Free Flight Duration - 2010/02

126 MODEL AVIATION
How many blades on a propeller is right for you?
[[email protected]]
Free Flight Duration Louis Joyner
Also included in this column:
• Carefully carving propellers
• Coupe survey
• Nats video available Henry Spence’s F1C Power model uses a geared 2.5cc engine to turn a larger, more
efficient propeller. Four blades allow the diameter to be shortened slightly for clearance
of the landing skid and wing.
Wings fold to reduce drag during the highspeed,
near-vertical climb and open to
double the span for the glide. The model is
from a former World Champion, Artum
Banenko of Ukraine.
You can carve accurate, repeatable
Rubber-model propeller blades from 1/2
balsa planks using fixtures. The 1/4 plywood
pattern (top) is for cutting the 1/2 sheetbalsa
blade blank (center) and 3/4 plywood
fixture base (bottom). A scientific
calculator and dial calipers are used to plot
curves on 1/32 plywood fixture sides.
The fixture (right) is used to locate 1/32
basswood strips along LE and TE of 1/2
balsa blank. After removal from the
fixture, the blank is turned over and the
underside is carefully carved and sanded to
desired undercamber. Laser-cut steel
templates are used to check upper and
lower airfoil shape.
FF MODELS HAVE traditionally used twoblade
propellers. Some rubber-powered
models from the 1930s through the 1950s
used single blades, often to simplify
construction (half the work) rather than for a
performance advantage.
For Gas models, manufacturers could
machine a two-blade propeller from a
relatively small piece of maple. When nylon
and other plastic propellers came along some
50 years ago, the two-blade tradition was
carried on.
For full-scale, piston-engine aircraft,
three-, four-, and even five-blade propellers
were developed. This was to absorb the
increasing power of engines without
increasing diameter and compromising
ground clearance.
The Supermarine Spitfire prototype first
flew in 1936 with a two-blade, fixed-pitch
wooden propeller turned by a 990-horsepower
engine. By the end of World War II, the
Spitfire Mk XIV used a five-blade propeller
turned by an engine that produced well more
than twice the prototype’s horsepower.
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February 2010 127
Components for F1G Coupe propeller fixture includes (from
bottom) 1/4 plywood pattern marked with radii stations, 1/32
plywood sides with TE and LE curves cut, 3/4 plywood base, and
stack of 1/2 balsa blade blanks.
A simple fixture holds a propeller blade blank in position for
drilling its mounting hole.
In the last decade, there has been some
revival of interest in single-blade propellers.
Rudolf Harbinger’s Carbonator F1K model,
detailed in the 2000 National Free Flight
Society (NFFS) Symposium, uses a highaspect-
ratio, single-blade propeller.
Rudolf’s balsa-and-carbon-fiber propeller
has a diameter of roughly 10.25 inches. He
claims an 8% increase in efficiency compared
to a conventional two-blade propeller.
A single blade’s efficiency advantage
comes from being able to increase blade
diameter using the same amount of power.
For propeller diameter, bigger is generally
better.
Rubber motors provide plenty of torque to
slowly turn a big propeller. Outdoor Rubbermodel
propellers typically average 600 rpm;
Indoor-model propellers often turn one-tenth
as quickly.
Compared with Gas-model propellers,
Rubber propellers are more efficient. Slowly
moving a lot of air takes less energy than
moving a small amount of air at high speeds.
For Gas models, modern engines develop
maximum power at extremely high
revolutions but low torque. A 7-inch-diameter
propeller turning 30,000 rpm affects a
relatively small piece of air.
Gearing down the engine allows it to turn
a much larger-diameter (13-14 inches)
propeller more slowly and more efficiently.
The slower-turning propeller also avoids the
problem of its blade tips approaching the
speed of sound.
But these bigger propellers can create
clearance problems—not ground clearance,
since the models are hand-launched vertically.
Instead, the difficulty is with folding the big
propeller for the glide.
A blade hanging up on the wing or
catching on the landing skid can act like a
forward rudder, messing up the glide. A
solution is to use a double-hinged
arrangement that moves the blade pivot point
forward, reducing the chance of a blade
catching on the wing.
Another approach is to use three or four
blades, with a slightly reduced diameter. At
the 2009 Nats, Henry Spence flew an Artum
Banenko model with a geared, four-blade
propeller.
“It clears the skid,” said Henry.
Careful Carving: Yes, some people still
carve Rubber-model propellers. Even though
the rule requiring the flier to make the
propeller has been lifted for most events,
carving your own propeller offers some
benefits.
Doing it yourself allows you to experiment
with various propeller designs and to produce
a propeller for an unusual or less-popular
design for which no commercial blades exist.
Besides, there is the reward of making
something yourself instead of simply writing a
check for it.
However, carving an accurate, matched set
of propeller blades can be daunting. There are
basically three approaches.
