RE: Motor - Prop Selection ???
Continuedâ€¦ Pulling it all together, sorta.
Well, I donâ€™t profess to be an expert on any of this, but those are the basics for the hardware on these toys as I have learned it. Forgive me if there are inaccuracies, but understand I am trying to present this for a noviceâ€™s perspective and skipping over some technical details to keep it simple and meaty. Now we can go back to the question that started me on this learning journey.
We decide we want to go electric for whatever motive, and we start getting all juicy over a particular plane, we now have to face the hardware selection head-on. Like Matt suggested, just go look up similar aircraft and see what they do, and duplicate it. The other choice is to put in the setup recommended by the manufacturer. Good ideas, but it didnâ€™t take long to discover that most of the guys out there are not that well educated, and most all the manufactureâ€™s recommendations perform well short of â€śawesomeâ€ť.
Itâ€™s obvious to me that manufacturers recommend â€śminimumâ€ť reasonable setups so that potential consumers are not too shocked by the cost of the equipment to put up the bird. This is definitely a bigger-costs-more hobby. You can easily spend 20-30% more for a good notch up to get that desirable â€śunlimitedâ€ť performance.
It seems there that it is common to overload equipment to save money. Seems we always plan with a budget, and then push it to ruin when we want more power. Then we spend more to upgrade or repair it later. My thing here is to learn how to avoid that circular trap. As I think many will agree, I would rather spend a little more now and buy the right goodies than replace/upgrade it later and buy twice. I have done this several times already in my short time in this segment of the hobby, and I donâ€™t â€śplanâ€ť on doing it any more.
Okay, now letâ€™s get a few issues on the table so we are not misled. We have a fundamental problem with this process that presents contradictory goals. We want incredible power, but a light aircraft flies much better. Powers means mass, get over it and deal with it. I suppose as these technologies advance, this will be less of an issue. Soon you will see 20C and 30C Lipoly batteries. However, we still drop mAH ratings to keep the weight down, even though we will have the big current capacity necessary on smaller batteries. Bad news is, when we do this, we sacrifice the other variable benefit, flight time.
So, now we have to deal with some balance of price, power, weight and flying time. I wonâ€™t debate that here, but you get the idea, it is a quandary. The point is, you sacrifice one for the other in every case. Until the â€śnext generationâ€ť battery and/or motor technologies become affordable mainstream items, this is the game we must play.
So, I think itâ€™s prudent here to focus on 3D flying requirements. I think itâ€™s the most demanding need to find the right mix of hardware, since the flying style is highly diverse. Experienced modelers in other segments of the hobby might argue.
Now for the confusing part. This is where I had to dig to gain some understanding. First, we have selected a model. There is generally a published weight for the aircraft with recommended hardware. My experience is brief, and limited to 12-22 oz aircraft. In most every case, the plane weighed more than the published weight by 1-2 ozâ€™s when I was done with it. So, letâ€™s factor that in right up front.
Second, as Matt suggested early in this discussion, to get good 3D performance you need 150 Watts per pound (9.4 Watts/oz). Power directly translates to thrust by the prop. Power, by definition, is the energy required to move a mass a certain distance in a certain timeframe. After using his benchmark, I have found that 9.4 Watts/oz represents good performance and you still get â€śdecentâ€ť performance as low as 8.0 watts/oz. to get â€śgreatâ€ť performance, you need over 10 Watts/oz.
I measure my assessment of very good as a thrust to weight ratio better than 1.6:1 and â€śdecentâ€ť as 1.3:1 to 1.6:1. IMHO (In my humble opinion) anything under 1.3:1 is not adequate for 3D flight. This is an opinion, not a recognized standard. Conversion of Watts-to-Thrust is the part of this whole thing that is not very well defined. Hence, the general guideline is given in watts, and the conversion to thrust is dependent on too many variables to define exactly without complex formulae that include air density, prop efficiency factors and a bunch of other stuff.
Now, we CAN translate Watts, as it is the product of Amps and Voltage. So, as an example, 12.2 Amps x 10.4 amps = 127 Watts.
We know we have a plane that letâ€™s say weighs 15 ozâ€™s, and we want â€śgreatâ€ť performance (10 Watts/oz). We want to fudge the weight an ounce (5-10%) for a cushion and a conservative calculation, then 16 oz x 10 Watts/oz = 160 Watts.
So, now we can calculate the Ampere Load this represents, assuming the battery voltage. A fresh 3S Lipoly actually measures 11.4-ish volts, and rapidly drops below 11 volts when loaded to near itâ€™s amp capacity, then decays steadily from there. Again, to be conservative so we overstate the real current, we use the fresh battery voltage 11.4V) to â€śpredictâ€ť current.
So, in our example, the current load is 160 Watts / 11.4 V = 14.0 Amps. So, we now know our minimum requirement for the ESC in this example. We also know have all we need to select a Battery. Again, simple math and your pocketbook help you decide. A good 15C battery costs a lot, and wonâ€™t live long, but will keep the plane light:
14A / 15C = 0.93 Amp-Hour capacity, or 930 mAH
So for extreme 3D with light wing-load and short fly-time, select a 15C battery at or slightly above 930 mAH.
A nice medium is a 12C battery.
14A / 12C = 1.17 Amp-Hour capacity, or 1170 mAH.
For better fly-time and moderate weight penalty, select a battery here in the 1300 mAH range.
In every case, you may go upscale on the current capacity (mAH rating) and extend flying time at the expense of weight, and hence a vertical performance. In this instance, an extreme might be to select a 2000 mAH battery. Fly long periods, and great wind stability, but a little more sluggish 3D performance unless the bird is very agile to begin with.
Next is motor selectionâ€¦