question regarding motor pinion gear

[rayshaw23]

sorry guys, dont know whether if this has been asked before?

but what is the advantage and disadvantage in having a motor pinion gear of 14T and one of 12T? the reason i ask this question is that i'm still learning hovering, but after every battery, the lipo feels hot and they are all starting to bloat. i'm currently using the stock 14T in my trex450. i was wondering by using a 12T pinion gear will improve the situation? and hopefully can extend my flight time too? someone mentioned to me that since i'm not learning 3D, i do not require such high head speed. thanks.

[VinCopter]

There are many causes to bloated lipo packs. Most common are ampere drawn over the pack rated capacity (C) and over discharge of the pack capacity (mAH). By using a smaller pinion gear, the load on the motor is reduced. Thus reducing the ampere drawn, and the flight time can be longer.

Just make sure that the head speed is enough, around 1800-2000rpm. Any slower, the heli will become sluggish to fly.

[rayshaw23]

then how to check my head speed without using a rpm meter? is there some mathematical formula?

[misato]

then how to check my head speed without using a rpm meter? is there some mathematical formula?

The full power theoretical calculation for electric 450 size heli is as below,

Pinon Teeth Count x (Motor RPM x Voltage of Battery Pack) / Main Gear Teeth Count = Headspeed

For practical reasons, we assume the nominal voltage of a LIPO cell to be at 3.7v. Thus a 3S LIPO battery will be based at 11.1v. Based on specs, a Trex 450 Pro uses a 150T main gear and the 450 Pro kit provides a 3700KV motor. Plugging in the values,

[14 x (11.1 x 3700)] / 150 = (14 x 41070) / 150 = 3833.2rpm headspeed at full power.

To estimate your theoretical headspeed at a given throttle point, you can use the value programed in your transmitter throttle curve. ie assuming linear curve, half stick (50%) will yield 1916.6rpm, three-quarter stick (75%) will yield 2874.9rpm.

What can not be calculated easily is the power loading of your setup, which will determine in practical how long you can fly at what type of throttle/pitch curve. A good ESC with logging capability or logging device will come in handly.

For starter, take note of how long you fly with each battery that when you touch feels hot and bloated. Reduce that by a minute and see if the symptom persist. For a average 2200mHA 25C 3S LIPO and non-extreme heli setup, just hoving around, you should be able to get somewhere between 5-6minutes. If your time is around this range, a good estimation would be your pinon is too high and draining too much power. Drop to a 13T or even a 12T to test. Pinon are cheap and are easy to change so try starting there.

[rayshaw23]

thanks for that very detail explanation.

i think i will change to a 12T motor pinion gear. i calculated, it will still have 3285rpm. one last qns, what is a good guage to know whether if the motor pinion gear is just nice lined up against the main gear? as in, i dont want to place the pinion gear too far or too close to the main gear which will result in too inefficient or too much power drawn due friction respectively.

[VinCopter]

cut a strip of paper, press it between the pinion and main gear and tighten the motor. remove the paper and that should be the right gap.

[rayshaw23]

cool! thanks! going down to fook hai for some parts now and there is something to do tonight!

[feudallordcult]

there is an app known as helitach to measure headspeed but I found it to be pretty inaccurate. For hovering, I prefer smaller pinion. Check yr throttle curve too.