Airplane on a conveyor belt?

[Turn7]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"


I say yes.

[racer2c]

I'm no phys ed teacher but I say no. Air movement across the wing causes the lift, not the speed of the wheels. Now, if there was a giant fan blowing 100mph wind at it while it was on the conveyer...

Is this a trick question? I hate math. :)

[Indy]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"


I say yes.

Interesting question. If the engines have thrust, then I think the plane will take off, barring some sort of wheel problem due to the conveyor.

[grungex]

I'm going with racer2c on this one...

[Cam]

I re-read the Q... Is the conveyor moving at the same speed as the ground speed or wheel speed?

I think it's cryptic and means ground speed so yes it can. :saywhat:

[grungex]

It sounded to me like the plane was standing still relative to the ground, just like a person running on a treadmill.

[Cam]

In addition to above I am guessing there would be more contact resistance between the wheels and the runway therefore the plane would need more runway? :confused:

[devilmaster]

[edit] re-read the question. I say no. Liftoff is an equation of wind speed over the wings - not ground speed of vehicle. If there is no wind, the conveyor keeps the foward ground speed of the aircraft at 0.

But, the way i originally read it says yes - According to a book I read about Air America.

One of the pilots that used to fly said that one of the smaller planes, (probably the helio courier) had such a large wing span that it only required something like 35-45 miles ground speed to take off. He said that it was possible to VTOL the aircraft with enough head wind speed.

Even if he was telling a lie - common sense tells us that aircraft carriers turn into the wind to take off. Why? Because head wind speed is added to the speed of aircraft at takeoff.

[Turn7]

Well, I say yes because it clearly states the the conveyor speed will match the speed of the plane. So, if the plane isn't moving forward then the conveyor will not move. Meaning that the plane has to be able to make forward progress to make the conveyor move.

Therefore, the plane will continue to gain ground speed since it can clearly out thrust the speed of the conveyor and minimal ground friction that the wheels will cause to even make the conveyor move.

Once it reaches the forward speed that is required for lift off, it will take off whether on a conveyor or stationary runway.

[Lizzerd]

Yes.

Here's why: The plane (let's assume a jet) will be thrust forward with respect to the ambient atmosphere, which is what gives the lift as the air passes over the top surface of the wings. The only difference would be that the wheels would be turning a hell of a lot faster.

In other words, as long as there is thrust, the conveyor would never be able to keep up with the wheels, but it wouldn't matter either way if it could. The plane would fly.

[Don Quixote]

Nope, no lift. Same reason why a plane couldn't take off on the moon, unless the engines could be directed downward. I read it as the plane is standing still relative to a fixed point on the ground.
:\ <-what does this smiley mean anyway?

[Lizzerd]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"


I say yes.

Another way to ask this question where the answer would be no is if the plane was on the conveyor (or not) and there was a giant fan behind it that matched the thrust of its engines. In which case the answer would be no, it could not take off, or move.

[devilmaster]

Think of it this way. Take a treadmill outside and run on it at a full sprint. Do you get extra wind in your face because you're running at top speed in the same spot?

No. And therefore the plane will not take off.

[Turn7]

Think of it this way. Take a treadmill outside and run on it at a full sprint. Do you get extra wind in your face because you're running at top speed in the same spot?

No. And therefore the plane will not take off.

That is because the propulsion force of you running on a treadmill is with the surface of the conveyor.

A plane does not create propulsion via the wheels. If a plane moves forward at 100 mph then it is moving forward regardless if something under the wheels is moving in the other direction. The wheels may be turning 200 mph becuse the forward speed of the plane is 100 mph and the speed of the treadmill is 100 mph in the opposite direction but, the speed of the treadmill does not negate the fact that the plane is moving in the opposite direction at 100 mph.

[Ankf00]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"

yes, because the engines impart the accelerative force, not the plane's wheels.

The relative velocity of the plane vs. ambient air is completely independent of any conveyor in all cases except for when the engines are shut down.

[devilmaster]

That is because the propulsion force of you running on a treadmill is with the surface of the conveyor.

A plane does not create propulsion via the wheels. If a plane moves forward at 100 mph then it is moving forward regardless if something under the wheels is moving in the other direction. The wheels may be turning 200 mph becuse the forward speed of the plane is 100 mph and the speed of the treadmill is 100 mph in the opposite direction but, the speed of the treadmill does not negate the fact that the plane is moving in the opposite direction at 100 mph.

