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The Multiplex Twinjet

Multiplex brings hot, jet-like appearance and performance together with high durability and simple assembly in the new Twinjet.

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Specifications:
  • Wingspan: 35.8 in.
  • Wing Area: 396 sq. in.
  • Length: 31.6 in.
  • Flying Weight: .36 oz with 8 SR Max 1300's, 37 oz with 7 Sanyo 1700's, 38 oz with 7 SR Max 2400's
  • Wing Loading: 13.5 oz/sq. ft.
  • Airfoil: flat bottom
  • Motors: 2, 6V Speed 400
  • Props: Gunther push-on props/ Master Airscrew 5.5x4.5
  • Cells: 8 SR Max 1300's, 7 Sanyo 1700's, 7 SR Max 2400's http://www.srbatteries.com/banners/efpacks.htm
  • Radio: Multiplex Cockpit Transmitter, Multiplex IPD receiver, Multiplex Pico 400 Duo esc, 2 Hitec HS-80 servos.
  • Available from: Multiplex USA, http://www.multiplexrc.com

 

 

Introductory Comments

Ever want a plane that you could truly assemble in an evening, fly about anytime, and which would shrug off impacts? Wouldn't it be even better if that plane would bring a smile to your face and gather plenty of attention when you go to the field?  Multiplex's new Twinjet (at least new to the U.S.) may be the answer to your dreams.  Molded of durable "Elapor" foam, the Twinjet evokes images of F-18's and Mig 29's, but with prop simplicity.  When you get this plane turnin' and burnin', you'll almost swear there's a gun button on your transmitter somewhere....

When the Twinjet arrived at my door, I was in the middle of another review, and I wasn't intending to take on anything else.  However, the box said you could assemble it in an evening, and it sure looked like the ideal plane to fly at our Wednesday lunch gatherings.  I get together with a couple of friends during our lunch break on Wednesdays (and sometimes other days) to fly at a nearby field.  The field is grass, but very lumpy, making it rough on planes with landing gear or of fragile construction.  The rugged foam and pusher prop design of the Twinjet definitely gives it an edge on keeping things together at this field.  It also looked fast and fun, and used 1200 - 2400 mAh battery packs - a plus in my book, since good quality smaller cells are getting harder to come by.  As you might expect by now, it didn't take a lot of extra convincing to "go for it!"

Kit Contents

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Upon opening the box, the first thing that strikes you is the utter simplicity of the design - there's very little to assemble.  Most of the Twinjet is a one-piece Elapor foam wing/fuselage.  Elevons and motor nacelles are molded in.  Beyond that the Twinjet has two fins, a nose cone, canopy, and a couple of other parts.  It's also a very complete package, including the motors, props, and wiring harness.  Add two servos, receiver, speed control, and a battery and you're pretty much in business. 

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Photo on the left shows the molded in nacelles and elevons.  On the right you can see the hand-hold molded into the fuselage - makes launching a breeze!  If you look carefully, you can also see a bump on each side of the wing just above the hand hold.  It lets you know where to balance the Twinjet.

One of the greatest things about this foam is the ability to handle Cyanoacrylate glue (CA, or :"super glue").  Called "Zacki" in the instructions, it's simply gap-filling CA.  It sure beats having to mix epoxy!

 

Basic Assembly

Fuselage (Wingelage?)

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This part was really easy!  Step 1 - glue the nose on.  As you can see from the photos, the nose has a neat interlocking design that really holds together well.  A little CA and you're done. 

 

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The next step was trimming the plastic tub that forms the fuselage liner.  Once it fits properly, it gets CA'd in place as well.  I followed the directions and used a minimum amount of glue, but I later went back and give it a more solid bond to keep the sidewalls from flexing.  Once the tub is in the fuselage, you'll need to make a hole in the bottom aft end to allow motor and servo wires to enter the cockpit area.

 

Control Surfaces

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Elevons can be cut free with a razor knife or a fine hacksaw.  Adding a layer of plastic packing tape over the top of the elevon keeps it from cracking (the foam acts as a hinge).  The original intent was to use a decal for hinging, but this method apparently didn't work very well and the supplemental instructions suggest using something like plastic packing tape.

