Okay, I admit that I'm a retread (been away since ~2004/05) coming back and wanting to get in electric (kinda have to - sold off most of my engines).

I have a design that I did about 11 years ago, originally designed around the ST60.  I scaled it down from a 62" span to a 36" span and built it as a .15-size stunter - sort of a "proof of concept" ship.  I was quite pleased with it as it flew like a big plane and handled the wind pretty well (for me, at least).

Now, I'm revisiting the design and want to build a whole new one for electric.  Only troubles I'm having are some sizing issues.  Most of the setups listed in the sticky are .40-.60 size ships, with a few 1/2As thrown in for good measure.  Almost nothing that would take the place of an OS Max .15FP, which is my goal here.

I know that Brodak has a setup for .15-.20 size stunters, but I'd like to learn the hows and whys of the individual parts and source them myself, if possible.  I've read through some of the threads discussing battery size and motor sizes, but to say I'm getting a little confused it putting it mildly.

Some specifics on the ship:

Profile, 37" span, 249 sq. inches (w/ flaps), and should finish out around ~20 or so (lighter would be better, I know).  Later, if it proves to be a good ship, I'll probably build a full fuse version, but for now, I'm sticking with the profile (simple and light).

Thanks.

Steve Berry

Welcome to the board, Steve!

I'm just starting in the "electric arena", so I can't be of much help.  But the best "electric" minds frequent here so help will be easy to get.

Good luck!
Big Bear

Hi Steve,
That's 20 ounces with what equipment in (or not in) the airplane.
Is that your guess at weight ready for motor/battery/ESC?
this will help get you in the neighborhood.
Dean

Unless someone has a better suggestions for estimating the weight?

Compare it to the last one you built!

I always underestimate weights.  Just going by total wood volume is going to get you a severe underestimate -- you have to account for glue, hardware, all that plywood, wheels, etc.

Your electric system will probably weigh in the neighborhood of your engine and a full tank.

John V
AWESOME combo & weight.  You have lots of RPM potential for the 4-pitch and still have a lot of motor for a 8x6 or even a 9x4.5

My brother put the PF system (above) on a Sig Akrobat, (goes like...HECK) I have a Circus Prince (same wing as the Flying Clown) ready to go when the snow stops for awhile..!

John V
AWESOME combo & weight.  You have lots of RPM potential for the 4-pitch and still have a lot of motor for a 8x6 or even a 9x4.5

My brother put the PF system (above) on a Sig Akrobat, (goes like...HECK) I have a Circus Prince (same wing as the Flying Clown) ready to go when the snow stops for awhile..!

Hi Denny,

As an aside,  I still have my PDQ Circus Prince from 1964.  At $1.95 I couldn't pass it up back then.   I have it stripped, straightened out, and repaired where necessary, waiting to compare it with the Brodak version of the same.  At the time (1964) I thought the Circus Prince looked WORLD'S better than the Flying Clown!  (started out with a McCoy .19RH, and it even survived a summer of flying it with a Fox .36X! LOL!!!!)

Mongo

Oddly enough, I am also dealing with sizing the components for a .15 size stunter.  Here is what I figure:

To figure out the motor size, take the glow motor size, say .15 cubic inches   (c.i.)   horsepower is (hp)

Required watts to produce equivalent power:   Watts = Engine c.i x 2 hp/c.i. x 750 watts/hp

So for our .15   .15 x 2 x 750 = 225 Watts

Now to calculate the battery capacity required:

Say we turn just under 5 second laps.  We need 5.5 minutes to complete the pattern from when the motor revs up.  We also want 20% battery overcapacity.

Battery capacity = 1.2 x Watts x Minutes x 16.66 / Volts          ( 16.66 = 1000 mAh/A  / 60 minutes/hour )

So again for the .15  with a 3s pack     1.2 x 225 x 5.5 x 16.66 / 11.1 = 2221 mAh battery needed.

So the 2200 packs are right on the money!

The next thing to consider is the prop.  There you have to take the "kv" rating of the motor, and the prop pitch you will use, the line length and planned lap-time, and come up with a useable combination.

The formula for lap time (approximately) is:

T = ( L x 4522) / ( RPM x Pitch)

T in seconds,  L in feet of line length, and pitch in inches

So for our .15 again

11.1 volt battery,  1000 kv motor  we get 11,000 rpm

Say an 9x4.7 APC-e prop and 55 ft lines

T = (55 x 4522) / (11000 x 4.7)  = 4.8 seconds

Just right! 

 

Hope this helps!  We can tune the numbers such as HP/c.i. as we learn more.

     
 

Darn I'm feeling ancient. I flew mine with a Ollessen (sp?) 23......

Just used what I had, and I didn't have an O&R! 

