Aircraft De-Icing Systems

Posted by PITHOCRATES - October 23rd, 2013

Technology 101

A build-up of Ice on Airfoils causes a Reduction of Lift and a Loss of Stability

In the classic movie Airport (1970) after the guy pulled the trigger on his briefcase bomb the plane suffered a massive decompression.  When Dean Martin got back to the cockpit he told the flight engineer to give them all the heat they had.  Because it’s very cold flying above 10,000 feet without pressurization.  That’s why World War II flight crews wore a lot of heavy clothing and thick mittens in their bombers.  As well as oxygen masks as the air was too thin to breathe.  The B-17 even had open windows for the waste gunners.  Making it very cold inside the plane.  Because the air is very, very cold at altitude.

There is another problem at altitude.  Because of these very frigid temperatures.  Water droplets in the air will freeze to any surface they come into contact with.  They can reduce engine power for both propeller and jet engines.  They can freeze on ports used for instrumentation and give inaccurate readings of vital aircraft data (such as engine pressure ratio, aircraft speed, etc.).  And they can freeze on airfoils (wings, rudder, tail fin, etc.).  Disturbing the airflow on these surfaces.  Causing a reduction of lift and a loss of stability.

Ice and airplanes are two things that don’t go together.  As ice forms on a wing it disturbs the airflow over the surface of the wing.  Increasing drag.  And reducing lift.  Causing the plane to lose speed.  And altitude.  If the ice continues to form on the wing eventually it will stall the wing.  And if the wing stalls (i.e., produces no lift) the plane will simply fall out of the sky.  In the early days of aviation pilots were highly skilled in flying their planes where there were no icing conditions.  Flying over, under or around masses of air containing water droplets in subfreezing temperatures.  Today we have anti-icing systems.

The most common Anti-Icing System on Commercial Jets is a Bleed Air System

One of the most common anti-icing systems on turboprop aircraft is the use of inflatable boots over the leading edge of the wing.  Basically a rubber surface that they can pump air into.  When there is no ice on the wing the boot lies flat on the leading edge without interrupting the airflow.  When ice forms on the leading edge of the wing the boot inflates and expands.  Cracking the ice that formed over it.  Which falls away from the wing.

Commercial jets have larger airfoils.  And require a larger anti-icing system.  The most common being a pneumatic manifold system that ducts hot air to areas subject to icing.  Which works thanks to a property of gas.  If you compress a gas you increase its temperature.  That’s how a diesel engine can work without sparkplugs.  The compressed air-fuel mixture gets so hot it ignites.  This property comes in handy on a jet plane as there is a readily available source of compressed air.  The jet engines.

As the air enters the jet it goes through a series of fast-spinning rotors.  As the air moves through the engine these rotors push this air into smaller and smaller spaces.  Compressing it.  Through a low-pressure compressor.  And then through a high-pressure compressor.  At which time the air temperature can be in excess of 500 degrees Fahrenheit.  It is in the high-pressure compressor that we ‘bleed’ off some of this hot and pressurized air.  We call this a bleed air system.  The air then enters a manifold which ducts it to at-risk icing areas.  From the engine cowling to the wings to the instrumentation ports.  Using the hot air to raise temperatures in these areas above the freezing temperature of water.  Thus preventing the formation of ice.

The Drawback of a Bleed Air System is Reduced Engine Efficiency

The bleed air system does more than just anti-icing.  It also pressurizes the cabin.  As well as keeps it warm.  Which is why we don’t have to dress like a crewmember on a World War II bomber when we fly.  It also powers the air conditioning system.  And the hydraulic system.  It provides the pressure for the water system.  And it even starts the jet engines.  With the source of pressurized bleed air coming from the auxiliary power unit mounted in the tail.  Or from an external ground unit.  Once the jets are running they disconnect from the auxiliary source and run on the bleed air from the engines.

There is one drawback of a bleed air system.  It bleeds air from the jet engine.  Thus reducing the efficiency of the engine.  And a less efficient engine burns more fuel.  Raising the cost of flying.  With high fuel costs and low margins airlines do everything within their power to reduce the consumption of fuel.  Which is why pilots don’t top off their fuel tanks.  They’d like to.  But extra fuel is extra weight which increases fuel consumption.  So they only take on enough fuel to get to their destination with enough reserve to go to an alternate airport.  Even though it seems risky few planes run out of fuel in flight.  Allowing the airlines to stay in business without having to raise ticket prices beyond what most people can afford.

