DescentBB

Awesome link Dedman!

Ferno: The \"Coanda Effect\" Only the wind moving over paper is the solid object and the paper is the \"water\" in the diagram in Dedman's link.

Bernoulli's principle states that \"an increase in the velocity of any fluid is always accompanied by a decrease in pressure\". And, since air is a fluid, blowing across the top surface of a sheet of paper will create lower pressure on top than the bottom, making the paper lift. This pressure difference is the basics of what makes planes fly.

HOWEVER, the air blowing across a wing of a large airliner at takeoff is nowhere near enough to make it lift. The plane could not get enough speed to create enough vacuum on top of its wing to take off.

So the angle of attack, or how much the leading edge of the wing is raised above the trailing edge, is needed to create an even lower pressure above the wing. But, its the much higher pressure under the wing, and the amount of air being forced downward because of that angle, that creates the majority of the planes lift. This also explains why flat wings without an airfoil can lift.

Once the plane is at cruising altitude, and flying at a much higher speed, less angle of attack is needed and Bernoulli's principle becomes more important than angle of attack.

Bettina

Ferno wrote:Then explain why a piece of paper rises when you blow over it. ;)

Bernoulli's principle works fine with a piece of paper. (I actually taught BP to a 4 year old earlier this year, during one of the classes I was teaching at the Museum of Flight. Have I mentioned I love my job?) But it's an incomplete description for why airplanes fly.

TY Dedman. I'll be passing that page on to my coworkers.

Bet51987 wrote:Once the plane is at cruising altitude, and flying at a much higher speed, less angle of attack is needed and Bernoulli's principle becomes more important than angle of attack.

Bettina

Actually, a lot of large planes have a neutral camber on their wings and use angle of attack almost exclusively. Next time you are on an airplane and you reach your crusing altitude, you will notice the plane is doing a slight "wheely" of a degree or 3.

Large airliners don't like Bernoulli and his fancy principle because although that principle is nice at lower airspeeds, it is very inefficient at higher airspeeds, such as what airliners and transports fly.....

Heh. Sorry I didn't read Dedman's link before. What I said is indirectly explained in there. I love you Dedman!!! Good link.

Hostile wrote:Next time you are on an airplane and you reach your crusing altitude, you will notice the plane is doing a slight "wheely" of a degree or 3.

Large airliners don't like Bernoulli and his fancy principle because although that principle is nice at lower airspeeds, it is very inefficient at higher airspeeds, such as what airliners and transports fly.....

I'm going to agree without an argument. even though you imply Bernoullis principle is something they can choose or not. Bernoullis principle does apply to the paper that has air blowing over it but is miniscule in scope to an aircraft which operates more on Newtons third law.

That 3 degrees BTW is small compared to the degrees neccessary at takeoff where they trade off forward speed for a lot of lift.

Bee

Lothar wrote:Bernoulli's principle works fine with a piece of paper. (I actually taught BP to a 4 year old earlier this year, during one of the classes I was teaching at the Museum of Flight. Have I mentioned I love my job?) But it's an incomplete description for why airplanes fly.

well, no argument from me. I tip my hat to you and duper. I feel like such a schlep

btw, i was hoping mobius would answer the question. then again that probably won't happen.


A question that just occured to me. if the angle of attack is important for generating lift, why is the airfoil shape still being used?

One other question: why in the hell is Bernoulli's Principle taught to just about everyone as the reason for aircraft lift if it's dead wrong? Seriously, I'm pretty pissed off about it. If anything made any sense in this world, I'd have understood the correct reason well before 19 years of age. This is just plain stupid...

A lot of airplanes use a cambered airfoil to generate the lift they need. Take all of your smaller light civil aircraft (ie Cessna-172, or a glider for that matter). These planes do not create a lot of airspeed and rely on this principle a lot. There are just trade offs that occur depending on size, speed, etc. It is not wrong. It just depends on the application.

Top Gun wrote:If anything made any sense in this world, I'd have understood the correct reason well before 19 years of age. This is just plain stupid...

yea exactly! I blame the system!

Top Gun wrote:One other question: why in the hell is Bernoulli's Principle taught to just about everyone as the reason for aircraft lift if it's dead wrong?

