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Dymond Modelsports/Multiplex Twin Jet

I was lucky enough to buy one of the early production versions of the original Multiplex TwinJet and after flying it for nearly two years and 280+ flights, it succumbed to a speed controller losing it's "magic smoke". Trying to track down a replacement was an exercise in frustration until I came across a Dymond Modelsports ad in one of the model aviation magazines.

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Specifications

Wingspan:

35.8 inches (910mm)

Wing Area:

395 sq. inches (25.5 dmę)

Weight:

41.4 ounces (8 cell Sanyo RC2000 pack)

Wing Loading:

15.1 ounces/sq. ft.

Fuselage Length:

31.6" (802mm)

Servos:

2 Dymond Precision Micro Servos

Radio System:

Airtronics RD6000 Sport Radio and 92777 Dual Conversion FM Reciever

Speed Controller:

Jeti JES500 50 Amp Electronic Speed Controller with BEC

Propellers:

2 APC 5.5" x 4.5" Speed 400 Electric Props

Prop Adapters:

MPI MAXX Speed 400 Prop Adapters

Batteries:

8 cell Sanyo RC2000 mah NiCad Pack or 10 cell Sanyo CP1700 NiCad pack

Introduction

I was lucky enough to buy one of the early production versions of the original Multiplex TwinJet and after flying it for nearly two years and 280+ flights, it succumbed to a speed controller losing it's "magic smoke". Trying to track down a replacement was an exercise in frustration until I came across a Dymond Modelsports ad in one of the model aviation magazines. Not long afterwards, I had one of the new TwinJet "White Eagle" Racer's Edition models spread out on the living room floor being looked over and admired prior to starting assembly.

Kit Contents

The White Eagle Racer's Edition version of the TwinJet is molded in white ELAPOR foam instead of the gray colored version of the original TwinJet. ELAPOR foam is similar in texture and density to the ARCEL foam that FMA's Razor flying wings are molded from and is the equivalent of a molded EPP foam with the "bounce" characteristics that make for a durable foam model.

The TwinJet is packaged in a strong cardboard box with cardboard dividers around the foam parts which brace the box top and bottom and do an excellent job of protecting the contents. Upon opening the box, you'll find 6 ELAPOR foam parts consisting of the wing and rear fuselage, the forward fuselage or nose section, 2 parts making up a portion of the turtledeck and canopy and the two vertical fins. There's also a vacuformed liner for the fuselage cockpit area, a 10mmx10mmx800mm hard balsa stick used as the spine for the fuselage and an accessory package supplying the needed odds and ends such as control horns, wire pushrods and the like. The supplied power system consists of two Speed 480 7.2 volt motors, capacitors, wiring and two Gunther plastic props. The instruction booklet is excellent and well written with an easy to follow assembly sequence along with useful drawings to guide you through the assembly process. The instruction booklet is multilingual so the appropriate written portion anyone would use is actually about 5 to 6 pages long plus 4 pages of drawings.

The manual is quite straightforward and with the relatively low parts count, the TwinJet goes together quickly. In the manual, there is a reference to "Medium Zacki" and "Zacki Activator". This translate to medium CA glue and kicker for those of us on the west side of the Atlantic. Instead of a blow by blow account of the assembly, I'll touch on points to watch for and the minor changes or revisions I took in assembling the TwinJet.

Assembly

The first step in the manual is attaching the nose section to the wing using medium CA and kicker as directed. There is a nice, positive fit of the nose section to the wing which assures a straight alignment when attached. While the CA is curing, use a bit of tape to hold things tightly together and then move on to trimming the vacuformed liner to fit into the cockpit cavity of the assembled airframe.

Sometime back, I bought a pair of Lexan trimming scissors at a hobby shop that catered heavily to R/C cars. Many of the aftermarket car bodies are made of Lexan and these scissors have a short cutting edge with a bit of a curve to it which makes cutting vacuformed parts quite easy. Once trimmed, trial fit the liner in the airframe and make sure that the canopy and the turtledeck will fit flush with the liner in place. Once you're satisfied with the fit, you'll need to glue the liner into the airframe. The manual calls for the use of contact cement but I chose to use polyurethane glue instead. I sanded the outside of the liner with 220 grit wet&dry and then applied a thin layer of glue on the liner and in the cockpit cavity. Be sure to have a couple of wet paper towels on hand to wipe off excess glue as you push the liner in place. Polyurethane glue will foam and expand slightly as it cures so you'll need to keep an eye on exposed glue along the edge of the liner and wipe it off as it appears. To keep the liner in solid contact with the cockpit cavity, I placed the fuselage on a level surface and taped the top of the liner to the fuselage in spots around the perimeter and then filled the liner with lead shot. Once cured, I just poured the lead shot back into its container and I'll use it later when I need to balance my sailplanes.

