Schools not teaching physics ?

[RAF_Blackace]

My son arrived home from school yesterday. After a physics lesson his homework involved flight and the wing and why it flies.

After questioning him I find he was told that a wing flies because of the reduced pressure of air flowing over the top surface of the wing and hence lift is generated. Bernoulli in other words.

The problem I have with that is that it has been known for years that this is not the full story, the pressure difference explained by Bernoulli accounts for a very small percentage of the lift generated. By far the vast majority of lift is generated by the deflection of the mass of air downwards, Newton in other words.

Wings fly because a mass of air is deflected downwards mostly, the additional lift from Bernoulli just helps it.

I suppose the best example of this is when an aircraft takes off, if it was purely down to Bernoulli the aircraft would simply lift off the ground once it attained take off speed without having to pull back on the stick, but they never would because the lift generated is simply not nearly enough. Aircraft have to flair or give the wing an angle of attack to the airflow to generate real lift and get the plane off the ground.

Having looked around the web there still seems to be a raging debate about this for some reason.

What were you told at school, and where do you stand on the debate, with Bernoulli or Newton ?

[Polyrealastic Observer]

As a pilot, I've always tried to see the fluid dynamics of the situation. The images of a wing in a wind tunnel with smoke in the airstream help me mentally. The answer to the need for rotation and flair is that they are useful techniques. An aircraft will eventually fly off the runway with no increase in the pitch angle. Rotation is just a safe way to expedite departure from the runway surface and gain altitude for obstruction clearance. Try riding with your arm out the window, palm down, parrallel to the ground. tilt your thumb up, and your arm gets "sucked" back. To me, I don't feel a major increase in wind on my palm so much as a suction. My vote is for Bernoulli.

[Gillianren]

I don't think we ever got to flight at all in my physics class. We may have been supposed to (we missed a lot, and for excellent reasons), but mostly, we did lenses and resistance and maybe one or two other things. It's been a long time since I've really thought about it, though.

[Bearded One]

Planes, well some of them, can fly upside down! So much for Bernoulli.

I remember being taught the Bernoulli principal and being told that it was important to flight. As an avid paper airplane builder I knew there was more to the story.

ETA:

What grade level are we talking about here? My high school physics class never touched flight, just as Gillianren's didn't. It was discussed in earlier general science classes though.

[Ara Pacis]

Yeah, I seem to recall it was taught as Bernoulli, but I learned more about deflection a few months ago when there was a debate about it in a thread here at BAUT.

[Fendercaster]

I went to high school out in the sticks in the '60s. Physics wasn't offered as a course; I've always regretted that!

[cjl]

In high school, we didn't cover it much at all. In my recent aerodynamics class however (ASEN 2002), we were taught fairly clearly that there was more to it than bernoulli. This is a good site to help explain it fairly well: http://www.grc.nasa.gov/WWW/K-12/airplane/right2.html

[01101001]

Having looked around the web there still seems to be a raging debate about this for some reason.

Did you look around BAUT Forum to see what the denizens there have had to say previously on this topic? That should tell you where a lot of members stand.

[NEOWatcher]

Planes, well some of them, can fly upside down! So much for Bernoulli.

Actually, with the shape of the wings, it doesn't really matter whether they are rightside up or not, because you can still get a "Bernoulli friendly" profile with the right angle of attack while it's upside down

I remember being taught the Bernoulli principal and being told that it was important to flight. As an avid paper airplane builder I knew there was more to the story.

Same here, but they didn't seem to say "important to" they always seemed to say "only because".
I had always questioned that. Paper airplanes made me suspect it, but they did have a leading edge where the fold was. Where I was more convinced was the rubberband planes with the straight balsa wings.

What grade level are we talking about here? My high school physics class never touched flight, just as Gillianren's didn't. It was discussed in earlier general science classes though.

In my case, it carried right on through high school in aviation class.

[PraedSt]

What were you told at school, and where do you stand on the debate, with Bernoulli or Newton ?

Bernoulli at school-school, both at flying-school. I prefer Newton in my head. For rough calculations I also use the Archimedes principle (which is sort-of Newton).

[Fazor]

I almost said something in a thread last week (or so), but decided I'd probably just be exposing my ignorance. But this thread brings my question back up.

Technically, isn't moving through air the same as moving through water? Obviously it's much less dense, but a "wing" on, say, a submarine, allows it to navigate underwater. A blade cutting through air would work much in the same way, but to less effect, wouldn't it? Which, if I'm understanding any of this, would indeed seem more to deflection than Bernoulli's Principle.

