Blackrams, a helicopter is just an airplane running around in circles.
One more time, air speed and ground speed are equal in a theoretical calm.

quote Originally posted by blackrams: Quite frankly, I'm amazed at some of the responses here. Ground Speed has absolutely nothing with whether or not the aircraft will fly. It's all about airspeed.

Exactly, and I don't believe this the crux of the debate. I think we all agree that a plane requires airspeed to create lift although airspeed and ground speed can often be the same. (For the sake of argument, let's assume there is no wind and we are at sea level.) So let's just move on from there.

I've read several test scenarios mentioned here about toy cars on moving sheets of paper, model airplanes on treadmills, etc. These are the responses that I'm "amazed" at. The imaginary "conveyor belt" dreamed up in the original scenario is just that, a dream, totally fictional. Nothing in the real world could duplicate what this device would be physically able to do.

But let's say that through some miracle, one of these magical devices appeared. Let's say it could instantly track any forward movement (relative to the ground or air, should make no difference in this situation) and just as instantly rotate the conveyor belt in the opposite direction to negate this forward movement. Very very quickly, the tires and wheels on the plane would be spinning incredibly fast. Would there still be friction and rolling resistance? You bet there would be, for there would still be absolutely NO lift created at this point. The full weight of the aircraft, whether it be a Cessna or a 747, would still be on these incredibly fast rotating tires and wheels due to the plane's stationary wings. This is a fact that I believe so many of you are conveniently ignoring.

Now, because our magical conveyor belt is purely fiction, we can also freely state that it has bearings and rollers and belts that will never overheat or fail. Can the same be said of the plane that is on this imaginary device? No, nothing in the original scenario stated that the test plane could be endowed with fictional qualities. Therefore, at some point during the rapid acceleration on the conveyor belt, the plane's tires or wheel bearings would fail, at which point the plane would eventually be skidding along on its belly (or the props, if the plane had props, would probably be destroyed bashing against the surface of the conveyor belt). If the test was continued, I don't see anything positive occurring (to say the least), and that includes positive lift.

Of that, I'm positive.

quote 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). The question is: Will the plane take off or not?

[This message has been edited by Patrick (edited 12-05-2005).]

"boss, boss! de plane! de plane!"

quote Originally posted by blackrams: Sorry Cliff, way off on this one, if that was true, how would a helicopter ever develop lift.

That's what the "=~" was for. I know Air Speed isn't the same as Ground Speed, but Air Speed and Ground Speed are interchangeable in this problem. If Air Speed is measured instead of Ground Speed (so the belt changes its speed relative to the Air Speed) the results will still be the same. And under zero wind conditions, Horizontal True Air Speed equals Horizontal Ground Speed.

quote Originally posted by Patrick: Let's say it could instantly track any forward movement (relative to the ground or air, should make no difference in this situation) and just as instantly rotate the conveyor belt in the opposite direction to negate this forward movement.

This is the point you're missing. No matter how fast the conveyer reacts - no matter how perfectly it's speed is matched to the plane's speed - it cannot negate the plane's forward movement, it can only spin the wheels on the plane.

How does the belt affect the speed of the plane? If the plane's engine is generating a trust that would create a forward velocity of 10mph, and the belt is moving backwards at 10mph - does that mean the plane is stationary?

No. The plane will be moving forward at 10mph, the belt backwards at 10mph, and the wheels turning for an effective ground speed between the belt and plane of 20mph.

quote Originally posted by Cliff Pennock: At take off, Ground Speed =~ Air Speed.

quote Originally posted by jstricker: Nope. Wrong, wrong, wrong. If you think an airplane takes off at the same groundspeed in Denver, CO as it does at Houston, TX, you're in for a very rude awakening.

Clarification: You are both right. In a no-wind situation ... and, more specifically, at takeoff ...the ground speed (of an airplane) is indeed approximately the same as its TRUE airspeed. Conversely, ground speed is seldom the same as INDICATED airspeed.

In a nutshell, true airspeed is simply indicated airspeed corrected for differences in air density due to barometric pressure, temperature, and altitude/elevation.

[This message has been edited by Marvin McInnis (edited 12-05-2005).]

quote Originally posted by Wolfhound: Blackrams, a helicopter is just an airplane running around in circles. One more time, air speed and ground speed are equal in a theoretical calm.

OK, I'll give up. It isn't worth the effort.
Happy Fieroing.

------------------
Ron
Freedom isn't Free, it's always earned.
My imagination is the only limiting factor to my Fiero. Well, there is that money issue.

[This message has been edited by blackrams (edited 12-04-2005).]

quote Originally posted by Formula88: This is the point you're missing. No matter how fast the conveyer reacts - no matter how perfectly it's speed is matched to the plane's speed - it cannot negate the plane's forward movement, it can only spin the wheels on the plane. How does the belt affect the speed of the plane? If the plane's engine is generating a trust that would create a forward velocity of 10mph, and the belt is moving backwards at 10mph - does that mean the plane is stationary? No. The plane will be moving forward at 10mph, the belt backwards at 10mph, and the wheels turning for an effective ground speed between the belt and plane of 20mph.

No offense, but I believe you're the one missing the point.

