[Rhodes22-list] Re: Pointing

[Stephen Staum]

I'll drink to that.  SS
----- Original Message ----- 
From: "brad haslett" <flybrad at yahoo.com>
To: "The Rhodes 22 mail list" <rhodes22-list at rhodes22.org>
Sent: Thursday, September 23, 2004 4:17 PM
Subject: Re: [Rhodes22-list] Re: Pointing


> Boys, Boys, Boys!
> 
> You guys are starting to scare me!  Bernoulli?  Pizza
> maker, right?  Newton makes a damn fine fig bar!  I'd
> be willing to bet that the sailors on this list who
> race, Anne for example, give little thought to
> aerodynamic theory in the heat of battle.  Slim, do
> you think about music theory in the middle of a good
> lick?  Competence in any field comes from experience
> and practice.
> 
> You guys must drink less when you sail than I do!
> 
> Brad Haslett
> "CoraShen" 
> --- Steve Alm <salm at mn.rr.com> wrote:
> 
> > Roger and Peter,
> > 
> > A musician should know better than to talk physics
> > -- especially around
> > here.  I'll shut up now.  8-)
> > 
> > Slim
> > 
> > On 9/23/04 8:25 AM, "Roger Pihlaja"
> > <cen09402 at centurytel.net> wrote:
> > 
> > > Slim,
> > > 
> > > Actually, all of the foils can stall out, both in
> > the water & in the air.
> > > An object does not have to be a certain shape to
> > generate lift.  To prove
> > > this to yourself, stick your hand out of the car
> > window.  If you hold your
> > > hand at an angle to the air flow, do you feel a
> > force?  That's lift!  Is
> > > your hand shaped like an airfoil?  Even a flat
> > plate can generate lift if it
> > > is held at an angle of attack to the fluid flow. 
> > The fluid does not have to
> > > be a gas, like air, either.  It turns out liquids
> > obey the same laws of
> > > hydrodynamics as gases.  The only differences
> > between gases and liquids show
> > > up in the defining equations as terms for density
> > & viscosity.  Liquids are
> > > usually more dense and more viscous than gases as
> > the same temperature &
> > > pressure.  Without going into the physics, what
> > this means is that dense
> > > liquids will produce the same amount of lift
> > force/unit area at a lower
> > > fluid velocity than gases.  Or alternatively, at
> > the same fluid velocity,
> > > liquids require less surface area to produce a
> > given amount of lift force.
> > > For example, at room temperature & pressure, the
> > density of air is about
> > > 0.076 lb/ft^3 vs. water at about 62.4 lbs/ft^3, a
> > factor of about 800X.  So,
> > > the keel only needs to have about 1/800 the
> > surface area of the sails to
> > > generate the lift forces required to resist leeway
> > under sail.  Water is
> > > also much more viscous than air.  This has the
> > effect of making the
> > > underwater foils much more forgiving or less prone
> > to stalling out than the
> > > sails.  This is a good thing because it makes
> > sailing much easier.  If your
> > > underwater foils stalled out as easily as your
> > sails; then, every time the
> > > boat lifted in a wave or every time you moved the
> > rudder blade, these foils
> > > would stall out & quit generating lift.  However,
> > at a sufficiently high
> > > angle of attack, even your underwater foils will
> > stall out & quit generating
> > > lift.  This happens most frequently with the
> > rudder blade.  If we define the
> > > angle between the tiller & the centerline of the
> > boat as the angle of attack
> > > of the rudder blade; then, the rudder blade is
> > starting to stall out at an
> > > angle of about 30 degrees & completely stalled out
> > at an angle of about 45
> > > degrees.  At angles greater than about 45 degrees,
> > the rudder blade behaves
> > > more like a water brake or drag device than an
> > underwater foil.  So, unless
> > > you are trying to slow down the boat, putting the
> > tiller over more than
> > > about 45 degrees off the centerline is
> > counterproductive as far as steering
> > > goes.
> > > 
> > > People cite the analogy of airflow moving faster
> > over the curved surface of
> > > the top of a wing vs., the straight bottom surface
> > as causing a pressure
> > > difference between the top & bottom surfaces &
> > that's what causes lift.  In
> > > the middle 1700's, a Swiss mathematician &
> > scientist named Bernoulli Bioplus
> > > did a mass & energy balance on all the forms of
> > energy contained within a
> > > moving fluid.  These days, mass & energy balances
> > are fundamental to