If so, then on the most basic (Fazor's) level, isn't the key to flight being able to "push off" on a mass of air that's greater than or equal to the mass of the object in flight?

[Larry Jacks]

As for planes flying upside down, the competition aerobatic planes use symmetrical airfoils (close to equal curvature on top and bottom) and depend on the angle of attack to generate lift in either direction. Likewise, with enough power, I've seen airshow pilots fly for considerable distances in knife-edge flight (90 bank angle) with no lift from the wings. Here's a good video that demonstrates knife-edge flight with a model airplane.

You can fly upside down in airplanes with ordinary airfoils but you have to maintain a higher angle of attack. It's harder but anyone who has seen a good old Steerman or Waco flying aerobatics knows it can be done.

Edit: Wow! What happened to my link? The link is corrected now.

[HenrikOlsen]

Technically, isn't moving through air the same as moving through water? Obviously it's much less dense, but a "wing" on, say, a submarine, allows it to navigate underwater. A blade cutting through air would work much in the same way, but to less effect, wouldn't it? Which, if I'm understanding any of this, would indeed seem more to deflection than Bernoulli's Principle.

There's one very fundamental difference, which is that water is almost entirely incompressible.

[PraedSt]

Likewise, with enough power, I've seen airshow pilots fly for considerable distances in knife-edge flight (90 bank angle) with no lift from the wings. Here's a good video that demonstrates knife-edge flight with a model airplane.

That's a gun ad!

[PraedSt]

Technically, isn't moving through air the same as moving through water? Obviously it's much less dense, but a "wing" on, say, a submarine, allows it to navigate underwater. A blade cutting through air would work much in the same way, but to less effect, wouldn't it? Which, if I'm understanding any of this, would indeed seem more to deflection than Bernoulli's Principle.

If so, then on the most basic (Fazor's) level, isn't the key to flight being able to "push off" on a mass of air that's greater than or equal to the mass of the object in flight?

I look at it as technically the same, and it works ok.

It's not exact, but as long as you're nowhere near the speed of sound, it gives you good ballpark figures for lift and drag.

[HenrikOlsen]

RAF_Blackace, I've seen the teaching of incomplete truths in physics (and other subjects) described as lies-to-children.
The idea is that the incomplete description gives the foundation to later learn a more precise description which gives the foundation to later learn a yet more precise description . . ..

At the level your son is now, the purpose of teaching physics isn't to teach specific knowledge, but rather to teach general concepts and that's done by teaching those parts of physics that demonstrate the concept and ignoring the bits that are irrelevant to the concept being taught and therefore would confuse rather than enlighten.

This only becomes a problem when the teacher isn't honest about the process, since that can result in students who'll remain ignorant the rest of their life because they believe they already know all there is to know about the subject.

[Larry Jacks]

Originally Posted by Larry Jacks
Likewise, with enough power, I've seen airshow pilots fly for considerable distances in knife-edge flight (90 bank angle) with no lift from the wings. Here's a good video that demonstrates knife-edge flight with a model airplane.

That's a gun ad!

Man, that was wierd. I was viewing this video and copied the link. Somehow, a link I copied earlier today (a very funny video called "The World's Shortest Slasher Flick") was what I ended up posting. My apologies.

[Paul Beardsley]

I'd like to know more about the lies-to-children thing. I recall being present at a science fiction convention, sitting behind (the very likeable) Jack Cohen, who was talking about a book he was writing on that subject.

I am worried about the amount of anti-education going around where science in general and physics in particular is concerned. Astrology, moon hoax and similar rubbish is not being treated with the contempt and hostility it deserves.

[Moose]

I'll say this: my 300 pound oak entertainment cabinet¹ didn't fly out of the pickup bed because of deflection². It flew because the hollow opening³ in the back generated lift. That was all Bernoulli's fault.

¹: It was lying on its front. The back of the cabinet was exposed to the wind.
²: As soon as the driver hit 120kph, the cabinet flew. It survived with remarkably little damage. Not even the glass chipped, and it fell to the highway and dragged glass side down.
³: The opening is the part out which you stick the pointy end of the CRT television. Most TVs wouldn't fit in the cabinet otherwise.

[tdvance]

RAF_Blackace, I've seen the teaching of incomplete truths in physics (and other subjects) described as lies-to-children.

Sounds a lot like Ian Stewart and Jack Cohen's definition of a teacher: "Liar to children"

[HenrikOlsen]

That is where I got it from

[Moose]

The idea is that the incomplete description gives the foundation to later learn a more precise description which gives the foundation to later learn a yet more precise description . . ..

