Electric Ducted Fans - November 2002

This month, Chris relates his experiences building, flying, crashing, and repairing the Fouga Magister kit by Christian Diffusion, a French jet manufacturer.

Hello EDF fans. It was a great summer and fun fly season, and now the winter building season is approaching. This month, I'll relate my experiences building, flying, crashing, and repairing the Fouga Magister kit by Christian Diffusion, a French jet manufacturer.

The Fouga Magister kit has been around for quite a few years; it was originally designed for the WeMoTec RK-720 fan. The current version of the kit recommends an 86mm diameter HW-620 fan as a replacement for the RK-720 fan. There is also a version with ducting to fit the Schuebeler 90mm fan. The real Fouga was a jet trainer designed in the 1950's that used two of the same small jet engines used in the Cessna T-37 Tweety Bird. The kit is designed around a single fan installation, so there are side inlets that join a single fan then split again aft of the fan into two small diameter exhaust tubes. At approximately 80% of fan swept area, the exhaust tube diameter is quite small, but still considerably oversize as far as scale goes. Other than that, the plane has an excellent scale look.

The kit is excellent quality with a very nicely molded fiberglass fuselage, along with foam core wings and V-tails sheeted in 1/32" balsa. Substantial carbon joiner rods allow the wings to be removed. They slot into brass tubes in the fuselage, which are located in precut ply formers that the builder glues in place after drilling holes at the spots marked on the fuselage. Fan access is through a large center fuselage hatch. The inlet ducts are glued permanently in place, but the outlet ducting is removable along with the fan, which is retained in the fuselage by a rubber band and wire hook arrangement. Wing area is 460 square inches by my measurement, and the plane flies on 14 cells (recommended) to 16 cells (what I used). Four servos are required, one on each V-tail and one on each elevator. The exhaust rushes past the bottom of each V-tail, which gives excellent rudder authority, so plan to use a transmitter with V-tail mixing facilities.

I received my kit from Christen Diffusion in late June, and it took about a month of not too intense building to assemble it. Since that time, I have learned of a US distributor who goes by the handle of "DuctedFann" on the E-Zone. He can be reached at ductedfann(at)aol.com or his website www.ductedfans.com. It is worth going through the US distributor, as he can order multiple kits and split the shipping cost, which is quite substantial over shipping from France. Equipment wise, I used the HW-620 fan and Hacker B50-13L motor available from Electric Jet Factory. The ESC I used was a Schulze future 58bo, but any brushless ESC capable of 45-50 amps will be sufficient depending on cell count chosen. I used two JR 341 micro servos on the V-tails and two of the new Hitec 5125 thin digital wing servos on ailerons. The battery was a 16-cell pack of CP-2400's at first, and then a 16-cell pack of Sanyo 3000HV NiMH cells.

So, how did it build? I didn't plan to do a full construction article, so unfortunately, I did not take any construction pictures, but the procedure is fairly straightforward. First up is the fuselage construction, which consists of drilling the holes for the joiner tubes on each side. The big main joiner has a witness marked molded into the fuselage. The plans show the location for the rear anti-rotation pin, and the formers have slots for the brass tubes, which further shows you where to drill the hole. After drilling the holes, install the two formers after securing the joiner tubes in place, and then epoxy the formers in place. The rear of the fuselage needs to have a small hatch cut free to allow the installation of two-ply ring formers with stub spars that secure the V-tails. After installing the spars as shown by witness marks in that area, glue the foam cored V-tails in place with some epoxy and micro balloon mixture as a root fillet. Once that has set, the removed piece of fuselage goes back on by sitting it on the ring formers and securing with epoxy.

The wings are pre-sheeted, and the balsa leading edges are attached and sanded to shape. The ailerons are cut free with skin hinges in place. A pocket is cut for the servo to sit in, and the tubes that accept the carbon joiners are installed. The wing root is square cut, and since it does not match the fuselage side, a little work is required there. I taped a piece of wax paper against the fuselage side to act as a release, and then applied a thick layer of epoxy and micro balloon mixture to the root. Slide the panel firmly against the fuselage and tape in place until the epoxy cures. Once cured, remove the wing, remove the wax paper, and sand the epoxy that oozed out flush with the surface of the wing. Be careful, as the wing skins are only 1/32" or so thick. Once sanded flush, your panel will perfectly match the fuselage contour.

