Aerodynamics and Aircraft Performance
Characteristics of High-speed Flight

Embry-Riddle Aeronautical University

From the beginning of the age of manned flight, aviators and engineers have continuously sought to increase the performance envelope. Many parameters for defining aircraft performance exist, but here I’d like to focus on going fast. Since I was a child I’ve been fascinated with military aircraft and the pursuit of speed. From the Bell X-1 to the famed SR-71 Blackbird and beyond, high-speed flight has been a chase toward an ever increasing limit. Several factors contribute to the aerodynamics of supersonic flight and many limitations apply as the envelope is pushed. I will focus the perspective of this paper on design characteristics, engine technology, and atmospheric considerations and take a peek into the future of hypersonic flight. In order to discuss the design characteristics of high-speed aircraft, a definition for the speed regimes must be given. Supersonic flight is typically defined as greater than Mach 1 but less than Mach 3. “High” Supersonic flight is a narrow band of operation from Mach 3 to 5 and hypersonic flight is in excess of Mach 5 (Benson, 2013). Shape technology and wing design are the primary areas of concern in setting out to build an aircraft capable of supersonic speeds. If airflow velocities reach sonic speeds at some location on an aircraft further acceleration results in the onset of compressibility effects such as shock wave formation, drag increase, buffeting, stability, and control difficulties (FAA, 2008). The shock wave produced by transonic and supersonic flight is responsible for most problems with airflow and can ultimately lead to flow separation, which will destroy lift. Modern aircraft wings are designed to mitigate these issues. One of the most uniform characteristics of wing shape is “sweepback.” Sweepback theory is based upon the concept that it is only the component of the airflow perpendicular to the leading edge of the wing that affects pressure distribution and formation of shock waves (FAA, 2008). Essentially, a swept wing lessens the impact of the airflow against the leading edge. The formation of shock waves is delayed and the aircraft can fly faster. Examples of this principle can be seen in the construction of nearly every supersonic aircraft built after the mid 1950s. One of the most beautiful in my opinion is the SR-71 Blackbird. It looks like a dart or broad-head hunting arrow from afar. The Blackbird has a wing sweep of over 50 degrees with a surface area of nearly 1800 square feet (Kucher, 2010). For the size of the aircraft, the surface area isn’t overly large. This illustrates another design principle for high-speed aircraft in the reduced size of the wing. As speed increases, lift increases so the faster an aircraft can go the more lift it will produce thus requiring a smaller lifting surface to maintain flight. Of course, in order to go faster, more powerful engines are required. All supersonic aircraft utilize some type of turbine engine and typically have afterburning capability. Afterburning is limited in use since you’re basically injecting fuel into the exhaust nozzle, igniting it and turn an air-breathing combustion engine into a rocket motor. As you can imagine, this goes through the fuel rapidly and this technology is used primarily for take off under high load or to achieve escape velocities. Sustained flight in excess of Mach 1 requires a little extra “juice.” The SR-71 and F-22 are two examples of aircraft that can sustain supersonic speeds without the use of afterburner. Both aircraft use turbine combustion engines that produce over 30,000 pounds of thrust per engine. Unfortunately, there is yet another limiting factor in engine operation at high speeds. Typical turbine engines require subsonic airflow to operate, so bypass louvers or vent screens are employed to restrict airflow and slow it down which ultimately restricts the air speed attainable. However, there is another solution in the form of a ramjet if speed is the utmost priority. In a ramjet, “ramming” external air into the combustor using the forward speed of the vehicle produces the high pressure. In a turbojet engine, the high pressure in the combustor is generated by a piece of machinery called a compressor. But there are no compressors in a ramjet. Therefore, ramjets are lighter and simpler than a turbojet. Ramjets produce thrust only when the vehicle is already moving; ramjets cannot produce thrust when the engine is stationary or static. Since a ramjet cannot produce static thrust, some other propulsion system must be used to accelerate the vehicle to a speed where the ramjet begins to produce thrust (Benson, 2013). The SR-71 uses a turbo-ramjet to generate thrust throughout the operating speed range. The aircraft operates as a standard turbojet at subsonic and low supersonic speeds. However, as it approaches cruise altitude and operating speed it converts to ramjet technology. The big cones mounted in the front of the inlets are moveable and transition forward decreases the inlet opening and force the airflow to increase velocity. Bypass louvers open to allow the airflow to pass the compressor stage entirely and go directly into fuel mixing and ignition. Compression is now achieved by shear velocity “ramming” the air down the tube. A final consideration must be made for atmospheric conditions conducive to high-speed flight. Generally speaking, the faster you want to go the higher you must operate. As altitude increases, air density decreases and with it so does air friction. Low to mid supersonic speeds are maintainable at nearly any safe altitude without exceeding the temperature threshold of the airframe materials. However, another problem is that the cloud base for most types of cloud formations is anywhere from six to forty thousand feet. You can imagine that flying an aircraft at fifteen hundred miles per hour through a thunderhead is pretty bad news. The rain erosion alone would wreak havoc on the skin and sensor ports, never mind the turbulence that would probably cause structural failure. Just because the aircraft has enough thrust to achieve high-speed flight doesn’t mean it can safely do so unless the service ceiling is high enough to reach clean air. Now imagine pushing the envelope even further into the boundaries of hypersonic flight. Some interesting things occur when you thrust an object through the atmosphere at those speeds. The main characteristic of hypersonic aerodynamics is that the temperature of the flow is so great that the chemistry of the diatomic molecules of the air must be considered. The molecular bonds vibrate at low hypersonic speeds, which only changes the magnitude of the external forces on the aircraft. At high hypersonic speeds, the air molecules break apart producing electrically charged plasma around the aircraft. In addition to extreme heat, large variations in air density and pressure occur due to shock waves and expansions (Benson, 2013). The SR-71 used titanium alloy construction to withstand the temperate at speed. Unmanned aircraft capable of exceeding Mach 5 make use of exotic nickel alloys and active cooling to maintain operation temperatures on the skin. There are many variables in the pursuit of speed beyond design characteristics, engine technology and atmospheric issues. These are just a few of the problems encountered and overcome in over six decades of jet-powered flight. Imagine what we can overcome in another six decades. The leading edge of technology today, in the form of rare construction materials and scramjet engines, will one day become the operating standard of tomorrow. Will our children or grandchildren be able to circumvent the globe in comfortable passenger aircraft traveling in near space at more than 10 times the speed of sound? Every step forward in aeronautical science brings us just a bit closer to that future reality.

REFERENCES

Benson, T. (2013, February 22). Index of Aerodynamics Slides (Speed Regimes). NASA Glenn Research Center. Retrieved May 14, 2014, from https://www.grc.nasa.gov/www/k-12/airplane/short.html

Federal Aviation Administration. (2008). Pilot's Handbook of Aeronautical Knowledge (FAA-H-8083-25A pg. 66-111). US Department of Transportation, Flight Standards Service.

Kucher, Paul R. (1998 - 2010). Lockheed SR-71 Blackbird. Retrieved May 14, 2014 from http://www.sr-71.org/blackbird/sr-71/