Two Weeks to Go - Calibrating the Balance

By Fanny Zuniga

December 31, 1997

Two weeks to go before our test starts. We are busy right through the holiday getting ready for our test. A lot of work is going into getting our internal balance ready, so I want to explain some of it to you. Remember, our balance measures the aerodynamic loads (like Lift and Drag) on our model. Think of it as a very fancy, and expensive, bathroom scale that can measure forces in any direction very accurately.

In this picture the blue structure on the right is like a giant vice that is holding the balance which is encased in the grey metal box. The red cart on the floor is holding about a ton of metal plates that hang from the balance during the calibration.

This is a picture of our balance during calibration last week. What's that mean, you ask? First I need to explain how a balance works. The balance is a cylinder 5 inches wide and 18 inches long. Inside the balance are small metal parts that bend when a load is applied and send out a small electrical signal. The more it bends, the higher the voltage output. Fine, except that we have to calibrate the voltage signal. This is because if you double the load on the balance, you don't necessarily get twice the voltage out. When the balance is calibrated, it means that we can say "this many volts means this many pounds."

All of these weights were hanging from the balance when it was being calibrated!!

To calibrate the balance you have to hold on to one end of the balance, hang some "known" weight on the other end, and measure the output voltage. In these pictures the balance is actually inside the rectangular metal "block." The block is only there to provide a convenient way to attach weights to the balance. The blue structure on the right is like a giant vice holding the balance. Below the red cart hods about a ton of metal plates that we can hang from the balance. We hang up to 12,000 pounds because we expect the model to generate at least 10,000 pounds of Lift. That's 5 tons! All of the weights in the second picture were hanging from our balance! Are you surprised by how much weight there is? Looking at all that weight sure gives you an idea how much Lift our model can generate in the wind tunnel. Now you know why we had to make sure the model was so strong.

This is the balance in the prep room.

Now that its calibrated, we've moved the balance into a preparation room where it can get hooked up to the software that has been written for this test. The last picture shows the balance in the prep room. It is mounted on the two struts which will be used to hold the model in the tunnel. Remember, the balance is inside the metal block. You can see the electrical cable coming out of the back of the balance. Soon that metal block comes off and the model will be attached to the balance. When everything is ready, the model, balance, and these struts will all be carried into the tunnel.

Our software is nearly ready and it has been loaded onto a computer in the prep room which is just like the computer in the wind tunnel. One of the next big steps is to make sure our computer software understands the electrical signals from the balance and that it uses the balance calibration information correctly. To do this we will hang some weight off of the balance again to check that the software program records exactly the right weight. This should happen next week.

Also this week I'm reviewing the "plan of test" (or run schedule) and adding detail to this plan. I have to make sure that everyone is in agreement with the plan and that we make sure we set out our plan as efficiently as possible. Time in the tunnel is very precious. We only have five weeks to do everything we want to do including installation and de-installation so it's critical that we have an efficient plan.

Finally, this week I am also working on getting some software files ready for our use during the test. All this needs to be ready so that we can easily and quickly plot up information about our model as soon as we get data from the tunnel. Once the test begins, the researchers will want to look at the data as soon as we get it. This is important because, no matter how well we plan out this test, we will have to make some decisions about what we want to test based on how the model performs.

Well, that's it for this week. Things are getting very hectic and time is running out before the test begins, only two more weeks to go. I suspect I'll be working long hours the next couple of weeks. It's crunch time!!