Basic aerodynamics

Outline of Presentation

Introduction

The Atmosphere

Newton’s Laws of Motion

Bernoulli’s Principle

Airfoil

Parts of an Airplane

The Four Forces of Flight

Three Axes of Movement

Stability

Control

Aerodynamics

Aerodynamics is the study of objects in motion through

the air and the forces that produce or change such motion.

INTRODUCTION

It is unnecessary that a mechanic be totally versed on

Aerodynamics and Theory of Flight. However he must

understand the relationships between the atmosphere, the

aircraft and the forces acting on it in flight, in order to make

intelligent decisions affecting the flight safety of both airplanes

and helicopters.

The Atmosphere

Air is a mixture of gases composed principally of nitrogen

and oxygen. An aircraft operates in the air, therefore, the

properties of air that affect aircraft control and performance must

be understood.

Pressure – Atmospheric pressure varies with altitude. The

higher an object rises above sea level, the lower the pressure.

Density – It varies directly with the pressure and inversely with

the temperature. With the same horse power, an aircraft can fly

faster at high altitude because of less resistance of air at there.

Humidity – Humidity is the amount of water vapor in the air. It

varies directly with temperature.

Newton's First Law of Motion

According to Newton's first law of motion (inertia), an object at

rest will remain at rest, or an object in motion will continue in

motion at the same speed and in the same direction, until an

outside force acts on it. For an aircraft to taxi or fly, a force

must be applied to it. It would remain at rest without an

outside force. Once the aircraft is moving, another force must

act on it to bring it to a stop. It would continue in motion

without an outside force. This willingness of an object to

remain at rest or to continue in motion is referred to as inertia.

Newton's Second Law of Motion

The second law of motion (force) states that if a object moving

with uniform speed is acted upon by an external force, the

change of motion (acceleration) will be directly proportional to

the amount of force and inversely proportional to the mass of

the object being moved. The motion will take place in the

direction in which the force acts. Simply stated, this means that

an object being pushed by 10 pounds of force will travel faster

than it would if it were pushed by 5 pounds of force. A heavier

object will accelerate more slowly than a lighter object when an

equal force is applied. F = m × a

Newton's Third Law of Motion

The third law of motion (action and reaction) states that for

every action (force) there is an equal and opposite reaction

(force). This law can be demonstrated with a balloon. If you

inflate a balloon with air and release it without securing the

neck, as the air is expelled the balloon moves in the opposite

direction of the air rushing out of it. Figure shows this law of

motion.

ActionReaction

Balloon

Air

BERNOULLI'S PRINCIPLE

Bernoulli's principle states that when a fluid flowing through a

tube reaches a constriction or narrowing of the tube, the

speed of the fluid passing through the constriction is increased

and its pressure is decreased.

Pressure Drop in Venturi Tube

AirfoilAn airfoil is the shape of a wing or blade (of a propeller, rotor or turbine) as seen in cross-section. An aircraft's wings, horizontal, and vertical stabilizers are built with airfoil-shaped cross sections, as are helicopter rotor blades.

- The mean camber line is a line drawn midway between the upper and lower surfaces. - The chord line is a straight line connecting the leading and trailing edges of the airfoil, at the ends of the mean camber line.

Mean camber line

Chord line

kinetic energy(velocity)

potential energy(pressure)

velocity increases

pressure decreases

Airfoil as a Venturi Tube

Lift force appear

Cockpit

Fuselage

Wing

Flap

Aileron

Empennage

Stabilizers

Rudder

Elevator

Engine

Parts of an Airplane

Parts of An Airplane

The forces acting on an airplane in flight are lift, weight, thrust,

and drag. These forces are in equilibrium during straight-and-

level, unaccelerated flight.

The Four Forces of Flight

DRAG

WEIGHT

THRUST

LIFT

Lift is an aerodynamic force

Lift must exceed weight for flight

Generated by motion of aircraft through air

Created by the effects of airflow past wing

Aircraft lift acts through a single point called the center

of pressure.

LiftLift is the force created by the interaction between the wings

and the airflow. It always act upwards. It is considered to be

the 'most important force' as without it, an aircraft cannot

ascend from ground and maintain altitude.

Newton’s Third Law and Lift

Newton’s Second Law and Lift

Lift: Wing Section

Lift Equation: L=CLift Equation: L=CL L ×× ½ ½ ρρ × A × V × A × V22

• The angle of attack is the angle between the chord line

and the average relative wind.

