Thread: Variable Geometry Inlets

Originally Posted by JA Boomer

Just looking for some information on variable geometry engine inlets such as those found on the F-14, F-15, and B-1A. How exactly do they work, I mean I know they control the air flow into the turbojet engines by changing shape. But how specifically fo they accomplish this?

On the F-14, there are three ramps, made of metal. They are hinged, and actuated by three hydraulic actuators. The ramps move into pre-determined positions depending on an input from the airspeed sensor to the analog Automatic Flight Control System. The AFCS in turn works with the engine control system, and autopilot, all analog computers to control the aircraft.

At sub-sonic speeds, all three ramps are in the up position, with a small gap between two and three. The gap allows some intake air to escape through the bleed air door on top of the inlet housing. This lets sub-sonic, air enter the engine.

At transonic speeds, ramps one and two are slightly opened creating a shock wave that is deflected downward, away from the engine intake. Ramp three is slightly opened, allowing more air to escape through the bleed air door. The goal here is to allow only subsonic air to enter the engine.

At supersonic speeds, the ramps are fully deflected. This allows a lot of air to escape through the bleed air door. It also compresses the air right at the inlet. This compression accelerates the intake air and creates a large shock wave that is reflected away from the intake. Once the air passes the constriction of the ramps, it has more room and less compression. With less compression, the air slows to a sub-sonic speed, which is just what the engine needs. The shockwaves, are always deflected away from the engine air intakes.

Testing developed the three positions needed for optimal air intake speed on the F-14. Thus, an analog computer can do the work as it only needs to know one input, airspeed. It then picks between three positions, closed, slightly open or fully open. The bleed air door is a fixed position door. Again, testing developed the optimal setting. So once that was decided on, it was fixed and no further control was called for. This rather simple system results in some limits on the top end of the speed range for the F-14. In flight testing with some experimental intake ramp positions, it was discovered that the F-14 could easily hit Mach 2.6. However, the engines had real problems at transonic speeds where the airplane usually works. Thus it was decided to sacrifice some top end for simplicity and much better control in the middle of the speed range.

Thanks for the interesting info. So it sounds like the three ramps on the Tomcat were located inside the air intake for each engine. Was there any mechanism where the outside of the intake changed shape like the Eagle?

I'm still racking my brain as to how exactly the sides of the F-15 intakes compress like that. I'm hoping not to have to wait until next year's airshows to find out, anyone?

Also, what exactly about the variable intake is it that balloons the aircrafts RCS?