Can It Take Off?? (Full Version)

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bentwings -> Can It Take Off?? (12/2/2005 4:26:05 AM)

Here is one to use up band width. The diesel truck guys made 302 posts in 5 days. only 2 personal flames recorded [X(]
can the experts beat this???[>:] Let's have at it. Can the "wing guys" beat the "wheel guys"[:D]

*** Brain Teaser ***

Imagine an airplane is on the beginning of a massive conveyor belt, as wide and as long as a runway, and intends to take off. The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?



BMatthews -> RE: Can It Take Off?? (12/2/2005 5:40:23 AM)

Yes, The props cut away at the air or the jets still throw out thrust to the rear and the airplane goes foward just like normal. All that will happen is that the wheels will turn twice as fast as they would on a stationary runway thanks to the belt.

The logic is that the airplane, unlike a car or truck, does not rely on the wheels for propulsion so it'll still take off just fine.... as long as the wheel bearings don't overheat and seize from the extra RPM's.... :D

Do you have a link to the other thread?



3dbob37n -> RE: Can It Take Off?? (12/2/2005 5:45:25 AM)

Boy, Bent Wings, you are trying to keep me up all night thinking this over.

If the model cannot accelerate because the wheels are basically just spinning in one place, because the conveyor matches the wheel rotation, the aircraft will not begin its takeoff roll and will simply be batting the air.
Let's see, where can I get a big conveyor belt?
Yeeeegads, where did you get that one?


3dbob



da Rock -> RE: Can It Take Off?? (12/2/2005 11:56:10 AM)

3dbob,
BMatthews solution isn't really hard to see if you think about a couple of things. Our models fly straight up right. That'd be on thrust alone right. After all, the wing would be lifting horizontally and nothing in the a/c is designed to lift "forward" (which would be "up" in this orientation). And our models fly downwind with no problem. And they do it with the wheels up when they have retracts. And models accelerate in dead air, wheels up or down.
quote:

If the model cannot accelerate because the wheels are basically just spinning in one place,
Hey, the model doesn't accelerate because of the wheels. And no matter what the problem states about the conveyor belt's speed matching the wheels' rotation, that won't matter to the a/c or it's prop one bit. That a/c will move forward on it's thrust and bring the wheels along for the ride.




da Rock -> RE: Can It Take Off?? (12/2/2005 12:00:25 PM)

And BTW, the conveyor belt actually won't get up to very much speed. Think about this little hidden deal. If the conveyor belt is going to match the FORWARD speed of the wheels, then the wheels actually can't be turning at all right?

quote:

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.

So the a/c is taking off toward the "right". The wheels will rotate which way? Clockwise right. That means the belt has to go to the "right" since the wheels, where they contact the belt, are going "left". So the belt goes right the same speed the wheels are moving to the right. right? right!



starwoes -> RE: Can It Take Off?? (12/2/2005 12:43:36 PM)

hmmmm interesting. what if the aircraft in question has pontoons and is on a fast flowing river that is flowing opposite the heading of the aircraft. speed of the river matches and balances the forward motion of the craft.

BTW, in your question, i think the aircraft will fly....because they don't need wheels to fly...right?....just less friction with the surface.



MajorTomski -> RE: Can It Take Off?? (12/2/2005 1:27:07 PM)

Good grief! this thing hit on ezone last week and went over 300 posts in two days.

See here

http://www.rcgroups.com/forums/showthread.php?t=446465

First there is a phrasing problem in the question:

quote:

The conveyer belt is designed to exactly match the speed of the wheels at any given time, moving in the opposite direction of rotation.
There is no wind.

Can the plane take off?



Go down to post #8 of the above thread and you can read the original statement.

To some people the term moving in the opposite direction of rotation means the belt will move forward in the same direction as the airplane, because the wheels rotation is opposite to this direction. The problem is the question was poorly quoted from the original site, the intent of the question is to simply have the airplane remain stationary to the rest of the world while the belt moves under it. under that condition, the airplane cannot fly because it cannot accelerat the wings to generate lift. It will sit there rolling up to full throttle.

Now then there are the arguments of bearings holding up, limited thrust, limited belt speed, lots of folks read too much into the mind game. It won't fly.



juanes1969 -> RE: Can It Take Off?? (12/2/2005 1:37:24 PM)

Ýou are right Major!! If the plane is kept stationary because of the movement of the belt, it would be just the same as if we kept the plane on brakes in the runway with the engine running at full throttle. No speed on the wing equals no lift. The engine alone cannot take the plane of the runway. Right?



dick Hanson -> RE: Can It Take Off?? (12/2/2005 1:41:24 PM)

Maybe your plane won't take off - all of mine will- easily



HighPlains -> RE: Can It Take Off?? (12/2/2005 1:44:31 PM)

Second cut:

It would take off slower, as some energy is being stored in the wheels as rotational energy. But the airplane still has to accelerate down the conveyor or else the speed of the tires will not increase. If it doesn't get off soon enough, the tire/wheels will explode.



britbrat -> RE: Can It Take Off?? (12/2/2005 2:11:50 PM)

Airborn with no sweat (except cooked tires).