But you're assuming that liftoff is a function of foward movement of the wheels. Which it is not. Liftoff requires a certain speed of the air around the wings for takeoff. There are 2 types of speed pilots deal with. Ground speed and air speed.

They are basically different in the respect that air speed factors in wind speed. For example - a plane's takeoff speed is 150 knots. If the plane is moving forward at 135 and has a 20 knot headwind, it should have enough speed to take off. If there is no wind, it cannot take off at 135. Its ground speed is 135, but with a 20 knot wind its airspeed is 155. Its airspeed that determines lift.

So if you have a conveyor going the opposite direction to the direction of the plane, its ground speed (or more precisely - its speed relative to the ground) is 0. Which means it will need a wind speed of 150 knots to take off.

[nissan gtp]

nope :laugh:

next:

take a skateboard and attach a sail. put an electric fan on the skateboard and turn it on.

does the skateboard move ?

[Robstar]

Think of it this way. Take a treadmill outside and run on it at a full sprint. Do you get extra wind in your face because you're running at top speed in the same spot?

No. And therefore the plane will not take off.

If you were on the treadmill on rollerblades & someone pushed you from behind (like the planes engines) you would go forward as the rollerblade wheels would just spin faster & faster so yes the plane would take off...

[Ed_Severson]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"

Based on the assumption that the plane moves relative to a ground-fixed frame of reference, then sure, the plane can take off. The only effect the conveyor will have is to modify the rolling resistance, which is negligible on an aircraft anyhow. Of course, this assumes that any rotation of the wheels due to the motion of the conveyor will have no effect on the velocity of the aircraft.

On the other hand, if this little riddle is supposed to imply that the plane does not move relative to a ground-fixed frame of reference, but only with respect to the conveyor, then this plane isn't going anywhere. My contention is that this is an impractical form of the question, because the thrust produced by the aircraft would have to be exactly countered by rolling resistance in order for the plane to remain stationary from a ground-fixed frame of reference.

So, the correct answer is that this airplane is going to take off regardless of what you put underneath it, unless the designers ****ed up and didn't achieve enough lift. Poorly worded riddle, though ... looks like whoever wrote it was more interested in watching people on the internets argue about it than in finding the correct answer to whatever scenario he envisioned. :thumdown:

[devilmaster]

Liftoff is a direct result of air flowing over the wings, not thrust from the jets. This is called airspeed.

Thrust from the jets pushes a plane forward relative to a fixed point on the ground, that is ground speed.

Ground speed + wind speed = airspeed.
135 knots + 20 knots headwind = 155 knots airspeed.

If a plane is rolling on conveyor that is rolling the opposite direction and at the same speed, that plane's ground speed is 0. If its ground speed is 0, air is not being pushed over/under the wings. (jet thrust has nothing to do with this - an MD80's engines are not slung under the wings and cannot push air under it).

[indyfan31]

Yes.

Here's why: The plane (let's assume a jet) will be thrust forward with respect to the ambient atmosphere, which is what gives the lift as the air passes over the top surface of the wings. The only difference would be that the wheels would be turning a hell of a lot faster.

In other words, as long as there is thrust, the conveyor would never be able to keep up with the wheels, but it wouldn't matter either way if it could. The plane would fly.

.... THAT's the answer right there.

[nrc]

Are there snakes on the plane?

[Lizzerd]

.... THAT's the answer right there.

Thank you. That aeronautics engineering course I took in college as an elective finally came in handy.

[Ed_Severson]

Liftoff is a direct result of air flowing over the wings, not thrust from the jets.

I would wager that the people who spend their days in cubicles designing aircraft engines would differ with that opinion. No thrust = no flight except in the extreme case where you have Cat 5 wind velocities. :tony:

Ground speed and air speed are equivalent if the wind velocity relative to a ground-fixed frame of reference is zero. Since no wind velocity was given in the problem statement, that seems like a good assumption, don't you think?


If a plane is rolling on conveyor that is rolling the opposite direction and at the same speed, that plane's ground speed is 0.

The only problem with this theory is that the problem states that "the plane moves." Since it doesn't specify that the plane moves with respect to the conveyor only, this would typically be interpreted to mean that the plane moves with respect to a ground-fixed (i.e., universal) frame of reference, in which case the ground speed is most definitely not zero, and neither is air speed.

[racer2c]

Are there snakes on the plane?

:rofl: You need your own HBO special!

[extramundane]

nope :laugh:

next:

take a skateboard and attach a sail. put an electric fan on the skateboard and turn it on.

does the skateboard move ?

