gunnut

I have some questions about variable air intakes for different fighters.

Why are they needed?

Which one of these have variable intakes?

F-4, F14, F-15, F16, F-18, F-18E, F-22, F-35
Rafale, Typhoon, Gripen, Tornado, Mirage 2000, Viggen
Mig-21, Mig-23, Mig 25, Su-27 family, Mig-29

Thanks in advance.


edit: oops, forgot to put the word "variable" in the first question. I was thinking a bit too fast for my fingers to follow.

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GGTharos

The F-15 and F-14, as well as Su-27 and MiG-29.

The others I don't really know, but I'm fairly certain the Mig-25 might have them also ... though it might be in the form of internal moving ramps.

The 22 and 35 have them, but not in the mechanical manner, or not all mechanical ... some of it is just using different surfaces or even sound, IIRC, to shape the shockwaves where moving parts were needed before.


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.

Where you can see this drastic difference is in F-15's vs. F-16's. F-16's will always drag F-15's down tot he ground to fight if they can, because up there, the F-16's immobile intake causes them to have a TWR disadvantage, and that directly translates into loss of performance for the 16's best characteristic: Its turn rate. You can do instantaneous butyou'll bleed speed like crazy, or you can try to do sustained, but the F-15 will keep its turn rate better up there simply because it doesn't cost it anything to add a bit of pull on the g-meter because its engines are powerful enough to maintain its speed in a faster turn!

Once dragged down to the ground, the F-15's TWR is nolonger over-powering, and the F-16 has a turn rate advantage.

Jimmy

For starters, they're needed because jet engines are still run by internal combustion. Fuel + oxygen + catylist = fire. Cooler air is denser than hot air, thus containing more oxygen (generally). So you want to scoop/suck the air from the opposite end of the exhaust.

Not on your list is the SR-71.

gunnut

I know air intakes are needed to keep combustion going. I just want to know exactly why variables are needed and what fighters have them.

SR-71 is kind of special. I know it has movable cones to keep the shockwave just outside the intake (I think). It also cruises on ram jets. Something not many other planes run on.

Here's a tangent. The B-70 has wings that can bend down to capture the shockwave it generates when it exceeds the speed of sound. It almost surfs the shockwave to maintain the mach 3 cruising speed. Does it have variable inlets as well?

Jimmy

Edit: I probably shouldnt be answering questions like this while drinking.

wabpilot

All current engines need a subsonic flow of air for entry into the compressor section. The variable inlets supply a flow of air at or near the optimal speed into the compressor section.

The F-4, F-14, F-18A-G, F-22, F-35, Rafale, Tornado, Mirages III - 2000, MiG-21 - 29 and Su-27 certainly have variable intakes. I am not sure about the others on your list.

gunnut

Nonsense. I find conversation with drunk people to be extremely entertaining. It's even better if they can't remember the conversation.

Shamus

I quite agree .

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Tin Man

Ok, basically, aero engines require their airflow to arrive at the compressor face at subsonic speeds. At high speeds a variable intake will decelerate the air flow to a speed manageable for the engine. Fighters that go Mach 2 plus really require variable inlets. You basically design an inlet to suit the envelope your aircraft is going to operate in.

Variable mechanical intakes are heavy and expensive. A variable inlet is VERY important for high speed aircraft with conventional turbofan engines. If you want to go to Mach 2.5 + and have efficient engines, you use VARIABLE intakes. Clever computer designed fixed shock inlets are sufficient for Mach 1.5 to 2 and below. The high thrust of today`s engines makes this possible to a degree along with clever inlet geometry. It is a complex field.


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".

The F-22 doesn`t need to go Mach 2.5 so doesn`t need variable inlets. It can cruise on mil` power at M1.7+. The F119 engines, which are beasts as far as thrust goes, may make up for any high speed inefficiencies to a degree. Another reason the F-22 won`t have variable inlets is that it difficult to make moving inlets stealthy, ie, in one position, they could be fine, in another position, they could reflect radar. Just speculation.
You will note that today`s latest fighters tend to limit their top speed to about Mach 2, hence fixed shock inlets.

On the aircraft listed,
F-22- FIXED inlet (according to Paul Metz.)
F-16- FIXED inlet.
F-14- VARIABLE.
F-35- FIXED inlet.(diverterless).
Rafale- FIXED inlet.
F-18- FIXED inlet.
Tornado- VARIABLE inlet.
MIG-25- VARIABLE inlet.
Gripen- FIXED inlet.
Viggen- FIXED inlet.
MIG-23- VARIABLE inlet.
MIG-21 FIXED inlet.
SU-27 FIXED inlet (With FOD screen).

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Tin Man

Gunnut, the B-70 had variable engine inlet ramps. Think the system was called the Air Induction Control System. (AICS). It did the same thing any variable inlet system does, slow the air flow down to subsonic speeds, automatically.

The outboard sections of the wings drooped down to 65* above M1.4 to capture shockwaves which generated additional LIFT.

The B-70 had the best lift/drag ratio of any aircraft ever built, utilised "compression lift" and basically rode it`s own shockwave at high speeds. This "Compression lift" configuration produces 30% more lift in the flying prototypes than the wing/body area alone could produce. You get compression lift by directing the shock outwards towards the down-turned wings, the conical shaped under-body was designed to do that.
If this method wasn`t used, the wing area would have to be huge, with the obvious penalty of increased drag, lower speed.

pdf27

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.

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...

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.

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Tin Man

Ok, insert best L/D ratio for a "SUPERSONIC" manned bomber! Would you buy that? I took the original quote from an article, on reflection, not the brightest idea!
Sounds like a pretty good ratio to me, although Comcorde must have been very efficient at Mach 2. I dimly recall Concorde being quoted at 7:1? Although not a bomber of course.

Jimmy

There havent been many supersonic manned bombers. Off the top of my head I cant name 4. (not counting multirole or fighter-bombers)

ArmchairGeneral

Psh. Still nursing that hangover? B-70 Valkyrie, B-57 Hustler, B-1 Lancer, Tu-22 Blinder, Tu-22M/Tu-something else Backfire, Tu-160 Blackjack. Oh, and the Myaschischev M-something or other. That last one was an incredibly bad design, IIRC.

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Jimmy

No, but I was drinking again. *sigh* I blanked on the Hustler and the variants of the Tu-22.