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Old 30th April 2019, 03:39 AM   #1
3point14
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Sucking or Blowing

Okay, this may be a really dumb question, but what the hell, I'm going to ask it anyway.


Concorde's four Olympus engines outputted 169 kN each (wet), pushing fuel and air out the back at an enormous rate and therefore pushing the craft forwards rather quickly.

In order to achieve this it's dragging in (I have no idea how much but it must be lots of) air at the front of the engine.


In what sense, if any, can Concorde (and other jets) be said to have been propelled by the suction in front rather than the exhaust behind. Are they secretly the same thing? Is the question meaningless? Am I missing something terribly obvious?
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Old 30th April 2019, 03:49 AM   #2
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Do you mean specifically for Concorde, or are you also asking about subsonic planes.

Concorde had very interesting intakes which accounted for up to 63% of thrust.

Quote:
The air-intake assemblies are the most critical part of the whole powerplant, assuming that they are operating correctly, with all the shockwaves in the correct positions, they produce 63% of the net positive thrust of the powerplant.
https://www.heritageconcorde.com/air-in-take-system
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Old 30th April 2019, 03:52 AM   #3
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Originally Posted by The Don View Post
Do you mean specifically for Concorde, or are you also asking about subsonic planes.
All jets in general, really. The conventional thinking, or at leas my conventional thinking is that throwing things out the back is what causes the thrust, but sucking stuff in at the front must provide some impetus.

Quote:
Concorde had very interesting intakes which accounted for up to 63% of thrust.
I think I was vaguely aware of that. It might have been what prompted the question.


Quote:

Thanks,
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Old 30th April 2019, 04:07 AM   #4
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Sucking or Blowing
She's gone from suck to blow!
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Old 30th April 2019, 05:08 AM   #5
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Originally Posted by 3point14 View Post
All jets in general, really. The conventional thinking, or at leas my conventional thinking is that throwing things out the back is what causes the thrust, but sucking stuff in at the front must provide some impetus.
This is why it doesn't work in a vacuum
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Old 30th April 2019, 06:31 AM   #6
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Originally Posted by 3point14 View Post
In what sense, if any, can Concorde (and other jets) be said to have been propelled by the suction in front rather than the exhaust behind. Are they secretly the same thing? Is the question meaningless? Am I missing something terribly obvious?
You can get some bounding values on what's possible with some back-of-the-envelope calculations. Let's look at some engine specs:
https://en.wikipedia.org/wiki/CFM_In...Specifications

The LEAP 1A engine has a diameter of 78 inches, which means a cross sectional surface area of about 4778 in2. Let's say we're at a cruising altitude of 35,000 ft. The air pressure here is about 3.2 psi, so that's a bit more than 15,000 lbs of force. So vacuum force is limited to less than 15,000 lbs of pressure (the inlet can drop the pressure by sucking, but not all the way to zero). The rated maximum continuous output power for the LEAP 1A engine is 31,690 lbs, so more than twice the maximum possible vacuum force. Again, the actual vacuum force will be less than that, but it's quite likely a significant fraction of 15,000 lbs. Let's suppose the engine sucks air enough to drop the inlet pressure in half (wild guess just to ballpark things). That means you would have about 7,500 lbs of sucking force, or close to 25% of the engine's thrust. That's very significant, and engineers would absolutely need to include that in their performance calculations. But most of the thrust is still going to come from the exhaust, not the inlet.
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Old 30th April 2019, 06:49 AM   #7
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There's intake, compression, ignition, and expulsion. The compression and the ignition are where the magic happens. They're what make the expulsion so forceful. It's the blowing, not the sucking, that makes a jet go.
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Old 30th April 2019, 07:01 AM   #8
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Third clickbait tile today
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Old 30th April 2019, 07:02 AM   #9
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Originally Posted by theprestige View Post
There's intake, compression, ignition, and expulsion. The compression and the ignition are where the magic happens. They're what make the expulsion so forceful. It's the blowing, not the sucking, that makes a jet go.
This. The volume leaving the engine is significantly greater than that entering, thus blowing.
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Old 30th April 2019, 07:30 AM   #10
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The air intake only lowers the pressure at low or zero speeds (where it has been known to suck in people, baggage carts, and other stuff ). You must remember that at cruise speed, air is literally rammed into the intake of the engine.

This does not mean that it does not contribute to the trust, however, because the ram effects helps compress the air before the combustion chamber, and this relieves the compressor stage of some considerable work, but, obviously at the cost of some drag.

