Brief History and Design credits for the Indoor Helicopter
When Jack Shafer and Bob Stalick asked me to come up with a helicopter design and
produce a dvd on its construction and flying, ready for sale in under two months, I
swallowed hard and enthusiastically said "yes."
Jack provided a drawing of a 1993 Nationals winning ultralight indoor helicopter design
by Andrew Tagliafico. I decided to start with his design parameters and use the
simplified building jig that Lew Gitlow used on his parlor copter. I also had already built
several sizes of a great flying outdoor helicopter designed by Bill Watson (of
Aerovironment fame, Dr. Paul MacCready's company) in the 70's. That little
helicopter was called the "PC-9" ("Pocket Chopper #9" - his ninth design attempt).
I methodically (frantically) began building and test flying, tweaking the design, adding
vanes, removing them, using stronger thicker motors, cutting down blade lengths, etc.
Since many of the flights ended with only 1/2 of the turns used on a 12" loop of 1/8"
rubber, I decided to design backward from that power source, not wanting to increase
rubber thickness, then motorstick thickness ad infinitum.
So I decided to build a new one with 13" rotor span (smaller than the 40 cm span
allowed) and used a much lower pitch. Lew's and Andrew's helicopters both used 12"
pitch. But this new helicopter had to weigh 4 gms. minimum, so I lowered the pitch so
it would use up most of the turns on the 1/8" wide motor. I tried creating a swept tip
shape to reduce flaring and washing-in on the very wide tips. Eventually, I cut the tips
back to the second outer rib and added a rounded tip made of sheet balsa. That
produced a wonderful increase in duration (broke the magic minute mark). I attribute
this jump in duration to the reduced drag from the thin, sheet balsa tips. The model
was still flying in wide swinging circles. I tried vanes again which reduced swinging but
hurt duration. Then I remembered Bill Watson's Pocket Chopper with dihedral in the
upper rotor (Bill Hannan graciously lent me a copy of the plan he drew up for Bill
Watson). So I reglued the sheet tips at modest dihedral angles and voila! Under full
power it ascended straight up (see flying sequence on video) to the ceiling. I got some
articles from Jack Shafer a day later, by William R. Bigge and Parnell Schoenky,
originally printed in Frank Zaic's Yearbooks '51 - '52, and '59 - '60 which mentioned
dihedral in both rotors, and interesting info on C.G., center of lateral resistance, etc.
The last modification was adding a sharpened leading edge extension to the rotors'
leading edges. That added 10 seconds to the average best times (about a 13%
improvement). So, going forward there is still much to learn. But this model indoor
helicopter is a stable and satisfying beginning point.
Thanks to all these pioneers in indoor helicopter design.
Mark Allison. October 6, 2010.
Tools and Supplies needed to make an Indoor Helicopter.
(*= useful but optional)
aluminum tube: 1/16" outer diameter (inner dia. ≈ .032") for rotor shaft bearings.
Comes in 12" lengths. Alternately: Peck Polymers small (for 1/32" shaft) nylon
bearings (comes in pack of 6 from A2Z Corp). Note: use .025" music wire for
shaft, (not 1/32" which is too heavy, thick and hard to bend).
balsa wood in these sizes: 1/16" thick sheets x 3" x 36" medium density (stripped to
1/16th inch square strips for rotors & ribs). You may instead buy 1/16th inch
square sticks (they come in 18" lengths and longer). 1/32"x3"x36" super light
density (for rotor tips and vanes). A few 1/8"x1/2"x36" strips of medium to heavier
density to strip for fuselage and vertical post.
building board: 1/16th inch thick cardboard, flat, (not corrugated). Picture framing
"mat board" works great.
eraser: this is a must have tool. Ideally, a brand new white rubber eraser. You'll cut
this to the dimensions shown in the illustration (see illustration pages), and use it
for your elevated rib-cutting surface and use one corner of it to make sure hub of
each rotor is vertical before glue dries.
thin plastic bag: super thin type that you pull off a roll, found at vegetable section of
supermarkets. For covering rotor frames. One bag is enough for two models.