Depends...does it have traction control?

[Lizzerd]

Did anybody actually read post #12?

I sent this question to a friend of mine, a retired USAF/Delta Airlines pilot for 39 years. I'll post his reply in its entirety.

[nrc]

:rofl: You need your own HBO special!

After your recent thread on HBO programming I'm not sure if that's a complement or an insult. :)

Dr. Lizardo has the correct answer. Think about the forces on the plane. The engines put 40,000 pounds of thrust on the wings trying to push the plane forward. What is holding it back? Unless the wheels are locked they will turn and the conveyor belt can impart very little force to the plane to resist it moving forward. The only thing having the conveyor belt match the planes speed will do is double the speed that the wheels have to turn on the plane to allow the plane to move forward.

[Ed_Severson]

Pop quiz for devilmaster ...

(Side note: This is not intended to embarass you or call you out -- this is only for instructional purposes, and only because you seem to be the only person who's really interested in discussing this).

The plane is sitting perfectly stationary on the conveyor belt -- zero velocity with respect to a ground-fixed frame of reference. The belt is not moving. The plane's wheels are not rotating. Instantaneously, the plane's engines go from producing zero thrust to producing 50,000 pounds of thrust. Does the plane accelerate with respect to the ground? Why or why not?

[devilmaster]

What gives a plane lift? Air flowing over and under the wings. If there is not enough air flowing over/under the wings, the plane will not take off (or in flight, will not fly)

Ever been on a plane just before it starts its takeoff run? They gun the engines with the brakes on, shouldn't it just take off right there? OF course not, that's why we have runways - or else every airport out there would have conveyors instead of long runways.

Jet thrust has nothing to do with this problem. Lift of a plane is derived from pushing air over/under the wings. A glider has no jet engines, but can fly for hours once its up there.

Another example. Take your car to a dyno shop. Put it on and run the car at 60 mph. The car is stationary but is running at 60 mph. Open a window and put your hand out. Do you have rushing air blow by you like you would on a highway doing 60? Pf course not.

If the basis of the question is that the plane is stationary when you look at it, because the conveyor runs at the exact same speed of the forward momentum of the plane, then its speed relative to normal ground is 0. The ambient air will not start pushing over the wings if the ground speed is 0.

The plane will not take off.

[Ed_Severson]

What gives a plane lift? Air flowing over and under the wings. If there is not enough air flowing over/under the wings, the plane will not take off (or in flight, will not fly)

Nobody is arguing this point, but it's completely inaccurate to say that thrust has nothing to do with this problem. Thrust is a force, and in order to solve for the motion of the aircraft, we have to sum the forces placed on it. Thrust very much does have something to do with this problem.


If the basis of the question is that the plane is stationary when you look at it, because the conveyor runs at the exact same speed of the forward momentum of the plane, then its speed relative to normal ground is 0.

But this is a trivial solution, where nothing moves, ever. If the plane is not moving with respect to a ground-fixed frame of reference, then it has no forward velocity (or momentum, which is not really relevant here), and the conveyor never moves either.

Trust me ... just indulge me for a minute and answer my question above. ;)

[devilmaster]

I stick by my view on this ed.

My understanding of the problem given - a plane is on a conveyor that runs the exact same speed opposite to the forward momentum of the plane, therefore the plane does not look like its moving forward at all if you were standing beside it.

That is the problem as I have understood it. A car on a dyno is the exact same example in this. Put your car on a dyno wheel and run it at 60 mph. Your car doesn't move, you do not feel air rushing by at 60 mph, yet your odometer keeps going up. Since you don't feel air rushing by at 60 mph, a plane won't either. And therefore will not take off.

[Ed_Severson]

I stick by my view on this ed.

That's fine, but it isn't consistent with basic mechanics.

As I said in my first post on the topic, if the plane is stationary with respect to a ground-fixed frame of reference, then you're absolutely right -- the plane will go nowhere. What I'm trying to tell you is that for any normal aircraft, when thrust is applied, the plane won't remain stationary with respect to the ground-fixed frame of reference.

Please, just for the sake of academia, answer the question posed above. In the scenario I gave you, would the plane accelerate with respect to a ground-fixed frame of reference, or would it not?

[coolhand]

[edit] re-read the question. I say no. Liftoff is an equation of wind speed over the wings - not ground speed of vehicle. If there is no wind, the conveyor keeps the foward ground speed of the aircraft at 0.