Instead of using the supplied decals to finish the model, I chose to paint the model using Testors brand Spray Enamel. This paint comes in small 3 ounce cans and our local Walmart carries a variety of colors at a very good price. I have also found it is safe to use with foam and it adheres well to the type foam used in the TwinJet. I did all of my painting immediately after the airframe (minus the vertical fins) was assembled. As with any painting, keep the coats light and evenly sprayed and be weight concious! Pick a paint scheme that easily defines top from bottom as the shape of the TwinJet can cause disorientation and the speed it flies at may give you little time for corrective action before the model meets terra firma.

Preparing the wiring
The next step is preparing the wiring for the motors. The pre-fabricated motor wiring that comes with the TwinJet uses the unique Multiplex connector. Since I use Sermos brand connectors on all but my smallest models, I removed the Multiplex connector and prepped the wiring to solder it directly to the speed controller.

There is a recessed bay in the belly at the rear of the fuselage where both the motor wiring and the servo leads are pulled through from the wings and then carried forward into the cockpit area. You'll need to drill a hole through the foam from this bay to the cockpit area. I used a 3/8" diameter brass tube with one end sharpened to make a clean cut in the foam. You'll also need to do the same on the lower surface of the wing at the fuselage to get the wiring into the fuselage. For those holes, I used a 1/4" diameter brass tube suitably sharpened.

Releasing the elevons & installing the control horns
I decided to not cut the elevons loose at this time, saving this for one of the last steps. The control horns that are supplied with the TwinJet will not fit into the molded pockets on the elevons without first being modified, something not specifically mentioned in the manual. You'll need to shorten the length of the plastic fin that inserts into the elevon so that the base of the control horn will sit flush in the pocket. You can either cut/saw this off or sand it off on a belt sander as I did. Be sure to trial fit the control horns prior to gluing them into place. I would also suggest fitting the pushrod connector on the control horn before gluing the control horn into place as it is much easier to handle as a separate assembly.

Installing the servos
I used Dymond Precision Micro Servos to move the elevons on the TwinJet. These fit the molded servo recesses on the lower surface of the wings. I chose to use 12" servo lead extensions to get the servo leads into the fuselage. To make this work, you'll end up cutting a small pocket in the wing surface to get the male/female servo connector to sit flush with the wing surface. The manual describes how to string the servo leads through a ferrite ring (not included with the kit) for added radio interference protection from motor noise. I did not do this on my first TwinJet and never experienced any glitching during the lift of that model so I skipped this step with this TwinJet. Prior to gluing the servos in place, fire up your transmitter and be sure you place the servo arms in the correct neutral position and that the servos operate in the correct direction.

Preparing & installing the motors
You'll need to solder the supplied capacitors to each motor - be sure to do this as it has a big impact on the amount of "noise" the motors create which in turn may have a detrimental effect on radio range and performance. With the motors used in a pusher configuration, you'll need to decide whether you'll switch the wiring around at the motor or at the speed controller. Since I hard wired the speed controller to the wiring (no connector used), I made the switch at that end. Before gluing the motors into their respective nacelles, be sure to check that your wiring is right and that you have the correct motor/prop rotation.

Installing the motors into the nacelles requires you to glue them into place. Believe me, this works well when done as described in the manual and it is more than durable enough for how you will fly the TwinJet.

Completing the fuselage
With your motor and servo wiring neatly routed through the fuselage and into the cockpit area, it's time to install the model's spine - a 10mmx10mm x 800mm piece of hard balsa stock. You'll need to first cut away a piece of foam in the forward fuselage recess (left as part of the molding process) to make room for a flush fit in the recess along the belly of the fuselage. Trial fit the balsa stock and mark the balsa at the forward nose section where it stands proud of the fuselage. Using a razor plane and sander, trim this portion of the balsa stock until you have a flush fit with the fuselage. Trim the rear end of the balsa stock to match the rear of the fuselage and then glue the balsa into place in the fuselage. When the glue is cured, use a sanding block to knock down any edges on the balsa or glue that has seeped to the surface and then cover the exposed balsa along the length of the fuselage belly with 3M brand #52 3/4" wide strapping tape. I like this tape as it has a higher tack strength (i.e. - it's stickier). This also makes for a smooth surface and it does a pretty good job of protecting the fuselage belly when landing.

When gluing the vertical fins into place, trial fit them to ensure the correct location. The fin tips angle inwards a few degrees from vertical.

Final assembly
At this point I hinged the elevons and cut them loose from the wing. The original TwinJet manual had you using part of the decal sheet as the hinge tape but this was inadequate in actual use. I use the hinge tape available from Planes, Wings & Things ( http://www.planes-wings-things.com ). The 1" wide tape works well and holds to the foam quite well. When cutting the elevons loose, be sure to cut out the small piece of foam molded at the outside end of the elevon hinge line. Work the hinge line back and forth a few times before completing the hook up of the pushrod to the control horn.

