120SR motor users - stock mcpx - what length boom?

[gunshine]

I just did this conversion and right now I'm sitting at 15.5 cm from the output shaft of the 120SR motor perpendicular to the vertical center of the main shaft. Even with the longer 300mah pack it sits on its tail.

I am using the stock motor and mostly e-flight batteries. I haven't had a chance to test fly it yet but I am using the in-line Astroid Designs tail mount. What do you guys recommend I start at? I don't do any 3d.

Thanks

[Ampdraw]

The Sr120 tail will not ever be over powered in a stock heli. It has double the stock tail's thrust (roughly).

Meaning you don't need an extended boom. In fact you are better going as short as you can without the tail blades hitting the mains. Lowers the mechanical gain of the tail, corrects the cog issue somewhat and get's the mass closer to the heli..so flips will be tighter.

Go short and shorten (and balance) the Sr120 rotor to 50-58mm

Sr120 tail is kinda overkill on a stock heli imo. Better with a stock, stock length and Kbdd/v3 or 4g3 motor setup.

Amp

[kokoki]

Hi,

I'm no specialist in avionics, THAT'S for sure...lol. All I can provide is my experience so take it for what it's worth. :-)

The stock boom is 125mm. I've had good luck with 145mm and 140mm, I'm currently running it at 135mm. Like you I noticed the 'tail-heavy' COG but you need not worry, the heli still flies well thanks to the 3-axis gyro.

I've read that running a longer tailboom with the stock motor helps with tail authority. Since you're running the SR120 mod, I'm not so sure having the extended tailboom is as important since the motor and rotor generate far more thrust than the stock setup.

You want to have the boom long enough to clear the rotor of course. Some people leave the rotor stock while others trim it slightly (1-2mm off each side). "Tail-Wag" is a problem and one that seems to be directly effected by the length of the boom and the rotor size. I can tell you my tail rotor is trimmed by 1mm and the length of my tailboom is 135mm. The setup seems to hold fine (albeit still tail-heavy). When I ran my heli with the stock SR rotor, I didn't have any issues with tail-wag but it did drift left during hover - once I trimmed the blades that was no longer a problem.

Hope this helps,
-dave

[gunshine]

Thanks for the responses. I'm running the kbdd tail rotor. Not having flown it I think ill shorten it to stock and go from there.

Anyone have the distance from tail output shaft perpendicular to the vertical center of the main shaft?

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