I am not really sure. My guess is it has to do with a lot of things:

- Most of the junior/senior highschool science teachers aren’t aerospace engineers.
- Bernoulli’s principal is very easy to teach through experimentation.
- Bernoulli’s principal is very easy for the student to visualize.
- Most people don’t question the obvious limitations of BP in explaining why planes fly.

Dedman wrote:

Top Gun wrote:One other question: why in the hell is Bernoulli's Principle taught to just about everyone as the reason for aircraft lift if it's dead wrong?

I am not really sure. My guess is it has to do with a lot of things:

- Most of the junior/senior highschool science teachers aren’t aerospace engineers.
- Bernoulli’s principal is very easy to teach through experimentation.
- Bernoulli’s principal is very easy for the student to visualize.
- Most people don’t question the obvious limitations of BP in explaining why planes fly.

You made a good guess I think. - Bernoulli’s principal is very easy to teach through experimentation.
I'm going to ask the teacher tommorrow. I'll let you know what he says.

Bee

Huh.

I remember learning, in Jr. High, that Bernoulli's principle was was made airplanes fly. Being a connoisseur of paper airplanes at the time, I made one with bulges on top of the wings, to see it in action. As I recall, the plane really sucked. My immediate analysis was that the drag caused by the broad wing was eating a lot of the energy the plane needed to go forward, and whatever gains I got from Bernoulli's principle weren't really helping.

I went back to using flat, super-aerodynamic-looking wings, and other methods (like the little 'winglets' & 'elevons' on the back) to keep my planes aloft, but I always figured that Bernoulli's principle must work--I was just doing it wrong.

... and then I come here and find this! I guess I probably was doing it wrong, since you say it works for gliders, but still... thank you, Mobius, you have solved one of my lifelong mysteries for me!

Top Gun wrote:One other question: why in the hell is Bernoulli's Principle taught to just about everyone as the reason for aircraft lift if it's dead wrong?

[Leading comment]

Yeah. I wonder how many other things they might be teaching in the schools that are scientifically bogus... (you know that of which I speak )

[/Lading Comment]

Drakona wrote:Yeah. I wonder how many other things they might be teaching in the schools that are scientifically bogus... (you know that of which I speak )

The Pythagorean Theorem

And Centrifical force

Duper wrote:And Centrifical force

a compound force that was mixed up with a fundamental force.

The correct force is Centripital. not CentriFical.

Ferno wrote:...A question that just occured to me. if the angle of attack is important for generating lift, why is the airfoil shape still being used?

My guess is it's efficiency, if the airfoil shape reduces the energy needed to maintain altitude at cruising speed, even by a little bit, it will save them literally millions on fuel costs every year.
As Hostile described it as a "wheelie" posture, imagine if they used a flat wing they would have to increase that pitch a few more degrees to maintain altitude at their desired speed. It would require a great deal more fuel/energy to slam that flat wing against the air to provide the lift needed. Change the shape of the wing a little bit and the aircraft doesn't need quite so much of an energy robbing wheelie posture to cruise....

I know we are all having fun bashing schools- but they arn't as wrong as you would think. It is true that angle of attack conributes significantly to lift in an airplane, but you guys are conviniently forgetting things. First of all, more angle of attack causes there to be more of a speed difference, and thus more of a pressure differential. Secondly, don't get \"Bernoulli's effect\" mixed up with \"airfoil.\" An airfoil is absolutely necessary to have any sort of efficient flight, because the flow over the top of the wing must stay laminar. If you start having turbulant flow over the top of the wing, you better have a beast of an engine, and be prepared to pay for lots of gas. Keeping the flow laminar depends on the coanda effect. (Air flow will \"stick\" to the curve.) The coanda effect is strong when there is more suction on the top of the wing, which is produced by the bernoulli effect. So, don't sell out completely on Bernoulli, because without the bernoulli effect, the downward deflection of air wouldn't happen. By the way, they explain things relatively well over at wikipedia.

Will Robinson wrote:My guess is it's efficiency, if the airfoil shape reduces the energy needed to maintain altitude at cruising speed, even by a little bit, it will save them literally millions on fuel costs every year.