Is not the speed of the plane being determined by it's relationship with us, on the ground?

If so, why in your scenario is the speed of the conveyor belt being limited to any speed? It is supposed to match whatever the forward speed is (relative to the ground) of the plane. (Airspeed is not an issue here as we've stated for the sake of argument that there is no wind.) If the plane was to creep forward (relative to us) then the belt would have to spin faster. As the plane quickly continued to accelerate, the belt would have to spin faster and faster as well. This could not go on indefinately without tire/wheel bearing failure.

[This message has been edited by Patrick (edited 12-04-2005).]

The plane WILL have a force pulling back on it though. Just picture that rc plane on the treadmill, it still has to overcome the friction of wheels.. It would take a hand to hold it there. I'm obviously not going to guess with numbers, but it's going to take more power to hold it stationary than if it was on solid ground (obviously).

quote Originally posted by JohnnyK: The plane WILL have a force pulling back on it though.

Of course it does. The full weight of the plane remains on the plane's tires/wheels as they spin faster and faster on the fictional conveyor belt. This friction/rolling resistance does not magically disappear somehow. The end result wouldn't be pretty. This is what I believe is being ignored by so many people in this thread.

quote Now, because our magical conveyor belt is purely fiction, we can also freely state that it has bearings and rollers and belts that will never overheat or fail. Can the same be said of the plane that is on this imaginary device? No, nothing in the original scenario stated that the test plane could be endowed with fictional qualities.

If you can imagine a fictional conveyor, to support the original question, then you also have to be able to imagine the same fictional qualities and capabilities for the plane. Unless of course, it causes problems for one's particular theory of the whole thing.
What you are doing is saying it is ok to imagine 1/2 of the equation (the irrelevant 1/2) with no limits, but everyone must deal with reality regarding the rest of the equation. You do this in order to support your theory.

quote 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).

It doesn't specify or limit what kind of plane or even if the plane is real--(you did that on your own) it just says "a plane". If you accept that the conveyor is capable of these speeds, you must also accept it for the plane. Otherwise, the answer is even more simple.

"Yes, the plane will fly because no such conveyor exists." (actually, there are conveyors capable of supporting a plane, in rock quarrys and such)

The fact is, it would be ten times+ a more intense & expensive problem to design and build a conveyor with this capability than it would to design a plane's wheel bearing system along with the tires capable of staying together at the rotating speeds the non-believers are talking about.

But, it's irellevent. The plane will be airborne, seatbacks reclined, and trays will be holding coctails long before wheel rotation speeds ever become an issue.

The Problem

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).

quote Originally posted by maryjane: What you are doing is saying it is ok to imagine 1/2 of the equation (the irrelevant 1/2) with no limits, but everyone must deal with reality regarding the rest of the equation. You do this in order to support your theory.

MJ, too funny for words! Let's all just pretend there's no air then and take it from there!

I reiterate my statement of dozens of posts ago. THE PLANE WILL NOT FLY!!!!!!!!!!! The energy produced by the propellers is only making the plane move forward AT EXACTLY the same speed the conveyor is programmed to go backward. The plane is stationary. The first rule of aviation is; no actual forward motion, no lift; no flight. The air flowing over the wing produced by the propeller is insignificant and has no affect on lift. Forget the propellers; if it was a jet, you could stand on the wing in your shorts and not feel a breeze. No breeze, no air flow OVER the wing; NO FLIGHT. Amen Ed

quote Originally posted by Patrick: No offense, but I believe you're the one missing the point. Is not the speed of the plane being determined by it's relationship with us, on the ground? If so, why in your scenario is the speed of the conveyor belt being limited to any speed? It is supposed to match whatever the forward speed is (relative to the ground) of the plane. (Airspeed is not an issue here as we've stated for the sake of argument that there is no wind.) If the plane was to creep forward (relative to us) then the belt would have to spin faster. As the plane quickly continued to accelerate, the belt would have to spin faster and faster as well. This could not go on indefinately without tire/wheel bearing failure.

The speed relative to the ground is irrelevant. Only the speed of the plane through the air matters. If the plane had to reach 80mph (just to pick a value at random) to take off, and there was an 80mph headwind - it would take off with a zero ground speed.

My whole point is that the belt doesn't slow the plane down. At the 80 mph take off speed, the belt is moving 80 mph backwards. The plane is still going 80 mph forwards (relative to the ground) and the wheels are turning as though the plane was doing 160 mph (relative to the belt).

Heck the earth is moving. That's a big, moveable runway. Now, does the plane require a different take off velocity based on if it's pointing East or West? No, it doesn't. The earth rotates to the East (counterclockwise about it's axis) and if your runway is pointing West, the plane is technically moving backwards as it takes off. Space craft actually take advantage of this. A Space Shuttle launching due East has a higher orbit and payload potential than one launched due North because it's starting at a higher "absolute" velocity.

The friction of the wheels can be neglected because the engine has the power to overcome that friction in reality - so it stands to reason it could in our example as well.

It's definitely been an interesting discussion, but it's quickly becoming a case of each party restating their viewpoint.

quote Originally posted by Patrick: MJ, too funny for words! Let's all just pretend there's no air then and take it from there!

