| Author | Thread |
|
|
12/06/2006 06:48:00 PM · #201 |
I did a bit of research and found this equation to determine how much lift given all the variables can be created.
L = lift
Cl = lift coefficient
(rho) = air density
V = air velocity
A = wing area
If ANY of the variables (L, Cl, RHO, V or A) = 0 (zero), lift would be impossible, since it would make the equation = zero. Since the plane isn't moving (stopped by the treadmill moving the opposite direction), it can not create air velocity (V). Thuis making the whole equation ZERO, making lift impossible.
Added: All this is assuming there's no wind that day ;)
Message edited by author 2006-12-06 18:52:37.
|
|
|
|
12/06/2006 06:53:12 PM · #202 |
Originally posted by MrEd:
I did a bit of research and found this equation to determine how much lift given all the variables can be created.
L = lift
Cl = lift coefficient
(rho) = air density
V = air velocity
A = wing area
If ANY of the variables (L, Cl, RHO, V or A) = 0 (zero), lift would be impossible, since it would make the equation = zero. Since the plane isn't moving (stopped by the treadmill moving the opposite direction), it can not create air velocity (V). Thuis making the whole equation ZERO, making lift impossible. |
That's all well and good, but that's not where the issue lies. Your assumption that the treadmill will stop the plane is wrong. |
|
|
|
12/06/2006 06:54:38 PM · #203 |
Like many others here, your assumption is that the plane's forward (through the air) motion will be stopped by the treadmill is incorrect. It will certainly not be.
If it *were* correct, your conclusion would then certainly be correct; no airspeed, no lift, no flight. Period.
Fact is, the treadmill does very little to impede the aircraft from gaining airspeed, which is the critical item necessary for flight. |
|
|
|
12/06/2006 06:59:30 PM · #204 |
Originally posted by legalbeagle:
A plane equipped with fixed horizontal engines and wheel landing gear is placed on a huge treadmill runway. The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction. |
Please read the inital question. It says the treadmill matches the speed of the airplane. Which, the way I read it, the plane never moves. Thus no air velocity (V) can be created.
Try running/jogging on a treadmill. The treadmill matches the speed your running/jogging at. Do you feel a breeze in your face? Same principle. Without forward motion, air velocity can not be created.
Message edited by author 2006-12-06 19:00:55.
|
|
|
|
12/06/2006 07:00:44 PM · #205 |
| Ye, it says the treadmill matches the speed of the airplane. It does *not* follow that the airplane does not move. Think about it. |
|
|
|
12/06/2006 07:04:04 PM · #206 |
Originally posted by MrEd:
Originally posted by legalbeagle:
A plane equipped with fixed horizontal engines and wheel landing gear is placed on a huge treadmill runway. The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction. |
Please read the inital question. It says the treadmill matches the speed of the airplane. Which, the way I read it, the plane never moves. Thus no air velocity (V) can be created.
Try running/jogging on a treadmill. The treadmill matches the speed your running/jogging at. Do you feel a breeze in your face? Same principle. Without forward motion, air velocity can not be created. |
Yes that's correct, but the reason you aren't moving forward is because you are applying a force to the treadmill. The plane is applying force to the air behind it, this just makes the wheels spin faster as the treadmill is going in the opposite direction. I'll see if I can make up a little diagram for those who don't understand what we're talking about...
|
|
|
|
12/06/2006 07:04:52 PM · #207 |
Originally posted by MrEd:
Originally posted by legalbeagle:
A plane equipped with fixed horizontal engines and wheel landing gear is placed on a huge treadmill runway. The treadmill has a clever design and always matches the speed of the plane, but runs in the opposite direction. |
Please read the inital question. It says the treadmill matches the speed of the airplane. Which, the way I read it, the plane never moves. Thus no air velocity (V) can be created.
Try running/jogging on a treadmill. The treadmill matches the speed your running/jogging at. Do you feel a breeze in your face? Same principle. Without forward motion, air velocity can not be created. |
Asked and answered.
