Motor voltage, kv, and battery weight saving

Question

I would like to ask about motor and prop size and efficiency.

So I was thinking to get either some 2203.5 motor with 1600kv wingspan of about 1,6m and 16x5 prop (prop length of 25% of wingspan), however manufacturers say that those motors are for 4-6s, isn't it weight wasteful to use 4s battery, when my aim is light slow cruising long range wing.

However I noticed that lets say that ummagwads aerolite 2004 in 3500kv can use 3-4s, so I would use smaller prop like idk 7x7 idk, but would it be more efficient or not, would it fly longer.

And well in the end, if I used that 4s motor and ran it mos of the time just on lets say 25% of throttle during flight, would it make difference against some 2s motor with weight of 2s battery on board, against that same milliamp-hour just in 4s battery build.

So the question is if that weight-saving of lighter battery is worth it, or if its not that significant, n i can run 4s n enjoy that head room of power when I would want to punch throttle?

Answer 1

Motors don't really have a voltage limit (until you get to hundreds of volts when the insulation breaks down). What you see in the advert is just a suggestion for where it is likely to work best. You can run them at very low voltage (but they won't make much power) or at very high voltage (with a very small prop, to prevent it making so much power that it overheats). Somewhere in the middle is a voltage where the appropriate prop size is useful to most people, and that's what you see in the advert.

The amount of power the motor makes is dependant on both the voltage and the prop size. So you if you change one, the other should change too. Otherwise the motor will be overloaded, or underloaded (i.e. not making enough power).

Using a higher cell count battery doesn't necessarily add any weight. You just use smaller cells. For example, a 4s 1000mAh battery weighs the same (and contains the same amount of energy) as a 2s 2000mAh battery. If you run the first one at half power, the motor would turn at the same speed and run for the same length of time as the 2s battery. The only problem is that you have to decide whether to use a big heavy motor that can handle the full power, or a lighter one that works for the amount you expect to use most of the time.

I'll add that 16x5 prop is far to big for a 1600Kv motor, even at 2s. eCalc suggests that it would be making 450W at 70amps, while your motor can probably only handle 50W and about 7amps. If you want to use anything other than the recommended battery and prop, you should get an RC wattmeter to check that you're not overloading anything.

Finding the optimum power system is quite tricky. Generally a big slow prop is more efficient, but it reduces your top speed. So you need to know your target cruise speed. Calculators like eCalc will get you close (if you know your wing area and weight) but it's worth experimenting. You should also consider that most common designs have been optimised over decades of use, and while there's some scope for customising them, big changes will probably make it worse.

Answer 2

Powering an aircraft comes down to one, relatively simple mathematical concept:

• Spinning a propeller of a given size at a given RPM requires a given amount of watts (power) input

Now we know that Watts(P) is Voltage(V) x current(I) How you achieve that is up to you (for the most part). You can use a high kV motor on lower voltage, and higher current, or a low kV motor on higher voltage and lower current, either way you get the same watts.

Now we know from some other mathematical concepts that heat comes from current flow, which means for the most part higher voltage and lower current is less stressful on components.

Running a higher voltage than necessary and keeping your throttle limited is theoretically acceptable, however as a relatively new pilot, having the additional mental load of having to closely monitor your throttle position to keep it out of the "light your plane on fire" zone is not a good idea.

As the other answer said, both of those motors are much too small for the propellers you've selected, and will quickly turn into a pile of melted goo. Play with ecalc, but I would recommend doing a few builds that don't stray very far from the recommended power system, until you get an understanding of sizing components properly