The type of wind tunnel test that was run would be called a “sweep test”. The glider was run through the same test twice. During the test, the angle of attack of the glider was changed from 0 degrees through to 12 degrees. So for test 1 (Run # 1) there were actually 10 points in the test when data was taken (Point 1 through 10). The test was repeated (Run #2) and at 10 points in the test, data was taken (Point # 1 through 10). For each point in the test, the angle of attack for the glider was changed (alpha: degrees). At each point in the test, measurements were taken for lift and drag. At each point, the amount of lift was measured in pounds (Lift: lbs.) and the amount of drag was measured in pounds (Drag: lbs.).

For example, in Run #1, Point #2 with an angle of attack of 1 degree, Martin’s glider generated 3 tenths of a pound of lift. It also created 4 one-hundredths of a pound of drag. Compare that to Run #1 at Point #8 with 10 degrees angle of attack when Martin’s glider generated 1 pound of lift and created only 16-hundredths of a pound of drag.

L over D (L/D)

The values of lift and drag are also shown as a ratio. We call it “L over D” (L/D). If the L/D number is high, then the design being tested can fly efficiently. When an airplane flies efficiently, it is creating the needed lift while creating little drag. This is important to flying because such an airplane can fly farther using less fuel.

The “L/D” column or last column on the right of the chart shows Lift divided by Drag. For example, look at the first line of data (Run #1 and Point #1):

This ratio tells us how efficiently lift is being made. Let’s look at four basic ways the lift and drag numbers could turn out in a test:

1. If the lift number is high and the drag number is low, then we say that L/D is high. That is a good thing!

2. If the lift number is high and the drag number is high, also, then we say that L/D is low and that the design is less efficient. That’s not a very good thing! We would need to redesign the airplane to lower the drag number. Or, we can increase the thrust by putting on larger (and heavier) engines.

3. If the drag number is high and the lift number is low, then we say that the L/D is real low. That means the design is very inefficient. That is not a good thing!

4. If the lift number is low and the drag number is low, then we say that the L/D is low and that it is not an efficient design. That’s not a very good thing! We would need to redesign the airplane to increase its lift.

Coefficient of Lift = CL

Knowing the amount of lift and drag that is generated by the design being tested is important. Just knowing how much lift is made though is not enough. We need to know other things, too. The amount of lift that is made during a test depends on many things. Here are some examples:

1. A big wing makes more lift than a smaller wing.
2. Flying faster makes a wing produce more lift.
3. Air of greater density will be able to make more lift than air of lesser density.

By combining all these variables into one number, researchers can compare test results more easily. This is done for lift by combining the following measurements:

1. Amount of lift (measured in pounds)
2. Wing area (measured in square feet)
3. Speed (measured in feet per second)

This one number is called the “coefficient of lift” or “CL”. The mathematical equation used to get this one number (CL) is given below:

This number is used to compare the airplane’s flight performance without having to run more tests. This means, for example, that a small-scale model of the space shuttle being tested in a wind tunnel will have the same CL as the full-size shuttle flying through the upper atmosphere.

Coefficient of Drag = CD

The CD is the coefficient of drag. It’s another calculation that is very important in wind tunnel tests. The CD includes the same measurements as CL except it uses drag instead of lift. This number can be used to make comparisons in much the same way as the CL.

Researchers make greater use of CL and CD because they work with models when performing wind tunnel tests. By converting a lot of the data to numbers like CD, researchers can compare data more accurately. Researchers can compare data from different tests. They can compare data of models to full size aircraft. And they can compare data from different aircraft designs. Researchers can then make predictions about how the model will fly based upon the CL and the CD.

Now that you have some insight into what the data means, help Martin Northrop answer some questions using the data table and the graphs that show the results of the tests.