Will it take off?

[Jeff Matthews]

You haven't read the thread yet, have you? [6]

Okay, CTBB, I haven't. Not to be hard-headed, but I don't want to take the effort to find the needle in the haystack. Just spit it out.

But, you'll be wrong.

[Jay481985]

Exactly, Jay. Where people (some) are getting confused is that they think that the thrust of the jet engine works on the air and NOT the ground. Let's get precise.

The jet engine pushes against the air SO THAT THERE CAN BE FORWARD MOVEMENT ON THE GROUND. It is not a direct relation, but is indirect.

a bit broad. with that statement you akin the airplane to a rocket in which they are similar but different at the same time, the plane has wings to cause lift

If you don't get moving forward, the plane will not fly. Pure and simple.

that statement is a bit to broad. take for instance a kite, it will fly even though not technically moving (the tether keeps the kite actually flying) also baloons but that goes into to bouyancy. Also the F-15 I stated before technically can lift off and it is a plane. If you put it on a vertical launcher (something like Nasa Spaceshuttle) it would get airborne due to the fact it can overcome its own weight, most planes do not overcome their own weights purely off the thrust of the engines.

Some people have a perception that with no thrust, the jet sits still on the treadmill. Wrong, the weight of the jet, along with coefficients of friction mean the jet moves backward with the treadmill if the thrust is not on.

but the bearing in the wheels will allow for a lot of movement. If you had a perfect bearing, no friction the plane would indeed be stationary though

Really, this problem is simple. No matter how fast you want to run on a treadmill, the wind will not blow any faster. Period.

but a person is not a fan, while a jet plane is a fan capable of moving the wind faster

[Jeff Matthews]

Solve the problem this way:

It's a still day. The treadmill is going 100 mph. The jet engines are not thrusting. The jet thus, move backwards at 100 mph. Therefore, there is a relative backwards wind at 100 mph. (You all are not accounting for that).

Now, if the plane needs 100 mph forward wind to achieve lift, the plane needs to have thrust enough to generate 200 mph to counteract the 100 mph backward wind and achieve 100 mph forward wind.

But.... when the plane accelerates to achive this 200 mph, the treadmill speeds up to 200 mph.

The goofy idea that some of you have is that a plane can move backwards and still lift.

Let's start Aggie Airlines, where the thrusters are put on backwards, and the pilot hits the brakes for take-off! []

[Jeff Matthews]

Exactly, Jay. Where people (some) are getting confused is that they think that the thrust of the jet engine works on the air and NOT the ground. Let's get precise.

The jet engine pushes against the air SO THAT THERE CAN BE FORWARD MOVEMENT ON THE GROUND. It is not a direct relation, but is indirect.

a bit broad. with that statement you akin the airplane to a rocket in which they are similar but different at the same time, the plane has wings to cause lift

But where are you going to get your wind? At least you realize you need wind, so we're half-way there.

If you don't get moving forward, the plane will not fly. Pure and simple.

that statement is a bit to broad. take for instance a kite, it will fly even though not technically moving (the tether keeps the kite actually flying) also baloons but that goes into to bouyancy. Also the F-15 I stated before technically can lift off and it is a plane. If you put it on a vertical launcher (something like Nasa Spaceshuttle) it would get airborne due to the fact it can overcome its own weight, most planes do not overcome their own weights purely off the thrust of the engines.

The tether keeps the kite flying because wind is passing under its wing. When you hold the kite STILL by hanging on to the tether, you create a relative wind force. If you let go of the tether, the kite will fall. Bouyancy has nothing to do with anything. It is the relative weight of the gases that is involved, and not relative wind speed. If you want to talk about firing off a rocket, you go beyond the problem. Lift off occurs because of the thrust against GRAVITY and not the wind. That is entirely beyond the scope of the problem.

