Plane Talk: Laser Design Services JetMach 60 - 2008/08

August 2008 71
Specifications
MODEL TYPE: All-wood, laser-cut RC kit
PILOT SKILL LEVEL: Intermediate
WINGSPAN: 80 inches
WING AREA: 1,300 square inches
LENGTH: 75 inches
WEIGHT: 15.5 pounds
WING LOADING: 27 ounces/square foot
ENGINE: 12- to 14-pound-thrust turbine
RADIO: Six channels (minimum),
nine servos
CONSTRUCTION: Laser-cut balsa and
plywood, aluminum (stainless steel)
heat shielding, aluminum tube spar
COVERING/FINISH: MonoKote,
UltraCote
PRICE: $320
smaller runway, both on takeoff and landing.
You can taxi to the beginning of the runway, hit
the brakes, run the engine up to full power, and
take off. On landing you can stop the model
within 50 feet as soon as it touches down.
If this is your first time dealing with
pneumatic brakes, have no fear; all will be
revealed. They are simple to set up and
maintain, not too expensive, and do not have to
be dealt with while flying—only before takeoff
and after landing. So although this is technically
a six-channel model, you only have to deal with
five (throttle, aileron, rudder, elevator, and
flaps) while flying. If you want, and if your
field is big enough, you can ignore the flap
switch altogether.
Construction: I do not intend to do a blow-byblow
account of the building and covering. A
good manual and a CD full of pictures are
provided. And if you are unfamiliar with the
basic rib-and-spar balsa construction this model
has, I suggest that you find a similar ARF
(although you will break our $3,000 budget);
you need to be confident in your ability to do
quality, sturdy construction on the airframe.
The JetMach 60 will be capable of 120-140
mph; loose glue joints and sloppy workmanship
are an accident waiting to happen. However,
you should be able to have it framed up and
ready to cover in a week of evenings.
I used conventional MonoKote; you may
select something similar. The structure is
designed to be strong enough with only plastic
covering. You could fiberglass and paint this
model if you like; it does not care about a little
extra weight.
I used four rolls of Yellow MonoKote and a
little Chrome UltraCote for the canopy area. I
trimmed it out a little with a black trim sheet—
just a few stripes here and there.
As I mentioned, the manufacturer provided
an exceptional photo-illustrated instruction
manual that covers every aspect of construction,
setup, and flying. The included CD contains
Top left: Friends of Laser Design Services—
we know him as Cel of Brazil—shared flight
shots of his JetMach. Flaps allow the
JetMach 60 to slow for landing and add lift.
Left: Flaps are unnecessary for takeoff.
Vertical performance can be impressive
with the throttle opened 100%.
Test-Model Details
ENGINE USED: Modellbau USA T-500
(13 pounds of thrust)
FUEL (KEROSENE): Du-Bro 20-ounce
tank, Du-Bro 50-ounce tank, with
gas stoppers
RADIO SYSTEM: Spektrum DX7
transmitter, Spektrum AR7100
receiver, four JR DS821 servos,
four JR JR591 standard servos, one
Hitec miniservo for brakes, two
TanicPacks 2000 mAh five-cell, three
Y harnesses, 10 18-inch extensions
READY-TO-FLY WEIGHT: 15.5 pounds
FLIGHT DURATION: Six to nine minutes
PETE OOCHROMA
Plane Talk: Laser Design Services JetMach 60
Photos by the author unless otherwise noted
Not all trainers are propeller driven.
This kit is a winner!
THERE ARE PLENTY of similar balsa jet
trainer kits out there, some of which are very
nice, but the turbine-powered JetMach is
simple, straightforward, and economical.
This high-wing airplane is of all-balsa, lasercut
construction. For the most part, it’s entirely
conventional; it builds identically to any other
balsa kit.
The only major difference is that the
propulsion system is mounted in the middle of
the fuselage and exhausts out the bottom of the
fuselage. A metal plate protects the bottom of
the fuselage from hot exhaust gases.
The JetMach 60 features fixed tricycle gear.
You could modify it to fit retracts, but you
would have to design the installation yourself
and, frankly, retracts are probably the biggest
source of headaches and maintenance with jet
aircraft. You will have enough on your plate for
now; leave the retracts for the next model.
This aircraft does have flaps, as most (but
not all) model jets do. The flaps are easy to
build in and set up and operate, and they
generally go a long way toward slowing the
landing speeds and making life easier on both
the pilot and the landing gear.
This project will use pneumatic brakes.
Brakes are required for AMA flying. The basic
reason is that most turbine engines produce too
much thrust at idle for the model to sit still on
the ground.
