Sep 2012 Lesson 2.1 Theory of Flight The Four Forces

Reference From the Ground Up Chapter 2.1.1: Forces Acting on an Airplane in Flight Pages

Introduction Theory of Flight is a study into the basic physics of why and how aircraft are able to fly, maneuver and keep stable in the air.Theory of Flight is a study into the basic physics of why and how aircraft are able to fly, maneuver and keep stable in the air. Aircraft fly and maneuver in the air by controlling the many forces acting on and created by them.Aircraft fly and maneuver in the air by controlling the many forces acting on and created by them.

Outline The Four ForcesThe Four Forces EquilibriumEquilibrium CouplesCouples

The Four Forces LiftLift WeightWeight ThrustThrust DragDrag

Lift Upward force that keeps aircraft in flightUpward force that keeps aircraft in flight Created by wing (airfoil)Created by wing (airfoil) Directly opposed to weightDirectly opposed to weight Acts perpendicular to the Relative AirflowActs perpendicular to the Relative Airflow

Airfoil Definition: Any surface designed to obtain a reaction from the air through which it movesDefinition: Any surface designed to obtain a reaction from the air through which it moves Camber – Curve of upper and lower surfaces of wingCamber – Curve of upper and lower surfaces of wing

Airfoil Chord – Imaginary line running from leading edge to trailing edgeChord – Imaginary line running from leading edge to trailing edge

Airfoil Boundary Layer – Thin sheet of air that sticks to wing as it moves through air.Boundary Layer – Thin sheet of air that sticks to wing as it moves through air. –When smooth, called Laminar Layer –At Transition Point, it becomes Turbulent Layer Laminar Turbulent

Airfoil Relative AirflowRelative Airflow –Direction of air flowing relative to wing (AKA relative wind) –Created by motion of airplane through air (it’s flight path) Flight Path Relative Airflow

Airfoil Lift acts perpendicular to the relative airflowLift acts perpendicular to the relative airflow Angle of Attack – Angle between relative airflow and chordAngle of Attack – Angle between relative airflow and chord

How Lift Is Created Newton’s Laws of Motion:Newton’s Laws of Motion: 1.An object in motion tends to remain in motion (i.e. inertia) 2.An external force must be applied to alter the state of uniform motion of a body 3.For every action, there is an equal and opposite reaction

How Lift Is Created When air hits wing, it’s deflected down (downwash). The opposite reaction is an upward force.When air hits wing, it’s deflected down (downwash). The opposite reaction is an upward force. Downwash

How Lift Is Created Bernoulli’s Principle:Bernoulli’s Principle: –The total energy in any system remains constant. If one element increases, another must decrease to balance it.

How lift Is Created Airflow over top of wing flows faster, therefore pressure dropsAirflow over top of wing flows faster, therefore pressure drops Airflow on bottom of wing flows slower, therefore pressure increasesAirflow on bottom of wing flows slower, therefore pressure increases

Centre of Pressure If we consider all distributed pressures to be equivalent to a single force, this force will act through a straight line.If we consider all distributed pressures to be equivalent to a single force, this force will act through a straight line. Point where this line cuts the chord is called the Center of Pressure (C of P).Point where this line cuts the chord is called the Center of Pressure (C of P). When angle of attack increases:When angle of attack increases: –Lift and drag increase –C of P moves forward –After point of stall (AKA stalling angle), C of P moves back

Weight Downward force due to gravityDownward force due to gravity Directly opposed to liftDirectly opposed to lift Weight of aircraft acts through Centre of Gravity (C of G)Weight of aircraft acts through Centre of Gravity (C of G) C of G is point through which resultant of the weights of all various parts of the aircraft passC of G is point through which resultant of the weights of all various parts of the aircraft pass Always acts towards centre of the EarthAlways acts towards centre of the Earth

Thrust Force exerted by engine and it’s propeller or jetForce exerted by engine and it’s propeller or jet Air pushed backward, causing an equal and opposite reaction, or thrust, in forward directionAir pushed backward, causing an equal and opposite reaction, or thrust, in forward direction Opposed to DragOpposed to Drag