Some people have a perception that with no thrust, the jet sits still on the treadmill. Wrong, the weight of the jet, along with coefficients of friction mean the jet moves backward with the treadmill if the thrust is not on.

but the bearing in the wheels will allow for a lot of movement. If you had a perfect bearing, no friction the plane would indeed be stationary though

The bearing quality has nothing to do with it. The bearing accounts for how easily speed can be achieved and maintained. Remember, the problem, as presented, mandates offsetting speeds.

Really, this problem is simple. No matter how fast you want to run on a treadmill, the wind will not blow any faster. Period.

but a person is not a fan, while a jet plane is a fan capable of moving the wind faster

A jet engine thrusts against the wind. People do, too. So, do cars. Ever notice when you sitting at a stop light, it always seems to be raining softer than when you're traveling down the freeway? The idea is to gain enough speed relative to the air. If you could run real fast and push the jet, it would take off. It has nothing to do with the jet engine. It is all about wind speed. The jet engine is only for raw forward power.

See the above.

[oldtimer]

I'll just repeat my question for dwilawyer:

How does the conveyor impart any horizontal force to the body of the plane that opposes the plane's thrust?

You're under the misconception that the plane remains stationary.

The real question here is does the plane remain stationary? Lets take the example of thrust regarding a rocket. The rocket's thrust overcomes gravity to propel it into the atmosphere. The thrust on the plane likewise propels the plane fast enough to create airflow over the wings creating lift. It overcomes the friction on the ground caused by the weight of the plane to propel it forward. Once the thrust is enough to counteract the friction of weight (gravity) it will move forward. It does not matter if the plane has wheels or skids or is on a rail. It moves forward, creating lift, and takes off. With enough thrust the plane does not remain stationary because it overcomes the friction of its own weight (yes gravity).

The car on the dyno does not move forward because the thrust of the car is sent through the wheels. If you can spin wheels on your car you know this intrinsically. The car will not move if the wheels have no traction. A plane does not rely on traction. That's why it can take off on ice as long as it goes straight enough for proper airflow over the wings. Does this make sense?

Jeff, here is how I understand it.

[Jay481985]

Exactly, Jay. Where people (some) are getting confused is that they think that the thrust of the jet engine works on the air and NOT the ground. Let's get precise.

The jet engine pushes against the air SO THAT THERE CAN BE FORWARD MOVEMENT ON THE GROUND. It is not a direct relation, but is indirect.

a bit broad. with that statement you akin the airplane to a rocket in which they are similar but different at the same time, the plane has wings to cause lift

But where are you going to get your wind? At least you realize you need wind, so we're half-way there.

• the jet engines

If you don't get moving forward, the plane will not fly. Pure and simple.

that statement is a bit to broad. take for instance a kite, it will fly even though not technically moving (the tether keeps the kite actually flying) also baloons but that goes into to bouyancy. Also the F-15 I stated before technically can lift off and it is a plane. If you put it on a vertical launcher (something like Nasa Spaceshuttle) it would get airborne due to the fact it can overcome its own weight, most planes do not overcome their own weights purely off the thrust of the engines.

The tether keeps the kite flying because wind is passing under its wing. When you hold the kite STILL by hanging on to the tether, you create a relative wind force. If you let go of the tether, the kite will fall. Bouyancy has nothing to do with anything. It is the relative weight of the gases that is involved, and not relative wind speed. If you want to talk about firing off a rocket, you go beyond the problem. Lift off occurs because of the thrust against GRAVITY and not the wind. That is entirely beyond the scope of the problem.

• umm the kite is a wing, the tether keep it stationary so in fact it is a sail.
• i was talking about bouancy as another way to get airborne, I think you were confused with why I was mentioning that.
• Rockets and jet planes have similarities enough to be introduced into the concept of getting the answer
• yes the rocket most overcome its own weight and gravity

Some people have a perception that with no thrust, the jet sits still on the treadmill. Wrong, the weight of the jet, along with coefficients of friction mean the jet moves backward with the treadmill if the thrust is not on.

but the bearing in the wheels will allow for a lot of movement. If you had a perfect bearing, no friction the plane would indeed be stationary though

The bearing quality has nothing to do with it. The bearing accounts for how easily speed can be achieved and maintained. Remember, the problem, as presented, mandates offsetting speeds.