Use latex contact cement (Locktite brand).
blades for cutting balsa: best is 1/2 of a double edge razor blade (see dvd for safely
breaking in two), or single edge razor, injector razor blade, #11 Xacto blade and
handle.
tape: to tape top spars in place at notches of the angled pitch templates. Can be
transparent, or magic tape.
thread: to wrap around bearing, bearing stand-off, and motor stick. Add a coat of
latex contact cement on top of thread. Thread is also used to reinforce the
thrustline adjusting cuts made in the motor stick.
sandpaper: 150 grit. Hardware stores have this and also inexpensive foam blocks
covered with sanding surfaces. If you buy sandpaper in sheets, adhere to a flat
block of wood. Don't try to use it folded, you need it flat.
cardboard: for angled pitch templates. Matboard scrap from a frame shop, or other
thick 1/16" cardboard.
glue stick: for adhering plans and angled pitch templates to 1/16th inch thick
cardboard.
bond paper: for gussets or reinforcing tabs for rib/spar joints.
compass (drafting type) for drawing round shapes for rotor tips, and vanes. OR a 2
5/8" (two and five eighths inch) diameter jar lid or measuring cup rim, etc.
waxed paper: kitchen variety. Use small 1" square pieces under lower spars at rib
joints to prevent them from getting glued to the plan.
pins: dressmaker's, steel, 1 1/16" length. For rear motor hook.
music wire: from hobby store or online supplier. Sold in 18" lengths. The thickness
or diameter needed is 25 thousandths of an inch, or 0.025". Used for rotor shaft
for the lower rotor. A short, sharp piece is chucked into the Xacto knife handle
and used to "drill" hole in the lower rotor's hub to accept the rotor shaft.
wood glue: Elmers, Wilhold, Titebond, etc. Wood glue is a yellowish, water-based
aliphatic resin glue, not the white casein-type school glue. Smallest bottle will
be more than enough.
Locktite Contact Cement, Latex formula ONLY. It needs to be thinned with water to a
thin cream consistency. For adhering plastic film to wing and stab. Used
undiluted for coating the bearing and its thread wrapping. Avoid the type of
contact cement with acetone, ketone, benzene, toluene, etc.
CA glue or Super Glue or Krazy Glue: For quick repairs. Comes in single-use tiny
containers, several to a package. Use with great caution to avoid adhering to or
bonding skin/fingers.
rubber strip: 1/8" (.125") wide FAI brand, Contest rubber strip. Can be purchased in
15 foot lengths from some suppliers, or in 1/4 lb. boxes from FAI Model Supply.
Do flight tests to see which width/length loop works best. Do NOT use regular
rubber bands, they are simply inadequate for our purpose and will put you at a
great disadvantage in competition.
rubber lube: The older type Armor All rubber protectant (yellow, black, white label)
makes a great lube that stays slippery even when dry. Spray some in a sandwich
bag for lubing and storing rubber motors. Lube motors every flight in a contest.
Every 5th flight when sport flying.
rubber winder: a must have, since the rotors are too fragile to wind by hand. 5 to 1 or
10 to 1 gear ratios are recommended because they are sturdy and can handle the
1/8" rubber motors. Available from A2Z Corp.
cardboard box: a large corrugated cardboard box with a styrofoam block in the middle
to support your helicopters for transport. Use a bigger one than you need and cut
and glue to the right size/shape to accommodate your helicopters.
*glass beads that fit onto the 0.025" wire. OR packaging plastic that can be
hole-punched to form discs that can be center-drilled for washers between lower
rotor and bearing to reduce friction. Use two of either beads or washers. None
needed if you use nylon bearings, but it wouldn't hurt to add them.
*scissors: sharp, for cutting plastic film, tape, etc.
