Flying Bird

Flying Bird
Type	Engineering
Category	Build

Flying Bird was a demo event for both divisions at the 2011 National Tournament. The top three overall teams in this event were awarded medals.

Overview

Flying Bird is an aviation event in which competitors build an ornithological flying device (i. e. a device which flies by the flapping of wings; hence the name "Flying Bird"), called an "ornithopter". Due to the difficulty of this event, it is unlikely that it will become a full event in the near future.

One version of the rules from 2009/2010 can be found here.

Construction

Simple free flight ornithopter

A basic ornithopter consists of a wing, stabilizer, motor stick, a flapping mechanism, and connecting rods. Very light balsa wood and lightweight covering are needed to construct the wing and horizontal stabilizer. Mylar film and Japanese Esaki tissue are good options for covering. Flapping mechanisms are more complicated, and typically require music wire, a metal bearing tube, wooden connecting rods, and a plastic bead.

Wing

The simple "membrane" type of ornithopter wing is the most commonly used. It consists of only a balsa leading spar and a wing membrane in the shape of a half circle from lightweight covering. The membrane portion of the wing will passively lag behind the rigid wing spar. When the wing is going up, it will also tilt upward. When the wing is going down, it will angle downward. This is called the twisting of wings.

Stabilizer

Demonstrating how a four-bar linkage works

The horizontal stabilizer or tail is usually cone-shaped. Using a flat triangular tail like in an actual bird will result in decreased stability and control. The stab usually provides a downforce to keep the nose up, so the incidence or angle relative to the wing is much more than you would find in an airplane. Typically the angle is around 15 degrees, but may need to be adjusted depending on the bird.

Flapping Mechanism

Most flapping systems are based on a concept called "four-bar linkage". In this system, the motor powers a crank shaft, which pushes the wings up and down via the connecting rods as the crank goes around. This is simple for one wind, but requires a little bit more aligning when a second wing is added. With identical connecting rods, the two wings will flap at different times, causing assymetric flapping, which lowers the efficiency and makes the ornithopter want to turn to one side. This problem can be remedied by using a staggered crank, in which the connecting rods are attached at different angles, but their timing is corrected by having them placed on separate crank throws. The staggered crank is most easily constructed from bent wire.

Flight

After your bird is built, it's time to fly and test it. Success will come from constant testing and adjusting. Remember to keep a flight log! Looking back on data will help you figure out which adjustment works best.

Materials

You will need a winder (5:1 ratio may work for ornithopters, but 10:1 or 15:1 ratio winders are faster), rubber for the motor, and a lubricant to allow the motor to crank more winds in. Do not use regular rubber bands for the motor (they will break easily and will not give you enough winds for a good flight). Use Tan Super Sport Rubber, or the rubber provided in your kit if you used one. A silicone based lubricant works best. Armorall works well, but avoid petroleum based lubricants such as WD-40, which will degrade rubber. Two little plastic or rubber o-rings strung onto the motor greatly help transfer the motor from winder to rear hook.

Winding

Apply the lubricant before the flight. Have one partner hold onto the motorstick gently. Attach one end of the motor to the crank hook, the back end on the winder. The partner winding should stretch the rubber back 3-6 times longer than the original motor size. When you have wound halfway the desired winds, slowly move closer while continuing to wind. This is called stretch winding and allows you to make the most of your rubber motor. Transfer the motor from the winder to the rear hook on the motorstick carefully. Hold the bird by the motorstick horizontally, and then let go. No need to thrust or throw the bird into the air.

Adjusting

Start out with just a test flight. Wind only around 250 winds at first. More winds come later, when your bird flies smoothly.

Sharp turn followed by crash

If this is the case, add weight to the wingtip on the outside of the turn. Adjust weight as needed. This may have also occurred because you have wound the motor backward. Make sure to wind clockwise.

Stalling

Stalling is when the nose of the bird points upward slowing almost to a stop and then quickly falls into a dive, losing height. This can be solved by carefully lowering the tail incidence. Errors in the strut length or the hole spacing of the connecting rods can also cause this.

Diving

Again, make sure to wind clockwise. If the bird is still nose-diving, increase the tail incidence. Errors in the strut length or the hole spacing of the connecting rods can also cause this.

Competition

At the competition, use time wisely. Only 8 minutes will be allowed for two official flights.

Scoring

Score will equal the time aloft in seconds in the longest flight. Ties will be broken with the lesser flight.

Violation	Deduction
Flight Logs are Incomplete	30% deduction from final score
Flight Logs are not turned in	50% deduction from final score
Building Violation	2nd tiered (scored below all legal devices)

External Links

The Ornithopter Zone