Helicopters

Helicopters is a National event in which students compete to build a balsa wood helicopter powered by a rubber band motor that has the longest flight time possible.

Key Components/Terms

There are some commonly used terms that are useful when describing these helicopters.

• Motor Stick: This is the body of your helicopter. It holds the rubberband, and the two rotors are attached to this. There are different methods and designs, which are discussed below.
• Rotors: These are the horizontally counter-rotating devices that transform the torque in the rubberband to thrust in order to lift the helicopter. Most designs utilize two of them, one fixed (glued firmly to the motor stick), and one free (attached to the motor stick with a shaft and bearing so that it can rotate when the rubberband is hooked onto it). However some designs use two free counter-rotating rotors, which may be more efficient.
• Spars: These are the long, thin wood pieces that span the length of the rotors.
• Ribs: these are used to brace the rotors, and are placed between the spars.
• Prop/Rotor Shaft: This is a metal wire that is used to connect the free-spinning rotor to the rubberband.
• Rotor Hangar: This holds the prop/rotor shaft, allowing it to spin freely.
• Rubberband: This is the power source for the helicopter. It is wound using a winder. More is discussed about rubber below.

Construction Process

A simple design you can build is the Wright Bat Flier.

It is important to emphasize that you cannot use commercially available propellers, as previously stated. This is to preserve the true spirit of Science Olympiad--the engineering creativity of students. You may, however, use a kit as long as there are no pre-built parts (also make sure they meet the dimensional requirements of the rules). Don't panic--take time to draw up plans and purchase the materials you need as quickly as possible. At times homemade helicopters can far outperform kit-built models.

You are allowed to use wood, paper, string, wire . . . basically anything you can think of, EXCEPT RIGID PLASTIC. You are disqualified if you use this material for any part of your helicopter.

Helicopter Body Building and Designs

Helicopter bodies should be simple and easy to repair if damaged. For the body use wood that is as light as you can get without being brittle; balsa usually works. The top of the helicopter body should be constructed out of stronger wood than the rest of the helicopter frame because it will be under immense strain during flight (balsa which is denser or has larger dimensions may work). Make sure it is thick enough to drill a hole through. This is where the motor hook will be attached.

Pictured at right is a simple but effective body design found at most helicopter competitions.

There are three common designs, each with its own benefits and issues.

• The most common design is a single-stick body. This is imply one piece of balsa, that is used as the main body of the helicopter. This helicopter exhibits the single-stick design. This design is very simple to build, and may possibly be lighter than the next one. However, balancing the helicopter with this method is slightly more difficult, and will result in a wavering helicopter. Carefully balancing the rotors will result in a smooth flight pattern.

• The second most common design is the split-stick body. Two thinner spars, instead of one long one, are used as the body. They are connected at the top and bottom by horizontal pieces. This frame is for a split-stick helicopter. Here is a video describing how to wind and fly a split-stick model. The advantages of this design is that it is much more balanced than the previous one. It is slightly more difficult to build, but is still in the grasp of many teams. This design could end up weighing more if not carefully built.

• The third design, possibly the most efficient, and also the most difficult to build is a hollow-body or rolled-motorstick design. It is made by taking a thin sheet of balsa wood, soaking it in hot water, rolling it onto a form, then allowing that to dry. This results in a thin tube, which is efficient in terms of weight and balance. The rubber band is wound and placed inside the tube, and the rotors are attached at each end. This video describes a hollow-body design. Once again, it is possibly the best design, but also the most difficult to build. Advanced competitors should try this design.

Main body of a typical helicopter. The motor hook on the left is fixed to the body (i.e it cannot move on its own), while the one on the right rotates with the rotors. In the center is the rubber band motor.

The rotors will be attached to the unfixed motor hook by a shaft in the middle. Often times the wire (paper clips work nicely for this purpose and they are quite pliable) is bent or glued around the shaft that connects the rotors.

