some more from this guy:
Quote:
The reason for the moving (variable geometry) intakes on aircraft like the F-15, F-14, and F-4 has to do not with AoA, but with speed.
Ahh…..precious speed.
You see, no matter how much thrust a turbine engine produces, it will start to run out of steam as it approaches Mach 1.8 to 1.9. The turbine engine can only use airflow that comes in at a certain velocity or slower. As aircraft go beyond Mach 1.8 to 1.9, the intakes must re-orient themselves to slow down air velocity to useable levels.
So, as an F-15 moves past Mach 1.8, the intakes adjust their ramp angle to slow down air velocity to useable levels. This allows the F-15 to continue through Mach 2.0 al the way to Mach. 2.5
This is the principal that applies to the following aircraft
B-1A (Mach 2.2)
F-15 (Mach 2.5)
F-14 (Mach 2.4),
F-4 (Mach 2.2)
F-111 (Mach 2.2)
SR-71 (Mach 3.4+)
And others from that design era.
Aircraft that have fixed geometry intakes are basically limited in their top speed due to the inability of the intakes to slow down air to useable levels at those speeds.
NOTE:
This is completely independent of thrust to weight ratio F-16 and F-15 have comparable T:W ratios, but the F-15 is WAY faster due to its VG intakes.
Aircraft with fixed intakes include:
B-1B (Mach 1.2+)
F/A-18 (Mach 1.8 )
F/A-18E/F (Mach 1.8 )
F-16 (Mach 1.9+)
F-35 (Mach 1.8 projected)
F/A-22 (Mach 2.0 released)
A major point to note here is that Variable Geometry intakes are no longer included in modern aircraft design. Their movement, by nature, greatly increases the RCS of the aircraft which is not permitted anymore. The F-18, F-16, F-35, and F-22 all have SIGNIFICANTLY lower RCS than the aircraft listed above.
The B-1 program is almost a perfect example of what the VG intake does for speed. The original B-1A called for an aircraft that had a Mach 2.2 dash capability and as such, the GE F101 engines were fitted with complex variable intakes that allowed such speeds. The revised B-1B proposal did not call for that capability, so the variable intakes were replaced with simplified fixed intakes (and RCS screens) that limited the top speed to about Mach 1.2. In both cases, the engine was a 30,000 lb. Thrust GE F101 with few differences so the entire difference between Mach 1.2 and 2.2 was the intake system!
A secondary function of the VG intake is to maintain a smooth flow of air over the compression sections so you will notice movement during BFM, and AoA situations, but those are secondary functions easily solved with a fixed intake of sufficient design.
Make no mistake, their main purpose is max speed, but that’s really no longer necessary in this world. That’s why the VG intake will cease production with the last F-15 that ever rolls off the line…..
The F-15's Variable Ramps are constantly moving except for while they have WOnW. With WonW the ramps are commanded full down by the Air Inlet Controllers or Air Data Processors when the ramp switch is set in the Auto position,hydraulic power is available and cooling airflow is sufficient enough for operation. Which type of computer is controlling the aircraft depends on what modification the aircraft has.
The ramps can also be commanded down by maintainers using the A/B test switches located under doors 6L/6R. There is also a safety cutout switch built into both 10L/10R which working independently of each other. This function is builtin to ensure if someone has 10L/10R open that the first ramp won't crush them under the door if the ramps are commanded down.
Once inflight the ramps will move independently depending on not only speed but AOA. Depending on the speed and how much the ramp has to actually move because of a bank the ramp opposite of the turn will actually adjust up accordingly to allow for sufficient airflow. This is due to the disturbed airflow that is being generated across the nose of the aircraft.
Also yet another thing to note about the F-15's Variable Ramps is that when coming through the 1.4 to 1.5 MACH range the Diffuser ramp and Bypass doors will begin to function. Reason for this is to bleed off the oblique shockwaves that are created at this speed. If the diffuser ramp or bypass door fail to operate properly then a Inlet failure light will illuminate and they aircrew will throttle back.
The First Ramps lip will disipate as much of the shockwave as it can and so will the lower lip of the intake. The diffuser ramp will then bleed off the remaing oblique shockwave as it enters the intake itself. The bypass door is there so the remaining shockwaves that are being bled off by the diffuser ramp will exit at the top the aircraft. Also built-in to the first, second and diffuser ramps is tiny holes to bleed of standing static pressures. This is to keep from getting whats called duct buzz pressures built up in the intake. Excessive static pressures will cause the engines stall or stagnate.
Meanwhile, the PS2 probe that exists on the front of P&W -220 and -229 will monitor the impact pressures and static pressures. If any abnormalities exist between the AIC or ADP's calculations and the DEEC and EDU's calculations then a fault will be tripped and a Engine Control Light and Inlet lights will illuminate.
Though these may seem as very basic it's a critical part of flight for the F-15
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