Ok so where on what part of the aircraft is the force applied to push the carft backwards?
vyVY
Let’s try and go to the basics, the molecular level, (layman’s terms as best we can please) I want to get your understanding of pressure… I see pressure as at least two ways, one where you force the air into a canister and it stays under that pressure until you release it, and the other where there is no pressure in the canister and you heat it and the pressure builds and if you take away the heat away as the air cools the pressure would go away…the combustion or heat induced pressure is where the atoms get exited and fly all over the place hitting the sides of the can trying to get out. This pressure to me has a perceived central point because the atoms would all fly away from each other in opposite directions if the sides of the canister disappear. The compression pressure I guess is the squishing of the air molecules and if the canister disappears the air goes back to it’s original size… in all directions away from a perceived central point.
can we agree on this ? if not elaborate please
vyVY
The reverser buckets. In the sense that they are the last part of the aircraft interacting with the exhaust, and it is the buckets that are accelerating the exhaust from going backwards to going forwards.
As far as I remember, that’s all I ever said.. We are in COMPLETE agreement, if I assume by acceleration you mean change of direction, or increase in speed but not increase in power. I.e the same lbs of force coming towards the buckets cannot increase after it leaves.
Hopefully we can agree that if 100 mph lbs of force hit the buckets, some of that was used/converted to try and push the aircraft backwards and there is less mph/ lbs thrust leaving the buckets than before the buckets. i.e 100mph exhaust exerting 100lbs of force going to the buckets and then less mph or lbs of force (or less of both) coming off the buckets. i.e. if you threw a ball at a movable object, the ball would come back at you at a lesser force. Because some of the force was used to move the object in the direction you threw the ball etc.
vyVY
Another on this, the turbojet: maybe I don’t understand the way you put it. You say the acceleration of the airmass through the core of the engine? My understanding is that the air introduced to the engine compartment (read combustion/expansion/acceleration chamber) by the fan is to mix with the gas to burn, and that burning is what does the work/acceleration/expansion (this is inside the engine aside from the bypass)
vyVY
I think this is part of your problem. “100 mph lbs of force” is a completely meaningless term. You insist on describing force with a speed, but force simply isn’t measured that way.
I am trying to use layman’s terms here. you answered but didn’t tell me what you say how force is measured..in aircraft terms we say 100 lbs of thrust. 100 lbs of thrust should move anything less than 100 lbs. i could lose the mph but wanted to keep it in the perspective of moving air. ie. the exhaust (air) moving towards the buckets at 100 mph carrying 100 lbs of thrust to it. 100 is just an arbitrary number for illustration purposes.’
vyVY
Actually, you can move objects much heavier than 100lbs with 100lbs of thrust, if the friction holding the object in place is low enough. F=MA and all that.
The problem here is that you are trying to understand some quite complex physics questions using ‘layman’s terms’ and a completely oversimplified understanding of the physics involved. You can’t do that. You need to actually understand the physics involved and, more importantly, the math involved in describing how those physics work. You can’t keep handwaving with terms like ‘100mph of force’ and expect to actually understand how a jet engine works.
Indeed. Airplanes typically have maximum thrust ratings that are far less than their weight. The exception is fighter aircraft, which typically have a thrust-to-weight ratio on the order of 1. But commercial airliners - the vehicles that tend to most commonly employ thrust reversers - usually have a thrust-to-weight ratio on the order of 0.2 (depending on how fully loaded they are).
For a more down-to-earth example, if I put my car in neutral I can move it across the garage with 100 pounds of thrust, despite the fact that it weighs around 3000 pounds.
agreed with both previous posters. that is why I was using mph thrust: i.e.To get your 100lb car moving at 20 mph would take x amount of thrust. If you attach another 100lb car to it would the same x thrust move the cars 20 mph? so the thrust it takes to move a car 20 mph I call 20 mph thrust.( the power it takes to move a car of a specific weight 20 mph).
