Does the intake produce thrust or drag?
Some sources say, without references, that
"It is commonly cited that a large amount of the thrust at higher mach numbers comes from the inlet. However, this is not entirely accurate. Air that is compressed by the inlet/shockwave interaction is diverted around the turbo machinery of the engine and directly into the afterburner where it is mixed and burned. This configuration is essentially a ramjet and provides up to 70% of the aircraft's thrust at higher mach numbers."
Ben Rich, the chief thermodynamicist who designed the SR-71 inlets, says the aircraft gets 65% of its propulsive thrust from the inlet, 25% from the engine and 15% from the ejector nozzle. He refers to the compressor as "a pump to keep the inlets alive". It is said the thrust produced by the inlet comes from the high pressure in the inlet acting on the spike.
SR-71 Retirement Ceremony speech by Ben Rich, Lockheed Skunk Works
So the answer would seem to be,
THRUST. Thats if you take the words of the designer at their face value. NASA also says the inlets produce thrust in this particular application (SR-71) and I would tend to think they know of what they speak.
http://www.nasa.gov/centers/dryden/p...ain_H-2179.pdf
The inlet spike (fig. 4) translates longitudinally, depending on Mach number, and controls the throat area. The spike provides efficient and stable inlet shock structure throughout the Mach range.
At the design cruise speed, most of the net propulsive force derives from flow compression pressure on the forward facing surfaces of the spike.