Yeah, the foil doesn't just increase efficiency by a little bit... Most airliners wouldn't have powerful enough of engines to maintain altitude without an airfoil.

I imagine the airfoil also causes the craft to be more stable than one with a flat wing. Probably takes much less input to change the direction of a flat wing airplane. It would be like when I set my sensativity on the joystick too high, twitchy instead of having some lag to the initial input.

(All that is total guess work on my part, no science has been used to form this opinion and no animals were hurt during the forming of this opinion other than the cow that ended up as my dinner last night...)

Duper wrote:The correct force is Centripital. not CentriFical.

yea true, if you're referring to a fundamental force. Like gravity.

No gravity. Centripital force is the tangent vector that is leading off of the \"circle\" the spinning object it making.

Classic bucket-0-water example:

The water and bucket, remember, are moving in an infinite number of straight lines. If that bucket had a release trap in the bottom and a ball instead of water; that ball would travel in a straight line tangent from it's point of release. It would not travel directly away from center on a radius.

you can do the same thing with a fast moving merry-go-round.

Duper, you're right about how the forces are working, but if I recall correctly, the terminology is as follows in your bucket 'o water example:

\"centripetal\" force: This is the force which is holding the bucket in, provided in this case by the tension of the rope. This is the real force; without it, the bucket would go flying off in a straight line (in the tangential direction you mentioned).

\"centrifugal\" force: This is the supposed \"force\" felt by the water, pressing it into the bucket. When someone is riding a merry go-round, they feel like they're being \"pushed toward the outside\", so it's been called a \"force\". Technically, it's not; it's just the combined effect of inertia and the centripetal force (above).

\"centrifical\" <-- Not even a real word.

heh, real or not, folks use it like crazy.

Great follow up. thanks!

Duper wrote:No gravity. Centripital force is the tangent vector that is leading off of the "circle" the spinning object it making.

Classic bucket-0-water example:

The water and bucket, remember, are moving in an infinite number of straight lines. If that bucket had a release trap in the bottom and a ball instead of water; that ball would travel in a straight line tangent from it's point of release. It would not travel directly away from center on a radius.

you can do the same thing with a fast moving merry-go-round.

Foil wrote:"centrifugal" force: This is the supposed "force" felt by the water, pressing it into the bucket. When someone is riding a merry go-round, they feel like they're being "pushed toward the outside", so it's been called a "force". Technically, it's not; it's just the combined effect of inertia and the centripetal force (above).

that's what i was sayin.

Bet51987 wrote: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

I believe rolling friction is proportional to weight... so you just need to make the plane really heavy, and the frictional force can overcome the force produced by the engines.

I imagine if the guy who put the model on the conveyor belt had dropped a dictionary on top, the plane would have move with the conveyor belt, due to high friction.

Also, here's a video (3 MB .mov) I took of a project for school where we added a small wing to the front of a speedboat to try to induce a down pitching moment, and put it in a wind tunnel with a smoke stream.

As you can see, we actually curved the airfoil the opposite way than normal, so that it would produce a downward force even at no angle relative to the wind. But you can see just how much affect the airfoil has on deflecting the otherwise horizontal flow.

Paul wrote:Also, here's a video (3 MB .mov) I took of a project for school where we added a small wing to the front of a speedboat to try to induce a down pitching moment, and put it in a wind tunnel with a smoke stream.

As you can see, we actually curved the airfoil the opposite way than normal, so that it would produce a downward force even at no angle relative to the wind. But you can see just how much affect the airfoil has on deflecting the otherwise horizontal flow.

I hope I can explain what I am seeing and I like the video a lot.

The airfoil on your boat is acting as it should. When the smoke stream is shown hitting the leading edge and then moving along its curved portion you will see it hugging the wing. This is due to the low pressure being developed there and you are indeed witnessing the bernouli effect. When the smoke exits the trailing edge, it is deflected skyward. This will cause your bow to move down.

Then, when the smoke screen is made to hit the flat part of the wing, you are increasing the pressure at the flat and decreasing it at the curved portion. Again, the stream of smoke is shown deflected skyward, causing your bow to move down.

The point is that the wing works as intended. No matter where you direct the stream of smoke, both sides of the wing will move the air upward forcing your bow down, but its the higher pressure that is doing the majority of the work, and the vacuum on the curved portion is helping, but very little.