DLSmith2 -> RE: Can It Take Off?? (12/2/2005 2:27:02 PM)

I think you might be able to visualize the problem better if you think of a full scale plane like a Cessna 152. If the 152 begins flight at about 60kts airspeed it doesn't matter what rate the tires are turning, other than like BMatthews said, the overheated bearing problem, or surface friction. You can lock the wheels of the Cessna, point the nose into a 60 kt wind, pull back on the yoke and the plane will lift off, because the airspeed over the wings will provide the lift, and the wheels aren't turning at all.

The question is misleading, because it implies that the speed of the wheels, or ground speed, determines an aircraft's ability to take off, instead of the airspeed.

If you take off from a grass runway that hasn't been mowed in a while, the friction of the long grass can increase the take off distance by 10%. This is because the grass prevents the plane from reaching the airspeed it needs to lift off, until the plane has gone a little farther. Wheel friction on the conveyor belt may increase the length of ground roll necessary to get 60kts of wind going over the wing, but that doesn't matter because it's a conveyor belt, right?

Interesting question, huh?

Blue skies,

DL

This is a job for the MYTHBUSTERS!!!



danny03 -> RE: Can It Take Off?? (12/2/2005 2:44:56 PM)

The aircraft will fly since its forward motion is produced by a prop, Fan, or turbine and not it's wheels. If we were talking about a car or truck then the vehicle will remain in the same spot.



Liberator -> RE: Can It Take Off?? (12/2/2005 4:34:05 PM)

Yes its all about air moving over the flight surfaces, not relational ground speed.

How about this.

Does a fly in a flying airplane add weight if the fly is flying inside the plane?



3dbob37n -> RE: Can It Take Off?? (12/2/2005 4:56:38 PM)

I think the comparison about a seaplane trying to take off in a river going in the opposite direction is a good one. It can't take off because the current is going the opposite way to the airplane. With no forward motion, no lift is generated.
Dick, you might have a problem too. I don't think my Yak 54 would take off because the tailwheel would keep it from obtaining enough angle of attack. I can hand launch, but not from a standstill on the ground. It has to bounce at least a little during the roll in order to get off.
Don't forget the question is described as "just as fast as the wheels turn, the conveyor always matches the wheel speed in the opposite direction." So the conveyor speeds up or slowd down.
Now I realize in reality, with enough power, the model would simply yank itself into the air but that is not what the question tries to acknowledge. The question assumes the model is just an ordinary every day type model and not super powered.

3dbob



iron eagel -> RE: Can It Take Off?? (12/2/2005 5:02:37 PM)

Of course, the airplane does not care how fast the weels are spinning all it cares about is airspeed. Plus the moving belt will cause the air to move relative to the airplane causing ground effect anyhow.



MajorTomski -> RE: Can It Take Off?? (12/2/2005 8:43:34 PM)


quote:

ORIGINAL: danny03

The aircraft will fly since its forward motion is produced by a prop, Fan, or turbine and not it's wheels. If we were talking about a car or truck then the vehicle will remain in the same spot.



But Dan in the problem no matter how much thust the airplane generates the roller rolls out from under it. If you are standing by the wingtip off of the belt the wingtip would never move relative to you, so the wing cannot be moving through the air, it cannot be generating lift. nO fly



danny03 -> RE: Can It Take Off?? (12/2/2005 9:05:36 PM)

I respectfully disagree, the way I see it since the wheels are free to rotate there is no force holding the aircraft still, thereby allowing the aircraft to move forward allowing airflow over the wings. The only thing that the belt is doing is spinning the wheels faster.



BMatthews -> RE: Can It Take Off?? (12/2/2005 9:07:52 PM)

quote:

ORIGINAL: MajorTomski


quote:

ORIGINAL: danny03

The aircraft will fly since its forward motion is produced by a prop, Fan, or turbine and not it's wheels. If we were talking about a car or truck then the vehicle will remain in the same spot.



But Dan in the problem no matter how much thust the airplane generates the roller rolls out from under it. If you are standing by the wingtip off of the belt the wingtip would never move relative to you, so the wing cannot be moving through the air, it cannot be generating lift. nO fly


You're falling into the trap that says what the treadmill and wheels do is important. It isn't. An airplane gets it's speed from the prop or jet thrust. The wheels can spin or not as much as they want and the treadmill can roll either way and none of that will make any difference. The airplane will still accelerate and lift off thanks to the prop or jet thrust and the speed that it generates. The only vairable will be the spin speed of the wheels of the landing gear. The only way it will not lift off is if the brakes are locked on



And getting back to the float plane scenario. The drag from the floats in the river is a whole lot more than any possible wheel drag if you do not include the brakes being on. So that scenario isn't really a fair comparison. But let's look at that case a little closer.

