Long rod question

[Houd]

Sorry for prolonging this but I just want to ask something to make sure I haven't massively missed the point.

So if you took a taut chain and chained the back of the plane to some immovable object so that the plane ACTUALLY couldn't move relative to anything relevant (which I think is kind of what Grrrrr was getting at), it'd just sit there spinning its wheels indefinitely?

See I always assumed the conveyor belt question was about somehow generating lift without movement when it seems it's actually about how conveyor belts aren't going to do much to stop a plane from moving forward. Is that right?

 

[box]

I thought it was important to get gas expansion and therefore increase the velocity of the exhaust gases (?).

Correct...which can therefore be transferred to the energy of the rotating turbine behind the burners.

Afterburner

Yup, now that IS newton's 3rd law.

Jet engines and rockets work in a very similar way at the level of basic physics.

A propellor is a completely different thing.

Well they do and they don't. Either way, jet engines are all a form of enclosed propeller really! The only exception to this is the turbojet.

Take a look at this image...

http://www.solarnavigator.net/aviat...aviation_space_images/jet_engine_diagrams.jpg

The turbojet is the engine you describe. It's thrust comes merely from the fuel burn and the 'push' of the hot gasses against the air outside. As you mention, very similar to rocket engines. However, this is a dated design and is rarely used any more - it is polluting and noisy!

The turbofan and turboprop engines are what we see most of today. The thrust effects of the fuel burn still exist, but the primary source of forward movement comes from a turning turbine/prop which moves air towards the rear of the engine in the same way as a conventional piston prop.

Fun eh?

 

[Katoom]

It wouldn't spin it's wheels.

 

[EffBee]

A propeller is not like a screw. A prop creates a low pressure infront which 'pulls' it toward.

Really? Thats interesting - I'd never stopped to think about the mechanism of exactly how a propellor on an aeroplane actually interacts with the air to do it's job. Thx.

We're sort of back to lift, and wings and wondering if you spin the aircraft one way and the propellor the other will the aircraft move forward <JOKE!>

 

[Katoom]

The bottom of a wing is flat, while the top is kind of an arc shape. The air moving over the top has to move further than the bottom, this creates a 'low pressure' on top which is lift.

This is propellers and choppers.

 

[Artheas]

Sorry for prolonging this but I just want to ask something to make sure I haven't massively missed the point.

So if you took a taut chain and chained the back of the plane to some immovable object so that the plane ACTUALLY couldn't move relative to anything relevant (which I think is kind of what Grrrrr was getting at), it'd just sit there spinning its wheels indefinitely?

See I always assumed the conveyor belt question was about somehow generating lift without movement when it seems it's actually about how conveyor belts aren't going to do much to stop a plane from moving forward. Is that right?

I didnt understand why the plane WOULD take off until i got the the 50th+ post in this thread, and the reason for that being i made the same assumption as you. They could have just changed the question into asking if a thrust powered car would move forward relative to the ground in that situation.

 

[p4radox]

The air moving over the top has to move further than the bottom,

That's not true.

The true aerofoil effect is predicted by analysing the curvature of streamlines on top of the wing compared with on the underside. The larger curvature causes/is caused by a lower pressure above the wing.

It's not really explained by distances over/under the wing.

 

[EffBee]

Correct...which can therefore be transferred to the energy of the rotating turbine behind the burners.



Yup, now that IS newton's 3rd law.



Well they do and they don't. Either way, jet engines are all a form of enclosed propeller really! The only exception to this is the turbojet.

Take a look at this image...

http://www.solarnavigator.net/aviat...aviation_space_images/jet_engine_diagrams.jpg

The turbojet is the engine you describe. It's thrust comes merely from the fuel burn and the 'push' of the hot gasses against the air outside. As you mention, very similar to rocket engines. However, this is a dated design and is rarely used any more - it is polluting and noisy!

The turbofan and turboprop engines are what we see most of today. The thrust effects of the fuel burn still exist, but the primary source of forward movement comes from a turning turbine/prop which moves air towards the rear of the engine in the same way as a conventional piston prop.

Fun eh?

Just to clarify for my own benefit, Turbo jets are easy enough to understand. Turbofan and Turbojet - the foward motion still comes from reaction against the air mass being shoved out the back (?) The difference from the Turbojet being the method of getting that air mass moving (?). Turbojets mostly burning and heat , from what you are saying I get the image of a turbofan being more like a giant compressor.

 

[Katoom]

Could I just say 'moves over the top'?

 

[p4radox]

Just to clarify for my own benefit, Turbo jets are easy enough to understand. Turbofan and Turbojet - the foward motion still comes from reaction against the air mass being shoved out the back (?) The difference from the Turbojet being the method of getting that air mass moving (?). Turbojets mostly burning and heat , from what you are saying I get the image of a turbofan being more like a giant compressor.

The thrust comes from kicking out high momentum gas from the back of the engine. A turbojet achieves this by kicking out high velocity gas. A turbofan achieves it by kicking out a high voumetric flow rate. The two both give you a large thrust, just in a different way.

 

[ChroniC]

if your on wheels on the treadmill like a plane then you wont move

Surely thats only the case if you were stablised from something above or to the sides that aren't moving.
The wheels would spin freely if the plane was being held for a stationary point, but remove this point and the plane will travel backwards if it had no thrust. (from the weight of the plane surely, i say that becuase if u put a shopping trolley on a belt it moves with it, it doesnt just sit there.)
However if the belt was infinitely long then the plane exerting a small amount of force would move forward relative to how fast it was moving backwards due to it pushing on the air not the belt and then if you applied more force would take off, but if it was runway sized and had an end, then surely you would have to counter the speed of the backwards motion with more thrust forwards before it could take off.

