airplane riddle (taken from another board)

[OldTownPilot]

BUT...at which point would you overspeed the tires as you are rotating with whe wheels spinning at twice the normal rotation speed?

[pullup]

Originally Posted by KLB

but the conveyer belt is changing its speed in order to keep the plane in the same position. true, if the plane can move forward and produce lift over the wings, enough for takeoff then yes it will get off the ground. however, if the plane does not move forward, which is what i gather from the riddle, then no, it will not take off.

But the plane IS moving forward!!!! The prop or the jet produces thrust with no involvement from the ground. The wheels are there to decrease friction, that's all. A plane does not produce thrust like a car (ie: the tires grip the road and the force causes acceleration). You could spin the wheels 1,000 mph and the plane would still take off. This is because the thrust system is independent of the wheels.

If you were roller blading down a hill your velocity would be your mass times accelration, correct? Now, if the ground was matching your speed in the opposite direction, what would happen? Your velocity and acceleration would still be the same as before but your wheels would be spinning twice as fast. Same concept! Again, because your thrust (gravity) is independent of the wheels. the wheels are frictionless for all intents and purposes, as are the aircrafts wheels. Whew! Anyone get all that?

[pullup]

Originally Posted by OldTownPilot

BUT...at which point would you overspeed the tires as you are rotating with whe wheels spinning at twice the normal rotation speed?

By overspeed do you mean when would the tires fly apart? If so then that is up to the company that makes them and how they engineered them.

[skywestseth]

This thread is crazy....so i see for all intensive purposes that we are assuming that outside variables are not a factor such as friction....but the question i think is still a little misleading....i do agree and believe it will takeoff given the explanations i have recieved in this thread...but the original question does state that the conveyor is matching the airplanes speed so technically if the aircraft is propelling forward and the conveyor picks up that and cancels it out all's i can see happening is the conveyor keeps accelerating as does the planes wheels and sooner or later something just gives out..

i dont know if that was easy to follow...it just seems that both the aircrafts wheels and the conveyor would just have to keep "auto-speeding" up to try and cancel out the never ending thrust from the plane...it doesnt make since to me saying that the aircraft will gain forward momentum and take off if the question itself says that the conveyor cancels any forward movement....therefore just leaving an ever-accelerating device.


-Seth

[SteveC]

Originally Posted by skywestseth

<snip>.. if the question itself says that the conveyor cancels any forward movement....therefore just leaving an ever-accelerating device.
-Seth

It doesn't say that the conveyor will cancel any forward movement. It simply says that it will run the same speed (as the plane) but in the opposite direction.

[skywestseth]

Which is what the misleading part is

This conveyer has a control system that tracks the plane speed and tunes the speed of the conveyer to be exactly the same (but in opposite direction).

Now is "plane speed" measured in

ground speed
wheel speed
air speed
speed relative to the treadmill.....


?

[B767Driver]

Maybe a good teacher out there can set me straight. I say it doesn't fly.

My rationale. The airplane has no forward movement...hence no relative wind and no airspeed. If I'm running on a treadmill...and I hold up a flag...the flag will drop straight down, dead. Hence, no relative wind acting on the flag. If I step off the treadmill and run holding the flag...it will blow in the breeze.

Let's say I'm a flying machine...built with a "strap on" set of wings that will allow me to fly once I reach 10 mph on the treadmill. I'll never fly...because I'll never produce a single knot of airflow over my wings because I'll always be at my starting point.

Much has been written about the wheels not being the propulsive source of the flying machine. However, on the ground, the wheels are the point of contact for the machine between the "mediums" it is operating in...the first being the ground...the second being the air. So until the machine becomes a flying machine...it is a ground machine.

As the conveyor negates the movement of the ground machine...it remains stationary...does not move...and does not produce an airspeed. Therefore no relative wind and no lifting force.

Can someone explain to me why the airplane will overcome the effects of the treadmill and advance it's position to produce an airspeed? If you can do that...I'll become a believer that it will fly.

Thanks.

[skywestseth]

also if the plane keeps accelerating, i interpret from the original question that the treadmill will in fact cancel forward movement by increasing its own speed to match the thrust from the aircraft.....so this is where i see the two machines just ever accelerating.... until one gives out...

