txdon
Super Star Member
"you will move forward no matter how fast the treadmill goes" MossRoad
So, the answer is YES, IF the treadmill is long enough?
So, the answer is YES, IF the treadmill is long enough?
will it take off?
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LeadPoison
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WVBill said:
Thanks chief. I thought that example would clarify things.
Tom_Veatch
Platinum Member
This is a very simple dynamics problem. Draw a free-body or force diagram of the airplane and you will see that the only horizontal force applied to the airplane by the conveyor is through the almost freely spinning wheels. That force resolves itself into a force applied to the a/c via the rolling resistance of the wheels. The only other horizontal force (neglecting aerodynamic drag) felt by the airplane is thrust from the engine.
Now, the engine thrust must greatly exceed the rolling resistance of the wheels or the airplane couldn't move even if the conveyor were dead still. Since the engine thrust pulling the airplane is much greater than the force of the wheels retarding the airplane, the airplane must accelerate in response to those unbalanced forces. Sir Isaac Newton taught us that back in the 1600's. F=MA where F is the unbalanced forces acting on the object, M is the Mass (or in very loose terms, the weight) of the object, and A is the acceleration of the object.
Don't confuse displacement with force. The conveyor can be "displacing" at a phenomenal rate, but the only force it can exert on the airplane is through those freely spinning wheels. It is FORCE which accelerates and moves unrestrained objects.
Said it before, and I'll say it again. The airplane doesn't care one iota what that conveyor is doing, anymore than if the airplane were flying with the wheels .001 inch above the conveyor.
If the engine is producing, say, 1000 lb of thrust, the airplane would act just like a stationary winch was attached to the airplane and pulling with a constant 1000 lb force. That 1000 lb force, reduced by the drag of the wheels, will accelerate the airplane forward regardless of what the conveyor is doing. Neither the a/c engine nor that imaginary winch cares about, or is even aware, of the existence of the conveyor, they just continue to apply that 1000 lbs of motivating force - and that a/c motivates!
But, since the wheels spin as if the a/c is moving at twice its actual speed (relative to a stationary observer) and a/c tires are rated for a specific maximum speed (RPM) you may ruin your tires or wear out the wheel bearings, but you will fly the a/c- if the tires/wheels last long enough.
I was an aeronautical engineer in the aviation industry for going on 40 years (MSAE), and a rated a/c pilot (Private, ASEL, Instrument) longer than that. I can categorically state that this is one of those logic problems in which your intuition leads you to a dead wrong conclusion. The key is to identify the forces acting on the a/c. Everything else is totally irrelevant.
Now, the engine thrust must greatly exceed the rolling resistance of the wheels or the airplane couldn't move even if the conveyor were dead still. Since the engine thrust pulling the airplane is much greater than the force of the wheels retarding the airplane, the airplane must accelerate in response to those unbalanced forces. Sir Isaac Newton taught us that back in the 1600's. F=MA where F is the unbalanced forces acting on the object, M is the Mass (or in very loose terms, the weight) of the object, and A is the acceleration of the object.
Don't confuse displacement with force. The conveyor can be "displacing" at a phenomenal rate, but the only force it can exert on the airplane is through those freely spinning wheels. It is FORCE which accelerates and moves unrestrained objects.
Said it before, and I'll say it again. The airplane doesn't care one iota what that conveyor is doing, anymore than if the airplane were flying with the wheels .001 inch above the conveyor.
If the engine is producing, say, 1000 lb of thrust, the airplane would act just like a stationary winch was attached to the airplane and pulling with a constant 1000 lb force. That 1000 lb force, reduced by the drag of the wheels, will accelerate the airplane forward regardless of what the conveyor is doing. Neither the a/c engine nor that imaginary winch cares about, or is even aware, of the existence of the conveyor, they just continue to apply that 1000 lbs of motivating force - and that a/c motivates!
But, since the wheels spin as if the a/c is moving at twice its actual speed (relative to a stationary observer) and a/c tires are rated for a specific maximum speed (RPM) you may ruin your tires or wear out the wheel bearings, but you will fly the a/c- if the tires/wheels last long enough.
I was an aeronautical engineer in the aviation industry for going on 40 years (MSAE), and a rated a/c pilot (Private, ASEL, Instrument) longer than that. I can categorically state that this is one of those logic problems in which your intuition leads you to a dead wrong conclusion. The key is to identify the forces acting on the a/c. Everything else is totally irrelevant.
Tom_Veatch
Platinum Member
MossRoad said:
Congratulations, MossRoad, you nailed it!
MossRoad
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Tom_Veatch said:Congratulations, MossRoad, you nailed it!
Now what about that bear in the woods question?
Tom_Veatch
Platinum Member
MossRoad said:Now what about that bear in the woods question?
I'm gonna have to pass on that one. We didn't study no bears in airplane school.
HTWT
Bronze Member
a plane is standing on a movable runway( something like a conveyor).as the plane moves the conveyor moves but in the opposite direction.the conveyor has a system that tracks the speed of the plane and matches it exactly in the opposite direction.
If a car is on the track will it move???
Yes it will. The track only moves when the car moves. The tires turnng doesnot constitute moving. The speedometer only tells how fast the tires are turning. If the speedometer says 50mph then the car is going 25mph and the convayer is going 25mph in the opposite direction. The convayer only moves if the car moves. If you spin the tires the convayer doesnot move. The car will move 1/2 the speed that the speedometer indicates. The tires will turn at twice the speed as the actual movement of the car. This also applies to a plane except a plane's speed is wind speed not ground speed.
If a car is on the track will it move???
Yes it will. The track only moves when the car moves. The tires turnng doesnot constitute moving. The speedometer only tells how fast the tires are turning. If the speedometer says 50mph then the car is going 25mph and the convayer is going 25mph in the opposite direction. The convayer only moves if the car moves. If you spin the tires the convayer doesnot move. The car will move 1/2 the speed that the speedometer indicates. The tires will turn at twice the speed as the actual movement of the car. This also applies to a plane except a plane's speed is wind speed not ground speed.
RobS
Super Member
rback33 said:
Same reason we make shipments by truck and transport cargo on a boat
BTDT
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BillyP said:
I thought all planes took off from a standstill!! Kind of like all cars, tractors, etc, take off from standstills.
RobS said:Same reason we make shipments by truck and transport cargo on a boat
About time.. I was wondering if ANYONE was gonna answer MY question...lol
TV>> I was guessing you you were in Aerospace, with your answers and being in Wichita.... I am just over an hour from you.... Your not a Friends supporter are you? Big game this weekend....