Originally Posted by
UAL T38 Phlyer
KN and Cub:
I'm a Mechanical Engineer (but have a bit of aero on the side), and that was the explanation I was given....seemed supportable with my knowledge of aero. But here is some info to ponder:
If we say the splitter is about 3 feet in chord, the front 1.5 feet are fixed. (The portion with the devil-head and logo in the photo). The hinge for the ramp is at mid-chord. The part of the ramp where the actuator is the "trailing-edge" of the external ramp, and is even with the intake lip; behind that, the ramp hinges and tapers back to full-opening of the intake tunnel. When it actuates, it moves the "trailing-edge" of the ramp outboard, and at full-deflection, restricts the intake to about half its area (an emergency if it malfunctioned and did it at subsonic speeds; thrust was severely limited).
KN, you are right, the ramp starts moving about 1.3 Mach down low (FL200 and lower): I would see it move at 1.7 to 1.8 on Functional Check flights at FL400 (it always moved at 500 kts indicated at that altitude).
At Mach 1.00, the shock wave is supposed to make a 45 degree angle. But if the portion of the ramp that moves is supposed to deflect the wave, that looks to me like it would still be inside the intake (as viewed in the picture above).
I thought the leading-edge of the splitter triggered the shock wave, which kept it out of the intake, and the ramp was an area-restrictor.
KN, I'm curious too about the F-104: I thought the shock-cones moved to keep the wave outside...true/false?
I know some airplanes (F-15; pretty sure the F-14) have a series of baffles that move internal to the intake to guide the wave around. but the Phantom just had the one ramp that is half external; half internal.
I think you are correct about the air splitter being an attached oblique shock trigger and strong bow shocks do not form ahead of airplanes like this at low mach numbers. But assuming the flow is subsonic behind that oblique shock extending from the side of the splitter plate would not be the whole story on what is happening there. The movable ramp is to place a second “normal” shock inside the inlet at an optimum place. The term for this situtation (I looked it up) is “critical mode”- what you get when the normal shock is ideally located for best stagnation pressures, flow attachment to the intake duct, and mass flow rates into the engine. Location of the shock would be determined by pressures existing in the intake scoop and it can be moved by adjusting the intake area.
To speculate on the action of this ramp, at design speed the intake shock is located ideally for "critical mode" and all is well and th engine gets good pressures and so forth. If the pressure drops due to a big increase in throttle setting, the shock would move forward which would place the shock sub-critical. The plate would move out reducing the area of the intake which would move the shock back again to the ideal location. Super-critical mode would similar but the opposite situation. I may have all this backwards but it is something like that.
It is also somewhat interesting why the intake duct on the F-4 is rectangular at the front rather than round as on many prior fighter designs. I think answer has to do with allowing a lower angle of flow at high angles of attack. Shape of the intake ducts in a rectangle or square was used in the F-14 and F-15 probably for the same reason. It is a little more curious on the F-15, assuming my theory is correct, because the 15 was not used for carrier landings.