Aerospace Stuff from A Previous Incarnation

NASA Langley Research Center 1989

I did my Master's research at NASA Langley in the 30 x 60 Foot Full Scale Wind Tunnel, and received an M.S. in aerospace engineering in 1990. My specialization was the aerodynamics and flight dynamics of subsonic airplane configurations. My research was on the flight dynamic characteristics of an "over-the-tail" (OTT) advanced turboprop configuration, as part of the Advanced TurboProp (ATP) program. The idea was to place the high speed propellors over the tail to: a) reflect noise away from the ground so the airplane wouldn't be so loud flying over your house; and, b) increase the effectiveness of the tail surfaces, allowing them to have smaller area and therefore produce less drag, possibly reducing fuel requirements. I'm not sure any of that really worked out, but it was a cool project.

Here are a couple of photos from my wind tunnel research. These are NASA official photos that I was able to get copies of. The propellors are made of carbon fiber. Eight blades per engine. For the wind-tunnel model, the blades are driven by air motors at, I think, 10 thousand rpm. We managed to crack one of the blades, which were not cheap or fast to make. Fortunately, we had one spare. We also ruined one of the threaded shafts of one of the air motors. They too were custom made in Langley's shop. Took a solid month to get a new part machined from an aluminum block.

Advanced Turboprop Over-the-Tail model in the Langley 30 x 60 Foot Wind Tunnel, 1990 - NASA Langley Photo 90-01876

Over-the-Tail TurboProp in NASA Langley 30 x 60 Foot Wind Tunnel, 1990

NASA Langley Photo 90-01876

Graham Rhodes Posing With Advanced Turboprop Over-the-Tail Wind Tunnel Model, 1989 - NASA Langley Photo L89-14718

Graham Rhodes Posing with Over-the-Tail TurboProp at NASA Langley, 1989

NASA Langley Photo L89-14718

North Carolina State University 1984-1988

I pursued a Bachelor's degree in aerospace engineering from 1984-1988 and received the degree in 1988. My focus was airplane design. In our senior year, we were given the option of completing an airplane design project or a spacecraft design project. The airplane design project enabled us to actually construct and fly our design, and at the time that seemed more interesting to me. My group was tasked to design a canard configuration, in which the main wing is in the back and the horizontal wing is in the front. There were two other designs, one a conventional airplane (that ended up being not too conventional) and a tri-surface design. The tri-surface design was called Tritanic and true to its namesake went down on the first flight. It was rebuilt and later flights were successful. The conventional design was not balanced correctly on the first flight, but our test pilot was able to bring it down safely. Our plane was dubbed the BFX-29.5 after the famous X-29 concept fighter (the extra 0.5 meant that we were a half step beyond the X-29, and the "BF" meant Blind Faith). An early proposed name was "Kniskern's Canard," after our fearless project leader, Mark Kniskern. (The "K" is usually silent, but was to be pronounced in the airplane name.) Like the X-29 that was our inspiration, our design had a forward-swept wing. The first flight was flawless, except for a rough landing that destroyed the cool fairing that we had installed on the landing gear. Our airplane was fairly aerobatic, and NCSU's test pilot, Robert Vess, was eventually able to do several fast rolls. The project was such a success that we were eventually invited to bring the airplane up to NASA Langley and test fly it at their Plum Tree test facility in Poquoson, Virginia. I attended these tests. Langley has some very nice video equipment with long zoom lenses, and I do have a tape of video footage of our airplane in flight. May try to capture and post some of that here eventually. Those of us who visited Langley in 1988 were absolutely thrilled to find that at the same test facility they had a 1/3 scale drop model of the X-29 that had inspired us. The drop model is balanced so that the inertia tensor accurately represents the full size aircraft. The model is then dropped from a helicopter at an altitude of 10 thousand feet, then flown down by radio control. For the drop models, there is an onboard camera and the test pilot sees, well, a pilot's eye view out of the small model. VERY cool stuff! There was a swamp nearby, and occasionally one of the drop models would land in the swamp (when, for example, an unstable configuration went out of control---better to have it happen with models than with the real thing!). There was a very large tractor, with, I think, 10 foot diameter wheels, which they would use to retrieve models that ended up in the swamp. The models were far, far from cheap, and they were not dispensible!

The forward-swept wing was contraversial. There were a couple of aerodynamic reasons for it, but at low speeds it didn't really do much for us aerodynamically. But, it gave us a huge structural advantage. Most canard style airplanes have rear-swept wings and must mount the vertical stabilizers on the tips of the wings to place them far enough behind the center-of-gravity to be effective at providing directional stability. This means the wings have to be beefed up structurally, which increases weight and makes for some very interesting intertia tensor characteristics that affect flight dynamics. With the forward-swept wing we were able to place the vertical stabilizer on the fuselage, like a conventional design, meaning the wings don't have to be stronger than usual, saving weight and making for more conventional inertia characteristics.

The canard concept also has its problems. For the airplane to be stable in a stall, the horizontal stabilizer, in front, must stall before the main wing. In a production airplane design, the stabilizer orientation and airfoil would be tuned carefully to maximize the amount of lift generated at very low speeds during landing----which occurs somewhat close to stall. The tuning is difficult, and we didn't get it right. Our stabilizer stalled first, but far too early. So if the test pilot came in to land too slow, the stabilizer (or stab for short) would stall, and the nose would drop hard on the runway. This was the cause of the destruction of the nose wheel fairing on the maiden flight. In an attempt to remedy the early stall, we place a line of vortex generators on the stab, about 30% back from the leading edge. The idea is that these will generate vortices at higher angles of attack. The vortices mix energy from the free stream flow and thus energize the boundary layer, delaying stall. At the same time, drag is increased. Increase lift by delaying stall---good for lower speed landing. Increase in drag---helps you slow down! But, that wasn't too successful. We really didn't see any change with the vortex generators. I expected to see some. I believe that either: a) we placed the vortex generators too far back from the leading edge; or, b) the offset angle of the vortex generators was ineffective. The placement and angle were taken from essentially textbook designs, without experiment. It was an ad-hoc fix that ultimately was a bad guess. Our most successful landings were hot, and the airplane invariably ran off the end of the runway (no brakes). At Plum Tree, where it was even a bit wet and swampy immediately around the runway, we had people wait at the end of the runway to try and catch the plane before it ended up in the mud or water. I think we could have done better by putting the stabilizer in the wind tunnel at NCSU to see what really was going on.

Here are some photos (list will grow) from the BFX-29.5 project, circa 1988.

BFX 29.5 at the flying field

Closeup of BFX 29.5

Oh, this last one isn't a photo of the BFX-29.5. It is a photo of a Personnel Launch Vehicle (PLV) prototype built by Robert Vess and students at North Carolina State University around 1989 (while I was at NASA Langley as I recall) or maybe 1990. This wasn't a flying model. It was full scale, and pretty. But primarily used to explore ergonomic issues and interior design/layout for the astronauts. This model was constructed for NASA Langley, and was used in research at NASA. It appeared on the cover of an issue of Aerospace America magazine.

NCSU Model of NASA Personnel Launch Vehicle

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Page updated on 9 December 2005. Text is copyright (c) 2005 Graham S. Rhodes. The photos are copyright (c) 1988-2005 Graham S. Rhodes except for the two NASA photos, which are in the public domain.