To help airlines squeeze out more costs Boeing designed their 787 Dreamliner to be as light as possible by using more composite material and less metal.  Making it lighter.  They are also using a more efficient engine.  Engines without a bleed air system.  In fact, they eliminated the pneumatic system on the 787.  Converting the pneumatic components to electric.  Such as using electric heating elements for anti-icing.  Thus eliminating the weight of the bleed air manifold and duct system.  As well as increasing engine efficiency.  Because all engine energy goes to making thrust.  Which reduces fuel consumption.  The key to profitability and survival in the airline industry.

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Phase Transition, Expansion Valve, Evaporator, Compressor, Condenser and Air Conditioning

Posted by PITHOCRATES - April 3rd, 2013

Technology 101

We can use Volume, Pressure and Temperature to change Water from a Liquid to a Gas and back Again

Liquids and gasses can do a lot of work for us.  If we can control three variables.  Volume.  Pressure.  And temperature.  For example, internal combustion engines work best when hot.  But excessive heat levels can damage the engine.  So we use a special anti-freeze/anti-boil liquid in the cooling system.  A pump circulates this liquid through the engine where it absorbs some of the excess heat of combustion that isn’t used in pushing the piston.  After leaving the engine it flows through a radiator.  Air blows across tubes in the radiator cooling this liquid.  Ejecting some of the heat of combustion into the atmosphere.  Lowering the temperature of the cooling liquid so it can flow through the engine again and absorb more heat.

Our first cars used alcohol in the winter for a lower freezing point.  So this liquid didn’t freeze in the engine and crack the block.  Letting the coolant flow out.  And with no cooling available the excessive heat levels would damage the engine.  In the summer time we used plain water in the cooling system.  And kept the cooling system sealed and under high pressure to prevent the water from boiling into steam.  But the high pressure often caused a hose or a radiator cap to fail.  Releasing the pressure.  And letting the cooling water boil out leaving the engine unsafe to operate.

If this happened on a hot summer’s day and you got a tow to a gas station you may have sat there waiting for them to complete the repairs.  Sipping on a cool bottle of soda from a refrigerated soda machine.  Soon drops of water would condense onto your cold bottle.  The cold bottle cooled the water in gas form (the humidity in the air) and turned it back into a liquid.  So in these examples we see how we were able to use pressure to keep water a liquid.  And how removing heat from water as a gas changed it back into a liquid.  This phase transition of a material has some very useful applications.

The High-Pressure Refrigerant Liquid from the Condenser loses Pressure going through the Expansion Valve

The phase transition between a liquid and a gas are particularly useful.  Because we can move liquids and gases in pipes and tubing.  Which allows us to take advantage of evaporation (going from a liquid to a gas) in one area.  While taking advantage of condensation (going from a gas to a liquid) in another area.  By changing pressure and volume we can absorb heat during evaporation.  And release heat during condensation.  Allowing us to absorb heat inside a building with evaporation.  And release that heat outdoors with condensation.  All we need are a few additional components and we have air conditioning.  An expansion valve.  An evaporator.  A compressor.  A condenser.  A couple of fans.  And some miscellaneous control components.

We install the expansion valve and the evaporator inside our house.  Often installed inside the furnace.  And the compressor and the condenser outside of the house.  We interconnect the indoor and the outdoor units with tubing.  Inside this tubing is a refrigerant.  Which is a substance that transitions from liquid to a gas and back again at relatively low temperatures.  As the refrigerant moves from the evaporator to the condenser it is a gas.  As it moves from the condenser to the evaporator it is a liquid.  The transition between these stages occurs at the evaporator and the condenser.

The refrigerant leaves the condenser as a liquid under high pressure.  As it passes through the expansion valve the pressure drops.  By restricting the flow of the liquid refrigerant.  Think of a faucet at a kitchen sink.  If you open it all the way the water flowing in and the water flowing out are almost equal.  But if we just open the faucet a little we get only a small trickle of water out of the faucet.  And a pressure drop across the valve.  With the full force of city water pressure pushing to get out of the faucet.  And a low pressure trickle coming out of the faucet.

