From the original post "You have a jet plane sitting on a runway which happens to be a conveyor belt, the wheels of the jet plane has encoders that relay wheel speed to the conveyor runway which instantaneously matches the speed but in the reverse direction to the way the jet plane is facing, "
To me this implies the condition that for a given rotation of the wheel resulting in a forward motion of dX, the runway will match that by moving -dX.
If we assume that the wheel cannot skid then no matter what justification is used to say that the plane takes of requires the condition to be broken.
C.W.
Not necessarily if there is no relationship between the runway and the plane itself due to the break of any relationship occuring at the bearings
Not necessarily if there is no relationship between the runway and the plane itself due to the break of any relationship occuring at the bearings
If there is a break of any relationship between the plane and the bearings then the plane is sitting on it's landing gear and is very unlikely to go anywhere.
The question has too many variables to simply ask will it take off.
As already seen it can be made as complex as you like with many variables thrown in but it's really simplistic if you know that the plane will take off as has already been stated due to the runway having absolutely no impact on the plane apart from spinning it's wheels
No, do some searching on the web regarding moment of inertia, wheel, axle, etc.
C.W.
I would be wasting my time simply because the axle and wheel are two different scenarios you cannot say that one influences the other the analogy of the ski on water applies here (the water is still and there is no frictional contact apart from the ability to float (don't let's get into molecular physics otherwise i'm out lol), If there was friction in the bearings then yes I would agree
C.W. you are correct. Because of the moment of inertia it is possible to apply a force to the plane that would counter the thrust of the engines. That's something I hadn't considered. The runway would have to continuously accelerate to produce a continuous force. For a fixed engine thrust the runway speed would increase at a constant rate to keep the plane in a fixed position. Of course this means that the runway speed would become unrealistically high in a very short amount of time. So in theory, it would be possible to keep the plane from taking off.
As already seen it can be made as complex as you like with many variables thrown in but it's really simplistic if you know that the plane will take off as has already been stated due to the runway having absolutely no impact on the plane apart from spinning it's wheels
Yeah but all that depends on the friction between the aircraft and the runway, which at this point seems like the variable. If it were clear I think everyone would lean one way.
If you imagine a person running in place on a treadmill their equal and opposite force acts upon their feet and the treadmill.
With a jet the power source is not coupled to the drag of the treadmill, and with this question in particular the drag cannot be measured. If there is no or little drag then we might as well assume there is no treadmill. If there is a lot of drag then we'd have to assume the jet was attached to the treadmill in which case why would it even try to take off?
I would be wasting my time simply because the axle and wheel are two different scenarios you cannot say that one influences the other the analogy of the ski on water applies here (the water is still and there is no frictional contact apart from the ability to float (don't let's get into molecular physics otherwise i'm out lol), If there was friction in the bearings then yes I would agree
Friction has nothing to do with it, however, like I said in #151, it doesn't matter. The only way the plane takes off is if the condition of runway speed matching wheel speed is violated, unless the wheels don't stay attached to the landing gear. Any foward motion of the plane means the wheel rotated faster than accounted for by the opposing motion of the runway, and the problem statement says this speed is instantly matched, so forward motion isn't possible. It's basically a trick question unless you allow the wheel to skid.
. The only way the plane takes off is if the condition of runway speed matching wheel speed is violated
But the plane can take off even if not violated as there is no relationship you could disregard the runway and wheels even existing bar then friction between the axle and ground may cause problems to take off
But the plane can take off even if not violated as there is no relationship you could disregard the runway and wheels even existing bar then friction between the axle and ground may cause problems to take off
If you tell me weight and balance info, fuel on board, indicated airspeed (at the point of rotation) the type of jet and length of runway I'll tell you if it can take off.
If you say I have a jet and a treadmill and some encoders with some crazy Propeller controlled treadmill and a hypothetical situation you'll get mixed results.
Is the jet getting any relative airspeed? Let's say it's loaded properly and the pilot is in it. It's on the treadmill and whatever mechanisms are in place. What does the airspeed indicator read? Is there forward relative airspeed?
Also the faster the treamill goes the more parasidic drag will create moving air around the wings and that throws the whole thing off even further.
Also, thanks because I'm going to be up all night thinking about this lol
@xanadu would it suffice to say that the jet plane is a standard one that flys in normal circumstances?
shall we say the runway has sufficient length if it was static to allow for that jet to take off.
If you say I have a jet and a treadmill and some encoders with some crazy Propeller controlled treadmill and a hypothetical situation you'll get mixed results
Hey there is nothing crazy about the prop! take that comment back immediately
You are so close to the real problem with the scenario but it takes time for others to see that as well
The aircraft is a red herring. Restate the problem as a wheel sitting on treadmill. There is a sensor that detects rotation speed of the wheel and drives the treadmill at a speed that matches the surface speed of the wheel. Can the wheel move forward?
The aircraft is a red herring. Restate the problem as a wheel sitting on treadmill. There is a sensor that detects rotation speed of the wheel and drives the treadmill at a speed that matches the surface speed of the wheel. Can the wheel move forward?
C.W.
No the runway and wheel are the red herrings in this situation, the plane has to exist as otherwise how would it take off?
