Free Flight Duration - 2009/02

THE 2009 FF World
Championships will
take place this
summer in Croatia.
In October, the US
Team Selection
Finals took place in
Lost Hills,
California.
The three team
members in the F1A
Towline Glider event
are Ken Bauer, Lee
Hines, and Steve
Spence. F1B Wakefield Rubber competitors are Alex Andriukov,
Walt Ghio, and Charlie Jones. The F1C Power team consists of Don
Chesson, Dick Mathis, and Gil Morris.
It’s an experienced team. Seven of the members have
represented the US in previous World Champs. Alex Andriukov has
represented the US, Ukraine, and the Soviet Union, winning the
World Championship three times.
Don Chesson and Charlie Jones are the only first-time team
members, but both have considerable experience in the America’s
Cup circuit. Five of the team members have been inducted into the
National Free Flight Society’s Free Flight Hall of Fame: Alex
Andriukov, Walt Ghio, Lee Hines, Dick Mathis, and Gil Morris.
The team is a bit more geographically diverse than in the past.
Also included in this column:
• Are two propellers better
than one?
• 2009 Sympo
• Beautifully joined balsa
146 MODEL AVIATION
Only four are from California (Alex, Ken,
Walt, and Lee), two (Charlie and Gil) are
from Ohio, and two (Don and Dick) are
from the new hotbed of F1C: Louisiana.
Steve Spence is the lone Texan.
Unlike many past finals, this one
required no flyoffs. In F1A, Ken Bauer
was the only one to max all the rounds in
the two days of flying. In F1B, Walt Ghio
and Charlie Jones maxed out. In F1C, the
top three maxed to make the team.
Don Chesson said:
“I was prepared for and very much
hoping for a big flyoff and confident that
this would produce a fine team composed
of any of the F1C participants. The overall
quality of flying is so high, anybody can
be in the winner’s circle. The finals were
tough, with very tricky air in the fifth
through seventh rounds. I was very lucky
to survive.”
Are two propellers better than one? One
of the more interesting airplanes at the
Team Selection Finals was Gil Morris’s
F1C Power model. As are many of the
latest generation of F1C aircraft, it was a
folder—but it’s a folder with a difference.
Gil calls it a “folder-flapper.”
He said:
“The wing chord is 51/8 inches and the span is 110 inches. The
foam core has a chord of only 4 inches and is covered with one
piece of bias-cut
carbon cloth so that
the last inch of the
cord is only two
thicknesses of
epoxy-impregnated
carbon cloth.
It’s a sort of
Jedelsky airfoil, and
when the wing is
folded, the two
trailing edges
collapse to form a
teardrop airfoil
thinner than the sum
of the two airfoils
when free. The
flaps drop down
when the wing is
opened, to provide
an undercamber in
glide.”
But the really
interesting aspect of
Gil’s model is the
propeller. Actually,
that is propellers—
plural.
As do many F1C
models, this one
uses a geared drive
to allow the engine
to turn a larger,
more efficient
propeller. The GK
Hummer 2.5cc
engine that Gil uses
Gil Morris primes the geared Hummer engine on his folding-wing F1C. This will be his
third World Champs team; he has won an individual Bronze Medal and two team Golds.
Aram Schlosberg photo.
Gil’s unusual two-propeller setup uses a
12-inch-diameter geared propeller and a
smaller direct-drive propeller. The smaller
propeller reduces vibration and improves
airflow to root air of the larger propeller.
Chesson photo.
has 4:1 planetary gears that reduce the shaft rpm from roughly
32,000 to 8,000 at the propeller. This allows the engine to swing a
12-inch propeller instead of the typical 7-inch propeller used with a
direct-drive engine.
The GK Hummer is available from Doug Galbreath. For more
information about geared F1C engines, see my Duration column in
the March 2002 MA.
But Gil has added a second, smaller nonfolding propeller
running at shaft speed. He had first tried the two-propeller setup as
part of a static-thrust test detailed in the 2006 Symposium Report.
He wrote:
“These tests were made on the hunch that the larger prop with its
higher pitch was stalled in the root area under static conditions and
that the small prop would feed a higher velocity of air to the root
area and thereby reduce the angle of attack of the large prop and
make it more effective.”
