Will the Plane FLY?

[Mobius]

The key thing to remember is that aeroplanes are NOT driven by their wheels - so the existence of the conveyor belt is a red herring. The plane will take off just fine: the only difference to a normal take off is that the landing gear wheels are turning twice as fast as normal.

It's not exactly rocket surgery now is it?

[Apogee]

Wait, wait.. I was under the impression that the conveyor belt was moving the plane backward at the same speed the engine were making it go forward? Not the case?

[Cuda68]

No - it says the moving conveyor belt is going the opposite direction and is designed to match the speed of the plane. The point is it makes no difference because the weels are free spinning and have nothing to do with holding it back. The plane is being pulled along by the props/jet engine pulling on air and will out pace the belt with no problem - it will then have lift.

[RoBoT]

Like I said before, it's the same principal that applies to bikes. A bike will only stay upright on its own if its forward motion is equal to or greater than the force of gravity pulling it down--if it is MOVING FORWARD. With planes, that same forward movement negates gravity's pull on the plane, making it harder to maintain traction on the ground. Therefore, the wheels do nothing more than smooth out the takeoffs and landings.

As for the lift issue: take a strip of paper and blow over the TOP of it, and it will rise. Again, same principal applies to plane wings: the air moving over the tops of the wings causes them to rise. The faster the plane goes, the harder the air blows across the wings, the higher they rise, until they take the rest of the plane up with them.


Now, if a plane crashes, and half of the plane is on the US side of the northern border, and half is on the Canadian side, where would you bury the survivors?

[Genghis]

Well hell, I sure didn't think that one through. Here I was all self-satisfied that everyone on this board was a bunch of half-witted uncle frackers, and it turns out I was the half-witted uncle fracker. I'm sitting here slapping my forehead and feeling quite chagrined.

However, since I still believe I am better than everyone else here despite the immediate evidence to the contrary, I will attempt to impart some wisdom despite (or as a result of) my folly.

(1) Everybody look at the guy who is admitting he is wrong. Now realize that in many arguments on this board, someone is wrong. Finally, consider how many folks here admit it when they were wrong. (Hint: it is a vanishingly small number)

(2) Apologies to Bubba, RC, and Bettina. When I read your responses they seemed like so many incoherent thoughts. Well it turns out you were right, but consider why your responses didn't convince me when others' did. The answer: your responses were unclear. I'm not blaming you for my idiocy, but good technical writing skills are key for any scientist, engineer, or professional smart-arse. Each of you had clear images in your mind, and I now that I'm enightened I can see those images, but your explanations did not convey your images to the average dummy. They could have been worded such that even a frack-tard like me could understand them.

My hubris knows no bounds.

[Dedman]

As a fellow aerospace engineer, I concur with Paul. Whether the plane will fly or not is solely determined by the speed the wing moves relative to the airflow (assuming we are not talking about an aircraft with VTOL capabilities).

The way I read Duper’s question, I see this as a vector problem. Let’s say the plane needs to reach an airspeed of 100 mph to produce enough lift to fly. Now let’s say that you (the observer) are sitting on an air molecule next to the plane. If the throttle is set to provide a ground speed of 100 mph in the forward direction and the surface of the conveyor belt is traveling 100 mph in the opposite direction, the plane stays stationary WRT the observer (100 mph + (-100 mph) =0). Unless the the plane is experiencing a 100 mph headwind, that is you go whizzing past the plane at 100 mph, it will not fly.

An easy way to think of it is this: when you are on a tread mill at the gym and it is set at 4 mph and you are running at 4 mph are you moving WRT to an observer standing next to you but not on the treadmill? Anyone who has been to the gym knows the answer is no. Even though the plane is not propelled forward by its wheels the effect is the same.

No forward motion WRT to the air means no take off.

[Xamindar]

Well hell, I sure didn't think that one through. Here I was all self-satisfied that everyone on this board was a bunch of half-witted uncle frackers, and it turns out I was the half-witted uncle fracker. I'm sitting here slapping my forehead and feeling quite chagrined.

However, since I still believe I am better than everyone else here despite the immediate evidence to the contrary, I will attempt to impart some wisdom despite (or as a result of) my folly.

