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Reading the question and answer to How does the Yak-130 fly with blocked engine inlets, I am now wondering why it - or any fighter jet - has forward engine intakes at all? The top and accepted answer indicates the forward engine inlets are closed when taking off from unprepared fields to avoid foreign object damage (FOD) to the engines, and inlets on the top are opened at the same time.

If the inlets on the top can supply enough air for the engine during takeoff - the phase of flight with the most demands on the engine - why not use them all the time and simply not have forward inlets at all? It would simplify the design, protect the engines from FOD all the time, and probably offer a few other benefits, but I'm sure there's a big trade-off - maybe stealth? Fuel consumption?

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    $\begingroup$ Already we have good answers, so just a tl;dr comment: FOD doesn't matter for most of the mission, so optimize airflow for the common case. $\endgroup$ Feb 3 at 6:32
  • $\begingroup$ HEH, I was going to ask the same thing after reading the same referenced question :) $\endgroup$
    – Fattie
    Feb 5 at 22:50
  • $\begingroup$ Interest only: B2 does not use taxiways, to limit FOD. They reverse taxi on the main runway and then turn round. (AFAIK) $\endgroup$ Feb 6 at 9:27

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When the inlets are on top, the air entering the engine must make a 90 degree turn to get into the engine. This will cause inefficiencies and might cause massive separation -- which would be very bad.

At high thrust and low speed, the flow can stay attached and the engine can tolerate inefficiency (takeoff and landing are a short period of the overall mission).

At high speed cruise conditions (for a subsonic aircraft), you need to use the flow decelerating into the straight-in ducts -- and you can't suffer the losses that would accompany a 90 degree turn.

If you're supersonic, it gets even worse -- fighter jet inlets are mostly designed around supersonic operation, with lots of doors, ramps and other contraptions to make them operate at other conditions. The faster the cruise, the more complex the inlets.

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  • $\begingroup$ Adding onto this great answer: as far as I understand them, you dont want supersonic airflow (specifically the shock front) entering the compressor stage of a gas generator (the 'jet engine'). Even the ramjet mode of the J58s (SR-71 engines) slows incoming airflow to subsonic utilizing the ram effect to feed air past most of the compressor stages and into the burner. However, keep in mind, air going into the big forward intakes is still used for thrust, so even if its slowed down it is adding pressure, thus gaining some 'ram air' efficiency gains even to non-ram designs. $\endgroup$
    – Tank R.
    Feb 5 at 12:24
  • $\begingroup$ Even subsonic aircraft use their inlets to slow down air. A transport aircraft at Mach 0.85 has a fan front face Mach number of about 0.6. $\endgroup$ Feb 5 at 18:09
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Forward-facing intakes provide ram air compression, which convert airspeed into additional pressure. This significantly improves engine performance if there's any significant airspeed.

It's true that takeoff is the most demanding phase in terms of static thrust. However, it's not always the most demanding overall. At altitude, air is less dense, so more air is needed by volume. Ram air effect helps collect and compress it.

The opposite happens to a side- or top-facing intake: air has to be accelerated with low pressure to "suck it in", reducing both efficiency and mass air flow, and thus thrust. A fighter with top-only intakes would underperform at high altitude, like a piston plane without a turbo- or supercharger. High-g turns would also be an issue.

Some Russian jets are built with rough field features like top air intakes after the lessons of Barbarossa, when Germany bombed most of their frontline airfields with a first strike, leaving many aircraft on the ground, flyable, but unable to take off due to runway damage. That's a useful feature in wartime, but even these jets switch to forward-facing intakes once they clear the runway.

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    $\begingroup$ It's true that takeoff is the most demanding phase in terms of static thrust. - Is that really true for a fighter? Their thrust-to-weight ratios are usually pretty high so getting off the ground isn't a big deal unless there's a limited runway length (or rough surfaces like you mention later). But intercepting other aircraft also has high thrust requirements, for rate of climb and for speed, plus dogfights to maintain airspeed in high-G turns. Unlike a transport plane, it matters what happens after getting off the ground. (I realize this is just a nitpick over phrasing.) $\endgroup$ Feb 3 at 12:24
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    $\begingroup$ @PeterCordes Yes, even for a fighter, takeoff requires the most static thrust. On some USAF bases, and needless to say in the USN, full afterburners are used for takeoff. Full afterburner at sea level is the most thrust the engines can produce. High-speed interception is the second-highest-thrust scenario. $\endgroup$
    – Therac
    Feb 3 at 15:56
  • $\begingroup$ @Therac I think the confusion is that it's not necessarily true that takeoff is the most demanding, it just happens to be true in essentially all cases. For an extreme example, the Bell X-1 had a very easy takeoff regime separate from the B-52, already at speed), and much higher thrust required later. There are doubtless other edge cases, but you are still correct in general that takeoff is most demanding. $\endgroup$
    – fectin
    Feb 4 at 15:50
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Takeoff is only a small part of the flight regime. And not necessarily the most demanding.

A high G turn , where the inlet on the top of the wingroot is in the shadow of the airflow...air intake is diminished, right when the engines need it the most.

The vast majority of fighter aircraft do not need such FOD protection in daily use. British, French, German, US, Swedish. Not everyone has FOD filled runways. Or can't spare a few maintenance people to do a FOD walk before daily ops.

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    $\begingroup$ "Lack of runway FOD walker personnel" is not why Russia builds FOD resistance into their jets. NATO aircraft require first-rate facillities, and Russia has those too but expect them all to be smashed in the first hour of a war. NATO spends a fortune protecting those bases and on ability to fight from distant bases (e.g. midair refueling) and Russia says, "we'll just base in a random farmer's barn and use his farm road as a runway, hence, rough field gear and good FOD protection". This doctrine saved Ukraine's air force, and may show the dainty F-16 to be a misfit in Ukraine. $\endgroup$ Feb 5 at 22:53

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