Explaining how planes fly to a seventh grader and the teacher

[Astronot]

My daughter came home from school yesterday saying that the teacher had told them about how airplanes fly. The text from the class uses the old air over the top of the wing speeds up to lower the pressure on top compared to the flat underside of the wing. One of the boys who is into airplanes pointed out that military fighter planes have a flat wing top. The teachers response was that it was just a little curved and that her engineer husband agreed with the view presented in the text book. My daughter was skeptical because I had previously discussed how airplanes do really fly, but she did not want to rely exclusively on that conversation. I did ask her to ask the teacher how airplanes fly upside down, we’ll see how that goes.

So do any of you have a good reference that a 7th grader could understand even if it needs some explanation. My preference would be a book that would be available in a library, even a university library, so I can take her on a field trip. We have not been to the Rice University or U of Houston libraries and this would be a good reason for a university visit.

[hhEb09'1]

"Bernoulli" vs "Newton"

This one is worse than "0.999...=1"

PS: I was going to post a link to a previous discussion here, but I couldn't find any right away. Perhaps we've been lucky?

[Matherly]

Okaaay... I'll bite.

How do airplanes really fly?

[01101001]

This nonpilot thoroughly enjoyed the 1944 classic Stick and Rudder (Amazon) by Wolfgang Langeweische. I think a child would enjoy it.

From the back cover (via Amazon):

The invisible secret of all heavier-than-air flight--the Angle of Attack. What it is, and why it can't be seen. How lift is made, and what the pilot has to do with it.

[Dave J]

This nonpilot thoroughly enjoyed the 1944 classic Stick and Rudder (Amazon) by Wolfgang Langeweische. I think a child would enjoy it.

From the back cover (via Amazon):

Dittos to this...an awesome piece of work on flying...written way long ago, but very accurate.

[01101001]

PS: I was going to post a link to a previous discussion here, but I couldn't find any right away. Perhaps we've been lucky?

We've been lucky that it didn't go on and on.

Something i've always wondered about jetfighters

[Dr Nigel]

Okaaay... I'll bite.

How do airplanes really fly?

Matherly, have a look here:

http://geocities.com/kfuller2001/tmyths.htm#myth10

In short, the curvature of the wing has nothing to do with it (otherwise, how could planes fly upside down?). What matters is the angle of attack. The wing deflects air downwards, creating the reactive force called that is known as lift.

[swansont]

The Bernoulli and angle-of-attack explanations do not contradict each other, if you have a valid description of the wing. Usually, though, the Bernoulli explanations oversimplify things like the wing diagram, which leads to the apparent contradiction of the inverted wing. They are both valid applications of physics, and are two different ways (conservation of energy vs conservation of momentum, neither of which is violated) of explaining the same effect.

http://www.amasci.com/wing/airfoil.html
http://hyperphysics.phy-astr.gsu.edu...s/airfoil.html

[Moose]

In short, the curvature of the wing has nothing to do with it (otherwise, how could planes fly upside down?). What matters is the angle of attack. The wing deflects air downwards, creating the reactive force called that is known as lift.

Paper airplanes, as well, couldn't fly according to Bernoulli. I suspect Bernoulli plays some role, especially when an airplane is at crusing speed, but angle of attack matters most when it comes time to get off the ground. And with helicopters. And paper airplanes. And hang gliders. And bird wings. And... And... And...

[peter eldergill]

Then why is there so much emphasis on the curved wing shape? I've discussed this at some point with one of my classes and gave the above explanation. I think I've even seen it in science centres.

Every wing I've seen have curved wings. Jet fighters don't?

Pete

Edit: From the link above

3. Most, but not all, wings are designed to take advantage of the Bernoulli effect for
more lift, especially during takeoff and landing.

I think I answered my own question...

Here's another one: Can all planes fly upside down?

[hhEb09'1]

Paper airplanes, as well, couldn't fly according to Bernoulli. I suspect Bernoulli plays some role, especially when an airplane is at crusing speed, but angle of attack matters most when it comes time to get off the ground. And with helicopters. And paper airplanes. And hang gliders. And bird wings. And... And... And...

OTOH, Nicolas, who appears to be an aerospace engineer in Belgium that contributed a lot to the previous BAUT thread that 01101001 pointed out to me (thanks!), says in that thread that it is all Bernoulli.

You have to be careful, many times it's just two different ways of saying the same thing, and people get very passionate about their way of saying it.

It's all relative.

PS: Astronot, you showed good sense in not trying to confront the teacher.

[cjl]

Every wing I've seen have curved wings. Jet fighters don't?

Here's another one: Can all planes fly upside down?

Jet fighters have curved wings, but often, it is a symmetrical airfoil, or one that has the same curve on the top and bottom. Clearly, this will not have any lift at a 0 degree angle of attack, unlike some other wings.