But, the way i originally read it says yes - According to a book I read about Air America.

One of the pilots that used to fly said that one of the smaller planes, (probably the helio courier) had such a large wing span that it only required something like 35-45 miles ground speed to take off. He said that it was possible to VTOL the aircraft with enough head wind speed.

Even if he was telling a lie - common sense tells us that aircraft carriers turn into the wind to take off. Why? Because head wind speed is added to the speed of aircraft at takeoff.

Back in WWII when the Limeys were still flying the swordfish they were light enough to practacly VTOL. When the carrier turned into the wind in the north atlantic there was enough airspeed for it to do this.

[devilmaster]

Please, just for the sake of academia, answer the question posed above. In the scenario I gave you, would the plane accelerate with respect to a ground-fixed frame of reference, or would it not?

Then we're argueing the question and not mechanics. My take on the question is that the conveyor will match any thrust the plane gives. Therefore, the plane will remain stationary to any fixed frame of reference.

[Ed_Severson]

My take on the question is that the conveyor will match any thrust the plane gives.

By what mechanism, exactly?

[devilmaster]

Back in WWII when the Limeys were still flying the swordfish they were light enough to practacly VTOL. When the carrier turned into the wind in the north atlantic there was enough airspeed for it to do this.

I should correct that story i read. The courier needed 70 knots to take off. So if you had 35 knot head winds and 35 knots ground speed then yes it would take off and it would do it VTOL.

[Robstar]

Another example. Take your car to a dyno shop. Put it on and run the car at 60 mph. The car is stationary but is running at 60 mph. Open a window and put your hand out. Do you have rushing air blow by you like you would on a highway doing 60? Pf course not.

But the wheels on the plane don't propel it...

[Robstar]

http://yoyo.its.monash.edu.au/~tbr/uploads/podge/planeflies.gif

:)

[Ed_Severson]

Eleventy billion pounds of thrust? :rofl:

[racer2c]

I feel stupid.:irked:

It makes perfect sense. Now if this were an automobile whose wheels propel the vehicle, then it would take off at around 32knots. Of course landing is the hard part.:tony:

[devilmaster]

I'll have the balls to hit this thread again and say i'm wrong. But not just yet ;)

Gonna go do some research into the old science laws on this one. (and maybe talk to a few of the superintendants from the school board)

I see what you're getting at though.

[Cam]

I may have been a little off with the wheel/runway friction thing, but I do wonder if the wheel bearings would handle spinning at twice the normal speed without overheating and siezing. :saywhat:

[devilmaster]

Man, I want the Mythbusters to do this now.....

mmmmm... Kari.

[Ed_Severson]

The only law you need to worry about is Newton's 2nd Law of Motion ... F = ma. ;)

If thrust is applied to the plane, the only way that plane remains stationary with respect to a ground-fixed frame of reference is if the thrust is exactly opposed by some other force. The motion of the conveyor does not oppose the thrust -- only force can oppose force -- so the only candidates are static friction (absolute aircraft velocity = 0) and rolling resistance (absolute aircraft velocity != 0).

The static friction case is really no different than a normal runway -- the thrust produced by the airplane will overcome static friction and cause the plane to accelerate from absolute zero velocity. Once the plane is in motion, rolling resistance is the only opposition to the thrust, and rolling resistance does not depend on the relative speeds of the wheel and road, so the conveyor motion does nothing to change the rolling resistance, and we're again back to a normal takeoff scenario.

If the torque applied to the wheels was related to the thrust produced by the engines (similar to a car, where the wheel torque is related to the engine speed) then it would be a different story. But since the wheels aren't driven wheels, they don't have any adverse effects on the motion of the aircraft when thrust is applied.

Trust me, dude. I wouldn't lead you wrong on a mechanics problem. ;) :thumbup:

[Lizzerd]

http://yoyo.its.monash.edu.au/~tbr/uploads/podge/planeflies.gif

:)

And I thank you too for verifying post #10. Exactly what I said the first damn time. :D

But alas, few seemed to listen.

[Lizzerd]

Think of it this way. Take a treadmill outside and run on it at a full sprint. Do you get extra wind in your face because you're running at top speed in the same spot?

No. And therefore the plane will not take off.

Your "thrust" is being provided by your legs. If a plane were to go down the runway "thrust" by its wheels, you have a point. However, if you were on a treadmill with a rocket strapped to your back, no matter how fast you ran, and no matter how fast the treadmill went, would you move?

