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- Thread starter Chas
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Towpack
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Just think of the plane and the air around it and forget everything else. The jet engines push against the air behind the plane and it moves forward. If it’s takeoff speed in still air is 150mph and there was a 150mph headwind it could lift off without engine thrust. If there was a 150 mph tail wind the plane would have to move forward (relative to the ground) at 300 mph but it’d still be doing only 150 mph relative to the air it was in.
Digressing slightly, the inlet ducts to the engines on Concorde had to be modified to slow down the air entering them when travelling supersonically as they couldn’t cope with air entering them at such speed.
Digressing slightly, the inlet ducts to the engines on Concorde had to be modified to slow down the air entering them when travelling supersonically as they couldn’t cope with air entering them at such speed.
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I think when chas finally stops laughing there will be a one line twist to this..
Just a technical point TP and generally I bask in the shadow of your monumental knowledge, but jet engines don't actually push against the air behind them. It's the MASS of air (because the turbines compress air and air has weight) being thrown out of the back that uses Newton's 3rd law - mass backward = movement forwards. It's the old standing on roller skates throwing a brick experiment. The equation here given that energy cannot be created nor destroyed means that the jet fuel is burned in the compression of massive volumes of air being drawn in and forced out of the back. You see, in space (where no one can hear me scream) thrusters still work in the vacuum because they use the same law of motion. Or rockets wouldn't work in space.
MarkW
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This question has been generating internet arguments for 14+ years, some pretty epic trolling by whoever originally posed the question.
A quick google reveals hundreds of links explaining the physics some even written by proper physics people, not many actually give a clear answer. Mythbusters looked at it in an episode but then they addressed a different question
A quick google reveals hundreds of links explaining the physics some even written by proper physics people, not many actually give a clear answer. Mythbusters looked at it in an episode but then they addressed a different question
Towpack
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An additional point re the bullets from a plane, the plane of course gets slowed down by each round it fires. Ok, by an infinitesimally small amount being a 747 with huge mass, but it will still be slowed.
The 20mm cannons on the Spitfires could lose them significant airspeed if used for long bursts.
The 20mm cannons on the Spitfires could lose them significant airspeed if used for long bursts.
We're not talking about arming a Jumbo Rich. it's a Warbird capable of about Mach 3. A Spitfire could manage about 400 mph. Rifle bullets are travelling at around 38oofps at the muzzle. Significantly less on a Spit.
The question really is really about nose cone mounted machine guns on a plane. Does it have to slow to a max speed before it fires or will it run into its own bullets. It's a nonsense but another long running debate. If the target plane in front is going faster than a bullet, you're not going to be able to hit it.
The question really is really about nose cone mounted machine guns on a plane. Does it have to slow to a max speed before it fires or will it run into its own bullets. It's a nonsense but another long running debate. If the target plane in front is going faster than a bullet, you're not going to be able to hit it.
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Why does it need a runway - because its not a jump jet . The (circa) 120 tonne of thrust provided by the 747's fixed jets can only supply forward motion , but to go forwards the wheels have to turn .
A conveyor belt runway being more effective than brakes in preventing forward motion means the aircraft would refuse to move .
Even if the jets were on a swivel like a jump jet 120 tonnes of thrust could not lift the (circa) 400 tonne aircraft to flight .
Gravity is the most powerful force in the equation
A conveyor belt runway being more effective than brakes in preventing forward motion means the aircraft would refuse to move .
Even if the jets were on a swivel like a jump jet 120 tonnes of thrust could not lift the (circa) 400 tonne aircraft to flight .
Gravity is the most powerful force in the equation
I know Chris, I just got the zip stuck on the anorak I was wearing to reply to Keira’s thread.
You're not getting this I can see. Gravity is not the most powerful force in operation. That's why planes fly. Lift overcomes gravity. I've said before, the conveyor idea is a total red herring. it has nothing to do with whether the plane flies or not. The wheels are just spinning in neutral effectively.
OK last example. Fast flowing river like the Amazon. Take an airboat. Those great big skiffs with a propeller on the back that they use in the Everglades. If the river is flowing at 10 m/s and the boat fires up, it has to do at least 10 m/s to stay still, yes? Then when it revs up it moves forward. When it has enough revs on for say 20 m/s it will only be doing 10m/s forward, yes? Cool.
However, there's a major difference here. In the river, the force is acting against the hull, pushing it. With the plane, because the wheels aren't connected to anything (a bit of friction in the axle) the conveyor cannot exert a force on the plane. Nothing of any significance at least. The plane will move in relation to the airport. It doesn't matter how fast the belt goes, all the engines have to do is over come the few pounds of resistance in the wheel assemblies. Given that a man has pulled a jumbo jet for a world record, you can see that the motor off a mobility scooter would be enough to do that! So four bloomin great jet engines will be more than enough. So that's all they have to overcome to be stationary. A bit of rolling resistance on the wheels. One the plane throttles up there's no force holding it back. The wheels will be spinning like crazy but the plane will take off like normal minus the power of a shopping scooter.
OK last example. Fast flowing river like the Amazon. Take an airboat. Those great big skiffs with a propeller on the back that they use in the Everglades. If the river is flowing at 10 m/s and the boat fires up, it has to do at least 10 m/s to stay still, yes? Then when it revs up it moves forward. When it has enough revs on for say 20 m/s it will only be doing 10m/s forward, yes? Cool.
