The Impossibility of Flying Heavy Aircraft Without Training

I'd like to take a moment and wander off into fantasy land here...
For what it's worth?

In respect to whatever bizarre principles are really at work holding
an airplane up in the sky, I think some of us only are only seeing
the "down" side.

I'd like, for a moment, to address the "up" side.

We can easily imagine a column of air that extends from the bottom
side of the wing - all the way to the ground.
Very Newtonian.

Sure, the high pressure under the wing PUSHES UP on the wing.

But at the same time the low pressure above the wing PULLS UP on it.

And I suspect that "pulling up" part is a little harder to visualize.
What, for instance, is it pulling AGAINST?

There doesn't seem to be anything solid up there to pull against.
Except a lot of empty sky?

Theory:
This is my over simplified "localized pressure field" theory.

In flight our wing is "compressing" the air under it (down), while
at the same time "stretching" the air above it (also down).
(Both are allowed by the compressibility of subsonic gassious fluids)
The combined reaction is, of course, Lift (UP).

These pressure fields, while strong near the wing are proportionally lower
as we get farther from the wing surface (unlike a simple spring network).

I think of it as the pressure field being spread out over a larger volumn,
rather than being dissipated in a smaller column.

This also might address the question of what happened to the "momentum"
imparted to the air that we might expect if we only consider the down
side. The two pressure fields pretty well cancel each other out after
the wing has passed by.

Stalls:
At some value of high alpha, the low pressure ABOVE the wing exceeds
the shear value (viscosity) of air, and the flow "tears" loose.

For fat airfoils this happens much further aft that for thin airfoils,
which tend to separate nearer the leading edge.


Psychic Hotline:
Why does the air flow start to rise BEFORE it meets the wing?

If we look at an idealized flow diagram in a text book, (which usually
implies that the air is moving and the wing is stationary!?) we notice
that there is a high pressure area right at the leading edge of the wing.
Think of it as impact pressure.

Combine this with the low pressure area above and high pressure below and
we can see why the air seems to start rising BEFORE it gets to the wing.

Scope:
Now, how far these forces extend above and below the wing depends on
several factors - weight, CL, velocity of the wing, etc.

In straight and level flight, the wing generates as much lift as the
aircraft weighs. (Lift = weight)

So?
Higher speed, lower CL - smaller pressure field disturbance?
Lower speed, higher CL - higher pressure field disturbance?
(PER unit volumn / time)
The net work is the same same though. Lift = Weight

Tip Vortices:
Wing tip vortices are a direct result of these two pressure areas.
The high pressure below and the low pressure above tend to pull the air
_around_ the wing tip, inducing the circulation for the vortex.

This also rolls back into the question of why the vortex is a lot stronger
at low speeds. There is simply a much higher pressure difference between
the top and bottom fields.

And explains why winglets can help to reduce the vortex size, and the
induced drag that comes along with them.

Ground Effect:
As we get down to within a wing span or so of the ground, and the higher
pressure on the bottom side actually does come into contact with the ground,
said high pressure area under the wing gets trapped and is noticibly stronger.

Anyone who has tried to plant a Taylorcraft a little too fast knows this
*problem* intimately!


Summation:
This is, obviously, NOT the final word in aerodynamic phenonomnon, and
I offer it only as a (hopefully) useful picture of the forces in play.

I've ignored a lot of other obvious stuff - like Drag (because it's such a -
DRAG!), circulation theory, etc.

Like I said, "for what it's worth".

Richard

Disclaimer: All puns are the sole responsibility of the author and do
not imply any such foolish thoughts on behalf of the management...