That means as soon as you let go of it, the
airplane will not stop rolling until it is either
airborne or hits something, such as a fence, a
car, or a human being. Unless you land with the
engine stopped, the same thing is going to
happen on landing.
Brakes let you stop a model when and
where you want. They also let you use a much
72 MODEL AVIATION
Pluses and Minuses
+
• Easy construction.
• Inexpensive.
• Superb flier.
-• You have to build it. (That’s not
a minus for many.)
Dollars and Sense
JetMach 60 kit . . . . . . . . . . . . $ 320
Spektrum DX7 radio . . . . . . . $ 350
Extra servos . . . . . . . . . . . . . . $ 70
Covering . . . . . . . . . . . . . . . . $ 50
T-500 turbine . . . . . . . . . . . . . $ 1,995
Wheels and brakes . . . . . . . . $ 100
Miscellaneous hardware . . . . . $ 40
Wiring . . . . . . . . . . . . . . . . . . $ 30
Start tube . . . . . . . . . . . . . . . . $ 10
Adhesives . . . . . . . . . . . . . . . . $ 10
Total . . . . . . . . . . . . . . . . . $ 2,975
That’s hard to beat, and all this is
new equipment. Some dealers offer
specials, such as a free kit if you buy an
engine or free covering if you buy a kit,
so check the list of sources to find out
about the latest offerings. MA
—Pete Oochroma
The model’s shoulder-wing layout promotes
upright flight stability. The oversized
stabilizer improves slow-speed handling.
An aluminum heat shield protects the
covering and wooden frame from exhaust
heat.
The control horns provided in the kit are
laser-cut composite. Pull-pull control is
used on the rudder.
videos and more construction videos and
photos—top-notch stuff.
If you have the experience of a bunch of
balsa kits under your belt, you can forge ahead
and build the model in the sequence you prefer.
Not much can trip you up.
My enormous kit box of balsa was reduced
to a ready-to-cover airframe in approximately
10 hours of furious gluing and wood-thrashing
with various carving and sculpting tools. The
only modifications I made included
constructing the wing LEs from 1/2-inch stock I
had instead of laminating it from two layers of
supplied 1/4 balsa. I also fully sheeted the wings
instead of leaving open bays between the ribs.
It’s not that I thought the wing was too weak
as it was. My method was just easier than
cutting the sheeting to the plans. The upside of
fully sheeting the wing is that you can
fiberglass the aircraft instead of covering it, and
it may be a bit stronger. The downside is
slightly more weight.
The designer will be adding the option of
full sheeting to the plans and instructions for
those who want it.
The kit was a delight, with superb parts fit,
clever engineering, quality materials, great
instructions, and fast assembly. Laser-cutting is
so nice.
Pay attention to the adhesives.
Cyanoacrylate does a poor job of gluing these
plywood parts; aliphatic resin does much better.
I used cyanoacrylate, aliphatic, or epoxy,
depending on the particular application.
A nice thing about most turbine models is
that weight is not such a big issue. So go ahead
and use enough glue; strength is more critical
than lightness. I took extra care gluing in the
main gear mounts, and I took the designer’s
advice of beefing it up with scrap basswood.
There is a ton of room for installing the
turbine gear. For a turbine-powered model, you
need a header tank of some sort to keep bubbles
from getting to the engine. You can use a
commercial unit, such as the Bob Violett
Models (BVM) Ultimate Air Trap, but I simply
used a Du-Bro 20-ounce tank with a
geometrically centered pickup. As long as that
tank is at least half full of fuel, the clunk will be
sitting in fuel.
The 50-ounce Du-Bro main tank and half
the header tank equals 60 ounces of fuel—
plenty for a seven- to 10-minute flight with
mixed throttle usage. If your JetMach is tooling
around at low throttle, you can go much longer
than that.
There is not much in the directions about
where to install what with regards to engine
accessories. It’s not that big of a deal, though.
Look at the pictures and see where I put the
engine control unit (ECU), fuel pump,
solenoids, and propane tank. It’s easy, and it’s
not particularly critical. But try to keep the
fuel pump away from the ECU and receiver,
to prevent electromagnetic interference from
the pump.
I made a start tube with a $5 piece of metal
ducting from The Home Depot and some scrap
aluminum. It prevents the bottom of the
airplane from getting scorched if you overprime
the engine and have a wet start with flames
coming out the back. I did that once, before I
made the start tube, and had to replace the
elevator linkages, the metal heat shield, and a
bunch of covering on the bottom. Live and
learn.
The requirement for a start tube is unique
for this aircraft configuration, with the engine
sticking out the bottom; most aircraft don’t
need this sort of thing. The start tube solves the
problem.
