Model Building (but not with plastic and glue)

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To continue to my discussion with Burt... ;-)

As noted in the previous thread, we all need to build an aerodynamic
model to inform our decision making process. I'd like to offer up some
notions that might make examining these models a little easier:

First, a model is only as good as its ability to predict outcomes
accurately.

Second, all models are flawed.

Third, a model for airmanship is only useful if it allows quick
reference and quick action.

Fourth, a good model does not purport to tell the truth.... that is the
realm of philosophy.

Fifth, an aviator's model will be very different from an engineer's
model; however, they should not vary in substance, only in application.

Sixth, because of the third point, it will be necessary to create a
system of simplified models. None of these models should conflict in
substance or application.

The object here is to agree that because all models are artficial
constructs, they are all flawed, and therefore, all open to
improvement. So the comment, "Your model is flawed..." should be
universally acceptable. "Of coruse it's flawed. It's a model." Then we
can get on with the business of whether a particular element can stand
improvement.

Let me introduce a last notion to help the discussion along. One model
that borders on axiomatic is F=ma. If a aircraft is subject to an
unbalanced force (Fnet does not equal zero), the aircraft will must
accelerate: that is, it must be changing its speed or direction. If a
force is acting on an aircraft it must accelerate. Try integrating that
with some of your models to see if they suffer.

(Yeah, I know... hurry up spring!)

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OK, maybe this is too esoteric. Let me ground it. I have often heard
professional pilots say that a side slip is used to counteract the
force of the wind. This comment implies a model in which the wind
applies a force to an aircraft in flight and that the horizontal
component of lift of the tilted wing (in a slip) allows the aircraft to
fly straight.

While this model works, it is flawed (as all model are). But is there a
better model to be had? The problem with this model is that it is not
compatible with other models. For instance, F=ma. If the wind applies a
force to an aircraft in flight, where is the acceleration?

The obejct here is to deconstuct some of our more cherished models to
see if there is room for improvement. And remember, there are very few
universal models. Most have subtleties based on who taught you, what
books you've read, your own experience...

[Marian Aldenhövel]

Hi,

> The problem with this model is that it is not
> compatible with other models. For instance, F=ma. If the wind applies a
> force to an aircraft in flight, where is the acceleration?

The wind applies a force. That force is cancelled by the horizontal
component of lift. So the net force is zero and the acceleration turns
out to be zero too.

If you like to you can also calculate two accelerations. As the mass
stays the same those would cancel each other in the same way.

Ciao, MM
--
Marian Aldenhövel, Rosenhain 23, 53123 Bonn. +49 228 624013.
http://www.marian-aldenhoevel.de
"Wir brauchen keine Opposition, wir sind bereits Demokraten."

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If the wings are level, then, the wind applies an unbalanced force?
Please show me the resulting acceleration (changing speed and/or
direction) that results during wings level flight. Remember, so long as
the force remains unbalanced, there will be a continuous change of
speed and/or direction.

Does everyone see the problem with this model of crosswinds?

Marian, thanks for responding.

[Marian Aldenhövel]

Hi,

> If the wings are level, then, the wind applies an unbalanced force?

If you start from an aircraft at rest or one that enters the crosswind
suddenly from a crosswind-free area (all theoretical of course) it will
be accelerated sideways. You can feel that. F=ma holds true.

> Remember, so long as the force remains unbalanced, there will be a
> continuous change of speed and/or direction.

The F is only non-zero until the wind has accelerated the aircraft
sideways to it's own speed. After that you move sideways above ground
but the crosswind does no longer excert force on the aircraft and so
the acceleration stops.

If you want to keep a straight track over the ground in the face of
a crosswind you need to accelerate into the wind. To do so you bank,
the horizontal component applies an F to the aircraft, you accelerate.
At this moment the aircraft can "feel" the wind and you get your F_wind
back.

If you bank by just the right amount this F and the F_wind balance each
other at the exact point where you travel above ground in the direction
you want to go.

> Does everyone see the problem with this model of crosswinds?

I don't.

Ciao, MM
--
Marian Aldenhövel, Rosenhain 23, 53123 Bonn. +49 228 624013.
http://www.marian-aldenhoevel.de
"Wir brauchen keine Opposition, wir sind bereits Demokraten."

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Todd wrote: "Why is it I can never write the short answer I intended
when
I started out.? "

Because we've made it more complex that it needs to be. Here's a
shorter answer.

A slip occurs when the wings are banked and the fuselage is
intentionally yawed to counteract the resulting turning force. This
results in increased drag and a misalignment between the aircraft's
path through the air (or direction) and the orientation of the
fuselage centerline (alternately "landing gear" or "heading"). A slip
may be used to decrease glide slope (at the cost of a misalignment of
the gear at landing) or to align the landing gear with ground track in
a crosswind (at the cost of additional drag).

>From this there are only two types of slips: balanced (a slip) and
sloppy turns. Calling it a forward slip or a side slip confuses the
maneuver with the varying circustnaces under which it might be applied.
A slip has two symptoms, increased drag and misaligment between
direction (as measured by the velocity vector) and heading. Therefore,
a slip never compensates for crosswind... it simply aligns your gear
with your ground track.

Compensating for the crosswind is achieved by changing your direction.
(This is another place where the model fails. Think about it.)

Again, do you see how subtleties in the model can lead to
misinterpretations and the creation of workable models? The very fact
that we differentiate slips into two types shows a flaw in the model.
There's only one type of slip, and different circumstances for its use.


I know some of you want to talk about turning slips, but they tend to
confuse what should be a simple difference. Once we can bifurcate slips
into balanced and turning, then we can talk about the uses of (and the
models related to) turning slips.

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Marian,

read Todd's post. He works through it nicely. I've commented on it as
well.

[Jack]

wrote:

> It's important to remember that glider pilots get little opportunity to
> practice slips on final. Having a sound model is critical because we
> simply don't have the luxury of teaching slips through all their
> variables by rote.

All the opportunity in the world, if they want it.

Why don't they?


Jack