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*From*: John Denker <jsd@av8n.com>*Date*: Fri, 21 Dec 2007 17:10:09 -0500

On 12/21/2007 09:05 AM, Rick Swanson wrote:

1. Flying things interact with a lot more than the near air — the

interaction of the near fluid with all the rest of it is extremely

important. Pictures of vorticies, contrails and other neat things help.

I also mention the "ground effect" observed when you see water fowl

skimming across the lake. The air knows that the rest of the air is

there.

That's all good. Other useful things to mention include:

-- Why do gliders have long skinny wings?

-- Why do geese migrate in that V formation?

And BTW beware that ground effect is widely misunderstood,

even among pilots.

http://www.av8n.com/how/htm/airfoils.html#sec-soft-field-why

http://www.av8n.com/how/htm/takeoff.html#sec-soft-field

2. If something interacts with the air and goes up, some air must go

down. It's a conservation of momentum thing.

I wouldn't have said that. That confuses force with motion.

In level flight, the airplane /pushes/ down on the air, but

that does not mean the air /goes/ down. Newton's cradle provides

a good way to visualize momentum transport without mass transport.

Helicopters help this discussion.

Or not. A helicopter in hover is a very nasty thing. I'm

not sure anybody really understands it. I would never bring

it up in an introductory discussion. A helicopter in level

cruising flight is not much different from an airplane, in

terms of wake vortices, induced drag, and all that.

My most difficult calculation in a fluid dynamics class,

which I never quite understood, was a 2-D calculation of the momentum of

the air after interaction with an airfoil. Good luck with a 3-D

treatment.

Actually the 3D calculation is in many ways easier. The

2D wing is /always/ in ground effect, which can be confusing,

and which makes it utterly impossible to discuss induced

drag. In 2D there are lots of integrals that converge poorly

if at all, whereas in 3D the corresponding integral is well

behaved. (I'm talking about integrals such as the total

amount of momentum in a given column of air.)

http://www.av8n.com/fly/vortex.htm

==============================

Another point to keep in mind, and to impress upon students,

is that fluid dynamics is just plain difficult and tricky.

Students sometimes get the impression that physics is a "dead"

subject, in the sense that all the answers can be looked up

in a book somewhere. Fluid dynamics easily disproves this.

There are lots of fluid dynamics questions where the student

can understand the question but nobody knows the answer.

I once asked Feynman if he were forced to start over, and

couldn't do particle physics, what would he do. He immediately

replied "fluid dynamics". Why? Because it's important, and

because there are lots of unsolved problems.

People spend their lives surrounded by fluids, and they think

that makes them knowledgeable about fluid dynamics, but it

really doesn't. I had a really good education, literally

beyond what most people can imagine, leading to a PhD in

physics and a research career. But when I got interested in

fluid dynamics, it took me a year of hard work to get up to

speed.

CFD helps a lot. Computational Fluid Dynamics. I reckon most

people would rather look at a picture of the airflow than look

at the partial differential equation that describes the airflow.

For serious research it's a two-way street; the CFD tells you

what to look for in the equations, and vice versa. I for one

would never have figured out how wings work if I hadn't been

able to write a bunch of CFD programs.

=======================================================

On 12/21/2007 03:00 PM, Robert Cohen wrote in part:

Suppose you hold a wood board out the car window while driving down the

road. If the board is oriented with its plane oriented vertically, what

"causes" the pressure on the leading side to be greater than the

pressure on the trailing side?

This is called pressure drag. The hand-wavy explanation is

that Bernoulli's principle more-or-less applies on the front

side, so you get the full static + dynamic pressure P+Q, while

on the back side the air is "spoiled" and all you see is the

static pressure P, so the drag force is nearly equal to the

dynamic pressure times the area, i.e. the coefficient of drag

is on the order of unity.

If you want the mathematical details on this, I don't know,

and it is possible that nobody knows.

Is your explanation of this much

different when the plane of the board is at an angle?

Completely different. Lift is incomparably easier to explain

than form drag. The basic explanation of lift survives in the

face of all sorts of simplifications and approximations, while

pressure drag does not.

You might think aircraft designers would just ignore the issue,

because nobody in his right mind would take a flat plate and

orient it face-on to the airstream -- but in fact there are

things like speed brakes that are oriented so. Similar issues

arise when considering the "all aspect" behavior of the aircraft,

such as might be relevant during a spin, or during taxi in a

crosswind. Until a few years ago designers didn't even try to

model such things analytically; they took a principled approach

to basic lift and stability, and then built a scale model to

obtain the all-aspect behavior data. Nowadays you can just do

ab-initio CFD for things the size of an airliner, if you have a

big enough computer.

**References**:**[Phys-l] Bernoulli's Principle***From:*Richard Blade <richardblade@comcast.net>

**Re: [Phys-l] Bernoulli's Principle***From:*John Denker <jsd@av8n.com>

**Re: [Phys-l] Bernoulli's Principle***From:*"Rick Swanson" <swansonr@sandhills.edu>

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