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Design of Balsa Wood Gliders: A lesson in the engineering process

By Michael F. Kamprath

This is a lesson in which you will cover many aspects of the engineering process while designing and building a balsa wood glider. Many topics will be covered, including aerodynamics, computer aided design, and experimental testing. These topics are all being taught, but, in themselves, they are not being emphasized. What is hoped that you will get out of this lesson is the methodological approach one would take in designing and building an object—here, it is a balsa wood glider. Engineering an object can be split up into five definite steps:

Research
Design
Prototyping
Testing and refining
Actual production

In this lesson, we will only get as far as step four—testing and refining.

As previously stated, you will be experiencing these engineering steps through building a glider. Shortly, we will cover step one by discussing the aerodynamic theories involved with a glider's flight. After that, we will then try designing a glider on a computer with the help of a program call Glider Design!. The computer program will then print up plans to your glider from which will allow us to go on to step three—prototyping (that is, building a test model). Once we are finished with building our glider, we will then test it.

The following sections will be split up into each of the engineering steps.

Research

The theory that describes glider flight (or any type of flight) is really beyond the scope of this lesson. But it can be broken down into two simple, yet important, ideas. Theses ideas are that of lift and stability. I will discuss each independently.

Lift

As you may know already, lift on any airplane (gliders included) is created by the wings. But how is lift produced by the wings? The single most important reason wings produce lift is called the wing's "Angle of Attack." The Angle of Attack (AOA) is essentially the angle that the wing makes with the oncoming airflow (see figure below).

But how does this cause the wing to produce lift? What happens is when a wing flies at a (positive) AOA, air is force to flow faster over the top surface than the bottom. Now, when air flows faster, its pressure drops. This results in a higher pressure over the bottom surface of the wing than the top surface. The pressure difference between the top and bottom of the wing cause a force to push up on the wing and thus lift is created.

Stability

The stability of a glider is derived from the glider's ability to "balance" all the forces that act upon it. Examine the picture below. It is a side view of a typical glider in flight. Notice that there are three forces that act upon a glider (their names are underlined).

A real airplane, such as a Boeing 747, has a fourth force acting upon it called Thrust. Thrust comes from an airplane's engines, whether they be jet engines or propellers. But gliders have no engines, and thus no Thrust.

You will notice that Lift acts directly up from the glider's glide path (its direction of flight). Lift is actually a combination of the lift produced by the Main wing and the lift produced by the stabilizer. Because it is actually a combination of two different lifting forces, Lift acts somewhere between the glider's main wing and stabilizer. This point is called the glider's "neutral point." Now notice that the glider's weight acts from a point somewhere forward of the neutral point. The place from where the glider's weight acts from is called its "center of gravity."

Now, without going into too much detail, we are ready to explain how to make a glider stable. It is really quite simple—make sure that the glider's neutral point is behind the glider's center of gravity. This ensures that the glider will not "nose up" or "nose down," or, in technical terms, have uncontrolled pitching motion. "Yeah, right! How am I supposed to do that?" you may ask. In order to determine the neutral point and center of gravity positions, quite a few equations are required to be solved. But do not worry, the computer program Glider Design! solves these equations for you and tells you whether or not your glider will be stable.

It should also be pointed out that the glider's vertical tail should not be smaller than a certain size in order for the glider to be directionally stable. Again, Glider Design! checks to make sure that the vertical tail is large enough in order to ensure directional stability.

Design

At this point in the lesson, you'll get to use a computer program called Glider Design! Your teacher will demonstrate to you on how to use this program. Pay attention to his instructions. But before you use this program, their are some more term that you should become aware of. The first set of terms to be described are used in the description of the shape of a wing. Study carefully the picture below of a wing.

With the aid of Glider Design!, you will be able to adjust the shape of the wings on your glider by varying the dimensions of their root - and tip - chords, tip - sweep and wing span. Find what these terms describe on the picture above. The word chord is a term used in aerodynamics to describe the width of a wing at any one point along its spanwise length. The root chord refers the the chord of the wing at the place it is attached to the fuselage (main body), and the tip chord refers to the chord of the wing out at its tips. The tip sweep is a measurement of the vertical distance between the start of the wing's root and the start of the wing's tip. Finally, the wing span is distance from one of the wing's tips to the other.