I think what im trying to say is that as long as the weight of the plane is being transfered onto the wheels, for the wheel to spin freely underneath the plane, near equal amounts of forced have to be applied from both the belt and the jet to keep it stationary. The all the jet has to do is apply more force than the belt. No? Surely?

 

["andy"]

aah you caught me off guard there like. in most my other posts i was mentioning that the bearings where frictionless just for making it simpler when demonstrating

your right though. the friction in the wheel bearings is probably so little that it takes about 1% of the jet power to keep it from moving backwards.

 

[Jonny69]

While in the AIR?

The spinning wheel has angular momentum. If the aircraft tries to slow the wheel it will turn about the axis of the wheel in the direction of movement of the wheel.

 

[ChroniC]

aah you caught me off guard there like. in most my other posts i was mentioning that the bearings where frictionless just for making it simpler when demonstrating

your right though. the friction in the wheel bearings is probably so little that it takes about 1% of the jet power to keep it from moving backwards.

Ok so in theory frictionless bearings would spin, but in the real world the plane is going to be exerting pressure on the belt through the connection to the wheels. It would have to counter the force which the belt is apply to them to become stationary, it may not be the equal amount but it would have to be some. In either case surely the forced applied to the air is having to counter that.
What is confusing me is that if the belt went mental fast to counter the thrust, and the planes bearings took the strain so that the plane only has to exert some force, the wheels would then be spinning so fast they would melt and the plane would crash. I assume this because its not rising, its just staying stationary relative to the air its in.

 

[Dist]

The way it was explained to me was this, imagine you are swimming in a pool next to the side, and all along the side is a treadmill. In your hand you are holding a trolly wheel (which is similar to a plane wheel in that its free moving), now if the treadmill is moving back at 5mph, and you hold the trolly wheel onto it, will it push you back at 5mph? NO! So if you swim at 5mph, the speed of the treadmill will not be sufficient to counter the motion generated by swimming, you will swim forwards and very slightly under 5mph, and the trolly wheel held in your arm will be moving forwards at the same speed with the wheel spinning as if it was moving at 10Mph.

 

[box]

Just to clarify for my own benefit, Turbo jets are easy enough to understand. Turbofan and Turbojet - the foward motion still comes from reaction against the air mass being shoved out the back (?) The difference from the Turbojet being the method of getting that air mass moving (?). Turbojets mostly burning and heat , from what you are saying I get the image of a turbofan being more like a giant compressor.

Yup, bang on.

Put very crudely, in turbofan jet engines, the 'jet' merely does the same job as the piston in a conventional combustion engine. ...it goes bang and makes the thing turn...

The way it was explained to me was this, imagine you are swimming in a pool next to the side, and all along the side is a treadmill. In your hand you are holding a trolly wheel (which is similar to a plane wheel in that its free moving), now if the treadmill is moving back at 5mph, and you hold the trolly wheel onto it, will it push you back at 5mph? NO! So if you swim at 5mph, the speed of the treadmill will not be sufficient to counter the motion generated by swimming, you will swim forwards and very slightly under 5mph, and the trolly wheel held in your arm will be moving forwards at the same speed with the wheel spinning as if it was moving at 10Mph.

Well maybe I'm just tired...but WHAT?!?

Is that meant to be a simplification? lol

 

[ChroniC]

The way it was explained to me was this, imagine you are swimming in a pool next to the side, and all along the side is a treadmill. In your hand you are holding a trolly wheel (which is similar to a plane wheel in that its free moving), now if the treadmill is moving back at 5mph, and you hold the trolly wheel onto it, will it push you back at 5mph? NO! So if you swim at 5mph, the speed of the treadmill will not be sufficient to counter the motion generated by swimming, you will swim forwards and very slightly under 5mph, and the trolly wheel held in your arm will be moving forwards at the same speed with the wheel spinning as if it was moving at 10Mph.

Thats because you have a stationary fixed point away from the belt, i.e the pool. If like the plane your exerting forced onto the belt via gravity its different.
Im not denying its not possible, but im saying that it become confusing and complictaed because unless you have frictionless bearings (which is impossible unless your at zero g) the plane is going to have some forced on the belt and thus be moving backwards. But because of the bearings it also doesnt require an equal amount to keep it staionary.

 

[Dist]

Thats because you have a stationary fixed point away from the belt, i.e the pool. If like the plane your exerting forced onto the belt via gravity its different.
Im not denying its not possible, but im saying that it become confusing and complictaed because unless you have frictionless bearings (which is impossible unless your at zero g) the plane is going to have some forced on the belt and thus be moving backwards. But because of the bearings it also doesnt require an equal amount to keep it staionary.

How is floating in a pool while using an arm pushing down a wheel into a treadmill different from a plane on a treadmill? the only difference i see is the medium in which we move, the body of a plane is surrounded by air, and i am surounded by water, the only point of contact with the ground is the wheels on the runway.

 

[Katoom]

Now wonder your confused

 

[box]

Im not denying its not possible, but im saying that it become confusing and complictaed because unless you have frictionless bearings (which is impossible unless your at zero g) the plane is going to have some forced on the belt and thus be moving backwards.

Friction may be influenced by gravity, but since when is it dependant on it?

Are you trying to say that in deep space everything suddenly becomes slippery? lol

I'm just nit-picking now