> > > engineering calculations.  But, in Bernoulli's
> > time, this was a completely
> > > new & creative approach!  Bernoulli found that, if
> > you keep a running tally
> > > on all the forms of energy in the fluid as it
> > flows from place to place;
> > > then, total energy is conserved.  The energy can
> > change form - i.e. kinetic
> > > energy can be traded off for pressure &/or
> > potential energy & vice versa;
> > > but, the total amount of energy remains constant. 
> > Bernoulli expressed this
> > > idea in the form of an equation that now bears his
> > name.  Bernoulli's
> > > equation is one of the 1st things students learn
> > in any class on fluid flow
> > > or hydrodynamics.  Naval architects, aeronautical
> > engineers, & chemical
> > > engineers have it tattooed on the inside of their
> > eyelids so they see it in
> > > their sleep!  Macroscopically, one of the things
> > Bernoulli's equation
> > > predicts & experimental measurements have verified
> > is that there is a high
> > > pressure region on the windward side of a sail, a
> > low pressure region on the
> > > leeward side of a sail, & greater air velocity on
> > the leeward side vs. the
> > > windward side - hence the common analogy cited
> > above.  The difference
> > > between these two air pressures, multiplied by the
> > surface area of the
> > > sailcloth over which the pressure difference is
> > acting, is a force, which we
> > > call "lift".  Although Bernoulli's equation is
> > correct, it doesn't provide
> > > much insight into what's actually going on,
> > physically.  Physically, what's
> > > actually happening is Newton's Laws of Motion are
> > at work, as always.  The
> > > air flowing over the sail is being forced to
> > change direction by the shape
> > > of the sail.  Since the air has mass & Newton's
> > Laws state that it doesn't
> > > "want" to change direction, forcing the airflow to
> > change direction requires
> > > that work must be done.  The only source of energy
> > available to do this work
> > > is the kinetic energy of the moving air itself, so
> > that's where it must come
> > > from.  Macroscopically, we observe this work as an
> > increase in the air
> > > pressure on the windward side & a decrease in
> > pressure on the leeward side
> > > of the sail.  The speed of the windward side &
> > leeward side airflows adjust
> > > themselves in response to these new pressures.
> > > 
> > > So, what the heck is stalling out?  Well, back to
> > Newton's Laws again.
> > > Remember the fluid flow does not want to change
> > direction.  Forcing the
> > > fluid to change direction too abruptly will cause
> > the more or less orderly
> > > flow of molecules to break down into a more
> > chaotic pattern.  The fluid
> > > molecules sort of get in each other's way when
> > they are forced to change
> > > direction too abruptly & go bouncing off in random
> > directions.  This process
> > > turns the kinetic energy of the fluid flow into
> > random molecular vibrations
> > > or heat.  We call this process "turbulence". 
> > Bernoulli's equation doesn't
> > > "care" what form of energy we convert the fluid's
> > kinetic energy into, heat
> > > is just as good as pressure.  So, at the onset of
> > turbulence or stalling,
> > > the pressure difference across the sail goes away
> > in favor of a slight
> > > temperature increase in the airflow.  Again, this
> > has been verified
> > > experimentally.  Around the turn of the 20th
> > century, a British physicist
> > > named Osborne Reynolds came up with the concept of
> > a dimensionless parameter
> > > which could be used to predict the onset of
> > turbulence under any set of
> > > fluid conditions.  This dimensionless parameter is
> > now called the "Reynold's
> > > Number" in his honor.  (NOTE: In engineering, one
> > of the highest honors is
> > > to have a dimensionless number or fundamental
> > defining equation named after
> > > you!)  The Reynold's Number is given by:
> > > 
> > > Re = (L * V * ro) / mu
> > > 
> > > Where:
> > > Re = Reynold's Number
> > > L = Characteristic Dimension Or Length Of The
> > Flowing System (ft)
> > > V = Fluid Velocity (ft/sec)
> > > ro = Fluid Density (lb/ft^3)
> > > mu = Fluid Viscosity (lb/ft-sec)
> > > 
> > > Note: all the physical parameters that go into
> > this calculation must be in
> > 
> === message truncated ===
> 
> 
> 
> 
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