So again, what I meant when I said a 100 lbs is the force to move a 100 lb mass 20 mph, it if met up with another 100lb mass the speed would be diminished (in half maybe), the two would not move at 20mph, you would need more thrust for that…
And I do believe that complex physics can be explained in laymans terms once they understand it. Maybe I’m wrong but from what I have seen so far, this seems to be the case.
vyVY
Yes , but my point is if the car wasn’t there you would be moving along a lot faster, to your full potential.
vyVY
my point is mass on the force: If you put the car on top of you you couldn’t budge the car. if you put 100 lb weight on you you couldn’t move it with 100lbs force they neutralize themselves . with 99 lbs you would get movement..Turn the plane on its tail with the nose at the sky and see if it you move, you would need more power than the weight of the plane to move it (harrier).
vyVY
Where ‘x’ in this case would depend on a large number of factors, including how smoothly your tires roll on the ground and how much air resistance your car has. If you open one of the car’s doors, the increased air resistance will increase the amount of thrust required to keep the car moving at the same speed. If you increase the pressure in the tires, the decreased rolling resistance will decrease the amount of thrust needed. If you apply the parking brake, you will GREATLY increase the amount of thrust needed. Different cars will require different amounts of force to reach the same top speed, and even things like the presence or absence of a headwind/tailwind or the temperature of the ambient air will have an effect.
Furthermore, it is an oversimplification to say that a certain amount of thrust will cause the car to move at a certain speed. When you apply force to the car, it will start accelerating. It certainly won’t suddenly jump to 20mph, rather it will gradually build up speed. As the speed increases, various forces will build up opposing further acceleration - air resistance, rolling resistance from the tires, etc. Eventually you will reach a point where the drag forces exactly equal the force you’re applying, and the car will have reached a constant speed. The amount of force applied to the car is your ‘20mph of thrust’, but the actual amount of force this represents will be highly dependent on a lot of different factors.
If you could completely eliminate all sources of friction - have a car with perfectly frictionless bearings and infinitely rigid tires in a vacuum - then the car would never stop accelerating, even if you’re only applying the same amount of thrust as needed to get it going 20mhp under normal conditions. (Technically the acceleration will slow down as you approach the speed of light, but we can ignore relativity for this thought experiment).
If you double the weight of the mass being pushed, the acceleration will be halved. The top speed that the mass reaches before the force you’re applying equals the drag is going to depend on rolling resistance, air resistance, and all those other factors. In a real-world condition, it will probably be less than the 20mph of the single car mass, since resistance on the tires will increase with the mass being pushed. If you can somehow keep the rolling and air drag the same while doubling the mass, then the same thrust would get the cars to 20mph, it will just take about twice as long to do so.
And as pointed out, this is a gross oversimplification and not at all accurate or useful for understanding how real-world systems work. Furthermore, you are now introducing power, which is a completely different measurement than force or speed. Don’t mix units like that.
This is all fairly basic high school level physics, but you need to actually understand it and the math involved to understand how the physics of airplane thrust and thrust reversers work.
For a 100-pound car, X could be any number at all, depending on how much time or distance you have.
Sure, it’ll just take twice as long to achieve that speed (since we are now trying to accelerate twice the mass).
You are confusing the bejeezus out of everyone (or maybe just me???) with your bizarre mingling of terms that have very specific meaning in physics and engineering. Power, thrust, and speed are all different quantities, and the units are not interchangeable. “Move” implies a simple linear displacement, measurable in distance units (feet, meters, etc.). If you are talking about accelerating a mass to a specific speed, specifying thrust is not enough: you need to also specify either time or distance (e.g. 100 pounds of thrust applied over a distance of X feet, or over a period of Y seconds).
Frankly the physics of a jet engine are not all that complex: air comes in at some speed, engine sends that airstream[sup]*[/sup] out the back at higher speed, and the difference in the speed of those streams results in thrust. A fundamental fluid mechanics analysis would draw the engine as a “black box” (i.e. we don’t give a rat’s ass what goes on inside it), and would only consider the momentum associated with the fluid flows into and out of that black box. You have a little rearward momentum coming into the box at the intake, and a lot of rearward momentum leaving the box at the exhaust. Net result is forward thrust, calculable on the basis of the properties (mass flow rate and velocity[magnitude AND direction]) of those two streams and nothing else.
*Given the amount of air that moves through a jet engine (especially for a high-bypass turbofan), the added mass of the fuel stream is pretty much negligible.
I agree and I think you understand now what I mean by 20 mph thrust, although you may say it is not a good way of putting it.
I disagree,I see it as it could not go more than 20 mph, because that is the only force available to make it go 20 mph in that configuration/weight/mass and so on. To go faster you would need more force, energy, faster force so to speak, or i.e.reduce the mass etc., then it would go to 30 mph or so and stay there.
