hi everybody

I am really stuck in motor selection for a semi racer plane i made recently and been testing dozen of different motors and props on it.
so here is my latest decision should I use a motor with higher KV (thicker wiring lower resistance) on 6-8S or should i go for its lower KV version and go for 12S?
my motors are these
Turnigy SK3 5055 KV320and Turnigy SK3 5055 KV430 they are identical motors with different KV ratings. so I have tested the 430kv with APC 13x13.5N and aeronaut CAM 14x12 and with the CAM14x12 on 9S the motor was on 3500W and 9800RPM static. in flight it handled short bursts of full throttle for a dive and low-pass and we managed to land safe. the APC has much less power demand static and it kind of had worse performance speed wise compared to a APC 14x12 which i tested them on a different 500kv on 6S setup.
so now when it come to over loading like this should I go for KV320 (should buy it first) and a 12S (just to reduce amps) against the 430KV and 9S or it would not be much of a difference is performance , efficiency and safety anyway.

A larger diameter propeller is more efficient and silent than a small one, btw.: single propeller blade is more efficient than 2.
A motor has a maximum efficiency point, and maximum power point just below the point it melts.
The efficiency of the motor is lower at higher speed, but the torque is limited it can produce, so you have to speeed it up to get more power out.

Dear Z-matrix

I know that the larger the dia the more efficient it is and I know that I running my motor way out of their most efficient point which would be around 45amps. but I want to go as fast as possible with my current plane and don't want to change my airframe. so I searched the available APC props and chose the ones with the most pitch/dia so ended up with 14x14 prop and some near it . i wanted to be able to rev it up to 12000-14000 rpm so now I have these 2 choices 5055 430kv on 9 or maximum 10S (which i thick is beyond its final limits and may destroy the motor) or go for a 5055kv 320 and a 12S. so what so you suggest now ?

Don't forget that 'pitch speed' is very important for high speed. Reducing the prop diameter could increase the rpm.

Best place to ask is the -- High Performance forum

For motors in the same family but different Kv there is no great difference in efficiency between low and high voltage setups of the same potential rpm. Sure the higher Kv motor has thicker winding with less resistance, but copper losses are only part of the total loss. Iron losses (voltage x zero load current) are the other and as you increase voltage, they also increase.

Your problem, as in the previous thread is not efficiency, but effectiveness. You need to find the combination of pitch and RPM to get the pitch speed you need, probably 20% over target airspeed, and then find a prop of that pitch with LOW enough diameter that you don't over load the system to destruction. That is not going to be achieved for 200 km/h plus with pitch:diameter less than or close to 1:1.

Dear scirocco
since you are really hard to find and I would really dig your experience so would you please let me know
wouldn't it be better to choose high cells with lower KV to minimize the waste in wiring of the batteries and the batteries themselves ?
yet I have reached a point that i GUESS a smaller diameter prop would not provide enough thrust to achieve the pitch speed even though my motor is turning for example a 12 by 12 at 14000 but my final speed would be lower than a 14 by 12 at 12000 !
I mean since my plane is fairly big for a racer ( it is not comparable with those tiny racers in any way) I will hit the Drag/thrust equilibrium barrier prior to a pitch speed barrier.
have you every encountered such case ?

Originally Posted by mohsenfjs

Dear scirocco
since you are really hard to find and I would really dig your experience so would you please let me know
wouldn't it be better to choose high cells with lower KV to minimize the waste in wiring of the batteries and the batteries themselves ?
yet I have reached a point that i GUESS a smaller diameter prop would not provide enough thrust to achieve the pitch speed even though my motor is turning for example a 12 by 12 at 14000 but my final speed would be lower than a 14 by 12 at 12000 !
I mean since my plane is fairly big for a racer ( it is not comparable with those tiny racers in any way) I will hit the Drag/thrust equilibrium barrier prior to a pitch speed barrier.
have you every encountered such case ?

I think your GUESS is wrong and continues to lead you down an unachievable path.

We absolutely know that the model will not go any faster than pitch speed and in fact probably about 15-20% slower. So if you have a target speed of 200km/h, let's assume a required pitch speed of 240km/h. Doesn't matter what the prop diameter is, how much static thrust etc - if your system isn't capable of achieving that pitch speed, you CANNOT achieve the target speed.

