How Things Work: Supersonic Inlets
- By Diane Tedeschi
- Air & Space magazine, November 2002
(Page 2 of 2)
During some Blackbird flights, however, the harmonious working of the spike and the forward and aft bypass doors broke down, and all too quickly the inlet was filled with more air than it could handle. When the air pressure inside the inlet became too great, the normal shock wave was suddenly belched out of the inlet in an unstart, accompanied by an instantaneous loss of air flow to the engine, an enormous increase in drag, and a significant yaw to the side with the affected inlet. Unstarts occurred “when you least expected them—all relaxed and taking in the magnificent view from 75,000 feet,” wrote Graham in SR-71 Revealed. If the crew’s attempts to restart the inlet’s supersonic flow failed, they would have to slow their aircraft to subsonic speeds.
With a top speed of Mach 1.6, the Lockheed Martin F-35 Joint Strike Fighter has an inlet design that is far simpler than that of the Mach 3-plus SR-71; the single-engine JSF inlet cannot vary its geometry. The F-35’s engineers could get away with a less complicated design because at vehicle speeds up to about Mach 2, the shape of the inlet itself can slow down much of the supersonic air before it enters the inlet. The JSF inlet is, however, a breakthrough design: It has no diverters. Traditional fighter inlets, such as those found on the F/A-18 and F-22, have slots, slats, and moving parts to divert or channel airflow. The F-15 inlet has ramps and doors that alter its external and internal shape to adjust airflow as needed.
Many other currently operational fighters also have boundary layer diverters. Air that clings to the surface of an aircraft in flight is known as boundary layer air, and it tends to cause turbulence in the air flowing into the engine, especially when it interacts with shock waves. Inlet designers try to keep out as much boundary layer air as possible, frequently positioning the inlet several inches away from the surface of the fuselage and its boundary layer air and employing a duct system to whisk the undesirable air away. (The SR-71 inlet rids itself of boundary layer air by sucking it in through slots on the spike and passing it through ducts that exit the nacelle.)
The F-35 inlet, however, is positioned flush against the fuselage, and just in front of the inlet opening is a raised surface, or bump, that pushes much of the boundary layer air off to the sides and away from the inlet. The bump serves another purpose: During supersonic flight, it compresses and slows the air passing over it into an oblique shock wave. The air is still moving supersonically, however, and it is slowed down to subsonic speeds after passing through a normal shock wave that forms at the mouth of the inlet. The simplicity of the JSF design makes for an inlet that requires less maintenance, reduces aircraft weight by 300 pounds, and costs $500,000 less than a traditional fighter inlet.