As for the other question: theoretically yes, actually no. Some do not have the control authority or power to do it, but theoretically, if they had enough power and control authority, they could.

[Moose]

Perhaps, but if it were "all Bernoulli", hang gliders and paper airplanes wouldn't fly. I've never heard a Bernoulli-not-angle-of-attack proponent even attempt to explain that. I would welcome an answer.

I'd also fling this question into the ring: if flight were "all Bernoulli" thus having little-to-nothing to do with angle of attack, why are avionic surfaces and propellers designed to deflect air? And sea-screws?

[Moose]

Jet fighters have curved wings, but often, it is a symmetrical airfoil, or one that has the same curve on the top and bottom. Clearly, this will not have any lift at a 0 degree angle of attack, unlike some other wings.

I was just looking at wing surfaces from older aircraft right back to the Wright Flyer.

I'm seeing a lot of curved wings, but not in the modern way. The Wright Flyer, in fact, has was looks like wings made of some sort of canvas, but attached to a curved frame only on the top. This makes the underside of the wings (and those of a number of stunt biplanes I'd looked at) look like the inside of a spoon. Or a parachute. Or a kite.

Here's the thing. It's not so much that I'm seeing the angle-of-attack idea as an actual deflection of air (although that's part of it at low speeds and while maneuvering), but as an increase of air pressure on the under/forward side of the wing, like under a parachute, with the corresponding decrease of pressure on the overside and behind the wing. At speed, a relatively (or even very) small angle of attack is sufficient to produce this effect. With a modern wing, lift can be produced, at speed, with nearly no angle of attack other than the curvature of the wing itself.

[cjl]

Agreed, although again, that is not the case with a fully symmetrical airfoil.

Another thing to note is that wings on jetliners tend to be mounted such that when the plane's body is completely level, the wings are at a 1 or 2 degree angle of attack.

[NASA Fan]

Have you tried?


http://www.howstuffworks.com/

http://howthingswork.virginia.edu/

I think I have seen books on this as well. Since it appears from your location that you live/work in Clear Lake, try going to Half Price Books on the corner of NASA Parkway and Egret Bay Blvd (was El Camino on the other side of NASA Parkway), they often have a good selection of books at a reasonable cost.

[Van Rijn]

Perhaps, but if it were "all Bernoulli", hang gliders and paper airplanes wouldn't fly. I've never heard a Bernoulli-not-angle-of-attack proponent even attempt to explain that. I would welcome an answer.

I'm not a hang glider expert, but all the hang gliders I've seen have pretty obvious airfoils designed to provide lift.

As for paper airplanes, the best ones have proper airfoils for lift, the better ones approximate proper airfoils. As for the simplist ones, don't forget that even a rock will fly if you throw it. For the simplist planes, the structure mainly directs the flight, it doesn't do much for lift.

I'd also fling this question into the ring: if flight were "all Bernoulli" thus having little-to-nothing to do with angle of attack, why are avionic surfaces and propellers designed to deflect air? And sea-screws?

Propellers are wings. A helicopter has a wing, the difference is that it provides lift by being rotated about, rather than by the forward motion of the aircraft. There is also thrust, by the air being moved. Rotate that forward and it pulls the helicopter horizontally through the air. You can also have a pusher prop (just put it behind you) which is like a sea-screw. But angle of attack on the propeller blades does matter as it does with a wing.

[cjl]

I would debate that a paper airplane has to have an airfoil to make true lift. The best one I have ever made had flat wings, and it worked better than any ones I have ever tried with airfoils. At the very back of the wings was an approximation of elevators which I had bent up. This causes it to fly with an angle of attack, and this creates lift, not an airfoil shape.

[hhEb09'1]

I think we oughta add Bernoulli to the list, alongside religion and politics and the 0.999...=1.

[JohnD]

Sigh.
It all a matter of lift over drag.
A cambered wing profile, one with a curved top surface and a flat bottom, provides a lot of lift with minimal drag at low speeds with no attack angle. Increase the attack and drag rises, a lot. A plane can fly upside down by using a greater attack than it would right way up, but at the cost of increased fuel consumption to overcome the increased drag.

Drag rises as the SQUARE of speed, and linearly with frontal area. A cambered wing has more frontal area, but lift also rises as the square of speed, so a high speed wing can to be much flatter, even with no attack.

All bets are off however beyond the speed of sound.
My knowledge is restricted to car technology and apart from one, cars don't break the sound barrier. But hypersonic wing designs are very different, and need to be very flat to minimise drag.

John

[Astronot]

A cambered wing profile, one with a curved top surface and a flat bottom, provides a lot of lift with minimal drag at low speeds with no attack angle.