• Again I was just stating if there were a perfect bearing........ and no a bearing does not account for how easily it can be achieved as that would be counter to what a bearing does.

Really, this problem is simple. No matter how fast you want to run on a treadmill, the wind will not blow any faster. Period.

but a person is not a fan, while a jet plane is a fan capable of moving the wind faster

A jet engine thrusts against the wind. People do, too. So, do cars. Ever notice when you sitting at a stop light, it always seems to be raining softer than when you're traveling down the freeway? The idea is to gain enough speed relative to the air. If you could run real fast and push the jet, it would take off. It has nothing to do with the jet engine. It is all about wind speed. The jet engine is only for raw forward power.

• a jet engine creates thrust in which the wings create lift in order to achieve flight.
• I do not quite undersand the driving in the rain example. The reason why it seems to rain harder when you are moving as opposed to the standing still is a simple idea. The basis is that on average if you were to warden off say 100 cubic feet of space, there are only so many rain drops within that space, but since you are traveling you will ignore the 100 cubic feet because you are moving, its not the fact its raining harder when you move and the clouds hate you, its the fact that you are hitting more planes (vertical planes) then when you are standing still.

See the above.

[sputnik]

You haven't read the thread yet, have you? [6]

Okay, CTBB, I haven't. Not to be hard-headed, but I don't want to take the effort to find the needle in the haystack. Just spit it out.

But, you'll be wrong.

Jeff,

Read through the thread. You're analyzing the problem incorrectly and just embarrassing yourself.

[Jeff Matthews]

I'll just repeat my question for dwilawyer:

How does the conveyor impart any horizontal force to the body of the plane that opposes the plane's thrust?

You're under the misconception that the plane remains stationary.

The real question here is does the plane remain stationary? Lets take the example of thrust regarding a rocket. The rocket's thrust overcomes gravity to propel it into the atmosphere. The thrust on the plane likewise propels the plane fast enough to create airflow over the wings creating lift. It overcomes the friction on the ground caused by the weight of the plane to propel it forward. Once the thrust is enough to counteract the friction of weight (gravity) it will move forward. It does not matter if the plane has wheels or skids or is on a rail. It moves forward, creating lift, and takes off. With enough thrust the plane does not remain stationary because it overcomes the friction of its own weight (yes gravity).

The car on the dyno does not move forward because the thrust of the car is sent through the wheels. If you can spin wheels on your car you know this intrinsically. The car will not move if the wheels have no traction. A plane does not rely on traction. That's why it can take off on ice as long as it goes straight enough for proper airflow over the wings. Does this make sense?

Jeff, here is how I understand it.

Hey, Oldie. Good to hear from you. The flaw in the reasoning above is this:

"The rocket's thrust overcomes gravity to propel it into the atmosphere. The thrust on the plane likewise propels the plane fast enough to create airflow over the wings creating lift. It overcomes the friction on the ground caused by the weight of the plane to propel it forward. Once the thrust is enough to counteract the friction of weight (gravity) it will move forward."

This is all true, EXCEPT, if you read carefully, you will see that it gets the cart before the wheel.

"Once the thrust is enough to counteract the friction of weight (gravity) it will move forward." This assumes a constant treadmill speed. It will move forward IF the increased thrust is not OFFSET by an increase in the antagonistic speed of the treadmill.

The only way it would work is if the treadmill pulled the air with it. But it does not. Think "Vectors," Victor!

Using the same explanation, you can see that there are VARIABLE amounts of thrust needed. If the treadmill is moving against the plane's direction at 100 mph, how much thrust is needed to make it move forward? The obvious - AND INDEED ONLY - answer is "more than 100 mph of thrust."

But what happens when, as you increase thrust, the treadmill speeds up against you? You need yet MORE AND MORE THRUST. If the added amounts of thrust are infinitley offset, you go nowhere.

Suppose there is NO WIND on a given day. You want to fly a kite. Do you think if you get on a treadmill and run with the kite string in hand, the kite will fly?