You've already done that. And there lies the problem. All you see is the imaginary conveyor.

The plane will get foward motion no matter how fast the conveyor moves backwards, the wheels will simply spin and the effect of the conveyor will be neglible. The propeller on the plane is a wing itself, as long as there is air the plane will move foward, eventually gaining enough speed through the air to create lift and fly.
The only time the plane would not be foward is if it were in a vacuum, then the propeller will have nothing to push against, but it's motor would not be able to run in a vacuum because of a lack of air.
As I stated before, the only way the plane would not fly, besides a vacuum, would be if the plane was tethered so it couldn't move foward. It would not generate enough speed through the air to create lift and fly.
If you want to see an airplane fly with zero ground speed check out a wind tunnel. The wind going over the airplane model makes it "fly" even though it has no thrust of it's own and is not moving relative to the ground. If you turn off the wind tunnel fan and put a conveyor under the model, you could make the conveyor run as fast as you want and the model would not fly. This is because there is no air speed being created around the wings.

I'm not even sure I should still be posting in this thread, but here goes.

First of all, the plane will move forward, or have a ground speed or true airspeed, no matter what the belt below it is doing. The two do not cancel each other out. Think about it. The whole purpose of an airplane's engine is to create a vacuum in front of the engine and an overpressure behind it. In other words, the engines are pushing/sucking against the air, not the belt. So the belt's motion will not have any effect on the plane's motion (or at least a neglectible) effect.

quote Originally posted by Firefighter: The energy produced by the propellers is only making the plane move forward AT EXACTLY the same speed the conveyor is programmed to go backward. The plane is stationary.

No it is not. It would only be stationary if the plane was getting its forward motion through it's wheels (like a car). And it's not. The plane is not pushing against the belt, it's pushing against the air. So it will accelerate, no matter what the belt is doing. I will explain a bit more below.

quote Originally posted by JohnnyK: The plane WILL have a force pulling back on it though. Just picture that rc plane on the treadmill, it still has to overcome the friction of wheels.. It would take a hand to hold it there.

Yes, but it isn't enough to cancel out the forward motion of the plane. Besides, the belt is reacting to the forward motion of the plane, not vice versa. So if the plane does not have a forward motion, the belt will not move. To stop the plane from moving, it would need to create a bigger force than the plane's forward motion, which is not possible since the belt can't move faster than the plane.

But the hand is a very good analogy to the plane's engines. Suppose you put that RC plane on the belt. Your hand is in fact the plane's engine. If you move your hand forward, the plane will move forward, no matter what the belt does, because the plane is pushing against your hand, not the belt. And that's the mistake everybody is making that thinks the plane will not fly. The plane does not excert force against the belt to gain forward momentum, it excerts force against the air.

Another analogy. You are standing on rollerskates on a conveyor belt. The conveyor belt moves in the same direction you are facing. In front of you and the belt is a wall, and you are pushing against the wall. Will you move backwards? Yes you will. Now the conveyor belt is some smart thing that increases its speed directly proportional to your backwards speed. You again push against the wall. Will you move backwards? Yes you will. You might need a little bit of extra force to compensate for the extra drag in your rollerskates, but you will move backwards nevertheless. Why? Because you are pushing against the wall, not the belt.

Now in above story, replace yourself with an airplane, your arms with the plane's engines and the wall with the air.

quote Originally posted by Patrick: If so, why in your scenario is the speed of the conveyor belt being limited to any speed? It is supposed to match whatever the forward speed is (relative to the ground) of the plane. (Airspeed is not an issue here as we've stated for the sake of argument that there is no wind.) If the plane was to creep forward (relative to us) then the belt would have to spin faster. As the plane quickly continued to accelerate, the belt would have to spin faster and faster as well. This could not go on indefinately without tire/wheel bearing failure.

The maximum speed of the tires/wheels is twice the plane's take off velocity. Suppose the plane's take off velocity is +140 kts, then the belt will have a speed of -140 kts so the tires will rotate at a speed of 280 kts. An airplane's landing gear can easily withstand twice the drag during ground-roll, The tires/wheels can easily withstand twice the take-off speed during ground-roll. So they will not fail. So even if you use a non-fictional plane, the plane will still take off before any failure.

quote Originally posted by Patrick: Of course it does. The full weight of the plane remains on the plane's tires/wheels as they spin faster and faster on the fictional conveyor belt. This friction/rolling resistance does not magically disappear somehow. The end result wouldn't be pretty. This is what I believe is being ignored by so many people in this thread.

No, the full weight of the plane does not remain on the plane's landing gear. As air speed builds up, so does lift. As air speed builds up, the weight of the plane is distributed among the landing gear and the wings. Only drag and frictional forces build up, but as I said before the landing gear can withstand that without a problem. For instance, a Boeing 747's empty weight is about 160 tons. Its maximum take-off weight is 340 tons. In order to take off with maximum weight, it would need twice the lift than when it's empty. To generate twice the lift, it needs at least twice the take-off speed than when empty.

So the forces on the planes landing gear on our conveyor belt can be compared to the plane taking off with a full load (actually much less, but you get the idea).