Originally posted by Spazmo99:
Originally posted by jahoward:
Originally posted by Spazmo99:
Read the thread, learn why you're wrong. |
I'd be interested to read the whole thing. Maybe tonight, but all I can say is the plane will not fly without air speed. If it is sitting still, the only airspeed it has is a head wind. If the headwind is strong enough, the plane might fly. But maybe I'm misunderstanding the scenerio. It the plane on a treadmill much like when I'm on a treadmill? I'm running as fast as I can, but not going anywhere. |
When you run, your foot pushes on the ground to propel you forward. If you are on a treadmill, that same push only keeps you from going backward.
The plane does not push on the ground. It pushes the air behind the plane, which is not affected by the treadmill and generates thrust. The treadmill only serves to make the wheels on the plane turn faster, but does not retard the forward motion of the plane. |
|
|
|
|
12/06/2006 07:09:14 PM · #208 |
Originally posted by Spazmo99:
Originally posted by Spazmo99:
Originally posted by jahoward:
Originally posted by Spazmo99:
Read the thread, learn why you're wrong. |
I'd be interested to read the whole thing. Maybe tonight, but all I can say is the plane will not fly without air speed. If it is sitting still, the only airspeed it has is a head wind. If the headwind is strong enough, the plane might fly. But maybe I'm misunderstanding the scenerio. It the plane on a treadmill much like when I'm on a treadmill? I'm running as fast as I can, but not going anywhere. |
When you run, your foot pushes on the ground to propel you forward. If you are on a treadmill, that same push only keeps you from going backward.
The plane does not push on the ground. It pushes the air behind the plane, which is not affected by the treadmill and generates thrust. The treadmill only serves to make the wheels on the plane turn faster, but does not retard the forward motion of the plane. |
|
Whether there is pressure on the ground pushing the treadmill backwards OR a motor spinning the treadmill....either way, the plane does not move. I'm assuming 'thrust = treadmill speed' (or whatever the equation would be).
Added: Now I have a headache....
Message edited by author 2006-12-06 19:10:32.
|
|
|
|
12/06/2006 07:13:28 PM · #209 |
The trick is to remember that wheels spin freely with negligible drag. The thrust of the engine (prop or jet) on an aeroplane is sufficient to overcome the very significant drag caused by wind resistance, and will be barely affected by the additional drag caused by a treadmill, no matter how fast.
Drag is caused by the runway moving backwards on the wheels, but a negigible amount (wheels are designed to overcome friction).
Therefore, the plane will move forwards at, say, 10 mph. The runway will move at -10mph. But this will only cause the aeroplane to slow from 10mph very slightly. The plane will not be stationary (it is moving at +10mph) - but the difference between the speed of the plane and that of the treadmill will be 20mph.
Therefore, the plane moves frwards at 10mph, with a small amount of drag caused by the increased speed of the wheels (which rotate at 20 mph) and the treadmill operates at -10mph.
The plane can accelerate to 600mph. Again, the treadmill is at -600mph, the wheels at 1200mph. However, the airspeed of the vehicle is still 600mph, so it will take off normally.
The question is not vague, but it is misleading - the treadmill would have little effect on an aeroplane (apart from possibly exceeding the maximum capacity of the wheels/tyres). An aeroplane's lift is generated through its relationship to the surrounding air, not the ground moving beneath it (whether 60k feet below or 10 feet and guided by some wheels).
If you are still having difficulty, imagine an aeroplane flying at 600 mph 10 feet above the treadmill (at -600mph). Then imagine it lowering its undercarriage - the wheels would touch down and immediately accelerate to the difference between the treadmill speed and the airspeed (1200mph), but would not cause the plane to stop altogether (ie lose 600 mph of forwards momentum). The plane could continue to travel at 600mph, even though the treadmill (by its opposite force) was accelerating its wheels to 1200 mph.
|
|
|
|
12/06/2006 07:26:55 PM · #210 |
Ok, here we go:
As you can see, even though the plane and the runway are moving in opposite directions, the wheels are still rolling forward. Now because the plane is not applying any kind of torque to the runway, it is not giving any kind of resistance other than the minimal (in comparison) rolling friction. So (this is the part that took me a while to get) even though the runway is moving backwards, it doesn't mean the wheels are turning backwards. They are just turning twice as fast as they should be. So as long as the tires don't blow out, it'll keep moving forward since the forward speed of the airplane is not in any way connected to the ground (the wheels spin freely). Make sense?
Message edited by author 2006-12-06 19:31:00.
|
|
|
|
12/06/2006 07:34:35 PM · #211 |
NB the answer is pretty much the same whichever direction the treadmill is running - a runway travelling at 600mph in the direction of take-off is never going to impart enough energy to make planes take off like gliders - not enough energy would be imparted through the wheels (assuming no brakes!).
|
|
|
|
12/06/2006 07:40:51 PM · #212 |
|
|
|
12/06/2006 07:52:37 PM · #213 |
Unfortunately, this would require the aircraft already be airborne (i.e. - would have overcome gravity, created lift, and BE IN FLIGHT).
The original question says [implies?] a stationary beginning, and THAT is the key. In order to create an uplifting force on the wings, the stated horizontally mounted engines would have to create foward motion. Remember that every action will have an equal and opposite reaction - that is where the energy from the engines is being transferred to the wheels, and the aircraft will NOT move forward (and therefore never generate lift beneath the wings).
Think of this example... have you seen the insurance commercials about cavemen (Geico, I think)? The most recent one features a man travelling forward on a moving sidewalk in an airport. When he travels in the same direction as the sidewalk, he moves forward at a combined speed of his pace and that of the moving sidewalk. However, when he changes direction & goes back to the poster on the wall, he matches the speed of the sidewalk (in the opposite direction) AND REMAINS STATIONARY.
The entire problem is TOTALLY dependent on being ground based, as that is where the energy is expended and equalized to zero forward motion. If one were to suspend the same aircraft from a giant string it would naturally fly around on the axis of the string's anchorpoint.
Essentially, without forward movement the aircraft will never achieve takeoff. Of course, a couple more beers and I really won't give a shit either way. (Anybody care to share a toast to my first ever paying customer?)
Next contestant! ;)
Originally posted by legalbeagle:
The trick is to remember that wheels spin freely with negligible drag. The thrust of the engine (prop or jet) on an aeroplane is sufficient to overcome the very significant drag caused by wind resistance, and will be barely affected by the additional drag caused by a treadmill, no matter how fast.
Drag is caused by the runway moving backwards on the wheels, but a negigible amount (wheels are designed to overcome friction).
Therefore, the plane will move forwards at, say, 10 mph. The runway will move at -10mph. But this will only cause the aeroplane to slow from 10mph very slightly. The plane will not be stationary (it is moving at +10mph) - but the difference between the speed of the plane and that of the treadmill will be 20mph.
Therefore, the plane moves frwards at 10mph, with a small amount of drag caused by the increased speed of the wheels (which rotate at 20 mph) and the treadmill operates at -10mph.
The plane can accelerate to 600mph. Again, the treadmill is at -600mph, the wheels at 1200mph. However, the airspeed of the vehicle is still 600mph, so it will take off normally.
The question is not vague, but it is misleading - the treadmill would have little effect on an aeroplane (apart from possibly exceeding the maximum capacity of the wheels/tyres). An aeroplane's lift is generated through its relationship to the surrounding air, not the ground moving beneath it (whether 60k feet below or 10 feet and guided by some wheels).
If you are still having difficulty, imagine an aeroplane flying at 600 mph 10 feet above the treadmill (at -600mph). Then imagine it lowering its undercarriage - the wheels would touch down and immediately accelerate to the difference between the treadmill speed and the airspeed (1200mph), but would not cause the plane to stop altogether (ie lose 600 mph of forwards momentum). The plane could continue to travel at 600mph, even though the treadmill (by its opposite force) was accelerating its wheels to 1200 mph. |
|
|
|
|
12/06/2006 08:05:23 PM · #214 |
Originally posted by rossbilly:
Remember that every action will have an equal and opposite reaction - that is where the energy from the engines is being transferred to the wheels, and the aircraft will NOT move forward (and therefore never generate lift beneath the wings). |
You are right till right about there. Remember that the energy from the engines is in no way connected to the wheels. The thrust of the engine is applied to the air behind it, not the wheels. The jet engine does not apply any sort of torque (or any force for that matter) to the wheels. So with that, the wheels are pretty much spinning freely (minus the friction from the bearings, rolling friction, etc...). Therefore, the wheels would just spin twice as fast in the forward direction (refer to my diagram, even though it's apparently too contrasty, in my previous post) and the plane would move forward.
|
|
|
|
12/06/2006 08:05:25 PM · #215 |
Originally posted by legalbeagle:
The trick is to remember that wheels spin freely with negligible drag..... |
I agree if you see my reply earlier but just to start this again :-) - I now suggest that the centrifugal forces that would result on the wheel bearings (even a small plane needs what 70knots of air speed to life off and a large plane requires more, so the ground speed would get fast VERY quickly) would blow the wheels off resulting in one of the best crashes of all time :-) |
|
|
|
12/06/2006 08:08:19 PM · #216 |
Originally posted by rossbilly:
Unfortunately, this would require the aircraft already be airborne (i.e. - would have overcome gravity, created lift, and BE IN FLIGHT). |
Not really - the only important relationship in a plane taking off or maintaining altitude is airspeed, whether 10 feet from the ground on wheels or 60k feet. The groundspeed is irrelevant.
A plane's wheels are relevant only in determining groundspeed and drag. Drag will be minimal and groundspeed is irrelevant.
The fact that the groundspeed may be higher than normal, creating a small amount of additional drag, will barely affect the ability of a plane to accelerate/thrust to a high airspeed and take off. |
|
|
|
12/06/2006 08:09:53 PM · #217 |
Originally posted by rossbilly:
Unfortunately, this would require the aircraft already be airborne (i.e. - would have overcome gravity, created lift, and BE IN FLIGHT).
The original question says [implies?] a stationary beginning, and THAT is the key. In order to create an uplifting force on the wings, the stated horizontally mounted engines would have to create foward motion. Remember that every action will have an equal and opposite reaction - that is where the energy from the engines is being transferred to the wheels, and the aircraft will NOT move forward (and therefore never generate lift beneath the wings).
Think of this example... have you seen the insurance commercials about cavemen (Geico, I think)? The most recent one features a man travelling forward on a moving sidewalk in an airport. When he travels in the same direction as the sidewalk, he moves forward at a combined speed of his pace and that of the moving sidewalk. However, when he changes direction & goes back to the poster on the wall, he matches the speed of the sidewalk (in the opposite direction) AND REMAINS STATIONARY.
The entire problem is TOTALLY dependent on being ground based, as that is where the energy is expended and equalized to zero forward motion. If one were to suspend the same aircraft from a giant string it would naturally fly around on the axis of the string's anchorpoint.
Essentially, without forward movement the aircraft will never achieve takeoff. Of course, a couple more beers and I really won't give a shit either way. (Anybody care to share a toast to my first ever paying customer?)
Next contestant! ;)
Originally posted by legalbeagle:
The trick is to remember that wheels spin freely with negligible drag. The thrust of the engine (prop or jet) on an aeroplane is sufficient to overcome the very significant drag caused by wind resistance, and will be barely affected by the additional drag caused by a treadmill, no matter how fast.
Drag is caused by the runway moving backwards on the wheels, but a negigible amount (wheels are designed to overcome friction).
Therefore, the plane will move forwards at, say, 10 mph. The runway will move at -10mph. But this will only cause the aeroplane to slow from 10mph very slightly. The plane will not be stationary (it is moving at +10mph) - but the difference between the speed of the plane and that of the treadmill will be 20mph.