We know less about thrust required. We do know that thrust must equal drag in steady state level flight and the drag equation is D= 0.5 x Cd x A x rho x V^2. So going faster we have drag increasing with the square of airspeed. The drag coefficient generally reduces slightly with increasing speed, but from an arbitrary high speed say 150km/h to a higher speed say 200km/h, Cd is already close to its minimum so airspeed completely dominates the drag equation. For our models, without wind tunnel testing we don't know Cd and we don't know frontal area A with any accuracy. BUT, because V^2 dominates drag, some assumption based modelling can achieve ballpark figures. Those assumptions are available in eCalc, which for speed calculations assumes a Cd of 0.05, wing frontal area 3% of area, and fuselage frontal area 90% of prop area. Now these may not be accurate, but they do provide a basis for comparing the potential of different props on the same airframe, at least to the extent of closing off dead end paths.

I accept that propellers from different makes and families within bands can't be compared identically, so props of similar dimensions of different brands may give anomalous results in testing. But for the sake of argument and to better understand the diameter, pitch, rpm and speed interaction, assume that they are from the same family, same sort of profile and aerofoil etc - basically these get wrapped up in the prop constants used in the calculator and plug into the reasonably well accepted propeller load vs rpm equations used in ECalc and other power system simulators.

Let's compare your 14x12 and 13x13.5, and also 10x16, 10x17 and 12x17 RFM folding blades, assuming the same prop constants for all, using the 430 rpm/V motor on 9S. Wing area is 37 dm^2 and weight 3.5kg (Sundowner 50 numbers). BTW, the RFMs are here for 29-31 euro a pair

Prop; Max Power; Static RPM; Pitch speed; Predicted max speed; Predicted thrust at 181km/h; Predicted Thrust at max speed (=drag)
14x12 3200W 11300 207km/h 181 km/h 1480g 1480g
13x13.5 2900W 11600 239 204 2625g 1566g
10x16 1580W 12700 310 244 2898g 1469g
10x17 1650W 12660 328 255 3285g 1646g
12x17 2740W 11760 305 247 4443g 2074g

The first thing to note is that the top speed drag figures are misleading because they are greatly influenced by prop area, due to the way eCalc assumes fuselage frontal area as a proportion of prop area. But there is much that can be learnt, especially from the pitch speed and predicted thrust at 181km/h figures, chosen because that is the predicted maximum of the 14x12.

First, the drag figures may be way off and a 10x17 MIGHT not get to 255km/h because there is more drag than assumed. But, regardless of whether the drag modelling is way high or even way low, the 14x12 CAN NOT get past about 180km/h simply because the battery voltage and Kv puts an effective max on the rpm, and then the rpm x low pitch limits the pitch speed. The ONLY way that 14x12 prop can go faster is to increase the rpm, either by higher voltage or higher Kv or both, but because power goes up with rpm cubed, that comes with a massive power penalty. Might not vs can not is compelling to me - at least might not gives you have an option for experimentation that is not blocked off by the underlying physics of the power system. Can Not is a dead end to give up on.

Second, while the airframe drag modelling may be way off, the prop modelling at least for the 3 RFM props with their similar blade profiles should be reasonably comparable, and clearly suggests that for a given motor and voltage, a relatively low diameter high pitch prop is capable of developing more thrust at high speed, which is where you need it, than a much large diameter undersquare prop. I'd suggest that the model may not go as fast as predicted on either a 14x12 or a 10x17, but I'd be confident it'll go a damn sight faster on the 10x17 than the 14x12, and be lot easier on the motor and ESC and battery.

Again, the actual speed predictions may not be all that good due to the assumptions that have to be made about drag, but the comparison should be clear enough to indicate the path to pursue.

Note that there is another price to pay for potentially high speed with fixed pitch props, and that is that the prop blades will be partially stalled until relatively high speed, possibly over 150km/h. Stalled doesn't mean no thrust, just less than the simple rpm/diameter/pitch calculation, but don't expect takeoff in 20m.

