DIY Foam Board Gliders
General guidelines, measurements and technical specs for making your own DIY foam-board free-flight glider.
Ready to make your own exciting foam board airplane? Certain aerodynamic design principles should be followed to ensure stable and efficient flight. Here are some general guidelines and measurements for making a simple free-flight glider. The term "free-flight" simply means that that the airplane is not remote controlled. It is free to fly wherever it wants, after you release it. Although we will be using foam board to construct these airplanes, the guidelines would apply to any free-flight glider, whether it is made of foam, wood, paper or carbon fiber.
Foam board (also called foam-core) is an excellent building material for making larger airplanes. It's made by gluing a layer of foam between two pieces of paper. This makes a lightweight and stiff material that is easy to work with. Be sure to check out our advice regarding the Tools & Techniques that you can use to master the art of using foam board. If you aren't yet ready to DIY your own airplane design, check out our article on Foam Board Airplane Kits that has a design that you can download and make at home.
Wing Aspect Ratio
The wing's Aspect Ratio (AR) is a number used to describe the overall shape of the wing. It is the ratio between the wing's span (distance from tip to tip), divided by the wing's chord (average distance from front to back).
Short stubby wings have a small aspect ratio of 4 or less. Long skinny wings have a high aspect ratio of 10 or higher. High aspect ratio wings reduce drag and improve glide efficiency, however they are also more fragile and prone to warping and bending. Making long wings that are strong enough to support the stresses of flight often requires adding so much extra structure and weight that it becomes impractical. Moderate aspect ratio wings between 4 and 8 strike a good balance for simple free-flight models.
Dihedral Angle
The dihedral angle is the amount of upward tilt that the wings have relative to a horizontal center line. The wingtips should be slightly higher than the root of the wing where it connects to the body of the airplane. Positive dihedral improves the roll stability of an airplane. If air turbulence or some other factor causes the airplane to roll to one side, the positive dihedral angle will cause the glider to naturally roll back to level flight.
This works because the lower wing will be more level with the horizon (see picture) and so all of its lift will be vertical. The other wing will be at an angle, so some of the lift force will be directed horizontally. Because the lower wing produces more lift in this scenario, that wing will rise and automatically level the airplane.
Without enough dihedral, the glider will be unstable and may even roll upside down! Too much dihedral can cause it to overshoot when it corrects, which can cause a side-to-side wobbling called a "Dutch Roll". A dihedral angle of 5° to 10° per wing is a good range to aim for.
Wing Shape
The "airfoil" of a wing refers to its shape when looking at a cross section down the length of the wing. Many simple foam board gliders use a flat wing, as opposed to a curved or "cambered" wing. Flat-plate airfoils are quick and easy to build but less efficient than a cambered airfoil. Adding a slight curve to the wing can improve its performance. Beveling or rounding the leading and trailing edges can also improve the efficiency of the wing by reducing drag. You can also build up a thicker, more complex wing by folding the foam board into an airfoil shape.
The wing "planform" is a term used to describe the shape of a wing when looking from above. Wings can be rectangular, elliptical, tapered, swept back, and even swept forward. The easiest wing to build is probably a rectangular wing with slightly rounded tips. An elliptical wing will have better performance but is more difficult to construct. Pick whatever shape you like the most.
| Suggested Measurements for DIY Wing | |
|---|---|
| Wingspan | 30cm |
| Wing Chord | ~7cm |
| Wing Area | 210cm2 |
| Aspect Ratio | 5 |
| Dihedral | 8° |
| Airfoil | Flat-plate wing with beveled leading and trailing edges |
| Planform | Rectangular with rounded tips, or slightly tapered |
If the wing has movable control surfaces on the back, these are called ailerons. They can provide some roll stability.
Tail Size
The horizontal tail surface (called the horizontal stabilizer) provides pitch stability. This is the up and down tilting direction. If the stabilizer has an adjustable section, this is called an elevator. Adjusting the elevator will cause the nose to go up or down. If the stabilizer is too small it will not be very effective; if it is too large it adds extra weight and drag. A good size for a horizontal stabilizer is around 15% to 30% of the wing's area. The aspect ratio of the stabilizer is usually in the 2-5 range.
