Will it take off?

[Jeff Matthews]

Just remember when test-taking, the first guess is the best guess. The plane will not fly. I liked Travis's effort to use the bicycle example where you stand in the street pushing the bike along a moving sidewalk. But that example was faulty. He asserted you can achieve forward motion while pushing the bike with free-spinning wheels on the conveyor belt. IF YOU MOVE FORWARD against the force of the conveyor belt, your speed opposite the conveyor belt EXCEEDS the conveyor belt speed by the speed at which you move.

Some of you might remember these back in the school days as VECTORS. Anybody remember that a "resultant vector" is just the sum of one or more vectors? Then, we had all those airplane problems like "If John is flying due East in a jet at 300 mph, and there is a tailwind blowing precisely North at 25 mph, how fast is the plane going East?"

Well, let's not make that the question (because you'll attack the wording, but you get the idea).

In our example, the plane has zero speed relative to the earth. It will not fly because even a strong wind that has a relative speed of 30-40 mph. is not enough to give lift. (Andf if I recall, the conveyor belt accounts for wind - nullifying it as well).

Where you "pro-yessers" are wrong is that you assume the jet engine will push the jet along the conveyor belt DESPITE THE WHEELS MOVING AT THE PRECISE OPPOSITE MAGNITUDE AS THE BELT.

When the wheels (which are affixed to the plane) rotate at the exact opposite magnitude as the belt, the plane does not move. You can hop behind the plane and give it a push, but the conveyor belt just adjusts to the speed of your pushing - and still, NO MOVEMENT.

You can add a jet engine to - according to popular belief - OUTSPEED this conveyor belt - and still, the conveyor adjusts its speed. That's right. The vector of the conveyor belt adjusts to OFFSET the vector of the jet engine. The resultant vector is ZIPPO!

The plane will not fly.

[Jeff Matthews]

A couple more thoughts for clarity (hopefully). Back to Travis's example, if you had a conveyor belt moving backwards at 100 mph, and the plane (with its free-spinning wheels) is sitting still, you must remember, the plane is sitting still because its free-spinning wheels are moving forward against the belt at the SAME 100 mph. Now, it was suggested that if you pulled the plane forward - say with a rope - you could easily pull it because it has free-spinning wheels.

The question is not "How easily can you pull the plane?" In the problem presented, if you pull that plane forward along the conveyor belt, the wheels must BY MATHEMATICAL CERTAINTY - move faster than the belt. That contradicts the assumption forced on us in the problem.

The problem calls for the wheels of the jet never exceeding the speed of the belt. Therefore, the two vectors always cancel each other out.

Another way to know this is the result is because I've seen some of you suggest air speed is independent of ground speed. Those of you who said that are wrong. Air, like just about everything, has mass. Gravity operates on it. So does the earth's rotation.

Did you know that at the equator, the earth rotates at about 24000 miles an hour? Well, if the wind speed was independent of the earth, all the people in the tropics would just be blown off the earth, and the poles would have the most expensive real estate there is. Gravity pulls the wind along the earth's surface. Just like it pulls all of us people. Actually, we are all moving at the speed the earth is moving. We do not move independently of the earth. Nor does the air.

[Champagne taste beer budget]

Oh My Goodness....[:|]

[Jeff Matthews]

Oh My Goodness....[:|]

For what? Because it was solved previously, or because I'm wrong? I didn't read all the thread - just the first few and last few pages.

[sputnik]

I thought that we got them all but they just keep coming.

[Champagne taste beer budget]

I'm sorry, Jeff. You really should take the time to read the whole sordid mess, it is quite the tale with more twists than a Hitchcock thriller. I meant no disrespect, sir, just surprise that the "arguments" were starting up again.

Basically, the power of the planes engine are exerted against the air, which the moving conveyor belt has no effect on, thus the plane is forced forward through the air. Forced fast enough, it will fly.

[Jeff Matthews]

Now, I agree with what you said if you can get air to pass by the wings quickly enough. But you cannot - or if you can, tell me how.

As I understand the jet engine, its purpose is to create forward thrust. Just so you can see my point, assume it is a still day, and there is a square meter of air just sitting stagnate right above the belt. When you crank the engine, the jet's wheels spin. The belt revolves in the opposite direction. There is no forward movement at all, and the square meter of air still sits there in relation to the jet. You have to get the jet past that air, but the conveyor will not let you.

Why is this wrong? I am not a design engineer, but maybe it has something to do with the jet engine. Does the engine pull enough air past the wing? It was always my belief that it did not.

[Jeff Matthews]

I Googled and found this question is all over the place. Basically, I have seen that I am right, though. Just think of it this way - the plane's forward thrust - being countered by the conveyor - is sitting in park.

If this was not the case, they would not spend all that money on long-a$$ runways - i.e. that plane has to move forward relative to the earth (and ergo, the wind). The jet engine is only for forward thrust to get the plane rolling along and get some wind up under the wing. For the jet engine to really cause the effect of flight - as opposed to forward rolling - you'd need a rocket engine.

As one example I saw, think of it this way. If you could roll down the passenger seat window and stick your hand out, all would be calm because the plane is simply not in motion relative to the earth and wind.

It's really kind of funny. All you "it will flyer's" have just solved the problem on how to save millions and millions on concrete. Just sit still and crank that jet engine up full blast. "It's the air, stupid! Not the ground." []

[Jeff Matthews]

Here's what somebody else said:

"Another way of looking at it - if you've ever been to the gym and gotten on a treadmill do you feel the wind on your face like you do when you run outdoors? No.

Moving forward does not create lift. Moving forward relative to the air creates lift."

