Helicopters

SOSI 2012Matt Chalker

chalker7@gmail.com

Rotors

Fuselage/Motorstick

Thrust Bearings

Spar

RibMotor Hooks

• Two Overall Components–Helicopter(s)– Flight Log

Construction Rules

• Maximum Rotor Diameter – 30 cm• Minimum Weight – 3.0 g• No boron on helicopter• Rubber powered (rubber weight doesn’t

contribute to overall weight, but no limit on the rubber itself)

Flight Log

• Minimum of 10 entries• Minimum of 6 data points– Motor size before windup– Number of turns before flight– Flight Time– Any three others

• 10% deduction for incomplete log, 30% for none• Most important part of practice and competition!

Flying - Basics

• Wind rubber motor, attach one end to each hook, let go! Pay attention to winding direction!

• Look at the ceiling, make sure you launch in a clean spot.

• Be prepared for repairs.• Don’t grab a flying helicopter from the air.

Flying – More advanced• Critical parameters affecting duration

– Turns on motor– Torque of motor– Pitch and diameter of rotors– Weight of overall helicopter (motor and airframe)– Drag

• Rubber motor is the most easily adjustable factor• Torque can fairly directly convert to thrust.• Thus, knowing how much torque the motor outputs, you

essentially know how much thrust the helicopter has.• A torque meter is the most essential tool to increasing

flight times.

Sample Torque Curves

Construction - General

• Anticipate building many helicopters throughout the year

• Priorities– 1 Build quality/trueness/straightness– 2 Strength– 3 Weight– 4 (and a distant number 4) Design

Supplies• 4 sticks of ~40 cm long 1/16” square balsa (can

be stripped from sheet stock)• 1 piece of ~3” x 1/16” sheet balsa• 1 piece of 1/8” x ¼” x ~15” balsa (motor stick)• 1 36” piece of 0.020” music wire• Covering material (condenser paper, mylar,

tissue)• Glue (Titebond, use superglue extremely

sparingly), 3M 77 spray adhesive• Tools (templates, razor blades, rulers, pliers, etc)

Cut out and fold templates

Trim long sticks to required length for spars (longitudinal supports). Mark spars in 6 equal lengths with marker (rib locations)

Using curved template (can be cut from soda cans, French Curves, sheet metal), slice curved balsa ribs from 1/16” sheet stock

Lightly tape spars to template (I use drafting tape)

LIGHTLY dip one end of rib in a small puddle of glue. Just enough dipping to cover the tip (I can’t emphasize how much glue is overused).Place the glued end on the lower spars, resting the rib on the top spar.

Allow the lower glue to dry (it should not be long if you are using the right amount). Cut the scrap end of the ribs off at the spar to miter the joint. Apply a similarly small amount of glue, using a scrap piece of wood as a brush.

Once dry, you have a completed rotor frame! If covering with paper, cut a piece a bit larger than the section to be covered.

LIGHTLY dust the area to be covered on the frame with 3M Super 77. Roll frame on covering to firmly attach paper.Trim excess with a brand new razor blade(the sharpest you can find).

Cut a piece of music wire ~4” long for the motor hook.Bend a complete circle on one end.Bend base of circle back to make the rotation symmetrical.Bend a small flare on the end of the wire loop to allow motors to attach (not pictured)

Cut two pieces of ~1/8” square to 3 cm long. Mark the center.Piece the pieces with the motor hooks, bend the wire as shown to latch on the stick.Glue the sticks and wire to the center of the spars (the top spar for the bottom rotor, the bottom spar for the top rotor).

Cut a stick of 1/16” x ~2” long balsa. Attach a disc larger than a dime atop. Glue assembly to top rotor.

Cut two pieces of hardened music wire off in order to make a thrust bearing. Follow directions at: http://www.youtube.com/watch?v=y8YL3kYLTVk

Cut the 1/8” x ¼” piece of wood to ~15” (this is something to play with a lot)Cut a notch in each end and glue the thrust bearings into the notches (make sure the center thrust line is perfectly straight on each side!)

Assemble and fly the finished helicopter!

Links

• Covering material, Teflon washers, winders, various tools: www.f1d.biz

• Rubber: www.faimodelsupply.com (buy 1/8” stock unless you have a rubber stripper)

• Various tools: www.indoorspecialties.com• Bulk Balsa: www.lonestar-balsa.com• www.indoorduration.com• www.indoorfreeflight.org• www.indoornews.com