The vertical tail surface (called the vertical stabilizer or fin) provides yaw stability. This is the left and right direction. If the fin has an adjustable section, this is called a rudder. If the fin is too small, the airplane may have difficulty flying straight, especially if there is significant wind. If the fin is too large it adds extra weight and drag. A good size for a vertical fin is around 5% to 15% of the wing's area or 1/3 to 1/2 the area of the horizontal stabilizer. The aspect ratio of the fin is usually in the 0.5-2 range.
Some designs don't have a typical separation between the wing and the tail. These designs are called flying wings or delta wings because there is one continuous surface all the way back. Because the wing and horizontal tail surface are combined, you can typically add the area of the horizontal stabilizer to the wing to make it bigger. The movable flaps on the back of a delta wing are called elevons because they act as both elevators and ailerons to provide both pitch and roll stability.
Tail Placement
When deciding how far behind the wing to place the tail (effectively setting the length of the fuselage) there are several trade-offs to consider. A longer fuselage increases the moment arm of the tail, which gives it more leverage. As a result, smaller tail surfaces can be used, but a longer tail also adds weight and potential structural fragility. A shorter fuselage can be easier to build, structurally stiffer and more durable, but the tail will have less mechanical advantage so the surfaces will need to be larger to achieve the same stability. Longer fuselages tend to be better at dampening undesirable pitching and yawing movements. Short fuselages tend to make the airplane twitchier and more difficult to control.
For a good balance, with small free-flight models, the horizontal stabilizer should be placed about 1-3 times the wing chord length behind the wing. This typically offers a good balance between stability and ease of construction. The vertical fin should be mounted high enough to be out of wing's turbulent wake where it can see cleaner airflow.
For flying wing designs, you can picture an imaginary dividing line between the wing and the tail with the tail being placed 0 wing chord lengths behind the wing.
| Suggested Measurements for DIY Tail | |
|---|---|
| Horizontal Stabilizer Size | 10 cm span with a 4 cm chord |
| Stabilizer Shape | Rounded at the tips or slightly tapered |
| Stabilizer Placement | About 10 cm behind the wing |
| Vertical Fin Size | 4cm by 4cm |
| Fin Shape | Shaped like a triangular shark fin |
| Fin Placement | Mounted on top of the stabilizer |
Fuselage
The length of the fuselage will mostly be determined by the size of the wing and therefore the size and placement of the tail. The part of the fuselage that sticks forwards of the wing is called the nose. The purpose of the nose in a small free-flight airplane is mostly to balance the Center of Gravity (CG) in the right place (see below). If the nose is too short, you will have trouble moving the CG forwards without adding too much extra weight. If the nose is too long, it might move the CG too far forwards which will require adding weight to the tail, which is never desirable.
The fuselage will take a lot of stress during crash landings, especially the nose. The nose acts like a car bumper and will often crumple. To prevent damage and improve strength, use two layers of foam board glued together to make the walls double thick. You can also reinforce the nose of the airplane with some tape to improve its durability.
Our suggested fuselage length for a DIY foam board airplane is around 30cm, if you use our other suggested dimensions mentioned in this article.
Center of Mass / Gravity Position
The "center of mass" or "center of gravity" (CG) is the point on the airplane (front to back) where it balances on your fingertips without pitching forwards or backwards. The "center of lift" (CL) is the point where all of the distributed aerodynamic lifting forces are considered to act as a single average force. In order for the airplane to be stable, the CG should be placed ahead of the CL. This will allow the airplane to self-stabilize when disturbed by a gust of wind. If the CG is too far forward, the glider will need more elevator to maintain level flight, which will increase drag and reduce performance. If the CG is too far back, the glider can become unstable and tend to stall or dive in unpredictable ways. A good rule of thumb is to place the CG about 25-35% back from the leading edge of the wing.