I rest my case.

[B3RACER1a]

Here's another brain teaser type problem that us engineers love:

A man is sitting in a boat in the middle of a lake. In the boat with the man is a big rock. The man tosses the rock out of the boat into the lake. The rock sinks to the bottom. Does the water level of the lake change? If it does, is the water level higher or lower? Explain. If the water level doesnt change, explain.

[nrc]

Here's another brain teaser type problem that us engineers love:

A man is sitting in a boat in the middle of a lake. In the boat with the man is a big rock. The man tosses the rock out of the boat into the lake. The rock sinks to the bottom. Does the water level of the lake change? If it does, is the water level higher or lower? Explain. If the water level doesnt change, explain.

The rock doesn't sink because the boat is on a conveyor belt. :gomer

The water level is lower.

With the rock onboard the boat must displace it's total mass - including the mass of the rock - to stay afloat. The rock sinks because its volume will not allow it to displace a large enough mass of water to keep it afloat. So less water is displaced and the water level is lower.

[Lizzerd]

Here's another brain teaser type problem that us engineers love:

A man is sitting in a boat in the middle of a lake. In the boat with the man is a big rock. The man tosses the rock out of the boat into the lake. The rock sinks to the bottom. Does the water level of the lake change? If it does, is the water level higher or lower? Explain. If the water level doesnt change, explain.

Throw the rock out of the boat, and level of the lake goes up. The rock weighs more by volume (which is why a rock doesn't float) than water. The amount of water displaced by the rock in the boat would be much more than the amount of water replaced by the rock if it was on the bottom of the lake.

[Lizzerd]

Here's another one for you.

A ship is docked. There is a rope ladder hanging down the side. Each rung on the ladder is one foot from the next one. There are 12 rungs of the ladder above water at high tide. The tide goes out at one half foot per hour. How many rungs of the ladder will be above the ocean at low tide, six hours later?

[chop456]

Uh...12?

Unless the mooring rope is too tight. :D

[Indy]

If the plane is a single engine jet, and Tony George stands behind it, snorting the jet exhaust through a straw, will the plane take off? :tony:

[Robstar]

He wouldn't let it... It may have Charlie on board as the fumes just aren't doing it for him...

[Warlock!]

Ok... say you're drivin' in your minivan and a fly buzzes around your right ear. You shoo it away and it comes right back, continuing to buzz around your right ear. After a half-dozen shoo-ings and the stoopid fly keeps coming back to buzz your ear, you can't take it anymore.

If you slam on the brakes as hard as you can (taking care that no one is behind you) will the fly be flung forward into the rearview mirror, knocking him unconscious, allowing you to tear his wings and legs off and sit him on your dash so that when he regains conciousness the first thing he sees will be you looking at him?

You then can flip him off and slowly bring your index finger towards him, resting it on his back so he can anticipate what you are planning to do, then slowly crush him in a way that you can hear each piece of his exoskeleton crack before the yellow stuff comes out the tip of his abdomen.

Can this be done?

[Turn7]

House fly or horse fly?:irked:

[chop456]

http://www.brightlightsfilm.com/18/18_images/18_blax2x.jpg

[Ankf00]

european or afric... oh wait, wrong flying critter. :gomer:

[Hard Driver]

Skipped the whole thread, but the answer is:

Impossible, ficticious situtation.

You see the thrust of the plane is generated through the engine that work on the air, not the ground. SO the conveyor belt does nothing.

Imagine, a car on a rolling dyno. If you speed up the wheels the dyno goes faster, because force is being applied BY the wheels to the dyno. If you take the same car on the same dyno and push it from the bumber, you will push it right off the dyno. Why, the force is being applied to the body, not the wheels.

A Plane has it's force applied to the body through the engines. The engines use AIR to create the force, whether prop or jet. The wheels simply roll, but have no propulsion. SO when the engines ramp up, the plane will move forward, no matter how fast the dyno turns, the plane will move forward. Whether the conveyor is running at the same speed of the plane, in reverse, at a standstill, it doesn't matter. The wheels just hold the plane up and keep it from scraping along the ground. THey do not have anything to do with taking off. That is just the windspeed over the wings.

[Wheel-Nut]

Ok... say you're drivin' in your minivan and a fly buzzes . . . .

Minivan? :laugh:

[Ankf00]

Skipped the whole thread, but the answer is:


something that's been stated 03q284083243 times in the thread already :gomer: :)

[racer2c]

I have the answer! The wheels aren't producing the force! It's the thrust from the jets/prop! The wheels and conveyor belt are irrelevent! The plane would take off as normal!