So, we have established that:

A) the speed of the conveyor belt hardly has any influence on the air speed of the airplane (again, this is not a car, this is an airplane, it uses the surrounding air to gain air speed, not the ground)
B) The extra forces on the airplane's undercarriage can be withstand easily by the airplane.

Therefore, the plane will lift off.

well said cliff

wow....talk about beating a dead horse. No matter how fast the conveyor is moving, it wouldn't be able to keep the plane stationary unless the pilot chose to stay stationary.

The real question is...(IF you believe the plane couldn't go forward)...if the conveyor tracks the plane speed, and adjusts to that speed in the opposite direction.....then how is the conveyor even moving in the first place if the plane is not moving, has no velocity, thus having no speed?

quote Originally posted by FierceGT: wow....talk about beating a dead horse. No matter how fast the conveyor is moving, it wouldn't be able to keep the plane stationary unless the pilot chose to stay stationary. The real question is...(IF you believe the plane couldn't go forward)...if the conveyor tracks the plane speed, and adjusts to that speed in the opposite direction.....then how is the conveyor even moving in the first place if the plane is not moving, has no velocity, thus having no speed?

What they are saying is, it tracks the planes forward speed. There will always be weight on the wheels of the plane, therefore the plane must have slightly more acceleration than the belt (moving in opposite directions obviously), therefore if the belt has an infinite amount of speed, it will always be able to keep the plane stationary, because as soon as the plane speeds up to move forwards, the belt will move even faster to keep it stationary, and since all the planes weight is on the wheels at this point, as I said before the plane will always need more (however slight) power to move forwards, it will happen infinitely.. (at least that is my understanding of where both sides are going with this).. As I said, there are so many ways to interpret this question..

quote Originally posted by Formula88: The speed relative to the ground is irrelevant. Only the speed of the plane through the air matters.

No one is arguing that, and it's probably been stated a dozen times by now that ground speed would be the same as airspeed in a non-wind situation. Why are you posting something we all agree on (rather than perhaps answering the question I earlier asked you)?

quote Originally posted by Formula88: My whole point is that the belt doesn't slow the plane down. At the 80 mph take off speed, the belt is moving 80 mph backwards. The plane is still going 80 mph forwards (relative to the ground) and the wheels are turning as though the plane was doing 160 mph (relative to the belt).

This is where the disagreement starts. Why are you limiting the belt speed to 80 mph in this example? If the plane is still moving forward (relative to the ground), then the conveyor is not spinning fast enough. If the conveyor belt was to go a million miles an hour in the opposite direction, are you suggesting that the plane would still be going forward relative to the ground? I'd say that if nothing else, tire/wheel bearing friction would put an end to the test in very short order.

quote Originally posted by Formula88: The friction of the wheels can be neglected because the engine has the power to overcome that friction in reality - so it stands to reason it could in our example as well.

It's not the engine that would be the problem. It's the ability of the tires and wheel bearings to survive incredible speeds, which they wouldn't. Saying that the "friction of the wheels can be neglected" because of whatever reason is bogus. Friction is something that needs to be dealt with, not casually dismissed.

[This message has been edited by Patrick (edited 12-05-2005).]

quote Originally posted by Patrick: Cliff, why are you fixated on this idea that the conveyor belt is limited in its speed? If it is monitoring the forward motion of the plane (relative to the ground) and is being adjusted accordingly, then WHATEVER speed it takes to slow the plane down is what it will do.

Why are you fixated on bending and twisting the original question and rules? The question clearly states:

"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).

Is clearly states that if the plane moves in one direction at speed X, the belt moves in the opposite direction at speed X. And nothing else. The belt will never move at a speed greater than the plane's speed. What's so difficult to understand about that?

quote Who the heck is limiting anything in this scenario to "twice the take-off speed"?

Errrm, the scenario itself? Are you saying the plane does not take off at take-off speed? That it will increase in speed indefinitely? That some invisible force is keeping it on the ground even when it exceed it's takeoff speed? No, the plane takes off at take off speed. Period. Therefore the belt's speed will never increase beyond (minus) take-off speed. Therefore the wheels will never spin faster than twice the take-off speed. This is so basic, I really don't understand why you insist the speed of the belt will increase indefinetly.

quote I'd agree with you IF there was any airspeed, but there wouldn't be. The full weight of the plane would therefore remain on the tires/wheels.

Of course there will be airspeed. Again, the plane is pushing against the air and not against the belt! The belt will never be able to counteract the forward motion because due to definition of the original statement, the belt can't move faster than the plane. What you are saying is that it I tie a rope to the nose of the airplane, and I stand in front of the belt (not on it), I will never be able to pull the plane forward. Of course I can (for argument's sake, let's say I'm strong enough to actually move an airplane). The speed of the belt only adds a slight drag. According to you, it doesn't matter how strong I am, I will never be able to move the plane.

I'm sorry Patrick, but you are just "plane" wrong.

Patrick, The conveyor speed rearward is limited to the forward speed of the plane.
It's so stated in the problem.

if there is no air flowing over it's wings......... it wont fly.

But air is flowing over the wings, and it flys.

quote Originally posted by Wolfhound: But air is flowing over the wings, and it flys.

NO IT WON"T!

