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Old Aug 30, 2013, 01:01 AM
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derp express's Avatar
Joined Dec 2012
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Geared Quadcopter

Hey All,

I was just wondering if anybody has ever experimented with using gears between quad motors and props in order to get a larger/slower prop out of a very small motor to get efficiency? I don't have a huge amount of experience with quads, but I know from basic physics that if you had, say, a tiny 3000kv motor that would usually swing a 4-5 inch prop at a certain voltage, and then you had a 10:1 gear reduction to the prop, 3000 kv would effectively become 300 kv at the prop itself (though the motor would still rev quickly), allowing you to spin a bigger prop at a lower speed and get more efficiency, despite having a very high-RPM, relatively low-torque motor. Since higher torque at the prop from gear reduction equates to a somewhat proportionally higher acceleration, this should theoretically mean the FCB should be able to rev the prop up/down fast enough not to wobble like crazy, the way you would with a direct-drive big prop on a low KV motor. Is this idea at all viable, or would you lose too much power between the gears, and the fact that the motor itself is still running at a very high RPM to produce a low prop RPM, thus still incurring the friction loss in bearings form high revs?

I know of at least one example of this idea, in the AR Drone, using tiny brushed DC motors with very, very high KV to drive geared props. Granted, the AR Drone is meant more as a toy than a real quad, but from a mechanical standpoint, the idea seems to work very well for it. I believe its an 8.5:1 reduction



I'd like to know if anybody has some thoughts/opinions/experiences with this idea, since I'm very interested in going deeper into it. My goal is to have a very efficient quad via using tiny, tiny motors (18 or 22 mm diameter brushless) to spin very big props at a very low speed on a very light weight frame. Since there's not really any micro-motors slower than around 1500kv, this is where the gear reduction comes in


Regards,


-DE
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Old Aug 30, 2013, 08:19 AM
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Joined Apr 2010
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As for cars, tiny motors will not give good performances.

Flight ability depends on power.
Prop diameter depends on RPM and RPM depends on KV.

You need around 200W to lift 1 kg and around 500W per kilogram to get good flight performances.
Motors' average power to weight ratio is 3W/g.
That means that if your quad weighs 1kg, you need minimum 42g motors.

Prop diameter and pitch will be choosen according to max RPM and power.
High KV, small diameter, more pitch. Low KV, larger diameter, less pitch.
Sensitivity to wind increases with diameter.

You may use gears, but, with all these motors available, is it necessary?
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Last edited by Mortimer; Aug 30, 2013 at 09:50 AM.
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Old Aug 30, 2013, 09:38 AM
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United States, OR, Portland
Joined Apr 2013
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High pole count in a motor functions as "electronic gearing", trading top speed for torque, but without the added weight, vibration, and mechanical complexity of a gearbox. The only reason to use a gearbox is for cost, since high pole count, low kV motors are still a pretty small niche in the overall brushless motor market, and are therefore relatively expensive.
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Old Aug 30, 2013, 10:04 AM
person
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Joined Dec 2012
160 Posts
Quote:
Originally Posted by Mortimer View Post
Same as for cars, tiny motors will not give good performances.

Flight ability depends on power.
Prop diameter depends on RPM.

You need around 200W to lift 1 kg and around 500W per kilogram to get good flight performances.
Motor's average power to weight ratio is 3W/g.
That means that if your quad weighs 1kg, you need minimum 42g motors.

Prop diameter and pitch will be choosen according to max RPM and power.
Small diameter, more pitch, larger diameter, less pitch.
Sensitivity to wind increases with diameter.
You may use gears, but, with all these motors available, is it necessary?


EOD's quad hovers at about 85 w/Kg, and max power is slightly over twice that, because of a low-revving, large-diameter prop

http://www.rcgroups.com/forums/showthread.php?t=1880665

It's not that I'm necessarily implying that little motors are better - it's just that it's very difficult to find motors of small sizes that are a low-KV, thus making it impossible to direct-drive a big prop. So if you're trying to make a very light weight micro-type quad, for instance, but want it to be efficient, it's tricky.

Don't get me wrong here - I understand that you're sacrificing max speed and aerobatic capability when you do something like this, but that's an acceptable sacrifice to me, because I'm just looking to build a long-endurance camera platform.

Let's just take a random tiny motor as an example. Say, this thing

http://www.hobbyking.com/hobbyking/s...er_2900kv.html

8 grams, 2900 KV. Pretty tiny little guy. Let's say my quad is 400grams AUW (Also very small - mini size). On a 2S, the biggest prop you can safely run on this thing would be about a 4" - maybe 5. Using some E-calc numbers (which I understand have a 15-20% error), four of these with 4x4 props will run at approximately 12,000 RPM to make a 400 gram quad hover. The pitch speed is 49 mph - meaning we're wasting a lot of energy accelerating the air. The static thrust efficiency is something like 3 g/W. If we want to fly forward fast, this is great - but for hovering we're wasting a lot of energy.

Now let's throw a 5:1 gear reduction into the mix, and throw a 12x4.5 onto the big gear. To generate the same amount of hover thrust (100g / motor), the motor is still spinning at around 12,000 rpm, but the prop is around 2,400. Pitch speed is now 11 mph, meaning less energy is being wasted to accelerate air (at the cost of a maximum top speed), and efficiency is about 12 g/W.

Granted, I don't completely trust e-calc numbers, especially when you go to extremes (very small, very big), which is pretty much what I'm doing. I HIGHLY doubt that I can increase static thrust efficiency by a factor of 4 with gear reduction - that's a pretty immense jump. The big point is that bigger props at lower rpms are better for static thrust efficiency, which means low-KV motors are a must, but since there isn't a huge availability of low-KV motors in small sizes (yet), tiny multirotors looking for good hover efficiency should resort to gear reduction

Quote:
Originally Posted by T0nkaTruckDriver View Post
High pole count in a motor functions as "electronic gearing", trading top speed for torque, but without the added weight, vibration, and mechanical complexity of a gearbox. The only reason to use a gearbox is for cost, since high pole count, low kV motors are still a pretty small niche in the overall brushless motor market, and are therefore relatively expensive.
You're totally right, and that's what I'm saying. It's ALWAYS better to direct-drive a prop on a low-kv motor, but the availability of low-kv motors isn't so great at the moment

I'd really like to see examples of quads that use gear reduction. I'm thinking I may order some pinion gears and big heli rotor gears and experiment with it. I know that 70-80 W/kg is totally possible with low kv motors, but i dont know about gears
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