In recent years, there have been significant technoloy advances in the areas of high efficiency electric motors and the batteries that power them. This has “sparked” a quite extraordinary increase in the reliability and affordability of electric powered RC model aircraft. These days, there are not many areas of the sport where “electrics” can’t compete on equal terms with with internal combustion models.
How to Balance an EDF Unit
How to Balance and EDF unit by 4 Stripes on RCGroups.com
Dynamic balancing an EDF fan/motor is the hardest thing about EDF models but it is critical if you want the fan to sound its best and not break itself to pieces. Running 30-40 thousand RPM can create some serious forces if not balanced.
Number one, you have to try and get the adapter on the motor shaft as true as possible. If it is out, the fan and spinner will wobble with large imbalance as a result. You may have to try each possible set screw position on the motor shaft flat spot. The FLAT SPOT is necessary on the motor shaft to keep the adapter from twisting or flying off. File one flat spot for the adapter set screw (you don’t need more than one). Make a mark with a felt pen on the other end of the motor shaft in line with the flat spot. This mark is needed to check orientation of the fan/adapter in the following steps. Some adapters have only 1 set screw but others like the Changesun have 4 (allowing 4 possible positions). Just turn the motor shaft and watch the end of the adapter to see if it rotates in the center. A very small amount of error can be offset with extra weight but the more true the adapter runs, the less weight you will need. Some adapters don’t fit tight enough on the motor shaft which will cause shifting of the fan and balancing impossible. If loose, I add a very small amount of epoxy inside the adapter just enough to go around the tip of the motor shaft when fully inserted. Wax the motor shaft to help make removal possible.
Try and statically balance the fan as good as possible. Magnetic static balancers can be flawed but the shaft with cones can still be used to balance the fan if placed on top of two flat surfaces (like coffee mugs). Add epoxy or drops of crazy glue (with kicker) inside the hub where needed (on the Changesun fan add glue to the backside of the hub where the blades form a lip). Then mount the fan and mark one blade (so you know where the various positions are relative to the motor shaft when you use the “clock method”). I use a dab of whiteout. You can hold the adapter from turning by having a hole in the shroud that lines up with one of its set screws. Stick an allen key in and the adapter won’t turn while you loosen and tighten the fan screw or nut.
DO NOT RUN THE FAN IN YOUR HAND! Run it in an enclosed mount, like the model or a custom made mount. I made a mount from scrap foam packaging that has a removable top for access. It gets mounted to a table and safety goggles and ear protection is necessary. If the fan or shroud lets go, it will happen in an instant unleashing a +2 hp meat grinder! Scary and very dangerous. I cannot over state this BE CAREFUL. Never assume the fan is sound, as I have seen perfectly smooth fans let go.
Now run the fan SLOWLY and feel for vibration. You don’t need much power to feel an imbalance. If run at full throttle and you have an imbalance, the shroud can be broken which is disastrous. Loosen the fan (not the adapter) enough to rotate it 180 degrees on the adapter (6 oclock) and run again. Check for vibration and see if better or worse than 12 oclock position. Apply same technique for 3 and then 9 oclock positions. By now you will see a better running quarter of the possible fan positions. Now fine tune it to the best running position (at all 12 “hours”). You will be amazed at how much better the fan will run. A 10 degree turn can make all the difference, but find that best position. Once found remove the white out spot and put a new one that lines up with the motor flat spot.
This next part is for fans that have a large separate spinner like the wemo midi fan and typical fans supplied with Flyfly and Hobby topgun kits.
Now put the spinner on and there may be a chance it too is out of balance (most of my aluminum spinners were and some of my plastic ones as well). This complicates things a little but just apply the clock method to the spinner until you find the smoothest running position. If it still vibrates, now you must add some weight to fine tune the complete fan. First add a small square of duct tape to the spinner at the 12 oclock position then apply the clock method until you find where the weight must be added. The tape will stick better if you clean the spinner with alcohol first. You must be careful not to give too much throttle as the tape will fly off into the fan. Take the spinner off and add epoxy to the inside of the hub. Re-attach the spinner in the same smooth running position found at the beginning. Run it, and if its smooth at all throttle settings, you are done. If it still isn’t perfect apply another piece to the same location and see if the fan begins to run smoother. Tape is just a temporary step to see where the weight must be added. Once you know, add some more glue inside the spinner at this location. Run and if still out, apply tape to see if more weight is needed. If so add more epoxy.
I think you get the idea…
With the changes on fan you use the same clock method with tape added on its small spinner but the weight is added at the back of the fan hub on the lip just behind the blades. I find CA (with kicker) works really well for this. Run, see if more tape added in same area helps, and add more CA as necessary. The CA can be scraped off fairly easily with a knife should you put too much on.
It is tedious but the results are worthwhile. Out runners are easiest as you know the motor shaft position, and it is easier to loosen and tighten the fan nut. Inrunners require you to use an allen key in the adapter set screw to hold the shaft while you change the fans position. I also mark the motor shaft on the rear shaft end to keep track of the fans clock position. This may be tricky if the lander motor is covered with a fairing.
At some point you will get the fan running smooth, but the more you work at it the better the results and the quieter it will run.
Best of luck!
Useful Links
| Link | Description |
|---|---|
| AEFA | The Australian Electric Flight Assn (AEFA) is an Australian organisation fostering and promoting aeromodelling using electric power. |
| Drone Flying Guide | A basic guide to buying a drone. Contains brief descriptions of the types of Drones available and contains links to online shopping sites. |
| EFO | Electric Flight Only - An established US club site with links to many useful Electric Flight resources, |
| Electric Flight eBook | An introduction and some basics of Electric Flight by Ken Meyers |
| How to Balance a Propellor | A video tutorial on prop balancing. The presenter uses a Dubro balancer but the principles can be applied to any balancer. |
| Ivan's Plans | Electric Powered Scale Models |
| Peter Rake | Electric powered models from bygone era's. |
| Prop Calc | eCalc tool for calculating prop size/pitch that contains a good list of motors, includy many HobbyKing models |
| RC Advisor | A forum for UAV and Model Airplane Design Building and Flying |
| Web Calc | WebOcalc is a useful tool for determining appropriate motor/esc/prop sizes for electric models. |