I think I have mentioned a couple of times (at least!) that I have been winding some motors. Yesterday and today I got a chance to do a some testing. I learned a little, and got confused a lot.

In these examples, I was looking at the motors I've been using for my E-Nobler, specifically the Scorpion line. They are characterized by a numbering scheme similar to the AXI. In my case, the motors are the 3020 and 3014. The "30" is the diameter of the armature, and the "20" (or "14") is the magnet length. Finally the number of turns are appended at the end. What I tested was the stock wound 3020-12, my own wound 3020-16 and 3014-16. Except for the magnet and armature length, the 3020 and 3014 are mechanically identical.

What I have been trying to figure out is how to select a motor from among the myriad options one is confronted with. Most manufacturer's have "lines", based primarily on diameter (stator or sometimes the outer can), then lengths (8 mm to 32 mm in the case of Scorpion) and finally the number of wire winds. In any diameter and length category,the "kV" of the motor is set from the number of winds, the lower "kV" having more turns of finer wire (or the equivalent). As a note, within a diameter class, a longer motor will have a lower "kV" than a shorter motor with the same number of winds. So my 3020-16 has a "raw" kV of 880 while my 3014-16 has a raw kV of 1265. By raw kV I mean it is just the max rpm I measure with no prop divided by the applied voltage as measured by my Astro Whattmeter.

So how do you choose? First it depends on what your battery is going to be. I make that decision based on space and weight availability. In an ARF you more or less have to go with what you have, unless you want to do some major surgery. Also batteries are really the major expense, each one costing abut what the motor costs (or more). I will also mention that these decisions are not linear, since at this stage of the game, we are all going a little on other's experiences and more than a little guessing. It is certainly better to error on the over-capacity than under, since "over" means that you may be carrying around a little weight penalty, but "under" means you can't make the full pattern--either by length or by power. I admit I cut a little close to the limit, and have been burned at least once (so far). I also decided to try and fix on the 2100mAHr cell since it really seems to be one of the most popular and least expensive sizes. So once I choose the cell size, the next issue is the number of cells and how you arrange them. What ultimately matters is the total energy that the battery pack contains--it doesn't matter if you arrange things in series or parallel, until you chose the motor that is.

So on one of the other threads, a reasonable rule of thumb is to have a kV so that the target rpm is ~70% of the no load rpm for a given battery pack. The other thing to remember is that the pack voltage drops over the run, so don't choose the max voltage of the pack at the start, but rather the "safe" voltage near the end of a run. Another rule of thumb is that you  should never use more than 80% of a pack's capacity (this is for Lipo packs) in a run. For a lipo, this "safe" voltage is 3.7V per cell. For a 4s pack, that means the "safe" voltage is 14.8V, and for a 3s pack 11.1V.

For the props I use, the target rpm's are "typically" in the 8000-9000 rpm range  for props with diameters from 10" to 12" and pitches from 5" to 7" (although I might start trying higher pitches). So with a 4s pack, that implies a kV greater than 770 rpm/Volt (for rpm target at 8000). The 70% number gives some overhead so that when more power is needed in the overheads, you can still get it from the system.

For a 3s battery, the target kV would be ~1030 rpm/Volt.

For the ENobler I started out with the 3020-12 stock motor (kV=1088 rpm/V) and a 3s battery---two 3s2100mAhr packs hooked up in parallel (~3s2p4200 mAHr). This was originally a guess--I was sure it would be enough to power the plane. And it was, but I discovered that I was only using about half the total capacity, but carrying around the full weight of 6 cells. So I thought that I could replace the setup with a single pack made up of 4 2100mAHr cells (a 4s1p 2100mAHr for 2/3 the total weight -6.7oz). I should mention that the 2100mAhr cells have a continuous "C" rating of 18 (~38A). Therefore the 4s pack has a 38A max continuos draw while the two 3s1p 2100 packs in parallel can supply twice that. In flight I am drawing about 18A in level flight, and up to 30A in the overheads from the 4s pack.

However I was now wondering if the original 1088 rpm/V was using my precious "Joules" efficiently, since it wasn't well matched to the 4s battery. I know from previous experience that my CC 35A ESC seemed to be pretty good in efficiently running at partial throttle, but there may be some room for improvement by going to a lower kV motor---but how much?

Actually I already had a Scorpion Motor kit that I bought for the ENobler, even before I bought the 3020-12 motor (I was in a hurry to get the ENobler flying before winter set in last year). So I thought I would wind it for a lower kV. I thought a 16 wind motor would get me a kV in the 800 rpm/Volt range, and I also saw that I could get more copper in the wind than the stock 16 wind motor (using a lower gauge wire--19 AWG). So that is what I did. It turned out that the raw kV ended up as noted above 880 rpm/V (showing it all isn't quite a science).