The two tail servos go in the rear of the fuselage behind the split in the exhaust duct. I used a couple of large ball links to hook up the V-tails and 4-40 rods. The new Hitec digital wing servos are VERY thin! This is a thin wing and the Hitec servos are thinner. I secured them in place with ply blocks cut in thin circular profiles, and then epoxied them into the round servo pockets.

The wings were covered with .75-ounce glass cloth, and the tails are covered with a coat of epoxy and no glass, just to toughen up the surface. After sanding and priming, everything was painted with Crossfire brand automotive Acrylic Enamel. The equipment for the first flight was an HW-620 fan, Mega 22-30-2 motor, and 16 CP-2400 cells for an all up weight of approximately six pounds.

Test Flights

The Fouga is several inches wider than my El Bandito, which means it doesn't sit too well on the launch ramp. Instead of the ramp, I made a Jepe style dolly. I used an approximately 8" long piece of 5/32" music wire for the axle. I put two wheel collars in the middle and three on each end of the dolly for the plane's hook. Two wheel collars locate the wheels at the ends, and a third holds the looped ends of the V shaped bridle wire in place. The V shaped bridle is made from .047 music wire. It joins the bungee rubber with a carabineer style link.

First Flight

She came off the ground fine with a bit of a left bank. Within one circuit at high throttle, I had the ailerons trimmed and throttled back to half to work on the elevator trim. That took a while with digital trims, since I needed about 80% of the available up trim. Once trimmed, I pulled the nose up to 70 degrees at full throttle to check the vertical performance, which was very good. I rolled it over and pulled over the top for a high-speed pass. The Fouga has a decent turn of speed, but isn't even in the same class as the El Bandito. After a nice 4-point roll and loop, I throttled way back and cruised around for a while doing an occasional aileron roll and checking for low speed quirks. I none found. At three minutes, the timer went off, so I set up a low approach followed by a landing approach. The landing was very easy. I didn't pop the ailerons up as spoilers, and it was still a snap to put it down at a very reasonable speed where I wanted it. The Fouga has a little plywood sponson and 1" diameter wheel on the belly, so it bounced once and rolled out smoothly. I think touch-n-goes are eminently possible here. Total time on the clock was four minutes, with almost a minute left in the pack.

The second flight went even smoother since it was in trim. Rolls were quick at high speed and still doable at very low throttle settings, and point rolls were precise. I did a couple of loops, split-S type turnarounds, and a very pretty stall turn. Those v-tails really work well for the stall turns; I guess the dual exhaust must give some induced flow over the stabs, because the stall turn is MUCH easier than on the El. After the maneuver sequence, I throttled back to somewhere between a one-quarter and one-third throttle and flew low fairly slow figure-8's over the runway and beyond. The timer went off, and after flying around a bit more, I landed. Total time was 4:30 with only about 15 seconds left in the pack.

It's a great flying plane! It's not hard to fly at all, and the landings and speed management on approach is not a problem at all. Sixteen cells and 800 to 1000 watts are probably overkill. I think I will switch to a little softer motor and a 16-cell pack of 3000HV at 40 to 45 amps. The current setup is over 50 amps at full throttle, and I am flying much less than full throttle for most of the flight.

That was a week before NEAT, which is a major electric fun fly on the East coast. The next weekend, I packed up the Fouga Magister and El Bandito for the trip to NEAT. Friday at noon, I was scheduled to fly an EDF demo flight, so I charged both planes. When the time came, I picked up the Fouga and started towards the impound when the wind picked up considerably. Hmm…The El Bandito was faster, and this was serious wind, so the Fouga was sidelined, and the El Bandito made the demo flight.

Later that evening, after the wind died down, I brought the Fouga out for a flight. With John Dukowski doing the holding and releasing chores, we waited for a break in the action and launched. The launch was perfect, with the plane climbing away at a 20+ degree angle. At the far end of the field, I started it turning towards the mountain, leveling off as I did so, when some radio glitches suddenly hit. The motor shut off, and since the nose was still up, the plane eventually stalled and spun. I did seem to get the signal back for a short while before impact, and I stopped from the spin. I wasn't sure I could recover it, and if I had recovered towards the field and snapped into a spin again, there was water between the plane and me. I at least wanted the equipment back! I closed the throttle, and it went into the trees going straight down.