[nrc]

Minivan? :laugh:

Classic post:
http://www.offcamber.net/forums/showthread.php?p=54125#post54125

[Joelski]

You guys are all on cheap drugs for arguing this point this long.:rofl: Regardless of how much forward motion the plane achieves by thrust alone, it's mass will prevent it from taking off because the conveyor belt will limit it's ability to generate takeoff speed. Eventually, the belt will be able to compensate for any forward motion and the plane will come to a stop as long as the belt is long enough to contain the plane's movement. Given a scenario with a very short belt, the plane might be able to blow itself off the belt with a blast of thrust against locked brakes. Thus, the conclusion is there is not enough date to formulate a concrete conclusion.

[racer2c]

Classic post:
http://www.offcamber.net/forums/showthread.php?p=54125#post54125

Best post evah!:thumbup: :rofl:

[Joelski]

AHA! It will take off if it's a propeller driven aircraft! Missed the the first time.:p

[Joelski]

DM and I both got roped into thinking "jet" immediately. You cheap bastages need to get with progress. :D

[Wheel-Nut]

Best post evah!:thumbup: :rofl:


I laughed again!! :rofl: BTW . . . we still don't own a minivan!

[Ankf00]

DM and I both got roped into thinking "jet" immediately. You cheap bastages need to get with progress. :D

what does it matter if it's a jet or a prop? it's force imparted at some point along the plane's fuselage or wings, but none the less completely independent of the ground.

and the locked brakes are a trivial solution, they address a singular case instead of the entire range of possible velocity...

[Gangrel]

I stick by my view on this ed.

My understanding of the problem given - a plane is on a conveyor that runs the exact same speed opposite to the forward momentum of the plane, therefore the plane does not look like its moving forward at all if you were standing beside it.

That is the problem as I have understood it. A car on a dyno is the exact same example in this. Put your car on a dyno wheel and run it at 60 mph. Your car doesn't move, you do not feel air rushing by at 60 mph, yet your odometer keeps going up. Since you don't feel air rushing by at 60 mph, a plane won't either. And therefore will not take off.

The thrust of the car is exerted directly on the dyno through the wheels. What you are missing in this equation is that the thrust of the airplane is not exerted through the wheels, it is exerted on the atmosphere behind the engines. The wheels do not factor into the thrust.

You are correct that thrust does not make the plane take off, airflow above and below the wings makes the plane take off. However, thrust moves the plane foward, creating a significant amount (nearly all in the case of a headwind, truely all in the case of no wind or a tailwind). Since the wheels are only a factor in thrust in that they allow the plane to move freely on the ground, what is the conveyor belt going to accomplish? The plane moves relative to the atmosphere, not relative to the object under the wheels. The conveyor will not accomplish the mission of keeping the plane stationary relative to the ground. It will only cause the wheels to move twice as fast (i.e. engines thrust the plane 100 mph forward, conveyor moves 100 mph backwards, plane actually moves 100 mph, wheels sping 200 mph). You are getting caught up in the plane staying stationary to the earth, but it wouldn't work that way. The plane moves relative to the earth, regardless of what the conveyor belt is doing.

[Gangrel]

Here's another brain teaser type problem that us engineers love:

A man is sitting in a boat in the middle of a lake. In the boat with the man is a big rock. The man tosses the rock out of the boat into the lake. The rock sinks to the bottom. Does the water level of the lake change? If it does, is the water level higher or lower? Explain. If the water level doesnt change, explain.

Water level does not change because the water was already displacing for the rock when it was in the boat. The force exerted downward by gravity's pull on the rock caused the boat to sit lower in the water, displacing water and raising the level of the lake. When the rock is removed from the boat and thrown into the water, the boat sits higher in the water, displacing less water. Net effect: No change in water level.

[Insomniac]

"A plane is standing on a runway that can move (some sort of band conveyer). The plane moves in one direction, while the conveyer moves in the opposite direction. 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 the opposite direction). Can the plane take off?"


I say yes.

No. The lift is provide by the air moving under and over the wings. If the plane is stationary, there is 0 lift to overcome gravity.

[Gangrel]

No. The lift is provide by the air moving under and over the wings. If the plane is stationary, there is 0 lift to overcome gravity.

