Again, a jet engine does not operate in pulses. Other than when the engine first starts up on the ground, the chamber is not empty, and the thrust does not come from anything expanding and “hitting” the chamber wall. A jet engine operates with a continuous stream of air and fuel being injected into the chamber and burning, resulting in a high pressure in the combustion chamber, which drives the burnt air/fuel mixture out through the nozzle and produces a reactive force which drives the plane forward.
It is the exact same principle as a solid fuel rocket, except the fuel is kerosene and air instead of solid propellant. Solid-propellant rocket - Wikipedia
For clarification, what do you picture. what force and from where is pushing on the aircraft to move it? I hope you agree that some type of force has to be acting on the actual aircraft to move it
vyVY
The pressure (i.e. force) of the exhaust gases on the walls and solid surfaces on the engine is what propels it forward. This same pressure is what drives the exhaust gases backward.
When you and I are standing on an icy lake, and you push me - I feel a force pushing me, you feel a force resisting your push. I go sliding one way, you go sliding the opposite way. These are the “equal and opposite” forces.
Yes and the exhaust opening is behind you and the aircraft metal is behind me, I hit the ac and push it forward and you go out the back pushing nothing. we are the expansion..
vyVY
I have no prob with that, but what you don’t see is before that happens the thrust reversal pushing the ac back as it hits the curvature of the buckets, what you are see is what’s bounced off the buckets not converted into backward motion. vyVY
In between me and you is the center of the expansion. we are expanding away from each other me hitting the aircraft and you heading out back. the force begins INDEPENDENTLY from the aircraft..THEN it acts on it!! . vyVY
Same page here. but the pressure is independent of the engine parts. it happens in the space of the chamber then it acts on the metal albeit continuously.its a continuous expansion as gas continues to be burned. so there is a continuous force acting towards the plane pushing it and another out the back.. vyVY
No. There is no force independent of the aircraft, and there is no “center of expansion”. You cannot have an unbalanced force, i.e. a “free” force not acting on anything initially, and then acting on something later. All forces are pairs.
I want to deal with the turbofan also, if I remember someone said in here that it is the thrust out back that pushes the plane, no! the blade is like a screw trying to screw into the air, if the air was fixed as a fixed piece of wood getting a screw put into it, the plane would go forward as fast as the blades were PULLING or CORKSCREWING it through the air! If you use a power drill and screw a screw into an unattached piece of wood the screw would go in some and the wood would come towards your direction, that is the thrust you feel, which is just a by product of the screw not being able to go all the way into the wood at once because the wood (or air) moves also… So for propeller aircraft and turbo fans they are trying to PULL into the air or corkscrew into the air. it is a pull not a push…
vyVY
yes there is. the ac dumps gas and air into an EMPTY chamber..when it is lit, it is in that space that the fire starts. the ac is not on fire, the suspended gas and air is separate from the ac. then the expanding gases act on the ac!!
vyVY
I guess you have never seen a fireball suspended in the air. That is independent expansion of gases! Away from anything! That is what’s going on in the combustion chamber, a continuous fireball! Anything in the way gets pushed. put that fire ball inside the engine of the plane and the force in one direction acts on the plane and the force in the other direction goes out through the exhaust!! the same forces if there was the fireball in the air not contained by anything!
vyVY
No, it is not. For the last time, the combustion chamber is not a continuous fireball. It does not have a center of expansion.
Or, if you insist on thinking of it in this way, the center of expansion is on the forward-most wall of the combustion chamber, not some arbitrary location in the middle of the chamber. The expansion only occurs in one direction - backwards, out through the exhaust.
well we’re getting there… the combustion chamber IS a continuous fireball albeit a controlled one. the second one we are in agreement on half. I used the middle to illustrate, if you want to say it is closer or even 1 millimeter AWAY form the metal that serves my argument. expansion is still in all directions: 110 lb thrust towards the millimeter to push the plane and 100lb thrust out the back.. remember I am not saying the whole 100lbs thrust is used by the plane. It would be the same basically if the combustion happened right at the tail of the exhaust: 100 lbs thrust towards the plane and 100lbs thrust out the back.. it would travel a few extra feet to get to the metal of the chamber and be a bit less efficient but the plane would move vyVY
So what pushes the plane forward?.. you seem to think that the plane is on fire producing the thrust.. its the gas that is on fire. and when gas is on fire, expansion is in all directions, even if it is an atoms distance away from the metal of the plane expansion is going in that direction. vyVY
Another point hopefully of benefit: if the thrust is 100mph and the plane was going 100 mph (using all the thrust) there would be nothing coming out the back…the thrust is relative to the airplane.
vyVY
Your muddled use of terminology is confusing and makes it difficult to engage in meaningful discussion.
Let me rewrite what you wrote above to see if I can make it clearer, and then you let me know if I correctly understood what you meant to say.
What you wrote:
What I think you mean:
If the exhaust plume exits the engine at 100 MPH relative to the plane, and the plane has an airspeed of 100 MPH, the engine would produce no thrust.
If that’s what you meant, then yes, that’s correct. If that’s not what you meant, then you’ll need to clarify.
If the engine produced no thrust, what thrusts the plane forward ? You have it backwards, you think that because you feel some breeze out the back of the cowling that that is the thrust. that thrust pushes YOU back. The plane feels the thrust in the opposite direction, this is what pushes the aircraft forward. If the engine was open on both sides you would see the flame of the combustion and feel thrust out of both sides of the engine front and back, but because the metal is there instead of you, the thrust that you would feel if you were standing at the front of the aircraft is pushing the metal of the plane instead of you. And the plane is pushed forward by that force vector.
remember that the combustion is independent of the aircraft, it happens in space.. if you lighted the fuel in a rocket and had two ends open the thrust would come out equally in both directions. it’s the firewall (metal of the rocket)that is there that blocks or captures the forward stream and that is what pushes the rocket, not the stream that is coming out the back,. that stream out the back would happen anyway if both side were open.. I hope you see my illustration.
vyVY
NO! Explosion has nothing to do with a jet engine.
A good common sense analogy for jet engine is a a hair dryer. Fan/compressor sucks in air and increases its pressure. The high pressure air then passes through a chamber where its temperature is increased. Jet engine burns fuel, hair dryer uses electric resistance heater. The effect is the same: temperature increases and pressure remains the same. Afterwards, air is directed through a nozzle where its high pressure and some of its heat is converted into kinetic energy.
You could argue that thrust is produced by the compressor or by the nozzle, but certainly not by the combustion process!