I Intro to EDF part 3
As promised, this time we will discuss radio equipment, the common mixing requirements needed, and getting reliable power to that radio system.
Starting with the simplest jets, you will only need 3 channels for control. Most small EDFs don’t have, and don’t require, a rudder. So you have real jet ‘bank and yank’ controls. Aileron, elevator, and throttle are all you need. Some jets fly with tail surfaces only, which typically are all-moving stabilizers. These provide all the control you need when mixed using the elevon mix on just about every radio out there.
As one moves up the scale the next control might be the rudder, with flaps and landing gear added. If a jet has flaps then ailerons will be incorporated as well. Some use flaperons, some are separate surfaces. Remember that as you are selecting your jet. The channels add up quick; you may be limited by your current radio. You can fly a simple 3 channel EDF with a basic radio, but even a mildly equipped ViperJet will need at least a 7 channel computer radio. The more channels you have, the less y-harnesses you’ll need. Some jets might use 2 elevator servos, and one might need to rotate the opposite way. Having an extra channel available goes a long way here. If you want the nosewheel steering to turn off when the gear goes up, add another channel and mix them in the Tx. This can go on and on. If you don’t use retracts (fixed gear or no gear) that channel can go towards lights, a bomb drop, etc, but if a jet is small enough to hand launch weight is probably pretty critical.
Be prepared to spend a bit of money on servos. EDF’s typically us micro or mini servos, a lot of them when you get into four surface wings. Standard sizes just won’t fit and are too heavy for the application. With jets the surface deflections needed are a lot smaller than one would think. The flying surfaces are more highly loaded, and thus that much more effective. Don’t go overboard here; work your way up, use dual rates and especially expo. I know, I know, “I have never needed that stuff before.” I am here to tell you, use expo. It’s impossible to fly a high speed jet without it. The servos need to be small, but strong and accurate. You don’t want a full-flying stab to recenter to a different spot each time it moves. Hitec’s micros are a goldmine without costing as much. I use the plain jane HS-55s on everything from tailerons on smaller jets, to the flaps and rudder on larger jets. Even Hobbyking’s HXT-900s work on my MiG’s ailerons and elevators. You have to look at the surface size and the anticipated speed it’s going to see. Don’t skimp, but put enough grunt behind them. There is absolutely no reason to buy $60 digitals unless you are equipping a hot rod 90mm sport jet. Even then it’s just a matter of common sense. If the ailerons are an inch wide and 4 inches long, there isn’t going to be much load on the servos when they only move 1/8 inch each way.
Now let’s talk about powering these servos. This applies to all electric powered airplanes as well. Today’s 2.4 Rxs (ANY brand) need a steady and reliable source of power. This does not mean the ESC. You are asking the ESC to take whatever voltage system you are using and turn it on and off between 8000 and 24000 times per second. Most ESCs have a pwm rate selectable between 8, 16, and 24 khz. And you want that ESC to do this switching while pulling a large current load, say 90 Amps. Yes you’d get hot too. Then if you ask the same ESC to provide steady, reliable power to your RX and servos, your new EDF may be on borrowed time. The manufacturer says that it can do it. You see it work on the bench. You see it work a few times in the air. Then while flying one day, you watch your beautiful jet emit a pretty stream of white smoke out the tailpipe, and all of a sudden, you have no control over the jet. What just happened? Perhaps the motor had a bad winding and in the process of shorting out, the ESC, while trying to switch a shorted winding, decided that it had enough and self-destructed. Well, that one component was supplying power to your radio. It isn’t doing that anymore so your radio won’t be listening. Or, the ESC got a bit on the warm side and hit thermal shutdown. When that happens all power going thru the ESC stops. So does your radio.
How do we prevent that? A little bit of insurance that is cheap and easy to do. While soldering up your ESC with its connectors, on the battery side, you add a BEC. This is basically a SWITCHING voltage regulator, not a LINEAR regulator. The Castle 10 amp unit is $20 and will handle anything you can throw at it. Up to 6s systems this little gem does wonders; it weighs a few grams, can be stuffed anywhere, is fully programmable, and only needs a breath of air every once in a while. Above 6s systems I still use the same unit, except that a small 1000 mah 2s lipo provides the power. It is a completely independent power source so no matter what happens to the flight packs, the ESC, the motor, or the fan your radio will always have the set amount of power. The only downside is that you have to remember to charge it. Fortunately, the switching regulator is pretty efficient and the lipo barely gets a workout after a full day of flying. Add to that the fact that the servos don’t draw a whole lot of power in a flight since they don’t have a large load on them, remember?
That all being said, I do use the BEC built into the ESC on 3s powered jets. This is mainly for weight and space considerations, although I have set up some foamy jets with BECs just because. The reason one can get away with this is that the voltage differential isn’t great enough to cause undo stress on the ESC, provided it has proper cooling. Jets are nice in that they have access to a concentrated, high speed flow of air perfect for cooling an ESC. Above 3s the voltage differential gets to be a bit on the high side, and one is wise to add insurance. Remember, you can run off the flight pack up to 6s with the CC BEC. Over that, the jet will be big enough to handle a separate lipo for the radio. The really good news is that electric retracts are the cat’s meow for jets, and by using a 10 Amp BEC, you are all set to use the electric gear.
Let’s talk for a minute about using your left thumb. The throttle on most radios has either 1024 or 2048 resolution, which translates into either 1,024 or 2,048 discreet steps of proportional control. That is an extremely fine resolution in which to control something. Don’t use the throttle as an on/off switch. Throttle control in EDFs is critical to component life, flight time and the sanity of your fellow fliers. Mixed throttle flying is the key. Your ESC gets the hottest at ½ throttle, so don’t stay there too long. Jets provide the perfect platform in which to fly a varied routine.
In the next go round we’ll discuss flying techniques, some dos and don’ts, flight patterns, takeoffs and landings.