 

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Plastic control horns are CA'd in place.  I found that I needed to cut the control horn slot all the way through the elevon to properly fit the horn.  The Twinjet comes with nice hardware, including pushrods cut to length with a Z-bend in one end.  A quick connect fitting is supplied for the horn end.   I ended up using a Hitec HS-80 servo and a Tower CS-11 (equivalent).  These servos are comparable to the current Hitec HS-81's, and were a little larger (and shaped differently) than the supplied servo openings.  I enlarged the servo openings and cut new channels a short distance for the servo wire.  The instructions recommended CA'ing the servos in place with tape or shrink wrap around the servo.  I basically did this, although I used masking tape only on the glued side of the servo.

 

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Here you can see where I cut a little extra space in the wire channel to make room for my servo extension connector.  I covered the channel and the servo with plastic tape to keep the wire in place and as extra insurance for the servo.  Possibly one of the trickiest aspects of this operation was getting the wire into the cockpit area.  A hole has to be punched through the end of the servo wire channel into the bay in the aft end of the fuselage, then you need to route the wire into the previously made hole in the base of the fuselage tub.  It's not hard, but it does take a little working to get it through the holes properly.

 

Fins

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You can't get much simpler than this.  Make sure you have the correct fin for each side (the fins will only fit the impressions in the foam correctly if they are on the correct side), and CA them in place.  The twin fins with an inward slant give the Twinjet a distinctive profile.  From some angles of flight it looks like an SR-71 coming at you.

 

Radio System

Cockpit Transmitter

I used my Multiplex Cockpit transmitter to guide the Twinjet.  I've already reviewed this radio in the past  http://www.ezonemag.com/articles/2000/oct/cockpit/cockpit.htm, so I'll just say it's easy to setup the elevon mixing and its thin, light package makes a great match to the Twinjet..

IPD Receiver

This was a new item to try out - Multiplex's IPD receiver.  IPD stands for Intelligent Pulse Decoding.  According to Multiplex, the IPD receiver incorporates a micro-processor which makes it possible to assess/decode the signals from the transmitter.   Signal strength is constantly monitored, with strong signals being passed on to the receiver.  As signals become weaker, the receiver calculates the intended servo position from several groups of signals.  If no valid signals are detected for 0.5 seconds, the servos are set to the fail-safe position.  I've heard a lot of discussion about this receiver on the E-Zone Eflight mailing list, so I was very interested in trying it out.  Multiplex claims their IPD receiver combines the best of PPM (fast control signal response and compatibility with all current FM/PPM 2- 9 channel transmitters) and PCM (only valid control signals are passed to servos and programmable fail-safe positions) technology, giving the user faster model response, reduced sensitivity to interference, and no servo jitter when the transmitter is switched off.

How does it work?  Well, it's hard to say how much better it is than other receivers, since I can't see how much interference the receiver has to deal with, but I will say that it has been absolutely glitch free in all my flying to date (22 flights).   In the area where we fly at lunch, I've frequently encountered motor glitching with other planes, where I'll hear the motor momentarily cut out, usually at certain parts of the field.  I haven't seen any indication of that problem with this setup.  It seems to be a flawless system so far.

Pico Controller

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For a speed control I used a Multiplex Pico 400 Duo ESC.  The nice part of this setup was that the connector for the motor side matched up with the wiring harness connector, saving the hassle of having to change connectors (although I did change the battery side to Anderson Power Poles to match up with my batteries).  Throttle control is quite smooth, and everything seems to stay fairly cool.  Mine was given to me by a friend because the auto cutoff feature wasn't working, but it isn't a problem with this plane - by the time the battery is that low (5 - 5.5 Volts), the Twinjet isn't flying anyway.

