Radio Control Jets - 2010/06

IN THE
PREVIOUS
column (in the April
issue) I mentioned
my latest project:
converting a BVM
Jets Maverick to
turbine power. I
have been pushing
that a little, but I
can’t resist the urge
to change things.
This Maverick
was changed to
turbine power in the
simplest way; a
tailpipe was added, a
turbine was installed
in front of the
tailpipe, and a 50-
ounce fuel tank was
placed in front of the
turbine. The original
inlets had
approximately 2
inches removed,
which made them
basically
nonexistent.
This setup
worked because the
jet did fly, but man
was it slow—it flew
maybe 130-140
mph—not up to jet
standards. That was the state of the Maverick when I got it. Now
it’s time for some fun.
The first thing I did was remove the 50-ounce fuel tank and
made two fiberglass saddle tanks to accommodate 32 ounces each,
freeing space for proper inlets. Then I fabricated the new inlets
from fiberglass, in a typical “Y” setup, running back to the turbine.
With that work done, I took the model out to the field for a few
flights. Oh yeah! What a performance difference it makes to
manage the air coming through the airframe back to the turbine.
The Maverick obtained speed and much better vertical
performance.
But alas, all was not well. The saddle tanks feeding directly into
a 2-ounce header tank left me with big air bubble issues. That little
header tank wasn’t up to the task.
Now the Maverick is in the shop for Round Two, to make room
for a bigger header tank, new wings, and a complete repaint.
New wings? Why not? I like them. The stock straight wings on
a Maverick never thrilled me; swept wings look fast. I’ll give you
more information about the conversion as it happens.
Molding your own fiberglass parts is not that difficult; it’s merely
different from working with balsa. Go ahead and try it.
A great place to start is to fabricate some simple flat fiberglass
sheets for use as hatch covers. Use a piece of fiberglass as a simple
mold. Sheets can be made using fiberglass cloth that weighs
roughly 6 ounces per yard, Z-Poxy Finishing Resin (available at
Also included in this column:
• Molding fiberglass
• How to make repeatable
good landings
[[email protected]]
Radio Control Jets Jim Hiller
An oldie gets the total turbine workup
Above: Typical supplies required to work
with composites: resin/hardener, fiberglass
cloth, wax, latex gloves for protection.
Quality materials and tools make the job
more enjoyable.
Below: Proper use of mixes aids in precision
flying. This flap mix is used to trim the
elevator and provide a good glide angle for
landing. A good landing results from a good
approach. Use landing mixes for consistent
approach speed control.
Left: Working with fiberglass is easier than
you might think. These hatch covers went
from plug to mold to complete in only five
evenings. Simple parts such as these are a
great way to develop composite skills.
most hobby shops), a piece of fiberglass as a mold surface, and
quality wax.
Apply a couple coats of wax to the fiberglass and the flat mold
is ready. I usually size my fiberglass sheets using a roughly 12-
by 12-inch piece of 6-ounce-per-yard fiberglass cloth.
A layer makes approximately a .010-inch-thick sheet, which is
good for servo hatch covers. A second layer of cloth will bring
this up to almost .020 inch in thickness, which is good for larger
covers such as for wheel wells.
118 MODEL AVIATION
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Put on a pair of latex disposable gloves
before you mix the finishing resin. That
will keep your hands clean, which is a must
if working with fiberglass and resin is to be
enjoyable.
Position the first layer of fiberglass
cloth in place on the fiberglass mold area,
and then pour mixed finishing resin onto
the cloth. Spread the resin with something
similar to an old credit card. I use hard 1/8
balsa cut to 3-inch lengths, but almost
anything will do.
Work the resin through the cloth. Work
the excess off of the edges or soak up extra
resin with paper towels. Minimize the resin
content, to keep the weight of the finished
panel to a minimum.
Add the second layer of cloth if a
thicker piece is required. Use the same
technique as before, but add the second
layer when the first just gets tacky; the
second layer will bond better.
That’s it. Let the whole thing cure for
24 hours and then remove it from the
fiberglass. Now you have some nice
fiberglass material for hatch covers.
That worked well. How about getting
more complicated and molding shaped
hatch covers that also hide the control
linkage? That’s what I did last winter to my
old BVM BobCat; I fabricated new elevator
hatch covers to hide my oversize JR8411
servos I had installed.
With this level of work, I suggest
ordering your supplies from a composite
supplier to get more specialized materials. I
chose to use West System Epoxy 105 Resin
with the slow 205 Hardener, for maximum
working time. Those products are available
from The Composites Store.
For a wax release agent, I have been
using Rexco Partall mold release wax. One
container will last decades, because of the
few composite parts I make.
A selection of 2-, 4-, and 6-ounce
fiberglass cloth will go a long way toward
providing options as you tackle different
projects, along with CAB-O-SIL that is
used as a filler.
To start this project, I measured the
shape required to clear the servo and
linkage and used this information to make
the plug. Don’t overcomplicate the
materials required to make the mold; use
whatever is easiest to shape.
The base of my plug is a piece of 1/8
plywood, to which I added balsa to provide
servo clearance humps and a 5/16 triangle
balsa piece to clear the elevator linkage that
extends all the way back to the control horn. Phone: (951) 654-8785
Website: airlandhobbies.com
Store: 1357 S. State St.
