RE: Can It Take Off??  
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RE: Can It Take Off?? - 1/12/2006 4:59:57 PM   
CHassan



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Status: offline 		Does the speeds match round per round, or inch per inch.

Does the earth spin faster at the equator or tropic of capricorn?



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RE: Can It Take Off?? - 1/12/2006 5:09:41 PM   
PowerPlay


 

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The question begins with 'Imagine" not " With in the laws of physics". You are correct that the wheels will be spinning faster than the belt if the plane moves forward and the belt is traveling in the opposite direction of the plane. The belt could never match the speed of the wheels on a plane as it could for a car. However, that is not the intent of the question.

If a car were placed on this runway belt it would not be able to move forward. The question is asking you to determine if a plane would be able to make forward progress and take off on the same runway. That's it , nothing more.


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RE: Can It Take Off?? - 1/12/2006 5:13:31 PM   
CHassan



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From: Beavercreek, OH, USA
Status: offline 		But how are we matching the speed of the wheels? Rotational or linear?

< Message edited by CHassan -- 1/12/2006 5:14:19 PM >

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RE: Can It Take Off?? - 1/12/2006 5:53:58 PM   
Tim Wiltse-RCU



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Status: offline 		Nope.. the plane will never leave the ground. With the belt going back equal to the speed of the wheels going forward due to the thrust of the plane all that it will do is sit in one spot with a forward speed of ZERO.

Think of a JetSki out on the lake. It uses a jet to move around. Maybe it is easier to see this in our heads being we can "see" water. On a flat calm lake running at idle speed the impeller is making thrust and the ski moves forward. More thrust the faster you go. Now take the JetSki out to a fast moving river. With the same idle speed you can't move forward but rather you are going backwards. You have to increase thrust to move forward, the thrust must be greater than the water speed around it. If the river was able to match the speed of the jetski's thrust the jetski would again just sit in one spot.

Also with the plane and the belt. If it was able to put it's breaks on and stop the wheels all that would happen would be the belt would stop too. The only way the plane would fly is if the wheels turned faster than the speed of the belt moving backwards. Without the wheels overcoming the speed of the belt the plane will never see any forward motion PERIOD.

Also another way to think of it is this. The only way a plane can take off in a windless day is by moving forward to gain airspeed. Even with the breaks on with enough power some plane can skid down the runway gaining airspeed and take off. Let's assume this plane can't drag itself down the runway with the breaks on. Put the breaks on, add full power but now start moving the runway backwards under the plane, where is it going to go...backwards right. Say the runway(belt) is moving the plane backwards at 30 mph. Pop the breaks and adjust the power to what would equal 30mph of forward motion on a standing still runway where is the plane going now...nowhere because it is just equal to the ground speed.

Bottom line the plane never leaves the ground!

LAter,
Tim

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RE: Can It Take Off?? - 1/12/2006 5:59:18 PM   
jamieduff1981


 

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Status: offline 		But Tim your analogy of a jetski in water is no different to the car analogy. Both are wrong.

Both jetski and car impart their thrust upon the medium beneath them (ie the belt OR the moving water)

The plane imparts it's thrust upon the atmosphere which is not connected to the belt. Yes there will be some dragging due to skin friction on the belt but this can be safely ignored as required belt speeds would need to be very large to impart a measureable airflow over the plane.

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RE: Can It Take Off?? - 1/12/2006 6:15:47 PM   
dick Hanson



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RE: Can It Take Off?? - 1/12/2006 6:42:31 PM   
mr_matt



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RE: Can It Take Off?? - 1/12/2006 6:43:20 PM   
Liberator


 

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Status: offline 		Hmm let's see......yup...your wrong.


Plane flies. Always will.



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RE: Can It Take Off?? - 1/12/2006 6:46:16 PM   
Rat1



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quote:

ORIGINAL: CHassan

SO one is rotating clockwise the other counter clockwise.


Do you still think 1= (-1)?




Why is this important???? No 1 is not equal to -1 as 1 is a greater number then -1.

Can 1-1= a number greater then 0???? For the aircraft to make forward movement on a conveyor that is spinning at a velocity in a counter clockwise direction its wheels would have to be free rolling at a velocity greater then 1 in a clockwise direction. Once this happens the the conveyor has failed to do what it was designed to do and that was to exactally match the wheel speed at all times.

As I said forward movement/Airspeed is =C in A-B=C

A is your wheelspeed in terms of velocity
B is your conveyor speed in terms of velocity
C is your airspeed/forward momentum/run out speed in terms of velocity

A-B=C





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RE: Can It Take Off?? - 1/12/2006 6:48:39 PM   
Rat1



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quote:

ORIGINAL: Liberator

Hmm let's see......yup...your wrong.


Plane flies. Always will.




Not in your description it won't. You say the conveyor is moving the aircraft forward and the wheels are doing nothing. This breaks the rules of the question in which the conveyor speed and wheel speed must match exactally.





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RE: Can It Take Off?? - 1/12/2006 7:00:59 PM   
mesae



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quote:

ORIGINAL: mr_matt

dick are your posts always this informative?



I'm with Dick on this one...

No dog hung up.

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RE: Can It Take Off?? - 1/12/2006 7:02:19 PM   
Tim Wiltse-RCU



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Status: offline 		Nope...I'm right ! The plane NEVER has any forward motion!!!! HELLO! It's forward airspeed is equal to the ground speed. Gees.