When I had completed the installation of the battery pack, speed controller and receiver in the fuselage, and was satisfied with the proper CG location, I wanted to add some cooling inlets to the cockpit area. Foam models aren't much different than a foam cooler in keeping food cold (or in this case, equipment hot) and getting at least some air to move through the cockpit area is vital to the longevity of the equipment there and the model as a whole. I chose to use two inlets drilled into the leading edge of the wing and one larger opening through the turtledeck to

exhaust that air out of the cockpit area. Using that trusty piece of 3/8" diameter sharpened brass tubing, I carefully cut into the leading edge of each wing so that air spills across the speed controller on one side and across the battery pack on the other. I used a 3/4" piece of brass tubing to cut the exhaust outlet in the turtledeck prior to gluing it permanently to the fuselage. I've found that this arrangement does help to remove the heated air in the cockpit area and goes a long way towards adding to the longevity of the speed controller and model. When choosing a speed controller, I erred on the side of conservatism and went with a Jeti JES500 50 amp speed controller with BEC. With adequate cooling, you should be able to use a speed controller with BEC rated for 35 amps minimum. Just remember, foam is an insulator!

I have used the supplied Gunter props on a number of previous models and I'm quite surprised at how well they work for something so simple and cheap. I have also damaged a previous model - a Multiplex Twinstar - due to a Gunther prop being thrown - not a good thing at low altitude! Seeing as the TwinJet can move along at quite a good clip, I had used on my previous TwinJet a pair of APC 5.5"x4.5" Speed 400 Electric Propellers mounted on MPI Maxx Speed 400 prop adapters. This turned out be an excellent combination and it was easy for me to decide to use the same set up on this model. APC has a couple of different prop sizes in their Speed 400 Electric series of props so there is room for some experimenting with prop sizes if you wish.

Flying

For the first few flights, I stuck to the suggested balance point for the model and Multiplex makes doing that very easy with the two small points molded into the lower surface of the wing. When the TwinJet is correctly balanced, the model will sit on your fingertips when they are placed on the molded points. As you become more accustomed to the TwinJet, you may wish to move the CG back just a bit, taking a bit of the reflex out of the elevons to increase the sensitivity of the model when flying.

I used my trusty Airtronics RD6000 Sport radio system to control the TwinJet. After charging up an 8 cell 2000mah NiCad pack and placing it in the TwinJet, a range check was made both with motors on and off and all was well to go and fly. Winds were light and with full throttle, a short run and firm and level throw, the TwinJet was climbing and accelerating away at a good clip. After a turn back towards the field, a click of left trim and three clicks of down trim was dialed in and the TwinJet was holding steady on heading. The TwinJet has a good turn of speed for a model of its size and you'll find that you need to make plenty of turns to keep it within a reasonable visual distance. With the balance point at the recommended location, there's a fair amount of reflex in the elevons and rolls take a bit of down elevator being held in to get a axial roll. Power off flight shows a good glide ratio and stalls are straightforward with no real tendency to roll off to the left or right. Landings are easy - make a straight in approach throttling back as you approach the landing area. Go to throttle off and slowly pull the nose up to bleed off speed and let the TwinJet settled on to the ground for the landing.

Loops take some getting used to as the TwinJet will try and roll a bit on the climb (possibly torque induced?) and a bit of aileron is needed to keep the model tracking straight. The elevon control throws spelled out in the manual work well and as you get more comfortable with the TwinJet, you can increase these throw amounts a bit more for quicker roll and pitch rates. Inverted flying at the recommended balance point takes more down elevator than the recommended throws will provide for but as you move the balance point back and take some of the reflex out of the elevons to compensate for the rearward balance point, inverted flying becomes quite straightforward and easy to do. Flight times are in the 6+ minute range using full throttle 80% of the time on the 8 cell pack.

Later flights with a 10 cell Sanyo CP1700 NiCad pack showed improved top end speed and essentially equal flight times. I was a bit surprised to find that when I weighed the two different battery packs, there was only .4 ounces difference in weight with the 8 cell NiCad pack being the heavier of the two. Use of a 10 cell pack will likely results in lower motor life but considering how inexpensive the motors are, the added performance may be worth it.

Conclusion

The Multiplex TwinJet is a fun model that goes together quickly, is durable and flies very well. The kit quality is excellent and Multiplex does a great job both with the packaging and presentation of the kit. The TwinJet makes the perfect model to put in the back of the car and make a quick run to the field to get a flight or two in after work and before the sun sets, or to spend the whole day at the field with flying. I have had numerous compliments and questions asked of "where to get one" when I fly it at the local wet fuel power field and when I head out of town for various events. The White Eagle Racer's Edition, with its supplied Speed 480 motors, has just a bit more performance than the standard Speed 400 powered TwinJet, but either way you won't be disappointed with this fun to fly model!

For further information and feedback on the TwinJet from other electric modelers, I would highly recommend checking out the EZone's Foamies discussion group.

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