quote:

ORIGINAL: OhD

Benoit,

I ran some calculations and figure you are putting out 3.28 HP or 2433 Watts. This is great for the airplane but...... you are putting in roughly 3010 Watts and getting about 80% efficiency. That doesn't sound bad but it means you are dissipating 577 watts in heat. Think about what six 100 Watt light bulbs do in a bathroom. You really want to run that motor with a prop that will run at a higher rpm and lower torque which would mean lower current.
I bought a Hacker A60 with a Kv of 190. I figure it will run that prop at 6270 rpm at 75 Amps. I'm going to put it on the test stand to see if it comes out as calculated.
The problem with the Hacker is it is heavy. It would be interesting to see if the EVO could be wound with a lower Kv to achieve the same numbers.

Regards, Jim O


Hi Jim,
This is very difficult to make statements on a set-up as usually, we don't have all the accurate data.
Anyway, on a "philosophical" point of view I try to find out the best set-up for my personal feeling flying F3A, not for the best electrical efficiency.
Nevertheless, a good electrical efficiency is needed to achieve my first goal as the actual available power/energy density is enough but not exessive.

Here are some mesurements I did with my Power Tree analyser, the Plettenberg 30-10EVO and the Flight Power 10S 4900Mah batteries.

With the APC 21/13W, the static Amps are of course impressive I got 6300rpm and 87A up to 6400 and 100A (+3600W ) with 20°c FP batteries...
You can think that the motor is overloaded But, in the air, you get 80A max and you don't need it exept in strong wind.
With 70A, the climbs are already impressives! The motor T° stays quite low with a max of 70°C at 25°c ambiant.

With the APC 20/12W, the ground test seems more acceptable with 6500RPM and around 80A
But the final consumption for the P07 is the same for the 20/12W as with the big 21/13W!!! (from 3200Mah up to 4000Mah in strong wind for both props)
It means that the electrical efficiency of the static ground test is not that interresting . The electrical efficiency at partial loads and the prop efficiency are more interesting and realistic.
As I have a better feeling with the 21/13W, I prefer to fly that one.
And like Chad told us, the plettenberg 30-10Evo can accept a lot of different props size. For me as well, a lower KV is not needed.
I am pretty sure that you will not get 6270RPM or 75A on the Hacker A60 (which one is it?). If you get 75A with 190KV, it will be at lower RPMs (just my guess, keep us informed)
But I also think that the efficiency of a heavier motor can be better for the same torque and power. (You have to stay within 5kg anyway...)
It's just one more choise and compromise
The Plettenberg 30/10 EVO is just amazingly powerfull and efficient for 550gr (and comfortable as I like the quiet powerfull noise just coming from the prop

About the FP 4900Mah, the low internal resistance keep the batteries very cool and the voltage very high. So the power is wonderfull, the batteries efficiency is better, and it seems that you don't need a bigger capacity. Of course, I need more time to see how those new generation Flight Power batteries will get older. (35cycles on my oldest pack)

Cheers,
Benoit


_____________________________

Just Fly

(in reply to OhD)

quote:

ORIGINAL: OhD

Hi Chad,

My understanding of Kv is that it is a motor constant and says rpm is constant regardless of load as long as the voltage is constant. This is only true of an ideal motor. We make a correction for a real motor by subtracting the IR loss from the applied voltage so the rpm is equal to the Kv*(V-(I*Rm)). So you are right the rpm drops as the load increases (I goes up) but Kv does not change. It is a characteristic of the motor.
The reason the rpm drops under load is the copper loss, the IR drop in the windings, and this is also what cuses the heat. You want to minimize this loss which can be done by reducing the load and by selecting the right Kv for the prop/rpm you want to run.
From what I hear I tend to agree with you that we need more speed in the wind. Being retired I am a fair weather flier but I understand how speed could help in the wind. One could elect to use a prop/rpm similar to a glow engine but an electric motor matched to that prop would not be efficient with a 22 inch prop. Perhaps we won't care about efficiency if the motors don't burn up and the batteries get better. This remains to be seen.

