Replies to This Discussion

Permalink Reply by Ellison Chan on September 16, 2012 at 12:33pm

I had this discussion non  AeroQuad, and to summarize, the following item help to increase endurance:

1. Low RPM, high torque motors allows the use of larger props.  The larger props mean higher larger surface area, and thus more efficient lift production.
2. The more efficient motors use higher voltage batteries.

I'm sure there are more factors, motors and props are the major factors.

Permalink Reply by Philip Sulikosky on September 16, 2012 at 3:57pm

A few other thoughts, larger props are quieter, but less suited to sport flying, higher voltages mean less Amps for the same wattage (IE Watts = Volts x Amps) so savings in weight from smaller cable guages, and possibly lower rated ESC's

Permalink Reply by Chris Gough on September 16, 2012 at 6:54pm

longer props have a higher aspect ratio (better lift:drag ratio, more lifting force for a given ammount of work. i.e. better prop efficiency). But longer props require a bigger frame, which is more weight.

Higher voltage batteries/motors means less amps (thinner wire, smaller mosfets/ESC, less weight) for the same ammount of work. So, A High voltage motor with long props would be best - e.g. low KV motor. But low KV motors are bigger and heavier (more copper). Relativelty small/light low KV motors are the most expensive kind (strongest magnets, etc).

So there's a ballancing act between prop efficiency (long props, low kv motors) and motor efficiency (thin wires, light motors). If you bring costs into it, there's a sweet spot between 800-1400kv motors, 8"-13" props, and 2-4 Cell lipos. outside those bounds things start to get a bit exotic (expensive).

There's another thing. Imagine a multicopter that hovers at 100 percent throttle; it's brushless motor is being used most efficiently, and there is minimal dead weight (all of the capacity of the motor, wire, esc, etc is being fully utilised). But if it's hovering at 100% throttle, it can never climb. What's more, it needs a certain "stability margin" for stabilisation. For sedate flying, I think about 10% throttle is a usefull stability margin (as in 10% in reserve at "full throttle", not hover). For acrobatic flying style or gusty conditions, the autopilot probably need more like 15% "unused" capacity for stabilisation. If this imaginary hovvering quad is very highly tuned and flying in a windless stadium maybe it could get away with only a few % stability margin - but how is that contrived situation better than simple pole? :)

It's the same for strength - to maximise hover time you could make a frame that's exactly strong enough to hover, using expensive super light materials. But if you want to manouver, you would need to make it slightly stornger (heavier). If you wanted it to survive the rigours of practical use, it would need to be stronger still. And of you wanted it to be affordable, you have to pay even more weight penalty.

Similar deal with batteries. If you drain every last drop of power out of your LiPos you ruin them. If you use Lithium Phosphate batteries (which can be safely discharged more fully), you pay a 20% reduction in energy density. Personally I plan to limit the drain on my LoPos to ~60% capacity, to prolong their life and leave some capacity for emergency. (e.g. 4Ah 3Cell Lipo theoretically has about 48 Watt-hour capacity, I'd plan to land by the time it's done 30 Watt-hours work).

In practice, with the energy density we have with todays batteries, 5-10 minute flights with a utility airframe of 1-2 KG is practical. You can get away with slightly mismatched components and 20% AUW of payload (cameras etc) if you are happy with 5 minute flights. If you want 10 minute flights, you need reasonably well matched, good quality components, and not to much payload relative to the size of the quad. If you want to push past 10 minutes, you need to make carefull compromises.

Permalink Reply by Stefan Gofferje on September 16, 2012 at 9:17pm

Ok, I think, I'm getting somewhere here...

The lift/drag ration thing is only valid for actually LONGER blades, right? It does not apply to e.g. a 3-blade instead of a 2-blade?
My trigonometry is a bit rusty, but there was something with surface increasing by the square...

Permalink Reply by Ramesh Tahlan on September 16, 2012 at 9:31pm

lifr drag rules apply to any thing and every thing that moves in the medium of air or water.... 

