Air, Low Pressure, High Pressure, Lateen Sail, Flight, Wing, Lift, Drag, Leading Edge Slats, Trailing Edge Flaps and Angle of Attack

Posted by PITHOCRATES - October 10th, 2012

Technology 101

There’s more to Air than Meets the Eye even though it’s Invisible

When you take a shower have you noticed how the shower curtain pulls in towards you?  Have you ever wondered why it does this?  Here’s why.  Air has mass.  The water from the showerhead sends out a stream of water drops that also has mass.  So they fall to the floor of the shower.  Pushing air with it.  And pulling air behind it.  (Like drinking through a straw.  As you suck liquid out of the straw more liquid enters the straw.)  So you not only have a stream of water moving down alongside the shower curtain.  You also have a stream of air moving down alongside the shower curtain.

As the falling water sweeps away the air from the inside of the shower current it creates a low pressure there.  While on the outside of the curtain there is no moving water or air.  And, therefore, no change in air pressure.  But there is a higher pressure relative to the lower pressure on the inside of the shower curtain.  The low pressure inside pulls the curtain while the high pressure outside pushes it.  Causing the shower curtain to move towards you.

There’s more to air than meets the eye.  Even though it’s invisible.  It’s why we build modern cars aerodynamically to slice through large masses of invisible air that push back against cars trying to drive through it.  Making our engines work harder.  Consuming more gas.  And reducing our gas mileage.  While race cars will use spoilers to redirect that air up, forcing the weight of the car down on the tires.  To help the tires grip the road at higher speeds.  We even design skyscrapers to be aerodynamic.  To split the prevailing winds around the buildings to prevent large masses of air from slamming into the sides of buildings, minimizing the amount buildings sway back and forth.

We put the Engines on, and the Fuel in, the Wings to Counteract the Lifting Force on an Aircraft’s Wings

Air can be annoying.  Such as when the shower curtain sticks to your leg.  As it steals miles per gallon from your car.  When it shakes the building you’re in.  But it can also be beneficial.  As in early ship propulsion before the steam engine.  Large square-rigged sails that pushed ships along the prevailing winds.  And triangular lateen sails that allowed us to travel into the wind.  By zigzagging across the wind.  With the front edge of a lateen sail slicing into the wind.  The sail redirects the wind on one side of the sail to the rear of the boat that pushes the boat forward.  While the wind on the other side follows the curved sail creating a low pressure that pulls the boat forward.  Like the inside of that shower curtain.  Only with a lot more pulling force.

Harnessing the energy in wind let the world become a smaller place.  As people could travel anywhere in the world.  Of course, some of that early travel could take months.  And spending months on the open sea could be very trying.  And dangerous.  A lot of early ships were lost in storms.  Ran aground on some uncharted shoal.  Or simply got lost and ran out of drinking water and food.  Or fell to pirates.  So it took a hearty breed to travel the open seas under sail.  Of course today long-distant travel is a bit easier.  Because of another use for air.  Flight.

Like a lateen sail an aircraft wing splits the airflow above and below the wing.  And like the lateen sail an aircraft wing is curved.  The air pushes on the bottom of the wing creating a high pressure.  While the air passing over the curve of the top of the wing creates a low pressure.  Pulling the wing up.  In fact, it’s the wind passing over the top of the wing that does the lion’s share of lifting airplanes into the air.  The low pressure on top of the wing is so great that they put the engines on the wings, and the fuel in the wings, to counteract this lifting force.  To prevent the wings from curling up and snapping off of the plane.  Planes with tail-mounted engines have extra reinforcement in the wings to resist this bending force.  So those lifting forces only lift the plane.  And not curl the wing up until it separates from the plane.

To make Flying Safe at Slow Speeds they add Leading Edge Slats and Trailing Edge Flaps to the Wing

Sails can propel a ship because a ship floats on water.  The wind only propels a ship forward.  On an airplane the wind moving over the wings provides only lift.  It does not propel a plane forward.  Engines propel planes forward.  And it takes a certain amount of forward speed to make the air passing over the wings fast enough to create lift.  The faster the forward air speed the greater the lift.  Today jet engines let planes fly high and fast.  In the thin air where there is less drag.  That is, where the air has less mass pushing against the forward progress of the plane.  At these altitudes the big planes cruise in excess of 600 miles per hour.  Where these planes fly at their most fuel efficient.  But these big planes can’t land or take off at speeds in excess of 600 miles per hour.  In fact, a typical take-off speed for a 747-400 is about 180 miles per hour.  Give or take depending on winds and aircraft weight.  So how does a plane land and take off at speeds under 200 mph while cruising at speeds in excess of 600 mph?  By changing the shape of the wing.

We determine the amount of lift by the curvature and surface area of the wing.  The greater the curvature the greater the lift.  However, the greater the curvature the greater the drag.  And the greater the drag the more fuel consumed at higher speeds.  And the more stresses placed on the wing.  Also, current runways are about 2 miles long for the big planes.  That’s when they land and take off at speeds under 200 mph.  To land and take off at speeds around 600 mph would require much longer runways.  Which would be extremely costly.  And dangerous.  For anything traveling close to 600 mph on or near the ground would have a very small margin of error.  So to make flying safe and efficient they add leading edge slats to the front edge of the wing.  And trailing edge flaps to the back edge of the wing.  During cruise speeds both are fully retracted to reduce the curvature of the wing.  Allowing higher speeds.  At slower speeds they extend the slats and flaps.  Greatly increasing the curvature of the wing.  And the surface area.  Providing up to 80% more lift at these slower speeds.

At takeoff and landing pilots elevate the nose of the aircraft to increase the angle of attack of the wing.  Forcing more air under the wing to push the wing up.  And causing the air on top of the wing to turn farther away for its original direction of travel as it travels across the top of the up-tilted wing.  Creating greater lift.  And the ability to fly at slower speeds.  However, if the angle of attack it too great the smooth flow of air across the wing will break away from the wing surface and become turbulent.  The wing will not be able to produce lift.  So the wing will stall.  And the plane will fall out of the sky.  With the only thing that can save it being altitude.  For in a stall the aircraft will automatically push the stick forward to lower the nose.  To decrease the angle of attack of the wing.  Decrease drag.  And increase air speed.  If there is enough altitude, and the plane has a chance to increase speed enough to produce lift again, the pilot should be able to recover from the stall.  And most do.  Because most pilots are that good.  And aircraft designs are that good.  For although flying is the most complicated mode of travel it is also the safest mode of travel.  Where they make going from zero to 600 mph in a matter of minutes as routine as commuting to work.  Only safer.


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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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