I challenge you to cut out the side of a ball of your shape, go there, then let someone add fuel (which the injectors of the plane does) and air (which is what the compressor does) into the space inside the ball (which is the combustion chamber of the engine) and then light it (which the igniters do)
And see if from that combustion you are not thrust away..
vyVY
Okay, I think the concept of combustion is causing us problems here. So let’s drop it, and go to an even simpler system.
Let’s say you have a tank of compressed air. Just ordinary air, nothing flammable. You knock the valve of the top of the tank. The tank will go shooting across the room. Can you explain what you think is happening inside the tank?
Here is such a video. Jump to 0:50 seconds.
Yes but what turns the fan in the hair dryer and provides the heat is the movement of electrons.. what turns the compressor and provides the heat is the movement of air (air = the electrons) The air moves because of the combustion that expands, moving the air that makes everything else move, like the electrons do.. maybe a crude analogy maybe not but I like to try and keep things on layman terms.
vyVY
When the exhaust is the same speed relative to the aeroplane as the aeroplane speed is relative to ambient air then the thrust is zero. Why? Because the air flow through the engine is not being accelerated. What is pushing the plane forward? Nothing, it has no thrust at that point, until it slows down a fraction, then it will have a small amount of thrust again so it speeds up till it has no thrust. This happens continuously so the speed is stable rather than constantly fluctuating. In reality this speed is never achieved because thrust is always required to overcome drag so the speed stabilizes below the point that thrust would be zero.
The principle is the same. air is PUSHING on all sides of the tank trying to get away from a central point.. if not from a central point why do force vectors move away from each other when expansion is concerned? The reason there is no movement is because the force in the north direction is balanced by the force in the south direction holding back each other.. so the insides of the tank are being forced south and north (in all directions of course but because the south side is being opened I illustrate this way) When you open the south tank, you have stopped the counteracting force on the north side force, so now it can go freely(but not as fast as the pressure pushing it because of mass, friction etc.) on it’s merry way.. if the north side could use the full potential of the force inside instantaneously, you would never feel air exiting the south side. i.e. if the forces are 100 mph in each direction and the ball went from zero to 100 mph in an instant there would be nothing coming out the back, the force would have been 100 % utilized
vyVY
Air moves because of the pressure difference, which is first produced by a compressor and then reduced by a turbine and the nozzle. The only effect the combustion has is to increase the air temperature.
Yes, air expands during combustion = rapid increase in temperature results in increase of volume. However, and this is important, pressure remains constant and increase in cross sectional area prevents significant increase in flow speed. Therefore, no thrust is produced in the combustion chamber!
Nope. Pressure acts on a surface, perpendicular to it, not from anywhere. The ‘center’ is irrelevant.
From Wiki
Fair enough.
Still ok.
This would be impossible. The force pushing the tank forward must be countered by the force pushing air out the back. It has to. Its not a matter of friction or anything else. Until you accept this, you are never going to get it.
No, compressed fluid exerts force normal (perpendicular) to the surface that contains it. There is not central point. Fluid will move in the direction of lower pressure. In an explosion it is outwards from the source of energy. In a jet engine, it is downstream from compressor.
put an empty cream can on the ground. put some gas in and light it. the combustion the expansion of gases is what pushes the can. I don’t picture what you see
vyVY
As far as I know only light has these properties: that no matter how fast you go in one direction the light will always leave at the same speed from the object.
I don’t think you get my point, yes there is an opposite relative force from the north push, but if the object is moving at 100mph how would you feel it? the force coming out opposite would have to be at 200 mph for you to feel a 100 mph breeze. Think of a 100 mph aircraft going into a 100 mph wind. the airspeed indicator would read 100 mph but it would be stationary relative to the earth. It would appear to be hovering above you, and if it was going the other way it’s airspeed would still read 100 mph. but it would be going 200mph relative to the earth..
vyVY
If pressure wasn’t in all going directions from a central point why if the walls of the container were instantaneously removed the force goes EQUALLY in all directions. from which point is the pressure emanating from?
If you are saying it is not like that, try to make me see what you are seeing… in layman’s terms. or better still disprove what I see, in layman’s terms.
vyVy
Why does the pressure have to “emanate” from a single point?
Yes, sometimes when you are drawing diagrams and doing calculations, you can treat a force as originating or being exerted on a single point for the purpose of simplifying your calculations. But you can only do this when it is a valid assumption. In the case of an expanding gas, it is not a valid assumption. The gas exerts a pressure normal (perpendicular) to the wall surface, along the entire wall surface.