The whole purpose of the plane having wheels is to help the engines pull enough air over the wings to create lift. Without lift, the wheels can be going a million miles per hour and still not effect whatever air flow the wings experience to gain the neccessary lift. The wings are still experiencing the same forces regardless of what the wheels are doing because they are not getting the air flow needed to create the vacuum above their surface to overcome gravity.

"Aerodynamic Forces
Before we dive into how wings keep airplanes up in the air, it's important that we take a look at four basic aerodynamic forces: lift, weight, thrust and drag.

Straight and Level Flight
In order for an airplane to fly straight and level, the following relationships must be true:

* Thrust = Drag
* Lift = Weight

If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up.

Similarly, if the amount of lift drops below the weight of the airplane, the plane will descend. By increasing the lift, the pilot can make the airplane climb.

Thrust
Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag (notice that thrust and drag act in opposite directions in the figure above). Airplanes create thrust using propellers, jet engines or rockets. In the figure above, the thrust is being created with a propeller, which acts like a very powerful version of a household fan, pulling air past the blades.

Drag
Drag is an aerodynamic force that resists the motion of an object moving through a fluid (air and water are both fluids). If you stick your hand out of a car window while moving, you will experience a very simple demonstration of this effect. The amount of drag that your hand creates depends on a few factors, such as the size of your hand, the speed of the car and the density of the air. If you were to slow down, you would notice that the drag on your hand would decrease.

We see another example of drag reduction when we watch downhill skiers in the Olympics. You'll notice that, whenever they get the chance, they will squeeze down into a tight crouch. By making themselves "smaller," they decrease the drag they create, which allows them to move faster down the hill.

If you've ever wondered why, after takeoff, a passenger jet always retracts its landing gear (wheels) into the body of the airplane, the answer (as you may have already guessed) is to reduce drag. Just like the downhill skier, the pilot wants to make the aircraft as small as possible to reduce drag. The amount of drag produced by the landing gear of a jet is so great that, at cruising speeds, the gear would be ripped right off of the plane.

But what about the other two aerodynamic forces, weight and lift?

Weight and Lift
747-400 Facts

* Length: 232 feet (~ 71 meters)
* Height: 63 feet (~ 19 meters)
* Wingspan: 211 feet (~ 64 meters)
* Wing area: 5,650 square feet (~ 525 square meters)
* Max. takeoff weight: 870,000 pounds (~ 394,625 kilograms)
* Max. landing weight: 630,000 pounds (~ 285,763 kilograms)
(explains why planes may need to dump fuel for emergency landings)
* Engines: four turbofan engines, 57,000 pounds of thrust each
* Fuel capacity: up to 57,000 gallons (~ 215,768 liters)
* Max. range: 7,200 nautical miles
* Cruising speed: 490 knots
* Takeoff distance: 10,500 feet (~ 3,200 meters)

Weight
This one is the easiest. Every object on earth has weight (including air). A 747 can weigh up to 870,000 pounds (that's 435 tons!) and still manage to get off the runway. (See the table below for more 747 specs.)

Lift
Lift is the aerodynamic force that holds an airplane in the air, and is probably the trickiest of the four aerodynamic forces to explain without using a lot of math. On airplanes, most of the lift required to keep the plane aloft is created by the wings (although some is created by other parts of the structure).

A principal concept in aerodynamics is the idea that air is a fluid. Let's investigate that concept more closely.

A Few Words About Fluid
As we mentioned, a principal concept in aerodynamics is the idea that air is a fluid. Like all gases, air flows and behaves in a similar manner to water and other liquids. Even though air, water and pancake syrup may seem like very different substances, they all conform to the same set of mathematical relationships. In fact, basic aerodynamic tests are sometimes performed underwater.

Another important concept is the fact that lift can exist only in the presence of a moving fluid. This is also true for drag. It doesn't matter if the object is stationary and the fluid is moving, or if the fluid is still and the object is moving through it. What really matters is the relative difference in speeds between the object and the fluid.

Consequently, neither lift nor drag can be created in space (where there is no fluid). This explains why spacecraft don't have wings unless the spaceship spends at least some of its time in air. The space shuttle is a good example of a spacecraft that spends most of its time in space, where there is no air that can be used to create lift. However, when the shuttle re-enters the earth's atmosphere, its stubby wings produce enough lift to allow the shuttle to glide to a graceful landing.

Popular (and Imperfect) Explanations of Lift Creation
If you read any college-level aerodynamics textbook, you will find plenty of mathematical methods for calculating lift. Unfortunately, none of these explanations are particularly satisfying unless you have a Ph.D. in mathematics.

There are many simplified explanations of lift that appear on the Internet and in some textbooks. Two of the most popular explanations today are the Longer Path explanation (also known as the Bernoulli or equal transit time explanation) and the Newtonian explanation (also known as the momentum transfer or air deflection explanation). While many versions of these explanations are fundamentally flawed, they can still contribute to an intuitive understanding of how lift is created.

The Longer Path Explanation
What is it?
The Longer Path explanation holds that the top surface of a wing is more curved than the bottom surface. Air particles that approach the leading edge of the wing must travel either over or under the wing. Let's assume that two nearby particles split up at the leading edge, and then come back together at the trailing edge of the wing. Since the particle traveling over the top goes a longer distance in the same amount of time, it must be traveling faster.