So as mentioned before, I finally tested things out yesterday and today, and I will post the results such as they are in the next post--but need a rest right now!!

The test was done on 3 motors, the stock 3020-12, and my own wound 3020-16 and 3014-16. The latter motor was wound to try on my Brodak Super Clown, but I was also curious how it would fare. It is lighter than the 3020 motors.

I first measured the 3 motors, using an APC Thin Electric 11-5.5 prop. I chose this prop because it gives a static draw similar in power as what I see my ENobler is drawing in level flight. The motors were tested with both a 3s1p 4000mAHr battery (the one that Brodak sells for the Super CLown) and one of my FMA 4s1p 2100mAHr batteries I use for the ENobler. This test was done yesterday in my garage with the setup you see in the photo. I used the JMP-2 timer and CC35 ESC (running in governor mode) from the actual E-Nobler setup. Currently this is set for 8100 rpm. I verified the rpm with my TNC tach. I measured the power (V, A, and Watts) with an Astroflight Whattmeter.

Saturdays results were as follows

motor           3s Watts measured          4s Watts measured
3020-16              230 Watts                      250 Watts
3020-12              240 Watts                      250 Watts
3014-16              250 Watts                      260  Watts

I was a little disappointed by these results which were at best a 5% effect for my own hard work. However you can see the 3s is generally more efficient, and this I assume is due the the ESC being more efficient at a higher throttle setting. But is really isn't a big deal.

I was also a little puzzled, since my 3020-16 seemed to be a little better at 3s, but the same at 4s. So the more I thought about it the more I though that maybe I wrote down the numbers wrong. Also I thought I should see what happens at a higher load setting, so I decided to make a measurement with the 12-6 APC Thin Electric prop I am now using on the ENobler.

So I went out today to the garage and set things up again. This time I let the motor run a little longer to make a few more data recordings to see if things were really stable. On the first measurement I noticed that the watt measurements were moving + 10 Watts or so, and I realized that occasionally a light breeze would come in the garage and this caused the difference. I was even more surprised when I was walking behind the prop (between the test stand and the back wall of the garage) and discovered the reading were changing---and that the prop wash was actually causing quite a back breeze from the wall!. Anyway I turned the test stand around so I was blowing it out the garage door, and also tried to make measurements when the outside breezes had died away. Now I understand why I have been noticing the upwind/downwind oscillations in amperage and power when I look at my onboard data recorder while flying!

So here is today's data. I did not measure the 3014 today or use the 3s battery. I recorded several measurments, to give some idea of the scatter.

motor         APC TE 11-5.5 prop          APC TE 12-6 prop
3020-16            246W, 240 W               397W, 385W, 400W
3020-12         263W, 268W, 263W          440W, 438W, 420W

So the 3020-16 is better, and I will mount it, sinceI am cutting it close on my 4s batteries in a typical flight, so it will help a little. What I am not sure of is where the advantage is coming from---a better match of kV to the 4s battery, or the better packing (more) copper around the armature teeth.

So my bottom line is that if I had a motor already, it is not worth buying a newer one to match to the battery if all you are trying to do is to make things more efficient. Also it is tough to setup a good test stand to measure things like this because it is very sensitive to environmental conditions.

Dennis,

It has been claimed that a "square'r" prop (pitch closer to diameter) is more efficient. That's all I meant. So of course I would run at a lower rpm for that test.
 
There is nothing god-given about pitches in the 6" range---except that's where glow engines rpm sweet spot use to be --(insert your favorite Stunt-God's name here  !) Now it seems closer to the 4" range since the newer (in last 25 years or so!) engines prefer higher rpm to make their power.

Of course electric motors have freed us from this tyranny, by choosing kV, you can make your torque and power almost anywhere. Yet we still seem to stick with it--out of force of habit?? . I am not sure how it will turn out for the stunt application.

At some  point I think Dean will "pipe" in and tell us that's where the RC Pattern guys have gone! Iam guessing (or slowly recalling) that is because the 4 stroke engine tends to make its torque at lower rpms, in a way similar to the electric motor.

Alan,

Very interesting information on the motor test. Did you measure the resistance of each motor? It would be interesting to see how the stock motor compares to the rewound ones. My guess is that the one with the lower resistance will likely be more efficient and would run cooler.