We've already covered this, bud...the conveyor belt can't make the plane remain stationary because the wheels do not provide the propulsion. The plane takes off, but the wheels are moving twice as fast as the ground speed. Keep up with the conversation. :D

[velocity boy]

When the rock is in the boat it is displacing its own weight in water, maximum displacement of water possible for the rock, when the rock is in the lake it is only displacing its own volume of water. If the rock's volume were say 1L and it weighed 2kg it would displace 2L of water when in the boat and only 1L of water at the bottom of the lake.
Less water displaced, lower water level of the lake.

[Insomniac]

We've already covered this, bud...the conveyor belt can't make the plane remain stationary because the wheels do not provide the propulsion. The plane takes off, but the wheels are moving twice as fast as the ground speed. Keep up with the conversation. :D

Hehe, I'm still not convinced. :) What happens to the plane if there is no thrust and the coveyer starts moving?

[Insomniac]

When the rock is in the boat it is displacing its own weight in water, maximum displacement of water possible for the rock, when the rock is in the lake it is only displacing its own volume of water. If the rock's volume were say 1L and it weighed 2kg it would displace 2L of water when in the boat and only 1L of water at the bottom of the lake.
Less water displaced, lower water level of the lake.

It took a riddle to get this boy to post after 2.5 years!

[Insomniac]

Hehe, I'm still not convinced. :) What happens to the plane if there is no thrust and the coveyer starts moving?

If you stand on a treadmill with rollerblades on you will roll off the back end of it. You would need to hold onto something normally to stay where you were. So it makes sense that some force pushing you forward (equal to the same force needed to hold on) would be able to keep you where you were.

[velocity boy]

Ya, I know... but if you include all my posts from all my years lurking at sector 7G I probably have 5 or 6 total by now.:gomer:

[Joelski]

what does it matter if it's a jet or a prop? it's force imparted at some point along the plane's fuselage or wings, but none the less completely independent of the ground.

and the locked brakes are a trivial solution, they address a singular case instead of the entire range of possible velocity...

DM is correct: You need air movement over the wings in sufficient quantity to overcome the effect of the plane's own mass. If you break the rule and add so much thrust that mass is irrelevant, you have a rocket, not a plane.

Hence, a propeller-driven aircraft can possibly achieve this. Where is rocketdoc?

[Joelski]

We've already covered this, bud...the conveyor belt can't make the plane remain stationary because the wheels do not provide the propulsion. The plane takes off, but the wheels are moving twice as fast as the ground speed. Keep up with the conversation. :D

Even on a catapault, the plane has to roll (on it's wheels) gaining forward momentum to achieve takeoff velocity. The wheels reduce the drag on the mass; look at floatplanes for another example, but a plane needs forward momentum to take off. Increase the drag (skis) when was the last time you saw one of those?

[Ed_Severson]

DM is correct: You need air movement over the wings in sufficient quantity to overcome the effect of the plane's own mass.

Fascinating stuff. It'd be even more entertaining if only every single person in the thread wasn't already aware of that. :tony:

Lift will be achieved in sufficient quantities. Resistance is futile. :thumbup:

[Joelski]

Fascinating stuff. It'd be even more entertaining if only every single person in the thread wasn't already aware of that. :tony:

Lift will be achieved in sufficient quantities. Resistance is futile. :thumbup:So what's your point? The plane won't take off if it's a jet. Do you work for airbus or something? :gomer:

[Wheel-Nut]

. Resistance is futile. :thumbup:


No, Foyt is Futile. :D

[Ed_Severson]

So what's your point? The plane won't take off if it's a jet.

Utterly ridiculous. Jet or prop, it makes no difference. Force is force, regardless of where it comes from.

[Joelski]

Utterly ridiculous. Jet or prop, it makes no difference. Force is force, regardless of where it comes from.

If it takes off without lift, it's a rocket, not an airplane. Aside from that fact, without trajectory, it won't go in the air. :gomer:

[Ankf00]

DM is correct: You need air movement over the wings in sufficient quantity to overcome the effect of the plane's own mass. If you break the rule and add so much thrust that mass is irrelevant, you have a rocket, not a plane.

Hence, a propeller-driven aircraft can possibly achieve this. Where is rocketdoc?

you don't get it. the propellor is not designed to move air over the wings. the propellor is designed to provide thrust, to impart a forward force.



The conveyor can move at 1000 mph, but the conveyor cannot move until the plane moves. Assuming the conveyor's speed instantaneously matches the plane's speed, the plane is still moving through the air.

