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In the context of a common airliner (say Boeing 777), when the airplane is on the ground, and the turbines are on, air is getting sucked in and compressed/bypassed. The engine has to do work to accelerate steady air in front of the engine in order to get enough in per second.

But I can imagine a speed at which a flying airplane no longer has to do work to accelerate air into the engine, as enough is getting rammed in per second.

My questions are:

  • Do any turbofan powered airplanes go fast enough that they no longer need to "suck" air in to maintain power?

  • At what point, if any, does air stop getting sucked into the turbine and is just getting rammed in?

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  • $\begingroup$ Some high-supersonic jets work like this. But for almost all jet engines the air velocity at the entrance of the engine and the air velocity that the aircraft sees is different. So there is always some acceleration of the airflow. $\endgroup$ Aug 30 '21 at 15:17
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    $\begingroup$ Congratulations! You have reinvented the ramjet. $\endgroup$
    – StephenS
    Aug 31 '21 at 16:30
  • $\begingroup$ @StephenS All we need now is a Proton Energy Pill. $\endgroup$ Sep 1 '21 at 3:18
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Your questions

At what point (if any) does air stop getting sucked into the turbine and is just getting rammed in?

On a CFM56-7B engine (Boeing 737 NG) pressure is provided by the fan, the low pressure compressor and the high pressure compressor with respective pressure ratios of about 1, 3 and 9, that is pressure is increased by a factor of 28 before air is delivered to the combustor for a cruise speed of about Mach 0.8.

This required compression ratio is indeed reduced as airspeed increases and pressure from speed increases. This natural compression occurring at the engine inlet is called ram pressure:

Ram compression ratio vs airspeed

Ram compression ratio vs airspeed, source

About Mach 2.5 or 3, air can be delivered in sufficient quantity to the combustor without a compressor. The fan is not needed anymore, all air entering the engine is mixed with fuel. The turbine can be removed too as there is nothing to spin anymore.

Such engines without compressors are named ramjets and scramjets. They have no rotors or stators. In theory a ramjet could work starting at Mach 0.5, but it doesn't produce any useful thrust:

Rocket, turbojet, ramjet, scramjet efficiency

Rocket, turbojet, ramjet, scramjet efficiency, source

Do any turbofan powered airplanes go fast enough that they no longer need to "suck" air in to maintain power?

A turbofan combines a core engine and a fan. The fan produces most of the thrust, the core engine mostly turns the fan.

Fans cannot work at supersonic speed, they wouldn't be efficient. therefore when the compressor is not required, airspeed is too fast for a fan. Actually neither the ramjet nor the scramjet have a fan, they are pure reaction engines.


Ramjet

The first engines to operate on this principle is the ramjet. The difficulty is to maintain a combustion at this speed (this is a 3,000 km/h wind), to solve this problem, a ramjet slows air down before it is mixed with fuel in the combustor.

Ramjet

Ramjet princple, Wikipedia

Nord 1500 Griffon II

Nord 1500 Griffon II, one of the first ramjet aircraft, source

Scramjet

With some improvements, supersonic burners have been built, though very inefficient ones. This engine is known as scramjet. It works at higher speeds than a ramjet.

Scramjet

Scramjet principle, Wikipedia

Need a little help

However no ramjet, and even less a scramjet, can produce static thrust. Both need some initial forward speed for to start the combustion. They can be launched from another aircraft:

Leduc 010 on top of SE-161

Leduc 010 on top of SE-161 for an air-launch, source

Aircraft can be fitted with an additional subsonic engine, a convertible engine (turbojet or ramjet) or with rocket boosters.

J-58 turbo/ramjet

J-58, convertible turbojet/ramjet used on SR-71 and other aircraft, source

In spite of their inconveniences, ramjets and scramjets are used on a few fast military aircraft, but their kingdom is missiles where larger airspeed often means larger altitude and extended range.

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    $\begingroup$ "Actually supersonic (and hypersonic) engines have no bypass." This seems to imply that all supersonic engines are turbojets (and not turbofans). But for example F-16 fighter has a max speed of around Mach 2, it is powered by F110-129 which does have bypass (now it is low bypass, only 0.76:1, but still it is bypass). Maybe you just meant to say hypersonic here? $\endgroup$
    – Daniel K
    Aug 30 '21 at 18:21
  • $\begingroup$ @DanielK, I made a mistake in the wording, what I wanted to say (I'll update) is neither the ramjet (subsonic combustion) nor the scramjet (hypersonic combusion) have a fan. $\endgroup$
    – mins
    Aug 30 '21 at 18:46
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    $\begingroup$ What's the X-15 photo doing here? That was a pure rocket spaceplane, never operated with any kind of air-breathing jet. An SR-71 would be a better illustration, with its turbo-ramjet (switching over between turbojet and ramjet somewhere above Mach 1). $\endgroup$
    – Zeiss Ikon
    Aug 30 '21 at 18:55
  • $\begingroup$ Don't forget hybrid engines like the turbo-ramjet $\endgroup$
    – slebetman
    Aug 31 '21 at 8:13
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    $\begingroup$ The link underneath the lsp efficiency plot seems to not work. $\endgroup$
    – Koyovis
    Dec 19 '21 at 5:39
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Like any kind of vacuum system, vacuums cannot “suck“ anything. They just remove air from an enclosed space. This causes a pressure differential between the evacuated space and the surroundings. The higher air pressure outside forces air into the cassion; it is not “sucked” in.

This is similar to the diffuser on a jet engine. The fan or compressor is drawing in air which creates a low pressure region in the diffuser, thereby causing air at local atmospheric to rush into the space.

What you’re describing here would be the concept of a ramjet, where the stagnation pressure of the diffuser is enough to compress the incoming air to the point where it can be burned and produce useful thrust without the need for augmentation by a mechanical compressor. Turbofans and turbojets cannot do this efficiently at their operating speed ranges but do get a performance boost via diffuser compression of incoming air.

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It is certainly possible to 'ram' air into a turbine. The maximum pressure that can be achieved is called the 'stagnation pressure', which is the point at which all incoming kinetic energy is converted into pressure energy.

For some applications, this is good enough. For example, the Meredith effect is responsible for a slight amount of net thrust in the radiator of the P-51 Mustang. For other applications, like a (subsonic) turbofan, while it helps a little bit in increasing the compression ratio, it cannot replace the compression section. The reason is that the downstream pressure will accelerate air in front of the engine away and outwards: air will simply 'spill' past the engine. Engines can be designed such that they exactly neither 'suck' nor 'spill' air at the design point.

The good news is that in supersonic applications, the downstream pressure cannot 'travel' upstream (because pressure travels upstream at the speed of sound). Here, incoming air is truly 'rammed' and has no choice but to undergo compression. (Sc)ramjets (and in lesser extent, inlets of supersonic turbojets) are based on this principle, but to achieve a meaningful compression ratio, they ideally would travel at hypersonic speeds for which they need a subsonic-capable engine.

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