As the Warm Air blows across the Evaporator Coil any Humidity in the Air will condense on the Coil

As the liquid leaves the expansion valve at a lower pressure it enters the evaporator coil.  A fan blows the warm air inside of the house through the evaporator coil.  The heat in this air raises the temperature of the refrigerant.  And because of the lower pressure this heat readily boils the liquid into a gas.  That is, it evaporates.  Absorbing heat from the warm air as it does.  Cooling the air.  Which the fan blows throughout the ductwork of the house.

As the gas leaves the evaporator it travels through a tube to the condenser unit outside.  And enters a compressor.  Where an electric motor spins a crankshaft.  Attached to the crankshaft are two pistons.  As a piston moves down it pulls low pressure gas into the cylinder.  As the piston moves up it compresses this gas into a higher pressure.  As the pressure rises it applies more pressure on the spring holding the discharge valve closed.  When the pressure is great enough it forces open the valve.  And sends the high-pressure gas to the condenser coil.  Where a fan blows air through it lowering the temperature of the high pressure gas enough to return it to a liquid.  As it does it releases heat from the refrigerant into the atmosphere.  Cooling the refrigerant.  As the liquid leaves the condenser it flows to the expansion valve to repeat the cycle.  Over and over again until the temperature inside the house falls below the setting on the thermostat.  Shutting the system down.  Until the temperature rises high enough to turn it back on.  A window air conditioner works the same way.  Only they package all of the components together into one unit.

There is one other liquid in an air conditioning system.  Water.  As the warm air blows across the evaporator coil any humidity in the air will condense on the coil.  Like on a cold bottle of soda on a hot summer day.  As this water condenses on the evaporator coil is eventually drips off into a pan with a drain line.  If the evaporator is in the furnace this line will likely run to a sewer.  If the evaporator is in the attic this line will run to the exterior of the house.  Perhaps draining into a gutter.  If it’s a window unit this line runs to the exterior side of the unit.  These simple components working together give us a cool and dehumidified house to live in.  No matter how hot and humid it gets outside.

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Kerosene, Jet Fuel, Lockheed Constellation, Boeing 707, Boeing 747-400, Newton’s Third Law of Motion, Turbojet and Fan Jet

Posted by PITHOCRATES - October 3rd, 2012

Technology 101

The only way to make Flying Available to the General Public is to put as many People as Possible on an Airplane

Refined petroleum products have made our lives better.  We have gasoline to drive wherever we want.  We have diesel fuel to transport things on ships and trains like petroleum oil, iron ore, coal, food, medicine, smartphones, coffee, tea, wine, scotch whisky, bourbon whiskey, beer, fresh fish, sushi, etc.  Pretty much everything we buy at a store or a restaurant got there on something powered by diesel fuel.  And sometimes kerosene.  If it must travel fast.  And if it does then it finds itself on a jet aircraft.

Today aviation has shrunk the world.  We can order a new smartphone sitting on a shelf in California and have it the next day in New York.  We can even travel to distant countries.  Some in the time of a typical working day.  Some a half a day or longer.  When but a 100 years earlier it took a couple of weeks to cross the Atlantic Ocean.  While 200 years ago it took a couple of months.  We can travel anywhere.  And get there quickly.  Thanks to the jumbo jet.  And not just the super-rich.  Pretty much anyone today can afford to buy a plane ticket to travel anywhere in the world.  And one thing makes this possible.  The jet engine.

Airplanes are expensive.  So are airports, air traffic control and jet fuel.  Airlines pay for all of these costs one passenger at a time.  Their largest cost is their fuel cost.  The longer the flight the greater the cost.  So the only way to make flying available to the general public is to put as many people as possible on an airplane.  Dividing the total flying cost by the number of passengers on the airplane.  This is why we fly on jumbo jets for these longer flights.  Because there are more people to split the total costs.  Lowering the cost per ticket.  Before the jet engine, though, it was a different story.