(1) Everybody look at the guy who is admitting he is wrong. Now realize that in many arguments on this board, someone is wrong. Finally, consider how many folks here admit it when they were wrong. (Hint: it is a vanishingly small number)

(2) Apologies to Bubba, RC, and Bettina. When I read your responses they seemed like so many incoherent thoughts. Well it turns out you were right, but consider why your responses didn't convince me when others' did. The answer: your responses were unclear. I'm not blaming you for my idiocy, but good technical writing skills are key for any scientist, engineer, or professional smart-arse. Each of you had clear images in your mind, and I now that I'm enightened I can see those images, but your explanations did not convey your images to the average dummy. They could have been worded such that even a frack-tard like me could understand them.

My hubris knows no bounds.

You are weird.

[Cuda68]

This was an awsome post. Really made me think

[Bet51987]

Well hell, I sure didn't think that one through. Here I was all self-satisfied that everyone on this board was a bunch of half-witted uncle frackers, and it turns out I was the half-witted uncle fracker. I'm sitting here slapping my forehead and feeling quite chagrined.

However, since I still believe I am better than everyone else here despite the immediate evidence to the contrary, I will attempt to impart some wisdom despite (or as a result of) my folly.

(1) Everybody look at the guy who is admitting he is wrong. Now realize that in many arguments on this board, someone is wrong. Finally, consider how many folks here admit it when they were wrong. (Hint: it is a vanishingly small number)

(2) Apologies to Bubba, RC, and Bettina. When I read your responses they seemed like so many incoherent thoughts. Well it turns out you were right, but consider why your responses didn't convince me when others' did. The answer: your responses were unclear. I'm not blaming you for my idiocy, but good technical writing skills are key for any scientist, engineer, or professional smart-arse. Each of you had clear images in your mind, and I now that I'm enightened I can see those images, but your explanations did not convey your images to the average dummy. They could have been worded such that even a frack-tard like me could understand them.

My hubris knows no bounds.

What an honest person! Geez, your ok.

I should have made it clear what I meant when I gave my answer, but I looked at it as a very simple problem, and it was. Lots of people here read way too much into it because they think technically... while my mind is still in its uncluttered youth.

For what its worth, a while back this problem was on a physics forum I go to and you would be surprised how many of these very intellegent, science minded people, got it wrong.

My favorite scientist was Albert Einstein and he once said: "Make things as simple as possible, but no simpler" and thats how I approach all problems.

Bettina

[Duper]

LOL Genghis, you shouldn't post when you're drinkin! ;D

(j/k) good form bro. You didn't go off that badly. I've seen folks on other forums TEAR into others for less than this topic. this is \"home\". Nothing ventured, nothing gained.

I posted this just to stir some brain juices. Like I said, the question is very poorly structured. So, more than anything, it's a good excuse debate.

[Ferno]

Everybody look at the guy who is admitting he is wrong. Now realize that in many arguments on this board, someone is wrong. Finally, consider how many folks here admit it when they were wrong. (Hint: it is a vanishingly small number)

I am also one of those people who will admit when he's wrong.

[Top Gun]

As a fellow aerospace engineer, I concur with Paul. Whether the plane will fly or not is solely determined by the speed the wing moves relative to the airflow (assuming we are not talking about an aircraft with VTOL capabilities).

The way I read Duper’s question, I see this as a vector problem. Let’s say the plane needs to reach an airspeed of 100 mph to produce enough lift to fly. Now let’s say that you (the observer) are sitting on an air molecule next to the plane. If the throttle is set to provide a ground speed of 100 mph in the forward direction and the surface of the conveyor belt is traveling 100 mph in the opposite direction, the plane stays stationary WRT the observer (100 mph + (-100 mph) =0). Unless the the plane is experiencing a 100 mph headwind, that is you go whizzing past the plane at 100 mph, it will not fly.

An easy way to think of it is this: when you are on a tread mill at the gym and it is set at 4 mph and you are running at 4 mph are you moving WRT to an observer standing next to you but not on the treadmill? Anyone who has been to the gym knows the answer is no. Even though the plane is not propelled forward by its wheels the effect is the same.

No forward motion WRT to the air means no take off.

