Free Flight Duration - 2011/02

130 MODEL AVIATION
In F1B the first-round max was set at 270
seconds. Four fliers of 24 maxed out. In the
flyoff, three attained the 300-second max to
make the team; Bill Booth finished fourth and
will be the alternate.
Of 17 fliers in F1C, four maxed, each with a
total of 2,760 seconds (two rounds of 300
seconds plus 12 rounds at 180 seconds). Henry,
Randy, and Mike made the 300-second flyoff
round; Faust Parker dropped 3 seconds and will
be the alternate.
The World Championships will be held May
2-9. More information will be available on the
organizer’s Web site.
As I was preparing this column, I received
word that Henry Spence passed away roughly a
month after making the US F1C team.
Six Instead of Four: For years most FF models
have used some variation of the polyhedral
wing with four panels. Compared to the Vdihedral
wing, the polyhedral offers theoretical
as well as practical advantages.
Theoretically a polyhedral wing more
closely approaches a more efficient elliptical
dihedral. The shorter individual panels make
building a bit easier, especially for long wings that would require
materials to be spliced in a V-dihedral.
In addition, a four-panel wing allows you to use lighter wood in the
tip panel for reduced weight on a constant-chord wing. Tapered or
rounded tip panels are another option.
You can construct a wing with elliptical dihedral, but it requires a
dedicated form to build the wing upon, and the LE, TE, and spars must
be laminated to match the curve. A 1/2A design with elliptical dihedral
was published in the mid-1950s; I doubt many were built.
THIS PAST FALL a nine-man team was selected to represent the US
at the Free Flight World Championships, to be held this May in
Embalse, Argentina. The members are:
• F1A Towline Glider: Brian Van Nest, Robert Sifleet, Jim Parker
• F1B Wakefield Rubber: Alex Andriukov, Robert Tymchek, Dave Saks
• F1C Power: Mike Roberts, Randy Secor, Henry Spence
Contestants flew seven rounds in each class on each of two days. For
F1A the first-round max every day was 240
seconds; the rest of the flights were flown to
a 180-second max. Brian Van Nest was the
only flier to max out, Bob Sifleet dropped 5
seconds, and Jim Parker finished third with
a total of 2,580 seconds of a possible 2,640.
Junior team member Miles Johnson
finished fourth, 10 seconds behind Jim, and
will be the alternate. A total of 17 fliers
participated.
FF World Champs team selected for 2011
[[email protected]]
Free Flight Duration Louis Joyner
Also included in this column:
• Advantage of a six-panel wing
• The radio DT
• Make our most basic building
material better
• The video magic of Alan
Abriss
Right: Models with six-panel wings, such as
Eddie Vanlandingham’s Gorban F1G
Coupe, are increasingly popular for FAI
events.
Left: Detail of the six-panel wing shows
how structure changes from carbon-Kevlar
D-box on the main panel to carbon-fiber
tubes on mid and tip panels.
Henry Spence, shown at the 2008 Nats, was one of three F1C Power fliers to max out at the
fall Team Selection Finals. He died approximately a month after making the US team.
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With materials in hand, I looked at six-panel
wings in earnest. The first panel is normally
nearly 46%-48% of the wing semispan, the
middle panel is approximately 35%-37%, and
the tip panel is 17%-18%.
Wing chords on Eugene Verbitsky’s #72
F1C Power model went from 100% at the root
to 87.5% at the first break, to 62.5% at the
second break, and down to 37.5% at the tip.
A root chord of 160mm (close to 6.4 inches)
gives a tip chord of 60mm (2.4 inches). For my
F1G I intended to use a root chord of 120mm
(approximately 4.72 inches); using Eugene’s
37.5% for the tip chord would yield a tip chord
of 45mm, less then 2 inches.
That might work fine for a fast-gliding F1C
model, but I was afraid that the Reynolds
number at the tip on a slow Coupe would be
lower than I felt comfortable with. So I fudged
the tip chord up to 70mm (roughly 2.75 inches).
The other design decision was how to
arrange the wing taper. The first design had a
straight TE, set at 90° to the fuselage. But that
would require the main spar to angle back
slightly, making installing the wing wire tubes
problematic with the narrow spar I planned on
using.
A change to a 90° spar in the main panels
would simplify the tube situation. Then I
decided to continue the line of the TE of the main panel in a straight line
that angled forward slightly. The spars and LEs of the middle panel and
tip panel would angle back as needed.
Look for updates on construction in future columns.
RDT: Originally developed by Danish F1B and F1C World Champion
Thomas Koster as a safety measure to save an off-pattern model from a
crash, the radio dethermalizer (RDT) has gone mainstream.
February 2011 131
Eugene Verbitsky’s #72 F1C is typical of the high-aspect-ratio FAI
models that use six-panel wings. The design was featured as a
Model of the Year in the 2005 NFFS Symposium.
