Kerry'd Away

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.

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.

Welcome back. This time we are going to start off by talking about the various classes of jets. Just like anything else in this hobby there are all sorts of flavors from which to choose. The main breakdown would be foamies and wood/composite. Both types come in all sizes and models; both scale and what are called ‘Sport jets’. Most are ARFs as this hobby has been veering in that direction for some time. There are still some kits around, but one must look for them. I, personally, enjoy building. Unfortunately I don’t have many options in the way of kits for jets.

Let’s start with the sizes. EDFs are broken down into what fan size they accept. Most are in millimeters, but there are a few inch sizes out there. By far the most plentiful is the 70mm and 90mm classes. The latest in micro technology is in the 25 to 30 mm class, while at the other end we are seeing fan sizes approaching 130mm in production. Cost is not linear. The cheapest option is the 64mm foamy. The prices are higher for both those that are smaller or larger than 64mm. The 64mm won’t have jet-like speed or performance, but it will be a jet.

The 70mm class is probably the most popular in EDF as the comparative cost is reasonable. The jets themselves rarely go to $300 as most are between $100 and $200, while the fans and motors can be anywhere from $20 on up. The 2 most popular fans are  the HET and Wemo minifan. The HET cost a bit less, but both perform within a few percent of each other. These 2 fans have been around a long time and are guaranteed performers, bulletproof, and can really take a beating. Motors for either can be had for under $70, or much less depending on the source. HET motors in the 28mm (70mm fan) class are almost the standard, especially for the price.

One real problem with the 70mm jets is the space available for equipment, especially landing gear. Most of my 70mm stuff is either hand launched or bungee launched. The wheels end up being too small for grass, and often the structure just can’t cope with the loads a rough field imposes in both takeoff and landing.

The 70mm size jets will use anywhere from 3s 3000s to 6s 4000s, depending on the size of the battery compartment. Typically one of these will use between 500 and 1500 watts of power with just over 1000 being a fairly good compromise. Anything more starts getting heavy.

The 90mm class is right behind the 70s in terms of volume. The larger size opens up possibilities for less than ideal flying fields, and they tend to give more presence in the air. Also, with EDFs being grossly inefficient compared to propeller driven birds, moving up to the 90mm size gains some of that efficiency back. It’s better to move a larger volume of air slower, than to move a small amount really fast. Going to the 100-120mm sizes gains that much more, but the investment goes up substantially.

Fan choices for the 90mm start with, again, HET and Wemo, RCLander, Stumax, Changesun, and a few other odd balls. There is a move towards the quieter fans that have more of the ‘whoosh’ sound over ‘screaming cat’. The tradeoff is sound versus efficiency. You get one, you lose the other. An exception is the Stumax fans, but they are in a price class of their own. With the Stus you get both.

To power an average 90mm model consider the weight range. I have seen between 6.5lbs up to 12lbs. Take 9lbs as the average, and you thus need somewhere around 2500 watts to get you going. Yes there are jets flying on much less (and much much more), we are just using an example. Anyway, you need to get 2500 watts out of your power system. A 6s gets you 22 volts, so the amp draw comes in around 110Amps. That is a bit on the high side. I try to stay with 100 amps max. The price on everything starts going up faster than I like past 100 amps and 8s. So on 7s you get 96amps, and 8s gets you 83. Remember, amps=heat and wasted power.  The higher the voltage, the lower the amp draw needed to get the same result. Every little bit of efficiency gain helps here.

Now we are getting aircraft of a large enough size to be able to mount retracts with decent size wheels. Good for grass, and with more structure, the gear stay with the airplane thru much more abuse. The 90’s are easier to see, easier to take out a distance to line up for a burner pass, etc. Landings still can be tricky, but some practice is all one needs there. The detail work on some kits out now is pretty phenomenal and they can be had for the $200-$400 range. Keep in mind this is for an ARF with a glass fuse that is painted and finished, and the wings may be built up or sheeted foam. New kits are coming out almost every day so there is no telling what may be available in the coming weeks or months.

Next in line are the 100-120mm classes. Here the selection thins out and what is out there is getting in the pricey range in terms of the jet and the stuff you have to load into it. For example a BVM Electra kit will cost $1400 alone, and that is before $750 landing gear, $1000 fan unit, radio etc. Fan choices include Dynamax (from the old glow days, same fan, but different motive power), Stumax, RCLander, Tamjets, and a few others.

Having attended a few jet meets I have seen the various sizes of EDFs and their capabilities. One would think that with a smaller field that is rather boxed in the 90mm class is at the upper end of the range that you would want to try to fly. Most EDFs are capable of at least 100mph, and some very easily can approach 160mph. If you have a total of a half mile for a field, you can see that one runs out of room REALLY quick.

Another thing to consider: Grass fields are not the place to try and fly an F-16 with scale gear. You must choose a subject that is tolerant of rough fields by its nature. Wide stance landing gear, lots of suspension travel, larger wheels, etc are a necessity. With well manicured grass, a slight wind in the right direction, and a healthy power system, a 70mm jet can get off the grass. In this case choose a jet carefully to give you every chance for success with each attempt. I have had a few that would go one day and not go the next.

The above is not intended to sway one away from jets, but rather give some ideas to ensure the best chance of success. These lessons have been learned the hard way at times. The reward for the research and work involved in setting up an EDF is seeing it whizzing by on a low fast burner pass, pulling vertical, and watching it go straight up while seemingly not losing any speed. It is well worth the effort.

A note on speed… The primary purpose of flying a jet is speed and smooth maneuvers. A nifty little benefit on some jets is flying them at the ragged edge of a stall in slow flight. I say some jets because a highly loaded sport jet is not the airframe to try and see how slow it will go. A light, airy foamy is perfect. There are some things you can do going slow that cannot be done while on the step at speed. Add thrust vectoring to the equation and you have a whole new dimension with which to play.

Next time we will talk about radios and some of the common mixing options needed to fly some jets.

~Jay Hot Pants