1. This approach is popular for many older
designs. Begin with a rectangular block that is
tapered in front view from the midpoint of
each blade to the center and then tapered in
side view from the midpoint of each blade
toward the tip.
The blades are carved from the back to the
edges of the block. Then a paper pattern is
used to mark the outline of each blade. They
are trimmed to this shape, and the front face is
carved to airfoil shape. (A variation of this
method uses an X-shaped block.)
2. This technique uses a block that is
carefully cut to both the front and side profile
of the finished blade, and then the back face
of the blade is carved away. (With all carvedblade
methods, the rear face of the blade is
carved first.)
While the profiled-blade method does
produce an accurate blade in both outline and
pitch, it depends on careful cutting for
accuracy. And as does the first method, it
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requires a large block of balsa.
3. This method utilizes a balsa plank,
typically 1/2 inch thick. Rotating the blade
from the design pitch allows it to fit within the
thinner plank. For each blade, the plank is cut
to outline shape. Then curved guidelines are
marked along the LE and TE of the plank. The
plank’s rear face is carved away to the lines,
and then the front face is carved and sanded.
But accurately laying out the curved
guidelines is difficult, and carving to a pencil
line on the balsa also can introduce additional
error. In the late 1990s, a Danish modeler,
Jørgen Korsgaard, developed a plank method
that uses a fixture to accurately locate the
curved line.
I used Jørgen’s method to carve some F1B
propeller blades then and recently produced a
new fixture for a set of F1G blades. Accuracy
and repeatability are the two main advantages
of this method; the biggest disadvantage is that
a separate fixture is required for any change in
blade outline or pitch distribution.
Following is a step-by-step description of
Jørgen’s method.
• Determine the blade chord and blade angle at
various radii out from the center of the
propeller—typically every inch. (Blade angle
is not the same as pitch. The angle is the actual
angle of the blade. Blade angle typically
increases from the tip to root, often by as much
as 40° or more. Blade pitch will be constant at
each point along the blade for a helical
propeller and vary slightly at different radii for
nonhelical-pitch-distribution propeller
designs.)
Many recent published designs provide
chord and blade-angle information in tabular
form. For older designs, you can calculate
blade angles from the width and thickness of
the rectangular block.
Using a scientific calculator (which costs
less than $10 at Wal-Mart), divide the block
thickness by the width. Then the co-tangent of
that number is the blade angle. (On most
scientific calculators, you have to hit the
second function key and then the “Tan” key to
get the co-tangent, which is often marked as
“Tan-1.”) A 1 x 2-inch block has a 26.57°
blade angle.
• Draw a full-size outline of the blade with
128 MODEL AVIATION
radii stations marked. This should be the front
view of the blade. The chord of the outline
should be reduced by a total of 1/16 inch, to
allow for the 1/32 basswood edge strips that
will be added later.
Attach the paper outline to a piece of 1/4
plywood. Cut and sand to exact shape. Use
this pattern to cut a fixture base from 3/4 birch
plywood. Use the same pattern to cut blade
blanks from 1/2 balsa.
(I’ve found that the most accurate way to
make duplicate parts from thick wood is to
rough-cut the piece and then use a router
mounted in a router table and fitted with a
flush-trim bit. This bit has a ball bearing that
rides against the pattern, while the router cuts
the part to the exact size and shape of the
pattern.)
This is a good time to drill holes for blade
fittings or to add a hardwood root
reinforcement.
• Measure the exact chord width of the
pattern at each of the radius stations. (If the
blade has a nonsymmetrical area distribution,
you’ll have to measure the semichord from
LE to centerline and from TE to centerline.)
Also measure the exact thickness of the
balsa blade blanks (close to 1/2 inch) and the
exact thickness of the 3/4 plywood base. (This
will usually be slightly less than 3/4 inch.) I
use a dial caliper for maximum accuracy.
• Figure out how much to rotate the blade so
it will fit within the 1/2 balsa plank. Rotating
the blade so that the blade angle at maximum
blade width goes to zero will typically allow
the blade to fit. Then reduce the blade angle
at all of the other stations.
If the blade angle is 30° at the station with
maximum width, you would deduct 30 from
all other blade angles. The rotated blade
angles from that point out to the tip will be
negative numbers.
(It helps to look at the end of a propeller
and see how it looks as you rotate it. Also
look at the propeller LE and see how it curves
up from tip to root; the TE will curve down
from tip to root.)
• Use the chord measurements and rotated
blade angles to calculate the exact location of
the LE and TE relative to the zero rotated
angle station. For a blade with symmetrical
area distribution, divide the chord by 2 and
multiply by the tangent of the rotated blade
angle at that station. (For a nonsymmetrical
area distribution, use the semichord
measurement.)
Check to make sure that the total LE and
TE offset at each station is at least 3/32 inch
less than the 1/2 balsa-blank thickness. If it is
not, select another station, rotate the blade
angle to zero, and try again.
