At one point I went through the "list your setup" and calculated the average power/pound for a bunch of planes, and I came up with something like 100-150W/lb for big planes, and more for 1/2-A sized.

Then someone pointed out that that's average power, and that one must allow for about twice that for peak power, at least if one is using an ESC in governor mode.

Can this be distilled into some rules of thumb, beyond just "look at 'list your setup' and guess"?  Has someone already posted them, and I missed it?

It looks like the following should get one into the ballpark, although I'm not sure of my RPM figure and I'm really not sure of it for small planes and I'm really not sure if 80% throttle is enough.

There's too many planes and motors and batteries out there for me to want to take a "copy your neighbor" approach -- I'd much rather have some sort of "design handbook" approach, which is what I'm trying to find, here.

• Size the battery so that 150W/lb for six minutes uses no more than 80% of capacity, i.e. (weight)(4000mAh/lb)/(cells)
• Size the motor and ESC for 300W/lb, i.e. current = (weight)(80A/lb)/(cells), power = (weight)(300W/lb)
• Size the motor for 8000 RPM at 80% throttle*, i.e. Kv = (2700 RPM/V)/(cells)

* Is 80% right?  70%?  50%?

Yes, Tim.
I'll reply in full later in the day.
Dean

Wow! took too long to get back: sorry, Tim.

Okay, for starters, a Stunter consumes between 0.6 and 0.7 Watt-hours of energy per ounce of ready-to-fly weight to actually fly the schedule. The difference between the two figures is mostly accounted for by airframe drag, differences in line length and also differences in powerplant efficiency which has a lot to do with prop and RPM choices.

Anyway, let's use the example of an "average" 55 ounce airplane on 65 feet of line. Assuming 0.65 W-h/oz I get 35.75 W-h.
Now I'm going to make an assumption that we will be running a 4S battery. It could be a 5S ...

Because the nominal battery voltage under load for a 4S pack is 14.4 Volts, we will divide Watt-hours by Volts to get Amp-hours consumed.
I get 2.48 Amp-hours and I'll round to 2.5.

If we fly a 55 ounce ship with a 4S setup, we need a 3.3 Amp-hour battery. (3300 milli Amp-hour)
This is lucky 'cause they are made in that size: otherwise we would have to look for the next size up.
Why 3300? You should not use more than 75% of the battery capacity during a flight.
Yes, some folks run 80% just to get by with the next smaller size battery, but they will suffer shorter battery life as a result, and hot weather operation may lead to unhealthy battery temps.
If you use much less than 75% of the battery capacity, there is no downside other than that you are carrying around needless battery weight, but using only 2/3 of capacity will lead to very good cycle life, so that you might loose them due to age first unless you fly a lot.

Okay, so now we have a 55 ouncer with a 3300 mA-h battery using 2.5 Amp-hours of capacity in about 6 minutes.
The average current will be 25 Amps or 2.5 A-h divided by the running time of one-tenth of an hour, or 6 minutes.

Depending on your setup, the peak currents will be maybe 50% greater than the average or somewhere near 37 or 38Amps. The minimunm currents will be maybe half of that during dives.

Now we pick a motor Kv and prop and RPM to get a sensible combination.
The target running RPM is : Rpm(target) = 0.75 * Kv * V(battery).
Let's assume I have a motor with a Kv of 920 RPM/Volt in my grubby paws. I get 9,900 RPM as a target RPM.
You can safely go 10% lower and maybe 10% higher than this target figure in order to accomodate the available prop pitches, and desired lap times.

Your actual running voltage (or equivalent throttle) will be maybe 6% to 10% higher than the 75% figure we used, because the motor resistance will "waste" something like 1 volt.
Yes, if you put 3.3 cells into one of the motor simulators you will get about the right results. Yes, many of them accept fractional cell counts!

So now we are running around 9900 RPM in flight, and for a sensible lap time an effective pitch of roughly 5-1/2 " is needed.
Unfortunately, the number stamped on the prop and the effective pitch are only neighbors ... not twins. You'll need to experiment or lean on the prior experience of others.
For example, if you add 5% to the RPM and run the fashionable 13 X 4-1/2 APC pusher that pulls like it has most of another inch of pitch in it, then you'll be okay.

How much diameter? Up until the point where diameter kills the turn, adding diameter adds power consumption in the climbs, and reduces it in the dives while only very slightly affecting the level lap power consumption.