I am currently building a Midwest Magician 15 that will be electrified.  Since this bird is the same size/weight as the 1/2A Pathfinder I plan on duplicating that installation.  However I thought it might be fun to run the numbers - as Larry did.  As you will see one or two different assumptions can lead to substantially different results.

The Magician 15 was designed in the era of the Fox 15 Steelfin (the one that preceded the Fox 15X).  With that engine we would have likely used either a 7x4 or 7x6 prop to happily stunt the day away on 52 foot lines. Another way to look at it is that the old Fox is comparable to a modern OS 10 or TT-10 engine.  I know that APC makes a pursher version of their 7x5 prop, and that is the same realm as what we want to replace, so let's run some numbers on replacing that equivalent to the .10 IC engine with an electric motor will turn a 7x5:


Critical Juncture 1:
My first difference vs Larry's calcs is to assume that I am replacing a 10 and not a 15.  To figure out the motor size, take the glow motor size, .10 cubic inches   (c.i.)   horsepower is (hp). Required watts to produce equivalent power:

   Watts = Engine c.i x 2 hp/c.i. x 750 watts/hp

So for our .10:   .10 x 2 x 750 = 150 Watts

 
Now to calculate the battery capacity required:

Critical Juncture 2:
We know if we fly a 5 sec laps on 60 feet of line we usually have plenty of speed and it will take us around 5.5 minutes to comfortably do the pattern.  The Magician I want to replace flew on 52' lines.  First lets calculate lap time.  I am going to assume that even though I plan to fly on shorter lines, that I really cannot slow the airplane down a lot - it will fall out of the sky.  Thus the conservative approach is to duplicate our airspeed on 60' lines when using 52' lines then it is a simple scaling excercise:

New lap time = (52/60) * 5.0 = 4.3 seconds per lap.

Now mind you if we determine that we actually CAN fly slower then that will be good news, we will use less battery.  So our 4.3 sec speed is conservative.


CRTICAL JUNCTURE 3:
Now the flight duration:  If we are turning 4.3 second laps instead of 5 second laps we will need less time to complete the pattern.  Again, by scaling the lap times we will get:

New flight duration = (4.3/5) * 5.5 minutes = 4.7 minutes.

Next we know We also want 20% battery overcapacity.
Battery capacity = 1.2 x Watts x Minutes x 16.66 / Volts          ( 16.66 = 1000 mAh/A  / 60 minutes/hour )
 
So again for the .10  and assuming that we will use a 3S Pack rated @ 11.1V nominal.

 1.2 x 150 x 4.7 x 16.66 / 11.1 = 1269.7 mAh battery needed, the 1300 in the Brodak EPS-10 package in my 1/2A Pathfinder is spot ON!


CRIICAL JUNCTURE 4:
Since we know our target lap time, line length and prop Pitch, lets determine what RPM we need to run.  The formula for lap time (approximately) is:

T = ( L x 4522) / ( RPM x Pitch)

Rearranging terms will let us solve for our target RPM:

Target RPM = (L x 4522) / (Pitch x T)

We would be VERY pleased to run a 7x5 prop on our old Fox 15X at 4.3 seconds/lap on 52' lines, thus:

Target RPM = (52 x 4522) / (5 x 4.3) = 10,936 RPM  -> lets call it 11,000 RPM
 

Into the home stretch!  We know that the motor will run the fastest with a fresh battery but that it will slow down over the course of the flight.  Therefore (as John Hammonds suggested) lets reserve some "overhead" for the governor or "Throttle-up timer" (like the Hubin FM-0c) to maintain the speed over the course of the flight - that way we start of at part-throttle and let it throttle up to match the deterioration of the battery.  How much is enough?  Castle says reserve use 70% - 85% of max available RPM - Lets use 75% as a comfortable "minimum":


kv = 1.25 x 11,000 / 11.1 volt battery = 1238 kv (minimum) The Arrowind 2210 used in the Brodak EPS 10 & EPS 15 systems is kv= 1560.  To generate 11,000 RPM it needs to run about 64% throttle.  That gives us plenty of room to adjust our "Rule of thumb" into practical real world Magician 15!


The results I generated using the same formulas are very different than what Larry generated.  Rather than obsessing over which is the "correct" answer I think the lesson is to bash around a few different assumptions and see how it shakes out.  A little spreadsheet routine (or a simple pocket calculator) that let you change displacement, line length, lap times and flight times and prop pitch could let you "build" a lot of different scenarios pretty quickly.


Of course there is no substitute for "committing aviation", and I am pleasantly surprised (shocked acutally) that the Brodak EPS-10 System on the 1/2A Pathfinder comes out as close as it does to fitting the ROT prediction.