The frame of reference for both airspeed & conveyor speed is the fixed-ground. If the plane isn't moving anywhere, neither is the conveyor.

If the plane is moving 5mph forward wrt ground, then the conveyor is moving -5mph wrt ground. But it doesn't matter how fast the conveyor goes, rolling friction is so negligible it will never matter until the wheel bearings burn away. The plane will keep increasing in velocity wrt ground, the conveyor will match, but this will not affect the thrust imparted by the jet engines on the plane resulting in further acceleration.

[Ed_Severson]

If it takes off without lift, it's a rocket, not an airplane.

Nobody believes it's taking off without lift but you. Jet or prop doesn't matter. Either way, the plane moves and lift is achieved.

Take a mechanics course and get back to us. :tony:

[Ankf00]

So what's your point? The plane won't take off if it's a jet. Do you work for airbus or something? :gomer:

Yes it will, propellors don't provide enough airflow over the wing's surface area to generate sufficient lift.

Prop or jet, lift is generated independent of the engine exhaust. The lift generated by the exhaust flow of a prop is negligible compared to the rest of the wing's surface.


Do you work for Ganassi or something? :gomer:

[Ed_Severson]

I'd like to propose a moratorium on use of the following words until those using them demonstrate an understanding of their meanings:

-momentum
-drag
-trajectory

[Ankf00]

Actually, if the conveyor instantaneously matches speed, how can you ever gain any velocity wrt ground?

Now if the conveyor is controlled by a control system, you have response time & system lag/error and the conveyor can never match the jet's speed beyond it's initial lag if it's a 2nd order? controller...

This thread has involved more thinking on my part than the past 3 months of work :laugh:

[Joelski]

Gravity exerts the same influence on a mass at 170 MPH as it does at 0 MPH (Just for you, Ed :gomer: ), so, if you negate a planes forward momentum (with a super-duper ultra converyor belt), how will it move? How will it overcome the effect of gravity, and if it does, how will it stay aloft?

KK and GF have already proven this theory; you can not reason with anyone from Gomer Central.

:D

[racer2c]

I'd like to propose a moratorium on use of the following words until those using them demonstrate an understanding of their meanings:

-momentum
-drag
-trajectory

What's 'moratorium' mean?:)

[racer2c]

Gravity exerts the same influence on a mass at 170 MPH as it does at 0 MPH (Just for you, Ed :gomer: ), so, if you negate a planes forward momentum (with a super-duper ultra converyor belt), how will it move? How will it overcome the effect of gravity, and if it does, how will it stay aloft?

KK and GF have already proven this theory; you can not reason with anyone from Gomer Central.

:D

Psst, you're debating actual engineers.;)

[Ed_Severson]

Gravity exerts the same influence on a mass at 170 MPH as it does at 0 MPH

What, are we just saying obvious things now?

It's Thursday!


if you negate a planes forward momentum (with a super-duper ultra converyor belt)

Please explain, in detail, the mechanism by which this occurs.

[Gangrel]

Hehe, I'm still not convinced. :) What happens to the plane if there is no thrust and the coveyer starts moving?

Problem states the conveyor moves as fast as the plane in the opposite direction. If there is no thrust, the plane isn't moving, so conveyor isn't moving either. ;)

[Ed_Severson]

What's 'moratorium' mean?:)

mor‧a‧to‧ri‧um -- a suspension of activity. :)

[Joelski]

Actually, if the conveyor instantaneously matches speed, how can you ever gain any velocity wrt ground?

Now if the conveyor is controlled by a control system, you have response time & system lag/error and the conveyor can never match the jet's speed beyond it's initial lag if it's a 2nd order? controller...

This thread has involved more thinking on my part than the past 3 months of work :laugh:


The original question implied that God designed the conveyor and it reacts at teh exact same time as the wheels start to spin. No, God doesn't have an AE degree either..:yuck:

[Gangrel]

So what's your point? The plane won't take off if it's a jet. Do you work for airbus or something? :gomer:

You guys are still stuck in the rut of thinking the plane remains stationary while on the conveyor. It won't. The plane moves forward regardless of the conveyor.

[Joelski]

What, are we just saying obvious things now?

It's Thursday!



Please explain, in detail, the mechanism by which this occurs.

Please read and understand my second post.

[Ed_Severson]

Please read and understand my second post.

Your second post does not explain the mechanism by which the conveyor belt keeps the plane stationary. Neither do any of your other posts.

Newton's Second Law ... look into it. :thumbup: