IMPROVED SHEET STUNTER
(Reproduced from
Airborne
magazine August
1990)
This
article is an update on the latest changes, refinements and improvements
to the sheet wing stunter concept, as introduced to me by Frank Coombs
in Airborne No. 59, 1983, and the plans that were part of that article.
Having been associated with Frank for a long time. I have been fortunate
enough to see the gradual development of the flat plate aerobatics model
right from the beginning.
Quite a few examples have been built by various people over the years,
and nearly all of them have been highly successful. The couple that were
less successful had badly warped wings, developed a tank leak or tried
using too small a bellcrank and small horns. I enjoy watching the amazed
expressions on the faces of the uninitiated as they gaze upon one of
these models flying a very presentable FAI aerobatics schedule in the
hands of a good pilot. Make no mistake readers, if built correctly, with
a suitable 2.5 to 3.5 cc motor, you too can fly or learn to fly
aerobatics with one of these simple models. Don't worry about the wings
bending during manoeuvres, as this is normal, and be ready to whip the
model around after the engine cuts to assist with the glide.
I want to tell you about the most important considerations and changes
that have been developed over the years, because there were a few
accidental omissions from the original plans, as Frank will acknowledge.
Firstly, no engine offset was shown, yet this has been found to be very
important for adequate line tension, especially with the smaller
engines. About 5° out-thrust is needed.
Figure
1 shows how it is done. Simply join the engine bearers to the balsa
spacer 4 mm closer than the crankcase width of the engine so that
crankcase clearance cut-outs can be made at an angle. If the bearers are
too far apart, mounting holes can end up beyond the edge of the bearer.
To avoid the model having a funny cranked nose it is possible to
camouflage the 5° offset by making the inboard cut-out much deeper than
the outboard, and adding sheet balsa to the width of the nose on the
inside. In other words, the motor crankcase is biased inboard of the
fuselage centre line except at the spinner. By carefully shaping the
nose and building up with balsa at the right locations, you can make the
offset less noticeable.
Secondly, a pushrod guide is needed to prevent the pushrod from flexing
under load. We use either one mounted to the fuselage at the half-way
position (see Fig. 2), or two spaced apart. They take very little time
to make and install, and are well worth the effort. Make from small
safety pins (see Fig. 3). Cut away the head leaving the eye and two legs
about 16 mm long. Form two feet by bending the legs in opposite
directions about 6 mm down from the eye. Two small slots are cut into
the fuselage side and the guide is pushed into position to about 1 to 2
mm under the surface with a screwdriver. Fix in place with epoxy
adhesive. Two more, similar, guides are used for the lead-outs on the
wing. Mount half way between the bellcrank and the wingtip guide (see
Fig. 2).
The
third omission was the wingtip weight detail. Plenty is needed for good
performance, as the wing has no asymmetry. Use 30 grams of lead or
tungsten tool inserts, let into the outboard wingtip. (Six 2¢ coins
weigh 30 gm.) Position tip weight in line with the centre of gravity.
I should also discuss the use of Kwik Links, as they were mentioned in
Frank's article. He has stopped using them, and so have I after losing
my first sheet stunter on two occasions. The Kwik Links fatigue and
break, so don't use them. Ask Ted Fancher! I now use 2.4 mm piano wire
retained in the horns and bellcrank by small washers. Please don't use
the safety pin guides on the wingtip, as they can wear through the
control line wire or lead-out wire. This has happened, and we now use
the method described later in this article.
The sheet stunter, like most of our models, has grown in size. The
original designs were about 1066 mm span for 2.5 cc motors; probably a
little small for the more powerful 2.5 cc motors. I think that 1090 to
1145 mm span is about right for 2.5 cc engines like the G20D. I had two
versions which were 46 inch span for a Super Tigre G20-15 diesel. These
were good except that my old G20 protested somewhat at pulling around
that load. It tended to load up during manoeuvres although other G20
diesels have been very good.
For appearance and extra smoothness in flight, the fuselage became
longer or, more specifically, the flap hinge to elevator hinge dimension
was increased. I think the original was 317.5 mm. My later 2.5 cc
versions were 343 mm and Frank's Classic Yardstick was 406 mm. To
compensate for the increased weight at the back, the nose was also
lengthened. The 4.8 mm (3/16 inch) thick wing became inadequate for the
larger versions, so I use 6.3 mm (1/4 inch) instead, still retaining the
4.8 mm thickness for the flaps, tailplane and elevator. On my later
versions I reinforced the centre section with epoxy resin and 1.5 oz
fibreglass cloth out to the end of the spars, top and bottom. This is
done before the wing is joined to the rest of the model, and it adds
considerably to the stiffness and strength.