I made a couple of assumptions in my calculations but if you have all the numbers the calculations are fairly easy. Here is my approach:

Kt= 1355/Kv
Kt= 6.159 in-oz/amp
Kt= .0321 ft-lb/amp
T= .0321*(I-Io)
I assumed Io was one amp but it is probably a little higher
T= .0321*(86-1) =2.7267 ft-lbs
HP=2.7267ft-lbs*6300rpm*2(PI)
Hp=3.26
Watts=3.26*746=2433
I assumed the input voltage was pulled down to 35 volts at 86 amps. It could be higher or lower depending on the batteries.
Eff= 2433/(35*86)=80%

Another approach is as follows:

Pout= Kt(Iin-Io)*(Vin-(Iin*Rm)

Without knowing Io, Rm and Vin I was reluctant to use this formula. If you have the numbers you can tell us the answer.

It sounds to me that the big time competitor will need two airplanes/motors with different Kvs if he wants the advantage speed in the wind and the big prop in calm air or he will have to live with the inefficiency, if the equipment can take it.

Jim O

quote:

ORIGINAL: Malcolm H

Chad,

I'm interested in your ducting to cool the motor. I have a plastic spinner with the front cut off to get air into the motor and a copy of the ZN firewall with the same holes to let the air out. I have also opened the cheek cowl fronts on my Impact to let air in but haven't put any internal deflectors in to direct air towards the motor can yet.

I have still to fly this puppy but the problem is that the model with 21 x 13W is 40g over weight. I have a very light carbon spinner which will wipe out just about all this but it uses a central bolt to hold the cone on so I can't cut the front off. How do you get air into your motor using an alloy spinner?

Cheers

Malcolm

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quote:

ORIGINAL: Malcolm H

Thank you Benoit,

I am amazed that you don't have significant cooling air over your batteries. In my Hacker model and in this new one I have most of the chin intake air going over the batteries. I know that when I separated the 2 halves of the battery by 20mm the temeprature went down by nearly 10 deg C.

I am surprised you are happy with motor temp of 70 deg C. I think this is a bit too hot.

Malcolm

quote:

ORIGINAL: DPappas_atwork

Hello All,
Hi Jim and Chad, let me add my two cents worth on the Kv issue.
Kv is only a constant because it is defined at no-load! I'll bet that you suspected that already. The effective Kv under load will get less. ( Well, when Professor Sandler taught the course, he said that Kv rose, but there it was defined as radians per second per volt!) Depending upon the motor construction, magnetic flux density and pole/winding geometry the magnetic field of the windings will distort the permanent magnet fields. This is called "armature reaction", and the primary effect is to angle the fields as they exit the face of the magnet. Perpendicular creates the "best" performance. Also there are hard to calculate fringing effects at the pole edges, and if the motor is close to pole saturation, then the uneven flux distribution at the pole faces can creat local saturations at high current/torque.

I've seen about 8 percent reduction of effective Kv / No-load Kv for my Neu 1515 at 75 A, and close to 10% for the AXI 5330/18. To figure effective Kv, you subtract I*Rwinding from the actual voltage getting to the motor, then divide into the actual RPM. Since Pletty are reluctant to advertise the Rwinding for their motors, you will need to take a 1-Ohm- 10Watt resistor, a car battery and a voltmeter/ammeter to measure the small winding resistance. Hook the battery and resistor through the winding and measure the approximately 1-Amp that flows througn the winding. Then measure the voltage (close to 30 milli-volts) right at the motor terminals. I will be interested to see what figure you get for both the Pletty and Hacker.

My work, so far, dictates a sweet spot of 200 < Kv < 205 and a 20-13 prop pulling about 85A with a fully charged battery. The goal will be to have a sustained (5 seconds) of 75A available at the end of P-07.

Hope that helps some, and best regards,
Dean Pappas