• Add each measurement from the previous
step to half the balsa-blank thickness plus the
exact thickness of the 3/4 plywood base. (It
helps to keep all those numbers neatly
organized in tabular form.)
To make the sides for the fixture, cut a
piece of 1/32 plywood to approximately 4
inches wide, its length should be slightly
longer than that of the 1/4 plywood pattern.
Carefully curve the 1/32 plywood around the
pattern and mark the location of each station.
Extend the station lines across the 1/32
plywood sheet, perpendicular to the long
edge. Mark one long edge as LE and the other
as TE. Carefully transfer the final
measurements to the 1/32 plywood. (Offset
plus half balsa-blank thickness plus 3/4
plywood thickness.) Connect these points to
create a curved line for the LE and another
for the TE.
(The LE line should curve down from the
root toward the tip; the TE line should curve
up from the root to the tip. The height of both
LE and TE curves should be the same at the
station where the rotated blade angle is zero.)
Cut along the curved line to make each
fixture side. Glue the 1/32 plywood fixture
sides to the 3/4 plywood block, flush with the
bottom.
• After the glue dries, fit a balsa blade blank
into the fixture. Cut some strips of 1/32
basswood to approximately 1/8 inch wide and
slightly longer than the blade blank. Position
a basswood strip against the balsa blank and
tight against the LE 1/32 plywood fixture side.
Carefully glue the basswood to the blade
blank using thin CA. Repeat for the TE.
Remove the blade blank from the fixture.
Use a modeling knife to carve away the back
face of the balsa, using the 1/32 basswood
edge strips as guides.
Sand in undercamber, checking frequently
with templates. (I use excellent laser-cut steel
ones from Alex Andriukov.) Turn the blade
over to carve and sand the front face.
• Finish the blades as desired, then mount on
the hub so each blade is at the correct pitch.
You put back in the degrees of blade angle
you took out when you rotated the blade to fit
it into the 1/2-inch balsa blank.
For more about blade-carving methods,
see “Propeller Construction” in the 1988
NFFS Symposium. For some tips about
molded blades, see “Composite Construction
of Propeller Blades for F1B” in the 1997
Symposium.
Coupe Survey: Sergio Montes and the rest of
the people at Free Flight Quarterly have
released an updated Coupe d’Hiver (F1G)
Survey 2009. The new version is a twovolume
set that totals more than 110 pages
and includes two full-size Coupe plans.
Maurice Bayet, a French modeler and
magazine editor, originated the Coupe
d’Hiver event in 1939 as an FF competition
to be held in the winter. (Coupe d’Hiver
means “winter cup.”)
In the last few years, it has become one of
the most popular mini FAI FF events. (The
“mini” events typically use smaller models
and are flown to two-minute maxes instead
of the three-minute max used for the “big”
FAI events.) Rules for F1G are simple; they
allow 10 grams of rubber, an 80-gram
minimum all-up weight, and five two-
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February 2010 129
minute flights performed in rounds.
With virtually no restrictions on
model size, this makes Coupe one of the
most fertile events for design innovation.
That variety of approaches certainly
shows through in Coupe d’Hiver (F1G)
Survey 2009.
The designs collected from around
the world range from simple stick-andtissue
models to carbon-fiber and Kevlar
high-tech creations. Most highlighted
aircraft include three-view drawings,
sketches of details, photographs, and
compressive text descriptions of the how
and why of the design.
The publication also includes several
articles about design theory,
construction, and trimming. If you are
interested in purchasing an RTF F1G,
there is even a roundup of available
designs. Priced at $25 including postage,
Coupe d’Hiver (F1G) Survey 2009 is a
true bargain.
If you are considering trying F1G or
are an experienced Coupe flier who
wants to improve your model’s
performance, order a copy. Check out
the Free Flight Quarterly Web site for
more information. US readers may find
it easiest to send a $25 check, payable to
Free Flight Quarterly, to Chris Stoddart.
Nats Video: If you missed the 2009 FF
Nats, or if you were there and you want
a permanent record, Alan Abriss has put
together a two-hour video of the
activities for Homegrown Television
Productions. As do Alan’s previous Nats
videos, this one includes interviews with
modelers, flying action, and detail shots
of all types of FF airplanes.
Also available are videos of 1999
through 2008 FF Nats and a video of the
1992 edition: the last unified Nats. That
video includes RC and CL activity, as
well as both Indoor and Outdoor FF. The
2009 Free Flight Model Airplane
Championships is available for $20 plus
postage and handling. MA
Sources:
NFFS
www.freeflight.org
Alexander Andriukov
(805) 577-1349
http://home.pacbell.net/andriuko
Free Flight Quarterly
www.freeflightquarterly.com
Chris Stoddart
8400 Woodbrook Dr.
Knoxville TN 37919
Homegrown Television Productions
www.homegrowntv.com