Dear Scirocco
thnx for your detailed information yet you didnt give me your opinion anout going higher on viltage and lower ok KV to minimize the loss even in the witing of battery and ESC if no the motor itself .
finaly think experience is a precious thing a modeler could have
for example today I tested and compared for the 3rd time the APC 14x12e and APC 13x13.5n props (on a turnigy 5045 kv500 on 6S and they both draw around 1500W and around 8500 rpm). and clearly I saw that the 14x12e can go faster in any way you compare those ! it was clear to me that the plane could not reach the 13x135 pitch speed ! yet even it was clear that i could no even reach the 12 pitch speed so the plane was behind the lack of propulsive thrust barrier so just going up in pitch speed in the dark wont do the job for me a Clear example of this is those foam board jet models some guys build most of them use 8 by 6 props with massive high KV motors reving up the 8x6 to almost 20k and I can clearly state that those foam board models wont exceed 100km/h while they have a pitch speed of 183 km/h . so now i guess the best approach for me would be to achieve 3000w on 12k-13k rpm in a reliable fashion then experiment with different props I have that would be 14x14 or 13x13,5 on a bit higher rpm. yet the best way to investigate this is to have a in flight data logger which i have ordered and eagle tree Elogger V4 with additional GPS and RPM sensor so I will have firm data in 3 weeks.

Originally Posted by mohsenfjs

... going higher on voltage and lower Kv ...

... thus keeping same rpm.

Makes no difference for the motor, same amount of copper- and iron-losses, therefore same efficiency in both cases. Assuming both versions of that motor, only differing in number of winds to get desired Kv, have same amount/mass of windingwire copper in the slots. More longer (and thinner) coils/wire for lower Kv, shorter (and thicker) coils/wire for higher Kv.

Also makes no difference for losses in battery. Assuming same amount of energy in both cases.

Wiring losses will be lower at higher system voltage.

Vriendelijke groeten Ron

Originally Posted by mohsenfjs

Dear Scirocco
thnx for your detailed information yet you didnt give me your opinion anout going higher on viltage and lower ok KV to minimize the loss even in the witing of battery and ESC if no the motor itself .
finaly think experience is a precious thing a modeler could have
for example today I tested and compared for the 3rd time the APC 14x12e and APC 13x13.5n props (on a turnigy 5045 kv500 on 6S and they both draw around 1500W and around 8500 rpm). and clearly I saw that the 14x12e can go faster in any way you compare those ! it was clear to me that the plane could not reach the 13x135 pitch speed ! yet even it was clear that i could no even reach the 12 pitch speed so the plane was behind the lack of propulsive thrust barrier so just going up in pitch speed in the dark wont do the job for me a Clear example of this is those foam board jet models some guys build most of them use 8 by 6 props with massive high KV motors reving up the 8x6 to almost 20k and I can clearly state that those foam board models wont exceed 100km/h while they have a pitch speed of 183 km/h . so now i guess the best approach for me would be to achieve 3000w on 12k-13k rpm in a reliable fashion then experiment with different props I have that would be 14x14 or 13x13,5 on a bit higher rpm. yet the best way to investigate this is to have a in flight data logger which i have ordered and eagle tree Elogger V4 with additional GPS and RPM sensor so I will have firm data in 3 weeks.

In post #5, I already said much the same as Ron - no great efficiency difference between high and lower voltage setups with the same family of motors. You just need to get the rpm however you can.

Good luck as you add cells and rpm

I recommend you run the biggest prop you can that will give you adequate ground clearance and pitch speed. Then pick your battery and kv to match. And from my experience, it seems to me that higher voltage, lower kv performs better than higher current, higher kv.

I forgot this ...
.... current is proportional to pitch¹, voltage², Kv³ and diameter⁴.

Prettig weekend Ron

once again new data and NEW HEADACHE !
ok here is the new phenomenon I see ! I have the Turnigy SK3 5045 KV500 and the turnigy SK3 5055 KV430 and static tested both of them along side some other motors with some props.

so I tested the 5045 500kv on 6S with APC 13x13.5n on 7000 RPM watt meter showed 970W. the 5055 kv430 on 6S on the exact same prop and 7000rpm and the watt meter shows 1050W and that motor on 9S with the same prop and again 7000rpm showed 1200w !
tested the same prop with a turnigy L5055C 700kv and on 6S and 7000rpm watt meter shows above 1500W !!!!!
and tested an EMAX BL4030 on a props with a specific RPM and on 6S it was 700w and on 9s with the exact prop and rpm it was 800w !!
so increasing the cell count (voltage) on a motor simply shows massive drop in efficiency in a specific point (same prop same RPM) for example around 20% (from 1000w to 1200w) on the turnigy SK3 5055 kv430. and there was no logical relation between KV and power draw for that specific working point and the SK3 5045 kv500 was on 950w yet the high kv L5055C kv700 on that exact point was over 1500w ( that is severely poor efficiency !!!) so who can explain all these non scenes to me !?!!