So .... bottom line: It is a complex equation, but no the intake does not actually suck the craft forward. It does, however play a role in producing the trust.

In fact, there is a type of jet engine, aptly called a ramjet, that depends entirely on air being rammed into the intake for compression, thus doing away with both turbine and compressor. Obviously, this only works at high speeds (mainly supersonic), so such a craft has to be accelerated so sufficient speed before the jets can kick in.

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Old 30th April 2019, 08:29 AM   #11
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Originally Posted by Thermal View Post
Third clickbait tile today
I was pretty sure it was going to be an air pressure/airflow thread.
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Old 30th April 2019, 08:33 AM   #12
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Originally Posted by theprestige View Post
I was pretty sure it was going to be an air pressure/airflow thread.

It was only accidentally clickbaity.

My apologies to those who expected something much more exciting. )
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Old 30th April 2019, 08:49 AM   #13
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At least the Concorde has air to push against....
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Old 30th April 2019, 09:40 AM   #14
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Originally Posted by Lothian View Post
This is why it doesn't work in a vacuum
Well, actually, turbojet or turbofan engines used on planes don't work in a vacuum, as, unlike a rocket, they don't carry their oxidizer with them, but rely on oxygen from air to burn the fuel.
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Old 30th April 2019, 09:41 AM   #15
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The reason that Wyoming is so windy is that Nebraska and South Dakota suck, and Utah and Idaho blow.
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Old 30th April 2019, 09:48 AM   #16
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Originally Posted by CORed View Post
Utah ... blow[s].
You have no idea.
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Old 30th April 2019, 09:50 AM   #17
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Originally Posted by MRC_Hans View Post
So .... bottom line: It is a complex equation, but no the intake does not actually suck the craft forward. It does, however play a role in producing the trust.
Does anyone have a new link to the video explaining the intakes on the SR-71? I think it went down the bit bucket when Jeff Quitney's YouTube channel got nuked.
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Old 30th April 2019, 11:11 AM   #18
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Originally Posted by MRC_Hans View Post
The air intake only lowers the pressure at low or zero speeds (where it has been known to suck in people, baggage carts, and other stuff ). You must remember that at cruise speed, air is literally rammed into the intake of the engine.
Good point. But on the runway (where you would be at minimum velocity), my numbers are off. Air pressure at sea level is about 14.7 psi, which means that for a cross section the size of a LEAP 1A engine, you can get up to about 70,000 lbs of vacuum pressure. The max takeoff thrust of a LEAP 1A engine is about 32,000 lbs of force. Obviously the engines aren't creating an actual vacuum at the intake, so I don't know what fraction of the 32,000 lbs of force is coming from suction, but it could potentially be a pretty big fraction.
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Old 30th April 2019, 11:57 AM   #19
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Originally Posted by Ziggurat View Post
Good point. But on the runway (where you would be at minimum velocity), my numbers are off. Air pressure at sea level is about 14.7 psi, which means that for a cross section the size of a LEAP 1A engine, you can get up to about 70,000 lbs of vacuum pressure. The max takeoff thrust of a LEAP 1A engine is about 32,000 lbs of force. Obviously the engines aren't creating an actual vacuum at the intake, so I don't know what fraction of the 32,000 lbs of force is coming from suction, but it could potentially be a pretty big fraction.
Yes, sure. You will notice that the intake of many (military) jets is slanted, to give it a higher frontal area at a high angle of attack, i.e. low speed.

Again, it is a complex equation, and suction/ram effect is certainly taken into account.

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Old 30th April 2019, 01:22 PM   #20
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Originally Posted by MRC_Hans View Post
Yes, sure. You will notice that the intake of many (military) jets is slanted, to give it a higher frontal area at a high angle of attack, i.e. low speed.
Hadn't thought about that before, but it makes sense for fighter jets. Can't do that on a B2, but I also can't see a pilot ever wanting to put a B2 in a high angle of attack either.
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Old 30th April 2019, 04:06 PM   #21
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Originally Posted by 3point14 View Post
In what sense, if any, can Concorde (and other jets) be said to have been propelled by the suction in front rather than the exhaust behind. Are they secretly the same thing? Is the question meaningless? Am I missing something terribly obvious?
To some extent, we could call the distinction meaningless because it's all one continuous movement in the intake, through the tube, and out the back. But there are a couple of technicalities along the way that we can also say make it significantly more suck than blow. (I'll get to what those two things are after a couple of paragraphs of setup.)