Bernoulli's equation, a fundamental of fluid dynamics, states that as the speed of a fluid flow increases, its pressure decreases. The Longer Path explanation deduces that this faster moving air develops a lower pressure on the top surface, while the slower moving air maintains a higher pressure on the bottom surface. This pressure difference essentially "sucks" the wing upward (or pushes the wing upward, depending on your point of view).

Why is it not entirely correct?
There are several flaws in this theory, although this is a very common explanation found in high school textbooks and even encyclopedias:

1. The assumption that the two air particles described above rejoin each other at the trailing edge of the wing is groundless. In fact, these two air particles have no "knowledge" of each other's presence at all, and there is no logical reason why these particles should end up at the rear of the wing at the same moment in time.

2. For many types of wings, the top surface is longer than the bottom. However, many wings are symmetric (shaped identically on the top and bottom surfaces). This explanation also predicts that planes should not be able to fly upside down, although we know that many planes have this ability.

Why is it not entirely wrong?
The Longer Path explanation is correct in more than one way. First, the air on the top surface of the wing actually does move faster than the air on the bottom -- in fact, it is moving faster than the speed required for the top and bottom air particles to reunite, as many people suggest. Second, the overall pressure on the top of a lift-producing wing is lower than that on the bottom of the wing, and it is this net pressure difference that creates the lifting force.

The Newtonian Explanation
What is it?
Isaac Newton stated that for every action there is an equal, and opposite, reaction (Newton's Third Law). You can see a good example of this by watching two skaters at an ice rink. If one pushes on the other, both move -- one due to the action force and the other due to the reaction force.

In the late 1600s, Isaac Newton theorized that air molecules behave like individual particles, and that the air hitting the bottom surface of a wing behaves like shotgun pellets bouncing off a metal plate. Each individual particle bounces off the bottom surface of the wing and is deflected downward. As the particles strike the bottom surface of the wing, they impart some of their momentum to the wing, thus incrementally nudging the wing upward with every molecular impact.

Note: Actually, Newton's theories on fluids were developed for naval warfare, in order to help decrease the resistance that ships encounter in the water -- the goal was to build a faster boat, not a better airplane. Still, the theories are applicable, since water and air are both fluids.

Why is it not entirely correct?
The Newtonian explanation provides a pretty intuitive picture of how the wing turns the air flowing past it, with a couple of exceptions:

1. The top surface of the wing is left completely out of the picture. The top surface of a wing contributes greatly to turning the fluid flow. When only the bottom surface of the wing is considered, the resulting lift calculations are very inaccurate.

2. Almost a hundred years after Newton's theory of ship hulls, a man named Leonhard Euler noticed that fluid moving toward an object will actually deflect before it even hits the surface, so it doesn't get a chance to bounce off the surface at all. It seemed that air did not behave like individual shotgun pellets after all. Instead, air molecules interact and influence each other in a way that is difficult to predict using simplified methods. This influence also extends far beyond the air immediately surrounding the wing.

Why is it not entirely wrong?
While a pure Newtonian explanation does not produce accurate estimates of lift values in normal flight conditions (for example, a passenger jet's flight), it predicts lift for certain flight regimes very well. For hypersonic flight conditions (speeds exceeding five times the speed of sound), the Newtonian theory holds true. At high speeds and very low air densities, air molecules behave much more like the pellets that Newton spoke of. The space shuttle operates under these conditions during its re-entry phase.

Unlike the Longer Path explanation, the Newtonian approach predicts that the air is deflected downward as it passes the wing. While this may not be due to molecules bouncing off the bottom of the wing, the air is certainly deflected downward, resulting in a phenomenon called downwash. (See NASA: Glenn Research Center for more on downwash.)

How Lift is Created

Consider This
It is important to realize that, unlike in the two popular explanations described earlier, lift depends on significant contributions from both the top and bottom wing surfaces. While neither of these explanations is perfect, they both hold some nuggets of validity. Other explanations hold that the unequal pressure distributions cause the flow deflection, and still others state that the exact opposite is true. In either case, it is clear that this is not a subject that can be explained easily using simplified theories.

Likewise, predicting the amount of lift created by wings has been an equally challenging task for engineers and designers in the past. In fact, for years, we have relied heavily on experimental data collected 70 to 80 years ago to aid in our initial designs of wings.
Pressure Variations Caused By Turning a Moving Fluid
Lift is a force on a wing (or any other solid object) immersed in a moving fluid, and it acts perpendicular to the flow of the fluid. (Drag is the same thing, but acts parallel to the direction of the fluid flow). The net force is created by pressure differences brought about by variations in speed of the air at all points around the wing. These velocity variations are caused by the disruption and turning of the air flowing past the wing. The measured pressure distribution on a typical wing looks like the following diagram:

A. Air approaching the top surface of the wing is compressed into the air above it as it moves upward. Then, as the top surface curves downward and away from the airstream, a low-pressure area is developed and the air above is pulled downward toward the back of the wing.

B. Air approaching the bottom surface of the wing is slowed, compressed and redirected in a downward path. As the air nears the rear of the wing, its speed and pressure gradually match that of the air coming over the top. The overall pressure effects encountered on the bottom of the wing are generally less pronounced than those on the top of the wing.