Permalink Reply by Chris Gough on September 16, 2012 at 10:03pm

Yes, aspect ratio (AR) is wing (or blade) length / wing width. So longer prop blades have a higher AR, and therefore (all else being equal) you can expect a better lift to drag ratio (LTD). That's why sailplanes have long skinny wings (high AR), and biplanes are draggy.

The most efficient prop would be a single blade, but that's not usually practical for a veriety of reasons (counterweight, radius, supply). Tripple blade props have a smaller circumference, so you can fit them on a smaller frame, saving some weight... there seems to be some good 3 blade solutions at the smaller end of the spectrum, but you don't often see them bigger than 11" multicopters, so I guess that's not the sweet spot.

You can't be to litteral with geometry and propellers, because the outside part of the blade is traveling through the air much faster than the inside part of the blade. So intuition about "the surface of the disk" might not be so helpful. If you think in 3 Dimensional geometry you will be closer to the truth, the lift distribution is similar to the surface of a rotated parabolic segment (because lift is approximately proportionate to the square of velocity). Don't worry about that, basically the outer portion is the most important part of the blade because it's going fastest.

Another consequence of the power square law and tip velocity is if you go from a 2 blade prop to a three blade prop (that provides the same thrust on the same motor), you don't reduce the diameter by a third. You reduce it be quite a lot less than 1/3.

Permalink Reply by Stefan Gofferje on September 17, 2012 at 11:01am

Ok, the prop stuff has sunk in, now I have an issue with the motors. You said, low KV motors run on higher voltage are better, which makes sense if you look at the physics strictly from an electric point of view ( W=V*A ). However, during my research today, I found this table, which seem to say that the motors efficiency (g/W || oz/W) is pretty much going down at high voltages. How do we account for that?

Permalink Reply by Ellison Chan on September 17, 2012 at 11:08am

That's thrust, not endurance.  Endurance is measure in terms of  (g/W)/Hr.

Permalink Reply by Stefan Gofferje on September 17, 2012 at 11:14am

g is thrust. g/W is efficiency - as I wrote in my post (and as the table says).

Permalink Reply by Ellison Chan on September 17, 2012 at 11:29am

Yes, but efficiency does necessarily translate to endurance.  You can have 13 g/W, but draw more W/hr, and have 6 g/W but draw less W/hr.  And if all you really need to hover is 6g, then your battery will last longer.  The key is to not expend more energy than needed, and also be as efficient as possible.

Permalink Reply by Stefan Gofferje on September 17, 2012 at 11:37am

Yeah, but my point is - it was said earlier, that high voltage is better, which makes sense from the W=V*A point of view.

If, however, motor-efficiency goes down with voltage...

As an example, if I need 6g thrust to hover and the efficiency is 12g/W with a 2S-Lipo but 6g/W with a 4S-LiPo, then I need 0,5W to hover with a 2S-LiPo but 1W with a 4S LiPo, which means that the Amps used to hover would be the same.

So effectively, I have gained no advantage from using the higher voltage.

Hence my question, how do I account for motor efficiency in the design?

Permalink Reply by Ellison Chan on September 17, 2012 at 2:35pm

Logically what you say makes sense.  But from an engineering point of view, you control the ESC, using PWM, and the ESC will send a voltage to the motor to drive the motors according to the PWM signal.  How much power is used is dependant on the resistance/impedance of the motor coils.

The motor running at 12g/W is going to have a higher resistance/impedence, and use up more power than a motor that's running at 6g/W.  It's kind of equivalent to running a car in a lower gear to get better acceleration, and lower mileage than running at a higher gear and getting better mileage.

But, talking is talking.  You could run some static prop motor combination bench tests, and measure endurance.

The MD4-1000 motors are 72kv, compared to the motors you linked above which is 650kv.

I found the link to the discussion on Aeroquad.  There some interesting information there:

http://aeroquad.com/showthread.php?4528-a-problem-about-power-syste...

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