Well this may be pertinent to the point, the reason the acceleration slows down as you reach light speed is because the mass somehow increases, so this proves my point that mass and available energy/thrust which would cause that particular configuration to go at a certain speed and no more. Which I’m calling mph thrust. (crude to some but hopefully effective)
This is why they theoretically talk of light sails so to speak, I forgot the correct terminology but it was proposed that if you could make a huge sun sail and harness those protons or whatever coming from the sun at or almost the speed of light you could attain high speeds. i don’t think regular combustion of today can push an object at those speeds. You need big bang technology for that, or warp power hehe.
Ok I see what you say, but i consider speed and force loosely in a crude sense to be the result of power.. You have power-you have force-you have force you can get movement/acceleration/speed. What’s the sense of having power and force it there is no motion (speed). So speed and force can be the result of using the power?
I guess by now some of you don’t need to go to a barber: hair all pulled out..
I still think everything can be explained to the common man for him to grasp what’s going on. It takes a smart genius to do this. vyVY
I believe until the folks in here understand that the force that works on the plane are those atoms going in all directions trying to find a way out as a result of rapid heat expansion, some hitting on the metal to the front of the plane and some going out the back, they will never see what I see. something has to push the physical plane, and it is the atoms moving at tremendous speeds in the direction of the plane that is doing it. anything out the back is atoms that were haulin’ butt in that direction ANY way from the result of the heat.the atoms don’t know which way is up or down, left or right. remember this is the pure turbojet I are talking about. I explained in another post how the bypass fan of a turbofan or prop plane is not pushing but pulling, or corkscrewing into the air.
Think of the atoms as blind people cooling out but not attached to in the chamber, put some heat in there and all they run in all directions trying to get outta there.some hitting the front of the engine or compressing the air coming from the turbine therefore putting pressure/force on the aircraft, forcing it forward, and everyone else hightailing it out the back. Every man for himself. vyVY
Then either you have a fundamental misunderstanding of physics, or you have misread AndrewL’s example. He described the elimination of all sources of friction, such that the only force acting on the car was the thrust that is currently the subject of discussion. Given that F = m * a, if F (the thrust) is applied continuously, then a (acceleration) will be also be nonzero continuously; the car will not stop accelerating when it achieves 20 MPH. The only way to get a frictionless car to keep moving at 20MPH is to stop applying thrust once it achieves that speed.
Relativistic effects come into play at speeds very close to 186,000 miles per second; we are discussing the physics of objects with speeds on the order of a few hundred miles per hour, so no, relativity is not at all pertinent to this discussion.
No, it’s not effective. Not at all.
I agree, but that doesn’t seem to be the direction you’re headed. Either you do not grasp the fundamental physics of jet engines, or you have extreme difficulty expressing your ideas in plain English; I’m not sure which.
How if the only force available is to move the object 20 mph it can go faster and faster than 20 mph under non friction scenario? i.e if x pounds of force was needed to make an object in space go 20 mph and continuously applied, then 20 mph is the limit, the object can’t go faster, you would be burning fuel/applying force for no reason. Other wise when they send an object into space , when they are out of gravity they could send a trickle of power out the back and accelerate to almost light speeds.. To get more speed you would need more force, more power. you are forgetting the mass of the object, friction and gravity is gone but the mass is there. If I am the force and moving at 20 mph and start push the object and attain 20mph, i don’t have anymore to give it, we are stuck at 20mph. maybe I’m wrong.. vyVY
what do you mean? i said it is the expansion of gases from the burning gas that works on the movement of the aircraft, what is all that gas in the wings for then? combustion is gas and air. what’s not to understand about that?. can you deny this? The problem is, you all are not peering inside the combustion chambers. You seem just to be concerned with what you feel or see coming out the back or the air generated by the fans. if you shut off the fuel and used an electric motor to turn the engine at the same speeds are you saying the same thrust would be produced?
vyVY
You are correct, this is all one need be concerned with when understanding the fundamentals of jet propulsion. Refer back to my description in an earlier post:
A jet engine that’s designed around heat addition (whether via burning of hydrocarbon fuel or via the use of powerful electric heaters) won’t operate the same way in the absence of heat addition. But you could instead design a jet engine that uses mechanical impellers that are driven by electric motors, and if it had the same mass through-flow rate and inlet/outlet velocities as the fuel-burning version, it would product exactly the same thrust.