[Jeff Matthews]

You haven't read the thread yet, have you? [6]

Okay, CTBB, I haven't. Not to be hard-headed, but I don't want to take the effort to find the needle in the haystack. Just spit it out.

But, you'll be wrong.

Jeff,

Read through the thread. You're analyzing the problem incorrectly and just embarrassing yourself.

Just point it out. Why send me to look for a needle in a haystack? The thread is long. Do me a favor since you know where it is.

[oldtimer]

Some people have a perception that with no thrust, the jet sits still on the treadmill. Wrong, the weight of the jet, along with coefficients of friction mean the jet moves backward with the treadmill if the thrust is not on.

Actually, if the plane has no thrust it will sit still, and therefore the conveyor belt will not move. The problem states that the conveyor matches the speed of the plane: 0=0.

[oldtimer]

"Once the thrust is enough to counteract the friction of weight (gravity) it will move forward." This assumes a constant treadmill speed. It will move forward IF the increased thrust is not OFFSET by an increase in the antagonistic speed of the treadmill.

I believe your logic is flawed. The tread mill can spin as fast as it wants. The wheels can spin as fast as they want. Once the gravity is overcome the plane will move forward because the conveyor belt will be unable to counteract the plane. The belt/wheel interaction cannot offset this much thrust. Edit: This is because the engine does not depend on the wheels to push the plane forward. Tangential interaction with the belt does not counteract the engines thrust once the thrust is enough to overcome the weight of the plane.

[Jeff Matthews]

In physics, the problem is one of vectors. A vector is a combination of magnitude (i.e. speed) and direction. It's solved like this:

Let's say you have a wind speed of 40 mph. That is NOT a vector. It is only a magnitude. When you say a wind speed of 40 mph east, then, you have a vector.

40 mph east would be represented by an arrow, like this: -------------------->

80 mph east would be represented by an arrow, like this: --------------------------------------->

If you had 2 separate winds blowing, one 40 mph east, and the other 80 mph east, the resultant vector is a 120 mph wind blowing east (all you do is add them).

Now, say you have an 80 mph wind blowing east, and a 20 mph blowing west. The resultant is a 60 mph wind blowing east. So, these two winds have the same effect as a single 60 mph wind blowing east.

There's Vectors 101 for you.

Now, we need a certain magnitude AND DIRECTION of wind in order to lift a jet. Let's say 100 mph headwind.

Using that example, let's assume it is a still day. So, if you move that jet 100 mph in ANY DIRECTION, it will lift.

Let's now assume, the wind is aimed in the same direction of the plane and is blowing 40 mph. Now, the plane needs to go 140 mph to lift. That is because we need a 100 mph headwind, but we are needing to first offset a 40 mph tailwind.

Let's assume the wind is blowing opposite the plane's direction by 20 mph. Now, we need only 80 mph of speed from the plane to get our 100 mph headwind.

Let's now get back to a still day with a treadmill. The wind is NOT blowing. We get on a treadmill and stand still. If the treadmill is moving us backwards 20 mph, we have.... a 20 mph wind. It would be a tail wind. If we want to get back to having no tail wind, we need to start running on that treadmill and achive a running speed of 20 mph. Then, we will offset the tail wind, and experience a resultant zero wind.

Now, let's say we do not want to run. Let's put on roller skates. Will it make any difference? No, we need to skate 20 mph to offset the 20 mph treadmill and have zero resultant wind.

Let's say we don't want to have to exert effort by skating, so we keep the skates on and we attach a mini jet engine backpack. We crank the thrust to achieve the 20 mph speed. Still, the resultant wind is the same - zero. It does not matter what type of propulsion we use, the result is the same.

If we crank the backpack up to achieve 30 mph on our 20 mph treadmill, we have a resultant headwind of 10 mph. To fly a plane, the example I used above requires a 100 mph headwind to achieve lift. So, if the treadmill is moving us backward at 20 mph., if we use enough thrust to achieve 120 mph, then, we will begin to rise because the resultant vector is a 100 mph headwind.