And with that I must call it time for tonight as I have to be up early, I will continue with the debate tomorrow, that's if erco hasn't hacked it into oblivion.
He's not a moderator is he?
@xanadu would it suffice to say that the jet plane is a standard one that flys in normal circumstances?
shall we say the runway has sufficient length if it was static to allow for that jet to take off.
Hey there is nothing crazy about the prop! take that comment back immediately
Okay, I take it back. Also I enjoyed reading this and probably have the least engineering experience here, so thanks for listening
Sorry I'm too mixed up on the details to give a real answer. It got me thinking about a lot of airframe stuff, and I wouldn't expect any less of a conversation on this forum....
My answer is - Yes the jet will take off, until someone shows me a significantly large enough treadmill to counter some of the finest engineering we have available today, and the jet owner is cool with testing it all out. Also the FAA can't know.
Geez... In a practical sense, the wheels and treadmill are irrelevant. Unless the wheels are locked, the jet is going to move forward and take off.
The speed of the wheels does not affect the the thrust of the engines.
I'm going to recant my position about the rotational inertia. I think ctwardell is right. As long as the wheels have mass and the belt is accelerating, there will be a force on the axle in the direction of the belt's acceleration. From a practical standpoint, the force on the axle would be insignificant compared to the thrust of the engines, but it's not zero.
If the plane moves forward the wheels HAVE to be going faster than the belt and as such the premise of the problem is violated. Since the thrust for moving the wheel forward does not come from the contact with the ground it WILL move forward so the problem is flawed.
After many hours of calculations, I have determined that the air craft will NOT take off under the following conditions:
1. It is on the moon.
2. It was trying to take off from planet earth before the evolution of photosynthesis.
3. It is on planet Mercury.
4. Its warranty expired one hour ago.
5. The British are shooting frozen chickens at it instead of raw chickens (again).
Comments
No friction needed, it's a moment of inertia thing. If no force is applied to the axle the wheel would translate instead of rotate.
C.W.
The question has too many variables to simply ask will it take off.
If there is a break of any relationship between the plane and the bearings then the plane is sitting on it's landing gear and is very unlikely to go anywhere.
C.W.
No, do some searching on the web regarding moment of inertia, wheel, axle, etc.
C.W.
Yeah but all that depends on the friction between the aircraft and the runway, which at this point seems like the variable. If it were clear I think everyone would lean one way.
If you imagine a person running in place on a treadmill their equal and opposite force acts upon their feet and the treadmill.
With a jet the power source is not coupled to the drag of the treadmill, and with this question in particular the drag cannot be measured. If there is no or little drag then we might as well assume there is no treadmill. If there is a lot of drag then we'd have to assume the jet was attached to the treadmill in which case why would it even try to take off?
Friction has nothing to do with it, however, like I said in #151, it doesn't matter. The only way the plane takes off is if the condition of runway speed matching wheel speed is violated, unless the wheels don't stay attached to the landing gear. Any foward motion of the plane means the wheel rotated faster than accounted for by the opposing motion of the runway, and the problem statement says this speed is instantly matched, so forward motion isn't possible. It's basically a trick question unless you allow the wheel to skid.
C.W.
If you thought the jet wouldn't take off, that it is because you assumed the treadmill was keeping it in place.
If you thought the jet would take off, that is because you assumed the treadmill wouldn't hold it in place.
The problem is, hypothetically speaking of course, both of these situations can exist.
So the real question should be, can a jet takeoff from a standstill? The answer is it depends on the jet.
If you tell me weight and balance info, fuel on board, indicated airspeed (at the point of rotation) the type of jet and length of runway I'll tell you if it can take off.
If you say I have a jet and a treadmill and some encoders with some crazy Propeller controlled treadmill and a hypothetical situation you'll get mixed results.
Is the jet getting any relative airspeed? Let's say it's loaded properly and the pilot is in it. It's on the treadmill and whatever mechanisms are in place. What does the airspeed indicator read? Is there forward relative airspeed?
Also the faster the treamill goes the more parasidic drag will create moving air around the wings and that throws the whole thing off even further.
Also, thanks because I'm going to be up all night thinking about this lol
shall we say the runway has sufficient length if it was static to allow for that jet to take off.
Hey there is nothing crazy about the prop! take that comment back immediately
You are so close to the real problem with the scenario but it takes time for others to see that as well
C.W.
And with that I must call it time for tonight as I have to be up early, I will continue with the debate tomorrow, that's if erco hasn't hacked it into oblivion.
He's not a moderator is he?
Okay, I take it back. Also I enjoyed reading this and probably have the least engineering experience here, so thanks for listening
Sorry I'm too mixed up on the details to give a real answer. It got me thinking about a lot of airframe stuff, and I wouldn't expect any less of a conversation on this forum....
My answer is - Yes the jet will take off, until someone shows me a significantly large enough treadmill to counter some of the finest engineering we have available today, and the jet owner is cool with testing it all out. Also the FAA can't know.
The speed of the wheels does not affect the the thrust of the engines.
@
-Phil
1. It is on the moon.
2. It was trying to take off from planet earth before the evolution of photosynthesis.
3. It is on planet Mercury.
4. Its warranty expired one hour ago.
5. The British are shooting frozen chickens at it instead of raw chickens (again).