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Optimum Kv for 2 meter motor?

Old 05-05-2009, 08:59 PM
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OhD
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Default Optimum Kv for 2 meter motor?

I was talking about power systems with Jerry Budd last night and he mentioned that even the C50 14XL was hauling with the new low resistance batteries available. I told him that I had to crank the high end of my throttle curve down to 80% when I got the Plettenberg and that got me thinking today. If the motor had a lower Kv it would have a higher Kt. Higher Kt means more torque per Amp. Therefore one could crank the endpoint back up to 100% to get the desired rpm and pull less current at that rpm. It looks like the Plettenberg should be rewound for the new batteries. What do you think? Can we get the motor suppliers attention?

Jim
Old 05-05-2009, 09:43 PM
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can773
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Default RE: Optimum Kv for 2 meter motor?

Hi Jim

If you rewind for a lower kV that might work great on a 21x14, but anyone that wants to run say a 20x15 is now likely lacking performance, so you get into the area of maybe needing two winds for different props. You are quite right though, as packs improve the winds need to be re-evaluated, and that is certainly a discussion we can have while in Portugal since the Pletty's will be there and there will be opportunity for a lot of feedback. Right now I know they are busy with other projects like the thrust vectoring assembly

An interesting development is going to be coming from Schulze as well, I think they are calling it volt-control and its available on their larger ESC's but are likely going to produce an F3A size controller with the same. Essentially as I understand it you dictate to the ESC what full throttle RPM you wish to have and as long as your battery is of sufficient voltage to deliver that the ESC would automatically calibrate the throttle during flight to ensure you always have the same top end RPM and therefore response. This is will be a lot more reliable and repeatable than setting the ATV. Although not as direct a solution its would provide a bit of compromise.
Old 05-06-2009, 07:38 AM
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Default RE: Optimum Kv for 2 meter motor?

Why is it so desirable to operate large pitch propellers at low revs?
The IC guys operate 10-11" pitch at 7500-8500 rpm, while we operate 12-15" pitch at 5500-6500 rpm.
Do you not achieve (generally) the same result by allowing the el. motor to spin slightly faster, and compensate by using a lower pitch for a similar pitch speed?
There may be a difference in efficiency, but is it significant, assuming similar prop. diameters?
At least you should get better braking from a lower pitched propeller. (Many people use regulators without brakes.)

An interesting development is going to be coming from Schulze as well, I think they are calling it volt-control
I have many battery packs of different age, and the older ones are significantly lower in performance than the newer ones. I have therefore always wanted a "governor" system incorporated in the regulator, that would ensure that every stick position equated to a given rpm, irrespective of the battery voltage, internal resistance etc.
Maybe not exactly what Schulze are working on, but maybe similar. Ideally this governor should work through the whole rpm range, and give you full braking power without stopping the propeller with a stationary model. (i.e. maintain a given idle rpm, irrespective of the model speed.)

Regards,
Magne
Old 05-06-2009, 09:18 AM
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Default RE: Optimum Kv for 2 meter motor?

Actual RPM control via stick position (pulse width to RPM control) would seem to be very desirable and would minimize the battery condition variable. Not sure one would want to be limited to the max performance attainable with the weakest pack though. Maybe RPM control to 80% or so and unlimited after that.

Magne, I've measured vertical performance in the old Humpty with snap up in P07 using EagleTree equipment in a Partner equipped with YS 160 & Hacker C50x14C (same airplane) @ 4.5 Kg + fuel glow & 5 Kg E. Props were 15x11-4b, 17x13-2b glow & 21x14 E. I don't have the exact numbers any longer (hard drive crash), but both airplanes came out of the snap at equal speed. The glow maintained that speed with the 4b and increased a little with the 2b. The E increased speed significantly. Not sure if the E benefits from slower prop speeds being more efficient or just a larger prop, as the calculated pumping volume of the latter is higher.
Old 05-06-2009, 09:49 AM
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Default RE: Optimum Kv for 2 meter motor?


ORIGINAL: Magne

Why is it so desirable to operate large pitch propellers at low revs?
The IC guys operate 10-11" pitch at 7500-8500 rpm, while we operate 12-15" pitch at 5500-6500 rpm.
Do you not achieve (generally) the same result by allowing the el. motor to spin slightly faster, and compensate by using a lower pitch for a similar pitch speed?
There may be a difference in efficiency, but is it significant, assuming similar prop. diameters?
At least you should get better braking from a lower pitched propeller. (Many people use regulators without brakes.)