Generally the CG for a freshly crafted airplane will be too far back and will require adding small weights to the nose of the airplane to move it forward. The lighter the tail, the less weight that will need to be added to the nose to get the CG in the correct spot.
When testing your airplane, if it dives sharply with a neutral elevator then you need to move the CG backwards. If the airplane rises and stalls then you need to move the CG forwards. Make small adjustments. Moving the CG just a few millimeters forwards can have a large effect.
Wing Loading
Wing loading is a measurement of the weight of the airplane divided by the area of the wing. A lighter airplane will have better glide efficiency and travel further in calm air. If you are flying in wind, you may want to add ballast to intentionally increase the weight of the airplane so it is not thrown around by the wind as much. Ballast should be added in such a way that the position of the CG is not affected.
Weight Distribution
Concentrating most of the weight of the airplane around the CG makes the airplane more responsive and easier to adjust. Try to keep the heaviest components near the CG for better handling. For small free flight foam board airplanes this generally means keeping the wings and tail surfaces thin and lightweight. When adding weight to adjust the CG, place the weights in the fuselage instead of on the wing tips.
Incidence Angles
The incidence angle is the angle that the wing or horizontal stabilizer makes with the fuselage. A positive incidence means that the wing's leading edge is tilted up slightly relative to the trailing edge. Generally for these small gliders, the wing should have a neutral or very small up tilted incidence of 0° to 2° and the horizontal stabilizer should have a neutral or slightly down tilted negative incidence of -2° to 0°.
For a simple DIY free-flight foam glider, we recommend setting the incidence of both horizontal surfaces to 0° and see how it works. If the airplane needs some adjustments to control its pitch, this can be done with an adjustable elevator.
Launch & Adjustments
Gently hand-launch your new glider at a slightly downward angle and observe the flight. What did you see?
- Airplane turned left or right.
- Bend the rudder slightly to control yaw. If the airplane turns left, bend the back edge of the rudder to the right.
- Bend the wings to control roll. If the airplane rolls left, bend the trailing edge of the right wing up slightly.
- Airplane pitched up and stalled.
- Move the center of mass forwards by adding some weight to the front nose of the airplane.
- Add some "down elevator" by slightly bending the back edge of the tail downward.
- Airplane dived sharply.
- Move the center of mass backwards by removing nose weight or if that isn't possible by adding some weight to the tail of the airplane.
- Add some "up elevator" by slightly bending the back edge of the tail upward.
- Airplane rolled and flipped over.
- Check for warps or twists in the wings.
- Increase the upward V-Shaped dihedral angle of the wings.
Basic Foam Board Glider Design
We used the above guidelines when we designed our BFG "Basic Foam Glider". This simple foam board airplane works very well as a hand tossed airplane and it has a hook on it for rubber band catapult launch too!
For those of you with a Pilot's License, you can download and print the plans for the BFG at home. You can also purchase this design from the Fold'N Fly Store. Glue or tape the plans to a piece of foam board and then use a hobby knife to carefully cut out the pieces. It goes together very quickly with hot-melt glue and then you can give it a hand toss to test it out. Once you get a little experiene, you'll be ready to modify it and/or design your own foam board glider!
| BFG Measurements | |
|---|---|
| Wingspan | 31.5cm |
| Mean Chord | 7cm |
| Wing area | 220cm2 |
| Aspect Ratio | 4.5 |
| Dihedral | 8° |
| Stabilizer Area | 39cm2 |
| Stabilizer Aspect Ratio | 2.5 |
| Fin Area | 11cm2 |
| Tail Placement | 10cm behind wing |
| CG | 12cm from nose |
Conclusion
By using these guidelines and by learning different foam board tools & techniques, you'll be ready to design your own simple glider! It's not as hard as you think, so give it a try. Start with the dimensions listed on this page and see if you can construct a glider that flies nicely. You can refer to our How to Steer an Airplane article for tips on how to adjust the control surfaces to get level flight. Once you understand how it all works, then you can start experimenting with different wing shapes and sizes. Have fun and don't forget to contact us and share what you built.
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