An electric ducted fan (EDF) is a propulsion arrangement whereby a fan; which is a type of propeller; is mounted within a cylindrical shroud or duct. The duct reduces losses in thrust from the tip vortices of the fan, and varying the cross-section of the duct allows the designer to advantageously affect the velocity and pressure of the airflow according to Bernoulli’s Principle:
Then using Newton’s second law of motion; we can define a force F to be the change in momentum of an object with a change in time. Momentum is the object’s mass m times the velocity V. So, between two times t1 and t2, the force is given by:
F = ((m * V)2 – (m * V)1) / (t2 – t1)
So……are you asleep, yet? While all the stuff, above is true…and interesting to some; most of us are flyers…and flyers just want to have fun. Today, we are going to talk about our EDFs and how to have more fun, more often, with fewer problems.
A few years ago EDFs were big, expensive, and most of the time did not work very well. Today, there are lots of really great EDFs on the market like Hobby Lobby’s EPO jet series including the MIG-15, F-86, and F-9F Panther….. for a little more than 100 bucks. These jets look great, fly great and are an incredible bargain!
Because they are relatively inexpensive, sometimes we tend to expect a little too much from them, and also do not pay as much attention to proper care and maintenance as we should. Here are some tips for EDF’s that will ensure a long and happy life for these great little jets.
1. “Run the Fan in” before Flight – after assembly and adjustment, turn on your transmitter and connect the battery in your jet. Hold on to the jet firmly, and cycle the throttle, up and down, for 15 to 30 seconds. Many EDFs have very close tolerances between the fan blades and fan housing. The purpose of this is to allow the fan to “seat” into the fan housing. If the fan rubs on the housing, we want to make sure that all blades are “seated’ and are free and clear to turn in the fan housing, before we fly. Any rubbing sounds should quickly disappear with this step. Disconnect the battery, and visually inspect the fan blades for flashing or melted residue on the tips of the fan blades. Remove any flashing or residue carefully.
2. Balance that Fan! – Most of us would not think about flying an airplane without balancing the prop first, but many of us totally skip this step with an EDF. Kinda funny as many prop planes are turning around 10,000 RPM……and some of our EDFs are spinning the fan at speeds upwards of 40,000 RPM! So get yourself a prop balancer. I have used the Top Flite Power Point Precision Magnetic Balancer for many years (part # TOPQ5700 available from Hobby Lobby for only $19.99). When I fly my EDFs people often ask what type of fan I am flying because they are so quiet and vibration free. Balancing makes a ton of difference, even in very inexpensive EDFs.
3. Properly re-install the Fan – Even inexpensive EDF motors are machined to very close tolerances. Many of the motor shafts are chromed and hardened, but do not grip collet type adapters very well. I use a small piece of emery cloth to slightly roughen the shaft. When finished, wipe the motor shaft carefully with alcohol to remove any residue. Now reinstall the fan using a couple of drops of Loctite. Just don’t get any Loctite in the motor.
4. Use the correct Battery! – Every EDF comes with a battery or a recommended battery from the manufacturer. Hobby Lobby’s EPO jet series (like the F-86) comes with a 3 cell 1600MAH lipo. Lipo battery prices have dropped dramatically over the past couple of years, and it is not unusual for many flyers to have a bucketful of batteries available to them. Please remember that all electronic devices (brushless motors, ESC’s, etc) are not 100% efficient. The result is that some portion of the battery’s energy is “wasted” and generates excess heat, both in the motor and ESC. If you have a bucketful of batteries and continually “refuel” as soon as you land…..you will eventually have a meltdown or failure in the EDF system. Let your EDF cool down between flights for 10 minutes or so! Many manufacturers have seen more failures from this than any other issue.
Cheap lipos can be found everywhere today. If you substitute, please make sure you use the correct size that meets the minimum “c” rating. Many flyers also like to use higher capacity packs (I have seen 3S 2200’s flying in the Hobby Lobby F-86), but the same issues apply here. Typically, more motor run time equals greater amounts of waste heat. Heat kills! Please let your EDF cool a bit between flights, and you will enjoy your EDF for many years to come.