*balsa stripper: "Master Airscrew Balsa Stripper" very handy, good investment if you
think you'll be making more models. If you already have a cork-backed metal
ruler and razor blade you don't have to get the balsa stripper. But the balsa
stripper is much easier to strip balsa with and does a cleaner, neater, more
accurate job. Not needed if you buy pre-stripped balsa sticks (1/16th inch
square).
*18 inch metal ruler, cork-backed: use if you have one, but don't go out and buy one.
Get the balsa stripper instead. If you have a metal ruler that isn't cork-backed you
can use rolled masking tape under it to keep it from slipping as you strip balsa.
Not needed if you buy pre-stripped balsa.
*cutting board: inexpensive kitchen type as seen on this video. You can use thick
cardboard but it will dull your blade quickly.
*wire cutters: for cutting music wire. Note: some needle nose pliers have wire
cutting jaws.
*needle nose or round nose pliers: for bending rear motor hook.
*hanging postal scale: digital or spring type weighing down to tenths of a gram. Or
go to your science teacher and ask if you can use the gram scale: for weighing
and grading the density of balsa sheets, weighing rubber motors.
*syringe & needle: for applying thinned wood glue. Get the largest bore needle
available (20 gauge is best, but the smaller 25 gauge will still work), cut off sharp
tip and sand or grind tip smooth and round. Keep water in the cap and keep
syringe capped when not in use. Or simply use a toothpick, strong balsa stick
(1/16"), or bamboo skewer, etc. to apply thinned wood glue.
*calculator: for figuring density, turns per inch on your rubber motors, etc.
Glossary of Aeromodeling Terms
This glossary is intended to explain terminology and concepts
used in this DVD and in the flight trimming & trouble shooting section.
Most of the parts are labeled on the Helicopter plan.
airfoil - usually refers to the shape of a wing especially referring to the cross-section.
In indoor aeromodeling it is usually curved or cambered to form an upward
arc. In flight, when air flows over and under the airfoil it produces lift, an
upward force that opposes gravity. Very low cambered ribs are used in the
ultra light expert class helicopters (usually 3° arc airfoils). For beginner's
helicopters flat plate airfoils using straight sticks for ribs on the rotors is
simpler to construct and still very efficient. There are more important factors
than slightly curved ribs in creating a high performance beginner's helicopter.
aliphatic glue - Carpenter's glue, yellowish, water soluble/thinnable, aliphatic resin glue.
Brands: Wilhold, Elmers, Titebond, etc. It is non-toxic, non-aromatic and
much safer than the Duco or Ambroid type. Should be thinned with water
(up to 1/3 water, by volume) for model building. Do not use regular white
school glue (casein-type) for general building, due to inferior strength. It may
be used in a pinch for repairs, but the carpenter's glue is much stronger.
CG or Center of Gravity - the point where the forces of gravity are balanced. The CG
can be moved with small weights (clay). It's better to locate the CG at the
spot indicated on the model plan with rubber motor installed. On the
Helicopter the CG is best located lower than 50% of the motor stick length.
If clay ballast is needed to bring the helicopter weight up to the minimum
weight required by the rules, it's best to attach it to the hub of the lower rotor.
chord - refers to an imaginary straight line that joins the leading and trailing edges of a
rotor, wing or stabilizer. Constant chord means the rotor, wing or stabilizer is
rectangular, not tapered in planform.
CYA - (NOT RECOMMENDED due to skin bonding hazard) - cyanoacrylate glue,
sometimes called "CA glue," Krazy Glue, Super Glue. Comes in a thin, fast
curing forms and a thicker "gap filling" form. Used for quick repairs in
aeromodeling. Not the best for building. Once it is cured, more glue added
will not adhere well. Use with great care. It can glue fingers to solid
objects, other fingers and skin. Keep this glue away from eyes!
dihedral - the upward angle of an airplane's wings when seen from front or back. In the
helicopter, dihedral is used on the tips of the upper rotor to create and
maintain stability in the vertical flight mode.
dive - a sudden plunge with the nose pointing downward. In the helicopter, this
happens if the C.G. is too high, too near the upper rotor.
drag - an aerodynamic force parallel to the air flowing over the rotor. It can be thought
of as pulling back on the rotor, slowing it down, diminishing lift, wasting
energy. (see lift)
flight circle - a circular flight pattern. A well balanced and adjusted free flight helicopter
will ascend quickly, smoothly and vertically to contact the ceiling with its post.