It should be noted that Glider Design! only displays half of the wing at a time. This is because it is assumed (and pretty much required) that the wing will be symmetric about its root chord, that is, either side of a wing will be the mirror image of the other. Study the picture above in order to get a feel for what this means.

The next set of terms you should know are used to describe the various parts of an airplane. Study the picture below in order to get a feel for the terms Main Wing, Stabilizer, Vertical Tail, and Fuselage.

Please notice that the Stabilizer is typically the smaller wing in the back of the glider. The stabilizer is needed in order to ensure that the neutral point is behind the center of gravity (see the THEORY section if you forgot these terms). The term nose mass refers to the weight of the clay that you will need to add to the nose (front) of the glider.

The final term you show know before using Glider Design! is called the stabilizer incidence angle. Notice in all the pictures above that the gliders' stabilizer is angled downward a little bit. This needs to be done to ensure that the plane will fly stably. Glider Design will give you the angle your glider's stabilizer needs to be set at in order for it to fly stably.

Prototyping

Once you are finished with your glider's design, and Glider Design! says that it will be able to fly, print up the plans for your glider (your instructor will tell you how). From these plans, you will be able to build your glider. Carefully follow your instructor's instructions on building the glider.

Some key points to remember on building your glider:

Draw the outline for your wings ahead of time on the balsa wood. This will make cutting them out a lot easier. Also, be very carefully to cut along the lines you drew, otherwise your glider may not fly properly.
Before you glue your glider's wings to the fuselage, be sure to mark the positions on the fuselage they will be attached to. Have your instructor help you with marking the stabilizer incidence angle at which the stabilizer will be mounted.
When gluing the wings to the fuselage, use as little glue as possible. Too much glue will throw off the glider's weight, and thus make it unable to fly correctly.
When adding your glider's nose mass to its nose, be sure to make it into a smooth, aerodynamic shape. Your instructor will explain this to you better.

Testing

Now that you have your glider built, you'll want to throw it to see if it flies. Before you throw it for the first time, consider this: you set the throwing speed for the glider with Glider Design! to some value (if you don't remember what that value was, look at the bottom of the first page of your plans). Let's say you picked 22 kilometers per hour. You think, "How am I supposed to know exactly how fast I threw my glider?" You can not know, unless you have a radar gun. But there is one way you can know if you are throwing your glider hard enough. You do it by the following method:

Throw your glider.
If the glider "pitches" up and then falls to the ground, you threw it too hard. Return to step 1, but throw it softer this time.
If the glider seems to quickly head for the ground, you threw it too soft. Return to step 1, but this time throw it harder.
If your glider glides smoothly on a slow approach to the ground, you found the proper strength with which you should throw your glider.

Step 4 implies something in its statement: that a properly thrown glider will slowly head for the ground. This is true. A glider flying at proper conditions will always follow what is called a glide path, which is always angled towards the ground, not up towards the sky. Can you figure out why this is so? The angle the glide path makes with the horizon is called the glide angle. Study the picture below to see exactly what is meant by glide angle.

As part of your glider testing, we are going to figure out exactly what your glider's glide angle is (call the value of the glide angle by the Greek letter B. Say "bay - ta"). In order to carry out this experiment, we need to find collect two pieces of information. They are the distance your glider flew for a particular throw (call this number D), and the height at which you released the glider when you threw it (call this number H). You will need a friend to help you carry out this experiment (and you can help your friend). The method you will use to conduct this experiment is outlined below.

Find the proper throwing strength for your glider as outlined above.
Throw your glider at the proper throwing strength, and hold your hand still exactly where you let go of your glider. This is called the release point.
Have your friend measure the distance from where you released your glider to the ground. Fill this value in on the attached worksheet under the column for H.
Now have your friend measure the distance from where you were standing to where the glider first hit the ground. Fill this value in on your worksheet under the column for D.
Compute the value for B. You do this by evaluating the following formula: You will need to use a calculator to evaluate this equation. Your instructor will help you do this.
Repeat steps 2 through 5. two more times (your worksheet has room for this). By having a total of three values of B, we can find an average value for it. Why would we want an average value of B?
Calculate the average value for B and fill this value in on the worksheet.