Electric Ducted Fans have opened many doors in the RC hobby, yet they remain somewhat intimidating. Here's how to use them.
For a long time, I stayed away from EDFs. Told horror stories of fast, expensive RC jets crashing on their maiden flights, the prospect of flying an electric ducted fan airplane was not one I fancied. However, when flown right, it turns out that jets aren't really that scary. All you need is a bit of knowledge of the essentials. As long as you have some experience of flying 4-channel airplanes, you should be good to go. This article covers what you need to know about building, tuning and flying EDF aircraft. First, though, let's start with the basics.
What is a ducted fan?
A ducted fan is a conventional arrangement of a fan (or multi-bladed propellor) and brushless motor within a shrouded duct. By shrouding the propellor, the ducted fan reduces losses in thrust from the propeller tips. In addition to this, they can run at higher speeds (or KVs) allowing the EDF unit to produce the same amount of thrust as a larger prop. For this reason, EDF's are often used to simulate full-scale jet engines in RC models.
Tip 1: "Make sure the intake path is clear and free of large sharp turns. Make sure the exhaust tube and nozzle are internally SMOOTH as well as clear and as straight as possible." - Ben Harbor
When it comes to equipping a plane with an EDF, it's quite essential to ensure that as much air as possible is fed to the motor. It's also important to make sure that you have a clear 'exit' for the air to be blown out of. This means having a short exhaust tube with no obstructions to foul the airflow. Sometimes you'll see scale model jets that have extra 'cheat' holes cut into their fuselages. This is mostly because, with the scale intakes, not enough air would be sucked into the EDF without them. Check out the first of the articles on Ben's gigantic XB-70 Valkyrie build here.
Tip 2: Make sure that you are using the right BEC
Most Electronic Speed Controllers (ESCs) these days include a battery eliminator circuit (BEC). However, when you get to the high powered end of the RC hobby, you'll need to make sure that you're using one that's up for the job. If you're thinking of building your own EDF powered jet plane, make sure you get yourself a UBEC or SBEC to handle the high current required to turn these fans at such high RPMs. If you want to know more about BECs and how they work, read this article.
Tip 3: Balance your blades
Like with any prop, it is important to ensure that you balance your EDF. Vibrations caused by an unbalanced fan can lead to all sorts of problems, so you'll need to tune out these imperfections before you get airborne. Here's a great, straight-forward video tutorial from one of our community members - Nerdnic.
Tip 3: "An EDF does not give instant thrust like a prop" - Chris Raynolds
When it comes to flying techniques, you'll need to keep in mind that piloting a jet is completely different to flying a prop plane in many regards. Mainly, when it comes to giving it the beans, you'll realize that there is a noticeable lag between punching the throttle and seeing any acceleration. Sam Lane likens it to the feel of a powerful car - "Treat it like a car with a big turbo, loads of power available, but only when running at very high RPM." Keep this in mind when it comes to flying at low airspeeds - you'll find it difficult to recover from a stall at low altitude as you can't always rely on just blasting out of it. This takes us nicely onto our next tip all about landing.
Tip 4: "It’s best to carry in the landing with power still applied, as opposed to fully cutting and gliding in. That split second can be enough between a stall and recovery." - Sam Lane
When you see a full-scale jet coming in for a landing, be it a passenger liner or an F-16, you'll never the pilot simply cut the throttle and glide in. Often, the aircraft will deploy landing flaps, assume a high, nose-up angle of attack whilst keeping the power on. Due to the previously mentioned 'thrust lag', this is a method that allows the aircraft to get out of a bad situation in an instant. In RC, it's just the same. Sometimes you won't have flaps at your disposal, so just keep that nose up, flair it in and cut throttle right at the point that you touch down.
If you have any tips of your own, post them in the comments!
Article by James Whomsley
Editor of FliteTest.com