As the motor unwinds, the spin of the rotors begins to slow down and there is
a reduction in the stabilizing gyroscopic forces. The helicopter will start to
move in small but increasing circles, with the post still in contact with the
ceiling. The bottom of the helicopter usually swings in a wider circle than the
top, making a kind of cone-shaped flight pattern. If the circular swinging is
excessive, efficiency is reduced and flight duration shortened. The flight
circle can be adjusted using sidethrust (see illustration of that adjustment).
As the motor continues to unwind, the helicopter post will lose contact with
the ceiling and the circular pattern may increase in diameter. The swinging
direction of the bottom may reverse (say from counter clockwise to clockwise,
seen from below) as the helicopter runs out of power and starts to descend.
This is normal. (see flight trimming & trouble shooting section).
helical pitch - (see rotor)
latex contact cement - a water thinnable, non-toxic glue used for adhering thin plastic
covering to the rotor blade frames. It can be used with thread binding to
secure nylon or aluminum bearings to the motor stick. Locktite brand has
this cement available in small tubes.
lift - an aerodynamic force perpendicular (up) to the direction of the air flowing over the
rotor. The L/D ratio or lift-to-drag ratio is a measure of a rotor's efficiency.
rib - usually refers to a wing rib, but most free flight helicopter rotors have ribs too. It
is usually a straight or slightly curved balsa stick that lines up on a wing chord
and joins the leading and trailing edges of the rotor. The rib helps hold and
shape the covering of the rotor. Too much curve (camber) or rib height will
create a rotor with a lot more drag than lift.
rotor - the free flight helicopter's propeller - sometimes referred to as an airscrew by
the British, a term that reflects the design and function of the propeller.
Leonardo da Vinci proposed a "helical airscrew" in 1493. The rotor provides
propulsion upward through the air similar to a threaded screw advancing
through wood. There are a pair of numbers that distinguish one rotor or
propeller from another: Pitch and Diameter, usually included together as the
P/D or Pitch/Diameter ratio, measured in inches for models. The pitch
describes the angle of the propeller blade and the diameter describes the
length of the whole propeller. The pitch of a model's propeller is the distance
that the "airscrew" moves forward in inches with one complete revolution.
Another feature of a good rotor or propeller is that the blade angle changes
from hub to tip while the pitch/diameter ratio remains constant, making the
blade look like a twisted wing, which it actually is. This quality is called
"helical pitch" and it means that all points along the blade push the same
volume of air. If the propeller worked like a screw, in one complete 360˚
rotation, all points on the blade would move forward the same distance. In
a free flight helicopter the upper and lower rotors counter rotate. They spin
in opposite directions using all the energy of the natural torque reaction (see
torque). Another way to understand propellers is to think of them as
spinning wings with lots of wash-out at the tips. The "lift" rotors produce is
called thrust, and moves the model upward against gravity. In general, you
want to use the largest diameter rotors allowed by the rules. Beginner's
should use different motor widths and lengths adjusted to weigh the maximum
allowed by the rules. Many test flights should be done to see which motor
matches the rotors and gives the best duration or flight times.
rubber lube - a lubricant applied to a rubber motor to keep it slippery so it can be wound
tighter. A dry, unlubed motor will get nicked, torn, and will break with much
fewer turns or winds and will not release it's stored energy efficiently. You
can get a good commercial rubber lube from the same folks who sell rubber
strip but Armor All (commercial rubber protectant) works fine as a lube. Just
spray a bit into a ziplock bag, put the motor in the bag and massage the
motor so it gets coated thoroughly. Armor All stays slippery even when dry,
but it's a good idea to lube the motor before each flight in a contest, every 5
flights for sport flying. If the two strands of a motor loop adhere to each
other, that motor needs to be lubed before winding again.
rubber motor - one piece of contest quality rubber strip purchased in lengths or 1/4 lb.
quantities, usually from an online source (FAI, A2Zcorp, etc.), that has been
tied in a large loop and lubed with a good quality rubber lube (available from
the same sources). It's width is measured in fractions of an inch or decimals.