But what if as we keep adding thrust to outpace the treadmill and gain this headwind, the treadmill keeps speeding up against us. The tail wind created by the treadmill offsets any of our gain, and we never achieve the amount of headwind we need. In fact, in the example, there will never be any headwind or tailwind because the opposing movement of the treadmill is always EQUAL to the thrust of the jet engine. Therefore, the resultant is always ZERO - a perfect calm.

Try Googling on vectors, and you will see.

But, Hey! If I'm wrong, I'm wrong. Somebody just show me. It has nothing to do with pride.

[Jeff Matthews]

Some people have a perception that with no thrust, the jet sits still on the treadmill. Wrong, the weight of the jet, along with coefficients of friction mean the jet moves backward with the treadmill if the thrust is not on.

Actually, if the plane has no thrust it will sit still, and therefore the conveyor belt will not move. The problem states that the conveyor matches the speed of the plane: 0=0.

Actually, you're 100% right on that, but it does not change the result. The plane will not fly.

[Jeff Matthews]

"Once the thrust is enough to counteract the friction of weight (gravity) it will move forward." This assumes a constant treadmill speed. It will move forward IF the increased thrust is not OFFSET by an increase in the antagonistic speed of the treadmill.

I believe your logic is flawed. The tread mill can spin as fast as it wants. The wheels can spin as fast as they want. Once the gravity is overcome the plane will move forward because the conveyor belt will be unable to counteract the plane. The belt/wheel interaction cannot offset this much thrust. Edit: This is because the engine does not depend on the wheels to push the plane forward. Tangential interaction with the belt does not counteract the engines thrust once the thrust is enough to overcome the weight of the plane.

But you assume "once the gravity is overcome......." How do we overcome it? With lift. How do we get lift? With a headwind. How do we get this headwind?

Now, the engine does not depend on the wheels, but it is irrelevant. You have to get lift in the first place. Where will you achieve this? You need wind. How will you get it?

Research vectors.

[oldtimer]

See my edit in my previous post for why the belt has no effect against the plane once the thrust required to overcome what should be a constant number is achieved. Try to think of a frictionless environment, because that is essentially what happens.

[oldtimer]

You will get lift because the tangent between the plane's wheels and the belt does not have any effect against the plane's propulsion once enough thrust is made.

[Jeff Matthews]

Where people are getting confused is they think the wheels are irrelevant. They are perfectly relevant.

Let's say, again, you need a 100 mph headwind. That means, on a still day, you need to achieve 100 mph speed. Suppose there were no wheels. How much thrust would you need to get that plane to achieve 100 mph on its belly? I bet it will be a he77 of alot more.

All the wheels do is enable the plane to reach a speed with less thrust than would otherwise be needed. You need to stop thinking wheels and start think "offset speed." The thrust increases to increase the speed, but the increased speed is OFFSET by a counter-speed.

Vectors..... Thrust is a vector. It has speed and direction. A treadmill, just like wind, can offset thrust. The problem creates a scenario where the resultant thrust is always zero.

[oldtimer]

But you assume "once the gravity is overcome......." How do we overcome it? With lift. How do we get lift? With a headwind. How do we get this headwind?

Now, the engine does not depend on the wheels, but it is irrelevant. You have to get lift in the first place. Where will you achieve this? You need wind. How will you get it?

Thrust is how you achieve this. How would the plane take off on ice, assuming it can go straight enough long enough? Thrust overcomes the coefficient of friction right? Why do you think the conveyor can oppose thrust through a spinning wheel, once the magic number of thrust is achieved? This question has been discussed in this thread.

[oldtimer]

The wheels are relevant to a point, Jeff. Just as the ice is relevant to a point, just as sliding on its belly is relevant to a point.

[oldtimer]

Thr treadmill counteracts the runner's thrust against the ground. The belt can not counteract thrust over and above the the friction of the wheels, because the power from the engine is directed against the air not the ground. There is a limit after which the belt will have no effect.