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by Susan Lee

January 22, 1999

Just before Christmas I went over to the Balance Calibration Laboratory with my two high school interns in tow. See their version of this visit at http://quest.arc.nasa.gov/aero/wright/tunnels/balcallab.html We wanted to see if we could get some video tape of the laboratory and the automatic calibration machine. One of the students was interested in video on the Internet and this was her chance.

Most people say that the balance works like the bathroom scales. It's a little bit more complicated than that. Scott Lijon showed us some of the strain gages that will be used for the Wright Flyer Test. These looked like little metal tubes with tiny multi-colored metal wires coming out of them. Phil Luan explained to us that one of the gages measures normal forces, and another measures axial force. These will be translated into lift and drag of the airplane. This information is relayed as an electrical signal that gets translated by software into pounds (lbs.) of lift and drag. Altogether the 6 gages in the balance will measure lift, drag, and side forces as well as pitch, yaw and roll. Basically if you make these measurements you will know if the plane will fly and when it will stall and how to operate it safely, like whether it has enough lift to carry elephants or just people.

They had lots of different balances there; each is designed to work within different ranges of measurement. Think of this as the difference between a baby's scale and a grown-up's scale.

So what's all this about calibration? Phil explained that they want to make sure that the measurements made by the balance are accurate and repeatable. That means they can trust the results they get in the wind tunnel.

How can they be sure that the balance measures 100 lbs. of weight as 100 lbs. of weight? They have metal disks that they hang from the balance. They know how much each disk weighs and then they check the measurement the balance makes.

Well if that wasn't exciting enough, they have built a new machine that uses actuators (a device designed to create motion, "little push-and-pullers" if you will) to test the balance. This is one of the new wonders of modern science. They won't have to hang weights from the balance they will be able to just put it in the automatic calibration machine and test away!!

The good news is: we expect to get accurate measurements for the Wright Flyer test when we turn on the wind in the wind tunnel. I wonder how the model will hold up. Here's hoping it comes through with flying colors!!


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