Liberator,

Tell me how the plane ever gets any forward motion? Remember the belt is moving backwards at the the same speed the plane is rolling forward. Who cares how much thrust there is. The fact that the wheels aren't the driving force makes no difference here.The only way a real plane moves forward on a runway is because the runway is not moving under it. Without a headwind you need the groundspeed to get airborn. You can put a Merlin on a tricycle but as long as the reverse belt speed is equal to the forward motion you will always be at a standstill as you have ZERO ground speed. You only have a screaming prop and a moving belt...no more no less. Heck we did this in high school with electric motors and prop rods.

LAter,
Tim

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RE: Can It Take Off?? - 1/12/2006 7:13:12 PM   
CHassan



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quote:

ORIGINAL: Rat1


quote:

ORIGINAL: CHassan

SO one is rotating clockwise the other counter clockwise.


Do you still think 1= (-1)?




Why is this important???? No 1 is not equal to -1 as 1 is a greater number then -1.

Can 1-1= a number greater then 0???? For the aircraft to make forward movement on a conveyor that is spinning at a velocity in a counter clockwise direction its wheels would have to be free rolling at a velocity greater then 1 in a clockwise direction. Once this happens the the conveyor has failed to do what it was designed to do and that was to exactally match the wheel speed at all times.

As I said forward movement/Airspeed is =C in A-B=C

A is your wheelspeed in terms of velocity
B is your conveyor speed in terms of velocity
C is your airspeed/forward momentum/run out speed in terms of velocity

A-B=C





If 1 does not = -1 then you broke the rules of the question by using these measurements.





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RE: Can It Take Off?? - 1/12/2006 7:21:45 PM   
mesae



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quote:

ORIGINAL: Tim Wiltse-RCU

Nope...I'm right ! The plane NEVER has any forward motion!!!! HELLO! It's forward airspeed is equal to the ground speed. Gees.

Liberator,

Tell me how the plane ever gets any forward motion? Remember the belt is moving backwards at the the same speed the plane is rolling forward. Who cares how much thrust there is. The fact that the wheels aren't the driving force makes no difference here.The only way a real plane moves forward on a runway is because the runway is not moving under it. Without a headwind you need the groundspeed to get airborn. You can put a Merlin on a tricycle but as long as the reverse belt speed is equal to the forward motion you will always be at a standstill as you have ZERO ground speed. You only have a screaming prop and a moving belt...no more no less. Heck we did this in high school with electric motors and prop rods.

LAter,
Tim


Tell us what force is holding the airplane stationary with respect to the earth, against thrust.

Unless you are invoking the arbitrary "The plane can't move because the problem (supposedly) says it can't", your argument shows a fundamental and hopefully momentary misunderstanding of how an airplane moves. The propeller moves air back. The reaction is thrust. Without some force to oppose the thrust (Newton's Third Law), the airplane will move forward. The movement or non-movement of the belt is nearly irrelevant.

Have you ever taken off westbound? The surface of the earth is moving at a tangent velocity at the equator of about 1000 mph from west to east (angular velocity 15 deg/hr). According to your argument, this shouldn't be possible. Everybody knows airplanes can't fly backward, right? You must know this argument is flawed, so how come you insist that the belt will prevent takeoff?

I can't believe I let myself get suckered into writing again.

< Message edited by mesae -- 1/12/2006 7:29:46 PM >

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RE: Can It Take Off?? - 1/12/2006 7:23:03 PM   
CHassan



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quote:

ORIGINAL: Tim Wiltse-RCU

Also another way to think of it is this. The only way a plane can take off in a windless day is by moving forward to gain airspeed. Even with the breaks on with enough power some plane can skid down the runway gaining airspeed and take off. Let's assume this plane can't drag itself down the runway with the breaks on. Put the breaks on, add full power but now start moving the runway backwards under the plane, where is it going to go...backwards right. Say the runway(belt) is moving the plane backwards at 30 mph. Pop the breaks and adjust the power to what would equal 30mph of forward motion on a standing still runway where is the plane going now...nowhere because it is just equal to the ground speed.

Bottom line the plane never leaves the ground!

LAter,
Tim



I can't believe any pilot would claim this.

Aparently you measure the speed of your plane in ground speed? What do you do when the wind picks up?







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The knack of flying is learning how to throw yourself at the ground and miss.
--Hitchhikers Guide to the Galaxy--

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RE: Can It Take Off?? - 1/12/2006 7:29:01 PM   
PowerPlay


 

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Status: offline 		The best way to illustrate what is (tricky) and wrong about the the question concerning the plane is to turn it around with the car example.

Could a car reach the other end of the runway if the belt matched the speed of the car in the opposite direction.? And the answer is---------- that the question doesn't make any sense. Because as soon as the car accelerated the belt would have to move faster than the car to keep it stationary , so it could never match the speed of the car.

Under the constraints of the question. The belt could match the speed (MPH) of the airplane and either prevent the planes wheels from spinning at all or cause them to spin twice as fast, depending on which way the belt was spinning, But, the belt could not match the RPMs or how fast the wheels are spinning if the plane were moving forward under power.

The belt could prevent the car from moving at all or make it travel twice as fast but it could not prevent the wheels from spinning. The belt could match the RPMs or the MPH that the wheels are turning but it could not match the MPH of the car if the car was moving forward under its own power.

If the original question stated that the belt would match the speed of the airplane (instead of the wheels) in the opposite direction I don't think there would be much debate that the plane would take off or not.

< Message edited by PowerPlay -- 1/12/2006 7:30:19 PM >

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