Paul Andrade now uses RDT on all of his models, including his Mulvihill Rubber design.
The system has become mainstream, and all FF pilots should consider employing it.
For more modern wings with carbon-fiber D-boxes, a curved mold is
needed for each wing half in addition to the curved building board.
Although six-panel wings with two dihedral breaks on each side have
been around for a number of years, their popularity has been increasing—
especially in FAI events. One advantage of using six panels is that it
allows the dihedral to more closely approach elliptical dihedral. By using
two dihedral breaks on each wing half, the angle between two adjacent
panels can be kept lower, to reduce interference drag at the break.
Interference drag is caused by air flowing over the upper surface of the
adjacent panels having to crowd into less space spanwise as it flows back
toward the wing high point. Henry Cole and Lee Hines have suggested
that keeping the dihedral angle between adjacent panels below 15° helps
reduce interference drag. With a six-panel wing you can use two shallower
dihedral angles to get the necessary rise at the tip.
Six panels also allow a triple-tapered planform that more closely
matches an elliptical planform while using straight components. Now FF
models typically use something that resembles a half ellipse, with a
straight TE, rather than the traditional full ellipse of a Spitfire wing. One
advantage of an elliptical wing or a six-panel wing is that the tip chord can
be quite small, to reduce drag.
Perhaps the biggest practical advantage of a six-panel wing is that you
can build a longer wing using available components such as D-box skins,
spars, etc. And you can do it on an existing building board or
undercambered wing fixture.
Any type of construction can be used for a six-panel wing. And, as
with the Gorban F1G model shown, different construction methods can be
used in one wing.
So far the only six-panel wings I’ve built have been on tip-launch
gliders, where adding a second dihedral break on each side is quick and
easy. Building a carbon D-box six-panel wing didn’t seem worth the extra
effort. However, the need to build a new pair of F1G Coupe wings from
one set of F1B Wakefield D-box skins did provide the impetus to try sixpanel
wings.
I had picked up the set of carbon-fiber skins approximately 10 years
ago. Since I already had three serviceable F1Bs, I studied how I could turn
the skins for one F1B wing into enough material for two F1G wings.
The D-box skins I had measured 530mm for the main panels and
448mm for the tip panels. After calculating all options, I went with a sixpanel
wing for each Coupe.
Each of the two 530mm panels would yield two 260mm main panels
for each Coupe wing. Cutting each 448mm tip panel in half provided two
220mm midpanels. That left the tips, which needed to be close to 100mm
each in length.
At first I planned on using a more traditional balsa structure, but after
some rummaging I found a carbon-fiber skin that I had made some years
back. The length was 370mm—just long enough from which to get four
90mm tip D-boxes. An angled tip would take care of the rest.
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More and more fliers are using this in all
types of FF aircraft. RDT allows safe testflying
on small fields, terminating a test flight
to make needed trim adjustments and
preventing an out-of-trim airplane from
crashing.
At a contest, RDT can be used to DT a
model early if it is booming out in a big
thermal, preventing a fly-away. It can also be
used to spot-DT the aircraft on the field rather
than in a tree, pond, or cornfield.
RDT systems work in a similar manner to
remote garage-door openers. A transmitter that
the flier carries sends an encoded radio signal at
the push of a button. A small receiver in the
airplane then activates the DT.
Some RDT systems are designed to be
incorporated into an electronic timer system.
Since the RDT uses the model’s battery and DT
servo, weight gain is only 1-2 grams. Standalone
RDT systems require a microservo and
battery, pushing weight up to nearly 8 grams.
Note that if a stand-alone system is used in
conjunction with a mechanical timer, some sort
of mousetrap system is needed to allow either
the timer or the RDT receiver to activate the
DT.
Ken Bauer produces one of the more
popular systems: the Airtek Radio DT (RDT).
His basic system is compatible with Red Magic
and Smart Magic electronic timers.
Several stand-alone versions are also
available; one uses a microservo for activation
and the other utilizes a pager motor. The standalone
systems do not include servo or pager.
An Airtek transmitter costs $175; the
receivers are $125 each. Visit Ken’s Web site
for detailed information.
As with airplane trackers such as the
popular Walston unit, many modelers have one
or two RDT receivers that they can switch from
aircraft to aircraft. This is usually easier with a
stand-alone system.
Better Balsa: No, I don’t have a new source
for 4-pound wood with perfect C-grain. Rather,
I have a few suggestions for how to make our
most basic building material better.
As are most woods, balsa is stronger when
bending lengthwise than across the grain. So
instead of using a single thickness of balsa for a
former or sheet pylon, make your own plywood
to get the grain running both ways.
A look through some of Frank Zaic’s old
Year Books will show that this is nothing new.
