Electric Stunt Models

Mike Palko electric stunters ('Model Aviation', March 2005)

Silencer

Brian Scott electric stunter ('Aviation Modeller International',
February 2005)

Nobler ARF

Jan Odeyn electric stunters

Watty

Watts-more 2 - 'Ghost of Peterborough'

Dave Day electric stunter

Here's what Mike Palko (see above) posted on the Stuka Stunt Forum regarding his approach to the design of an electric stunter:

I have been asked to make a post on how to convert a glow powered stunter to electric, so here is a short write-up giving you a starting point. This will also work for a scratch built electric model and in turn will be easier than doing a conversion.

Converting an exsisting airplane is not the ideal situation because more than likely there are areas that are over built, oil soaked, or are going to need to be modified to accept an electric power system. Also, batteries may be hard to mount depending on balance point and room constraints.

To determine whether or not the airplane can be sucessfully converted follow these steps.

1) Determine the target weight ready to fly. (My example will be a .40 size stunter.) The target weight I chose is 46oz. I will set this as my upper limit.

2) Now you need to calculate the power needed to fly the stunter at a satisfactory performance level. We want a minimum of 160-170 watts/lb input. We have a 46oz stunter or 2.875lbs so we are looking at 2.875lbs x 160 watts/lb = 460 watts input power minimum.

3) Pick the current draw you want the power system to run at and then divide the total number of watts needed to fly the stunter by this number. I will use 35amps, so 460watts / 35amps = 13.14. This number is the minimum number of volts you will need to get the power input we are looking for.

4) So now we know to fly a 46oz model we will need to draw 35 amps from a battery source of at least 13.14volts. Since we are using Li-Poly batteries we have to choose packs with voltages divisable by 3.7. Each Li-Poly cell is 3.7 volts so 13.14 volts / 3.7volts = 3.6 cells. Remember we are using minimums to calculate our setup so we don't want to drop down in cell count, so we will round up to a 4 cell pack. Most packs on the market with discharge rates high enough for our use are 2P packs or 2 cells in parallel. Putting the two together gives us a 4S2P pack. (the number of cells parallel depends on the C rating needed)

5) Now that we know what we are looking for we can decide on what motor and battery combination to use. This is where things become more difficult. The selection may seem endless when it comes to motors, so how do you know which one to pick? If you read up on electric power systems you will find the most popular or best performing motor brands. (Ex. AXI, Plettenberg, Aveox, Hacker, Jeti, Mega) to name a few. I have used mainly AXI and Plettenberg outrunner style motors from the beginning. There are also a number of motors that are the conventional "inrunner" style that will work well, but I found good results with outrunners so I have stuck with them for the time being.

Batteries are the same situation. There are many good brands out there and if you follow electric power at all you will pick up on them. I prefer Thunder Power because they seem to have the highest energy density per weight, and also have proven to be extremely reliable.

6) After you decide which brand or brands you want to look into you need to determine what size motor will handle the voltage you will be using. We are using 14.8volts or 4S Li-Poly pack. Motors are normally rated at a min and max voltage, max continuous amperage, and max efficancy @ a given current level, or at least that is what we will be looking at mainly. We need to be sure we fall in between them.

As far as the battery pack goes you need to be sure it is a 4S2P (for this setup) and can handle the 35amps continuously that we will be drawing. Do not pick a pack that says it can handle 35amps continuous and say 50amps for short bursts. We want some room for error and a little safety cushion so look for a pack that can handle at least 40amps continuously. This will let you pull more current later if needed and also keep the pack from working at it's limit extending it's life and keeping it cool during use.

7) So you have picked the motor brand and size that will handle our input voltage of 14.8volts. For a .40 size stunter we will be using a 10-12" prop with a pitch of 4-6" more than likely. I will pick an 11" dia and a 4" pitch. (I prefer lower pitch props, but you can use what you prefer) Now you can either look up the manufacturers specs for the motor (they usually give you a few examples of performance levels for a given prop at a given voltage) or you can buy a motor calcualtor program to help you along (motocalc is an example). Lets say the first motor you choose turns an 11x4 @ 14,000rpm and draws 47amps. The first problem is the rpm. It is to high for our application and the current draw reflects this. To fix this we will need to pick another motor (same size and manufacurer) with a higher number of winds. The higher the number of winds the lower the rpm for a given voltage. This will also lower the current draw if you keep the input voltage constant. You would do just the opposite if the motor turned to few rpm. If you go through each motor in that size range (from the same manufacturer) and you still can't find one suitable for your application you will need to vary the prop dia. and pitch to adjust the pitch speed and amp draw until it is usable. If you can't get it to a satisfactory level you may need to change motor manufacturers. Even though two different motors have the same size, weight, number of winds etc.... they may still perform differently. The reason being different manufacturers hold different tolerences, use different materials, have different designs and so on...

8) Ok, we now know our target weight, power system requirements, battery voltage needed and amp draw that is required. We are pretty sure the motor we picked is what we need or is close enough that a few prop adjustments will get us there and the battery pack will deliver the power. That leaves us with an ESC and timer.

ESC's for the most part all perform very close to one another. A few examples are Castle Creations and Jeti. The biggest difference is the programming options. All you really need to be concerned with is that is capable of handling the current levels we will be pushing, has a governor and has a BEC built in. I recommend like the battery going a little higher on the rating than needed. For a 35 amp setup go with a 40 or 45 amp controller. Again we don't want to push the system to the limit and cause heat problems. Just like an IC setup to much heat is bad for electric power systems as well. The governor will be used as a break to prevent whip up. The BEC is short for battery eliminator circuit. This will be set at the appropriate voltage incase of any type of timer failure the ESC will shut the motor off to prevent damage to the battery pack or airplane. I won't go into detail because each pack 2S 3S 4S 5S etc... needs a different cutoff voltage and each ESC is different when it comes to programming.

Timers are limited right now to the Z-Tron and the JMP. I only have experiance with the Z-Tron timer and can say it works very well although the JMP has gotten very good reviews as well.

9) Now we know the motor, battery pack, ESC, timer and prop to be used. Now you can weigh the entire setup and add a little extra for the connectors, solder, shrink wrap, prop, prop adapter, timer and other misc mounting hardware. When your total is tallied subtract it from your ready to fly weight. I will call this setup weight 22oz. 46oz - 22oz = 24oz bare airframe weight.

I am sure you can guess what to do next. Pull that engine, prop, tank, fuel lines, filter, etc... out of the airplane and weight it. Above 24oz and you may be a little on the heavy side. 24oz or less and you are on your way to a successful conversion. If you are close to the weight you may want to pull the engine beams/crutch assembly and tank mount/floor out to get you to your target weight or maybe even below it. Don't forget to pull out any nose or tail weight. You will be able to shift the battery to compensate for the added balast previously needed.

If you are using this step by step for a scratch built project you can build the model to suit the weight needed making it easier on yourself.

Like I said earlier this is just a quick rundown on how to get started. I didn't really go into any detail on any one subject. Electric power systems have so many variables one could write page after page about them. There is still a lot of theory behind it all (for C/L use) and there are many things I still don't know myself. Also, the figures I used may not hold true. They are just examples to show the math involved.

I think this will give you a good starting point on designing your own power system. I am sure I missed some things, and may have even been a little off on somethings I mentioned. I apologise ahead of time if I did.

Mike