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The Multiplex Twinstar

A review of the ARF twin by Steve Kranish.

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Specs:
  • Supplier: Multiplex/Critter Bits via Northeast Sailplane Products
  • Manufacturer: Multiplex (Germany)
  • Price:US$95 with motors, props, wiring
  • Wingspan: 55.9 in 142cm
  • Wing Area: 620 sq. in. 40sq dm
  • Airfoil: Semi-symettrical
  • Weight: Suggested 51oz 1450g, As Reviewed 43.3 oz 1230g
  • Wing Loading: 11.8 oz/sq ft 30.7g/sq dm as reviewed
  • Channels: Ailerons/Spoilerons (2 servos), Elevator, Rudder, Throttle
  • Recommended Power System: 2 supplied Permax 400 motors, props, 7 cell 1400mAh or larger pack

 

Introduction

In the last year or so, Multiplex has introduced their new 'Pico Line' of inexpensive, easy to build, and easy to fly electric planes. The Pico Line started with the Teddy, a small single engine pusher, and the Smiley, a rudder-elevator model with twin motors, both meant for beginners. The most recent addition to the line is the Twin Star, a twin motor plane with ailerons, which is more appropriate for an intermediate pilot. All the kits in the Pico Line are suitable for a very inexperienced builder.

The Twin Star is manufactured in Germany, and is available from Multiplex dealers and distributors around the world. See the Multiplex web site at [http://www.multiplexrc.com] for your nearest dealer. My Twin Star was purchased from Northeast Sailplane Products [http://www.nesail.com].

Multiplex describes the Twin Star as 'The electric model with that certain something'. Read on!

 

Kit Contents

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The Twin Star arrived in a large box (7" x 11" x 41", 18cm x 28cm x 104cm) with very colorful graphics and lots of photos. This box is clearly meant for display in a hobby shop, and there is no way you will sneak it into the house without someone noticing what it is.

Inside the box are instructions, decorative stickers, a small box containing the drive train components, a few wood pieces, linkages, and lots of moulded foam airframe pieces. Lots of moulded foam airframe pieces, because the entire airplane is made of foam.

The hardware package is very complete. Pushrods, special control horns designed for foam surfaces, special hinges designed for foam surfaces, and even the screw eyes for the rubber band that holds the hatch in place are included. Velcro for holding the motor control and the battery is also included.

 

What you need in addition to the kit

The Twin Star is about as complete a kit as you could as for. This is everything that I added:

  • slow set epoxy ( the instructions call for 5 minute epoxy, but I prefer the slow set kind)
  • motor control rated at more than 20 amps (Viper Model Products MD-102)
  • Astro Flight connector pins, to replace supplied Multiplex connector
  • radio (receiver, 4 mini or micro servos, I used Hitec HS-80s)
  • 2 aileron lead extensions, 18"
  • 1.5" shrink tubing, for mounting aileron servos
  • strapping tape (not called for) for reinforcing the wing and tail
  • sticky back film covering (not called for, I used Ultracote Plus)

 

Construction

The Twin Star is not an Almost-Ready-to-Fly (ARF) kit, but it certainly deserves the description of 'Schnellbaukasten' or 'Quickie Kit'. If you don't care about how it looks, and use 5 minute epoxy for everything, you could probably build it in an evening or two. An hour or two an evening for about a week is a more realistic approach to building it, but either way there is not much to do, and it does not take much time. The assembly process consists of:

  • Glue wing halves together
  • Install motor and servo wiring in wing
  • Glue spars into wing
  • Install motors in wing
  • Glue aileron hinges to aileron and wing
  • Install pushrods in fuselage
  • Glue horizontal stabilizer to fuselage
  • Glue vertical stabilizer to fuselage
  • Glue rudder and elevator hinges to tail surfaces
  • Install radio gear
  • Check balance
  • Charge the battery and go flying!

There are no complicated or difficult tasks, and I think the plane could easily be built by someone who has never assembled a model before.

The instructions booklet is written in four languages - German, French, English, and Italian, but if you are reading this, English will probably do. The instructions include many assembly sketches, but no photographs. All components are numbered, and there is a parts list with all of the numbers, so there should be no problem identifying any parts.

 

Wings

The Twin Star wings are the most complicated part of the plane, and they are pretty simple at that. There are two moulded foam wing halves, with integral motor nacelles, and moulded slots for a pair of spars. The wing halves are simply epoxied together. The motor and aileron wiring is then installed in the spar slots, and the the preshaped spars, which overlap in the center, and epoxied into place. I did sand the spars a bit to get them flush with the surface, but everything fits well as supplied.

motors.jpg (22367 bytes)

This photo shows the wiring and motors that have to be installed in the wing. The aileron servo extensions are not included in the kit.

