I disagree. the only thing the aircraft throws is gas, air and a spark into the empty space of the burner cans. what happens after is independent of the aircraft. an explosion in space so to speak.the force is the reaction of the gas air and spark igniting. (which of course you know) that explosion is what pushes on the front end of the burner cans. that is the first force, as the force pushes the other way it is harnessed by the turbines which transfer that into the fan rotation. the rest is left over what could not be converted to motion in respect to the plane. the works is done already and has been converted to motion. vyVY
The way you say it, the less exhaust there is (ie “wasted energy”), the more energy is being used to propel the aeroplane forward. But that is not true, the more air that gets thrown out the back and/or the faster it is thrown out the back, the faster the aeroplane will go. The exhaust is fundamental to creating net thrust, without it the net thrust is zero.
You are also using the ball analogy incorrectly. The balls are the air being accelerated, you can’t throw a ball at the front of the engine and have it disappear, it has to go somewhere. Where does it go? Out the exhaust having “bounced off” the front of the engine. But that is still not the right way to use the analogy. The balls are actually only being thrown backwards, and as Absolute says, the force required to throw the ball backwards is what pushes the aeroplane forwards. At this point using the ball analogy is of little use because you are extending it beyond what it was originally designed to demonstrate. I think you’ve been trying to use the balls as an analogy to the pressure differential within the engine, but other people have been using the balls as an analogy to the air mass, and the air only goes one way, out the exhaust.
Lets go back to what is really happening. Air is compressed, mixed with fuel, and ignited. The air then expands rapidly and is accelerated out the jet exhaust resulting in forward motion of the aeroplane. If you block the exhaust pipe so that no air escapes then you get no thrust and no forward movement. The more exhaust you let out, the more thrust you get, up to a point. If you open the exhaust so much that the air starts being accelerated sideways THEN you have wasted energy that is not being used to propel the aeroplane forward.
So what is happening with reverse thrust? In this case the accelerated air is accelerated further by changing the exhaust direction from rearward to forward. Remember that “accelerated” means a change in velocity and velocity is both a direction and speed, if you change the direction of the air without increasing the speed, it is still being accelerated and can still do work. Because it is the final vector of the exhaust leaving the aeroplane that defines the thrust, the resulting thrust is reversed. In simple terms, if you reverse the exhaust you reverse the thrust. In the case of thrust reversers a lot of the exhaust actually goes sideways and that energy really is wasted.
A final word on wasted energy. If you direct all of the exhaust in all sideways directions, the plane won’t move and the energy is wasted, if you send half of the exhaust forward and half backward, the plane won’t move and the energy is wasted. If you control the exhaust so it is all directed rearward, the plane moves forward and the energy is NOT wasted. Yes the exhaust going out the back represents energy that is not being absorbed by the plane’s mass, but if it was being absorbed the plane wouldn’t move anywhere.
like i said in my previous post the reaction has taken place, think of the explosion with a nucleus in the center of the burner can in space.. force is emanating from the center of the explosion is in ALL directions that’s why if you stand around a suspended grenade everyone gets hit.. it is not because I stand opposite on the side of the grenade you are getting hit. You are getting hit because the force is going towards you also from the CENTER of the explosion. now if i stand my ground, the energy will bounce off me and go back towards you, yes, but but if i am blown away at the same speed of the force in my direction, you wont feel anything from my end and vice versa.
vyVY
Yes but in order to get movement you need net thrust in a particular direction, the only way you get that is by having part of the explosion being un-contained. This is not “wasted” this is “necessary”, without it you don’t get movement.
Lets use your grenade and people analogy.
In the case of an aircraft engine we need a grenade and we need people standing around it, all of them holding hands. For the purpose of the analogy the people are strong enough to hold hands through the force of the grenade explosion. If you have a complete ring of people all holding hands, then when the grenade explodes, no one will move any where because the group experiences equal force in all directions. However if you remove one of those people and create a hole in the group, when the grenade explodes there will be explosive debris exiting through the hole in the circle of people and the people will all be moved in the opposite direction. Because we made a hole on one side of the people circle, the people moved in the other direction.