Near the back of the engine, right after the part where fuel gets added & ignited, there is a turbine. A turbine looks like a fan but works the opposite way: instead of using power to move a fluid, it gets spun by the movement of fluid, and thus draws power, which is used to drive the compressor at the front of the engine, which sucks air in & compresses it. In a plain turbojet engine, that's all there is to it, but other categories of turbine engines also use the power from the turbine to drive something else:
•Turboprop: the turbine's power drives a propeller (not a piston engine)
•Turborotor: the turbine's power drives a helicopter's rotor
•Turbofan: the turbine's power drives a big fan at the front of the engine
•Any type: the turbine's power might also drive a generator for onboard electricity, or not

Most of what we call jet engines are turbofans; the fan in question is what you can see easily at the front of passenger & cargo jet engines. In a turbofan, the engine core, where the compressor and turbine are and where ignition happens, is inside a larger second tube, called a bypass duct because the air going through it passes by the engine core. If you look at the back of a passenger or cargo jet engine, you can usually see the distinction between the core and the bypass; the protruding conical piece is the nozzle where air & burned fuel exit the core, and the gap around it between the cone and the outermost sleeve is the exit opening for the bypass, where air flows through & out without having ever been mixed with fuel. You can't see this distinction from the front because the drive fan covers both the core and the bypass duct, so it can pull air into both.

So, back to those two things that, I said before, make a jet engine's action more suck than blow:

1. The turbine draws energy by taking kinetic energy away from the post-ignition gases; they blow out the back slower than they would without a turbine present. Blocking & reducing that flow isn't something you'd want to do if that were your main source of thrust.

2. Where that kinetic energy is diverted to instead is the compressor, and a fan or propeller in a turbofan/turboprop engine, and thus the air that's moved by those things. That's where most of the thrust for that engine type comes from, and, since that's at the front of the engine instead of the back, the normal word for what they do would be sucking, not blowing.

Point #2 gets more important the bigger the bypass duct is. The airflow through the bypass duct divided by the airflow through the engine core is an engine's bypass ratio, so bypass ratio ends up as an indicator of how much more of the thrust is from sucking in at the front instead of blowing out the back. Higher bypass ratios are more efficient, but lower bypass ratios allow greater speed, so they get used on different kinds of planes. Of course, the ratio on a turbojet, as opposed to a turbofan, is zero by definition. Turbofan bypass ratios from about 0.3 to about 1˝ ("low") are typical for fighter jets, and passenger & cargo jets' ratios are typically from about 3 up, with older ones generally lower than the latest ones, which have ended up in the 9-12 range, meaning only about a tenth to a thirteenth of the air they move ever gets fuel added! (I'm not even sure whether anybody in aviation today would call anything less than 7 or 8 a "high-bypass" engine anymore, but they would at least have been called that back when they were the newest & highest-bypass engines around.)

Concorde had turbojets, not turbofans, so point #2 was not an issue like it is for most jets now, but there was still point #1, the issue of deliberately diverting power from where it would blow to where it would suck instead. A new engine created for supersonic use today would be likely to be a low-bypass turbofan instead of a turbojet, just to use the bypass flow for core cooling.
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Old 30th April 2019, 05:10 PM   #22
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Originally Posted by Delvo View Post
To some extent, we could call the distinction meaningless because it's all one continuous movement in the intake
Nope, we can very easily make the distinction. Any reduction in pressure at the front end of the engine below atmospheric pressure is suck. Any higher pressure or increased momentum of air at the back of the engine is blow.

And MRC_Hans is right, at higher velocities there won't be any suck, there will only be blow.
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Old 30th April 2019, 05:41 PM   #23
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Originally Posted by Delvo View Post
To some extent, we could call the distinction meaningless because it's all one continuous movement in the intake, through the tube, and out the back.
I don't think it is one continuous movement.

There's a flow into the ignition space, and a compression there. Then the ignition and the exhaust. The way I understand it, there's a flow discontinuity at the point of compression, and it's this discontinuity that is central to the operation of the jet engine.

I think it's two linked systems. One system for feeding (relatively) low energy air into the compression-combustion chamber, and one system feeding (relatively) high energy air out of the chamber.

A jet ski or a vacuum cleaner would be an example of a continuous flow device.
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Old 30th April 2019, 09:10 PM   #24
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Kind of like harmonicas- no matter whether sucking or blowing, the note is the same.
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Old 1st May 2019, 12:53 AM   #25
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Originally Posted by ahhell View Post
This. The volume leaving the engine is significantly greater than that entering, thus blowing.
To be strict. The mass of air entering must be the same as the mass of air leaving the engine. It could not be otherwise.

