Flight Research in the '70s

The late '40s through the the '60s is often considered the golden age of aviation, epitomized by the X-planes flown at the flight research center in New Mexico originally called Muroc and later as Edwards. For those born in the early '50s, you saw articles about these X-plane in the paper. But I was born in 1959, and while I saw the Mercury, Gemini and Apollo Missions, my teenage years were during development of the Space Shuttle which didn't fly until I was in college in the early '80s. So what did happen out in the desserts of New Mexico in the long decade between the last Apollo and the first Shuttle?

The Dryden Flight Research Center web-site ( http://www.dfrc.nasa.gov) is very interesting web-site. It provides photographs of the research aircrafts and several on-line books describing the research that took place during my teenage years, including Wingless Flight by Dale Reed and Darlene Lister. I found that while research may have been less exotic, it was fundamental to the success of the Space Shuttle.

When visiting the Dryden web-site and looking at the projects in the '70s, I continually overlooked the lifting bodies and digital fly-by-wire projects. The site has on-line books describing these projects, as well as the X-15. After reading through these books I realized how significant these three projects were to the success of the Space Shuttle.

X-15

The X-15 is probably the most famous X-plane after the X-1. It still holds altitude and speed records for such an aircraft. Ironically, it's contributions to the space shuttle has little to do with it's aerodynamic properties. However, it's aerodynamic properties and strength allowed it to go where no man had gone before, where the Space Shuttle was headed. Surprisingly, it's contributions to the Space Shuttle were not in aerodynamics, but in the study of the thermodynamic aspects of hypersonic flight. It is surprising to see how big a part thermodynamics, the study of heat transfer, plays in higher speed flight .

As modelers, we consider the aerodynamic forces on our models, and the designers of commercial aircraft also only consider aerodynamics forces because they are subsonic. But while the Space Shuttle does have to sustain severe aerodynamic forces, it travels at hypersonic airspeeds (greater than mach 5) and must also endure extreme thermodynamics forces. At these speeds, the friction of the air on the surface of the aircraft causes temperatures no other aircraft has had to endure.

As a research aircraft, the X-15 was built strong enough to withstand unknown aerodynamic forces, and built with titanium to withstand the heat. This allowed it to study the thermodynamic characteristics of hypersonic flight. It was also used to study decent profiles to minimize the impact of both thermodynamic and aerodynamic forces on the Shuttle. All surfaces of the X-15 were thoroughly instrumented with temperatures sensors to determines both what the maximum temperatures were as well as where they were. Later flights tested oblative materials which flake off taking heat with them. So while the X-15 was a revolutionary aircraft that proved in aerodynamic concepts allowing high alitude and hypersonic flight, it's most significant contribution has been in the study of the thermodynamics of flight.

Digital Fly-by-Wire

R/C modeling is inherently based on digital fly-by-wire concepts. But in the early days of fly-by-wire (FBW) research it was an enormous risk to fly an aircraft where a simple electronic component failure could render the aircraft unflyable, not only risking the life of the pilot and the investment in both money, but especially time in the research aircraft. The initial FBW research had to be done with extreme attention to safety, achieved with the use of extremely reliable and redundant systems.

It's also interesting to realize how inexpensively much of the research was done. An unused F-8B was made available by the Air Force at no expense. And while discussing funding for the project before a committee including Neil Armstrong, Armstrong suggested using spare Apollo flight computers. The F-8's mechanical flight control systems were left in place as a back-up, and five flight computers were used to provide the required reliability.

Most flight research at that time was primarily intended to support the testing of new concepts too expensive for any one commercial aircraft manufacturer to do. While some exotic concepts may be funded by the Air Force, most were not. The commercial benefit for FBW, which is common in most large commercial airliners today, is a significant reduction in the cost of the flight control system while maintaining the required reliability of the older mechanical systems. However, there are two other benefits to digital fly-by-wire.

The first was described as control augmentation, the ability to sense the movement and orientation of the aircraft and provide limited control input to maintain the stability of the aircraft in order to reduce the workload on the pilot. Significant experimentation was required to prove in the computer algorithms to keep the plane stable and provide only enough control input without requiring the pilot to work against the the computer for control of the aircraft. Experiments often included cockpit switches allowing the pilot to disable the computer.

The F-16 was the first production aircraft to use a fly-by-wire system, and because such a system can make an aircraft more stable, the F-16 was designed less aerodynamically stable making it more agile and maneuverable. Ironically, the Wright brothers designed their airplanes to be less stable to improve maneuverability. This capability is also used to maintain stability in the forward swept wing design of the X-29, although wing strength was a more fundamental research goal.

In combination with the self-stabilizing capability of a FBW system, remote flight is also possible, and this is significant to the Space Shuttle. With the capability to remotely pilot the Space Shuttle from the ground, NASA has the ability to return the Shuttle to the ground in the event of an accident where the crew is unable to pilot the Shuttle. The digital fly-by-wire research therefore provided a significant safety feature for the Space Shuttle, as well as making the Space Shuttle more stable.

Lifting Bodies

The third project affecting the Space Shuttle is the lifting bodies research. A lifting-bodies aircraft broke it's landing gear on landing, and its tumbling crach is immortalized in the opening of the Six Million Dollar Man TV series. The lifting bodies (LB) research project is based on the concept of using a half-cone shaped space capsule hanging sideways from a landing chute to provide limited maneuverability. From orbit, this limited maneuverability could guide it to a landing site within 500 miles of where the chutes are deployed.

A researcher at the flight test center who was a modeler first proposed the LB research project after building a model and seeing the maneuverability that was possible. The director of the flight test center, who was also a sailplane pilot, was a whole hearted supporter of the project. He started the project by having the first lifting-body aircraft built out of plywood by a local builder of sailplanes using less than $10,000 in discretionary funds. The use of discretionary funds ment he didn't need authorisation from his superiors, and intended to keep them ignorant of the project for as long as he could.

Initial testing was performed by towing the unpowered aircraft behind a suped-up Pontiac and evaluating it's flight characteristics. Since such an aircraft was intended as a space capsule, it could not have any protruding control surfaces such as a horizontal stabiliser and elevator. Instead all control would be provide through multiple and independently controllable vertical fin and rudders. The independently controllable rudders on opposite side could be used like ailerons to provide roll and yaw control. Initial flights were not immediately successful.

Digital flight simulators were also being developed around the same time as the LB project. Once the LB aircraft was measured in a wind tunnel, the simulator was programed with its parameters and used by the LB pilot to evaluate various control systems and to familiarize himself with the flight characteristics of such an unusual aircraft.

Ultimately, the lifting-bodies research reached the point of being towed from an aircraft and flown and landed like a glider. Several additional aircraft were built and tested, including the study of a powered landing required to sustain airspeed while landing at high angles-of-attack. This proved significant to the Space Shuttle design, convincing the Shuttle designers than it could adequately control its decent from orbit as an unpowered glider, and land safely on a sufficiently long landing strip.

I now know why there were no new X-planes in the '70s, and realise that flight research was not standing still. I also realise that flight research involves many things other that exotic looking aircraft that fly faster or higher. The problems with hypersonic flight have more to do with thermodynamics rather than aerodynamics. Digital fly-by-wire incorporating computer technology allow less aerodynamically stable but more maneuverable aircraft designs. And it is amazing that the idea for the Space Shuttle reentry is based on sailplane techniques and benefitted from a plywood aircraft built for $10,000. I think that the study of aircraft is facinating and is not just limited to piloting and aerodynamics.

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