OT: Any mensa members here? ;)
 

Here is a question that will for sure get you guys thinking.

If you have a large jet plane (747) sitting on a runway that was actually a giant conveyor belt (go with it). And there is also a device on the plane that communicates with the conveyor belt to tell it how fast the plane is traveling, which would then make the conveyor belt match the speed IN REVERSE.

Can the jet take off?

currently i am not sure if the take off speed is measured on wind speed or ground speed.
The device you described create a differential ground speed with wind speed =0 , since the plane itself is not moving at all.

That's a good question. Ground speed.

Just to keep things simple...

If the jet was traveling at 250mph that would mean the conveyor belt would be going in 250mph in the opposite direction.

yes, you described the same device that some people use to run indoor.
they can run as fast as they want, they are not moving.

you can put the plane in front of a big fan and have a wind speed that allow the plane to lift vertically while not moving at all again.

No no... no additional devices. Just givin what I said in the first post....... can the jet take off?

if your plane roll forward on a track that runs backward at same speed, you plane is not moving. There is no chance it is taking off that way.
Actually, you have to know how the device on the plane is recording speed, since speed is recorded by unit of distance divided by unit of time.
if the reference is the surface of the conveyor belt, the speed will be different than if measured from its start position.

It would not. The wheels would be turning like mad but the plane would otherwise be still, so there would be no lift.

Okay, I'll bite
 

If, as you say, the "runway" is a conveyor belt, then we could assume that the conveyor belt is as long as a runway for a 747, so, more than 10,000 feet. As the plane's thrust increases and the belt begins to move, it will also move the air above it. Not efficiently, but when the belt is moving at high speed, it will generate quite a wind. So, I'll say, given the implausible "ifs," a lightly-loaded 747, and a conveyor capable of keeping up with it, the wind created by the moving belt could eventually let the wing produce enough lift to raise the plane off the belt. At that moment, when contact is lost with the belt, the plane would accelerate rapidly and climb away.

In aircraft, you are only concerned about ground speed in determining how long it will take you to go from point A to point B. For making the aircraft fly, you must have airspeed, measured by the differential of a static source and a pitot tube which faces the oncoming wind. Airplanes fly when air moves over their wings at sufficient speed to cause lower air pressure above the wing and higher pressure on the bottom (Bernoulli's principle). The pressure difference is created by air being forced to travel a greater distance over the curved top of the wing which increases its velocity because it will try to reach the rear of the wing at the same time as the air moving below it. Air moving at a higher velocity will have lower pressure.

A fun way to demonstrate Bernoulli's principle is to take a piece of cardboard or any lightweight flat object and place it with your hand an inch or so above it. Now, take a compressed air gun and place it between your second and third fingers and blow straight down on the paper. It will lift up and stick to your hand because you are creating high velocity air traveling outward over the top of the paper.

As for the original question, it still comes down to airspeed. If the plane was facing into a headwind with sufficient velocity, the plane will fly regardless of its speed on the ground.

-gb-

Yes, the plane will take off.

The aircraft develops thrust by pressure WITHIN the engine. It is independent of contact with the ground.

Unlike people on a treadmill, who must push against the ground to move forward, the airplaine's thrust is generated WITHIN the engine.

It will fly.

But the thrust of the engine is used to overcome the friction of the ground and move the plane forward until sufficient airspeed is achieved for flight. If that forward inertia creates airspeed which is then fed to the conveyor to move it backwards at that speed, the wheel speed should increase, but the aircraft will not move forward.

However, if a big gust of headwind came along and got the wheels off the conveyor, the thrust of the engines would then be the sole factor and the aircraft would accelerate above the ground until it could lift off.

Just playing devil's advocate here, Richard.

-gb-

Planes take off from rivers all the time.

Planes take off from frozen lake beds.

The thrust from the engine, pushes the aircraft in the opposite direction THROUGH THE AIR.

What's happening beneath it is irrelevant.

A fixed wing (Non Harrier) plane takes off based on the air flow over and under the wings. Lift off speed is typically measured in terms of speed over the ground but only for convenience. This is why the plane takes off into the wind, and air craft carriers point into the wind, and power head.

If the airflow over/under the wing is not sufficient to create lift, the plane will not take off

Sharyn
Having flown turbo prop/jet aircraft for years ;-)

Another Brain Teaser
 

A jet flying at mach 3 crosses over laser tower on the ground. At the same precise moment a laser beam is flashed from the Jet and the tower on a target the same distance ahead. Which laser beam hits the target first ?

Sharyn

Have to disagree Richard (respectfully of course). Say my aircraft is sitting in a muddy field and the tires are sinking in. I put the throttle to the wall. The excess ground friction will not allow the aircraft to move forward and it will never get off the ground.