When you glue the joints together, USE CA GLUE (not gorilla glue, which takes too long to dry)! The helicopter will be taking some rough falls and you don't want it to get destroyed every time you test fly it! In addition, super glue dries somewhat slowly and it can be hard to hold in place while you glue joints. Buy CA glue accelerator from your local hobby shop. Apply it on wet super glue and it instantly sets. This is essential to achieving a perfect angle when gluing.

Helicopter Rotor Building and Designs

There are many different approaches to building helicopter rotors, but all successful designs have a few things in common:

• A regularly shaped frame that can be easily replicated. NO ODD SHAPES!

• A strong frame on each rotor. You can put in cross beams if you want for extra support. If you curve the balsa, the rotor becomes an airfoil and therefore has higher efficiency, but this adds some unnecessary weight to your helicopter.

• There are many ways to attach the free rotor to the body. One popular option is to use a pigtail hangar, as can be seen here and here. Another common method is to use a a propeller hangar from an Ikara prop.

• Reducing friction is key to enhancing flight times. Tiny craft beads can be placed on the rotor shaft along with a couple drops of lubricant.

• Thin, durable, smooth covering over each rotor. Using solid wood rotors is often a good idea for beginners. Otherwise, build a solid frame and use Mylar or tissue paper to cover your rotor. When covering with mylar, building a covering cradle and using spray glue to attach the mylar is the best way to do it. Super gluing mylar adds a lot of unnecessary weight.

• Rotor angle (or pitch). All rotor are angled such that the broad surfaces of the rotors are pointing away from their neighbors. You want the rotors to push against the air vertically. You will discover quickly that flight is impossible without this in mind at all times. Remember: The greater the pitch, the more torque required to turn (thicker rubber) and more lift.

• All good rotors are light. If you have materials to make your 1.2 gram rotor weigh 0.8 grams, by all means do so. Every bit of weight shaven off helps tremendously. However, make sure they can support the weight of the rest of the copter in flight!

Building Techniques and Tips

• Measure, measure MEASURE! Set a fixed size for everything and write the data down. Your helicopter WILL BREAK EVENTUALLY, and you may need to entirely replace parts. You will need to know how large to make them.

• Double check before you glue. Ask your partner if everything looks right before pouring on the superglue. It is a pain to rip rotors off and re-angle them because neither of you noticed that the wings were upside-down.

• TRIM! Trim everything. Shave off every bit of extra glue, and use an Emory board to gently sand off the extra tissue paper, glue and wood from the edges of your rotors. (Ask your mom or sister for one. They have many and are usually willing to sacrifice one in the name of science.)

• Weigh before you glue! Pile your parts on your scale and make sure you are okay with the weight before you glue them. This saves time, energy, and money. Plus, you should get a good idea of how much your helicopter will weigh. Seeing that (for example) one wing may weigh more than a gram will probably prompt a drastic design change.

• Keep an abundance of paper clips. They are good for everything from unclogging superglue to pressing down things you don't want to touch. Smaller pieces of wood with which to apply glue are also useful.

• Reducing weight is key! Make sure you use a glue applicator (see the above link) to reduce the amount of unnecessary glue used on the helicopter.

• Have acetone ready at all times. Superglue has a nasty habit of getting caked on your fingers, and at times acetone is the only way to get it all off.

• When building rotors, build a jig first. This saves A LOT of time.

• Build without excess weight! (Yes, it has been twice already, because it is so important)

Glue

CA glue (cyanoacrylate, a key ingredient in many super glues) and Ambroid Balsa Cement are among the best to use. Here is an article discussing more into glue. To achieve the strongest bond, with least weight, connect two pieces of wood with as tight a gap as possible. Therefore you can use less glue, saving weight.

CA glue accelerator, or kicker, is a valuable thing to have at competitions or even at home. It will cure the glue instantly (within two seconds), and doesn't add too much extra weight (as it is thin enough that the extra liquid evaporates). However, if there is enough time, attempt to build the helicopter without using accelerator, as it may result in a stronger joint.

When applying accelerator, do not use the spray attachment on the bottle. Instead unscrew the top, and use the end of the tube to simply drop a tiny amount of accelerator onto the glue joint. Spraying uses far too much and makes a mess of your work area.

Tools

You will need a few materials (besides those that will end up on your helicopter to get started.