On the props one thing that was learned with pipe setups was that lower pitch at higher rpm allows the plane to re-accelerate out of the corner smoother. With the electrics we seem to be able to hold rpm even better than the pipe engines and can take a little higher pitch and still accelerate smoothly. My feeling is that up to about 6 ish should work OK. In strong wind we may want to use a little less pitch just to make sure we get the prop pulling a little stronger. I have had very good performance with the APC E 12x6 cut and repitched to 11 3/4 x 4 3/4. This prop pulls very strong in the wind. My supprise was when trying the APC E 11 x 5.5, it had no bite at all and lost almost a full second lap time even those the pitch is higher. It might be that the APC E 12x6 has more undercamber then the 11 x 5.5 and therefore more effective pitch but it is really noticeable on a larger, draggy ship. Did you fly the 11x5.5 ?

Best,        Dennis

Should I?
    Dean

Sure!

Alan,

Agree on the electrics giving us the ability to pick a wider range for where the power is applied. I think that we have a wider range of usable pitches also. The higher pitch will likely work on the low drag designs but if you have a ship with a very thick wing we may still fine the lower (< 6") to have some advantage to accelerate out of the corner better.

On the APC E 12x6 I agree there is something about that prop that gives lots of thrust. I did some prop test over the weekend and found that the Rev-up 11 1/2 x 5 was about the same as the APC_E 11x5.5 lost almost a sec of lap time for the 8800 rpm set point. The tried a Top Flite Power Tip 12x6 it was as good as the APC_E 12 x6 at 4.92 sec lap time @ 8800 rpm (I think it was right at the edge of the governor). The last one I tested was a Zinger 12x5 that was cleaned up a little (rounded the LE instead of the stock square edge). This prop was also pretty good at 5.3 sec lap time @ 8800 rpm. It seems the thick airfoil props pull better then the thin one (ie rev-up & APC_E 11x5.5) and the ones with lots of undercamber and thin airfoils also pull with lots of thrust. My guess is the undercamber on the 12x6 APC_E and the philips entry on the TopFlite give more "effective" pitch then the measured 6" and that's were the additional thrust is coming from. But they work!!!!

Best,         Dennis

Nowadays, all electric pattern setups have killer brakes because of the regenerative aerobatic brakes that many ESCs have in airplane mode. This feature is not available from anybody in heli governor mode and it isn't likely, either!

The problem with low pitches is that electric motors absorb more power when loaded down. It is erroneous wet-hangover thinking to get more acceleration with low pitch, unless you can carry more diameter as a consequence.

later,
Dean

Hi Dean,

Just to clarify and amplify a couple of points.

The Schulze 18.46K F2B ESC runs in heli governor mode WITH BRAKE ENABLED. This is how we stop the prop at the end of the flight. As we move forward we will be working to develop intelligent on board control of the brake from our processor. (being aerobatics we want to control the horizontal and the vertical)

Low pitch high rpm is simply a remnant of the need to prop the plane to operate in the power band of the engine. If the engine will not bog down and will not accelerate away, there is no need to worry about the wind or accelerating out of corners.

The biggest bang for the buck with any propeller is more diameter. Since we are limited in diameter by gear height and prop clearance we then must get the next biggest bang for the buck and that is more pitch since it is much more efficient. (high rpm low pitch setups simply whip the air white causing drag)

I think you can easily run eight inches of pitch if you have the power to turn it. I have and I still have all of my fingers and toes. Brain is a bit cloudy but ..........


Kim.
 

ESC governor with brake - this past weekend I did some prop testing, it was pretty strong winds > 15mph and on one flight the motor cut just before down wind. The motor was still at a fair rpm and acted as a big brake at the front of the ship, as the ship slowed and came to down wind the wind hit the back end and flipped it up and in. This caused a momentary lose line condition and the ship then lost almost all airspeed and just about fell out the air, I just caught it and it pancaked in.

Point being that if the motor cuts in the wrong spot in high winds with the prop free wheeling it could cause lose of control and a crash or at least a poor landing. If the Schulze 18.46K F2B ESC has the governor w/brake feature it is a strong advantage in windy conditions for landing points.

Dean - is there no chance that CC will provide special CL software add-in feature to allow the governor-brake mode? I can see that they would not want this to be a standard feature but as a special add-in for us CL people to purchase it would keep the Heli guy from activating it by accident and still allow us to get what we need.

If no - once we know the % throttle setting using the governor for the needed rpm, could we set up the motor start and end settings in the timer to simulate the flight power settings that the governor would have provided. ?Then not set the ESC to governor mode, but set it to use the braking mode? Can we get close to the same performance?

Best,           Dennis



Best,          Dennis

Dennis, Dean

You should know that we first tried to work with CC when we were developing our package. They had less than zero interest. Even if you do get them to enable the "Brake" function in governor mode, there are so many other parameters that will not be totally optimised for C/L PA. (even with all of the settings CC allows you to adjust)

I am not sure what the hesitation is in using the Schulze ESC other than cost. (you get what you pay for) My guess is that we can get more done by supporting the company that made the first dedicated C/L ESC than by trying to get five other companies who are not interested, to make one. (isn't going to happen  :-(   )  Please note that I do not receive ANY compensation from Schulze.