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This photo shows how the motor and aileron wiring is installed underneath (well, really above) the wing spars.

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This photo shows the pre-tapered ends of the wing spars, which allows the spar halves to overlap in the middle.

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This photo shows how I used clamps to hold the forward spar in place while the epoxy cured. I did not have any clamps large enough to do this with the rear spar.

The motors are installed in the nacelles with a few spots of epoxy, and then the rather oddly shaped nacelle bottoms are installed. The shape of the nacelle provides noticable upthrust, and based on the flight performance, the upthrust is intentional. The motors are not easily removable, but it should be possible to remove them from the wing without any real damage. The motor wiring harness has push-on connectors for the motors, but I soldered these in place to make sure I had a solid, low resistance connection.

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This photo shows the installation of the bottom half of the motor nacelles.

The top center section of the wing is then covered and reinforced with a clear plastic cover. I sanded the underside with fine sandpaper before epoxying it in place. The center section cover provides reinforcement for the two long nylon bolts that hold the wing to the fuselage.

The ailerons use the same unusual, designed-for the purpose hinges that are used on the tail surfaces. See the section on 'Empennage' below for a photograph. These flexible plastic hinges are simply glued to the outside of the surfaces, and their shape holds them in place until the epoxy cures.

The aileron servo wells are (like pretty much everything else in the Twin Star) pre-molded into the underside of the wings, at the end of the rear spar channel. The openings are probably large enough for mini-size servos. I installed Hitec HS-80 servos by putting a piece of 1.5" diameter shrink tubing around the servo, carefully (and slowly) shrinking it in place, and then gluing the shrink tubing into the servo well with epoxy. If I ever need to remove the servo, I just have to cut the shrink tubing.

The aileron horns are the same as used on the tail surfaces (see photo below), and they are simply epoxied into the pre-moulded slots in the ailerons. The wire linkages are pre-bent, with a Z bend at the servo, and a clamp type connector at the control horn, so they do not need any additional fitting.

The completed wing seemed adequately strong, but I did add a single piece of 0.75" strapping tape from tip to tip on the bottom of the wing as reinforcement. The only likely weak spot is at the servo well, which is also the end of the spars. The strapping tape definitely reinforced this area.

That is it. Pretty simple!

 

Fuselage

The fuselage (excluding the tail surfaces) consists of a single very large piece of moulded foam, a moulded foam hatch/canopy, and a number of small pieces of die cut plywood. All of the die cut plywood comes from a single sheet, and the die cutting is, well, OK, but none of these pieces needs to be a precision fit. One of the pieces had a hole drilled in the wrong place, so Multiplex also included a new piece with the hole in the correct location. Thoughtful!

The forward and rear wing holddowns are made from several pieces of die cut plywood that are epoxied together, and then epoxied into the fuselage. The holes for the wing holddown bolts are predrilled, but I found it necessary to redrill the holes after every thing was assembled. Included in the kit is a self-tapping screw that can be used as a tap for cutting threads for the bolts, but I used an actual 4mm tap.

wingmnts.jpg (24614 bytes)

This photo shows how I used a piece of cardboard as a sort of internal expansion clamp to hold the rear wing mount in place while the epoxy cured. The forward and rear wing mounts can be seen.

A pair small screw eyes are inserted into a pair of small wood blocks, which are then glued into the fuselage and the underside of the hatch. A rubber band stretched between the screw eyes holds the hatch in place.

The pushrods are music wire (stiff steel wire) in a plastic tube. There is a channel on the top of the fuselage for installation of the pushrod housings, and the moulded foam dorsal fin is then glued into the channel to close it off. The pushrods have Z bends for the servo end, and use clamp connectors at the control horn end, so there is no need for cutting or bending them.

The battery is installed in an appropriately sized opening under the hatch, which extends to about the middle of the wing chord. The opening is large enough for the battery to be shifted forward or backward enough to balance the plane. I put some foam sheet in the front and back of the opening to fix the location of the battery. See the notes below on flying the Twin Star for an explanation of why you really should use velcro to hold the battery in place. Really.

The motor control is installed next to the battery and held in place with velcro.

The fuselage is setup to accept with the Multiplex 'EinStein' brick receiver with two servos, or a separate receiver and two micro or mini servos. Plywood rails are simply glued into the fuselage, and the servos are screwed to the rails. See the section below on 'Equipment Installation' for more details.

I added a piece of strapping tape to the bottom of the fuselage as a sort of landing skid.