You do not need an airfoil for this application since a flat plate would do just fine for the high pressure to do its job. However, at much higher speeds and much higher altitude where airliners fly and the air is thinner, the flat plate wing would be much worse because the airstream on top of the wing would be very turbulent instead of smooth. A greater efficiency would be gained if BOTH sides of the wing are used and the top is curved in an airfoil shape.

Bettina

Back to the original question:

Duper wrote:An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

Also, it's said that the conveyor is designed to "match the speed of the plane".

As Duper said, the question is flawed. Or more accurately, it's ambiguously stated such that it's a riddle, not a true question. You can interpret it how you want, such that either answer is correct.

What does "match the speed of the plane" mean? If it means match the speed of the wheels' rotation, then in fact, by definition, the plane is not moving. Because for the plane to move forward, it's wheels would have to move faster than the conveyor belt. Or the wheels would have to be skidding along the conveyor belt without turning any faster.

But this is the theoretical math/physics definition of the problem; kind of a thought experiment. As a group, we seem to have decided on the engineering defintion of the problem, as in what would actually happen in the real world.

I therefore declare this entire discussion moot. Also, it makes me slightly less wrong (which is the important part).

Genghis wrote:Back to the original question:

Duper wrote:An airplane taxies in one direction on a moving conveyor belt going the opposite direction. Can the plane take off?

Also, it's said that the conveyor is designed to "match the speed of the plane".

As Duper said, the question is flawed. Or more accurately, it's ambiguously stated such that it's a riddle, not a true question. You can interpret it how you want, such that either answer is correct.

What does "match the speed of the plane" mean? If it means match the speed of the wheels' rotation, then in fact, by definition, the plane is not moving. Because for the plane to move forward, it's wheels would have to move faster than the conveyor belt. Or the wheels would have to be skidding along the conveyor belt without turning any faster.

But this is the theoretical math/physics definition of the problem; kind of a thought experiment. As a group, we seem to have decided on the engineering defintion of the problem, as in what would actually happen in the real world.

I therefore declare this entire discussion moot. Also, it makes me slightly less wrong (which is the important part).

Never give up...ever.

Bee

Bet51987 wrote:

Paul wrote:Also, here's a video (3 MB .mov) I took of a project for school where we added a small wing to the front of a speedboat to try to induce a down pitching moment, and put it in a wind tunnel with a smoke stream.

As you can see, we actually curved the airfoil the opposite way than normal, so that it would produce a downward force even at no angle relative to the wind. But you can see just how much affect the airfoil has on deflecting the otherwise horizontal flow.

I hope I can explain what I am seeing and I like the video a lot.

The airfoil on your boat is acting as it should. When the smoke stream is shown hitting the leading edge and then moving along its curved portion you will see it hugging the wing. This is due to the low pressure being developed there and you are indeed witnessing the bernouli effect. When the smoke exits the trailing edge, it is deflected skyward. This will cause your bow to move down.

Then, when the smoke screen is made to hit the flat part of the wing, you are increasing the pressure at the flat and decreasing it at the curved portion. Again, the stream of smoke is shown deflected skyward, causing your bow to move down.

The point is that the wing works as intended. No matter where you direct the stream of smoke, both sides of the wing will move the air upward forcing your bow down, but its the higher pressure that is doing the majority of the work, and the vacuum on the curved portion is helping, but very little.

You do not need an airfoil for this application since a flat plate would do just fine for the high pressure to do its job. However, at much higher speeds and much higher altitude where airliners fly and the air is thinner, the flat plate wing would be much worse because the airstream on top of the wing would be very turbulent instead of smooth. A greater efficiency would be gained if BOTH sides of the wing are used and the top is curved in an airfoil shape.

Bettina

Yup, that's a pretty good explanation. One thing to keep in mind, though, is that there is constant airflow over the entire wing, the smoke just serves to highlight a portion of it, showing the streamlines. While you only see part at a time, the part you don't see is still there, acting just as it did when you saw it.

You're right that we didn't really need an actual airfoil, but the camber also helps turn the flow, and the extra thickness for using an airfoil instead of a curved plate doesn't really have much downside in this case.