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General RC Heli Discussion Discuss general topics on rotary wing aircraft, including construction, Servo's, Gryo's and Stabilizer discussions. Heli Pro Shop

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Old 05-11-2006, 07:09 PM   #11
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I'm thinking the OS46. GH is selling it for $150. I have two mufflers already so this will save me some cash. All I will need is the mount and the pinion. Is there a particular reason for changing the pinion?
The 46 should make the old bird RIP(hopefully that doesn't translate to R.I.P. HaHa!!!) Bigger is better as they say.

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Old 05-11-2006, 10:36 PM   #12
Phil Noel
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Most things in helis are a bit more complex then in airplanes and other nitro powered models.

The main reason for changing the gearing, relates to the RPM at which engines develop their best HP to torque compromise, for a given desired rotor speed. Im smaller engines this generally happens at higher rpms then it does with their larger brothers.

So a 9.78/1 ratio will mean that for an 1850 rotor speed, the engine would be turning at 18093/ (9.78 x 1850), which is about the top for a muffler equiped 30-40 engine.

A 9.07/1 ratio (as you would get using the 14T pinion) would mean an engine rpm of 16650, which is better for this larger engine. Your fuel economy will be a lot better also. The OS46FXH carburator will also prove to be more user friendly to set at this engine speed, specially in the hover .

ANother big part of this equation, is what you plan to do with the heli.... 3D, Sport fly or scale fly.

Absorbing this extra power is always going to be a consideration, so doing it in a way that is most efficient for the type of flying and weight/flight loads and rotor span of the heli is important.

Your Hawk III is a very light heli that runs on a short mainshaft and uses 550mm blades. The short mainshaft of these older helicopters do not provide the same pitch range as the later units with the longer and fatter 10mm mainshafts. (e.g. as seen on the subsequent Hawk IV, Sport and the current Hawk Pro).

As the max range is 18 degrees with this shorter mainshaft, in a 3D set up, you would have +9/0/-9, while on the newer ones with a 22 degree range you could have +11/0/-11. A properly set up OS 46FXH will easily pull 10.5 to 11 degrees of pitch on an efficient 550mm set of blades, without overspeeding when at full throttle. And it would also probably only require 50/55% throttle at hover (4.5 to 5 degrees pitch).

So as you can see, with 550mm blade, a whole new pitch curve and throttle curve will be required. And one would never be able to harness the power at a 9 degree top or -9 bottom collective on the 550mm blades. Consequently, one would have to set the throttle not to open fiull at these settings (maybe only 85/90%) and to use the aileron to throttle mixing and elevator to throttle mixing to use this extra power thorough maneuvers when one is loading the motor with a lot of collective & cyclic pitches.

Alternatively, if one is not setting for 3D, and is setting for scale (extra weight) then one would not need anywhere near as much negative collective pitch or head speed. Consequently, he can set for more pitch at the top and at hover and set the throttles for a lower head speed. e.g only use 16 degrees of the range and set it up for +11/6/-5. And use full power at wide open collective and with less cycic/throttle mixing.

Phil Noel
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