Roger Long wrote:
On Sep 29, 7:32 am, Goofball_star_dot_etal
wrote:

There are numerous examples of equating inconsistant units. Here is one
example of gobeldygook:

The reaction to these presentations on the web is always the same.
The professionals, especially teachers, like them and they gather all
sorts of nit picks from others. That particular bit of gobeldygook
came from an article published in a leading aviation Emagazine and,
last I heard, was being used as an introduction to the subject in at
least one college course.

These are not intended to be physics texts. There are plenty of
those. The intent is to provide a plain language viceral
understanding of the basic principles. Units and terms most
recognizable to the reader with little prior knowledge are preferable
in a quick and light treatment.

Why this kind of thing worthwhile? I've had a whole career (I'm
hardly "budding") to watch people with naval architectural degrees and
complete understanding of the math and unit consistency come to
really bone headed conclusions that have greatly hampered the
commercial and educational sail industries because they didn't start
with a gut understanding of the physics and let numbers and anal
attention to unit consistency lead them to absurd conclusions. If
they had first understood the subject on this kind of level, they
might have made better use of the mathematical tools. Most college
courses and texts start right off with the math.

These articles are just starting points and not intended to be much
above the level of Sunday newpaper supplement stuff. Professionals
tend to see them for what they are and their limited value and net
posters as opportunities to show how smart they are. Happy to have
provided the opportunity.

--
Roger Long

The point is not that I am a "clever clogs" but that you publish stuff
as an "expert" and get it plain wrong. As it happens I am just "an oily
rag".

The vertcal force on an airfoil (lift) is equal to vertcal *rate of
increase of momentum* of the surrounding air. Not displacement, work or
anything else.

Force has the dimensions (MLT-2) the same as Mass (M) times acceleration
(L-2) (from F=ma)or the same as mass flow rate (MT-1) times velocity
(LT-1). M represents Mass, L length and T time. 1/T^2 is represented as
T-2 etc.

In the case of airfoils, turbines etc. there is not a fixed mass but a
mass flow. It is a lot easier to measure the pressures over an airfoil
than the increasing momentum of the air but they are two sides of the
same coin.

No work is done on a plane in level fight at constant speed. Its
potential energy is not increasing with height and its kinetic energy is
not increasing with velocity. The lift is equal to its weight and its
drag is equal to the thrust. All the power ends up heating the air,
although initially some goes into increasing the kinetic energy of the
air you cannot get at this number by looking at the lift, as you
suggest, since kinetic energy and momentum are not the same thing.

Context re-inserted:

"Note the net downwards displacement of the air. The essence of all
Newtonian physics is the symmetry of energy conservation (the equal and
opposite reaction business). The work done by accelerating the mass of
air downwards is exactly equal to the work required to keep the aircraft
aloft. The work required to shift it from left to right in the
animations is an important aspect of the drag that the engine must
overcome."

http://www.rogerlongboats.com/Circulation.htm

Goofball_star_dot_etal wrote:

/snip/
No work is done on a plane in level fight at constant speed. Its
potential energy is not increasing with height and its kinetic energy is
not increasing with velocity. The lift is equal to its weight and its
drag is equal to the thrust. All the power ends up heating the air,
although initially some goes into increasing the kinetic energy of the
air you cannot get at this number by looking at the lift, as you
suggest, since kinetic energy and momentum are not the same thing.


Hmmmm. a mostly reasonable review - but the idea that force times
distance is not equal to work is somewhat radical, don't you think?

And if the force (usually called thrust in this context,)which was
provided to oppose the drag on the plane due to its airspeed, is
multiplied with the airspeed rather than some air distance - the work
becomes the power expended in opposing drag.

But you knew that, I'm sure - you were just trying to provide a gut feel
for the physics, huh? :-)

Brian W

brian whatcott wrote:
Goofball_star_dot_etal wrote:

/snip/
No work is done on a plane in level fight at constant speed. Its
potential energy is not increasing with height and its kinetic energy
is not increasing with velocity. The lift is equal to its weight and
its drag is equal to the thrust. All the power ends up heating the
air, although initially some goes into increasing the kinetic energy
of the air you cannot get at this number by looking at the lift, as
you suggest, since kinetic energy and momentum are not the same thing.


Hmmmm. a mostly reasonable review - but the idea that force times
distance is not equal to work is somewhat radical, don't you think?


I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

And if the force (usually called thrust in this context,)which was
provided to oppose the drag on the plane due to its airspeed, is
multiplied with the airspeed rather than some air distance - the work
becomes the power expended in opposing drag.

But you knew that, I'm sure

True.

- you were just trying to provide a gut feel
for the physics, huh? :-)

No I was responding to Roger's statement:

"The work done by accelerating the mass of air downwards is exactly
equal to the work required to keep the aircraft aloft"

I think he meant "force" but it is difficult to tell..

Goofball_star_dot_etal wrote:
....
Hmmmm. a mostly reasonable review - but the idea that force times
distance is not equal to work is somewhat radical, don't you think?


I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

If I push a sled over snow with force F for distance D
it's usually accounted that the work I did ON the sled is F X D

....but I won't beat the topic down....
:-)

Brian W

On Oct 2, 8:02*pm, brian whatcott wrote:
Goofball_star_dot_etal wrote:

...