Therefore, the plane moves frwards at 10mph, with a small amount of drag caused by the increased speed of the wheels (which rotate at 20 mph) and the treadmill operates at -10mph.
The plane can accelerate to 600mph. Again, the treadmill is at -600mph, the wheels at 1200mph. However, the airspeed of the vehicle is still 600mph, so it will take off normally.
The question is not vague, but it is misleading - the treadmill would have little effect on an aeroplane (apart from possibly exceeding the maximum capacity of the wheels/tyres). An aeroplane's lift is generated through its relationship to the surrounding air, not the ground moving beneath it (whether 60k feet below or 10 feet and guided by some wheels).
If you are still having difficulty, imagine an aeroplane flying at 600 mph 10 feet above the treadmill (at -600mph). Then imagine it lowering its undercarriage - the wheels would touch down and immediately accelerate to the difference between the treadmill speed and the airspeed (1200mph), but would not cause the plane to stop altogether (ie lose 600 mph of forwards momentum). The plane could continue to travel at 600mph, even though the treadmill (by its opposite force) was accelerating its wheels to 1200 mph. |
|
Again, the analogy between walking or running on a treadmill and this airplane on a treadmill is incorrect.
|
|
|
|
12/06/2006 08:15:10 PM · #218 |
Originally posted by robs:
I now suggest that the centrifugal forces that would result on the wheel bearings (even a small plane needs what 70knots of air speed to life off and a large plane requires more, so the ground speed would get fast VERY quickly) would blow the wheels off resulting in one of the best crashes of all time :-) |
Possibly very true! However, I have always been impressed with the technology that goes into aircraft wheels, ever since they placed a 747 undercarriage strut in the entrance hall to the science museum in London. It is amazing that the tyre can cope with the stresses generated by accelerating from 0 to 600mph while bearing hundreds of tonnes of force in the instant of touchdown.
Message edited by author 2006-12-06 20:16:46. |
|
|
|
12/06/2006 08:19:56 PM · #219 |
Originally posted by SamDoe1:
Originally posted by rossbilly:
Remember that every action will have an equal and opposite reaction - that is where the energy from the engines is being transferred to the wheels, and the aircraft will NOT move forward (and therefore never generate lift beneath the wings). |
You are right till right about there. Remember that the energy from the engines is in no way connected to the wheels. The thrust of the engine is applied to the air behind it, not the wheels. The jet engine does not apply any sort of torque (or any force for that matter) to the wheels. So with that, the wheels are pretty much spinning freely (minus the friction from the bearings, rolling friction, etc...). Therefore, the wheels would just spin twice as fast in the forward direction (refer to my diagram, even though it's apparently too contrasty, in my previous post) and the plane would move forward. |
I understand what you are saying, and agree that there is no sort of gearing/linkage/driveline connecting the engine & wheels. However, the entire point is that ground speed MUST increase in order to create the intial lift beneath the wings.
Please try to imagine the moving sidewalk example, and realize that forward movement of the aircraft is the ONLY thing that will allow for takeoff... and this scenario makes forward movement impossible.
Thank you all for discussing this in an intelligent manner (rather than the typical name-calling found on some other boards) - I'm really enjoying this!!! :) |
|
|
|
12/06/2006 08:27:11 PM · #220 |
Originally posted by rossbilly:
Originally posted by SamDoe1:
Originally posted by rossbilly:
Remember that every action will have an equal and opposite reaction - that is where the energy from the engines is being transferred to the wheels, and the aircraft will NOT move forward (and therefore never generate lift beneath the wings). |
You are right till right about there. Remember that the energy from the engines is in no way connected to the wheels. The thrust of the engine is applied to the air behind it, not the wheels. The jet engine does not apply any sort of torque (or any force for that matter) to the wheels. So with that, the wheels are pretty much spinning freely (minus the friction from the bearings, rolling friction, etc...). Therefore, the wheels would just spin twice as fast in the forward direction (refer to my diagram, even though it's apparently too contrasty, in my previous post) and the plane would move forward. |
I understand what you are saying, and agree that there is no sort of gearing/linkage/driveline connecting the engine & wheels. However, the entire point is that ground speed MUST increase in order to create the intial lift beneath the wings.