The Boeing 747-400 can take up to 660 Passengers some 7,260 Miles at a Speed of 567 MPH

One of the last intercontinental propeller-driven airplanes was the Lockheed Constellation.  A plane with four (4) Wright R-3350-DA3 Turbo Compound 18-cylinder supercharged radial engines putting out 3,250 horsepower each.  Which is a lot considering today’s typical 6-cyclinder automobile engine is lucky to get 300 horsepower.  No, the horsepower of one of these engines is about what one modern diesel-electric locomotive produces.  So these are big engines.  With a total power equal to about four locomotives lashed up.  Which is a lot of power.  And what does that power allow the Constellation do?  Not much by today’s standards.

In its day the Lockheed Constellation was a technological wonder.  It could take up to 109 passengers some 5,500 miles at a speed of 340 mph.  No bus or train could match this.  Not to mention it could fly over the water.  Then came the age of the jet.  The Boeing 707 being the first largely successful commercial jetliner.  Which could take up to 189 passengers some 6,160 miles at a speed of 607 mph.  That’s 73.4% more passengers, a 78.5% faster speed and a 14.1% longer range.  Which is an incredible improvement over the Constellation.  But nothing compared to the Boeing 747-400.  Which can take up to 660 passengers (506% more than the Constellation and 249% more than the 707) some 7,260 miles at a speed of 567 mph.

Now remember, fuel is the greatest cost of aviation.  So let’s assume that a intercontinental flight costs a total of $75,000 for each plane flying the same route.  Dividing that cost by the number of passengers you get a ticket price of approximately $688, $397 and $114 for the Constellation, the 707 and the 747-400, respectively.  So you can see the advantage of packing in as many passengers as possible into an airplane to lower the cost of flying.  Which is why the jumbo jets fly the longest routes that consume the most fuel.  And why we no longer fly propeller-driven aircraft except on short routes to airports with short runways.  These engines just don’t have the power to get a plane off the ground with enough people to reduce the cost of flying to a price most people could afford.  Only the jet engine has that kind of power.

The Fan Jet is basically a Turbojet with a Large Fan in front of the Compressor

Newton’s Third Law of Motion states that for every action there is an equal and opposite reaction.  Think of a balloon you just blew up and are holding closed.  If you release your hold air will exit the balloon in one direction.  And the balloon will move in the opposite direction.  This is how a jet engine moves an aircraft.  Hot exhaust gases exit the engine in one direction.  Pushing the jet engine in the opposite direction.  And because the jet engines move the plane moves.  With the force of the jet engines transferred via their connection points to the aircraft.  The greater the speed of the gas exiting the jet the faster it will push a plane forward.

The jet engine gets that power from the continuous cycle of the jet engine.  Air enters one end, gets compressed, enters a combustion chamber, mixes with fuel (kerosene), ignites, expands rapidly and exits the other end.  The hot (3,632 degree Fahrenheit) and expanding gases pass through and spin a turbine.  Then exit the engine.  The turbine is connected to the compressor at the front of the engine.  So the exhaust gases spin the compressor that sucks air into the engine.  As the air passes through the compressor it compresses and heats up.  Then it enters the combustion chamber and joins fuel that is injected and burned continuously.  Sort of like pouring gas on a burning fire.  Only enormous amounts of compressed air and kerosene are poured onto a burning fire.  As this air-fuel mixture burns it rapidly expands.  And exits the combustion chamber faster than the air entered it.  And shoots a hot stream of jet gas out the tail pipe.  Which produces the loud noise of these turbojets.  This fast jet of air cuts through the surrounding air.  Resulting in a shear effect.  Which the next generation of jet engines, the fan jet, greatly reduces.

The fan jet is basically a turbojet with one additional feature.  A large fan in front of the compressor.  These are the big engines you see on the jumbo jets.  They add another turbine inside the jet that spins the fan at the front of the engine.  Which feeds some air into the compressor of what is basically a turbojet.  But a lot of the air this fan sucks in bypasses the turbojet core.  And blows directly out the back of the fan at high speed.  In fact, this bypass air provides about 75% of the total thrust of the fan jet.  Acting more like a propeller than a jet.  And as an added benefit this bypass air surrounds the faster exhaust of the jet thereby lessening the shear effect.  Making these larger engines pretty quiet.  In fact a DC-9, an MD-80, a 707 or a 727 with standard turbojets are much louder than a 747 with 4 fan jets at full power.  They’re quieter.  And they can push a lot more people through the air at incredible speeds over great distances at a reasonable price per passenger than any other aircraft engine.

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