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Permalink Reply by Michael Pursifull on May 1, 2012 at 7:56am

what is the application? how do you plan to use them?

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Permalink Reply by etienne.t.micallef on May 1, 2012 at 8:00am

hexa copter.  

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Permalink Reply by Michael Pursifull on May 1, 2012 at 8:20am

how much does it weigh? What prop do you plan to use? This will decide if this is a workable/efficient/good motor, questions of reliability aside.

You should know that this appears to be an outrunner motor, with a shaft-side cord... this means it will most likely require special (compared with typical frames) mounting. 

It is a 20A motor, 189 Watts, with manufacturer recommended 10x5 to 12x6 Electric type props (but see my former statement, and my other posts about starting with the weight, not the propulsion solution, and matching the motor efficiency to the prop efficiency at the designated RPMs)

May I ask, why a hexa? There are better reasons and less good reasons to chose a hexa.... I am wondering what your reasons are...

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Permalink Reply by etienne.t.micallef on May 1, 2012 at 8:36am

the idea is to do a hexa.  a hexa  as it seems its more reliable, easy to control and can carry  some weight.  My first idea was a y6,  but  i was told that  a hexa is a better option.  it also has some reduancy built in, ie if a motor fails one can try to do a sort of emergency landing.  regarding props i still not decided wither 10x4.5 or 12 x  45.  

i need extra weight to carry some more lipos for duration - flight time

I am a a loss as i am a beginner.  already started with a y6 and it turned out a disaster.  ie no lift off due to bad motors... In this case i used 1200kv, as per the specification of the  frame supplier, but  it never flew.  in fact i have now the motors sitting  on my work bench.

I learn that the larger the frame is  the lower the KV and vice versa.

in your opinion  what is the best frame?  

Y6 is good for compact,  etc.

motor above is 120g it seems

Etienne

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Permalink Reply by Michael Pursifull on May 1, 2012 at 9:13am

If you are a beginner, then I recommend an established kit, either the classic jDrones quad or the 3DR quad. I recommend a quadcopter. The quad is a better fit for a first build because it is a very excellent compromise between simplicity and efficiency. If you want to keep it simple(st) [good for getting started] and efficient, build a pre-designed quad, and use the recommended props and motors. Someone else has already done good engineering for you, take advantage of it.

The reason I recommend this is because there are so many things that can go wrong, and as a beginner you want to leverage the most help and learn what you need... and what you really need to learn at this moment is how to get someone else's design working well. Once you have learned all the things you can do wrong... wiring, soldering problems, APM problems, preflight and flight problems, battery management, programming/configuration/tuning issues, etc... the list is very long... then you can learn how to design your own aircraft for you needs, and you will be able to make your aircraft work for all the problems I just mentioned. Trying to make a *new* design work, in the face of all these other issues that you don't have much experience with is like trying to learn to shoot skeet while learning to ride a horse at the same time....

That being said, an hexa is better than a y6 for your purpose, but I have not heard a good reason yet to choose a hexa over a quad for your application yet. 

In general, follow this rule - the fewer the blades, the more efficient you can get it. But below three arms (six blades, when using a two bladed prop, each prop as two blades...) the flight mechanics get more complex, so the aircraft needs special/non-symetric designs. So a quad can be built symmetrically, with four motors, four props, and it the flight mechanics are more simple. Drop down to three arms or less, and you have to deal with rotational forces generating yaw. Thus, Y frames have one or more arms that rotate, like your wrist, usually on the "rear" arm. Coaxial copters are very complex, and so are helicopters, there are additional requirements, so even though they are more efficient, it is usually not worth the complexity (for our general purposes.)

So a quad is really your best fit, unless you say the magic words... But those words mean, by trading up to adding more blades, you give up some efficiency. 

Here is the paradox. If you choose a pre-designed quad, but you want to add more weight than it can carry, then you can bump up to a pre-designed hexa to carry more weight. But this is only a good idea because the hexa was designed for a larger weight budget to begin with. If the quad was designed with that same weight budget, it would be more efficient by a significant amount over any hexa of the same design... since the designs are 'fixed' because they were done by someone else already with specific weight budgets in mind, it might confuse you.