Or, I pull into position on the runway and stand on the brakes while going to full throttle. The aircraft will just sit there as the engine produces full thrust. This is in fact the procedure for short-field take-off.

What's happening beneath an aircraft in contact with the earth is indeed relevant.

The question here is does the counter movement of the conveyor belt create the effect of zero forward movement upon the surface of the earth, like being stuck in the mud.

-gb-

But it's always the airspeed indicator you reference, correct? (It has been for me). If you have a 30kt headwind, you take-off roll is over the ground at 30kts less than your airspeed indicator. But as we know, the airplane only cares how fast the air moves over its wings.

-gb-

Greg.

Irrelevant was a poor choice of words.

We have determined that the mud and brakes PREVENT THE WHEELS FROM TURNING.

The conveyor belt will do no such thing. There is no more friction than when the plane is rolling along at any speed. The engines have the power to move the plane through the air, while the wheels are rolling along the ground.

The 'ground speed' indicator of the wheels on such a takeoff would indicate a nominal double of airspeed, assuming still air.

They both hit at the same time, as the speed of light is always the same, which I think is 186,000 miles/sec.

Use a bullet and you have a different story though!

Mike

I wish I still had my RC planes. I'd throw one on a treadmill and try it. And of course I would videotape it all for the amusement of everyone here. (-:

I just like to throw out the various thought processes to have a good discussion so don't take me too seriously on this.

-gb-

As far as the plane question______

It indeed would take off and fly and here is why.

The conveyor sensing or communicating device you said is measuring "how fast the plane is moving" not how fast the wheels are turning, so if the plane doesn't move the belt doesn't move.

Obviously the plane will move when the thrust is applied and when the belt moves all it will do is change the speed at which the aircrafts wheels are turning.

I'm assumming that you mean that the conveyor belt is moving forward to match the planes speed, so all that would happen is the the plane would take off without the wheels ever making a single rotation. If the belt is going in the other direction, the wheels would simply turn twice as fast until lift off, if the Goodyears hold up! :)

It would save a bunch of tire wear if they go in the same direction!

Mike

All in good fun Greg. I'm crunching renders, hence my terse replies.

Basically, the thrust from the engines is generated against the wings, transmitted into forward motion. It is incorrect, but often stated, that the thrust from the engines pushes 'against the air'... but for our purposes, that's close enough. The hot air is expelled, pushing the airplane forward. The wheels beneath the plane are there to ease the friction against the ground. They are still free to roll in this example. Hence, the thrust moves the plane forward, the wheels rotate twice as fast, and the plane gets enough airspeed to lift off.

A closer analogy, but still flawed, would be to imagine your person on a treadmill, wearing rollerskates. He holds in his hands, a rope connected to the far wall. The faster he pulls himself forward, the faster the treadmill rotates beneath him. BUT HIS PROPULSION comes from the interaction of his hands and the rope, not the wheels.

Make sense?

That would also make for a funny video. (hehe)

The demonstration of Bernoulli's principle I outlined in an earlier post always baffles the uneducated. They just can't believe you can lift something up by blowing DOWN on it.

-gb-

Nice call Richard. He nailed it from the get go... and with really good examples.

You guys wouldn't believe me if I told you how crazy convorsations get with this question. On another forum I saw one guy who claimed to be an engineer and refused to believe it would take off. That thread was like 20 pages long at least.

And for the record, in my experience probably 95% of people get this question wrong. It just feels so wrong to answer correctly. :)

I'm just curious... how many pilots answered incorrectly?

I'm not sure any pilots were on the forum I referred to. It was a motocross website.

I'll try and dig up the thread. It's worth a read just based on the fact that it will make people not be so hard headed when they are 'positive' they are right. :)

I got this question correct initially but once I thought about it I thought my answer was way to simple. So I second guessed myself thinking there was actually something more to it. So my official answer was wrong :) Next time I'm gonna stick to my instinct.

yes, the plane could take off as it doesn't exert force against the ground to gain momentum...making the treadmill irrelevant.

Not mensa but:

Plane: Unless conveyor belt is creating airspeed to flow across the wing, it will not fly. What is happening in the scenario provided is that the thrust from the engines trying to get plane up to airs speed, is being offset by an equal counter force, offsetting it.


Laser question: They tie. The speed of light is a constant. Laser is at light speed. Light wave emitted from a single point in time and space will reach a distant object at the same time. Now there may be some red shift, which would allow measurement of relative speeds of the sources, but both beams should arrive at same time.

Or you could have a rope tied to the back of the plane with engines at full thrust, still wouldn't lift off. The wings might rip off the plane, though :)

Chris my man,

There is no counter force applied to the wheels, they just roll. Thrust of the engines will move the plane. Read my full post for the explanation.

