Starr Ginn, Deputy Branch Chief, Engineering Directorate, Aerostructures Branch, Dryden Flight Research Center
- Tuesday, 01 December 2009
Ginn: Flutter is a natural phenomenon that happens, and it’s a case where you get just the right aerodynamic conditions where you’re exciting two different modes of a test article at the same time. Each mode is trying to beat out the other one, so what happens is, you have these two natural frequencies that are trying to occur at the same time and they just keep beating each other up until something structural catastrophically fails.
One example that everyone can relate to is the Tacoma Narrows Bridge. That was a situation where you had the wind speeds just right coming over the bridge that day, and it was trying to excite the bending mode and the torsion mode of that bridge at the same time. You saw the result of that. The two modes were trying to overtake each other, and neither of them won.
We do the same thing with aircraft. Any structure has natural frequencies. First of all, you have somebody who builds up a big finite element model and they say, “This is how I want you to build the vehicle.” And, of course, the vehicle gets built and, number one, maybe it’s not built exactly the same as what the model showed. Number two, anything in real life is not linear, and most all modeling techniques are linear-based. So what happens is, before we actually go to flight with a test article, we have to do ground tests to it to make sure that we do understand what it is that we’ve made, and then we try to update the model the best that we can – even though they are linear models – with what we know about the vehicle. So, when these structural models get updated, we run an aerodynamic code over the models and we do this flutter analysis. We tell it the flight conditions we’re going to have the test article at, and it tells us how the frequencies are going to be changing as the dynamic pressure increases. So what we look for are any of these natural frequencies going to go divergent or coalesce at these higher dynamic pressures that we’re going to flight test in.
NTB: Of all the disciplines that come under the general heading of mechanical engineering, what sparked your interest in vibration analysis, which is probably one of the more math-intensive subjects?
Ginn: I’m one of those people who think that everything just happens for a reason. I did most of my interning at Dryden doing mechanical design tests, and when it actually came time for them to give out job offers, they had to fill spots in certain areas and the offer that was sent to me was, we have a spot for you in structural dynamics – take it or leave it. And I absolutely was going to take it because this was my dream job and this was where I’ve wanted to work forever, so I took it.
It was very much a challenge for me because some students in mechanical engineering have to take one vibrations class – it just depends on the school you go to – but I didn’t even have a vibrations class. I came in completely open-minded to a brand new subject, so the first year was all on-the-job training. I spent most of the time doing all ground vibration testing, getting up to speed on different kinds of testing techniques in that area, and then I realized I’ve got to get some theoretical background so I really understand what it is I’m doing. So, about a year after starting in the group, I ended up slowly taking some masters courses to get myself up to speed on what my everyday job entailed. NASA has a graduate studies program where they’ll actually send you for a year to go get your masters degree, so I got into that program and I ended up going to UCLA and I got my masters in aerospace engineering, with an emphasis in aeroelasticity.
NTB: Here’s a chicken-and-egg question for you. When you’re not working at Dryden, you enjoy flying your own aircraft. Was it your work at Dryden that inspired your love of flying, or was it your love of flying that prompted you to pursue a career at NASA?
Ginn: It was absolutely my work at Dryden that made me want to start flying airplanes. Besides just being around flight tests – which I compare to the movie Top Gun, that’s how inspiring it is to me – there are actually a lot of engineers here who are all into flying personal aircraft, and it was just being around both of those environments. And, again, it’s really my competitive nature. One day somebody took me flying, I’d never been before, and this was actually when I was a student in the co-op program – I was 21 – and another full-time engineer took me flying, and I said, “Man, if this guy can fly an airplane, I can fly an airplane!” So that’s where it started. I decided I was going to go get my license, and it’s become more than just a hobby. My husband and I actually ended up building our house on an airport so that we have our airplanes in our backyard and we can taxi right out and go somewhere.