picture above created by Ecky Schaub
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.
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.
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.
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):
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.
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.
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 / 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.
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!