I stick the tube under the model when I start
it. As soon as the engine is up and going, and I
am sure there is no wet start, I remove the tube
while wearing a protective glove, and off we
go.
The provided heat shielding for the bottom
of the airplane is fine for normal use. The only
time the heat shield comes into play is in the
event of a wet start, which is almost invariably
the user’s fault—from getting too much fuel to
the engine before starting.
Make sure you shut off the main fuel valve
to the engine before fueling. And if you have
any ideas that the engine might be wet, hold
the model up and shake any fuel out of the
engine’s tailpipe before starting.
I used the Spektrum DX7 radio. It has
plenty of programming features and is more
than adequate for this airplane. The stock radio
package includes four nice digital servos that I
put on my JetMach’s elevators and ailerons.
The rest of the servo suite is rounded out with
four standard servos—for flaps, rudder, and
nose-gear steering—and a microservo for the
brake valve.
One nice thing about the JetMach is that it
calls for only standard servos. Many jets require
six or more expensive high-powered digitals;
this one has been flight-tested and proven with
standards.
I used two 2000 mAh five-cell Ni-Cd packs
going through two standard switches to the
receiver for redundancy. Later I ditched the
switches entirely and plugged the batteries
directly into the receiver.
You could go with even larger packs; I had
to add some lead to the nose anyway. The
model might as well carry battery power instead
of dead weight. You don’t need the battery
power—2000 mAh is good for flying all day—
but if you are ordering new packs anyway (you
should because this is no place to use old gear
from your scrap box), get something with 2500
mAh or more. There is plenty of room in the
nose.
I mounted the flap servos with both
servos’ arms going the same direction; that
is, both servos’ horns point to the left side
of the airplane. This lets me operate with a
Y harness and a single channel without
using a MatchBox or a reversed servo or
mixing them with the radio. After all, only
August 2008 73
Per AMA regulations, all turbine-powered
models must have wheel-brake systems.
JetLegend wheels are shown.
To prevent damage to the horizontal
surfaces from a hot start, a steel duct is
temporarily located over the turbine exhaust.
A 20-ounce hopper tank ensures that the
fuel flow is bubble free and increases flight
time.
A turbine needs no fancy ductwork;
however, binding wires and accessories
prevents intake-damage possibilities.
The JetLegend brake system that is used is
a simple on/off type. Proportional brakes
are available.
The front hatch is removed for start-up
and shut-down, for ECU access and
inspection.
seven channels are available.
I did mix the elevator servos using separate
channels, but if I had thought further ahead I
could have put the output of one elevator servo
on the opposite side, as I did for the flaps, and
saved another channel slot for future use, such
as a bomb drop or smoke system.
The JetMach does not come with hardware,
but it does include a complete list of necessary
hardware. You can find most of it at your local
hobby store or order it online. I used only
heavy-duty linkages for all flight surfaces. This
is not a place for cheap plastic clevises. I used
off-the-shelf Du-Bro heavy-duty hardware—
nothing exotic.
The rudder is designed as a pull-pull setup.
A neat fiberglass horn is provided for the
rudder, but the pull-pull cables themselves are
not included. Du-Bro sells a package that will
do the trick, or you can find some appropriate
material such as SpiderWire fishing line and
save a few bucks.
The Modellbau T-500 turbine is a drop-in fit
on the mounts. There is plenty of room for a
larger engine, such as a JetCat P-80, but beware
of the extra power.
If you are going that route, limit your
turbine’s thrust to approximately 16 pounds.
You could easily exceed the speeds intended for
the design and end up breaking something.
Performance is sparkling with 13 pounds of
thrust anyway. If you really need to go 200
mph, consider a sleeker, all-composite airframe.
Another issue is that larger engines weigh
more, and that weight is aft of the CG. You will
have to pile on more nose weight to get the
model to balance, and your flights will be
shorter with the increased fuel consumption all
around.
I used the excellent JetLegend brand of
main wheels and brakes. They are simple to set
up and operate, and they work with perfect
reliability. The JetLegend brake valve is
nonproportional; it’s either full on or full off.
But in practice I don’t find that to be a problem.
Just don’t hit the brakes going full tilt, or
you will lock them up, skid, and wear flat spots
on the tires. Wait until your JetMach has
slowed a bit.
Be sure to remove the O-rings from the
brakes and lubricate them with Lithium Grease
before installing them. That will offer much
smoother operation. If you want, you can
upgrade to a proportional brake valve such as
the BVM Smooth Stop or the Orbit electropneumatic
type. They are nice; they just cost
more.
For the nose gear, I used a 5/16-inch-gauge
wire strut from my scrap box and a Robart tire.