That's the exact question I had, Dedman. I'm a Physics major, I read the official explanation here, and I still felt as though I was missing something. I really didn't understand what friction had to do with anything until about 30 or so seconds ago. Even now, I really don't like the way that aspect of the argument was worded I'm still flipping it back and forth in my mind, but I think I've finally got it down. Let me try rephrasing things here for a second; if there's anything at all I'd consider myself remotely good at, it's laying out arguments in plain language.

You're absolutely right; it really is all about the airspeed. Below a certain speed of air moving relative to the plane, there's insufficient lift produced, and the plane won't get anywhere. (One of the problems I had with the arguments I read is that some make it sound as though the propellor/engine somehow provides the upward thrust, which is dead wrong; it simply provides the necessary motion forward to produce that proper relative airspeed.) Obviously, then, this whole question revolves around whether or not a plane in this situation will be able to generate that relative airflow.

You mentioned a person on a treadmill, which is the same thing as the car example posted before. In both of those cases, the static friction between the person's shoes or the tires is what creates the forward thrust that propels the object forward. (For anyone else following along, yes, I said static; when your car moves, it's actually the portion of the tire at any given instant that's stationary relative to the road that provides that force.) If the surface you're getting that thrust from is also moving backwards at an equal and opposite rate to the velocity that thrust is generating, then you'll simply stand still, just as that person on the treadmill does. In all of these cases, in a windless environment, there's no airflow relative to the person/car, as well, so a hypothetical plane-car (think Bond )on a dynamo wouldn't be able to get anywhere. No airflow = no lift.

Now, let's go back to that plane-on-a-treadmill example. As you obviously know, whether in the air or on the ground, a plane generates forward thrust through its propellor/engine pulling at the air in front of it and pushing it out behind. This generates the necessary speed relative to the air molecules that allows the plane to attain sufficient lift to fly. In this vein, a plane that requires 80 mph relative to the air to attain flight would be able to take off in a 70 mph headwind by traveling at a speed of only 10 mph relative to the ground, since the combined vectors produce that 80 mph airspeed. You'd think that this would mean that a plane on a conveyor belt on a calm day would have no chance to take off, since there would be no relative airspeed to produce lift. Obviously, the engines don't push air over the wings to produce that lift (though that would be a pretty cool, if most likely unfeasible, prospect ), so it seems as though there's no way to generate that relative airpseed, just as in the case of the car.

However, there's a fundamental difference between the car and the plane. The car relies on contact with the ground to generate any speed; lift a stationary car up in the air, rev it up, and it'll just freewheel in position. However, a plane requires no such contact to produce forward thrust; obviously enough, if you suspend a plane in midair and rev up its motor, the propellor/engine will produce forward thrust, and the plane will start to move forward, regardless of whether or not it has sufficient airspeed to maintain level flight.

Now, let's look back at that treadmill. When the plane is sitting stationary on the treadmill, there is some static friction between the tires and the belt; there's also some inherent friction in the wheel bearings, but for the sake of argument, let's call that negligible. This friction, though, isn't what produces forward thrust for the plane. It isn't as though a plane's wheels have built-in driveshafts that allow them to produce forward thrust when they taxi; it's really the prop/engines that's producing that thrust. The wheels are really acting as coasters, reducing friction enough for the plane to move forward. That's exactly how seaplanes and ice planes work without wheels; their pontoons/skis reduce friction between the ground and the plane and allow it to move forward.

Here's where we start the treadmill. Say the plane starts moving forward, at 10 mph relative to the ground. At that moment, someone starts the treadmill up; it moves at 10 mph in the opposite direction to the plane's motion. If this were a car, where the motion of the wheels relative to the ground is what drives it forward, it would obviously stop, since the "road" is moving equal and opposite to its forward thrust. However, this isn't a car. The propellor/engine is what produces the thrust that moves the plane forward, not any contact with the road surface. Provided there's no massive amount of friction between the track surface and the plane's wheels, the plane will keep moving forward regardless of what the treadmill is doing, since that prop/engine is still pulling on the air. That pull will drive the plane forward relative to the ground, and it will generate the necessary airspeed to attain lift and take off.