My own intent (if/when I get my hands on a physics class) is to explain the evolution of physical models. (Using inverted bowl -> geocentricism -> heliocentricism -> center of nothing in particular -> and hints of modern cosmology as a backdrop.

"We" teach Newtonian physics rather than Einstein's, because Newton's is plenty good enough for teaching the physics these kids need to know. No model is perfect, but if it works to the precision you need, within the bounds you care about, it's good enough to use. Special relativity is important if you're doing anything aerospace, but way beyond the reward-to-effort ratio for your typical high-school application.

[Gillianren]

That is where I got it from

Science of Discworld?

[HenrikOlsen]

Yep.

[RAF_Blackace]

An aircraft will eventually fly off the runway with no increase in the pitch angle.

That only happens because an Aircraft has a fixed angle of attack to start with. It would take off eventually, but the runway would be enormous. If the wing had no angle of attack, it would not generate any lift and the Aircraft would not take off.

RAF_Blackace, I've seen the teaching of incomplete truths in physics (and other subjects) described as lies-to-children.
The idea is that the incomplete description gives the foundation to later learn a more precise description which gives the foundation to later learn a yet more precise description . . ..

At the level your son is now, the purpose of teaching physics isn't to teach specific knowledge, but rather to teach general concepts and that's done by teaching those parts of physics that demonstrate the concept and ignoring the bits that are irrelevant to the concept being taught and therefore would confuse rather than enlighten.

I can see the point, but it is not correct to teach the wrong basic principle to start with. I would certainly not say that deflection of a mass of air is irrelevant to the way a wing flies.

This is a very good description that some Teachers really should read.

http://www.allstar.fiu.edu/AERO/airflylvl3.htm

[Ara Pacis]

I'll say this: my 300 pound oak entertainment cabinet¹ didn't fly out of the pickup bed because of deflection². It flew because the hollow opening³ in the back generated lift. That was all Bernoulli's fault.

¹: It was lying on its front. The back of the cabinet was exposed to the wind.
²: As soon as the driver hit 120kph, the cabinet flew. It survived with remarkably little damage. Not even the glass chipped, and it fell to the highway and dragged glass side down.
³: The opening is the part out which you stick the pointy end of the CRT television. Most TVs wouldn't fit in the cabinet otherwise.

My understanding of the aerodynamics of pickup trucks is that the space behind the cab generates a vortex and laminar flow moves above that turbulent layer. Would it be low pressure or the turbulence or both that resulted in the flying entertainment center?

[Moose]

My understanding of the aerodynamics of pickup trucks is that the space behind the cab generates a vortex and laminar flow moves above that turbulent layer. Would it be low pressure or the turbulence or both that resulted in the flying entertainment center?

It would be interesting to see a truck and my cabinet in a wind tunnel. I suspect the cabinet removed a great deal of the vortex behind the cab.

But yeah, it would have been low pressure caused by the fast air over the hollow in my cabinet.

[Ara Pacis]

It would be interesting to see a truck and my cabinet in a wind tunnel. I suspect the cabinet removed a great deal of the vortex behind the cab.

But yeah, it would have been low pressure caused by the fast air over the hollow in my cabinet.

I was thinking about the experiments from the Mythbusters on the "Tailgate Up vs. Tailgate Down" debate.

[Tobin Dax]

I was thinking about the experiments from the Mythbusters on the "Tailgate Up vs. Tailgate Down" debate.

I'm thinking about that same thing. The vortex wouldn't be present where the cabinet is, no matter where it is in the bed of the truck. (Presumably, the cabinet is up against the cab, though, which effectively extends the cab and shortens the horizontal length of the vortex.) So the reason the cabinet flew out is still going to be the pressure difference between the still air behind the cab/in the cabinet and the moving air above.

[Larry Jacks]

RAF_Blackace
Originally Posted by conniefan
An aircraft will eventually fly off the runway with no increase in the pitch angle.

That only happens because an Aircraft has a fixed angle of attack to start with. It would take off eventually, but the runway would be enormous. If the wing had no angle of attack, it would not generate any lift and the Aircraft would not take off.


Not necessarily. When I look at my airfoils book (Theory of Wing Sections), I see many airfoils that have a positive lift coefficient at 0 degrees angle of attack. These are the airfoil sections that have positive camber such as the old Clark Y. It's a little hard to read the graph but the coefficient of lift at zero degrees angle of attack is about 0.4.