Frank devised another method which works very well on his stunter. Lay
strands of carbon fibre at the wing centre section, top and bottom, with
epoxy resin. After this has cured, epoxy a diamond shaped piece of .4 mm
ply directly over the carbon fibre, one on top and one underneath. The
strands of carbon fibre run span-wise and are placed so that they
approximately match the shape of the plywood. All this strengthening is
positioned half way between the leading and trailing edges, as shown in
the diagram. The ply pieces measure 350 x 40 mm and can be feather
sanded on the edges before gluing in place for a neater job (see Fig.
4).
Perhaps
the single most important refinement has been the updated control
system. The latest one used by both Frank and me is almost the same as
that used by Brian Father. Brian presented an article in issue 77, 1986,
describing his system in some detail. Please refer back to it for more
specific information. I retain the 25 to 40 mm flap horn and 40 mm
elevator horn, from 1.6 mm steel sheet. The bellcrank became boomeranged
so that it can be mounted very close to the fuselage for correct
geometry. In other words, it is mounted so that the flap push rod is
parallel to the fuselage instead of at an angle as before (see Fig 2).
Looking
at the side view, I set up the system as in Figure 5. This is to ensure
equal controls both up and down. Look at Figure 6. This is the bellcrank
that I use. The 1.5 mm epoxy board (circuit board material) that it is
made from is very strong and wear resistant. Quite easy to cut out, too.
Epoxy board is available from electronics stores like Dick Smith and
Tandy Electronics. Nominal dimensions of the bellcrank are 112 x 25 mm.
I make the wingtip lead-out guide from this material as well. I cut out
a rectangle 40 x 15 mm, drill the holes and glue this into a slot that
has been cut into the wing. I feel that getting the control system
right, not just on this type of model but on any model, is of paramount
importance. It can make all the difference between having a successful
model and courting disaster.
Too
many people, for too many years, have been installing the same old
inappropriate control system in every model they make; almost as an
afterthought. Sure, vary the ratios of the bellcrank and horns if you
like more or less sensitivity, but keep the horns long, the bellcrank
large and the pushrod flex free. With Brian Eather's control system you
will be amazed at how easy it is to control your model with precision,
and how smoothly it will fly.
Finally, a few comments about my latest model, shown in the photos. It
is the largest one I have made, at 1193 mm span, 238 mm root chord, plus
63.6 mm flaps at the root. Tailplane is 597 mm span and 76 mm at the
root. Elevators are 635 mm at the root. Hinge to hinge is 419 mm and the
nose is 229 from leading edge to spinner backplate. Length of fuselage
from spinner tip is 1035 mm. The fuselage is made from a full length
piece of 9.5 mm (3/8 inch) x 76 mm medium weight quarter grain balsa
tapered behind the tailplane leading edge.
The design is virtually a copy of Frank's Classic Yardstick, only
larger, and is powered by an Enya 21X converted to diesel. This engine
seems to suit very well, as it is quite powerful. I run it with a
home-made tongue muffler and Bolly 10 x 6 prop cut down to 9-3/4 inch
diameter, thinned and balanced. The model flys on 18 metre long lines.
The entire airframe. except parts of the fuselage, is finished and
decorated with coloured tissue. The finish coat of clear two part
polyurethane is sprayed on. This model is rather extreme, with a very
long fuselage; however, it does fly very smoothly, noticeably so
compared with my old model.
It is recommended that builders start with an original plan using the
same general construction, and update the detail as described. You have
much to gain with one of these easy-to-build and inexpensive models.
Substituting a standard small wedge tank for the front feed,
inboard-mounted, chicken-hopper tank is detrimental. The recommended
tank is drawn on the Airborne plan. If you eventually have to replace
the model due to crash damage, then simply salvage all the hardware. The
control system, undercarriage and fuel lank can be re-used in your next
sheet stunter. Learn to fly the aerobatics schedule with a model that is
truly capable, and enjoy the experience.
The original sheet wing stunters are available as Airborne Plans:
Olympus, Mini Nobler and Mini Bandolero; at $13.00 each, including
postage, from Ropomod Productions, Box 30, Tullamarine, Vie., 3043.
