RE: Can It Take Off??  
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RE: Can It Take Off?? - 12/5/2005 4:26:04 AM   
David Cutler



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From: Los Angeles, CA, USA
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quote:

But, without forward movement, which the belt prevents,


Why does the belt prevent forward motion?What's to stop the plane from building up speed if there are no brakes on the wheels?

If you are saying the wheels have brakes then the whole story is different.

-David C.

(in reply to 3dbob37n)
       Post #: 76

RE: Can It Take Off?? - 12/5/2005 4:31:14 AM   
BMatthews



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From: Burnaby, BC, CANADA
Status: online 		Since this is a hypothetical question we can assume that the hypothetical parts will not fail from overspeed. If that is the case then the bit about the wheel speed not mattering is still valid. The belt can spin them backwards as fast as it wants to and the plane will still move forward thanks to the prop or jet thrust. The wheels still have nothing to do with the situation in this case. The wheels do not drive the airplane nor can they hold it back. The mechanism just isn't there for them to do so.

I'll also add a warning to keep things civil or the thread will be closed. I've deleted a couple of somewhat inflamatory posts. If any of you find that your posts are missing please take this as a warning.

This is intended to be a fun brain twister. Do not forget the fun part.


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RE: Can It Take Off?? - 12/5/2005 4:43:08 AM   
HighPlains


 

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Status: online 		The airplane will take off with a slightly longer rollout due the amount of energy the rotation of the wheels is storing.

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RE: Can It Take Off?? - 12/5/2005 8:27:23 AM   
Flyfalcons



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From: Bonney Lake, WA, USA
Status: offline 		The Avweb article is interesting but it only answers the question of if the belt will move relative to the airplane's speed, not the speed of the tires.

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RE: Can It Take Off?? - 12/5/2005 12:45:22 PM   
starwoes



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Joined: 10/6/2005
From: Maple Heights, OH, USA
Status: offline 		saying the belt makes a difference is like saying grabbing onto a falling tree trunk while going over a waterfall will somehow stop your downward fall. the airplane is pulling/pushing against the wind while the belt matches a spinning wheel which has been optimized to reduce friction between the aircraft and the contact surface....be it tarmac or conveyor belt....an obvious missmatch!

(in reply to Flyfalcons)
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RE: Can It Take Off?? - 12/5/2005 1:56:37 PM   
davidfee



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From: San Diego, CA, USA
Status: offline 		Break it down. The motion of the axle (airplane) is independent of the motion of the tire.

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RE: Can It Take Off?? - 12/5/2005 2:44:45 PM   
CHassan



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< Message edited by CHassan -- 12/5/2005 3:33:22 PM >

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RE: Can It Take Off?? - 12/5/2005 2:56:27 PM   
CHassan



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< Message edited by CHassan -- 12/5/2005 3:32:56 PM >

_____________________________

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?? - 12/5/2005 3:10:38 PM   
LarryC


 

Posts: 405
Joined: 10/9/2002
From: Brookland, AR, USA
Status: offline 		Although it's been answered, I'll chime in.

With a car or truck, rc or full size, the propulsion is applied via the wheels. Move the belt back at a speed matching the wheels and it's not going anywhere. Think about how they do dynomometer tests.

With an airplane, the propulsion is produced by the propeller. The prop provides the thrust that moves the plane; it moves it forward through the air. Movement through the air allows the wing to provide the lift for flight. If the wheels had anything to do with it at all, an airplane would settle immediately after lifting off.

To add another line of thought, assume your plane is a seaplane that can take off at 15 mph airspeed. Now you have a smooth river that is flowing at 15 mph. There is no wind. You turn loose of the plane going downstream. You plane has also a 15 mph airspeed. You can take off. The speed of the water relative to the plane is zero. Effective ground speed is zero. The plane doesn't care, it has airflow over it's wings and and it's happy.

Ya'll have fun!

Larry

(in reply to CHassan)
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RE: Can It Take Off?? - 12/5/2005 4:08:15 PM   
David Cutler



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quote:

Break it down. The motion of the axle (airplane) is independent of the motion of the tire.


Yes, but not driven by it. If the wheel is free to rotate it isn't going to affect how the prop accelerates the plane.

Incidentally, the aerodynamics of a rolling wheel is extremely complex as the bottom of the wheel is stationary (in relation to the ground, if there's no slip) and the top of the wheel is travelling about twice the speed of the vehicle It's not exactly twice the speed because the radius to the ground is less than the radius to the top due to the tire flattening somewhat where it touch the ground. This change in speed happens every revolution, the stresses of which which has to be taken into account in the structure of the wheel.