Please try to imagine the moving sidewalk example, and realize that forward movement of the aircraft is the ONLY thing that will allow for takeoff... and this scenario makes forward movement impossible.
Thank you all for discussing this in an intelligent manner (rather than the typical name-calling found on some other boards) - I'm really enjoying this!!! :) |
Groundspeed will increase, so will airspeed, the only effect the treadmill will have is to make the wheels turn twice as fast. It's irrelevant to the overall speed of the plane. |
|
|
|
12/06/2006 08:32:20 PM · #221 |
Originally posted by Spazmo99:
Originally posted by rossbilly:
I understand what you are saying, and agree that there is no sort of gearing/linkage/driveline connecting the engine & wheels. However, the entire point is that ground speed MUST increase in order to create the intial lift beneath the wings.
Please try to imagine the moving sidewalk example, and realize that forward movement of the aircraft is the ONLY thing that will allow for takeoff... and this scenario makes forward movement impossible.
Thank you all for discussing this in an intelligent manner (rather than the typical name-calling found on some other boards) - I'm really enjoying this!!! :) |
Groundspeed will increase, so will airspeed, the only effect the treadmill will have is to make the wheels turn twice as fast. It's irrelevant to the overall speed of the plane. |
Exactly, the speed of the plane will increase since the negative movement of the treadmill will be absorbed by the wheels. It won't be transferred to the plane. And no problem, I enjoy a conversation like this from time to time too. How about we discuss time travel next? :-P
|
|
|
|
12/06/2006 08:32:50 PM · #222 |
Another way to state this (since I'm not always great at explaining myself)...
The engine creates a force in one direction, the treadmill (and the aircraft's wheels) absorb that force in the OPPOSITE direction. The wheel spins around the bearings, and forward motion is ZERO.
No matter what, the aircraft has to begin moving forward BEFORE it can ever achieve takeoff. Without that forward movement, there will NEVER be an upward movement!
I'm afraid some of you are trying to make this more complicated than it is. Someone waaaaay back correctly stated that the plane has to move forward through the air, or the wind has to blow reeeaaalllly hard LOL.
Message edited by author 2006-12-06 20:36:34. |
|
|
|
12/06/2006 08:33:44 PM · #223 |
Originally posted by SamDoe1:
How about we discuss time travel next? :-P |
Cool but I am with Hawking on this. |
|
|
|
12/06/2006 08:36:48 PM · #224 |
Originally posted by SamDoe1:
How about we discuss time travel next? :-P |
Anything except religion......
Added: I can't wait for someone to actually try this.
Message edited by author 2006-12-06 20:37:45.
|
|
|
|
12/06/2006 08:38:45 PM · #225 |
Originally posted by rossbilly:
Another way to state this (since I'm not always great at explaining myself)...
The engine creates a force in one direction, the treadmill (and the aircraft's wheels) create a force in the OPPOSITE direction. The wheel spins around the bearings, and forward motion is ZERO.
No matter what, the aircraft has to begin moving forward BEFORE it can ever achieve takeoff. Without that forward movement, there will NEVER be an upward movement!
I'm afraid some of you are trying to make this more complicated than it is. Someone waaaaay back correctly stated that the plane has to move forward through the air, or the wind has to blow reeeaaalllly hard LOL. |
Right, but there is nothing holding the plane back... The wheels are still spinning in the forward direction without applying any negative force to the plane itself. So if you open up the engines (as someone said before) the wheels just spin twice as fast as they would normally (since the runway is applying a negative speed and again, look at the diagram I posted before). I thought of this the way you are too for a while before it finally clicked.
|
|