C. Lift component

D. Net force

E. Drag component

When you sum up all the pressures acting on the wing (all the way around), you end up with a net force on the wing. A portion of this lift goes into lifting the wing (lift component), and the rest goes into slowing the wing down (drag component). As the amount of airflow turned by a given wing is increased, the speed and pressure differences between the top and bottom surfaces become more pronounced, and this increases the lift. There are many ways to increase the lift of a wing, such as increasing the angle of attack or increasing the speed of the airflow. These methods and others are discussed in more detail later in this article.

Bla bla bla...

quote Originally posted by Wolfhound: But air is flowing over the wings, and it flys.

Ok..

If you were running on a tradmill how fast is the air moving past you? 0 MPH. So how can a plane fly with 0 airspeed?

In theory there may be enough mechanical wind from the tredmill and propeller over the wings to cause lift but it ALL depends on the stall speed of the wing. In the real world it's not possible to get a plane to fly in this way because there must be sufficient airspeed realitive to the wing and not the ground.

The ground can go 1000MPH realitive to the airplane but without sufficient wind speed to generate lift it will not fly, it's that simple.

quote Originally posted by 84Bill: The ground can go 1000MPH realitive to the airplane but without sufficient wind speed to generate lift it will not fly, it's that simple.

THAT's WHAT I'M SAYING! : pulls hair out : Using this as an example- put your arms out of the window while on your way to have your car Dynoed. Then put your arm out the window as the car sits on the Dyno and is running at 210 mph. Get the same effect? Noooooooooooooooo... So, guess what braniacs, neither does the plane, regardless if it's pointing east, north south, west or up your butt.

[This message has been edited by F-I-E-R-O (edited 12-05-2005).]

Running on a tread mill is not the same thing.
Thats the same as a axle powered plane, an idea that was abandoned early on after people laughed at them and called them puddle jumpers.
Now if you put on roller skate and a backpack with a prop and a hang glider. It flys

Your LEGS are providing the forward movement on the treadmill. If you put on roller skates and stand on the tread mill, and place your hands on the rails on either side, are you thinking the treadmill would still pull you backwards? No, you could pull yourself forward and back REGARDLESS of the speed of the treadmill, because the force holding, pulling, or pushing, are your hands on the RAILS, not the skates against the treadmill. The ONLY way to keep the plane form taking off would have nothing to do with the wheels, unless you have the brakes locked while on the conveyor, otherwise, as posted in Cliff's quote, the plane FREEWHEELS, and if provided forward movement by AIR, not motivation afforded by the wheels. Sheesh, c'mon you guys, this isn't hard.

[This message has been edited by Taijiguy (edited 12-05-2005).]

I truly can't believe people still think the plane won't fly.

I rest my case - no explanation, calculation or scientific proof will make them think otherwise.

Air Speed DOES NOT EQUAL GROUND SPEED

Example. A Jet is flying straight up, a blue angel perhaps. Its climbing at a rate at a speed of 350 MPH. Its flying STRAIGHT up. In this sense, its making absolutely NO DOG (Distance Over Ground) but its still flying at a air speed of 350 MPH. Can you then say the plane is completely stationary in the air?

A plane gets its thrust from the AIR, NOT THE GROUND. Any plane will be able to produce enough thrust from the AIR to compensate the drag from the free spinning wheels. How do you think a helicopter works? It has absolutely ZERO ground speed when it takes off, yet produces enough thrust to go straight up.

The plane would NOT fly if the wheels were somehow connected to its foward force, but this isn't the case. The wheels are free spinning.

Say the ground isn't moving, in such the wheels are still in sync with the ground speed (=0) all of the sudden a headwind of 200mph comes about, does this mean the plane isn't going to produce lift?

Someone worded it exactly, since the wheels are free spinning, say, you're moving at 70MPH foward, so in this situation the conveyor belt will be moving 70MPH backwards , 140 MPH total of wheel speed, but again, the only thing wheel speed has to do with taking off is drag. The thrust (lets say 20,000 lbs) foward is going to over compensate the say 500 lbs of drag created by the wheels.

Heres another situation on the same concept, say you're super strong and can hold a jet motor capable of producing 50,000 lbs of thrust. You're also in roller skates, and your standing on this hypothetical runway. You have that jet motor cranking out full blast. You say you're still going to be stationary? What happens if you jump? Do you accelerate foward then?

Anyways, scratch me down for you WILL take off.

quote Originally posted by Patrick: It's not the engine that would be the problem. It's the ability of the tires and wheel bearings to survive incredible speeds, which they wouldn't. Saying that the "friction of the wheels can be neglected" because of whatever reason is bogus. Friction is something that needs to be dealt with, not casually dismissed.

The speeds really wouldn't be that incredilble. Assuming a calm day, the wheel speed would only need to be twice what would be required for a normal take-off. For example, and realize I'm just pulling numbers out of the air here...