Results with the two-propeller setup were promising, yielding 98
ounces of static thrust—approximately 4.2% more than with the
single geared propeller. Propellers used were a 12.7-inch-diameter x
12-inch-pitch geared folder and a 5.7-inch-diameter x 3-inch-pitch
direct-drive, nonfolding propeller.
Gil said:
“If you read the prop thrust article you would note my comment
that the runs with the small turbo props were the smoothest. I had
forgotten that last spring I got fed up with the inherent vibration of
the geared engine and decided either to solve the problem or go
back to direct drive. They just aren’t fun as is. Then I recalled those
thrust tests and I focused on the small prop idea; they make the
geared engine run silky smooth.
“I think the reason is that without the small prop all the flywheel
action comes from the big prop transmitted through the gears and
that means chatter. With the addition of the small prop, much of the
flywheel action is directly attributed to it. Also the root area of the
big prop is stalled because of its 12-inch pitch and I suspect there is
buffeting.”
An unexpected advantage of the extra propeller is that it
eliminates the need for a propeller brake.
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February 2009 147
“With the added rotational drag of the
little prop, there is no discernible
rundown,” said Gil. “Without a brake, the
big prop hits compression and folds
nicely.”
Although the small propeller doesn’t
fold, Gil doesn’t think that the extra drag is
significant. But the fixed blade is
vulnerable to damage on landing, requiring
replacement every three or four flights.
You can see additional photographs of
Gil’s model at the FlyingFoam.com Web
site; click on custom projects.
2009 Sympo: John Lorbiecki will be
editing the 2009 Symposium Report of the
National Free Flight Society (NFFS). As
have past editions, this one will include
technical articles about all aspects of FF
design, construction, and flying. In
addition, the Models of the Year and new
Hall of Fame members will be profiled.
The 2009 Sympo will be available in
mid-summer. Look for additional
information in the May Duration column
and on the NFFS Web site. If you are
interested in contributing a paper to the
2009 Sympo, please contact John via email.
Going Wider: Balsa comes in standardwidth
sheets; 3 inches is the most common.
But what if you need something wider?
You can usually find 4-inch sheets and
occasionally 6 inches, but the selection of
wood grain and density may not be exactly
what you want.
The solution is to glue together two or
even three narrower sheets to get the width
you need. This also allows you to vary the
density or grain across the width of the
sheet.
On a tip-launch or Catapult Glider, you
could use a wide piece of soft balsa for the
rear portion of the wing and a narrower,
harder piece from the high point forward.
Some people suggest reversing that, using
the less dense wood in the front, to reduce
weight, and the harder wood in the thinner
aft section, to add stiffness. Another
variation is to use three or even four pieces
to put strength where you need it.
On some Glider wings, I have adjusted
the wing outline so that the airfoil high
point is a straight line. Two narrower sheets
are glued with the joint along the highpoint
line.
After shaping and sanding, I cut a small
triangle out of the center-section, to give
the typical straight-TE layout. Next time
I’ll add colorant to the adhesive, to make
the glue joint easier to see when sanding.
The quick-and-dirty way to join two
sheets of balsa is to shove the pieces
together and squirt some cyanoacrylate
along the joint, trusting it to fill in any
gaps. But this can cause several problems.
Cyanoacrylate adhesives have the habit
of hardening any surface fuzz or balsa dust
into the consistency of concrete. Good luck
with sanding that smooth.
The thin adhesive will wick to the sides
of the joint, creating a stiffer section along
either side of the joint. This can cause
problems if you need to bend the wood for
a rolled motor tube or tailboom. You’ll end
up with a slight flat along the joint.
But the biggest problem with the quickand-
dirty approach is that you can
introduce stresses into the joint that can
cause warps. Any time a piece of wood is
forced into a position and then glued, the
wood is going to want to spring back to its
original position.
The solution is to take the time to
carefully fit the joint and then glue it with
the minimum amount of adhesive. The goal
is a joint that is as strong as the wood and
virtually invisible.
To do that, I straighten and square the
edges to be joined. The factory edges are
usually square but seldom straight. There
are often saw marks along the edge that
will prevent a tight joint.
