Physics

Principles of Flight Thrust Drag Lift and Weight



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Flight is the result of an elegant balance of thrust, parasitic drag, weight and lift.  Any changes in this balance will result in climb or descent.  A discussion of these key ingredients is appropriate in understanding this ability to fly envied by man since before Icarus flew too close to the sun.

There are countless books and articles discussing flying, but one of the most authoritative sources is the “Theory of Flight” by Richard von Mises.  Dr. von Mises, brother to the famous Austrian economist Ludwig von Mises, wrote his first book on flight, “Fluglehre” after fleeing Germany in 1936.  The English version, “Theory of Flight”, was updated and written in the United States during WWII.  Within the context of his book Dr. von Mises assumes the reader has a college level understanding of calculus and fluid dynamics as it was intended to be used as a text book.  However, also within the book there exists a plain language explanation of the physics of flight.

In order for flight to be achieved “lift” must be generated.  In explaining this phenomenon, von Mises makes use of Bernoulli’s equation.  Simply stated, the movement of a fluid above and below an airfoil surface causes a pressure differential; the pressure on the upper surface is lower than that on the bottom surface.  This pressure differential is referred to as “lift”.  Lift increases as the flow velocity increases until the lift component is greater than the weight of the aircraft and flight is achieved. 

The airflow required to develop lift is generated by moving the airfoil through the air.  This is accomplished by moving the vehicle, the aircraft, through the air through the use of thrust.  Jet engine thrust was but a lab experiment when Dr. von Mises was compiling his book and teaching at Harvard.  He discusses the use of a propeller as the source of thrust and, again applies Bernoulli’s equation to describe how thrust is produced.  Thrust developed by the propeller moves the aircraft through the air once it exceeds the parasitic drag to be discussed next.

Parasitic drag is the result of both the frontal area of the aircraft which must push through the air, and drag that is generated as a component of lift.  Items such as control surfaces and flaps contribute to drag as they affect aircraft attitude and direction.

Picture the profile of an aircraft in flight.  The weight of the aircraft would be represented by an arrow pointing down, lift by an arrow pointing up, an arrow pointing forward represents thrust and the arrow pointing rearward represents drag.  When in stable flight the arrows would be equal, thrust equals drag, lift equals weight.  When the pilot increases engine RPM, thrust increases as does the aircraft speed.  The resulting increase in lift will cause the aircraft to climb unless the pilot also changes the aircraft control surfaces to alter the trim and maintain altitude at an increased air speed.  Likewise, decreasing thrust would cause the aircraft to descend.

These very basic factors of flight ignore the more complex facets of controlled flight.  What made the Wright Brothers famous was not the act of attaining flight, but rather the ability to control the flight of the aircraft in turns, climbs and descents.  Air travel is three dimensional and presented a world of opportunity to discover and solve the new problems uncovered that day in December, 1903. 

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