Multiplex Twin-Jet
Twin Power for even more fun and excitement ! 

picture above created by Ecky Schaub

Article Written by Jurgen Heilig and published in EFI issue 1/2002 (EFI is now Q&EFI) 

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Introduction

The Twin-Jet is the latest member of the fast growing family of foam jets from Multiplex. The trend started end of the last millennium (could not resist ;-), i.e. in 1999) with the Pico-Jet, a Styrofoam model powered by a single Speed 400, 6V (called Permax 400 at MPX) and was soon followed by a tougher version called Pico-Jet Combat with a more powerful motor. The quest for even more power and improved flight performance made modelers install high power Neodym-, Cobalt or even Brushless motors, and the Pico-Jets proved to cope well with the additional loads.

Another route to boost performance was chosen for the Twin-Jet. The additional motor definitely does more than just carry its own weight. With a bigger fuselage the Twin-Jet not only looks more grown-up and more elegant, but also allows the usage of standard SC-sized cells. Furthermore every Twin-Star pilot can upgrade to the Twin-Jet using the same equipment.   

Construction

Following the instructions the assembly of the model is straightforward. Just 1 major and 5 minor foam parts make up the structure. A balsa keelbeam and a plastic insert reinforce the fuselage. Medium thick CA is used throughout the construction and subsequently takes very little time. More time consuming are the necessary wiring extensions , especially when using two separate speed controllers for the motors. A total of 24 soldering joints are required for servos and ESCs, plus 4 to 10 for the motor cables. Using long extension leads this figure can be reduced, but as soldering can not be avoided anyway, I opted to go all the way and used twisted cables (personal preference).

No provision for battery or ESC cooling is foreseen and according to Multiplex not required. As I had plans to draw up to 50A from the battery, I did some modifications. Two holes at the bottom of the fuselage serve as cooling air inlets and an additional hole at the end of the battery bay allows for air to pass through a slotted hatch in the keelbeam. The hatch provides easy access for maintenance, i.e. when replacing motor or servo cables.

The complete surface of the model was given a treatment with #400 grit sandpaper to remove any small dimples from the manufacturing process. Deviating from the instructions, no activator spray was used. ZAP-A-GAP from Pacer worked fine for all glue joints.

Equipment- and RC-Installation

It proved to be a lot easier to route the servo extension leads into the fuselage from outside. You can use the same method for the motor cables. Just solder the ends to some surplus wire and pull them through the fuselage wall. Impressed and satisfied by the tiny HiTec’s HS-55 performance in various applications (including the Pico-Jet Combat), I used them despite the slightly larger control surfaces. Some might not like the position of the receiver in front of the battery pack, but from an interference point of view this position is pretty much ideal. I did not use the recommended filters and was still able to fly the model at a height of about 300ft with the TX-antenna fully retracted (Quite different from the Pico-Jet!).

All servo leads need to be extended and soldering is also required for motor/ESC installation. If you feel uncomfortable with soldering - have somebody experienced to do it for you. In order to facilitate easy motor replacement, the motor is wrapped in heat shrink before glued into place.

Drive concepts

Option 1:  7 cells Sanyo RC 2000, Permax 400, 6V (as supplied), Grp CAM Prop 5.5x4.3”

Deviating from the recommended Pico Duo 400 ESC both motors were equipped with Schulze slim-15be. The price for two of these tiny controllers is not much more than for a single, bigger ESC, but offers the possibility for differential thrust to owners of computer radios. Shutting down motors in flight is an interesting feature and allows for some spectacular maneuvers. The additional effort to install two ESCs is marginal. Just deepen the existing channels for the motor wires to accommodate the ESC leads (which need to be extended).

 Differential Thrust Set-up (requires three free mixers):

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How does it work?

Mix 1 ensures that both motors react on throttle stick movements as if connected to a single ESC. Moving the rudder stick to the left will reduce the power of the left motor and increase the power of the right motor (provided it is not already running at full power).

Flying at full power and applying full rudder will reduce one motor to half throttle. Maximum effect is achieved at half throttle where full rudder will stop one motor and bring the other one to full power. While conventional rudder becomes more effective with increasing air speed, differential thrust is most effective at low speeds. Stall turns are most impressive and you can even spin an otherwise extremely stable model.

It also makes a very effective training aide for engine out simulations, especially when using this set-up on the conventional Twin-Star. The Twin-Jet still handles very well on a single motor and will even climb, provided you keep the speed up.

