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Thunder Tiger Raptors 30-90 - Imperio > Which gear ratio best for the OS 50
 
 
Crashr30
Senior Heliman
Location: DFW

I am converting the Ratpor 30 to a 50. The stock ratio is 9.56:1 and the gears supplied in the conversion kit is 8.5:1. So here is the question. The peak H.P. on the OS 50 is 1.8 H.P. at 17,000rpm.(The practical operating range is 2,000-20,000.) If I want to maintain a headspeed at peak H.P. then would not the 9.56:1 be better? 17,000/9.56 is equal to about 1780 rpm headspeed. 17,000/8.5 is equal to 2000 rpm. From what I understand and I did not have the sheet to read, is that the conversion instruction sheet in the kit makes some sort of statement that 9.6 may be the preferred ratio for the 50 engines. The reason for the question is that I find it strange that we have to use a governor or cut back the max point on the throttle curve to run the engine in the heli without really overcooking the head speed. It would seem that the higher ratio will increase engine speed and maintain a reasonable head speed. I know it is an awesome motor and all but it seems to me that gear ratio of 8.5 is not right. I may not be thinking straight so help me out. The start of this question was an article in the latest RCM in the Hover section. Any help would be greatly appreciated.
05-06-2002 Over year old.
 
 
Doug
Elite Veteran
Location: Naples Florida....

The 8.5 and about 1900 works great and is quiet.
05-06-2002 Over year old.
 
 
Lwnemesis
Senior Heliman
Location: Seattle, WA

it does make sense when you go to an 8.5 gear ratio you are making the engine work harder. it would only turn the head faster if there were no blades attached to it the blades load up the motor and so does the 8.5 gear because the pinion is bigger. the OS 50 has so much torque it needs the higher gear to load properly. And its not the horsepower that turns the blades it the torque.
05-07-2002 Over year old.
HOMEPAGE  
 
 
Stet
Elite Veteran
Location: Long Beach CA

I'm running the helihut gear (I think it is 84 teeth-see his website for exact number) with standard pinion giving 9.33 ratio. Though I understand the 8.5 is better for the torque curve, this combo seems to be working fine. I have the OS 50, Hatori 666 muffler, SAB 600 blades and running a governor. I run the headspeed at 1650, 1750, 1850.

All I can say is from my limited experience basis, the heli will climb out at 10 degrees at any of these three speeds with no reduction in speed. This heli is really fast, and I've had no heating problems and am still running it quite rich.

It's easy to swap main gears, not so easy to change pinions. I think the 8.5 is more mandatory for the OS46, but the 50 handles the 9.33 ratio just fine (for me). These options are cheap, so try them all and see what you like.
05-07-2002 Over year old.
 
 
Smooth3D
Heliman
Location: San Francisco Bay Area

I believe the method of calculation is:
Rotor RPM x Gear Ratio = Engine RPM
Example:
1850 x 8.5=15725

We lower the gear ratio so that we don't over rev the engine. Doesn't matter if its an OS46 or OS50. Pick the Main gear tooth count devide by the pinion tooth count for desired gear ratio. Raptor 30 Gear ratio is 9.53-9.55.

To perform aerobatics we want the rotor RPM to be around 1800-1900 -- e.g. 1850 x 9.55=17667 -- This is beyond the max RPM OS50 is rated for. For best results, we run motors safely below the Max. RPM they are rated for.

I run the TT conversion with OS46, Hattori and 600 carbon blades @ 140 grams each. This combination is just a sweathart. I get a healthy 15min of Aerobatics, and its super quite.

Hope this will be of use.
05-08-2002 Over year old.
 
 
Stet
Elite Veteran
Location: Long Beach CA

Oops,

make that 1600, 1700, 1800 (I kept it below the max rated power/RPM)
05-08-2002 Over year old.
 
 
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Thunder Tiger Raptors 30-90 - Imperio > Which gear ratio best for the OS 50
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