Wow, this is a hot topic. I guess to clarify my point a little bit. The standard text book explanation has the air on top of the wing moving faster than the air underneath with no downward deflection. That is what I have read is really the over simplification with no reason ever given for the increase in speed.

[hhEb09'1]

Wow, this is a hot topic. I guess to clarify my point a little bit. The standard text book explanation has the air on top of the wing moving faster than the air underneath with no downward deflection. That is what I have read is really the over simplification with no reason ever given for the increase in speed.

I'm not sure what your text says, but most texts that I've seen with this sort of thing have pointed out that the distance is farther across the top--so to match up ahead and behind, the air has to move faster. Kinda like a pipe where the input velocity is the same as the outlet, but at an internal constriction, the air must move faster to keep up. But there's no doubt that elementary texts are oversimplified.

An example that I like to use sometimes illustrates the dichotomy: a floating balloon. In order to float, air molecules striking the bottom of the balloon must be deflected downwards in compensation of the force of gravity, to keep the balloon afloat. OTOH, the pressure on the bottom surface is exactly more than the pressure on the top surface by an amount that compensates for the force of gravity. Two different ways of explaining the same thing.

[Dr Nigel]

Then why is there so much emphasis on the curved wing shape?

I believe the curve exists to reduce drag and gain some additional lift from the Bernoulli effect and also to maintain laminar airflow over the wing surface. With a laminar airflow (which is related to reducing drag), your control surfaces (ailerons, elevators or elevons, dfepending on the wing and the aircraft) can deflect airflow from the back edge of the wing. Without this laminar flow, I do not believe the control surfaces have anything like as useful an effect.

Also, many aircraft have fuel tanks in the wings, so a thicker wing gives you more space for fuel. A thick wing must be curved, because a rectangular wing (rectangular in cross-section, that is) would create a lot of drag.

[swansont]

Perhaps, but if it were "all Bernoulli", hang gliders and paper airplanes wouldn't fly. I've never heard a Bernoulli-not-angle-of-attack proponent even attempt to explain that. I would welcome an answer.

I'd also fling this question into the ring: if flight were "all Bernoulli" thus having little-to-nothing to do with angle of attack, why are avionic surfaces and propellers designed to deflect air? And sea-screws?

It's not one or the other. It's all Bernoulli, and all angle-of-attack, depending on which way you solve the problem. Like many physics problems, one may be easier to solve than the other. Also, like many physics concepts, a simplified example is given as the basic explanation in books.

[cjl]

I'm not sure what your text says, but most texts that I've seen with this sort of thing have pointed out that the distance is farther across the top--so to match up ahead and behind, the air has to move faster. Kinda like a pipe where the input velocity is the same as the outlet, but at an internal constriction, the air must move faster to keep up. But there's no doubt that elementary texts are oversimplified.

Here's a question that I have never gotten a satasfactory answer for:

Why must they reach the back of the wing at the same time?

Why can't the air on the top reach it after the air on the bottom?

[hhEb09'1]

Here's a question that I have never gotten a satasfactory answer for:

Why must they reach the back of the wing at the same time?

Why can't the air on the top reach it after the air on the bottom?

At the risk of defending the indefensible, I think the usual take on it is that the wing is cutting through the air, so it's really just the back of the wing reaching them at the same time.

The way you phrased the question makes it seem like you're thinking of a wing in a wind tunnel. The wind tunnel explanation is a little bit different, but as we all know, if the engineers have set it up properly it will simulate real conditions but with the air moving instead of the wing

[cjl]

My take on it is that the wing is not perfectly free of parasitic (skin friction) drag, and since the air on top is moving faster initially, this slows it down more (the drag is both pulling on the plane and on the air). Therefore, I see the air on top of the wing, in both the wind tunnel and actual situations, as ending up at the back of the wing later than the air that took the bottom path.

Note: I am thinking of it from the wing's perspective, and as far as I know, air moving over a wing is identical to a wing moving through air as far as the effects go.

[hhEb09'1]

My take on it is that the wing is not perfectly free of parasitic (skin friction) drag, and since the air on top is moving faster initially, this slows it down more (the drag is both pulling on the plane and on the air).

Is that the sort of analysis you'd expect to see in an elementary textook?

I mean, we also know that things don't fall with an acceleration of 9.8 m/s/s either, because of friction effects. The Galileo explanation is often presented straight, with the introduction of the effects of friction later.

[cjl]

True...

I guess I just wish it were presented a little bit more clearly, without relying on the single explanation that only accounts for part of the truth.

[Moose]

The way you phrased the question makes it seem like you're thinking of a wing in a wind tunnel. The wind tunnel explanation is a little bit different, but as we all know, if the engineers have set it up properly it will simulate real conditions but with the air moving instead of the wing

General relativity says either situation would be identical in every way that matters.