However, there's a major difference here. In the river, the force is acting against the hull, pushing it. With the plane, because the wheels aren't connected to anything (a bit of friction in the axle) the conveyor cannot exert a force on the plane. Nothing of any significance at least. The plane will move in relation to the airport. It doesn't matter how fast the belt goes, all the engines have to do is over come the few pounds of resistance in the wheel assemblies. Given that a man has pulled a jumbo jet for a world record, you can see that the motor off a mobility scooter would be enough to do that! So four bloomin great jet engines will be more than enough. So that's all they have to overcome to be stationary. A bit of rolling resistance on the wheels. One the plane throttles up there's no force holding it back. The wheels will be spinning like crazy but the plane will take off like normal minus the power of a shopping scooter.
chadr
Well-Known Member
Right, I think I have (finally) got it.......yes, the plane will take off and fly. Thanks to those of you that persevered providing explanations. 
The mistake I was making was thinking that the plane would be static due to the counter movement of the conveyor belt against the rotation of the wheels - this, as kindly pointed out, is a complete red herring.
The mistake I was making was thinking that the plane would be static due to the counter movement of the conveyor belt against the rotation of the wheels - this, as kindly pointed out, is a complete red herring.
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I remain unconvinced , 400 tonne keeps the wheels planted firmly on the ground and so regardless of how the plane accelerates the ground will accelerate at an equal speed in the opposite direction - into infinity , and the plane will not travel at all .
Interesting hypothetical question though beats "what happens if an unstoppable force collides with an unmovable object " to which i reply "marriage i guess"
Interesting hypothetical question though beats "what happens if an unstoppable force collides with an unmovable object " to which i reply "marriage i guess"
But the ground doesn't Shayne. It's just the runway that's moving. The airport and the rest of the planet is the same as before. It's nothing to do with weight. This answer is the same as if it were a Cessna single prop 4 seater. The prop pulls the plane through the air of the planet. It has sod all to do with the wheels.
chadr
Well-Known Member
I found this to be one of the better explanations out there.....especially the 2nd para - which is when the penny dropped for me!
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
source - https://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/
The problem here, of course, is that the poster (and Neal) cannot disengage themselves from seeing the airplane as a car. The difference between a car and a grounded airplane is that a car uses its wheels to propel itself forward, and an airplane moves itself forward by moving air. They assume that the runway moving backwards would move the plane backwards. This is what would happen with a car (that is in gear), so why not for an airplane? Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
You can test this with a piece of paper and a matchbox car (which has free rolling wheels like an airplane… or like a car in neutral.) Place the paper on a table, and place the matchbox car on the paper. Take your hand, and hold the car still with a lightly placed finger on top of the car. At this point you are providing no forward thrust, and the “conveyor belt” is not moving. The car remains stationary. Now, continuing to hold the airplane with a lightly placed finger, and start to pull the paper out from under the car, in the backwards direction. According to Neal’s logic, the car should push back on your finger with the same force that you are exerting on the paper… but this is not what will happen. You will find that your lightly placed finger is not stressed to any noticeable extent. The paper will slide out, and the wheels will spin, but the car will not be propelled backwards. The reason for this is is that the rotation of the wheels is not related to the movement of the matchbox car except by the very small friction component of the axle, which your lightly placed finger can easily control.
So now we have established that movement of the surface beneath a free wheeling object does not exert a noticeable force on the object. Next, we’ll see what happens when the object is trying to move forward. Attach a string to the matchbox car. Place the car at one end of the paper, and use the string to start pulling the car forward with a steady force. As the car moves forward, start pulling the paper out from under the car, backwards. Do you feel increased resistance as you pull the string? Of course not. The wheels are free rolling! Spinning the wheels does not make the object move!
When an airplane takes off, there is one major forward force… the forward thrust. The main rearward force is air resistance. The turning of the wheels provides a small frictional force, but because the wheels are free-rolling, this friction is very small. Unless the wheels are locked, the friction is always going to be less than the thrust, which means that the overall force is still forward, and the plane will still move.
source - https://txfx.net/2005/12/08/airplane-on-a-conveyor-belt/
The thing is Shayne, when a plane is airborne, it has no relationship to the ground whatsoever.
Take the Red Arrows for instance. They don’t always fly with their wings parallel with the ground do they? In fact, I think there’s a move they do while flying through the air where they turn a full 360 degrees around the axis from nose to tail. Even upside down or straight up. At that point, the relative speed of the ground is immaterial. It’s the speed of the aircraft relative to the air it flies through that gives it lift and flight.
Going straight up is probably the best example there is as the plane’s speed relative to Mother Earth is zero mph across its surface. The air exists in 3 dimensions and the plane can go in any direction through it.
It matters not one wit what the conveyer belt does, what does matter is that the thrust from the engines, which will move the aircraft forward whatever the ground (conveyer belt) does so airflow across the wings will occur creating lift, she flies. Think of an aircraft on an aircraft carrier, they take off using the engines plus assistance from a steam? catapult but the ship (conveyer belt) is moving, it's the forward motion that matters.
Mind you the original question never mentioned if the engines were running, if they weren't the plane would not fly.
I didn't post this as a test, but I think Chris was the first with an explanation, sorry Chris, no prize but you have the kudos of being first, there were others too though.
Mind you the original question never mentioned if the engines were running, if they weren't the plane would not fly.
I didn't post this as a test, but I think Chris was the first with an explanation, sorry Chris, no prize but you have the kudos of being first, there were others too though.
Yay. Winner winner chicken dinner.
Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
See, that's what I said. It was in my book of physics for Boy Scouts.
Well, because an airplane’s wheels are free rolling. There is obviously some friction, so there would be some small backwards force, but it would be infinitely small as compared to the forward thrust of the airplane.
See, that's what I said. It was in my book of physics for Boy Scouts.
Now I was going to use an aircraft carrier as an example Chas. I discounted it because of the steam (yes) catapult which is an additional thrust, necessary because of the short runway, and as such adds another dimension to the conundrum.
Take away the catapult and assume planes can get going without and it’s the perfect analogy.
Take away the catapult and assume planes can get going without and it’s the perfect analogy.