-Seth

[SteveC]

Originally Posted by B767Driver

Maybe a good teacher out there can set me straight. I say it doesn't fly.

My rationale. The airplane has no forward movement...hence no relative wind and no airspeed. If I'm running on a treadmill...and I hold up a flag...the flag will drop straight down, dead. Hence, no relative wind acting on the flag. If I step off the treadmill and run holding the flag...it will blow in the breeze.

Let's say I'm a flying machine...built with a "strap on" set of wings that will allow me to fly once I reach 10 mph on the treadmill. I'll never fly...because I'll never produce a single knot of airflow over my wings because I'll always be at my starting point.

Much has been written about the wheels not being the propulsive source of the flying machine. However, on the ground, the wheels are the point of contact for the machine between the "mediums" it is operating in...the first being the ground...the second being the air. So until the machine becomes a flying machine...it is a ground machine.

As the conveyor negates the movement of the ground machine...it remains stationary...does not move...and does not produce an airspeed. Therefore no relative wind and no lifting force.

Can someone explain to me why the airplane will overcome the effects of the treadmill and advance it's position to produce an airspeed? If you can do that...I'll become a believer that it will fly.

Thanks.

I'll try with a new analogy since the old ones don't seem to be working.

Imagine an airplane on skis, sitting on a plate of ice. You could move the ice forward and backward underneath the plane and the plane would not move, it's inertia would hold it still and the ice plate would slide freely without affecting the airplane. Now, start up the airplane's engine and watch the plane very easily move forward, accelerate, and take off. It will do so if the ice is stationary, if the ice is moving in the same direction as the plane, or if the ice is moving the opposite direction. The ice cannot affect the airplane because it has no way to exert any force on the plane.

The bearings in the plane's wheels act exactly the same as the sheet of ice above. The lack of "drag" means that the treadmill cannot exert force on the airplane to oppose the propeller's force trying to move the plane forward.

Another analogy:
Instead of walking or running on the treadmill, pretend you are wearing roller blades, holding a rope which is tied to a tree 100 feet in front of you. Turn the treadmill on, and notice how easy it is to hold your position on the treadmill with a very low force against the rope. Now, turn the treadmill up to a much higher speed. Notice how the force needed to hold yourself in position has not increased? (This is because rolling friction is largely independent of the speed - take my word for it or get out a physics book ) Now, start to pull yourself forward by pulling on the rope with your hands. The treadmill can speed up, slow down, stop, whatever, and it cannot prevent you from pulling yourself towards the tree. The airplane does the exact same thing, except instead of pulling on a rope it is pulling on (pushing against) the air. Independent of the treadmill. The wheel bearings isolate the roller blader and the aircraft from the movement of the treadmill.

[SteveC]

Originally Posted by skywestseth

also if the plane keeps accelerating, i interpret from the original question that the treadmill will in fact cancel forward movement by increasing its own speed to match the thrust from the aircraft.....so this is where i see the two machines just ever accelerating.... until one gives out...

-Seth

This is where the disagreement comes in. In my world the original statement says nothing about cancelling forward movement, it simply says that the treadmill will run at the same speed in the opposite direction. Physics dictate that the treadmill cannot stop the plane from moving forward, so your statement is not possible in the real world.

Now, if you want to postulate a theoretical world in which a treadmill can magically induce such a force, well, then I'd say that you were right.....in your imaginary place.

[B767Driver]

That's an excellent explanation Steve...I think I'll buy that one. Good job.

[SteveC]

Originally Posted by B767Driver

That's an excellent explanation Steve...I think I'll buy that one. Good job.

Then my work here is done. Tonight I shall sleep soundly indeed!

[JaceTheAce]

Originally Posted by B767Driver

Maybe a good teacher out there can set me straight. I say it doesn't fly.

My rationale. The airplane has no forward movement...hence no relative wind and no airspeed. If I'm running on a treadmill...and I hold up a flag...the flag will drop straight down, dead. Hence, no relative wind acting on the flag. If I step off the treadmill and run holding the flag...it will blow in the breeze.

Let's say I'm a flying machine...built with a "strap on" set of wings that will allow me to fly once I reach 10 mph on the treadmill. I'll never fly...because I'll never produce a single knot of airflow over my wings because I'll always be at my starting point.