John, his son Don Belfort, and I trudged off towards the mountain to look for the plane. After a short search, John found the plane, or at least most of the plane. The plane had come down vertically with tree branches slowing the decent and breaking off the outer half of both wing panels and one v-tail. After clearing the branches, the fuselage hit the bank leading down to the water. The bank was covered in tall grass and weeds and was sloped at about 60 to 70 degrees. Since the bank was sloped and fairly soft, the fuselage was undamaged. The canopy was shattered by a branch impact, and we only recovered a few shards. One broken wing panel half was still loosely attached to the wreckage, and the other panel was 60+ feet up in a tree. Don kindly offered to climb the tree, which was much appreciated since my tree climbing days are over . Unfortunately, despite his best efforts, he could not get high enough to reach the panel. We walked out of the woods with the wreckage we could recover. I was a little upset. The plane was fixable, but not with that panel in the tree! That night, I got a spool of kite string and some of those nylon ropes with a stake on one end and a ring one the other. (Trout fisherman use these to secure their catch while they continue to fish.) The next morning, I made up a long rope of multiple strands of kite string and a few of the fisherman things. After tying a cheap pair of pliers to the string, I staked the other end to the ground. Using a swinging motion, I heaved the pliers towards the wing panel, which worked like a charm! The pliers looped the string over a nearby branch, and after pulling back and forth for a minute or two, the wing panel shook loose and fluttered to the ground. I was a happy man! The pliers were stuck in the tree, but it was a good trade.

The Phoenix Project

After returning home and letting the parts age for a week or two, I began putting things back together. Within two hours or so, the major pieces were all stuck back together. The two wing panels were glued together with 5-minute epoxy. The V-tail was similarly stuck back on once the mating surfaces of the fuselage and fillet were roughed up. Several large dings in the wing leading edges were filled.

Both control horns on the elevators were loose, so they were removed and the underlying structure consolidated with epoxy. Several breaks, splits, and outright missing sections of the elevators were repaired and/or built up with epoxy and microballoons.

Once these flaws were fixed, the sanding began. I wanted to remove as much of the paint as possible in order to minimize weight buildup. I spent at least two weeks wet sanding the entire plane with 220 grit sandpaper. I used two and a half entire packs of sandpaper; automotive paint is tough stuff!

Once the paint was sanded off, I applied a glass reinforcement patch over each break in the skin on the two wing panels.

The patches were sanded, and then automotive spot filler was applied and wet sanded.

At this point, I figured a test flight was in order before the final color was reapplied.

The chosen day was pleasant, but the winds were a little gusty and coming at an angle to the runway. One of my club-mates did the release honors, and after a bit of a wing dip, the plane was away. It took some aileron trim and a good bit of elevator trim to get the plane flying straight, but there were no serious problems. I did notice the Fouga tended to yaw off to one side in vertical pulls, but with the gusty crosswind, I chose to leave the rudder trimming for later flights. This time, the motor was the Hacker B50-13L, and the battery I used was Sanyo 3000HV 16-cell pack. With a full throttle, current draw of 45 to 46 amps, the potential flight time is much better than the previous setup. Actual power seemed about the same, with vertical pulls quite tall and 60 to 70 degree climbs sustainable for a long time.

After flying around for more than four minutes at mostly high throttle settings, I setup for a landing approach, which I preceded to blow big time. With the crosswind, I came in faster than normal and managed to flare late. The plane hit the ground moderately hard and bounced back into the air about five feet up in a 90+ degree bank! I applied full left aileron to correct. With the wings almost level, the Fouga suddenly started a snap roll to the left in the direction of recovery. I neutralized the ailerons, which stopped the snap. I then applied a bit of power and eased in some right aileron. The plane lost the last couple of feet of altitude as the wings rolled level, and it plopped onto the ground in about a 10-degree yaw. The ply sub-fin at the rear of the fuselage popped off, but other than that, no damage was incurred. The sub-fin was easily CA'd back in place, and it was time to start the final finish work.

Gray sandable automotive primer was used for the first coat, and then more sanding to feather in the repairs, as well as the remaining spots of old paint.

Next came more grey primer and more sanding.

A final coat of white primer was used as a base for the color coats.

Can you see the repair and reinforcement? I know it is there, but only by looking with my eyes right down along the surface of the wing can I see the repair. After waiting a couple weeks, I received a new canopy and plywood base from the manufacturer, which were the only new parts needed for the repair, as the old canopy was in tiny bits.

More Progress Pictures

These pictures were taken after the completed repair and repaint and the installation of the new canopy.

A lot of elbow grease went into the repair, mostly in the effort to wet sand off all the paint. In the end, it was certainly worth it. The plane actually looks better the second time around, and doesn't seem to have suffered any in it flying characteristics. That is all for now, so till next time, fly low to avoid the radar.