San Jacinto, CA 92583
Email: [email protected]
My plug is finished with three coats of
the finishing resin. I didn’t even bother to
paint it; I did a final-sanding with 600-
and 1,500-grit sandpaper, to knock off
some dirt, and applied six coats of wax.
Then it was time to make the mold.
I fabricated the mold by putting pieces
of 2-ounce cloth over the plug: one piece
for the linkage triangle and one piece with
a clearance slot for the flat hatch cover. I
used two pieces of cloth so I would not
have to force a single piece of fiberglass
cloth to confirm to this complicated
shape.
Practice getting the cloth to lay against
the plug before mixing the resin and
applying it to the material. Mix the resin
and apply the first layer. An acid brush is
good for applications on small parts such
as these.
Let the resin cure for roughly an hour,
till it gets tacky, then lay on multiple
layers of cloth to make the mold rigid. Let
this mold cure for approximately 24 hours
before removing the mold from the plug.
The mold is complete. It’s time to
make parts.
Apply six to eight coats of wax. For
this hatch cover, I cut two 6-ounce layers
of cloth: one for the deep draw triangle
linkage cover and one for the generally
flat main hatch cover, with the slot where
the deep draw of the mold is. Carefully
lay the fiberglass cloth in place and brush
in the resin.
If things don’t lay properly on the first
piece and you get voids, it’s no big deal;
learn from this bad piece and try again.
That’s what makes having the mold so
great. Now you can make as many
identical parts as you want.
Flying Tips: How do you set up a jet to
make good, repeatable landings? Nothing
is worse than fighting your airplane to
maintain proper airspeed, descent rate,
heading, and runway alignment, all at the
same time.
Varying airspeeds affect the final
flare; higher speed means more float,
more time, inches off the runway trying to
bleed off excess airspeed, or maybe not
enough speed to even flare at all. Get
behind the jet on approach and the landing
gets ugly.
Let’s work on airspeed. You don’t
have to fight it; set up your model
correctly and let it do the work for you.
My jets are trimmed so that when I
enter the traffic pattern for landing, I
don’t have to touch the elevator except to
hold the nose up in the turns. Then when
the aircraft is close to waist height, I
merely pull the elevator to initiate the
landing flare.
That takes the inconsistency out of
gliding the model in for landing, by trying
to hold a consistent elevator to control
angle of attack all the way around the
pattern to landing. Airspeed control
requires a great amount of our workload in
the traffic pattern; inconsistency in
airspeed greatly affects the reaction of the
elevator and the distance of float down the
runway during the final flare to landing.
When I was a kid, I trimmed balsa
hand-launch gliders to glide after gaining
altitude. I do the same for my jets, but
now I can do it with transmitter mixes. I’ll
use my BVM Jets T-33 setup as an
example of how it can be done.
I found that with flap application, the
nose tended to pitch down. So let’s use a
mix to apply up-elevator with flap
deflection.
My radio is a JR 10X, so the mix is
based from a center position or
approximately half flaps. This complicates
things a bit but adds greater flexibility for
trimming to the glide.
I start with the flaps retracted and
roughly 3% down-elevator mix. That
affected my normal trim, so I had to retrim
the elevator for level flight.
This gave a nice, level glide, although
a bit fast at half flaps. It was great for
entering the traffic pattern with minimal
effect to my level flight.
As the landing gear is extended, the T-33
slows. That means it’s time to fully extend
the flaps, and with that I have an additional
5% up-elevator mix. In this state, the T-33
will maintain level flight as it slows to a
comfortable glide speed.
The jet is now gliding with no elevator
input by me, I’ve used the mixes to trim the
T-33 for a perfect glide speed for the landing
approach, and my workload has been
reduced. I can now concentrate on aligning
the model to the runway for that perfect
approach to landing.
That sounds too simple, and it is. It took
quite a few flights before I got the mix setup
where I like it, but it is wonderful now!
One effect of this, though, is that I know
how much fuel is left when I enter the
crosswind leg for landing. Most of the fuel in
this model is forward of the CG, so the glide
speed is an indication of how much fuel is
remaining—more fuel, nose-heavy, faster
glide; less fuel, more tail-heavy, slower glide.
I can use this information to adjust my
initiation for the final flare to landing.
When the T-33 is gliding slower, I will
have less airspeed to fiddle with during
the flare. So I will initiate the elevator pull
slightly lower, to allow a gentle leveling
off during the flare before I run out of
airspeed.
I have one more trick. When the jet is
landing heavy, with more fuel onboard,
which is typical when flying at a jet meet,
my speed board function is programmed to
give me another 2% up-elevator trim, so I
can make another fine adjustment to the
glide speed by adding the speed boards.
Maybe I’ve been flying this T-33 for way
too long; I definitely have it dialed in.
It’s summer and the jets are packed, so
let’s get out and fly. MA
Sources:
BVM Jets
(407) 327-6333
www.bvmjets.com
The Composites Store
(800) 338-1278
www.cstsales.com
Rexco
(800) 888-1060
www.rexco-usa.com
Jet Pilots Organization
www.jetpilots.org