Originally Posted by mohsenfjs

once again new data and NEW HEADACHE !
ok here is the new phenomenon I see ! I have the Turnigy SK3 5045 KV500 and the turnigy SK3 5055 KV430 and static tested both of them along side some other motors with some props.

so I tested the 5045 500kv on 6S with APC 13x13.5n on 7000 RPM watt meter showed 970W. the 5055 kv430 on 6S on the exact same prop and 7000rpm and the watt meter shows 1050W and that motor on 9S with the same prop and again 7000rpm showed 1200w !
tested the same prop with a turnigy L5055C 700kv and on 6S and 7000rpm watt meter shows above 1500W !!!!!
and tested an EMAX BL4030 on a props with a specific RPM and on 6S it was 700w and on 9s with the exact prop and rpm it was 800w !!
so increasing the cell count (voltage) on a motor simply shows massive drop in efficiency in a specific point (same prop same RPM) for example around 20% (from 1000w to 1200w) on the turnigy SK3 5055 kv430. and there was no logical relation between KV and power draw for that specific working point and the SK3 5045 kv500 was on 950w yet the high kv L5055C kv700 on that exact point was over 1500w ( that is severely poor efficiency !!!) so who can explain all these non scenes to me !?!!

Have you ever looked at a motor efficiency curve in Ecalc or other? As you increase voltage, the peak efficiency current increases. Higher Kv motors of the same type have a higher peak efficiency current than the lower Kv variants.

on 6S, the estimated peak efficiency current and rpm for the 5045-500 is 50A @9550 rpm
on 6s, for the 5055-430, it is 54A 8215 rpm. Note that a direct comparison between these 2 is less valid as they are different sized motors. But in both cases 7000rpm will load the motor way below max efficiency, and the further the actual current is below the max efficiency point, the less efficient. So for the fixed output power of the same prop at a fixed rpm, those results are not inexplicable.

Going to 9S, the peak efficiency point for the 5055-430 increases to 79A /12000rpm. The 1200W input, say roughly 35A @ 9S, you required to get 7000rpm simply reflects that you are even further away from peak efficiency than the 6S case with the same motor. You didn't test the 55055-320, but its predicted max efficiency point on 6S is 38A @ 6230prm and on 9S is 52A @ 9270 rpm.

These results emphasise the point I made in post #5 that motor losses are BOTH the copper losses related to current and resistance AND the iron losses related to input voltage and zero load current. Increase input voltage for a given motor and the share of the loss attributable to iron increases, pushing the max efficiency point (that occurs where copper and iron losses are equal) to higher current.

So if you are trying to compare efficiency at higher or lower voltage, you need to understand the efficiency curve and where you are operating on it. By picking a fixed rpm, your experiment is invalid because you compared motors at different points in their efficiency range.

A more valid experiment would be to compare input power with output power at the expected peak efficiency current at different voltages, but that is exceptionally complex because different loads are required and measuring actual output power effectively requires a dynamometer. And the current required at 9S is probably beyond what you could expect the motor to withstand.

Dear Scirocco thnax for you info
I owe you big since you guided me through some fundamentals of BLDC which i didnt care much of that before .
I went trough some characteristic curves of some BLDC motors ( and i shall say what a shame hobby motor manufacturers dont support users with those curves) so It seems like most of the motors max their power peak at half the no load RPM and the max efficiency occurs at around 80% of the no load RPM.
is there any way I could find out the best efficiency RPM and the max power RPM in practice for a motor so I could design my power systems in a way that I could cruise on max efficiency and have my WOT at max power . ( yet this is only optimizing the motor and the prop and air-frame would need further effort and data logging!)