Is there a prescribed method for figuring out gearbox ratios - RC Groups
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Sep 27, 2001, 09:14 PM
Mike Swan

Is there a prescribed method for figuring out gearbox ratios

Does it pertain to wing areas,all up weight,or is it run what ya brung. mike.
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Sep 28, 2001, 12:50 AM
Motors beat engines!
Has more to do with stall speed and motor power and what rpm the motor is most efficent.

Do yourself a favor and download, it makes picking gears easy.
Sep 28, 2001, 09:11 AM
Product Manager at Hobbico
GWRIGHT's Avatar
I'm an admitted "motocalc junkie". I use it when designing a plane,.. and afterwards, attempting to optimize the drive system/etc. I've developed one rule of thumb after all the experimentation I've done. That rule is to use the absolute highest gear ratio possible that still allows an appropriate sized APC "E" prop to be used. I like aerobatic "3d" type planes, and pitch speed is not as important as static thrust. Another thing to take into account for this type flying is that a pitch to diameter ratio of one to 1.5 is about right,...i.e. 12X8, 9X6, 15X10, 18X12,..etc. This is the ratio at which the prop is not statically stalled and cavitating. I try to use the largest diameter prop that's available with a 1.5 to 1 diam. to pitch ratio, for the highest ratio available for the gearbox I'm using. This has to be tweaked sometimes, as it's hard to find the "perfect combination" even with motocalc (motocalc gets you in the ballpark, but you can't fly motocalc). The E3D uses 4.6 gearing (highest I can put in the inexpensive GD600 gearbox), and a 12X8. This prop was not the best according to motocalc but it was in the top 3. I went to the field with about a dozen props and tried them all, starting with what motocalc predicted 13X6.5 at first, and progressed through all the motocalc "good" predictions, arriving at the 12X8. This 1.5 to 1 ratio can be "tweaked" a bit sometimes for example, I've found a 13X7.5 sonictronics folder to work equal, if not a touch better on this airplane. A prop is always partially stalled (the entire blade is not working) unless it is being flown very very close to it's pitch speed, so a touch under 1.5 to 1 gives you more of the prop actually working in a static condition (for hovering and extremely low speed stuff). If you want very long flights for sport flying,.. a diameter/pitch ratio of 1 to 1 works well. More of the prop is stalled during takeoff/handlaunch, so the thrust is lower than when the plane is actually flying. However, you can fly at more reduced throttle settings because the pitch will keep the plane going at lower rpms, and you can sometimes use this part-throttle gain in efficiency to achieve much longer flights (assuming you use throttle appropriately). I've found the APC "E" props (and apparently this sonictronics folder also) to be so much more efficient than the normal "logs" that I tweak gears a touch if the right prop size is not available from APC in an "E" version. My big E3D had the MEC monsterbox on it, which allows up to 9.6 to one. I was flying it at 8 to 1 (lower than possible) because the largest APC "E" prop available is an 18X10. This was still a touch low, as an 18X12 should have been better at that ratio,.. and something in the 20" size at 9.6 gearing would have been even "more better" . Since 18X10 was the largest available, I geared appropriately to extract full power from the motor, up to it's limit at full throttle (40 amps and 10 cells), then using throttle to get flight times. Motocalc is really an wonderfull tool that is a requirement in my opinion if you're going to fly electrics. You still have to experiment, using a wattmeter, a tachometer, and actually flying the aircraft,.. but motocalc gets you in the ballpark to begin with. I get laughed at sometimes, when I show up at the field and use a full day trying different props, and sometimes gear ratios (my record is 30 props in a day on a glow plane), but my stuff is almost always outperforming anything else at the field. This is even more important in the electric arena due to the gearing and proping possibilities being almost endless (I've flown a single electric motor on props ranging from 7X5 up to 18X10, varying the cellcount and gear ratio for instance).With the help of motocalc, I've taken planes that I couldn't fly initially,(one would just have a rather lengthy glide to landing from handlaunch, another would fly at handlaunch altitude and not climb), and transformed them with gearing and propping to the point where they would ROG very well, and had extreme vertical for stall turns and such. This was with the same input power (watts), just being "optimized". Download motocalc, it's an eye-opener. Just don't get too addicted
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