 

Empennage

The Twin Star horizontal stabilizer and fin are simple molded foam sheets, with no reinforcement or other structure. The stabilizer is glued to the fuselage, and then the fin is glued to the stabilizer. There is also that cleverly designed dorsal fin that extends all the way forward to the wing trailing edge, covering the slot where the pushrods are installed. The control surfaces use the same cleverly designed hinges and control horns as the ailerons. We are talking serious clever here!

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This photo shows the stabilizer, elevator, and the special hinges and control horns used in this kit. Both are held in place with epoxy

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This photo shows how I used clamps to hold the stabilizer in place while the epoxy cured.

 

Covering and Finishing

Ahhh..yeah, right. Covering and finishing? You don't have to do any! I did sand most of the surfaces with 400 grit sandpaper to remove the mould bumps, but there is no need to do anything else.

I did give some thought to covering part of the wing and most of the tail with low temperature film, but after experimenting with covering part of the wing leading edge, I gave it up. I really wanted to cover the leading edge with something, just to protect it from bumps, so I did it with sticky-back Ultracote Plus. If you want to cover any part of the plane, this is the material I would recommend. I limited the covering to areas without any compound curves, so it was not necessary to use heat to get the covering in place.

The kit includes a sheet of decorative stickers that can be cut apart and scattered about the plane. There is also a sheet with two very large panels that are meant to cover the spars and servo opening on the bottom of the wing.

 

Equipment Installation

Installing the radio and power equipment in the Twin Star is very easy. There is a place for everything, and everything fits where it is supposed to go!

equipmnt.jpg (33786 bytes)

I used the following equipment:

  • JR R600 Receiver
  • Hitec HS-80 servos for all surfaces
  • Viper Model Products MD102 motor control with BEC and brake
  • 7 Cell Sanyo 1400SCR pack
  • Permax 400 motor as supplied, with supplied 'rubber model' propellers.

The battery well is large enough to handle an 8 cell pack and there is a recess to the side of it for installation of the motor control. I simply wedged some foam into the battery well to maintain the position of the battery.

The plane was balanced by adjusting the location of the flight battery. No ballast or trim weight was required.

The motors are wired in parallel to a single motor control. With a 7 cell 1400SCR pack, I measured the power consumption of the supplied drive train with my Astro Flight WHATT meter::

  • 7.9 V at battery
  • 22.9 A (remember this is two motors in parallel, so we have twice the current of one)
  • 185W input
  • 13.7K RPM, and surprisingly quiet!

 

Radio Setup

The Twin Star could be setup with a Y cable to run the two aileron servos in parallel, allowing the use of a non-computer radio. But the pop-up spoilerons (both ailerons raised about 30 degrees) really are a convenience for landing, so it really makes more sense to use a programmable transmitter. Even without considering spoiler operation, the ailerons must be setup with approximately 2:1 differential, with twice as much up travel as down travel. I set the initial surface travels as suggested in the instructions.

twindiag.gif (91928 bytes)

This diagram shows the JR783 transmitter setup for the Twin Star. If you would rather read a text summary of the setup:

  • set transmitter to glider mode, and reset all data
  • set flap-aileron mixer ON with Flap switch DOWN
  • set flap-aileron mixer to -42% with an offset of -170
  • set servo travel for the left spoileron to 35% right, 129% left, to provide differential
  • set servo travel for the right spoileron to 129% right, 35% left, to provide differential
  • set flap-aileron mixer to offset with spoilerons in neutral, and 30 degrees up when mixed in

 

Subassembly Weights

The following is a list of the important subassembly weights.  

Assembly Out of the Box Ready to Fly
Wing panels 6.9 oz / 196 g 17.9 oz / 510 g
Fuselage with Hatch 4.4 oz / 128g 11.8 oz / 334g
Vertical Tail   0.4 oz / 12g see fuselage
7 Cell 1400SCR pack   13.9 oz / 396g
Complete Plane 31.8 oz / 900g empty  43.3 oz / 1230g

The completed wing weight includes the the complete drive train, aileron servos and linkages. The completed fuselage weight includes the receiver, rudder and elevator servos and linkages, and the tail surfaces.

The completed weight is well below the weight specified in the instructions, and I do not feel I did anything unusual in equipping the plane. I would certainly NOT recommend letting the weight increase.

 

Flying the Twin Star

The Twin Star is launched by opening up the throttle, and simply pushing it forward out of your hand. It climbs right out, probably because of the built in upthrust. It is well behaved and easy to maneuver. The power available from those two tiny Permax 400 motors is really surprising, and I have frequently flown it for extended periods at about half throttle.

The climb rate is so good that most of my flights are spent pretty high up. With careful use of the throttle, I have had flights of over 8.5 minutes on a 1400SCR pack, so flights of 12 minutes or more with a 2000 pack seem pretty realistic. This is comparable to a lot of glow planes, and should end any discussion that long flights with an electric plane are only possible with slow flying gliders.