I use a wide aluminum straightedge that
is roughly 1/4 inch thick. Mine is a Swanson
CG100, available at most big-box home
centers for approximately $25.
This straightedge was designed as a
guide for cutting 4 x 8-foot plywood sheets
down to size and consists of two identical
straightedges that are roughly 49 inches
long. You could keep one long and cut the
other to a shorter, more manageable length.
For a cutting surface, I use 1/4-inch
medium-density fiberboard (MDF), also
from the home store. Compared to pine or
plywood, MDF has virtually no grain,
which could cause the blade to wander.
Thick cardboard would also work. Make
sure that every cut is over a fresh section of
the cutting board, which should rest on a
piece of 3/4 plywood.
Position the balsa sheet on the cutting
board, and then place the straightedge on
the balsa. Be sure to use the thicker edge as
the cutting guide. Carefully adjust the
straightedge so it follows the grain of the
wood and is at least 1/16 inch in from the
edge of the balsa at all points. Clamp in
place.
For clamping, it helps to have the
cutting board and plywood the same
approximate length as the straightedge. If
necessary, shim under the clamps to
prevent the straightedge from bowing up.
Carefully cut the balsa using a sharp
single-edge razor blade held flush against
the straightedge, keeping the blade vertical.
Make a series of progressively deeper cuts;
a good rule of thumb is no more than 1/16
inch per pass with soft balsa and half that
with harder balsa.
If you are cutting anything thicker than
roughly 3/16 sheet, the thicker backing on
the razor blade will hit the straightedge,
limiting the depth of the cut. You can
usually slide the backing partially off to
allow a deeper slice, but be careful. The
safer option is to use a modeling knife to
finish the cut.
After cutting a straight edge on both
pieces of balsa to be joined, square the
edges with a simple sanding fixture. This
consists of a sanding block, our faithful
aluminum straightedge, a few strips of
MDF, and clamps.
The sanding block is a piece of 2 x 2
lumber that is approximately 10 inches
long. Make sure that the 2 x 2 is square.
Select a smooth work surface, such as a
piece of 3/4 birch plywood or MDF, and add
a piece of 1/4 MDF to the top. Position the
balsa on top of that, with the straightened
edge overhanging 1/8 inch or so.
Add a second piece of 1/4 MDF, again
leaving roughly 1/8 inch of balsa showing.
Position the straightedge on top, with
approximately 1/32 inch of balsa showing
when viewed from above. Clamp
everything in place.
Cut a couple strips of adhesive-backed
150-grit sandpaper slightly wider than the
thickness of the balsa. Attach these pieces
to the 2 x 2 block so that the sandpaper will
only touch the balsa.
Begin sanding until the 2 x 2 block
touches the straightedge all along its
length. The result will be a straight, square
edge. Repeat on the second sheet.
Check the fit; temporarily join the
sheets with a few short pieces of tape
applied lightly across the joint. Hold the
joined sheets up to a bright light; any gaps
will be readily apparent. Be sure to check
from both sides. If necessary, repeat the
sanding step until the fit is perfect.
Now to glue it together. Ambroid has
been my adhesive of choice in the past, but
I recently switched to Titebond II because
it gives a bit more open time. Either will
work; the secret is to use the bare minimum
of glue. Using too much adhesive will add
weight and make the joint harder to
smooth.
Spread a generous bead of glue along
the edge of one sheet and spread it evenly
with a scrap piece of balsa. This is still way
more adhesive than is needed, so blot the
joint with a piece of paper towel to remove
any excess. Then wipe off any glue on the
face or back of the sheet, and press the two
sheets together on a smooth, flat surface
that is covered with waxed paper.
Be sure to press the sheets down firmly
so that the faces of both sheets are flush.
Weight or pin down until dry. Then
carefully sand both sides of the sheet using
a sanding block. MA
Sources:
Doug Galbreath Free Flight Model Airplane
Stuff
(530) 757-6058
www.the-printer.net/DookCat.html#
hummer
FlyingFoam.com
http://flyingfoam.com
NFFS
www.freeflight.org
John Lorbiecki
[email protected]
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