On a roll ! - One more week!by Fanny Zuniga
February 19, 1998
We are settling into a routine here, and making great progress on our test. We made our 400th run late this week. Things have really calmed down, so I'll just give you some highlights of the week. Our day team and night team are competing to see who makes more runs in the tunnel. The crews are also competing to see who can make the quickest model changes. It's like we are having some Wind Tunnel Olympics going on here!
We can see the light at the end of the tunnel. The end of the test means we all get a much needed rest. Meanwhile, it seems that now we could keep the model for an extra week if we need it. And, the next test to use this tunnel can give us a few extra days if we really need it. But, by the end of the week we caught up with our planned run schedule so we may not need extra time back after all. So, everyone is asking "When will this test end?" We are feeling a bit less pressure and are confident that we'll complete almost everything we had planned.
This also means we should get to do some of the interesting stuff we planned for the end (stuff we'd skip if we hadn't caught up). For example, on Thursday, we decided we could set up the model deformation system I described a few weeks ago. So we removed the mini tufts from some of the left hand wing, painted the the wing black to cut down glare, then applied reflective targets over that. By Friday we took some practice images to checkout the video imaging system that we will use next week to calculate how our wing bends and twists under high loads. As another example, if we keep making this great progress we should have time to use the Pressure Sensitive Paint (PSP) I talked about. So we are refining our preliminary plans for using PSP and really working out the details of when to paint, when to install cameras, and when to make the runs. And we still hope to just squeeze in the oil flow studies after the PSP runs.
We have also been able to do some of the things that we really wanted to do but that weren't the highest priority. We were able to study the effects of the model "skidding". This helps make sure the airplane will be controllable in flight. For this, instead of just tilting the model's nose up and down (angle of attack), we also swung the model's nose to the left and right. We also stuck some model wheels on to see how they effect the aerodynamic forces during takeoff and landing. We also got to test how effective the horizontal tail is. The tail is the primary way that the pilot controls the speed of the airplane, so we tilted the tail up and down for a bunch of runs to make sure it could control the airplane.
We have had the usual list of small problems. For example, we use wax to fill in holes where screws hold the flaps on the wing. We had some trouble keeping the wax from blowing off the model. We also had a small piece of the model come off during one of the runs. A big piece flying down the tunnel is a test team's worst nightmare because it can damage the tunnel. Since it was a small aluminum part, it wasn't a big deal. A bigger piece, or harder metal, might have damaged the giant fan that pushes the air around. Finally, remember those accelerometers we use to measure the model's angle of attack (a very important thing to know)? Also remember I said we double up on instruments when we can? Good thing we did! The main one we were using quit working properly, but we were able to keep on testing using the backup.
When everything is working well, here is a typical sequence of activities: We look at our run schedule and decided what we want to run next. Usually we follow our original run schedule, but we can make changes based on what we've learned about the airplane. If we are running behind, we can skip over something that is less important. Next, we tell the shift engineer what we want to do. He/she opens up the wind tunnel and the mechanics change the model to the next configuration. When the tunnel is finally closed up, several different conditions (like airspeed) are run. For each run we tilt the model through several angles of attack. As soon as signals start coming in, technicians process the data and send it to us to look at. We plot it up on our computers, check to make sure it looks OK, and use the information to answer our research questions and to help us plan the next set of runs.
We do the things that are fastest the most often, like change angle of attack (the tunnel controllers do this in 2 seconds). The things that take a long time, mainly changing the model configuration, we try to do less often. An example - - our Olympic Tail Changing Team can take the tail off in 5 minutes once the tunnel is stopped and the test section is open. Changing flap angles, however, means taking out a lot of screws, changing the flap angle, and putting all the screws back in - maybe one and a half hours. We generally try to sequence our list of runs in this order: for each type of flap we go through all the flap angles. For each flap angle we run with the tail on and off. For each of these configurations we get data for all the angles of attack and airspeeds we want. Finally we put on a new flap type and repeat the whole sequence. Once we make a model change, we want to get all the data on that configuration. We don't want to have to change the model back to the same configuration later just to get one more piece of information. That's why we spend so much time in the beginning of the test making sure everything is working properly. Once we move on, we want to be confident that we won't have to back up. One exception to this pattern is flow visualization studies which we save until the end of the test because they take so much time. Sometimes this means we have to go back to a configuration we tested earlier.