Drag Resistance to aircraft when moving forward through airResistance to aircraft when moving forward through air Opposed to ThrustOpposed to Thrust Types:Types: –Induced Drag –Parasite Drag Form DragForm Drag Skin FrictionSkin Friction

Parasite Drag Created by all parts of aircraft which do not contribute to lift (e.g. fuselage, wheels, antennas, etc).Created by all parts of aircraft which do not contribute to lift (e.g. fuselage, wheels, antennas, etc). Form Drag - Created by form or shape of a body as it resists motion through airForm Drag - Created by form or shape of a body as it resists motion through air Reduced by streamliningReduced by streamlining Skin Friction - Tendency of air flowing over a body to cling to surface. Made worse by dirt, mud, ice etc.Skin Friction - Tendency of air flowing over a body to cling to surface. Made worse by dirt, mud, ice etc.

Induced Drag Induced Drag - Caused by parts of aircraft which are active in producing lift (e.g. the wing).Induced Drag - Caused by parts of aircraft which are active in producing lift (e.g. the wing). Cannot be eliminated, but can be reduced.Cannot be eliminated, but can be reduced. Greater angle of attack = Greater induced dragGreater angle of attack = Greater induced drag Airflow over top of wing flows inward. Airflow under wing flows outwards. Therefore at trailing edge, small eddies are created. These move to tips, resulting in wing tip vortices, which are main component of induced drag.Airflow over top of wing flows inward. Airflow under wing flows outwards. Therefore at trailing edge, small eddies are created. These move to tips, resulting in wing tip vortices, which are main component of induced drag. Ground Effect – Vortices reduced very close to the ground, therefore induced drag is greatly decreasedGround Effect – Vortices reduced very close to the ground, therefore induced drag is greatly decreased

Aileron Drag When aircraft banks to make a turn, one aileron is down and other is upWhen aircraft banks to make a turn, one aileron is down and other is up Down going aileron = More lift = more dragDown going aileron = More lift = more drag Up going aileron = Less lift = Less dragUp going aileron = Less lift = Less drag Result is yaw in opposite direction to which bank is appliedResult is yaw in opposite direction to which bank is applied Fixed by using Differential Ailerons, where up going aileron goes up more than the other goes down.Fixed by using Differential Ailerons, where up going aileron goes up more than the other goes down.

Equilibrium When two forces are equal and opposite, object is in equilibriumWhen two forces are equal and opposite, object is in equilibrium Therefore, when thrust and drag are equal and opposite, aircraft will move forward at a constant speedTherefore, when thrust and drag are equal and opposite, aircraft will move forward at a constant speed Equilibrium refers to steady motion and not state of restEquilibrium refers to steady motion and not state of rest If either of these forces become greater than force opposing it, equilibrium will be lostIf either of these forces become greater than force opposing it, equilibrium will be lost

Equilibrium When thrust and drag are equal and opposite, they are in equilibrium.When thrust and drag are equal and opposite, they are in equilibrium. –If thrust is greater than drag, airplane will accelerate. –If drag is greater than thrust, airplane will decelerate. When lift and weight are equal and opposite, they are in equilibrium.When lift and weight are equal and opposite, they are in equilibrium. –If lift is greater than weight, airplane will climb. –If weight is greater than lift, airplane will sink.

Couples When two forces (such as lift and weight) are equal and opposite, but parallel rather than passing through the same point, they form a couple.When two forces (such as lift and weight) are equal and opposite, but parallel rather than passing through the same point, they form a couple. A couple will cause a turning moment about a given axis (couples act around the C of G).A couple will cause a turning moment about a given axis (couples act around the C of G).

Couples Weight ahead of Lift – Nose downWeight ahead of Lift – Nose down

Couples Lift ahead of Weight – Nose upLift ahead of Weight – Nose up

Couples Thrust below Drag – Nose upThrust below Drag – Nose up

Couples Drag below Thrust – Nose downDrag below Thrust – Nose down

Next Lesson Theory of Flight Wing Design From the Ground Up Chapter 2.1.2: Design of the Wing Pages