Windmills, Rotational Energy, Wing, Lift, Rotary Wing, Angle of Attack, Variable-Pitch Propellers, Drag, AC Power and Wind Turbine

Posted by PITHOCRATES - June 27th, 2012

Technology 101

When an Aircraft Rotates for Takeoff it increases the Angle of Attack of the Wing to Create more Lift

Early windmills turned when the wind pushed a sail or vane.  Thereby converting wind energy into rotational energy.  Mechanical linkages and shafts transferred this rotational motion to power a mill.  Or pump water.  As well as an assortment of other tasks.  Whatever the task it was important to regulate the speed at which the shaft rotated.  Which meant turning the windmill into the wind.  And adjusting the amount of sail catching the wind.  Much like on a sailing ship.  At first by shutting the windmill down and manually adjusting the sails.  Then later automating this process while the windmill was turning.  If the winds were too strong they’d lock the windmill to prevent it from turning.  To prevent damaging the windmill.

They regulated the speed to protect the equipment attached to the windmill, too.  To prevent a mill stone from spinning too fast.  Risking damage to it.  And harm to the people working with the equipment.  Or to protect a water pump form pumping too fast.  Even the small farm windmills had over-speed protection.   These sat atop a well.  The windmill drove a small piston to pump the water up the well shaft.  To prevent this windmill from flying apart in high winds over-speed features either furled the blades or rotated the windmill parallel to the wind.  Shutting the pump down.

But wind just doesn’t push.  It can also lift.  A lateen (triangular) sail on a sailing vessel is similar to an aircraft wing.  The leading edge of the sail splits the wind apart.  Part of it fills the sail and pushes it.  Bowing it out into a curved surface.  The wind passing on the other side of the sail travels across this curved surface and creates lift.  Similar to how a wing operates during takeoff on a large aircraft.  With the trailing edge flaps extended it creates a large curve in the wing.  When the aircraft rotates (increasing the angle of attack of the wing) to take off wind passing under the wing pushes it up.  And the wind travelling over the wing pulls it up.  These lift forces are so strong that planes carry their fuel in the wings and mount engines on the wing to keep the wings from bending up too much from these forces of lift.

A Pilot will Feather the Propeller on a Failed Engine in Flight to Minimize Drag 

When an aircraft carrier launches its aircraft it turns into the wind.  To maximize the wind speed travelling across the wings of the aircraft.  For the faster the wind moves across the wing the great lift it creates.  Commercial airports don’t have the luxury of turning into the wind.  So they lay their runways out to correspond to the prevailing wind directions.  As weather systems move through the region they often reverse the direction of the wind.  When they do planes take off in the other direction.  If the winds are somewhere in between these two extremes some airports have another set of runways called ‘crosswind’ runways.  Or trust in the highly skilled pilots flying out of their airports to adjust the control surfaces on their planes quickly and delicately to correct for less than optimal winds.

Helicopters don’t have this problem.  They can take off facing in any direction.  Because that big propeller on top is a rotary wing.  Or rotor.  A fixed wing airplane needs forward velocity to move air over their wings to create lift.  A helicopter moves air over its rotary wing by spinning it through the air.  To create lift the pilot tilts the rotor blades to change their angle of attack.  And tilts the whole rotor in the direction of travel.  The helicopter’s engine runs at a constant RPM.  To increase lift the angle of attack is increased.  This also creates drag that increases the load on the engine, slowing it down.  So the pilot increases the throttle of the engine to return the rotor to that constant RPM.

Propeller-powered airplanes also have variable-pitch propellers.  To create the maximum possible lift at the lowest amount of drag.  So it’s not just engine speed determining aircraft speed.  When running up the engines while on the ground the pilot will feather the propellers.  So that the blade pitch is parallel to the airflow and moves no air.  This allows the engines to be run up to a high RPM without producing a strong blast of air behind it.  A pilot will also feather the prop on a failed engine in flight to minimize drag.  Allowing a single-engine plane to glide and a multiple engine plane to continue under the power of the remaining engines.  A pilot can even reverse the pitch of the propeller blades to reverse the direction of airflow through the propeller.  Helping planes to come to a stop on short runways.

By varying the Blade Pitch for Different Wind Speeds Wind Turbines can Maintain a Constant RPM

Thomas Edison developed DC electrical power.  George Westinghouse developed AC electrical power.  And these two went to war to prove the superiority of their system.  The War of the Currents.  Westinghouse won.  Because AC is economically superior.  One power plant can power a very large geographic area.  Because alternating current (AC) works with transformers.  Which stepped up voltages for long-distance power transmission.  And then stepped them back down to the voltages we use.  Power equals voltage times current.  Increasing the voltages allows lower currents.  Which allows thinner wires.  And fewer generating plants.  Which saves money.  Hence the economic superiority of AC power.

Alternating current works with transformers because the current alternates directions 60 times a second (or 60 cycles or hertz).  Every time the currents reverse an electrical field collapses in one set of windings of a transformer, inducing a voltage in another set of windings.  A generator (or, alternator) creates this alternating current by converting rotational energy into electrical energy.  Which brings us back to windmills.  A source of rotational energy.  Which we can also use to generate electrical energy.  But unlike windmills of old, today’s windmills, or wind turbines, turn from lift.   The wind doesn’t push the blades.  The wind passes over them producing lift.  Like on a wing.  Pulling them into rotation.

The typical wind turbine design is a three-bladed propeller attached to a nacelle sitting on top of a tall pylon.  The nacelle is about as large as a big garden shed or a small garage.  Inside the nacelle are the alternator and a gearbox.  And various control equipment.  Like windmills of old wind turbines still have to face into the wind.  We could do this easily and automatically by placing the propeller on the downwind side of the nacelle.  Making it a weathervane as well.  But doing this would put the pylon between the wind and the blades.  The pylon would block the wind causing uneven loading on the propeller producing vibrations and reducing the service life.  So they mount the propeller on the upwind side.  And use a complex control system to turn the wind turbine into the wind.

When it comes to electrical generation a constant rotation is critical.  How does this happen when the wind doesn’t blow at a constant speed?  With variable-pitched blades on the propeller.  By varying the blade pitch for different wind speeds they can maintain a constant number of revolutions per minute (RPM).  For a limited range of wind conditions, that is.  If the wind isn’t fast enough to produce 60 hertz they shut down the wind turbine.  They also shut them down in high winds to prevent damaging the wind turbine.  They can do this by feathering the blades.  Turning the propeller blades parallel to the wind.  Or with a mechanical brake.  The actual rotation of the propeller is not 60 cycles per second.  But it will be constant.  And the gearbox will gear it up to turn the alternator at 60 cycles per second.  Allowing them to attach the power they produce to the electric grid.


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