Hmmmm. a mostly reasonable review - but the idea that force times
distance *is not equal to work is somewhat radical, don't you think?

I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

If I push a sled over snow with force F for distance D
it's usually accounted that the work I did ON the sled is F X D

...but I won't beat the topic down....
:-)

Brian W

In regard to sudden gusts, why not use a breakaway strap like some
rock climbers use whose stitches break at some load allowing the sail
to be let out all the way.
How many boats get knocked down anyway? I mean cruising boats, racers
intend to be on the edge. My own boat, a 28' S2, if a sudden gust
came up, I'd never be able to hold the tiller and she'd round up into
the wind before getting knocked down.

On Fri, 2 Oct 2009 17:42:17 -0700 (PDT), Frogwatch
wrote:

How many boats get knocked down anyway? I mean cruising boats, racers
intend to be on the edge. My own boat, a 28' S2, if a sudden gust
came up, I'd never be able to hold the tiller and she'd round up into
the wind before getting knocked down.

There are limits to that. If you get knocked down hard enough and
fast enough, the boom will hit the water and prevent the mainsail from
being eased. That's where the fun starts, and some boats will tend
to stay on their beam ends once they get knocked flat with the mast in
the water.

On Oct 2, 8:42*pm, Frogwatch wrote:

In regard to sudden gusts, why not use a breakaway strap like some
rock climbers use whose stitches break at some load allowing the sail
to be let out all the way.

That was a huge issue and we did a lot of analysis on it back during
the sailing school vessel research in the early 80's. The sail and
rig forces caused by wave motion and rolling often exceed the forces
that would cause knockdown although only for brief periods. If you
put "fuses" in the rig, you would have stuff breaking constantly.

--
Roger Long

Roger Long wrote:
On Oct 2, 8:42 pm, Frogwatch wrote:

In regard to sudden gusts, why not use a breakaway strap like some
rock climbers use whose stitches break at some load allowing the sail
to be let out all the way.

That was a huge issue and we did a lot of analysis on it back during
the sailing school vessel research in the early 80's. The sail and
rig forces caused by wave motion and rolling often exceed the forces
that would cause knockdown although only for brief periods. If you
put "fuses" in the rig, you would have stuff breaking constantly.

Most of the Nonsuch's (unstayed catboats) were built with aluminum masts
that tapered near the top. The marketing folks made a big point of how
the tip would flex to leeward and spill wind in gusts. Indeed, it
worked well and was helpful since it was easy to overpower the large
mainsail in gusty weather. (I learned to reef before going out, since
it was a lot easier to shake out than take in a reef!)

At the end of the production run they shifted to fiberglass masts that
were stiffer. The marketing folks stressed how this gave better
pointing ability in strong wind.

On Fri, 2 Oct 2009 17:42:17 -0700 (PDT), Frogwatch
wrote:

On Oct 2, 8:02*pm, brian whatcott wrote:
Goofball_star_dot_etal wrote:

...

Hmmmm. a mostly reasonable review - but the idea that force times
distance *is not equal to work is somewhat radical, don't you think?

I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

If I push a sled over snow with force F for distance D
it's usually accounted that the work I did ON the sled is F X D

...but I won't beat the topic down....
:-)

Brian W

In regard to sudden gusts, why not use a breakaway strap like some
rock climbers use whose stitches break at some load allowing the sail
to be let out all the way.
How many boats get knocked down anyway? I mean cruising boats, racers
intend to be on the edge. My own boat, a 28' S2, if a sudden gust
came up, I'd never be able to hold the tiller and she'd round up into
the wind before getting knocked down.


Not really a problem to design a boat that won't be knocked down. Good
form or ballast stability and a small rig.

Of course, as soon as the bloke buys this no-knock-down marvel he will
get a cruising gennaker, a storm spinnaker and fit a topmast so he can
fly a topsail. Probably it is impossible to build a vessel that is
impossible to knock down if the owner is in any way adventurous.

After all they once built an airplane that wouldn't spin and wouldn't
stall. It wasn't immensely popular.

Even full rigged ships quite frequently could send down their topmasts
in bad weather.

Cheers,

Bruce
(bruceinbangkokatgmaildotcom)

On Fri, 2 Oct 2009 17:42:17 -0700 (PDT), Frogwatch
wrote:

On Oct 2, 8:02*pm, brian whatcott wrote:
Goofball_star_dot_etal wrote:

...

Hmmmm. a mostly reasonable review - but the idea that force times
distance *is not equal to work is somewhat radical, don't you think?

I said no work was done *on the (air)plane*. Since we only have the
airplane and the air, the work done by the thrust of the engine moving
the airplane through a distance all goes into the air as (kinetic)
energy or heat in its wake.

If I push a sled over snow with force F for distance D
it's usually accounted that the work I did ON the sled is F X D

...but I won't beat the topic down....
:-)

Brian W

In regard to sudden gusts, why not use a breakaway strap like some
rock climbers use whose stitches break at some load allowing the sail
to be let out all the way.
How many boats get knocked down anyway? I mean cruising boats, racers
intend to be on the edge. My own boat, a 28' S2, if a sudden gust
came up, I'd never be able to hold the tiller and she'd round up into
the wind before getting knocked down.

Guess again!