There is not a lot of load on the nose gear and it
has worked out well, but the tire wears quickly.
You should consider getting a proper jet wheel
for the nose—something with a metal hub and
hard rubber composition. JetLegend has such
items in stock.
You cannot get away with cheap wheels and
tires on the mains. They will wear out
extraordinarily quickly, or your JetMach will
shed a tire or break a hub on takeoff or landing,
which will probably end up being a more
expensive proposition than using proper jet
wheels in the first place.
The other issue is that AMA requires brakes.
Most of the regular model-airplane wheels will
not accept brakes, whereas the proper jet wheels
have brakes integrated into their design.
If you are clever, you could make a servooperated
friction brake that would work only
on the nose gear. That would still leave you
with the issue of finding wheels that will
stand up to jet landings, and most of those
come with brakes anyway.
All told, I put maybe 50 hours into getting
the JetMach 60 ready to go. I was in no
particular hurry, and it could be done a bit
faster. I needed roughly 10 ounces of nose
weight to balance the model at the
recommended CG. That will vary based on
what batteries you use for the ECU and the
radio.
So how does it fly? Like a big trainer—a big
trainer that is capable of serious aerobatics, with
a top speed of approximately 130 mph. The key
element, though, is that at the lower end of the
speed range, the aircraft slows like a trainer and
becomes entirely docile. It is extremely easy to
land.
Most pilots with reasonable vision and
reflexes can fly the model just fine; the
difficulty is all in the landing. Full-scale pilots
often say that the landings are what they’re paid
for. And the JetMach is a real confidence
builder. With the flaps fully down, it will slow
to the proverbial, clichéd “walking pace.”
In a 15-knot headwind, you can practically
land this model going backward. All control
surfaces remain splendidly responsive to the
lowest speeds. If you work the brakes, you can
have the airplane down and stopped in 50 feet
of pavement.
Takeoffs are dead simple; point the JetMach
into the wind, give it the gun, and it climbs
away in close to 40 feet (maybe slightly more
off grass). I have not tried flaps on takeoff to
shorten takeoff distance; there is no need.
This model does pitch the nose up a bit with
flap application, so I put a down-elevator mix
on a switch to counteract that. I like to be able
to switch that mix on and off; I can pitch the
JetMach into high-alpha mode, add throttle, and
hover into a headwind like that.
Roll rate is great, roughly one per second,
and you can tweak that to taste. Loops can be
500 feet in diameter. The model won’t hold
knife edge. If you really want to do knife
edge, the designer suggests that it will be fine
if you add some area to the rudder. This
aircraft will perform snap rolls and outside
loops. Recovery from almost any awkward
attitude is only a matter of letting go of the
sticks.
Anybody who can handle an intermediate
airplane can handle the JetMach 60. The only
negative thing is that it does not prepare you
for the kind of high landing speeds you might
experience if your next jet is a heavy scale
model. But you can’t have everything!
This model will give you great confidence
and experience in flying, building,
maintaining, and handling installation in a
turbine-powered model, and there is much to
be said for that. It’s also a perfect way to get
your AMA turbine wavier.
If you have ever wanted to take the plunge
into turbines but felt they were too hard to
fly, too expensive, or too complicated, now is
the time. Finally, there is an aircraft that is
super easy to fly and tremendously
affordable; it’s a perfect way to get your feet
wet with a jet.
You could have this model airborne in
two weeks of evenings for less than $3,500,
even including a new engine. Dreamworks
Model Products has a deal in which the kit is
free if you buy a JetCat P-60 turbine. That’s
hard to beat!
The JetMach 60 has been flown at
locations around the world and has
introduced many new pilots to flying with
turbines. There have been no complaints from
those who have flown it or seen it fly.
Go for it! MA
Pete Oochroma
[email protected]
Manufacturer/Distributor:
Laser Design Services LLC
1823 Emerald Bay Dr.
Rockwall TX 75087
(972) 772-4326
www.laser-design-services.com
Sources:
Modellbau USA (T-500 turbine)
(954) 476-5572
www.modellbau-usa.com
Dreamworks Model Products (JetCat turbines)
(386) 852-4793
www.dreamworksrc.com
Spektrum (DX7 radio)
(800) 338-4369
www.spektrumrc.com
JetLegend (wheels and brakes)
86-20-8603 3112 (China)
www.jetlegend.com
TanicPacks (batteries, servo extensions)
(800) 728-6976
www.tanicpacks.com
Du-Bro (hardware)
(800) 848-9411
www.dubro.com
MonoKote
(800) 637-7660
www.monokote.com
UltraCote
(800) 338-4639
www.hangar-9.com
Other Review Articles:
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