)

Higher – more response & takes more power

Lower – less responsive & takes less power

If you want your heli to be more aggressive in the cyclic & collective response - raise the head speed. If you want your cyclic to be less aggressive, lower the head speed.

The higher head speeds take more power (burns more fuel or drains batteries faster), so if you want longer flights, then you can run lower head speeds.

Nitro & Gas power

At top stick where your throttle is already wide open, you may want to drop some of your top end pitch to keep it from bogging when at top stick. Of course if you are proficient at collective/cyclic  management then you can have more collective here for more collective pop but you will have to be proficient at unloading that before the engine really gets too loaded.

At hover and other points of the throttle curve, to raise the head speed, increase the throttle % openings at each point on your throttle curve. To keep your hover at the same place on the stick, you then will have to drop the pitch accordingly, ditto for the other points.

For the best results in maintaining power and rotor speed, when adding any type of cyclic load, you will have to also go into the mixing menus of your radio TX and mix for throttle to be added when you give fore-aft (elevator) commands and roll (aileron) commands.

Electric power

As most ESCs are most efficient when operating as close to a 100% setting as possible, and as it is usually advisable to run them higher then 80%, it is important to pick a pinion for your power system (motor and selected battery packs) that will give you your desired head speed with ESC settings within the range of 80% to 100%.

How to calculate: (examples using the Century 600+ motor and 5S Lipo and the 86T maingear of the Swift 16 and the Swift Carbon 550SE and 620SE)

NOTE: A good lipo pack will deliver 3.65V or better per cell, so a 5S pack would deliver 3.65 x 5 = 18.25V. An A123 pack will deliver only 2.8V per cell in this type of application, so an 8S A123 pack would be 22.4 volts

Calculate the rpm of the motor at 100%: KV of motor x voltage of pack (e.g. 1110 x  18.25 = 20,258). As the Century 600+ motor has a good 90% efficiency rating, that will probably be around 18,232 (20,258 x 90%).

Calculate the gear ratio needed for a given head speed. Take the motor rpm and divide by the desired head speed. So for 1900 you get 18232/1900 = 9.6:1. For a hotter 3D head speed of 2100 you get 18232/2100 = 8.68:1

Calculate for pinion requirement by dividing the number of maingear teeth by the gear ratio. So for 1900 you have 86/9.6 = 8.96. So the 9T pinion would be close enough. This will give a gear ratio of 9.55:1 for a head speed that will be pretty close to 1900 (18232/9.56 = 1907). For the hotter 3D head speed of 2100 then you would calculate the pinion teeth requirement as 86/8.68 = 9.9. So going to a 10T pinion would do the trick nicely resulting in a gear ratio of 86/10 = 8.6. The resultant head speed would be 18232/8.6 = 2120.

Of course these are calculations at the 100% ESC settings. If you want to fly at rotor speeds lower then 1900 for example, you can use the 9T pinion and an 80% ESC setting, which would deliver a relatively low rotor speed of 1900 x 80% = 1520. So using the 9T pinion and by setting your ESC up from there, you can tune for a rotor speeds from a docile 1520 to a much more lively 1900. And for real hot cyclic & collective, you can move up to the 10T pinion and also slow your rotor speed from 2100 by dropping the ESV settings to 80% for a selection from 1680 to 2100.

Of course the higher the rotor speed, the more power it takes to turn the rotor at any given pitch setting, even at zero pitch. So if the power system is overloaded with too much pitch, it will also sag (drop rotor speed). So you will have to go into your pitch curves and lower  them accordingly.