[Editors Note: Chris is a mechanical engineer who invented a robotic balance calibrator. His work is to make sure that the balance measures strains correctly. Read his bio at http://quest.nasa.gov/aero/wright/team/clockwood.html ]
PUSHING, PULLING AND TWISTING
by Chris Lockwood
March 23, 2001
My name's Chris Lockwood and I work in the balance calibration lab. The last time I did a web chat was during the Wright Flyer effort here at NASA Ames Research Center. I explained a little about wind tunnel balance at that time too You can check that info out on the web at: http://quest.arc.nasa.gov/aero/wright/team/fjournals/lockwood/index.html
A wind tunnel balance (or just balance) is a thing that looks like the cardboard tube from a paper towel roll with the cone part an ice cream cone, stuck in one end. The "cone" part fits into the end of a long pipe called a sting. The sting is mounted in the wind tunnel so the tip - the balance - is pointing up stream. The balance has electronics inside to measure forces. If you were to hang on the balance, it would tell you how much you weigh, where you were hanging and which direction. Now of course gravity only goes down so the direction would be down, but if you put your feet the floor and push up on the balance it would tell you how hard you were pushing, and the new direction.
So what? Well now if you mount a model of an airplane on the balance and blow some are air across the wings, the balance will tell you how much lift the wings are generating and which direction. In fact the balance tell you every thing that the wind is doing to the model, in all directions - pushing, pulling and twisting. The thing to remember about a wind tunnel is this: if the model is stationary, and you blow air over it at 100 miles per hour, model thinks it's flying at 100 miles per hour even though it's going nowhere (models don't really think, but you get the idea). A wind tunnel simulates flight without having to first make something fly. It's a tool for experiments.
The last few weeks have been very busy. We have had a company that builds airplanes here testing a model of a plane that already exist. Companies like this always want to improve the gas mileage of their planes. To do that, they make them as smooth and sleek as possible. During this test, they are looking at one thing in particular called "fillet drag". A fillet is a rounded corner where two surfaces meet. Like at the bottom of some swimming pools. Well the question is, how round do you make the joint were the wings meet the body (fuselage) of an airplane? This company brought lots of different fillet panels, each shaped slightly different from the others. They will test them all to find out which one will create the least drag (backward push from the wind). The more drag you have the harder the plane's engines have to push and the more gas you use.
We had to calibrate 6 different balances for this effort because of all the different model pieces they wanted to test (see the previous journal web site to find out what we do when we calibrate a balance). The biggest balance was high strength steel, as big around as a large plastic soda bottle, and twice as long. It will measure up to 8000 pounds of lift. That's the weight of about 3 cars!