If a plane requires an airspeed of 50 knots to lift off the plane will lift off at 50 knots. In our problem, the conveyor will be moving backwards at 50 knots. Because the wheels are free spinning and not related to the conveyance of the plane, the plane will move forward, but the wheels will be spinning as if the plane were moving at 100 knots, which it is, in relation to the conveyor. The takeoff wouldn't take any longer than a normal take-off, so for how many seconds would the wheels and bearings have to take these conditions?

Let's look at another problem--a slightly different scenario--to help illustrate why the plane would move forward.

Under "normal" conditions, the engines propel the plane forward in relation to the ground by means of the surrounding air. The wheels simply spin to allow the plane to move across the ground at whatever speed the engines dictate. The forward movement of the plane creates lift, but the engines are pulling it across the ground, not the wheels. The wheels make no difference in the forward movement of the plane...they just allow the plane to roll. The force for forward movement is created by the engines in the air.

Now, lets assume that you had this theoretical conveyor belt, and a large brick wall at the back side of it. If you positioned the plane so that the tail was resting against the back wall and released the brake, you could start the conveyor, and move it at whatever speed you wished. The wheels would roll with the speed of the conveyor, since they are free spinning, but the plane would not move backward, being stopped by the presence of the wall. Now we've established that the conveyor can will simply spin the wheels if there is a force to prevent the plane from moving backward greater than the friction of the bearings in the wheels.

We've established that the forward movement is generated by the engines in the air, not the wheels. We've also established that the conveyor is capable of moving backward and rolling the wheels at a velocity greater than that normal for the airspeed of the plane. Put those two together.

Say this plane was sitting here motionless with the wheels spinning backward at 10 knots. If you started the engines, could they generate enough thrust to move the plane forward, even if the wheels were already (free) spinning because the conveyor was moving?

Which of the above isn't possible? If the plane generates its forward movement by use of the engines in the surrounding air, and the wheels simply follow along, allowing the plane to move across the ground, and we agree that the wheels will spin at a speed greater than the plane's airspeed (remember the brick wall) then you have to conclude that the plane would move forward, regardless of the speed of the conveyor. At the point of liftoff, if the conveyor speed is equal to that of the forward speed of the aircraft, the wheels will be spinning at a speed twice that of a "normal" takeoff on a normal runway.

There...now that I've added my own $.02, let's sit back and keep reading!

quote Originally posted by RandomTask: Air Speed DOES NOT EQUAL GROUND SPEED Example. A Jet is flying straight up, a blue angel perhaps. Its climbing at a rate at a speed of 350 MPH. Its flying STRAIGHT up. In this sense, its making absolutely NO DOG (Distance Over Ground) but its still flying at a air speed of 350 MPH. Can you then say the plane is completely stationary in the air?

Au, contraire! In your example, you describe airspeed in 3-space (3 dimensions) while you arbitrarily limit the groundspeed to 2-space. You can't have it both ways ... at least not at the same time. In 3-space, "groundspeed" (speed with reference to the ground) and "distance over ground" (a strictly two-dimensional phenomenon) are not the same thing. Back to your example, a jet climbing vertically at 350 MPH not only has a true airspeed of 350 MPH, but it also has a vertical speed (with reference to the ground!) of 350 MPH.

In fact, climb and descent are both conventionally defined with reference to the earth, not the air. In a strong updraft, you can have the throttle at idle in steady-state flight and still be climbing. Similarly, several large and powerful aircraft have crashed because they didn't have enough power to arrest their descent in a downburst.

That said, we are both nit-picking here. Both of the terms "airspeed" and "groundspeed" normally assume a two-dimensional context of steady-state, unaccellerated, horizontal flight unless explicitly stated otherwise. I stand behind my original post:

quote Originally posted by Marvin McInnis: ... In a no-wind situation ... and, more specifically, at takeoff ...the ground speed (of an airplane) is indeed approximately the same as its TRUE airspeed. Conversely, ground speed is seldom the same as INDICATED airspeed....

[This message has been edited by Marvin McInnis (edited 12-05-2005).]

quote Originally posted by fierofetish: OK, in a nutshell. The speed of the plane is AT ALL TIMES relevant to the opposite speed of the conveyorbelt,

This is incorrect.

As I stated earlier. There is NO physical way for the conveyer, via speed variation, to oversome the thrust of the planes engines. The ONLY way the plan could remain stationary is if the plane where physically tethered to the ground. Then the tethers would be the restrictive force against the thrust. The plane will fly since it will overcome the conveyers resistance by virtue of the fact that the trust of the engines is acting against something that the conveyer is not related to...the air molecules.

This is truely sad.....I hope for the sake of human good, you people (The ones that are saying it WON'T fly) are not engineers.
If the plane is moving forward at 10 mph...it's moving at 10 mph...even if the conveyor is moving at 10 mph in the opposite direction. If it were a car and you would argue that the speedometer would then be reading 20 mph, then the conveyor would speed to 20 mph...so then you'd be going 40 mph so the conveyor would go to 80......and so on and so on.....THAT'S RIDICULOUS!!!!! The SPEED would not be relative to the conveyor, the speed is the magnitude of the velocity vector, and the conveyor would not affect that. If the plane is going to go 10 mph in the forward direction, it doesn't matter if the wheels are going 10 mph in the reverse direction....the speed of the plane is STILL 10 MPH IN THE FORWARD DIRECTION!!!