Monday March 16, 2020

Many people I meet ask me why there are so many pilots who also love
sailing?

The answer is simple. The physics is the same. It is the Bernoulli
Principle, named after the Swiss Daniel Bernoulli. He realised that a
shaped object moving through a fluid, be it water or air, exhibits certain
predictable results.

Lets look at an airplane wing. The “front” edge is called the leading edge.
The “back” edge is called the trailing edge. A line drawn from the leading
edge to the trailing edge is called the chord line. A line drawn
perpendicular from the chord line to the upper and lower surface of the
wing is the camber. An airplane wing has a greater camber on the top side
of the wing than the bottom side of the wing. Meaning the wing has more of
a curve on the top than the bottom. All airplane wings are designed
basically the same. They are more technically known as an airfoil.

So according to Bernoulli, as this airfoil moves through the fluid called
air, the air is forced to travel around our wing. Because of the greater
camber, or curve, on the upper section of the wing, the air has a farther
distance to travel over the top of the wing, then it does under the bottom
of the wing. Here is where Bernoulli’s genius comes into play. As a result
of the greater distance traveled, the air flows faster over the top of our
wing and as a consequence the top of the wing creates a slightly lower
pressure area than does the bottom of the wing, creating a lifting force
towards the lower pressure area. This and a bit of Air Isaac Newton”s
Second Law of Motion, “for every action there is an equal and opposite
reaction,” and you have the ingredients for flight.

Well, there are a few more important considerations to create an airplane,
but you do have an airfoil. Now, as our intrepid airfoil moves forward
through the fluid of air, it creates in the air a relative wind that is
equal in velocity but opposite in direction of the movement of the wing. It
also creates drag as a by-product of generating lift. Remember Isaac
Newton. Pilot’s know this is induced drag, which among other things reduces
our wings efficiency.

If we were to draw the chord line of our wing, and draw a line that
represents the relative wind, that angle formed by those two lines is
called the angle of attack of the wing. As the angle of attack increases,
initially so does both lift and drag, lift increasing fasterq than drag. At
a point these trends switch and now the wing generates drag faster than
lift (induced drag). This point is called the critical angle of attack. As
a pilot you have control over the angle of attack. You use a control
surface called an elevator. Attached at the tail of our airplane, the
elevator moves the nose of the airplane from the pilots head to the pilots
foot.

(Not to be confused with the ailerons on the trailing edge of the wing that
moves the nose of the airplane from the pilots head to the pilots hip, or
the rudder, also located at the tail end of the airplane and moves the nose
of the airplane from the pilots ear to ear. All 3 of our airplanes control
surfaces intercect at the center of gravity of the airplane).

Back to our discussion. As the pilot you have control over the angle of
attack, but not the critical angle of attack. This is a design feature of
the airplane. The critical angle of attack also is the point at which the
airplane will stall. Ominous words for staying the airplane now is
generating drag faster than lift and as such its aerodynamic behaviour
changes, and so does how the pilot controls the airplane. Pilots are
trained to recognise when you are getting close to the critical angle of
attack, and to promptly recover by adding forward elevator pressure, or at
least relaxing the back elevator pressure to decrease, or lower, the angle
of attack below critical. There are many aerodynamic and mechanical clues
that a pilot is trained to use to make this prompt and correct control
surface input.

Also when the wing stalls, the airflow cannot smoothly follow the upper
section of the wings surface and separates from the wings surface causing a
loss of lift, an increase in drag, and the aerodynamic buffeting pilots
associate with a stall. Many pilots are scared of stalls. Typically sue to
a lack of understanding and training. We aerobatic pilots love that
stalled flight region. So many possibilities!

A sail is basically a wing. Just look at the Americas Cup race sailboats
and you think a Boeing 737 is missing a wing.

A sail has a leading edge called the luff. A sail has a trailing edge,
called the leach. The apparant wind is redirected by the sail. Our sail
then takes its functional curved shape. It becomes an air foil. Like the
camber on our wing, the airflow over the sail creates a low pressure area
forward of the sails luff and is the primary forward motion force parallel
to the course steered. This is Bernoulli. A sailor trims the sails, less
they begin to vibrate and luff. Sounds very much like a wing stalling.

Newton has his input as well. Sailboats have a keel, which acts as an
airfoil in the water. The same wind that is the sails forward driving
force, also creates a side or perpendicular force to the course steered.
The keel converts this side force into forward thrust. Think of squeezing a
banana. Squeeze the middle of the banana and the banana shots forward. This
is Newton, and the job of our keel.

It’s with this basic understanding of Bernoulli and Newton that pilots seem
to have a good grasp of the physics of sailing. Add to that understanding
communications, navigation, risk management, engines, mechanical systems
and troubleshooting are basically the same, and our pilot steps onto the
sailboat with a wealth of knowledge.

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