Just thought I'd throw that in to confuse!



-David C.

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RE: Can It Take Off?? - 12/5/2005 10:34:21 PM   
kriegsmacht



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From: Castle, OK, USA
Status: offline 		Heres what would happen in the real world, no wind, plane sitting stationary on the belt, engine idling.

Gradually apply full throttle and the plane would quickly move forward. At this point the sensor unit would activate the belt drive motors. I am still unclear which way the belt is designed to turn, but I will cover both instances.

1. Belt goes same direction as plane body: Plane accelerates forward, wheels not turning, and measurably but not noticeably sooner than a normal takeoff, lifts into the air. If it is a tail dragger, you will not have to compensate for left pull (P-factor) because the wheels will not be rolling. The nose, on a plan with a free castering tail wheel will yaw left for a moment then straighten up before plane lifts off normally.

2. Belt goes opposite direction of plane body: A measurable decrease in acceleration due to wheel bearing friction, but with quality bearings it wont be very noticeable. Steering of either a tricycle or tail dragger will be greatly amplified and quite squirrelly. If you don't tip over or something it will take off normally when it reaches its normal takeoff speed.

Gyroscopic effect, and aerodynamic forces of spinning wheels would not be noticeable.

In other words the plane would take off fine, the only difference would be steering on the ground, and it would be a big difference.


< Message edited by kriegsmacht -- 12/6/2005 12:26:59 AM >

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RE: Can It Take Off?? - 12/6/2005 12:52:26 AM   
Flyfalcons



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quote:

ORIGINAL: kriegsmacht

2. Belt goes opposite direction of plane body: A measurable decrease in acceleration due to wheel bearing friction, but with quality bearings it wont be very noticeable. Steering of either a tricycle or tail dragger will be greatly amplified and quite squirrelly. If you don't tip over or something it will take off normally when it reaches its normal takeoff speed.



Ah but the question does not say the belt moves in the same speed but opposite direction of the plane body. It states it will go the same speed but opposite direction of the wheels.


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RE: Can It Take Off?? - 12/6/2005 2:44:07 AM   
David Cutler



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quote:

It states it will go the same speed but opposite direction of the wheels.


In that case the wheels would wear out and there would be a lot off smoke and squeeling!



-David C.

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RE: Can It Take Off?? - 12/6/2005 6:42:49 AM   
Craig-RCU



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quote:

ORIGINAL: Flyfalcons

The Avweb article is interesting but it only answers the question of if the belt will move relative to the airplane's speed, not the speed of the tires.

I don't understand how you are defining the speed of the tires. My definition of the speed of the tires is that since they are attached to the plane their speed is the same as the plane. What is your definition of tire speed that it can be different than the airplane's speed? If a plane is moving on the ground at 10mph, how fast are the tires moving by your definition?

If you are assuming that the speed of the tires means rotational speed, then you are confusing the original problem needlessly because the discussion here shows that the speed of the conveyor belt (no matter how it is measured or what it is matched to) is irrelevant to the airplane gaining airspeed enough to take off.


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RE: Can It Take Off?? - 12/6/2005 12:06:23 PM   
Phlip



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quote:

If you are assuming that the speed of the tires means rotational speed, then you are confusing the original problem needlessly because the discussion here shows that the speed of the conveyor belt (no matter how it is measured or what it is matched to) is irrelevant to the airplane gaining airspeed enough to take off.


Not to mention, this approach makes the problem a mathmatical and mechanical impossibility. Since the wheels of the plane are not driven, as soon as the plane begins to move, the belt would move the other way, thus accelerating the rotation of the wheels (but not the plane) and the belt would immediately have to rotate faster to catch-up thus accelerating the wheels more, and within milliseconds the belt would be moving at the limit of it's mechanical capability (assuming there is such a limit. If not, it would certainly blow the wheels apart from centrifical force).

edit: typo

< Message edited by Phlip -- 12/6/2005 12:07:57 PM >


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RE: Can It Take Off?? - 12/6/2005 3:09:14 PM   
kriegsmacht



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From: Castle, OK, USA
Status: offline 		Philip, good thinking. I also like the riddles about does a housefly inside an airplane add weight to the aircraft someone mentioned earlier. How about a helium balloon? I think it would add weight if sitting on the floor, not affect weight if floating in mid air and reduce weight (add lift) if it floated up and touched the ceiling.

How about if you had a large pan full of water on a scale and added a model boat? Or how about if you had an RC helicopter inside a really fast elevator.......

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