Much has been written about the wheels not being the propulsive source of the flying machine. However, on the ground, the wheels are the point of contact for the machine between the "mediums" it is operating in...the first being the ground...the second being the air. So until the machine becomes a flying machine...it is a ground machine.

As the conveyor negates the movement of the ground machine...it remains stationary...does not move...and does not produce an airspeed. Therefore no relative wind and no lifting force.

Can someone explain to me why the airplane will overcome the effects of the treadmill and advance it's position to produce an airspeed? If you can do that...I'll become a believer that it will fly.

Thanks.

I was once a believer that the aircraft would not take off. Then I came back an hour later after considerable thinking and reversed my answer and said it WOULD take off.

You are thinking the exact way I was thinking before I changed my mind. This question stumped my father (CFI, II, MEI, ATP guy) but he then changed his mind quickly after.

One thing you must realize is: true, the aircraft won't take off until it gains enough GROUND speed. I think a lot of people who think it won't take off misunderstand when people say it will - The people who don't think it'll take off believe that the people saying it'll take off mean that the airplane takes off "instantly". It just moves along forward and takes off at a normal rotation speed relative to the ground as it would in a normal takeoff situation...the wheels will just be twice the speed!

Realistically the tires will just blow before the aircraft gets to Vr and kill all on board due to a heartattack from being so scared by the tire popping.

[pullup]

Originally Posted by SteveC

Then my work here is done. Tonight I shall sleep soundly indeed!

Thanks for taking over...again. Thought I would blow a gasket trying to explain it again. Can we put this to rest or do I have to derive all of the equations?

[Fly4Pay]

Yeah, this one pops up every so often. It got a LOT of press a few months ago when Neal Boortz (an ATL based talk show host and militant pro-aviation voice) discussed it on the show and on his web site.

Fun to hear people try to "logic" their way through it.

[TheShortOne]

holy necropost batman!

[averyrm]

Noooooo stop posting! You're making it bounce back to the top!!!




... oh wait... DAMN!

[Raskolnikov]

Chris Ford never did give in. Is he still a disbeliever? A heretic to the god of physics? Shall we see if he weighs more than a duck?

Um, that was the beer posting, not me…

[Chris_Ford]

I was never really a disbeliever, I've stood by that this is a purposefully vague question that has both sides arguable.

The "plane"'s speed is being matched, which is generally perceived as the "airframe", which means that yes, the airplane would take off.

However, if the wheels' speed is being matched (and it does not say that it's not), then the airplane would not take off. Not to mention the issues of tire speed limitations, etc.

So much like many other physics questions, the answer is "In the physics world, yes. In actual reality it's highly unlikely" with the various minutae of the planet (no max tire speed I know can handle 2x Vr speed)

[Doug Taylor]

I hear they didn't play the national anthem during the game. True?

[Raskolnikov]

The speed of the treadmill actually has nothing to do with whether the plane will take off. Even if it were more than matching the airspeed of the plane. Let’s give this plane super freaky wheels that can spin with no limits. Let’s then build a treadmill that can move at up to 20000Mph with no problems. If we put the plane on the treadmill and started the treadmill (a different look at the old problem) and got ‘er going to 10000 Mph backwards, the plane would sit there with zero ground speed while its wheels rolled along the treadmill at 10000 Mph. Then if we fired up them engines on that bird and gave ‘er take-off power, down the rolling runway she’d go until she reached Vr and up she goes. Note I've changed the problem from the treadmill matching the speed of the plane to a crazy fast speed. Still the plane rotates and delivers its pax on schedule to LAX.

That's why it's called an "airplane" and not a "treadmillplane" because it flies through the air.

Wow this beer is good!

[TonyC]

It's ironic that this thread was resurrected at about the same time Chris is claiming to be Harvard material.






It's not about physics, it's about reading comprehension.


The original question: "The plane moves ...".



Case closed.




.

[Chris_Ford]

Originally Posted by TonyC

It's not about physics, it's about reading comprehension.


The original question: "The plane moves ...".



Case closed.

The wheels are just as much part of the plane as the airframe is. I still agree that the intent of the question drives you to what the most realistic solution is (the plane taking off) and I agree with the notions of physics being brought forth. But if the speed of the wheels is continuously being matched by the treadmill, there would be no way for the airplane to gain any relative motion on the treadmill (aka the wheels would just spin up to infinity)