[sputnik]

.......You have to get the jet past that air, but the conveyor will not let you..........

The conveyor is irrelevant. Look back into this thread more than just a few pages (go back to around the post where D-Man calls me nuts).

[Champagne taste beer budget]

Now, I agree with what you said if you can get air to pass by the wings quickly enough. But you cannot - or if you can, tell me how.

As I understand the jet engine, its purpose is to create forward thrust. Just so you can see my point, assume it is a still day, and there is a square meter of air just sitting stagnate right above the belt. When you crank the engine, the jet's wheels spin. The belt revolves in the opposite direction. There is no forward movement at all, and the square meter of air still sits there in relation to the jet. You have to get the jet past that air, but the conveyor will not let you.

Why is this wrong? I am not a design engineer, but maybe it has something to do with the jet engine. Does the engine pull enough air past the wing? It was always my belief that it did not.

I believe this is where most people get off track. When you crank the engine, the wheels DO NOT spin. I won't go into all the analogies that were made in this thread, but the task of the jet engine is not to PULL air across the wings, but to PUSH the plane forward. The engine IS NOT connected to the wheels.

[Jay481985]

Just like it pulls all of us people. Actually, we are all moving at the speed the earth is moving. We do not move independently of the earth. Nor does the air.

uhhhhhhhhhhh......................................... then I guess we would have no wind with a statement like that?

[Jay481985]

Now, I agree with what you said if you can get air to pass by the wings quickly enough. But you cannot - or if you can, tell me how.

As I understand the jet engine, its purpose is to create forward thrust. Just so you can see my point, assume it is a still day, and there is a square meter of air just sitting stagnate right above the belt. When you crank the engine, the jet's wheels spin. The belt revolves in the opposite direction. There is no forward movement at all, and the square meter of air still sits there in relation to the jet. You have to get the jet past that air, but the conveyor will not let you.

Why is this wrong? I am not a design engineer, but maybe it has something to do with the jet engine. Does the engine pull enough air past the wing? It was always my belief that it did not.

I believe this is where most people get off track. When you crank the engine, the wheels DO NOT spin. I won't go into all the analogies that were made in this thread, but the task of the jet engine is not to PULL air across the wings, but to PUSH the plane forward. The engine IS NOT connected to the wheels.

not exactly the het engine sorta has a weak connection as the bearings in the wheels are not perfect.

[Jay481985]

Jeff, I understand that you believe the force vectors cancel out but think of the wheel as a free moving object. If the conveyer belt were to be the only thing moving the plane would move backwards but not at the same speed the wheels are moving backwards due to the bearing in the wheels. Think of it more or less as a muscle car burning out, it might be moving forward just a bit but mostly the wheels are being stationary spinning.

[Jeff Matthews]

I know the engine does not spin the wheels. Use Jay's example. Jay, you're right, a car spinning out just means the car is sitting there. There is no more wind going past that car than had the car been in park. I don't care how fast you spin the wheels, if there is no forward movement, you will not get enough wind under the wing.

You guys have some wild theories. This is really very simple.

Let's say the wind is blowing outside at 15 mph relative to a person who is standing still. Now, put a treadmill next to that person, and introduce a second person to be a jogger on the treadmill. Turn the treadmill up to spin at 20 mph. The jogger is jogging away. Is the jogger now running into a 35 mph headwind? NOT!!!!

[Jeff Matthews]

Guys, just pick a number. Let's say it takes 120 mph wind under a wing to lift a jet.

How are you going to get that 120 mph of wind just keeping up with a treadmill? Won't happen. Fuggetabout it!

[Jay481985]

I know the engine does not spin the wheels. Use Jay's example. Jay, you're right, a car spinning out just means the car is sitting there. There is no more wind going past that car than had the car been in park. I don't care how fast you spin the wheels, if there is no forward movement, you will not get enough wind under the wing.

Jeff my car spinning wheels was an example that does not work in every case, the car has a direct connection (from flywheel, to transmission, to driveshaft and then to the wheel but non the less a direct connection) while the airplane has a mediocre connection, it pushes the air to actually move. The airplane when they start the engines should move the plane tecnically but there are many reasons. Brakes are on, just like you should have the clutch when you start the car in a manual tranny car. The force of the engine is not strong enough to overcome inertia, etc. The wheel example here was just the fact that the plane should move at a lesser speed than the wheels relavent to the conveyor belt.

You guys have some wild theories. This is really very simple.

Let's say the wind is blowing outside at 15 mph relative to a person who is standing still. Now, put a treadmill next to that person, and introduce a second person to be a jogger on the treadmill. Turn the treadmill up to spin at 20 mph. The jogger is jogging away. Is the jogger now running into a 35 mph headwind? NOT!!!!

again everything is relative to whatever. The the wind speed is relative to the earth hence its moving 15 mph. The jogger is running 20 mph on relative on the treadmill but relative to the earth he is 0mph. you have to keep everything relative just like the apples to oranges notion everyone from time to time says.

[Champagne taste beer budget]

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

[Jay481985]

Guys, just pick a number. Let's say it takes 120 mph wind under a wing to lift a jet.

How are you going to get that 120 mph of wind just keeping up with a treadmill? Won't happen. Fuggetabout it!

I never knew texans said fuggetaboutit [] thats a jersey word, take it back!

and think of you on a treadmill with rollerblades/skates on. BTW again it depends on the plane, a military plane could do it. The McDonald Douglass F-15 was the first military plane to have more engine thrust then weight so it could actually climb vertically (90 degrees) and not stall. This is how rockets must have greater thrust than weight in order to fly with no wings. While many commercial planes have no where near the ratio the F-15 does.

[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.

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

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.

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