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Originally Posted by GGTharos
The reason for their need is that as the schockwave shape of the incoming air changes in the duct, you lose pressure, which translates to loss of thrust. By re-shaping the shockwaves you can reclaim or preserve this pressure at higher speeds and altitude, thus effectively preserving more thrust at high altitude and speed.
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Mix of truth and rubbish there. The reason for variable inlets is that an oblique shockwave is more efficient than a perpendicular shockwave in slowing down air flows (less kinetic energy is converted into heat). These shockwaves are generated by turning the flow slightly.
Unfortunately, the amount you need to turn the flow by to achieve a shockwave of a particular angle varies with speed. Since you need to use multiple oblique shockwaves to get flow subsonic and you have space constraints, this means you need to keep the shockwave in the same place at all times and hence need movable inlets.
The ideal situation is no shockwaves at all. This is possible (rocket engines use the principle a lot) but not really practical for jet aircraft.
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Originally Posted by TIN MAN
The Typhoon has a fixed shock inlet, although there is a moveable "lip" at the base of the inlet to help mass flow at high Alpha. Fixed inlets tend to be inefficient above Mach 2 and tend to limit an aircrafts top speed due to lousy "stagnation pressure recovery".
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P0s/P0 at Mach 2 is 0.7209. Thus, even if Typhoon used a single plane shock (which it almost certainly won't) it would still recover 72% of the kinetic energy in the air at Mach 2. Moveable inlets might recover 90% if very well designed and if they have a significant weight allowance. Of course, they are also superb radar reflectors...
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Originally Posted by TIN MAN
The B-70 had the best lift/drag ratio of any aircraft ever built
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BS. Peak L/D for the B-70 was about 6:1, while modern airliners usually manage nearly 30:1 and open class sailplanes like the Eta reach L/Ds of nearly 70:1.