Thrust, Drag, Lift, Weight, Concorde, Center of Pressure, Center of Gravity, Boeing 747, Slats and Flaps

Posted by PITHOCRATES - January 16th, 2013

Technology 101

The Drawback to increasing Thrust and Lift with more Powerful Engines is the Weight of Greater Fuel Loads

To get an airplane off of the ground requires two things.  To produce thrust that is greater than drag.  And to produce lift that is greater than weight.  You do this and you’ll get any airplane off of the ground.  Of course, getting these two things is not the easiest thing to do.  Primarily because of the purpose of airplanes.  To move people and freight.  People and freight add weight.  Which increases the amount of lift needed.  And they make the plane bigger.  A bigger object displaces more air increasing drag.  And thus requiring more thrust.

Engines provide thrust.  And wings provide lift.  So the obvious solution to overcome greater drag is to produce greater thrust.  And the solution to overcome greater weight is to produce greater lift.  And we do both with fuel.  Greater amounts of fuel can power bigger engines that can produce more thrust.  And larger wings can produce greater lift.  But larger wings also produce more drag.  Requiring additional thrust.  And fuel.  Or, we can produce greater lift by moving air over the wings faster.  Also requiring additional thrust.  And fuel.

Of course, the obvious drawback to increasing both thrust and lift is the added weight of the fuel.  The more fuel carried the more weight lift has to overcome.  Requiring more powerful engines.  Or bigger wings.  Both of which require more fuel.  This is why our first planes were small by today’s standards.  The thrust of a propeller engine could not produce enough thrust to travel at high speeds.  Or operate at high altitudes.  And the first wings were relatively fixed.  Having the same surface area to produce lift at takeoffs and landings.  As well as at cruising altitudes.  Big wings that allowed the lifting of heavier weights produced a lot of drag.  Requiring more fuel to overcome that drag.  And the added weight of that fuel limited the number of people and freight they could carry.  Or they could trade off that fuel for more revenue weight.  The smaller fuel load, of course, reduced flying times.  Requiring an additional takeoff and landing or two to refuel.

A Wing that produces sufficient Lift at 600 MPH does not produce sufficient Lift at Takeoff and Landing Speeds

The supersonic Concorde was basically a flying gas can.  It was more missile than plane.  To travel at those great speeds required a very small cross section to reduce drag.  Limiting the Concorde to about 100 revenue paying passengers.  Its delta wing performed well at supersonic flight but required a drooping nose so the pilot could see over it to land and takeoff due to the extreme nose pitched up attitude.  As Concorde approached supersonic speeds the center of pressure moved aft.  Placing the center of gravity forward of the center of pressure.  Causing the nose to pitch down.  You correct this with trim controls on slower flying aircraft.  But using this on Concorde would create additional drag.  So they trimmed Concorde by pumping the remaining fuel to other fuel tanks to move the center of gravity to the center of pressure.

They designed Concorde to fly fast.  Which came at a cost.  They can only carry 100 revenue paying passengers.  So they can only divide the fuel cost between those 100 passengers.  Whereas a Boeing 747 could seat anywhere around 500 passengers.  Which meant you could charge less per passenger ticket while still earning more revenue than on Concorde.  Which is why the Boeing 747 ruled the skies for decades.  While Concorde flies no more.  And the only serious competition for the Boeing 747 is the Airbus A380.  Which can carry even more revenue paying passengers.  How do they do this?  To fly greater amount of people and freight than both piston-engine and supersonic aircraft?  While being more profitable than both?  By making compromises between thrust and drag.  And lift and weight.

Jet engines can produce more thrust than piston engines.  And can operate at higher altitudes.  Allowing aircraft to take advantage of thinner air to produce less drag.  Achieving speeds approaching 600 mph.  Not Concorde speeds.  But faster than every other mode of travel.  To travel at those speeds, though, requires a cleaner wing.  Something closer to Concorde than, say, a DC-3.  Something thinner and flatter than earlier wings.  But a wing that produces lift at 600 mph does not produce enough lift at takeoff and landing speeds.

Planes need more Runway on Hot and Humid Days than they do on Cool and Dry Days

The other big development in air travel (the first being the jet engine) are wings that can change shape.  Wings you can configure to have more surface area and a greater curve for low-speed flying (greater lift but greater drag).  And configure to have less surface area and a lesser curve for high-speed flying (less lift but less drag).  We do this with leading-edge slats (wing extensions at the leading edge of the wing).  And trailing-edge flaps (wing extensions at the trailing edge of the wing).  When fully extended they increase the surface area of the wing.  And add curvature at the leading and trailing edge of the wing.  Creating the maximum amount of lift.  As well as the greatest amount of drag.  Allowing a wing to produce sufficient lift at takeoff speeds (about 200 mph).  Once airborne the plane continues to increase its speed.  As it does they retract the slats and flaps.  As the wing can produce sufficient lift at higher speeds without the slats and flaps extended.

But there are limits to what powerful jet engines and slats/flaps can do.  A wing produces lift by having a high pressure under the wing pushing up.  And a low pressure on top of the wing pulling it up.  The amount of air passing over/under the wing determines the amount of lift.  As does the density of that air.  The more dense the air the more lift.  The thinner the air the less lift.  Which is why planes need less runway on a cold winter’s day than on a hot and humid summer’s day.  If you watch a weather report you’ll notice that clear days are associated with a high pressure.  And storms are associated with a low pressure.  When a storm approaches meteorologists will note the barometer is falling.  Meaning the air is getting thinner.  When the air is thinner there are fewer air molecules to pass over the wing surface.  Which is why planes need more runway on hot and humid days.  To travel faster to produce the same amount of lift they can get at slower speeds on days cooler and dryer.

For the same reason planes taking off at higher elevations need more runway than they do at lower elevations.  Either that or they will have to reduce takeoff weight.  They don’t throw people or their baggage off of the airplane.  They just reduce the fuel load.  Of course, by reducing the fuel load a plane will not be able to reach its destination without landing and refueling.  Increasing costs (airport and fuel expenses for an additional takeoff and landing).  And increasing flying time.  Which hurts the economics of flying a plane like a Boeing 747.  A plane that can transport a lot of people over great distances at a low per-person cost.  Adding an additional takeoff and landing for refueling adds a lot of cost.  Reducing the profitability of that flight.  Not as bad as a normal Concorde flight.  But not as good as a normal Boeing 747 flight.

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