What about the wheels, you may ask? Well, it isn't as though they're not affected. Let's go back to the start, when the plane's moving forward at 10 mph and we switch on the treadmill. Remember, unlike a car, a plane's wheels are no more than freewheeling coasters; they don't have to produce any sort of grip to move the plane forward. When the treadmill starts moving backward relative to the plane's motion, those wheels will remain in contact with the runway. However, since they provide no grip and no forward thrust to the plane, they're able to freewheel at any speed. Since they don't provide any (significant) grip, if the plane is moving at 10 mph forward, and the treadmill starts moving at 10 mph backward, the wheels of the plane will be moving at an equivalent speed to 20 mph. Meanwhile, the plane's engine will continue to move it forward through the air, without being affected at all by what its wheels are doing; the necessary airspeed will be produced, and the plane will attain flight.

It took me a while of racking my brain to come up with this; I kept getting hung up on, "Where the heck is the airspeed coming from?" My brain automatically wanted to leap to that car argument; it took me a while to make the connection that a plane is fundamentally different from a car in the way it produces its forward motion. A plane's wheels have nothing to do with the plane's forward motion, while a car's wheels have everything to do with it; that's the connection I had to make before I understood the answer. At any rate, I hope that I helped you (or anyone else) to wrap your head around this; it's definitely a doozy.

P.S. I think the whole rocket argument really doesn't do much to explain the solution either. While the principle of action-reaction is the same as an airplane's propellor, there's a fundamental difference between them: a plane requires an air medium, while a rocket can work in vacuum; besides, a rocket's thrust is generated straight down, which ignores the whole question of wheels. I know that I kept looking at that and thinking, "But what does a rocket have to do with an airplane?" I see the point now, but I don't think it does much for clearing things up for someone who doesn't.

P.P.S. Wow; this turned out to be a post of Draconian proportions.

[JMEaT]

There is no lift... Hence... no it won't fly...

[Dedman]

Great post Top Gun. I am still having a devil of a time visualizing how the plane could actually fly.

One guy even got one of those rubber band powered wood and plastic airplane that sell for about a buck, put it on the treadmill someone foolishly donated to the Lounge years ago, thinking that pilots might actually exercise. He wound up the rubber band, set the treadmill to be level, and at its highest speed. Then he simultaneously set the airplane on the treadmill and let the prop start to turn. It took off without moving the slightest bit backwards.

I am going to have to try this and see what happens. I'll let you know.

[Top Gun]

There is no lift... Hence... no it won't fly...

Oh, there is indeed lift. See above.

[Xamindar]

There is no lift... Hence... no it won't fly...

Somebody didn't read all the posts in this thread.

[Duper]

Well done top. +5 cool points for you ..er.. when I get some.

[MD-2389]

The play will fly people. There is nothing stopping it from moving! The thing isn't wheel driven, so it won't matter how fast the belt travels because all thats going to do is spin the tires a little faster. Once the engines fire up, there is NOTHING except for friction to slow the plane down. Anyone here thats actually ridden in a jet airliner knows how quick these things can move.

Now if we were talking about a car here, there would be no contest.

[Lothar]

I guess being a "head-in-the-clouds" student of Mathematics, I'm more interested in the hypothetical, where the conveyor keeps the plane motionless (which is theoretically possible, if the force of the thrust is not enough to overcome the friction).

REAL Mathematicians (TM) don't believe in friction.

-----

For anyone who doesn't yet know the answer:

It doesn't matter how fast the conveyer belt is turning or in what direction, as long as the landing gears are able to roll freely along the belt. If the brakes are on, the plane will never move, and therefore, neither will the belt -- so I'm going to assume the brakes are not on. In that case, all that matters is how fast the plane's engines are pulling it through the air. The belt doesn't matter.

Imagine the airplane kicks the engines up fast enough to get an airspeed of 200 mph. The plane will move forward that fast (or close to it, owing to some friction from the ground.) At that airspeed, most airplanes will take off. The conveyer belt will spin the opposite direction at 200 mph. This means the plane's tires will spin at 400 mph, but that's entirely irrelevant, unless you're the guy who has to inspect the tires after the plane's next landing.

Consider the opposite scenario: imagine a sports car driving through a strong headwind. Say, the car's wheels are turning at 60 mph and the wind is 60 mph. The car will move forward at 60 mph, because all that matters for the car is how fast the tires are moving along the ground. The wind will feel like it's at 120 mph and make the car difficult to handle, but unless it's so strong that it lifts the car off the ground, the car will still go forward. In this case, the wind doesn't matter because the car isn't using the air to move, it's using its tires. In the previous case the conveyor belt didn't matter because the plane isn't using its tires to move, it's using its engines in the air.

