A Tri-Copter Vs a Y6

[Phil Rosa-Leeke]

When fitted with FPV and something like an AEE or Gopro camera, what “extra” weight could a ti-copter like David Windestål’s V3 carry? (I’m looking at between 250 grams and 500 grams load to “drop”)

Could a ti-copter chassis, say a V3, be used to build a Y6?

Do Y6 set ups offer that much more lift than tri-copters?

How are Y6s steered, via the props or servo assisted?

 

[joshuabardwell]

Assume your copter's weight is approximately 1 kg not counting cargo (gimbal, camera, bigger battery, etc...). So your all-up including cargo would be perhaps as much as 1500 kg. You can easily get around 1 kg of thrust out of a motor, so with three motors, your TWR will be about 2:1. This is about the minimum I would personally want to fly, but it's 100% flyable as long as you don't get too crazy. With some careful engineering, you could get as much as 1.5 kg out of the motors typically used on this size of quad, which brings your TWR into more comfortable range.

A Y6 offers a little less than twice the lift compared to a tri. Co-axial motors don't exactly double thrust, because there is some inefficiency, but they'll substantially increase thrust. I couldn't say exactly how much.

A Y3 frame could be used to build a Y6. You'd just need different motor mounts, and you wouldn't buy a servo mount. If the frame had an integral servo mount, that'd be wasted and would need to be removed, so better to get a frame with generic arms, or one purpose-built.

A copter with coaxial motors typically has the motors on each axis counter-rotating, to cancel each others' torque. This is how a Y6 is steered; it doesn't need a servo.

 

[joshuabardwell]

Adding... the main advantage of a Y6 will be that you have additional redundancy in the event of a motor failure. You also get additional thrust without increasing your prop size, if overall size is important (e.g. for portability).

If your goal is to increase thrust, there is almost certainly a cheaper and less complex way to do it than by doubling the number of motors and ESC's. Go to larger, lower-kv motors with bigger props. Go to 4S. If you estimate that a motor is $15-$20 and an ESC is $15, you're adding $30-$35 times three = $90-$105 to the cost of your copter.

 

[Phil Rosa-Leeke]

If your goal is to increase thrust, there is almost certainly a cheaper and less complex way to do it than by doubling the number of motors and ESC's. Go to larger, lower-kv motors with bigger props. Go to 4S. If you estimate that a motor is $15-$20 and an ESC is $15, you're adding $30-$35 times three = $90-$105 to the cost of your copter.

Thank you for your feedback JoshuaBardwell.

This mutli motor lark is all new to me and lower/higher kv and things like 4S are things I have yet to get my head around, but I'll get there with a bit of help.

So, in other words, I could still used a Tri-copter as long as I "up-grade" a few things.

Excuse my ignorance, but I am trying to work the following out: "...Go to larger, lower-kv motors..."

Is a lower-kv motor stronger?

As I said above, I would like to be able to carry up to about 500grams extra on top of FPV and V Camera.

 

[makattack]

Generally speaking, the larger the motor, the stronger it is... the KV basically indicates how fast it'll spin (RPM's per volt) but it's a ratio of torque & RPMs... a larger motor will allow you to spin a larger propeller for greater thrust at lower RPM's, and generally give you better efficiency.

If you're relatively new to the hobby, rather than building a high thrust / high payload multirotor, you may do well with a simpler, lower cost training aircraft, then upgrade when you have become more experienced a pilot...

The more you add, obviously, the more complexity and the greater risk of something going wrong with either the build, configuration, tuning, or flight...

I just build a tricopter using the electrohub, and RTFquads kits, as my first. I approached it as a relatively low-cost training system for me to get familiar with the build, configuration, and flight.

My build:
https://store.flitetest.com/electrohub-quadcopter-kit/ ($38 w/o shipping because I bought it at FTFF2014)
http://witespyquad.gostorego.com/flite-test-electrohub-electronic-kit-1.html ($139 when I bought it, but when I price out the same stuff now, it's $150)
http://www.hobbyking.com/hobbyking/store/uh_viewItem.asp?idProduct=30686 ($19 + shipping)


So, I get a medium sized multirotor for a bit under $200 to train with.

 

[joshuabardwell]

The kv value is a rating of how many rpm the motor makes per volt, at no-load. A 1000 kv motor, with 10 volts applied to it, would spin at 10,000 rpm if it didn't have a prop on it. When you add a prop to the motor, the prop loads down the motor, and actual rpm is lower, but all else being equal (prop size and voltage), a higher kv motor will spin faster than a lower kv motor.

The current that a motor will draw is proportional to its speed (rpm's) and the size of the prop that's on it. A bigger prop will move more air and make the motor draw more current. A higher rpm will do the same. Therefore, you can see that there is a sliding scale wherein a low-kv motor with a big prop, and a high-kv motor with a small prop may draw the same current.

Here's the cool part: a prop's thrust is proportional to its area, not its length. And area scales as the square of length (again, this is a simplification, since the shape of the prop is not fixed, but go with it). What this means is that a long prop being pushed by a low kv motor will be more efficient (generate more thrust-per-watt) than a small prop being pushed by a high kv motor.

This is why, for heavier lifting, you tend to see low-kv motors with larger props, whereas for lighter copters, you tend to see high-kv motors with smaller props.

 

[joshuabardwell]

So, in other words, I could still used a Tri-copter as long as I "up-grade" a few things.

I don't have any direct experience with tri's, only quads, but I would think that if your all-up-weight (AUW) was below 1.5 kg, you should be able to handle things with a tri, as long as you selected the correct motors and props. Honestly, you need to try to get a better handle on your AUW before you really start designing. The difference between 1 kg, 1.5 kg, and 2 kg is substantial. It doesn't have to be exact, but anything you can do to narrow it down helps.