Installation

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I changed the installation arrangement a little from that recommended by the instructions.  As you can see from the photo on the left above, the wiring comes into the fuselage tub from a hole in the bottom of the tub.  According to the instructions, the battery should sit on a ramp molded into the rear of the tub, with the receiver ahead of the battery and the speed control in the front.  I really don't like having my electronics ahead of the battery - I've seen the results of a battery acting as a battering ram in a crash.  It may be that the ramp would launch the battery over the electronics in the event of a crash, but it still seems like an iffy arrangement to me.   As it turns out, I didn't have to worry anyway.  With the components in the recommended position, the CG was too far aft.  I ended up with the receiver at the aft end of the compartment and the speed control at the front, with the battery in the middle Velcro'd to the side of the tub (see photo on the right).  It actually works out quite well like this - the Velcro material is acting in the shear direction against vertical and forward displacement of the battery, holding it quite securely.  I'll also have to say that the hook and loop material supplied in the kit is very strong stuff, making it something of a job to remove the battery, even when you want to!

 

Power System

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Motors, props, and wiring harness installed on the Twinjet.

The kit I received had the standard 6V Permax 400 motors (the box also lists a hotter Permax 480 option), Gunther push-on props, and a wiring harness.   Capacitors were also included, to be soldered to the motors.  I was a little surprised that the Twinjet harness didn't have the push-on motor connectors like the Twinstar wiring harness, requiring instead that the builder solder the wiring to the motor terminals.  This isn't a big deal with most experienced e-flyers - we've learned to solder battery packs, connectors, etc. - but it might be a little intimidating to someone new to electric flying.  It also adds a little complexity to what is otherwise a very simple, almost no tools, assembly. 

Installation requires running the wiring harness from inside the fuselage liner into the cutout region in the lower aft part of the fuselage, and out the sides where the wiring grooves are cut.  Once the wiring is pulled through, the ends are stripped and the wiring (and capacitors) are soldered to each motor.  The wiring harness is wired up in reverse (red wire to negative motor terminal, etc.) to turn the motors backwards, since this is a pusher design.  In order to accommodate the pusher setup, the Gunther props must be reversed as well.  The prop hubs are pushed out, the props are turned around, and the hubs are pushed back into the props.  Motors are held in place by CA to the underside of the fairings.  I wrapped my motors in masking tape to make them easier to remove should I decide to do so in the future.  Actually, I didn't glue them in sufficiently the first time and I had the motors pop out during an unexpected impact with the earth (more on that later).  I glued them in much more solidly after that, and I have yet to see any hint of looseness, even after 50 mph landing (called getting too low on a low speed pass!).  As you can see in the photo, I covered the wiring harness with tape to ensure that it stays in the slot.

By the way, I advanced the timing on my motors to give them a little performance edge.   This is accomplished by making a mark on the backplate and the case, then making another mark on the case about 5 mm away (going around the perimeter of the case) in the direction opposite of shaft rotation (remember that they are reversed).  Next, I wrap the motor several times with a #64 rubber band to give my hand something to grip.   Then I work the tips of a needle-nosed pliers into the holes in the back plate and use the pliers to twist the plate until the mark on the backplate is aligned with the new mark on the case. 

As a side note about the props:  The Gunther props work quite well, in spite of their "toy" appearance.  I've used them on other applications in the past with very good results as well.  It helps to moisten the shaft a little to get the props to slide all the way on (they tend to be tight).  Hopefully this doesn't upset anyone's digestive system, but I found a little saliva will do the trick quite well!    I would recommend other props for cold weather applications, however.   The Gunthers tend to get brittle and break very easily in cold weather.  My first couple of flights were in sub-freezing weather, and I went through a set of props on each flight.  I switched to a pair of Master Airscrew 5.5x4.5 props (using prop adapters) and I haven't had any problem since then (20 flights later).  The MA props draw a little more current and probably give a little more thrust, but the speed and overall performance appear to be very similar.  I've heard that Graupner CAM 4.7x4.7 props work very well with the Twinjet, but I also know they're brittle in cold weather and I didn't want to go through any high-dollar props at the moment.  I should point out that the Twinjet design keeps the props free of the ground, but you can still get grass clumps sticking up that catch the props.

 

Finishing Touches

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To finish off the basic construction of the Twinjet, the builder installs a long square balsa keel to the underside of the fuselage in the pre-formed channel.   You'll have to open up part of the channel (part of the molding process), but it's easy to do with a razor knife.  Once the keel is installed, the excess gets cut off, and part of it becomes a retaining block on the back of the canopy, mating up with a notch on the back half of the rear cockpit cover (which is CA'd in place).  I covered the bottom rib region with a strip of fiberglass strapping tape just for a little extra "wear protection". 