[Foil]

I guess being a "head-in-the-clouds" student of Mathematics, I'm more interested in the hypothetical, where the conveyor keeps the plane motionless (which is theoretically possible, if the force of the thrust is not enough to overcome the friction).

REAL Mathematicians (TM) don't believe in friction.

Oh, how true! I think I'm gonna have to send that quote to a couple of my old professors.

Anyway, although I still think it was vaguely worded at first, it sounds like there's now a general consensus that the question was meant to say:

"the conveyor speed matches the airplane's airspeed" (so the plane actually can move forward)

instead of:

"the conveyor speed matches the airplane's wheel speed" (keeping the plane stationary)

And, yes, the plane does indeed fly.

[Bet51987]

I guess being a "head-in-the-clouds" student of Mathematics, I'm more interested in the hypothetical, where the conveyor keeps the plane motionless (which is theoretically possible, if the force of the thrust is not enough to overcome the friction).

REAL Mathematicians (TM) don't believe in friction.

Oh, how true! I think I'm gonna have to send that quote to a couple of my old professors.

Anyway, although I still think it was vaguely worded at first, it sounds like there's now a general consensus that the question was meant to say:

"the conveyor speed matches the airplane's airspeed" (so the plane actually can move forward)

instead of:

"the conveyor speed matches the airplane's wheel speed" (keeping the plane stationary)

And, yes, the plane does indeed fly.

Ummm... My bobblehead is saying no to the last part about keeping the plane stationary.

If we take a standard plane like a cessna that has a standard set of wheels and a standard set of wheel bearings, then place this entire scenario on a conveyor belt that is the length of the runway on a standard sunny day with no wind, the plane will still take off whether the conveyer matches the planes airpeed or the tires ground speed.

There will not be enough friction, at the speeds neccessary for the cessnas takeoff, to overcome the thrust of the propeller moving the airplane to takeoff airpseed. This airplane will take off with an airspeed of 80 knots and the worse case belt speed will be 160 knots. Not enough to burn out the bearings. (according to my dad who says those bearings are very rugged).

In the scenario as presented, the conveyor is still irrelevent and the plane is going to take off without any trouble at all.

Bettina

[Dedman]

(according to my dad who says those bearings are very rugged).

If ANYTHING on a prop driven Cessna goes anywhere near 160mph, sumthins gonna break

[snoopy]

Wait! They forgot to mention that this was on the surface of the moon, and that the wheels are actually bikes tires, and that the conveyor belt is covered with hundreds moose on acid, will it fly now?

[Bet51987]

(according to my dad who says those bearings are very rugged).

If ANYTHING on a prop driven Cessna goes anywhere near 160mph, sumthins gonna break

Yeah the takeoff speed is more like 62 making the wheel speed 124. He mentioned the bearings being rugged for landing shock. Either way, that plane is taking off.

http://www.bordenflyingclub.com/cmqperformance.shtml

bet

[Foil]

Ummm... My bobblehead is saying no to the last part about keeping the plane stationary.

If we take a standard plane...
There will not be enough friction, at the speeds neccessary for the cessnas takeoff, to overcome the thrust of the propeller moving the airplane to takeoff airpseed...

Quite true!

So... am I the only one interested in the hypothetical scenario where the conveyor is somehow able to keep the plane stationary?

Wait! They forgot to mention that this was on the surface of the moon, and that the wheels are actually bikes tires, and that the conveyor belt is covered with hundreds moose on acid, will it fly now?

See, there's one such scenario already!

[Bet51987]

Ummm... My bobblehead is saying no to the last part about keeping the plane stationary.

If we take a standard plane...
There will not be enough friction, at the speeds neccessary for the cessnas takeoff, to overcome the thrust of the propeller moving the airplane to takeoff airpseed...

Quite true!

So... am I the only one interested in the hypothetical scenario where the conveyor is somehow able to keep the plane stationary?

Wait! They forgot to mention that this was on the surface of the moon, and that the wheels are actually bikes tires, and that the conveyor belt is covered with hundreds moose on acid, will it fly now?

See, there's one such scenario already!