It is the series of compressors that squeeze the air into a progressively smaller volume that then have no route to equalise other than rushing out the back end at speed.

It is the addition of fuel in the combustion chamber that adds the extra mass ejected out the back end.

It is both processes that produce copious water vapour (combustion and compression) that produce copious water vapour and hence contrails.

Seems a pointless quibble, but simply pre-empting any chemtrailers that may show up.
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Old 1st May 2019, 01:41 AM   #26
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Originally Posted by casebro View Post
Kind of like harmonicas- no matter whether sucking or blowing, the note is the same.
Generally not the case. Blow and draw are usually a tone or two apart.
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Old 1st May 2019, 04:24 AM   #27
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Originally Posted by theprestige View Post
I don't think it is one continuous movement... there's a flow discontinuity at the point of compression, and it's this discontinuity that is central to the operation of the jet engine.
You're describing a pulsejet engine. Those exist, but they're rare, only used for aircraft that are built as cheap as they can be and still fly: drones for target practice, WWII flying bombs, and some modern remote-control model planes.
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Old 1st May 2019, 07:43 AM   #28
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I'm describing the kind of jet engine we've been discussing so far. Feel free to tell me it's a bad description. Feel free to tell me the physics of why "continuous flow" is a good description of such an engine. But please don't tell me I'm describing something other than what I'm describing.
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Old 1st May 2019, 08:46 AM   #29
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Originally Posted by theprestige View Post
I don't think it is one continuous movement.

There's a flow into the ignition space, and a compression there. Then the ignition and the exhaust. The way I understand it, there's a flow discontinuity at the point of compression, and it's this discontinuity that is central to the operation of the jet engine.

I think it's two linked systems. One system for feeding (relatively) low energy air into the compression-combustion chamber, and one system feeding (relatively) high energy air out of the chamber.

A jet ski or a vacuum cleaner would be an example of a continuous flow device.
Uhm, no. The jet engine has continuous flow. Air enters the intake and is compressed via a number of rotary compressor stages, and is then led to the combustion chambers where fuel is added and ignited with a spark. The exit pressure is not higher than the compressor output, but the volume and mass is far greater. On the way out, the exhaust drives a turbine which drives the compressor.

Actually, this arrangement means that if you throttle down a jet engine too quickly (or if the fuel supply is suddenly interrupted), you may experience a compressor stall, where the compressor becomes unable to sustain its own pressure, and air and sometimes exhaust blows backout through the air intake. Not something you will want to experience, especially at low altitude!

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Old 3rd May 2019, 09:27 AM   #30
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Originally Posted by theprestige View Post
I'm describing the kind of jet engine we've been discussing so far. Feel free to tell me it's a bad description. Feel free to tell me the physics of why "continuous flow" is a good description of such an engine. But please don't tell me I'm describing something other than what I'm describing.
You are in fact describing something other than what you intend to describe.

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Old 3rd May 2019, 11:41 PM   #31
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Originally Posted by theprestige View Post
There's intake, compression, ignition, and expulsion. The compression and the ignition are where the magic happens. They're what make the expulsion so forceful. It's the blowing, not the sucking, that makes a jet go.
In case you have any problem remembering the above, it's often taught as:

suck, squeeze, bang, blow.

For some odd reason, people find that easier to remember.

(Note this is also the four stages of a four-stroke reciprocating engine)
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Old 4th May 2019, 12:20 PM   #32
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Originally Posted by novaphile View Post
In case you have any problem remembering the above, it's often taught as:

suck, squeeze, bang, blow.

For some odd reason, people find that easier to remember.

(Note this is also the four stages of a four-stroke reciprocating engine)
This is exclusively a description of a piston engine.

In the jet engine there is, in principle, no explosion, just a VERY vigorous fire.

Hans
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Old 5th May 2019, 09:23 AM   #33
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There's not supposed to be an explosion in an IC engine, either. When that happens it's called "knock", which is not a good thing.
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Old 5th May 2019, 09:56 AM   #34
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Originally Posted by Trebuchet View Post
There's not supposed to be an explosion in an IC engine, either. When that happens it's called "knock", which is not a good thing.
A knock is when the explosion happens too early, before the piston has reached its top position (or, strictly speaking,
Quote:
too long
before it has).

We can discuss what is an explosion or not. It is certainly not a detonation, but it is an explosive combustion. Hence the rather loud bangs, if you don't have a muffler on it.

Hans
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Old 5th May 2019, 02:36 PM   #35
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Originally Posted by Thermal View Post
Third clickbait tile today
So, a slow day for input...………………..
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