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The front of the canopy is held in place by a rubber band between two screw hooks.  One hook is fastened into a dowel glued into the canopy; the other hook is fastened into the rib through the cockpit floor.  This makes a nice way to retain the canopy, yet allowing quick and easy battery changes.

One interesting aspect of the Twinjet is the lack of cooling provisions for the battery and esc.  Some modelers have created their own ducting in the canopy, but I haven't found cooling to be a problem so far (of course, I haven't flown in over 50 deg. F. weather, either!).  The motors are essentially out in the open, plus they have a duct on the top of the fairing to divert cooling air to the motors.  The motors on my plane never seem to get more than lightly warm.

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The antenna wire is routed out the side of the cockpit and along a channel in the wing (keeping it out of the way of the props).  As with the other wiring, I covered the channel with blue plastic tape.

Trim

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Multiplex's write-up on the Twinjet gives the impression that the kit comes with some colorful decals, but mine didn't (and I've heard others say the same thing).  It did come with some decals that said "Twinjet", but they wouldn't have stood out very well against the dark foam.  I was originally going to trim the plane with some self adhesive orange, but then I came across a sheet of self-adhesive trim with a yellow and black checkerboard pattern which had been lying around virtually forgotten.  It just looked "right", so in short order I gave my Twinjet something of a race plane look.  It still looked incomplete, but some self-adhesive black trim made the canopy stand out and gave me the look I wanted.  On a more practical note, it really helps having a bright color on the top of the wings - the blue-gray coloring of the Twinjet can easily cause you to lose orientation if you don't have a contrasting color scheme.

Control Settings

The instructions for the Twinjet recommend +/- 20 mm for elevator and +/- 15 mm for aileron, set at the widest point of the elevons.  I couldn't seem to set my aileron movement differently than my elevator (maybe I need to read the instructions again), so I set them both at +/- 0.6 in.   I later backed off the throws to 85% (using the radio's travel adjustments), which ended up being about right.  My initial settings were just a little too sensitive on roll control for comfortable flying (but it sure did roll fast!).

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Flying

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The Twinjet looks fantastic making fast passes.  The picture on the right looks like the Twinjet just intercepted one of our local aircraft, but Ken Johnson just happened to catch the Piper in the background as he snapped the snot.  Those motor pods on the back give the impression of jet nozzles in flight.

Ok, it builds easy and looks cool, but what happens when you actually commit aviation?  Well, it's an interesting mixture of mild and wild, with a few unique qualities to distinguish it as something different.  I was pleasantly surprised at how easily the Twinjet hand-launches (as a side note, the prop locations keep them sufficiently clear of your arm to minimize any hazards).  I had Shaun Cronin give the first toss for me, since I like to be fully on the controls at the inaugural launch of a new plane (especially if it looks like it might be sensitive), but I really didn't need to be concerned.  The Twinjet launched absolutely straight and true, with no quirks.  As I went to turn, it became obvious that this was no polyhedral glider - the Twinjet reacts almost instantly to control input.  All I could say was "Wow!".  Although I have to be on top of it, the control reaction is also reassuring, giving you the feeling that you control the plane rather than waiting to see what the plane will do.  My first flight ended fairly quickly with a test of the Twinjet's durability.  I was testing the low speed handling of the plane, when I turned downwind and learned something new - it really doesn't care too much for being flown slowly.  With no prop blast over the control surfaces, the control sensitivity goes down as you slow down.  Turning downwind on a windy day only made things worse, and I soon lost control of the plane (plus I lost orientation of it).  The end result was an impact with the earth on the underside of the nose.  Total damage?  Two broken Gunther props (which apparently happened when the motors popped loose) and the need to re-glue the motors into their mounts and the screw eye mount back into the canopy. 