If the float plane starts by floating downstream with the water at a ridiculous river current speed of 80 mph but is pointed back upstream then it's got an 80 mph tailwind and a zero water speed (let's forget about air drag for now). So the pilot starts the engine and guns it while pointed upstream. The prop thrust will slow the downstream movement. This will reduce the tailwind effect and also slow down the downstream velocity compared to the water so an effective forward water speed will develop even though the airplane is still seeing a tailwind effect. This forward hydrodynamic velocity of the water passing UNDER AND PAST the slower airplane will allow the floats to come up onto the step and greatly reduce the water drag. At some point the airplane will by hydroplaning at 80 mph across the water while traveling upstream but "flying" at 0 effective airspeed. But if they are lucky the reduced drag at the water thanks to the hydroplaning will allow the plane to continue accellerating until it has an 80 mph forward airspeed and a 160mph indicated water speed at which point the floats will shatter or the plane will lift off thanks to the airspeed.



MajorTomski -> RE: Can It Take Off?? (12/2/2005 9:23:11 PM)


quote:

ORIGINALYou're falling into the trap that says what the treadmill and wheels do is important. It isn't. An airplane gets it's speed from the prop or jet thrust. The wheels can spin or not as much as they want and the treadmill can roll either way and none of that will make any difference. The airplane will still accelerate and lift off thanks to the prop or jet thrust and the speed that it generates. The only vairable will be the spin speed of the wheels of the landing gear. The only way it will not lift off is if the brakes are locked on



If the wheels are not important, how well will the airplane fly if I lock the wheels? It won't because it cannot move relative to the air around it.

The hook in the stated problem is that the airplane cannot move. For every 100 miles an hour it is moving forward the belt underneath it is matching the speed exactly. The airplane to an outside observer is not moving relative to the air around it.



britbrat -> RE: Can It Take Off?? (12/2/2005 9:48:27 PM)

----and yet another thought to ponder. If the treadmill is moving in the opposite direction to the aircraft, it will create its own slipstream past the AC because the skin friction of the treadmill will drag the air along with it.

As I said, the bird's gonna fly.



Phlip -> RE: Can It Take Off?? (12/2/2005 9:57:05 PM)

Can an airplane with an 80 knot take off speed take off down wind in an 80 knot gale? Yes! But the relative ground speed (and wheel speed) when it becomes airborne will be 160 knots. Not a safe way to do it, but it could happen. And the motor on the plane isn't working all that much harder than it normally does (just enough to overcome the extra rolling friction of the wheels.

Or look at it this way. Hook a chain to the front of a tow glider, and anchor it securely to a monster truck. Now set the glider on the belt, and begin pulling it with the truck (which is not on the belt). Will the glider come up to speed and fly? Heck yea! But as people keep saying, the wheels will be turning twice as fast on the glider as on the truck.

If you're gonna lock the wheels on the plane, you're changing the rules of the original question.



Bax -> RE: Can It Take Off?? (12/2/2005 10:02:10 PM)

deleted



dick Hanson -> RE: Can It Take Off?? (12/2/2005 10:18:33 PM)

And -- if the aircraft is not moving -what turns the wheels ????
When I took the written test phase whilst applying to be a air force driver --they asked a number of rather trick type questions .- Some were sorta like this one -
think of this:
If the wheels are turning - the plane is gathering speed.
No speed =no turning of the wheels



BMatthews -> RE: Can It Take Off?? (12/3/2005 1:12:13 AM)


quote:

ORIGINAL: MajorTomski

.....The hook in the stated problem is that the airplane cannot move. For every 100 miles an hour it is moving forward the belt underneath it is matching the speed exactly. The airplane to an outside observer is not moving relative to the air around it.


Locked wheels is the only thing that would trip this up. But the problem does not state locked wheels. It also does not state that the airplane cannot move. It only asks if it will lift off.

So as the plane starts to move the treadmill reflects the motion of the wheels and moves backwards in response. This just makes the wheels roll faster but does not halt the airplane. It will still move forward. The effect is such that the treadmill will spin the wheels faster than the motion of the airplane would indicate but it will not stop the airplane from moving forward and accelerating to flying speed.

If it helps to think of it another way let's assume the wheels were replaced with ice skates and the treadmill is faced with ice. The treadmill can then turn as fast as it wants to but the airplane would still be able to "slide" forward and lift off. The airplane wheels are just another way of reducing friction.

Remember that the airplane bites into the air thanks to the props or is pushed thanks to the reaction of the jet exhaust That action and reaction does not depend on nor is it related to what the wheels and treadmill are doing. The treadmill and wheels can spin up to a bazillion rpm if they want but the airplane is still going to accelerate forward and roll on those wheels that have no choice but to roll faster than the treadmill speed.



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