[Rhodes22-list] Re: Pointing

[Steve Alm]

Well, Brad, you see that's exactly what gets me in trouble--I was drinking
when I sent my post about lift, as if I know anything about it.  8-)
I'll go practice my scales now.  Hey--isn't it five o'clock somewhere?

Slim

On 9/23/04 3:17 PM, "brad haslett" <flybrad at yahoo.com> wrote:

> 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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