Gerald Ritz’s Hot Head high-thrust Power
model used a three-ply pylon with a 3/16-inch
balsa core and 3/32-inch balsa sides. Although
the outer plies were glued up at 90° to the
thicker core, the whole thing was rotated so that
the grain of the sides followed the angle of the
forward-raked pylon.
For formers, a two-ply arrangement could
be used to give equal strength in both
directions. Here it is easier to make a sheet of
balsa plywood and then cut individual parts.
For wood-to-wood joints I prefer Titebond,
but epoxy would also give you the necessary
open time to assemble and clamp the layers. To
keep adhesive weight down, spread the glue
evenly and then blot with a paper towel to
remove excess. Clamp the plies between two
flat surfaces, protected with waxed paper or
plastic wrap, and allow to dry overnight.
Another use for balsa plywood, detailed on
132 MODEL AVIATION
Martin Gregorie’s excellent Web site, is for ribs
in a carbon-fiber D-box. He uses five plies of
1/16 balsa for the end ribs of each panel and three
plies for the intermediate ribs. Martin’s site
contains a lot of information about wing and tail
construction using carbon-fiber components.
Traditional Rubber model nose blocks are
made from multiple layers of thin balsa sheets,
usually 1/16-1/8, glued with alternating layers
oriented with the grain at right angles. The front
and back of the stack are often thin plywood.
But there is always the temptation to
substitute a single piece of thicker sheet for the
stack of balsa. It’s a mistake that many of us
have made, but only once.
The thick sheet lacks the compressive
strength of the alternating-grain stack and easily
compresses under the load of the wound motor.
Add some moisture and a crash or two, and the
nose block needs to be replaced.
An alternative to the stacked-balsa nose
block is to cut a block of the desired length with
the grain running fore and aft. It’s quick and
easy with a power saw but difficult to do
accurately with hand tools.
If you can’t find good C-grain balsa for ribs
or fuselage sides, consider covering both sides
of a sheet of A-grain with tissue and then
cutting the parts. The tissue will provided
stiffness across the grain and added strength
against splitting.
The dope used to attach the tissue will also
give some measure of moisture resistance. You
could use lightweight fiberglass cloth instead of
tissue.
In either case, make sure that the covering
material is oriented the same way on both sides
of the balsa to prevent warping. This technique
is also helpful for Rubber model fuselages, to
prevent absorption of rubber lube.
Instead of using a single thick sheet of
plywood, consider laminating a balsa core
between two sheets of thinner plywood. It
worked for the all-wood de Havilland Mosquito
fighter/bomber of World War II.
I’ve used that technique recently for the
pylon on some F1G Coupes. The pylon sides
are light 1/16 balsa; the structural tie between the
wing wire and the composite motor tube is
actually a thick balsa-and-plywood former.
The former is made from 1/4 balsa with 1/64
plywood glued on the front and back; on each
side a small piece of 1/32 plywood is used to
carry the concentrated load of the aluminum
tube that holds the wing wire. The bottom end
of the piece is shaped to fit the round motor
tube, to ensure a secure glue joint when the
finished pylon is epoxied onto the tube.
For I-beam type spars using either spruce or
carbon fiber for the top and bottom chords,
consider using two layers of balsa oriented
90% to each other and 45% to the spar. This
will better carry shear stresses than a single,
vertical-grained balsa web.
An easier but more expensive solution is to
use a carbon-balsa-carbon spar with the balsa
running spanwise and the same width as the
carbon-fiber top and bottom flanges. Then slip
a piece of carbon-fiber braid over the spar, pull
it tight at both ends, wet it with epoxy, blot, and
vacuum-bag on a flat surface. The braid
provides shear strength. This technique is also
detailed on Martin Gregorie’s site.
Nats Video: Alan Abriss has again worked his
video magic and compressed a week of the
Outdoor Nats into two hours of video. And this
year there’s an added treat: a video shot from a
model in flight.
The view is to the rear, so you see the
ground dropping away rapidly in the climb.
Then you get a 360° view of the International
Aeromodeling Center site in Muncie, Indiana,
as the airplane circles in flight.
The video is $20 plus $4 for shipping. You
can order it by sending a check to Alan Abriss
of Homegrown Television Productions or order
online with a credit card. MA
Sources:
2011 FF World Championships
www.embalse2011.com
Gorban F1B & F1G models and components:
Bob Tymchek
[email protected]
Airtek Free Flight Electronics
2306 Turquoise Circ.
Chino Hills CA 91709
http://bit.ly/hRCnGy
Martin Gregorie
www.gregorie.com
Homegrown Television Productions
94-20 66th Ave. Ste. 1G
Rego Park NY 11374
www.homegrowntv.com
National Free Flight Society
http://freeflight.org
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