Predesigned jDrones or 3DR hexa carries more weight than the same quad. However, it is also less efficient in general for the same weight carrying capacity. If you can carry the weight in the quad, and you can do it within the most efficient RPM range of the design, use fewer propellers - the quad design.

If you are designing your *own* craft, if you are designing for efficiency, set the weight range first. Then multiply that by 1.15 (hover) 1.6 (fast maneuvering) and 2.2 (acrobatic). Then divide that by four (the number of motor/props) and this is your target/peek efficiency goal for thrust for each motor/prop pair. Then find a prop that works most efficiently delivering the needed thrust at that RPM Then find a motor that can efficiently drive that prop at that speed. Since motor efficiency varies from 0 to 83% depending on speed and load, and typical operating ranges in even a well designed craft are 55%-83%, it is very important to get this right if you want good lift and good efficiency. If your prop or motor are operating out of sync with their most efficient range, you might be driving your motor at 80% efficiency, but your prop may be delivering 20% of its force, or vice versa. And even if you are matched at 80%/80%, if it is not enough collective thrust (or as bad, if it is too much thrust) then your aircraft will not fly (or will be large, expensive, heavy, and still shoot up into the air, and will chew through your battery when you reduce the thrust to controllable flight...)

That is a lot to think about, I realize. So here are your general guidelines to guide you:

* Design with the weight in mind first, if efficiency is the goal.

* Hover at 1.15x - weight x 1.15 = force required to hover

* Anything over 2.2x - weight x 2.2 - is too aggressive to control manually

* Fewer blades, more efficient

* Fewer than four arms on a multi-, more complexity

* The larger the blade, the greater efficiency (potentially, if everything else is matched)

* The slower the blade moves, the greater efficiency (potentially, if everything else is matched)

* Quads cannot recover from the loss of a motor/prop

* In practice, most hexas will not recover from the loss of a motor/prop (but at least they have a chance, can be designed to recover, sometimes)

* There are a great number of things to learn, and many, many things that can go wrong, therefore, build to someone elses' design first, so you can:

     * get help from more people, since they know your design

     * limit the number of things you are doing wrong to manufacture/build/flight issues, and not design issues also. This reduces the complexity by as much as 36 times, by making it a 3D problem, not a 4D problem...

PS The relationship between frame size and Kv is more accurately understood if you realize that a low Kv is *always* better (efficient) if you have a prop that will deliver the required thrust, and your motor can drive the blade. Thus, at some point, the frame is so small that you have tiny props.... and these tiny props and tiny motors must spin much faster to deliver any thrust because of their size. It is always better (i.e., more efficient) to have a larger blade, and to spin faster, if you can. But a larger blade takes more power to turn, even if it is more efficient. 

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Permalink Reply by Michael Pursifull on May 1, 2012 at 8:32am

Assessing motor merits (or, rather, communicating them) requires a common understanding of your value system. Does efficiency rule your value system (in an electric multi rotor, I believe it should given the state of battery technologies today) or do you other specific requirements? Flight time, lift capability, intended application (is this a fun fly/acro craft, or is it intended as a hover/photo platform, or will it be something in the middle, for example)

If efficiency is not a concern, then the engineering can be much less precise. 

With these motors, you have a weight issue, sure, but the mounting issue may be more significant. Questions of efficiency rely on all the things I have mentioned before. If you do want to pursue efficiency, I wanted to bring your attention to an invaluable resource http://www.flybrushless.com/

Spending sometime there looking at general thrust windows, RPM windows for different props, Watt requirements for different thrust yields, then matching a motor that can deliver its most efficient performance at those RPM/Watt ranges is what you want to do. The weight of the motors is not an issue if the system is efficient, most motors will easily lift themselves, at 200% or more of their own weight. As long as you've designed an efficient system everywhere else, choosing a more efficient motor for your weight range is vastly more important than the weight of the motor itself (in anything but mini/micro multi rotors...)

Just some things to think about.... and good luck!

PS - the 880 and 850 motors from jDrones are really very good motors.... I've done the math, and I encourage you to do the same....

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Permalink Reply by etienne.t.micallef on May 1, 2012 at 8:38am

thanks for your reply...

Etienne

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