Just for fun, I would be interested in trying to keep the plane from taking off but don't see how it could happen.

In the original scenario, the conveyer could be built and a real plane put on it, then the problem could be acted out and the plane would take off.

The hypothetical scenario of causing the plane NOT to take off would be a major problem....I don't know how you can make a plane stay stationary on the belt so in effect the belt speed and plane speed keep it from flying... This could get interesting if anyone wants to try.

The only rule? It would have to be able to be built in real life... like the original scenario. Nothing "magical" can be put in the equation.

Maybe another thread would be nice.

Bee

[Valin Halcyon]

God..I can't believe a debate started over this.

Aircraft fly based on airspeed, yes? That is, the speed at which air flows over the wings. if the plane is moving forward on a belt which is going the same speed in the opposing direction, THE PLANE IS NOT MOVING AND HAS NO GROUND SPEED. That said, we have no indication of what the airspeed is relative to the ground speed, which if high enough (around 130 knots for a cessna 172), the plane would actually hover.

A good example would be a R/C model aircraft. I've seen really light ones with high lift wings be simply hand released into the air...not throwing it all, simply letting go of it while facing into the wind. Again, airspeed, relative to the ground speed and related to the airfoil design ratio.

Clear enough now? If we don't know the airspeed and the airfoil's rated takeoff airspeed, we don't know if the plane will fly or not. Ground speed has nothing to do with it whatsoever.

[TIGERassault]

So... am I the only one interested in the hypothetical scenario where the conveyor is somehow able to keep the plane stationary?

What about if the tyres werent able to rotate? Would that help achieve your scenario?

[Bet51987]

God..I can't believe a debate started over this.

Aircraft fly based on airspeed, yes? That is, the speed at which air flows over the wings. if the plane is moving forward on a belt which is going the same speed in the opposing direction, THE PLANE IS NOT MOVING AND HAS NO GROUND SPEED. That said, we have no indication of what the airspeed is relative to the ground speed, which if high enough (around 130 knots for a cessna 172), the plane would actually hover.

A good example would be a R/C model aircraft. I've seen really light ones with high lift wings be simply hand released into the air...not throwing it all, simply letting go of it while facing into the wind. Again, airspeed, relative to the ground speed and related to the airfoil design ratio.

Clear enough now? If we don't know the airspeed and the airfoil's rated takeoff airspeed, we don't know if the plane will fly or not. Ground speed has nothing to do with it whatsoever.

No, its not clear. You need to go over the problem again. This plane will fly.

Bee

[Cuda68]

God..I can't believe a debate started over this.

Aircraft fly based on airspeed, yes? That is, the speed at which air flows over the wings. if the plane is moving forward on a belt which is going the same speed in the opposing direction, THE PLANE IS NOT MOVING AND HAS NO GROUND SPEED. That said, we have no indication of what the airspeed is relative to the ground speed, which if high enough (around 130 knots for a cessna 172), the plane would actually hover.

A good example would be a R/C model aircraft. I've seen really light ones with high lift wings be simply hand released into the air...not throwing it all, simply letting go of it while facing into the wind. Again, airspeed, relative to the ground speed and related to the airfoil design ratio.

Clear enough now? If we don't know the airspeed and the airfoil's rated takeoff airspeed, we don't know if the plane will fly or not. Ground speed has nothing to do with it whatsoever.

You said that THE PLANE IS NOT MOVING AND HAS NO GROUND SPEED, I made the same assumption and it was incorrect - this statement is where you are making a left turn. All planes have free moving wheels, the only thing they do is roll free or brake. Since the brakes are not on during this manuever they are free spinning. So no matter how fast the belt is going in the opposite direction the propulsion from the propellers will outpace the belt, always, and therefore move the plane forward to get the needed speed and lift. So the point is the belt can never hold the plane back.

[Mobius]

As for the lift issue: take a strip of paper and blow over the TOP of it, and it will rise. Again, same principal applies to plane wings: the air moving over the tops of the wings causes them to rise.

This is a common misconception, and simply is NOT true.

Please believe me, I was a paragliding instructor for over decade, and taught micrometeorology and L/D theory the entire time.

While a glider DOES work in the method you describe, it does NOT represent an aeroplane. The lift generated by the faster air over the top surface of the wing isn't even really worth talking about as far as forces maintaining a powered aircraft in flight.