I made one follow-on flight with another set of Gunther props, but one snapped on landing after brushing over a clump of grass on a cold day.  I switched to Master Airscrew props after that and I haven't had any broken prop problems since.    Now that I've made 22 flights on the Twinjet, I can pretty well summarize its flight characteristics.  The Twinjet likes to be flown fast, and it's most at home making low, high-speed passes.  I don't know how fast the Twinjet really is (I suspect it's in the 50 - 55 mph category), but it looks and sounds like it's going 100 mph when you take it up high, roll it inverted, and come screaming across the deck.  I felt very comfortable with the Twinjet almost immediately, confidently flying it quite low and fast (I got too low one day and ended up making a 50 mph landing!  It was unhurt and I just tossed it back into the air again).  As with many flying wings, the reflex in the elevons gives it a definite "pitch up" tendency at higher speeds.   You'll either have to give it some serious down trim, or hold down elevator to keep level flight.  The instructions tell you that you may want to give it down trim in flight, then remove the trim for launch, but I found it launches just fine with the down trim still in the plane.  I still have to hold a little down elevator, especially when I'm using the 8-cell pack, but it's not nearly as much as it was.  I understand that some Twinjet owners have removed the reflex from the elevons with good results, but I haven't tried that yet.  Another trick that's been helpful is keeping the CG just a little forward of the recommended location. 

Aerobatically, the Twinjet is a mixed bag.  Rolls are fast and axial, and it will loop easily from level flight, but the loops look a little funny.  I think it must be the natural tendency of a delta to shed speed, but I find that I can start a very pretty loop and keep it that way until just past the top, when the plane sort of falls for a moment, then resumes the line of the loop.  Going with the 8-cell pack seems to clean up the loop by keeping the speed up.  By the way, it looks like an F-18/Mig 29 pulling up into a climb as you start into the loop.  I haven't been able to successfully achieve sustained inverted flight yet.  I don't think the flat-bottomed wing and the elevon reflex really like to allow the Twinjet to hang inverted.  I suspect those who have removed some of the reflex and moved the CG further aft probably have greater success at inverted flight. 

Turns can be fairly tight and precise with the Twinjet - it would be fun to have pylon races with this plane.  Don't use it for All Up/ Last Down contests, however, unless you happen to have some secret battery technology giving you about 10000 mAh capacity in our current 2000 mAh battery pack sizes.  In spite of the low wing loading, the Twinjet has about zero glide.  This isn't a bad thing - it's actually great for allowing comfortable landings in small fields - but don't expect to soar the length of the field a few times before coming in.  With the power off, you can pull full up elevator and bring the Twinjet in for a smooth, short landing.

I've been flying the Twinjet on both 7 cell and 8 cell packs with good success.  I'm using an SR Max 2400 7-cell pack, a Sanyo 1700 mAh 7-cell pack, and an SR Max 1300 8-cell pack.  The Twinjet flies very well on 7 cells, but going to 8 cells gives it a notable boost in performance.  I don't notice the difference in speed as much (although it's definitely faster), but I do see stronger launches and better vertical maneuvers, plus you can throttle it back for longer flights and still keep the speed up.   Either way, it's a fun machine.

 

Recommendations

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For simple, sheer-fun flying the Twinjet is nearly unbeatable.  Not only is it simple to build (you really can build this one in an evening), it's simple to operate: no rubber-banding or bolting on wings, no worries about bending prop shafts (and not too much about breaking props), not much worry about crash damage, and a very simple means of changing batteries.  I like being able to grab a plane, a few charged battery packs, and my radio and run to a nearby field to fly without having to carry a bunch of support equipment or spend a lot of time with setup and teardown.  The other great asset of the Twinjet is its fun-to-fly factor - it's way up on my scale.  At first I thought the idea of naming a plane like this one TwinJET was a little silly - the pictures I saw on the web site didn't look too jet-like to me.  After I put mine together, however, I changed my mind - it really does have a jet look, and it flies like one as well   It's fast, cool, and just a heck of a lot of fun to turn and burn.  If these things are high on your priority list, give the Multiplex Twinjet a close look.  If it's not fast enough for you, there's always the option of going with 480's, or how about a pair of Velkom 2020/27's, or a couple of brushless........

 

Discussion

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Old Jul 19, 2010, 04:07 PM
"Where are the shovels?"
wise guy's Avatar
Joined Apr 2010
818 Posts
I know this is a discontinued product, however there is bound to be at least one seller who still has some in stock...
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