This was written in response to a number of confused newcomers to idea of using electric power in control line precision stunt, primarily based on information that I found in the "Gettin all AMP'ed up!" on the Stunthanger Forums.
You need a browser that has a Java plugin installed. If you're not sure, go here and see if things work as described.
If you don't have a Java plugin and don't want one, you can download the jar file and run it directly. It's right here. Download it, and run it the way you would any other jar file.
This program distills the computation of the necessary parts that you need for an electric control line stunter. Its current form is pretty rough: at the moment this is just a proof of concept calculator. I want to see if it comes up with good numbers, and if it is even remotely usable to folks who aren't computer experts.
I wanted a calculator that would let you answer questions in 'all directions'. Not just "I have an XYZ plane, what motor do I need to buy", but "I have X motor, Y batteries, and Z speed controller, do they match and what plane can I build?". This can make operation a bit confusing, but I've tried to make the program so it'll let you know if you're veering off into the weeds.
The calculator works by letting you enter data into any cell, then working out what the values of the other cells are. The problem with this is that the program needs just the right number of things specified to work out an answer: if there are too many things specified then two or more of the numbers specified must agree exactly (the problem is over-constrained), while if there are too few things specified then there's not enough information to figure stuff out (the problem is under-constrained).
As it comes up, the calculator is set to show the calculation for a "normal" situation: you have a 40 ounce plane that you want to fly with a 6-second lap time for 6 minutes. All of the boxes that are checked denote information that's 'pinned down' -- i.e., that the calculator knows beforehand that it is using to make the calculation. All of the boxes that aren't checked denote the places where the calculator has filled in the blanks.
If you want to do a calculation of this type, then enter in numbers in the checked boxes only, and see how things change. For instance, if you know that you have a plane that's going to build out to 50 ounces instead of 40, then enter that number in the "weight" box and see how the other numbers change.
In the case where you change to a 50 ounce airplane, you'll find that you need a four cell pack with 2956mAh. If you're happy, you can round that figure up to 3000mAh, and go buy stuff.
But what if you know you want a 5-cell pack? Then you can change the cell count (that's a pinned number), and see that now you need a 2364mAh or better pack.
But wait -- with that 50oz aircraft with its 5-cell pack, you see that you need a motor with a Kv of 650 RPM/volt. You don't have one, and the catalogs don't show them. But you can find one that's 600RPM/volt, and another that's 720. What happens if you use one?
To find the answer, enter 600 into the Kv box. As soon as you hit the enter key, a whole bunch of boxes light up -- that's to tell you that either the number can't be trusted (if it's blue), or that it can't be trusted and it's causing a problem because it's pinned down (if it's red). So you know you need to unpin something. You can either unpin the RPM and find out that the motor 'wants' to go at 8325 with this combination, or you can unpin the speed derating and find that things work if you use a speed derating of 81% -- but you know that an 81% speed derating is unwise, so you think that perhaps that isn't the right motor with this number of cells.
What happens with a Kv of 720? Try it! Do the same calculations as above, but see that in this case you can let the RPM go up, or you can run with a safer, lower speed derating.
But say, for instance, that you've got a specific motor laying around, or a speed controller. In this case you may want to know "What can I build with this combination?". This calculator lets you do that, too.
Let's say that you're building a smaller plane, and you've got a motor like the AXI Atlas 480 lying around. This motor has a maximum current draw of 20A, and a Kv of 1090 RPM/V. So you want to use these known factors to do your computation. In that case, you go to those boxes, and you enter them. Note that two things happened: the borders on several boxes changed colors, and now the numbers don't really make sense.
What happened was that when you specified more numbers than necessary, the remaining numbers became inconsistent. So what you need to do now is to free up the log jam. There are several boxes that have red borders and that are pinned down. Choose quantities that you think can change, and unpin these. We'll start with the RPM and see what happens.
What happened was that the RPM changed value, and we corrected problems with two boxes. But there's still problems elsewhere. The next big question is airplane weight. So let's uncheck that box.
BAM! Everything is back to being normal. Now we know that we can build a 20 ounce plane with the combination shown.
There's still something not right, though -- the Hobby Lobby web site says that the AXI Atlas 480 is only good for 3 LiPo cells. So, change that number (it's already pinned down).
Now we find that this motor will still work, if we can build a 15 ounce plane for it. Or we decide that we can just wind that sucker up to 12000 RPM and fly a heavier plane.
Something is still wrong, though: the lap time, flight time and line lengths are correct for a big plane, but not one this size. So we'll adjust them. Set the lap time to five seconds, the flight time to 300 seconds (5 minutes), and the circle radius to 55 feet.
Now look at the prop pitch. The slip of 1 is a guess, but we know that for CPLA we want it close to 1. But where are we going to find a 5.5 pitch prop? Let's change that to 6, and see what happens.
What happens, of course, is that a whole bunch of factors that we don't want to change light up as being in error! There is one thing we can do, however -- the 'speed derate' variable can be set to anythnig between 0 and 0.75. So let's unpin that, and see what results.
This fixes everything, and gives us a design with which we can go forward. Based on this we know that we need a three cell pack with 908mAh or better batteries, a speed controller that can handle 20 amps, and a plane that's going to build out to 13.8 ounces (the weight changed when we changed the prop pitch). If this looks good, we can go forward. If it doesn't, then we can think about different motors, or using the one we have differently.
Let me know how this goes for you -- suggestions for additions, deletions, plain ol' corrections and any other constructive criticism is welcome.