Remember that the battery voltage in a healthy setup in a climb near the end of the schedule can drop to 3.4 Volts per cell, (or 13.6V for a 4S)
and this eats into that excess battery voltage that the RPM governor needs in order to goose the throttle and fight gravity. If you use more than 75% of capacity, you may see even lower voltages.

more to come,
Dean

I ought to put it into a spreadsheet ../.
Dean

Doing the battery-growth calculation for cooling in the other topic got me wondering about how stuff scales.  What goes into the .65 W-hr./oz., and how does it go?   I think some calculation could be way more fruitful for electric stunt at this point than for glow-engine stunt: electric doesn't have the benefit of 60 years of trial and error, and everything is so much easier to measure.  All that effort I put into figuring out how to do airplane systems trade studies might not go to waste after all.  I look forward to that spreadsheet.

Dean,
GREAT response,, I have not seen these calculations before,,
However one question,,
you state
Anyway, let's use the example of an "average" 55 ounce airplane on 65 feet of line. Assuming 0.65 W-h/oz I get 35.75 W-h.
Now I'm going to make an assumption that we will be running a 4S battery. It could be a 5S ...

should this be 0.65 W-hr* oz
instead of 0.65W-hr/oz?

I see where its gone wrong,,
I assumed the 0.65W-0h/oz was the stated formula when in reality the formula is
0.65W-h/oz * oz,, the inclusion of the units ( oz) made it appear to the un-engineered back seat math guys,, like me, that this was the formula and he was dividing the Whours by the ounces,,
so there was a language barrier occuring,,

"MAX WATTS AVAILABLE THAT THE SYSTEM CAN DELIVER AT ANY MOMENT"!

At one point I went through the "list your setup" and calculated the average power/pound for a bunch of planes, and I came up with something like 100-150W/lb for big planes, and more for 1/2-A sized.

Then someone pointed out that that's average power, and that one must allow for about twice that for peak power, at least if one is using an ESC in governor mode.

Granted, it wasn't stressed as much as you have, but we most certainly did mention it.

Getting the motor size right, and sufficient current out of the battery, and having the whole thing crap out four minutes into every flight, without enough room to put in a bigger battery, would be a pain.

Hence the desire for a spread sheet that lets you get all the variables into the ballpark, so that when you go to the flying field for the first time you can at least be confident that you won't have to stomp the airframe to toothpicks and start over.

I never, ever, get things right when I do them by rote.  If that's what works for you -- great.  Me?  I need to understand the issues before I proceed, or I mess things up.  I'm working on a tool for people who think like me.

Me?  I want to figure out how to beat other people at flying stunt. 

now Howard, there are always people at stunt contests that you beat,, heck you beat most of the people at a stunt contest,, I suppose its just certain people you want to figure out how to beat then? 
so how about we concoct an EMI device to trigger at said portions of said persons pattern, say right at the intersection of the hourglass,, the EMI might cause a hickup,, oh wait, thats pretty specific huh,, ok well never mind,,

Thanks Rudy.

I do think your suggestion to check your motor manufacturer's suggestion for sizing for an equivalent weight 3D RC plane is a good one.  It's a nice clear direction that should certainly give some strong guidance!

Tim,

We are fortunate in ECL to have over 25 years of ERC experience to draw on. I, and thousands of others, started using LiPos in RC about 8 years ago. This and modern brushless motors changed everything. I first flew electric RC in the 1980s but they were heavy and primitive by todays standards. There have been many bright guys in ERC over the years that have perfected the E systems that we are now fortunate to be able to use in ECL.

I like to go through my old magazines, re-reading the construction articles.  Except that the electric power ones are downright painful.  Geared can motors, and NiCd batteries, and by gawd I'm getting 75W/lb oh boy look at the performance!!  Now you just take any old wet model, put in modern equipment, and you find that it's no heavier (lighter if you build for the lower vibration), quieter, and doesn't have slime all over it when you land!

The most puzzling part for me is that I still like flying wet power...

No big supprise.  Real airplanes make noise and leave a smoke trail, smell bad and need the oil cleaned off - its part of the air power culture.

 

Hi John,

I think a similar line was posted on another forum about 100 years ago.   "...... Real carriage power systems make noise, leave a dust trail, smell REALLY bad, need a LOT of really bad stuff cleaned off ....... it's part of the carriage culture" .... signed; Clyde "the scoop" Rawhide      

The trail of Progress does not always travel in a straight line! Or maybe: "you can lead a horse to water but you can't make him drink" applies? 

Rudy!
Don't you remember the old sales pitch from the same era?
"Fight pollution ... get rid of that horse and get an internal combustion engine." 

Dean P.

Thanks Dean, that's a great sales pitch, it worked .... well sort of? I'm going to use that in my Marketing class, my students will get a kick out of it. 

I can't cope with that now.  Visions of sugar plums are dancing in my head.

Sounds like Howard is working on the next paint scheme..!   

My guess is that it will be some significant amount of some shade of orange.   

Paul