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!