The reaction is not the explosion. The explosion itself is not what is causing movement. The reaction is the point at which the airmass is accelerated out the exhaust. By focussing on the forces within the engine you are working at a different level than others here. You can look at the pressure within the engine and see that there is high pressure at the front and low pressure at the back and therefore the engine will move forward, or you can look at the mass accelerated rearward and see that because a mass is accelerated in one direction there must be an equal and opposite reaction in the other direction and the engine moves forward. You are trying to do both, you are mixing the different principles. You are saying that there is force on all parts of the inside of the engine except the exhaust and therefore the engine moves forward. That’s ok, nothing wrong with that, but then you say that because the exhaust is an area of the engine where there is no force, that the exhaust gasses are “wasted”. But this is where you are crossing over into the other way of describing what is happening, and you can’t mix the two. The exhaust gasses are not wasted, they are the mass that must be accelerated rearward in order to get the equal and opposite reaction forward. Stick to internal forces, OR acceleration of the airmass, don’t try and mix the two, it is confusing.
Additionally, don’t go too far with the “explosions” analogy. An explosion produces sounds waves, shock waves, and debris which carry energy themselves, reflect, and interact with materials differently, and your intuitive sense of how explosions operate reflects that. Combustion in a turbine is a continuous, smooth, controlled process, that simply results in high pressures and temperatures, not a detonation or explosion.
Finally, you are correct that some of the energy of the fuel is wasted. But this waste comes because it transformed into heat and pressure, and the engine is not 100% efficient at transforming heat and pressure into backward movement of the air - the engine exhaust is hotter than the ambient air. All of the energy that is transformed into backward movement of the air is used to propel the plane forward, none of that energy is wasted. The waste is the heat of the exhaust that the engine could not transform into motion, not the motion of the exhaust gas. The motion of the exhaust gas is the whole point of the engine.
when I say explosion it is for argument sake, the reason they call it combustion if I remember my flight school days is because the explosion is consideredcontrolled. hence combustion but it is an explosion all the same or expansion so to speak
like you said there is force on each person. each want to move in the direction the force is pushing him, from the result of the force emanating from the center of the people which is going away from the center in all directions towards the people.. why each person doesn’t move is because each one is counteracting the other. when you move the person to the south, the pressure that was on him goes outside, so there is no more force counteracting the person to the north, so the force that was acting on the north person all this time can finally push him without the person to the souths force stopping/counteracting his force vector . the south person is not there anymore to stop the south force escaping which of course was counteracting the northward (and everyone else force to a degree but since its an engine with front and back i use north and south) force to stop him moving north. his tie down chain was removed so to speak so he can go the way he was trying to go all the time. he is free to use the north force so to speak. the force that was acting on the south person just leaves the area doing no work. remember the force acting on the south person emanates from the center of the people where the combustion happens, not from the north person.
vyVY
I put it to you that the combustion/thrust/exhaust has to touch a part of the plane to move it, what is coming out the back is not touching the plane. the part of the combustion that pushes/touches the plane is the combustion that is directed towards the front of the plane where the metal is for it to touch and push on. not the part that escapes out the back. that is the unavoidable exhaust which is the by product of removing the counteracting force by opening the back of the engine .
I say exhaust because if the front of the engine was open you would have exhaust out of both sides, technically speaking.(that would be of course if you fed air by a tube into the fuel mix) The reason you don’t have exhaust out of the front of the engine is because the metal of the plane is there and that is what is being acted on and pushing the plane forward. The exhaust that would have been wasted out the front if both side of the engine were open so to speak.. wasted because it didnt do any work, like the exhaust going out the back.
vyVY
sir with all respect, stand behind a jet on takeoff you will experience sound, shock waves and debris.. lots of rocks and sand to boot. I think you sell me short if you think I really mean an explosion. It was for illustration purposes that I assumed you would understand. maybe I am being held in less esteem in your view than I hold you 
By explosion i mean expansion, expansion in all directions.. as I mentioned in the recent post , if I remember from my flight school days 30 or so years ago we call it combustion because it is considered a controlled explosion. but by definition it is an explosion (expansion) all the same, and can produce shock waves, albeit continuous, noise (depending on how much you rev the power) and debris if you put it in there.
vyVY
Please.. stop… saying… this…
Why? there is no free lunch , if it was used, it wouldn’t come out the back, you saying that there is perpetual energy?
vyVY
No, but the force on the engine is caused by the pressure of the gas contained inside, and the pressure is the combined result of the presence of all the gas, not just the gas molecules that are actually colliding with the wall. The wall opposes expansion of the gas, and the gas expands in the backward direction while simultaneously “pushing off” the wall and imparting thrust to the airplane/engine. You cannot separate these two effects.