1/8th inch wide (.125") or say .093" (3/32"). Rubber strip can be purchased
in several widths and test flown for the best match for the propeller used.
Rubber strippers are quite costly ($250 to $300) but not needed because of
the availability of various rubber widths. The motor is weighed so it doesn't
exceed the rubber weight limit of that year's event. The motor dimensions
(loop length, width and weight) are recorded in a flight log along with flight
time, estimated circle diameter, etc. A rubber motor is not a typical rubber
band found at grocery stores or office supply stores. The motors need to be
very close to the maximum weight allowed by the rules to avoid a
disadvantage in competition. You may have to "cut-and-try" to get a motor
that weighs at or just under the maximum allowable weight. Cut a strip that
weighs .1 to .2 grams heavier than the max weight because when you trim off
the ends beyond the knot you'll get closer to the maximum weight. Motors
that end up too heavy can be used for test motors for getting the model in
trim, or winding to see how many turns they take before breaking.
sidethrust - the sideward angle of the propeller or propshaft. It is used to control the
diameter or direction of the flight circle.
stability - "stay-ability." The ability of an aircraft to stay on or return to its intended flight
path and resist the effects of turbulent air or other disturbances. Stability is
different than trim. Stability depends on things like Center of Gravity (See
Center of Gravity), dihedral, side area, etc. You cannot correct instability
troubles with trim changes. The helicopter will have stability if the C.G. is
located well below 1/2 the length of the motor stick and the upper rotor has
dihedral, and if the rotors have the same weight, stiffness, and pitch on both
ends of each rotor. "Trimming" or fine adjustments to the flight pattern can
then be made with the addition of vanes which create more "side area" to
reduce the sideways movement of the top or bottom of the helicopter.
thrust angle - refers to the angle of the propeller or propshaft. For free flight
helicopters it is used to correct the excessive circular swinging of the bottom
of the helicopter. (see flight trimming & trouble shooting pages)
torque - the tendency of a force to rotate an object about an axis. Just as a force is a
push or a pull, a torque can be thought of as a twist. Torque is a measure of
a twisting force on an object. In indoor modeling, torque or power of a
particular rubber motor at any number of turns or winds is usually measured
in inch/ounces using a device called a torque meter. As the number of turns
increases, torque increases. Just before a rubber motor breaks, the torque
rises much faster for each additional turn of the winder. Although it is a very
useful tool for maximizing flight times, a torque meter is not recommended for
beginners. For beginning modelers a test motor of a known length and of
the same width as the motors intended for competition is lubed and wound up
until it breaks. The total turns that broke that motor are divided by that
length in inches. The resulting number is "turns-per-inch-breaking" or TPIB
for short. If you multiply that number times the length of your other motors of
the same width you will know the maximum turns that motor will take before it
breaks. But you will only use about 82% to 90% of that number when going
for your best time in a contest. For sport flying or testing and trimming
purposes you can use about 75% of that maximum number. A motor will get
"tired" after being wound up to contest levels so for a contest, bring with you
the maximum number of motors allowed.
trim - refers to the angular settings (thrust angles) and adjustments in side area (using
vanes). Trim sets the flight pattern and swing-circle diameter. Once
trimmed, the model will fly this pattern very consistently - provided there is
adequate stability (also see stability, center of gravity). A model must be
stable and trimmed to achieve the best performance.
vane - a flat, thin, lightweight balsa sheet usually adhered to the center and top of the
upper rotor or the center and bottom of the lower rotor. A vane increases the
"side area" or lateral area of the top or bottom rotor, acting like a fin to reduce
swinging or sideways movement. (see flight trimming & trouble shooting
section and illustrated pages).