Adam, all the compressor is doing is providing the air part to the combustion chamber, consider it the intake valve. all the injectors do is provide fuel to the combustion chamber , consider this what it is, all the igniters do is provide the spark, consider this the spark plug. When the air/fuel mixture iginites, expansion happens in ALL directions. The plane is not on fire, the compressor is not on fire, the GAS is on fire..
vyVY
What are you on about? Why do I need to “feel it”? Your comment in no way relates to my post, or your post that i was responding to.
I mean no disrespect, but you started this thread because you didn’t understand what should have been a relatively simple effect, and many people were happy to help with what appear to me to be very clear explanations. The fact that you don’t think reverse thrust should work, when clearly it does, should be an sign to you that you don’t have a clear understanding of the forces at work. Maybe instead of vigorously defending your assumptions which have lead you to the wrong conclusion, you should listen to what people are saying and think about why they might be right.
It doesn’t. I can’t imagine why you think it does.
As we have told you earlier, pressure does not eminate from any point.
You need to feel it if you are standing behind it.. Feel is used for clarity purposes.
This is my original post, Didn’t you see where I said I KNOW IT WORKS SOMEHOW OR ELSE IT WOULDN’T BE USED?: Hi. The engine pushes and the fan pulls through the air then you reverse the flow,wouldn’t that be an equaling effect? or less power for the reverse due to the turning and not full directional control? I know it works somehow else it wouldn’t be used, but it is not the same as reversing the blades on a propeller.
virtually yours,
Virtually Yours.
That is all I am asking of everyone else, shouldn’t it go both ways? I said that i grasped what i was asking in a previous thread, now I am trying to explain what I grasp, shouldn’t you also be trying to understand what I am stating, or am I the continually dummy because I have the misfortune to actually ask a question? Just because someone answered doesn’t make it right. I need to see what they see. in laymans terms
I listened to them and it seems they are saying pressure is the like big bang it comes from nowhere, no distinct point of reference (even the big bang though had a central point hmmm) it was said pressure emanates from no specific point and you are actually in here seeming to say that a 100 lb of air pressure would not be exerting the same pressure in all directions. this is what you said It doesn’t. I can’t imagine why you think it does.
I can’t imagine why you think it doesn’t when the law of equal and opposite reactions come in to effect. if you have equal and opposite forces how can you have that without a middle point? when did the force start going north and the other going south there has to be a balancing point i.e the pressure in a ball. To me every vector appears to be coming from the middle of the ball if not why would the air expand equally in all directions away from a center if the ball was suddenly removed?
if I am wrong tell me why it is wrong just don’t say it’s not like that and expect me to accept it. if I’m right I’m right , if I’m wrong maybe right now I am not grasping why. contrary to popular belief I am trying to understand the other opinions.
vyVY
vy VY
re reading this.. my scenario is that if I was sitting inside the vessel going north at 10 mph and I blew 10 mph in the south direction that 10 mph breeze would not make it out the back relative to something unattached to the vessel. Instead of the air going out the vessel back at 10 mph the vessel is going away from the air at 10 mph.. if someone was standing at the exhaust they would not feel a thing. because the 10 mph south wind blow is relative to the vessel only. That’s why I gave the reference of the aircraft in flight. And of course as I understand light is the only thing that would leave an object at the same speed regardless of direction..
and on it goes..ugh
vyVY
And btw I started going through the posts again to re read, and the first two posts are at very clear explanational odds of each other.. so please don’t say that just because someone answered and it is a very clear explanation to you that means it is clear and right to everyone else and me. why dont you go to the third poster and tell him the same thing you told me, didnt the first poster after me answer and give a very clear expectation?
I think something else may be amiss here.
you take care
vyVY
No, the first two responses agree with each other. The third response nitpicks a point in the second post but it is ultimately not pertinent to the discussion.
Your right, Thanks for the correction . I meant 2 and 3.you call it nit picking I call it a refutation all 6the same. I agree with you not really pertinent. But I will look through the rest .
vyVY
All through this thread posters have generally been talking about a pure turbojet where all of the thrust is due to acceleration of the airmass through the core of the engine. Modern airliners actually use turbofans where a significant portion of the thrust is via acceleration of the airmass by the front fan bypassing the core. The thrust reverser still works the same way though, by redirecting the airmass forwards.