Imagine you are a standing on a frictionless surface - an icy pond, say - facing me. We are standing north/south. You give me a hard shove - equivalent to the “forward force” on the forward wall of the combustion chamber. I start sliding north, you start sliding south. Your southward motion is not wasted - it was essential in producing my northward motion! Conservation of momentum, and all that.
If you were to remove the front and back of the combustion chamber, you would have a force in the forward direction that is equal to the force in the backward direction, and each of these forces would be 1/2 the force in the forward direction in the normal configuration.
The exhaust is not “wasted”, it is necessary. Something that is necessary to produce the desired result can not also be “wasted”.
If I spend $1000 to buy a round-the-world ticket, that money is not wasted. The money may not be directly used to fly me around the world but it was a necessary expense to achieve the result of going around the world. Likewise the exhaust is not wasted, it is a necessary “expense” in order to achieve the result of forward movement of the aeroplane. The hot gasses that make up the exhaust were used for the purpose of moving the aeroplane forward, they were not used “carelessly, extravagantly, or to no purpose.” If there is no exhaust, there is no thrust. Something that is necessary can not also be waste. The purpose of the exhaust is to be expelled at high velocity from the rear of the engine, that is what the engine is designed to do.
Exactly. Unlike frictional losses, turbulence losses, noise emissions, etc., which are “wastes of energy” in a real-world system because they are not key to the functioning of the system and simply unavoidable side-effects, the kinetic energy in the exhaust stream is theoretically necessary in order for there to be any thrust. This is part of the fundamental nature of a reaction engine. It is not wasted, because it is necessary.
That is not my analogy. mine is that I am standing in between two people.(the combustion in the middle of the engine). because of the nature of combustion I have to push in both directions but I really only want to push the man to the north. but because I am combustion I have to push south also. so the north man goes north where I want it to go and the south man goes south where I am not interested. even if the south man wasn’t there the force would still go that way, from MY center MY south side, not from the side of the north man who is getting his push from MY north side (my expansion in that direction so to speak) . so the southward force from my center does nothing I need and the northward force from my center is used in the way I want. to push him north.
look at combustion as if you drop a pebble in the middle of a bucket of water. waves go in all directions because that is the nature of it.. you only want to push north so you put something in the path of the north wave. the south wave which is a result of the pebble hitting the water is not used.. the south wave has nothing really to do with the north wave. they never come into contact with each other so to speak. They are not a result of each other, they are a result of the displacement of water of the pebble hitting the water. that is basically the combustion that happens in the middle of the burner cans force in all directions with an imagined nucleus or central point so to speak. Now, place the pebble scenario in the middle of the combustion chamber of the engine and you will see what I see hopefully. ..vyVY
I wonder if he considers it “wasted” because it is not being used to drive the turbine?
That’s not right. If you remove the south man then YOU move in the opposite direction of the north man, you in effect become the south man moving south in an equal and opposite vector to the north man. You (the combustion) are the mass airflow moving out of the engine. Perhaps now you can see that you were not wasted?
The only action used is that which hits the in the direction forward, you seem to be saying that all the force of the combustion goes towards the front of the engine pushing it and then comes back out. Forgetting that the expansion of gases are immediately going out in ALL directions, hence only the force going towards the direction the plane wants to go is being used.. vyVY
vyVY
If the exhaust is not allowed out then nothing gets used either. I’m not sure how many times I have to say it. The exhaust is necessary for the thrust. If the airmass is not accelerated backward, there is no thrust and nothing moves. How can the airmass then be called “wasted”. It’s entire purpose is to be ejected forcefully from the rear of the engine, that is the design principle of the engine. The thrust of a jet engine is DEFINED by the acceleration of the airmass rearward.
yes but SOME of my energy north is used to move him north if he is on wheels (remember the wheels of the plane). if his brakes are on and mine, nothing happens we are counteracted…
vyVY