For those of you that want to check the numbers behind the fun

[Bobmic]

So today I decided to do a few experiments with my Sundowner as I wanted to push it a bit.
Since I was already using the recommended "speed setup" with an Eflite power 32 5S and a 10x10 APC I tested an 11x10 and a 12x12 ..
Unfortunately I could not find something like a 10x11... or 11x11 anyhow for those of you who are interested to get some numbers beside ground testing and just the feel of the plane in the air I was using my eagle tree and bellow are the results.
Even though it was a bit windy today the 12x12 was really nice but I will probably burn the plane with it so no go.
- By the way on the ground the 10x10 prop was generating just over 1000 Watt, what a difference in the air.

- One thing I was a bit disappointed was the actual KV of the motor which was about 15% lower than the spec.
I guess even if in theory KV should not be linked to load I might get other numbers with no load on the motor.

Anyhow any suggestions on the next step without going to some kind of an exotic super fast motor that will cost more than the plane (maybe shaving one of the 10x10s and make it a 9x10? or even better take the 12x12 and magically transform it into a 9x12  ) ?

*Each graph has three sections looking from left to right each the first flight was with the 10x10, second flight 11x10 third flight 12x12, the X axis is relative to the flight time in seconds.
All flights were done with the same batteries, all are relatively new, charged and used equally (2700mAh, 35C)

[pmackenzie]

Kv is defined as the ratio between rpm and voltage at zero current.
You get close at no load, but even then there is some error due to the voltage drop in the winding resistance.
You can make two (or more) loaded measurements and use them to calculate both Kv and Rm.
There is even a little spread sheet available (called MUMTATS  ) to do all the maths for you:

http://www.rcgroups.com/forums/showthread.php?t=233250

Pat MacKenzie

[Bobmic]

Thanks Pat, I will take a look.

What is Rm?

Bobby

[pmackenzie]

Rm is the other motor constant, the winding resistance.

The simple representation of a DC motor is a "perfect" generator in series with a fixed resistor.
A perfect generator produces voltage in proportion to the speed it spins at.

The equation is then   Vt =n/kV + I*Rm

Where Vt is the voltage
kV is the motor contstant
n is the rpm
I is the motor current
Rm in the winding resistance.

So for I=0 it simply becomes  n = Vt*kV, rpm = volts times kV

Our 3 phase brushless motors using standard ESCs have the same equation describing them. That is why they are called brushless DC motors

True AC motors driven by AC line voltage have very different equations and very different characteristics.

(The Kv we use is the inverse of the one used by most engineers, so they write the equation as Vt = n*k + I*Rm.
There is also an additional inductance term, but that is not as important for steady state operation)

Pat MacKenzie

[Bobmic]

OH MY.... So after reading way too much information about the kV factor I wish the manufacturers will just write a spec that will be a bit easier to understand.
So the question is how do we compare motors with the same theoretical data....as in real life they will spin at different speeds.
Any how - Thank you!
Since I did not feel like doing any additional tests I checked the data and as expected at Max RPM the loss is about 3-4v (I*Rm), when plotting the kV info again it align perfectly and is identical to the manufacturer spec of 770kV.
I am not sure why the efficiency number is linked to the load but I understand it has too...oh well it's too late now.

Thanks again
Bobby

[pmackenzie]

A simple way to compare motors (for the purpose of substitution) is to match kV and motor weight.
The premise is that motors of similar weight will have similar construction, so the Rm will be similar.

You can have several motors with the same Kv but vastly different sizes.
Only those of similar size will have similar Rm and be able therefore to swing similar size props.

Efficiency comes from low Rm and low no load current. No load current is "wasted" since it does no work and power is also lost in the winding resistance.
MUMTATS can estimate Rm and no load current from the loaded data.( Rm in this case will include the ESC resistance, it is basically part of the motor)

A very simple concept is that larger motors will in general be more efficient than smaller ones at the same power output. The trade off is extra weight.
Larger motors can also handle more power since they have more mass and surface area to dissipate waste heat.

Pat MacKenzie

[Ededge2002]

Some great data Bobby!  I enjoyed my Eagle Tree till it died....   One thing I changed that made the data easier to look at was i flew the plane at full power for a fixed period of time and used only that in the graphs.  You will actualy see the motor unload and load up from the turns!  Seems you need a pitot tube!!  Thanks Pat for your contributions as motor performance can make a big change in the power system.

[Bobmic]

Attached is a zoomed in portion of the RPM, I did hold max throttle for a while each time as I guessed I needed to look for an average reading.
X axis in the picture are seconds and the Y is RPM of the 10x10 prop keep in mind that crossing the field is quite fast so max the areas with max RPM include turns banks and  wind direction as well.
The capture rate was set to 10Hz and I measured the pulses to two phases for RPM not an optical sensor so for a setup in this price range I think it looks relatively stable.
As for the Pitot tube...Hmmm it will require more fiddling to attach it somewhere on the plane and than I don't know how to run the tube into the fuselage as I don't have any openings.
If HK had a cheap speed gun it could have been much simple

[pmackenzie]

Once you are in the air upwind/downwind direction is irrelevant to the plane/prop/rpm etc.
It only affects ground speed not airspeed, so it wont affect the logged data.

Pat MacKenzie