And as far the laser goes, I already answered it! :)

Mike

You must be Mensa, I'm not, but there has to be a counter force being applied to make the plane remain stationary. You have jet engines asserting force that will make the plane move forward by overcoming inertia from the plane's weight, friction from tires on ground, friction in the ball bearings, and, then you have conveyor belt running in an opposite direction overcoming the force of the get engines, to extent that it is not allowing the jet to move forward. That would have to be a counter force with respect to the planes mass being moved forward. In other words to drive that belt fast enough to offset the speed the plane would normally attain on stationary ground, energy is applied to creat a static stationary plane.....

So then the bearings would fail? ;)

The belt speed is inverse of the planes speed not the wheel speed. In order for the belt to move, the plane has to move and move it will.

The wheels, belt direction like I said, will either go twice as fast as normal takeoff speed or not move at all.

The belt has no real effect on the plane speed, just the wheel speed.

Mike

The initial post:

"If you have a large jet plane (747) sitting on a runway that was actually a giant conveyor belt (go with it). And there is also a device on the plane that communicates with the conveyor belt to tell it how fast the plane is traveling, which would then make the conveyor belt match the speed IN REVERSE.

Can the jet take off?"

Reach air speed to create lift to overcome gravity is the issue, not the speed of the ground underneath the plane. I still say in the scenario presented that every time the jets rolling speed was increased, there is a counter reaction with respect to the belt underneath, there would be virtually no change in speed of air over the wing. Therefore gravity can't be overcome, and flight not attained.

That is why planes take off into the wind. IF you take of with the wind behind you, you have to attain a higher ground speed to reach air speed, and vice versa.

Cole summed it up perfectly:
Yes the plane will take off.
Thanks to the wheels, the motion of the belt does not apply force/drag on the plane, so it continues to move forward, build up speed and fly.


Here's two more .... (I was actually asked these once as part of a job interview)

A) You have nine cubes that look exactly alike. Eight have exactly the same weight, one weighs more than the others. The only tool you have to measure the weight of the cubes is a balance (like the Justice statue), but you are only permitted to use it two times.

How do you find the one cube that has a different weight from the rest?

B) You're in a room with no clock, but you can hear a bell clock in the next room that you know rings once every 15 minutes, and rings out the hour at each o'clock (rings 5 times at 5 o'clock, 6 times at 6 o'clock, etc.).

You just woke up and hear the clock ring once. What's the longest amount of time you may have to wait before being absolutely certain about what time it is? (AM/PM not a factor)

Hehe, here we go. :D

Chris, the difference is that a plane isn't propelled by it's wheels. It's propelled by the jets which suck air in and blow it out... thus propelling the jet forward. The runway/conveyor belt has absolutely no effect. At all.

If you switched the jet with a car then yes. It wouldn't move anywhere because it IS propelled by it's wheels. Unlike a plane which has free spinning wheels.

Btw, in my initial post I didn't say anything about friction, ropes or any of the other things you guys are trying to introduce to the question. It's a set of widgets you have to problem solve with. And no you can't phone a friend either. ;)

I denied being mensa, but my common sense says:

Yes but initially, before flight is achieved, in order to fly, the mass of the plane must be moved through the air in order to gain speed. This is achieved by rolling against the ground to gain the air speed necessary. In this hypothetical, every time the plane is inched forward, a signal is sent to conveyor to speedup, and that pushs the plane back. The wheels are not without some friction to enable that to happen. As power is applied, it continues the forward thrust continues to match the reverse thrust of the conveyor belt. So plane is not moving with relation to surrounding air, so no lift-- no flight. I guess I need a MENSA guy to pound something into my head...

Chris,

Think of it this way. If that plane is sitting on the runway and all of a sudden the belt takes off at 200 mph, is the plane going to immediatly start moving at 200 mph, no. It has wheels and only a gradual increase in speed will occur, in proportion to the friction in the wheel bearings. But, the planes engines will move the plane no matter what direction the belt is going.

Mike

I was initially confused, thinking about the Bernoulli effect as it was explained to me some years ago; "The engines don't lift the plane, the wings do. If the air isn't moving over the wings, the thing won't lift off." But then I saw the explanations about a jet's propulsion, and kicked myself for being so dumb (of course the wheels don't move it forward, how could I forget a thing like that?).

To make it simpler, can't we ignore the conveyor belt? During a regular takeoff, the aircraft is being propelled down the runway with its engines. The wheels are rolling forward, and the ground is moving backward at the same speed (in a sense...it's not, really, but we can look at it that way, can't we?). There is friction between the ground and the tires, the tires and the wheels, the wheels and the axles, and even the surface of the vehicle and the air, but the thing still gets itself airborne, correct? Friction doesn't seem to matter, unless it's so strong as to prevent the craft from moving through the air, which in this case it's not.

Those engines are strong.

-and that pushs the plane back.

Why?