wash-in - a twist that increases the angle at the rotor tips. This is undesirable and very
inefficient. It is the opposite of helical pitch. It creates a lot more drag than
lift on a rotor. It is usually only used in indoor model airplanes to "hold up"
the inner wing panel (the half of the wing on the inside of a turn) to produce a
flat efficient turn.
wash-out - a twist that is built into a rotor with helical pitch. The outer ends of a rotor
are "washed-out" relative to the inner sections of the rotor. A flat, untwisted,
or non-helical rotor is only 2/3rds as efficient in producing lift as a helical pitch
rotor.
Resources
If you search online, go to: Wright Stuff/Science Olympiad - you'll get the official
website. Since many other events are listed under Science Olympiad, be sure to
include "Wright Stuff" in your search. Most of these resources are about airplanes but
the info regarding tools, materials, and building techniques applies to all types of indoor
modeling.
Here is a partial list from the first page of the articles available on that website [Mark
Allison's comments are marked with these brackets]:
"The information below should not be interpreted as an interpretation or extension of the
rules. The official rules in the current Rules Manual take precedence. Please read
the Disclaimer regarding Kits. This event is generously supported by the Academy of
Model Aeronautics (AMA)."
• Wright Stuff Construction tips by Chuck Markos: [a comprehensive 25 page article
on construction, aerodynamics, props, covering techniques, and flying. A "must
read" for anyone just getting started on indoor airplanes].
• Construction plans by Chuck Markos: [a super simple beginner's airplane design
with a "flat plate" airfoil]
• Articles by Ray Harlan [also available on his website: http://www.indoorspecialties]
• Build a Simple Torque Meter
• Build a Simple Pitch Gauge
• How to Cover with Plastic Films
• How to make a Flight Log
• Kits, plans, rubber strip, wood, tissue, and other Science Olympiad Wright Stuff
necessities by Bob Clemens. [Also includes suppliers, websites, materials
sources, etc.]
Websites
National Free Flight Society: http://freeflight.org/ The National Free Flight Society
(NFFS) is committed to the preservation and promotion of free flight model
aviation in all of its aspects and manifestations. The National Free Flight
Society is the only organization in the United States that serves the interests of
all free flight categories. The Society was formed over 30 years ago to promote
free flight activities and continues as a vital, growing organization dedicated to
the advancement of the free flight hobby and sport.
Indoor Specialties: http://www.indoorspecialties.com/ is Ray Harlan's Website:
Excellent articles, kits, supplies like propeller bearings. Good supplier for all
indoor modeling.
Thayer Syme's site: Free Flight Fantasies: http://www.gryffinaero.com/models/
Articles, links to other helpful websites.
Indoor freeflight: http://www.indoorfreeflight.com/ The Chuck Slusarczyk Indoor
Web Page: Excellent articles, links, photos, plans.
Napa Air Phantoms Association: http://www.napaphantoms.com/ Kits,
Suppliers, and links to many resources, suppliers, clubs, etc.
Supplies for Wright Stuff ("SciOly") and other indoor models
A2Z Corp (Englewood, CO, USA): http://www.a2zcorp.us/store/ This is the only
one you really need! They've got it all: balsa, propellers (including the red SIG
7" Prop with shaft and hanger), contest rubber strip (from F.A.I model supply),
glue, winders, plans, kits, books. They are also now the official suppliers of
Indoor Model Supplies, and Peck Polymers products.
FAI Model Supply: http://www.faimodelsupply.com/ CONTEST RUBBER, kits,
plans, propellers, glue, winders, more. Proprietor, John Clapp has been
providing the best rubber strip for the modeling community for many years. (His
rubber strip is also available through A2Zcorp.) FAI Model Supply: P.O. Box
366, Sayre, PA 18840-0366, phone: (570) 882-9873
Turner Toys: http://www.turnertoys.com/ Science Olympiad kits, supplies, articles,
updates, links.
