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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Airbus proposing Measures to Reduce the Aviation Carbon Footprint that may make Flying more Dangerous

Posted by PITHOCRATES - September 9th, 2012

Week in Review

Airplanes are very complex machines.  They fly at speeds 3-4 times the speeds they land and take off at.  Which requires leading edge slats and trailing edge flaps to curve the wing more at low speed to increase lift.  While flattening it out more at high speeds to reduce drag.  When landing pilots put the engines into reverse thrust to help slow the plane down.  So they even use fuel to slow down.

And speaking of fuel it’s expensive.  Airlines carry as little of it as possible in their airplanes to reduce weight which reduces costs.  Sometimes bad weather forces planes to go to an alternate airport.  Sometimes there are strong headwinds.  Sometimes they fly into Heathrow and have to circle for a half hour or so to land.  Because they only have two runways.  Compounding this problem planes are getting lighter and engines are getting more efficient.  Allowing airlines to carry even less fuel.  So it is not uncommon for a pilot to declare a fuel emergency because of unexpected additional flying time.

When flying in the air highways air traffic controllers keep airplanes separated by large distances.  To keep them from running into each other.  The more distance the better so they can take evasive actions to avoid bad weather cells.  Or allow a plane some leeway in case they have a system malfunction (like plugged pitot tubes feeding false air speed and altimeter readings into the autopilot) that takes the plane off course.  Or in case a plane flies into some clear air turbulence (CAT) and it drops out of the sky 1,000 feet or so.  Or rises 1,000 feet or so.  Two things that allow a plane to recover from unplanned events like these are empty skies around you and altitude.

Aviation has come a long way.  And Boeing and Airbus are making some incredible airplanes.  So they know a thing or two about flying an airplane.  And it shows in their planes.  Which makes it hard to take them seriously when they talk about ways to reduce their carbon footprint by making flying more risky (see Airbus To Present Measures To Reduce Industry’s Environmental Footprint by Jens Flottau posted 9/6/2012 on Aviation Week).

Airbus on Sept. 6 will unveil five measures it says will make the aviation industry environmentally sustainable by 2050 despite projected growth for global air transport…

Airbus also foresees a new method for takeoff, with renewably powered propelled acceleration allowing aircraft to climb steeper and reach cruise altitude faster. This in turn would allow airports to build shorter runways and minimize land use.

Once in cruise, aircraft should be able to self-organize and select the most efficient routes, says Airbus. On dense routes, aircraft could fly in formation, like birds, to take advantage of drag reduction opportunities.

In Airbus’ vision, aircraft will descend without using engine power or air brakes and would be able to decelerate quicker and to a lower final approach speed enabling them to use shorter runways…

Fuel is a key component of Airbus’ proposal, and the manufacturer says the use of biofuels hydrogen, electricity and solar energy will be required to reduce the industry’s environmental footprint.

You simply can’t build shorter runways.  Because planes aren’t perfect.  Sometimes things happen.  If we had shorter runways what would happen to a plane landing with damaged leading edge slats or trailing edge flaps?  And they have to land at a higher speed than normal because they can’t curve the wing to create more lift at lower speeds?  And what if a plane’s thrust reversers failed to deploy?  This is why we have long runways.  To give planes with problems a better chance to land safely.

Flying commercial jets in formation?  Not a good idea.  One of the most dangerous things to do in the Air Force is aerial refueling.  Where two large planes get real close to each other.  If they bump into each other they could cause some damage.  Even cause them to crash.  Flying in formation would be exhausting for a pilot.  Or they could entrust their formation flying to an autopilot.  But if they hit some CAT and get thrown around in that airspace they could get thrown into each other.  Even while flying on autopilot.  Planes also make their own turbulence.  Which is why there are larger distances between the big planes (i.e., the heavies) and the small ones.  So the small ones don’t get flipped over by some spiraling wingtip vortex turbulence off the heavy in front of it.

Solar energy?  Really?  How?  It’s not going to propel a jumbo jet.  And if they think they’re going to save on engine emissions by using solar panels on the wings to produce electricity for the cabin lights and electronics I don’t think that will work.  The emissions from the electrical load on those engines may be negligible compared to emissions they make producing thrust for flight.  And if they add more weight (solar panels) that will only take more fuel for flight.  Which will release more emissions.  Finally, a lot of planes fly at night.  When there is no sunshine.  What then?

Trying to reduce a plane’s carbon footprint will only make flying more dangerous.  It’s one thing to throw money away building solar panels and windmills on the ground.  For that’s just ripping the people off.  But applying this nonsense to aviation may end up killing people.  It’s hard to believe that Airbus is serious with these suggestions.  One wonders if they’re just proposing this to get those proposing that carbon trading scheme to back off as it will increase the cost of flying.  Which will reduce the number of people flying.  And reduce the number of planes Airbus can sell.  Perhaps by dangling this green future of aviation they may buy some time before the carbon trading scheme kills the aviation industry.

Fighting nonsense with nonsense.  It’s just as good an explanation as any.


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