Option 2:  8 cells Sanyo RC 2400, Permax 480, 7.2V, Grp CAM Prop 5.5x4.3”

This is the factory tuning option, using the same motor as in the Pico-Jet Combat. Durability put aside, this option was actually only emulated by running the Permax 400 on 8 cells. With 8 cells the Permax 400 produce the same rpm as the Permax 480, but consume about 2A more per motor. The improvement in flight performance is evident. The climb rate increases to almost 5m/s and the top speed is noticeably higher.

Warning: Using the Permax 400, 6V in combination with 8 cells is not recommended. Even at 8°C outside temperature, the motors get very warm. The Permax 480 has the better efficiency and a little more mass to dissipate heat.    

Option 3:  8 cells Sanyo RC 2400, 2 x SMILE 40-6-12 BEC, 2 x FUN400-28, Grp CAM Prop 5.5x4.3”

You may have already noticed that the prop for all drive options remains the same. You can change the flight characteristics/performance by using different props, but the question which prop is best has already been answered in the article “More Power for the Pico-Jet” in EFI 1/2001. With two motors you have a better power to weight ratio and therefore you can use smaller props with higher pitch without losing too much thrust for safe launches.

With two brushless motors the performance of the Twin-Jet becomes absolutely awesome. The difference in power is like going from 100% military to full reheat (afterburners for our American readers). Using the Cox 6x3” props you can get about 1600g of thrust, but top speed is limited to about 60mph. With the 5.5x4.3” props you still achieve a thrust/weight ratio of about 1:1 and the speed range is much wider. You can clearly see the wing bending from the flight loads during High-G maneuvers, but so far the structure appears to be up for the job.

Performance Comparison  

Flight performance / Handling characteristics

In standard configuration with the two motors supplied and 7 Sanyo RC2000 the Twin-Jet is adequately powered. The initial climb rate was measured at >3m/s and flight times of over 7 minutes at continuous full power are outstanding for such a model. At cruising power this time can be stretched to about 20 minutes. The sink rate was measured at 1.4m/s, which is about 10% better than the Pico-Jet, and the gliding speed is somewhat higher due to the increased wing loading.

Despite the higher wing loading the Twin-Jet is much easier to hand launch than the Pico-Jet. The massive amount of reflex causes a strong tendency for the model to climb. Although helpful for the launch, it is a bit of a nuisance during normal flight. Some of the reflex can be removed by bending the elevons - don’t worry; the material is tough and will keep the new shape. On newer production models a beefed-up section between fuselage and motor nacelles already reduces the reflex.

The model performs all maneuvers possible with elevon control with authority - with the exception of inverted loops (this comment does not apply to the new version with reduced reflex!). It is very agile despite the long nose and rolls are very axial. The recommended control throws appear to be on the high side. Using the outer holes of the servo arms, throws of about 17mm were achieved, which is more than enough. Actually 12mm of aileron will still give you a roll rate of more than 360°/s. Dual rate and exponential can be used to good effect here.

Slow speed handling is uncritical – the model will not tip stall. You slow it down too much and it will just drop the nose and pick up speed. 

Tips

• The length of the ramp for the battery pack may require extension - I used some scrap balsa for it.

• Before you line up the elevons with the wing section, apply full elevator up trim. Once airborne, you will then have plenty of down trim available to correct unwanted climbing of the model (this comment does not apply to the new version with reduced reflex!).

• In case you loose a prop in flight - no need to worry. Just keep the speed up and don’t try to apply power to stretch your glide on final when the nose is already way up. 

Summary

The Twin-Jet will give the Pico-Jet and similar competitors a good run for their money. It looks more impressive than the smaller models, takes standard battery packs, and outperforms them in every criterion (probably except ease of transport and crash resistance) and all that for very little extra cost.

In standard configuration with Permax 400 on 7 cells, the performance is already better than the Pico-Jet Combat with Permax 480 on 8 cells. Two brushless motors transform the Twin-Jet in an absolute powerhouse capable of outperforming most IC-powered models. The step of going from Option 2 to Option 3 is massive, and expensive! As an intermediate solution two FUN400-22 controlled by a single SMILE 40-6-12 should be considered. Kontronik is offering this combination as Twin-Drive at an interesting price. Another option worth considering is Graupner’s Competition Drive Set for the Star-Jet. Two Speed 480L (timed for reverse rotation), two aluminium spinners, two CAM props plus accessories offered at a very competitive price. No matter what drive combination you use, the Twin-Jet is a very robust model for everyday use with excellent flying & handling characteristics for lots of fun - I bet it will be a hit! 

Specifications