Kim.

 

Hi Kim,
The prospect of using a low percentage duty cycle brake to make the RPM control loop gain more nearly the same in both directions is fantastic.
Aside from expense, I have been trying to cultivate a domestic source for a Stunt optimized product. I may give up, but not quite yet. With that added feature, it sure will be hard to argue against using the Schulze. I don't get any compensation from Castle either, but I sure have been plugging their product.

As an aside, the system Kim is describing would be capable of dropping excess RPM as fast as it can add RPM when too low. Right now all we have is drive and coast rather than drive and brakes, kind of like the cruise control in your car. This will be very good in strong runway wind.

later,
Dean

The North-American dealer for Schulze is in Canada: www.icare-rc.com.
He also imports the Plettenberg line of motors. The Plettys are good stuff, if also not cheap.
The fellow in charge is named Etienne ... and I'm sure I've misspelt it, but he's a good guy.

Dean

Hi Mike,
Is that in governor high or governor low mode? On 70' lines with a 12 X 6 or 12-1/2 X 6 you shouldn't ever need to go below  9,000.
I seem to remember not being able to get above 9,000 RPM with a 14-pole motor in the low range, so the 8 ought to work with a reasonable pulse width.
later,
Dean

Hi Mike,
That is going to be tight, but it will work..
Using my rule of thumb of setting the RPM to 75% of Vbatt * Kv with the 675 Kv Neu 1907, you get 9300.
At 9300 RPM, with 70' of line and a 5.25 second lap you get a 6-1/2 inch pitch. APC make a 12-6E and a 13-7E.
You don't really have the ability to set for the 7" pitch, and 6" should be okay. I'd probably start with a 12-6 and try pitching a 13-7 down to 6-1/2 and clipping if necessary for current draw. You have a narrow sweet-spot to work in, but there is one!

I'll bet you didn't need me to tell you that,
Dean

Hi Dennis,
You should have no problem whatsoever.
Dean

Hi Dennis,
It looks like the Schulze works on 4 or 5 cells, based on the data I found at the Icare-rc.com site.
I don't think it has a BEC, so you'll need a commercial BEC regulator. Lots of outfits sell them.
I'll go look at the Schulze site from Germany to see if it works on 3 cells.
Right now it is the only one.
Dean

Hi Guys,
   You will need an external BEC to run the Schulze 18.46K F2B timer and it will work with 2-5S lipo's.

Mike

Thierry,
   What motor/battery combination are you using with the Schulze? During the flights that shut off due to thermal overload did you have air flowing over the ESC and can you confirm it was thermal related or was that just a guess?

   I haven't experienced this problem with my Schulze yet, but I will be sure to measure temps after the flights.

Mike   

Next,

On the D 520 the motor is a SMT 3600, it is a "french wired" AXI 28/26/10-12 with a Kv of 730 + Evolite 5S 4350 mah+ Schulze F2B+ U Bec+ APC-e 13 x 6.5
On the above graph, what you see after the 3 mn flight session are short ground motor trials in order to try understanding the reason of the cutt-off.
Rgds, Thierry

while your motor test stand looks very simple, what is attaching the 2 pieces of wood together is it just (2) pieces of 1x2 pine ?
For reassurance I'd add a 1/16  or 1/8 gusset on each side of the joint to make sure the lo9ad never causes that joint to give out.

Hi All,

Thanks again to Alan for starting this "juicy" thread. Igor has told me that the newer range of Jeti Spin esc's have the option of governor mode with brake. The Jeti range is a little cheaper than the Schultze, and with the optional Jeti Box, they are easy to program. If you fly FAI rules, then the brake is necessary for your landing points. As some have found out as well, it's also nice to have the propeller braked in lots of wind. The FAI ruling was in fact to prevent a programmed landing with a progressively slower rpm. This would be an unfair advantage over the i/c engines.

The only thing that I have against the Jeti esc's, is that they have lousy, or no short circuit protection. I have managed to burn out 3 of them already. Two of these were at the last world champs when my motor wires burnt, and the last one at home when I managed to jam the prop on start-up by accident. O.K.....I am naturally clumsy, but I think that if my el-cheapo Hobbywing esc can shut down, then Jeti can also get it right. Other than that, the Jeti Spins work very well and program easily with the Jeti Box. My last unit did have the brake option, and Igor talked the Jeti guys into making him just 10 units with the added brake feature. That is not too shabby in my book!

Keith R