Drag and Thrust

Up Weight and Lift Drag and Thrust Flight Control

Air flowing past an airplane's surfaces, even in laminar flow, produces drag.  Drag is generated by every surface on the airplane.  The drag depends on the properties of the surfaces and of the air.  Smooth surfaces produces less drag than rough surface.  Drag depends on the shape of the surfaces, i.e. on the airplane's form factor.  It also depends on the viscosity of the air, the velocity of the air, and the condition of the boundary layer.  Drag increases dramatically with airspeed.  Producing lift also generates induced  drag.  Swirling vortices are formed at the wing tips, which detach and carry away energy.

Thrust is generated by the engines of the aircraft.  All aircraft engines produce thrust by pushing air backwards.  The reaction force due to Newton's third law pushes the plane forwards.  The amount of thrust generated depends on the amount of air that flows through the engine and the change in the velocity of the air as it moves through the engine.

Propeller Engines

A propeller acts like a rotating wing.  Propellers usually have between 2 and 6 long and thin blades.  A perpendicular cut through a blade reveals a cross section shaped like an airfoil.  Because the blades rotate, the tip moves faster than the hub.  The blades are usually twisted, so that the angle of attack of the airfoils at the tip is lower than at the hub because the tip is moving at a higher speed than the hub.  These variations make analyzing the airflow through the propeller a very difficult task.

When a propeller rotates, it pulls in air from in front of the aircraft and pushes it back towards the tail.  The blades of a propeller acts like rotating wings.  The pressure near the top of a lifting wing is lower than the pressure below the wing.  A spinning propeller sets up a pressure gradient.  The pressure is lower in front of the propeller and higher  behind the propeller. 


Turbine Engines

Most modern passenger and military aircraft are powered by gas turbine or jet engines.  All jet engines have certain parts in common.

A diagram of a basic turbojet engine shows the inlet at the front, followed by the compressor. The compressor increases the pressure of the incoming air.  The compressor is connected by a shaft to the turbine. The compressor and the turbine are composed of many rows of small airfoil-shaped blades.  Some rows are connected to an inner shaft and rotate at high speed, while other rows remain stationary.  The rows that spin are called rotors and the fixed rows are called stators.  Between the compressor and the turbine flow path is the combustion section or burner.  This is where the fuel and the air are mixed and burned.  The hot exhaust then passes through the turbine and out the nozzle. The power turbine extracts energy from the hot flow and turns the compressor.  Work is done on the power turbine by the hot exhaust gas from the burner.  The nozzle is shaped to accelerate the hot exhaust gas to produce thrust.

Link:  Turbine Engine Simulation  
Link:  The Turbofan Engine