FREE FLIGHT INDOOR - 2003/05

122 MODEL AVIATION
CORRECTION: In the February 2003
column, a description defined propeller
pitch as pitch = 3.1416 x tan(y/x).
Actually, diameter is part of the equation,
as explained in detail in the following.
Propeller Formula: A reader asked for an
explanation of the basic propeller pitch
formula presented in a previous column:
pitch = 3.1416 x tan(angle) x diameter.
This formula expresses the pitch of any
individual station along the blade where:
• Pitch is expressed in inches.
• “Tan” is an abbreviation for the tangent
function.
• Angle is the angle of the station under
discussion, relative to the propeller shaft.
• Diameter is the diameter of the circle
traced by that blade station as the propeller
rotates.
A typical propeller assembly fixture
uses a 45° triangle to support a blade while
it is being attached to the spar. If that
triangle is placed at three inches from the
shaft, the diameter is twice that radius, or
six inches. The 45° angle makes pitch
calculation easy since the tangent of 45° is
1.0. So the pitch of the propeller blade at
that station equals 3.1416 x 1 x 6, or 18.8
inches.
Some events require a flat-blade
propeller (no blade twist or camber). In this
case, each station along the propeller has a
different pitch. Assume that the blade
angle is 45°.
Station Diameter Pitch
(inches) (inches) (inches)
1 2 6.28
2 4 12.5
3 6 18.8
4 8 25.1
5 10 31.4
Much of the thrust (and rotational drag)
is generated by the tip. Since the area near
the hub generates very little thrust, most
blade outlines have the greatest width at
midblade and taper to a more narrow tip.
The usual indoor propeller blade is
twisted so that the pitch remains the same
at every station. Solving the formula for
angle gives:
Tan(angle) = pitch/(3.1416 x diameter)
Pitch Diameter Tan(angle) Angle
22 2 3.5 74.1
22 3 2.33 66.8
22 4 1.75 60.3
22 5 1.4 54.5
22 6 1.16 49.4
22 7 1.00 45.0
When you consider blade outlines used
by fliers today, they typically taper to a
Bud Tenny, Box 830545, Richardson TX 75083
FREE FLIGHT INDOOR
Andrew Tagliafico won MiniStick with a
Jerry Powell (Yreka CA) with his 1.2-gram AMA Easy B at Albany, Oregon, contest. time of 8.58 at the Albany contest.
point near the three-inch-diameter mark,
with maximum width at midblade, then
taper toward a tip with a rounded outline.
Wingtips and propeller tips develop a
tiny vortex caused by spanwise flow that
spills off at the tip. Also, the propeller tip
travels farther each revolution than does
the center of each blade, where the
maximum width is located. So the blade
outline has that shape for reasons besides
appearance.
Tip vortices have severe effects in full-
Jim Longstreth competed at the South
Albany Gym with his MiniStick.
scale aircraft, varying in intensity with
the airplane’s weight. For years I was
convinced that Indoor models, especially
Indoor Rubber duration models, were
too light to generate the effects. I have
been disabused of that notion by two
trusted friends with considerable
experience in aerodynamics.
Wing wash (another name for wing
vortices) can affect the stabilizer list in
some flight attitudes. Some designs set
the horizontal stabilizer higher than the
expected vortex or set the wing just high
enough above the motorstick to allow it
to be removed from tissue sockets, then
tilt the tailboom down roughly 15° so the
stabilizer is below the expected
downwash path.
Indoor News and Views (INAV): For
those who do not subscribe to INAV,
Carl Bakay has created an exceptional
publication with the aid of Steve
Gardner and Dave Haught in the US and
Nick Aikman in the United Kingdom.
Unless you know all there is to know
about Indoor models, you need INAV.
They expect to publish six issues a year
for $15, payable to Tim Goldstein,
13096 W. Cross Dr., Littleton CO
80127.
Indoor Supplies: Tim Goldstein has a
steadily increasing number of Indoor
items, and he features precision-cut
select balsa. Contact Tim at tim@indoor
duration.com for more information.
Slanic Notes: Junior Doug Schaeffer
and Jim Richmond made the two longest
flights regardless of class at the 2002
Indoor World Championships in Slanic,
Romania. Doug and Jim led their
teammates by a good margin, and these
flights led the Junior team and Open
team to victory by a large margin and
have been homologated as US records.
May 2003 123
Tony Mula brought his Pfake Pfokker scalelike fun model to Albany, Oregon.
Caitlyn Gilbert’s first Science Olympiad
model was built from a Midwest kit.
Marty Thompson holds his Science
Olympiad model, which is a learning tool
for him.
Jessica Buffard—in her third year of
competition—with her Science Olympiad
model.
Science Olympiad (SO): I have noted
in previous columns that young participants
in (SO), with guidance from model builders
in the Wright Stuff event, have become
excellent Indoor fliers. All US Junior team
members at the Indoor Champs in Slanic—
with their towering win over the other
teams (50 minutes between first and second
place)—came from the SO program.
There are many SO model kits
available. One that I have been privileged
to review came from Ray Harlan; the
Bambino is carefully crafted to produce a
model capable of making good flights.
Two things are outstanding in this kit.
One, a detailed selection of wood sizes and
lengths gives the builder a head start in
constructing the model. (Each strip of
wood is selected by weight and colorcoded
to match the intended use.) Two, the
full-size plans are supplemented by 11
pages of detailed, step-by-step instructions
that can lead an inexperienced builder to
success with little outside mentoring.
Contact Ray at 15 Happy Hollow Rd.,
Wayland MA 01778-3521, (598) 358-4013
(voice), (508) 358-4013 (fax), or at
[email protected] for pricing and other
helpful information.
Testing: Having good rubber is critical to
making long flights, and Jim Richmond
([email protected]) is king of the long
flight. He tests all of the rubber he uses. I
asked him how he does the testing.
“My rubber test formula is the same as
the one I cooked up in the 1960s: ‘Torque
at 1⁄2 turns (unwinding) times total turns
divided by the loop weight.’
“This has proven to be accurate enough
for most evaluations even though it only
looks at the middle torque. When doing
rubber tests, I record the unwinding torque
in 200-turn increments.
“These readings can then be compared
124 MODEL AVIATION
with similar ones from other tests for
further evaluation. Additionally, I
sometimes use the torque measurements
to plot charts for really close
comparisons.
“For each test, I record the date and
ambient temperature as well as the rubber
batch and the number of wind-ups
represented. My torque readings are in
oz.-inches and the rubber weight is in
grams.”
Northwest Activity: Albany, Oregon, is
an active Indoor area, with five contests
each year in the South Albany Gym. Bob
Stalick and Andrew Tagliafico are the
leaders. The contests draw participants
from as far as Yreka, California (280
miles). I’ve included photos by Bob
Stalick from a recent contest.
A photo shows Jim Longstreth with
his MiniStick. Also shown is Jerry Powell
of Yreka, California, with his 1.2-gram
AMA Easy B. MiniStick winner Andrew
Tagliafico posted 8.58 in the 36-foot
ceiling.
SO models are popular for all ages;
one picture shows Caitlyn Gilbert’s first
SO model, built from a Midwest kit.
Marty Thompson, also shown, uses SO as
a learning tool to break into Indoor.
Jessica Buffard is in her third year of
competition and is a seasoned SO regular.
Another picture features Tony Mula’s
Pfake Pfokker scalelike fun model. MA