Aeroplanes are BIG and aeroplanes are HEAVY, and the only thing that stops them from descending at a high rate is THE LARGE AMOUNT OF AIR DEFLECTED DOWNWARDS BY THE ANGLE OF ATTACK ON THE WINGS. The "normal" angle of attack on a powered craft is very high, and of course, if the engines stop, then the plane will stall very rapidly because of the high angle of attack. In order to make the plane glide after the engine(s) fail, you have to push the stick forwards to poitn the plane downwards in order to lower the angle of attack and generate lift by forwards (and downwards!) movement. Most planes can glide at around 4:1 so if they are at 30,000 feet, they should be able to glide for about 120,000 feet across the ground before they have to ditch.

The plane deflects almost it's entire weight of air downwards in order to stay aloft. That is why there is such enormous turbulence behind a large plane, as opposed to a glider, which does NOT deflect air downwards, and relies purely on Bournelli's principle to stay aloft. Please note: a glider is ALWAYS descending through the air, even though it may climb large distances. It's simply that you find air going UP faster than you are going DOWN.

The turbulence behind a plane has little to do with the jets (or props) and it predominantly the turbulence caused by "tip vortex" and the amount of air deflected downwards, into a giant tumbling horizontal tornado appearing thing. You can demonstrate it for yourself by towing 200 metres of ribbon behind a small plane.

[Mobius]

Should be feeling sheepish Valin: the answer is very simple. You overcomplicated it for some unknown reason.

[Ferno]

the queston could have been answered with a simple yes.

man you guys are waay too cerebral.

[Stryker]

It's really quite simple to keep the plane from taking off. One of two ways: Put a brick wall in front of it, or a giant fan behind it.

[snoopy]

speaking of tip vortex- they have some sweet tip vortex effects in \"We Where Soldiers.\"

[Lothar]

if the plane is moving forward on a belt which is going the same speed in the opposing direction, THE PLANE IS NOT MOVING AND HAS NO GROUND SPEED.

What do you mean by "the plane is moving forward"? The body? Its tires? What's moving forward?

If the body of plane is moving forward (and the air is still, as has been assumed throughout) then the plane has airspeed. Period. It doesn't matter what's going on between the tires and the ground; it doesn't matter how fast the conveyer belt is moving backwards. The plane body has airspeed, and while the tires might be spinning awful fast against a conveyer belt, the plane will take off.

If the tires of the plane are spinning forward and the conveyor belt is moving backward such that the body of the plane is stationary, then it won't take off. But, this is a red herring, because airplanes don't use their tires to gain airspeed. It doesn't matter how fast you're making the tires spin, as long as the engines are generating enough thrust to make the airplane body move.

[Ferno]

It's really quite simple to keep the plane from taking off. One of two ways: Put a brick wall in front of it, or a giant fan behind it.

or throw a chain around the tail.


and mobius, as for the paper thing, it is true. i've seen it happen with my own eyes. The paper rises due to the air pressure below. That's how an airfoil works. Air moving over the top surface is forced to move faster, thereby creating a low pressure area. the air below the wing is at a higher pressure. so the air PUSHES up on the wing, and pushes the plane up.

"then the plane will stall very rapidly because of the high angle of attack"

If this was true, gliders would fall out of the sky once their tethers are released. but they don't.


you have no concept of basic physics. stop filling people's heads with garbage.

[Dedman]

and mobius, as for the paper thing, it is true. i've seen it happen with my own eyes. The paper rises due to the air pressure below. That's how an airfoil works. Air moving over the top surface is forced to move faster, thereby creating a low pressure area. the air below the wing is at a higher pressure. so the air PUSHES up on the wing, and pushes the plane up.

"then the plane will stall very rapidly because of the high angle of attack"

If this was true, gliders would fall out of the sky once their tethers are released. but they don't.


you have no concept of basic physics. stop filling people's heads with garbage.

As much as it pains me to say this Mobius has it right. The Bernoulli principal is a VERY flawed model of lift. A really good description of what causes a wing to produce lift can be found here .

[Ferno]

Then explain why a piece of paper rises when you blow over it.

[Xamindar]

Then explain why a piece of paper rises when you blow over it.

I don't think mobius knows that.