An interesting development is going to be coming from Schulze as well, I think they are calling it volt-control
I have many battery packs of different age, and the older ones are significantly lower in performance than the newer ones. I have therefore always wanted a "governor" system incorporated in the regulator, that would ensure that every stick position equated to a given rpm, irrespective of the battery voltage, internal resistance etc.
Maybe not exactly what Schulze are working on, but maybe similar. Ideally this governor should work through the whole rpm range, and give you full braking power without stopping the propeller with a stationary model. (i.e. maintain a given idle rpm, irrespective of the model speed.)

Regards,
Magne
My original thought on going electric was to have a system better than the IC guys rather than emulate them. I reasoned that the larger diameter props would provide more thrust at low speeds and that is what I wanted. Granted you need speed when it is windy but now I believe we've got both ends covered with the new batteries. The 20.5x14 and 21x15 are less load than the 22x12 and the Plettenberg doesn't have trouble turning these at over 6000 rpm. I haven't tried it but I'd bet it would be 6500 unloaded in level flight. So I think the electric now has a clear advantage.

Back to lowering the Kv. Think about this. If we operate at 80% throttle, whether we do it with the ATV or the throttle stick we are at an RMS voltage of .894 times the battery voltage. That means we are carrying an extra cell around for no good reason. I'm considering trying a 9 cell pack and go back to 100% throttle.

Now Magne's suggestion. I've been thinking the same thing and don't know why the governors in the ESCs right now wouldn't work. When I get my throttle curve finalized I plan to try it. Has anyone already tried it? If this works then we want to operate at something less than 100% so we have enough overhead to hold the RPM.

Jim
Old 05-06-2009, 12:11 PM
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Default RE: Optimum Kv for 2 meter motor?

I looked at using the govenor mode in some bench tests with a small Castle ESC (35A) & Hacker OR. It worked at WOT pretty well (as in a typical heli set-up) but would often oscillate when throttle position was changed. Generally it was way to inconsistent for airplane use. My suspicion is that the PID loop is optimized for heli applications and that the different accel rates & throttle changes typical of airplanes are a whole different ball of wax.
Old 05-06-2009, 12:54 PM
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Default RE: Optimum Kv for 2 meter motor?

Magne

That's essentially what the volt control is for, to remove the variable of battery voltage. Depending on how low your lowest pack is though, it may not be practical to force the controller to limit power so much, so the range of operation would be limited to an extent. My experience though is that typically static tests before and after a flight don't have more than about a 300 rpm change, so on relatively healthy packs you should not lose that much performance at the beginning of the flight.

I am going to ask Schulze about adding the braking feature to force the idle speed to be constant (ie: automatically adjusted braking power). We will see if they can do that.

A couple of years ago one of the Canadian F2B (CL stunt) guys ran electric and used a schulze controller with governor, it worked but he had to get schulze to adjust the governor to have many more steps to get the proper RPM. From what I understood the standard heli governor is pretty coarse and didn't offer very good control of rpm. The volt control feature is not a governor, but more like an automatically calibrated throttle except using a fixed top end RPM.
Old 05-06-2009, 01:46 PM
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Default RE: Optimum Kv for 2 meter motor?

Magne

I just heard back from Ulf at Schulze, and was told that they are unable to control the braking based on an idle RPM because its not possible to detect the rotor position at low rpm under braking.
Old 05-06-2009, 02:32 PM
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OhD
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Default RE: Optimum Kv for 2 meter motor?

I just ran some hanger tests and the governor looks like it will work. Fine response and no rpm sag with voltage over a few seconds. The real test will be a flight test and see what it looks like with a pack 80% discharged.

Jim
Old 05-06-2009, 03:35 PM
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Default RE: Optimum Kv for 2 meter motor?

Here's a look.
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Old 05-06-2009, 06:19 PM
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Default RE: Optimum Kv for 2 meter motor?


ORIGINAL: can773

Right now I know they are busy with other projects like the thrust vectoring assembly

What is the concept? Years ago I was thinking a tail rotor would work well on a pattern airplane. I guess it could be done in all three axes.

Jim
Old 05-06-2009, 11:33 PM
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Default RE: Optimum Kv for 2 meter motor?

I think the newer cells certainly change the power curve. But, why restrict to 80%? Why wouldn't you want to have it all available? I've been flying in some stiff wind lately and it is nice to have the punch. But, my power curve is not quite right. I find that I struggle to find the right speed. I try to fly the speed rather than the sound of the motor and I am constantly changing throttle setting throughout the flight. I guess the idea of the governor is to try and tame things down, but maybe that should be done with a flight condition for different power curves throughout the flight so you still have the max available at any time.

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