• Pliers; something to bend wire with. You will need these to make your motor hooks.

• Saw. You will use this when you cut a piece of thicker wood for the top of the base where the motor hook and wings will be attached.

• Wire cutters. Paper clips and wire are not always the right size. You will need to clip and trim.

• Drill with a VERY TINY drill bit. You will use this to drill your hole in the square of wood your motor hook will be coming through. If you can poke through the wood you are using with a paper clip, it is too weak. It needs to be so strong you need to use a drill to do any damage.

• Scissors. You may need these to cut your thinner balsa wood and wing covering.

• Xacto-knives or single-edged razors. These are vital for making clean, precise angles when cutting wood. They are also basically good for anything else you can come up with. Keep some at hand at all times.

Flight

Rubber

Matching rubber width and torque to rotor pitch is the key to being successful in this event. Rubber for indoor-duration models comes in many different widths that must be contemplated before purchasing. For example, if your rotors have a high pitch, then it would be wise to use a thicker rubber. However if your rotors have a low/shallow pitch, then you will be able to use a thinner rubber.

Thicker rubber will not take as many winds, but will provide much more power. Thinner rubber will store many more winds, but will release the energy slowly and at much less power.

Also, it would be wise to lubricate your rubberbands before flying them. You will able to place more winds into the rubber, and it will not cut itself over multiple uses. Do not use WD-40 for this, as it will degrade the rubber. A popular lubricant used by many is ArmorAll lube.

Simply spray the lubricant onto the rubber band, and rub it gently between your palms. Store in a plastic bag, to prevent the rubber from drying out.

Winding your Helicopter

Buy a winder or wind manually. Winding manually takes an agonizingly long time, so winders are worth the money. Do not buy a battery-powered winder that needs to be put directly on the rotor! Use a hand-crank winder with a 10:1 or 15:1 ratio.

Additionally, a good single person winding process is to construct a winding jig. It consists of a plank of wood with 2 perpendicular wood peices. In one of the perpendicular peices, drill a hole and put in a hook that can hold the O-ring. On the other end cut a hole so the winder and fit in while winding. The distance between hooks of the hook and winder should be equal to the length of the motor stick.

Flight Logs and the Test Flight Process

Data for 10 test flights is required before the competition. Get permission to go to your school gym and test. Your house is not an adequate test facility unless you have at least 18 foot plus ceilings. Fans, beams, contact with the ceiling, and furniture can negatively affect your helicopter's flight time. DO NOT TEST IT OUTSIDE.

If a suitable testing location is not available, or you want to test just rotor efficiency, consider building a static testing jig. Place the helicopter in the jig, place the jig on a scale, and zero it out with the weight of the helicopter too. Wind up the rubber, and let the helicopter spin. The negative amount the scale displays is the comparitive amount of lift the rotors generate.

6 data parameters are required for each test flight. 3 are required, 3 are any that you choose.

The required data parameters for each flight are:

• Flight time
• Number of turns in motor before liftoff
• Motor size (length) before windup

Some suggested parameters are:

• Torque at launch
• Number of turns remaining after helicopter lands
• Estimated peak flight height
• Variance from starting point (how far your helicopter drifted.)

You can pick three of the above, or any of your own. Feel free to add your own ideas to this list.

Scoring

Only the better flight (out of the 2) will be scored. You will have eight minutes to fly your helicopter once or twice. If that time expires and you still have not completed a second flight, you will not get test your helicopter another time.

Bonuses

There are no bonuses this year.

Penalties

Violation	Deduction
Flight Logs are Incomplete	10% deduction from final score
Flight Logs are not turned in	30% deduction from final score
Building Violation	2nd tiered (scored below all legal devices)

External Links

2010 National Trial Rules

2010 B/C Division New York Pilot Rules

Illinois Science Olympiad Helicopter Construction Instructions

North Carolina Science Olympiad Helicopter Event Page

How to wind and fly a helicopter

Helicopter demo

How to make a motor hook

Another good helicopter demo

How to attach wing covering

How to glue balsa wood

Rubber band motors