Landings are easy, just set it up and slide it on to the ground. The bottom of the fuselage is broad enough that it tends to stay upright, rather than settling onto a wingtip. The spoileron function slows down the plane a little, but is not very important. My Twin Star tends to nose up with the spoilerons deployed.

Although it appears to be a semi-scale transport plane, the Twin Star is very capable of un-scale-like maneuvers, including rolls, loops, immelman turns, and inverted flight.

Yes, inverted flight. Think about that. Upside down. One day I got a little too confident flying the plane, and decided to try some extended inverted flight. After a few seconds, I noticed something fall off the plane. At first I thought it was the hatch, but then I realized that it was trailed by ... some wires. It was... the battery. The only battery. I stood there and watched, because there was nothing else I could do! Although badly out of balance, the plane descended in a slow, inverted turn, until its path intersected with the ground. Total damage from this disaster? The stabilizer and elevator broke in half, a few dings, and a badly bruised ego. That was all. The damage was easily repaired with Titebond glue, and I have since added 3/8" strapping tape reinforcements to the stabilizer and fin. This incident confirmed that this lightly built foam airplane is actually pretty tough, and should stand up well to a lot of flying.

Remember from my above description of the fuselage that I simply wedged the battery in place, without using any velcro to hold it in place. Dumb! Velcro-like material is supplied in the kit. Use it!

I have used two different battery packs in my Twin Star, with very different results. One is a 7 cell Sanyo 1400SCR pack, which gives good climbouts and long flights. It was actually made for the RC car market, and is assembled with spot welded straps, but this type of construction is acceptable for the 22A current draw from a freshly charged battery.

The other pack is a 1500SC 'Sport Pack', as in "Why did you buy such a useless battery pack, SPORT?". Perhaps this pack has a weak or bad cell, but at 22A, it just does not put out enough voltage for the plane to fly well unless the battery is charged right before flight. If there is any delay between charge and discharge, this pack does not work well (See this month's From the Lab for more information on this problem). It is, however, suitable for making a lot of low passes. If I ever get any in-flight photos of my Twin Star, it will be with this pack.

I am looking forward to flying the Twin Star in the winter, when a coating of snow on the ground will make it possible to do touch-and-goes. I wonder how far it will slide when I finally land it?

 

Recommendation

The Twin Star is an inexpensive, easy to build, easy to fly electric plane. It flies very well equipped with the supplied drive train. It would make a great introduction to electric flight for someone who already has flying experience, and it would make a very good first aileron plane. The kit is magnificently engineered and well thought out. It includes a number of pieces (such as the hinges and control horns) that were clearly designed for this type of foam airplane, rather than just adapted from balsa kits. Multiplex has obviously invested a lot of engineering and money in the design and tooling for this kit.

The Twin Star is the electric airplane kit that should be on the display shelf of every RC hobby shop in the world. For any customer that has even the slightest doubt about the viability of electric flight, the Twin Star is an outstanding introduction. It climbs well, flies well, flies for a long time on a standard battery back, handles well, and does not require any expensive equipment or exotic materials. It also does not require any difficult building techniques to achieve the low flying weight required for good performance - it is light right out of the box, and as long as you don't load it up with overweight equipment or excessive epoxy, it stays that way. This plane could change the minds of a lot of people who don't yet take electric flight seriously. The semi-scale appearance will also appeal to those modelers that think that models should look like full size airplanes, and the twin motors will appeal to modelers who like multi-engine planes, but don't want to deal with multiple glow engines. Bravo for Multiplex! We need more kits like the Twin Star!

I have heard that in California some people are using the Twin Star in one-design pylon races. Sounds like fun!

If you feel I have to say something negative for you to believe this review, well, I am not sure how well the Twin Star will survive a cartwheel landing. But then I could say that about a lot of planes! That is all I can think of; it even stands up to hangar dings better than I expected.

There can be no doubt about it, the Twin Star is 'The electric model with that certain something'. A lot of certain somethings. I highly recommend it.

For ordering information or the nearest dealer in the USA, please contact:

Multiplex USA / Critter Bits
414751 Calvert Street
Van Nuys, California
(818) 785-2401 Voice
(818) 785-3946 Fax
Email: information(at)multiplexrc.com
Web site: http://www.multiplexrc.com

 

Additional Reviews

Don't just take my word for it! You can check out recent reviews of the Twin Star in two UK based magazines:

The reviews in both of these magazines include several airborne photos, some of which look surprisingly scale-like, and some which are decidedly not, such as the low inverted pass shown in RCM&E.

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