• Greater angle of attack creates more lift (up to a point).

Angle of Attack

High velocity

Low pressure

Low velocity

High pressure

Angle of Attack and Lift Force

• The angle of incidence is the angle between the chord line

and the longitudinal axis of aircraft.

• It is the angle of wing setting.

• When the leading edge of the wing is higher than the

trailing edge, the angle of incidence is said to be positive.

It is negative when the leading edge is lower than the

trailing edge of the wing.

Angle of Incidence

Chord line

Aircraft longitudinal axis

Angle of incidence

Horizontal Component of Lift

• Lift acts through the center of pressure, and perpendicular to the relative wind.

• This creates induced drag.

chord line

average relative wind

total lift

effective lift

induced drag

Lift and Induced Drag

Shape of the Airfoil

• The shape of the airfoil determines the

amount of turbulences or skin friction

that it will produce. The shape of a wing

consequently affects the efficiency of

the wing. A wing may have various

airfoil section from root to tip, with

taper, twist, sweep back and sweep

forward.

Wing Shapes

Weight is not constant

Varies with passengers, cargo, fuel load

Decreases as fuel is consumed or payload off-loaded

Direction is constant toward earth’s center

Acts through a single point called the center of gravity

(the CG)

WeightThis force acts on an aircraft due to the interaction between

the aircraft's body weight and Earth's gravity. Weight is a

downward force.

Forward-acting force opposes drag

Direction of thrust depends on design

Propulsion systems produce thrust

Equal to drag in straight, constant speed flight

ThrustThis force is created by an aircraft's engine and is required for

forward motion.

An aerodynamic force.

Resists forward motion.

Increases with the square of speed.

Two broad drag classifications.

– Parasite drag: drag created by airplane shape.

A result of air viscosity.

– Induced drag: by-product of lift generation.

Caused by the wingtip vortices.

DragThis force acts in reverse direction to that of 'Thrust' and hinders forward motion. Drag is considered as a negative force and all engineers try their best to reduce drag.

Drag Equation: D=CDrag Equation: D=CD D ×× ½ ½ ρρ × A × V × A × V22

Example of Drag Formation

Skin Friction Drag

Three Axes of Movement

Axis of Roll (Longitudinal Axis)

Axis of Pitch (Lateral Axis)

Axis of Yaw (Vertical Axis)

Pitch Around the Lateral Axis

Roll Around Longitudinal Axis

Yaw Around the vertical Axis

There are two types of stability Static Stability - The initial movement of an object after being

disturbed.– Positive Static Stability – returns to position before

displacement.– Neutral Static Stability – tendency to remain in displaced

position.– Negative Static Stability – tends to continue away from

displaced position in same direction. Dynamic Stability - The behavior of the object over time.

– Positive Dynamic Stability – the oscillations or phugoids dampen themselves out.

– Neutral Dynamic Stability – the oscillations or phugoids carry on with out increasing in severity.

– Negative Dynamic Stability – the oscillations or phugoids increase in severity and diverge.

StabilityAn aircraft must have sufficient stability to maintain a uniform flight path and recover from the various upsetting forces also to achieve the best performance.

Static Stability

Positive-Neutral-Negative

Dynamic Stability

Positive Dynamic Stability

Natural Dynamic Stability

Negative Dynamic Stability

Larger wing area

More lift

Smaller wing area

Less lift

Stability recover by a dihedral wing

Stability recover by a sweep back wing

Stability recover by keel effect

CONTROLTo achieve the best performance, the aircraft must have the

proper response to the movement of the controls. Control is the

action taken to make the aircraft follow any desired flight path.

Different Control surfaces are used to control the aircraft about

each of the three axes.

Flight Control Surfaces – Hinged or moveable airfoils

designed to change the attitude of the aircraft during flight.

1. Primary group

- ailerons

- elevators

- rudder

2. Secondary group

- trim tab, spring tab

- servo tab, balance tab

3. Auxiliary group

- wing flaps

- spoilers

- speed brakes

- slats

- leading edge flaps

- slots

Flight Control Surfaces

Flap

Flap

Spoiler

Spoiler

wing flaps spoilers leading edge slats

leading edge slots speed brakes

Ailerons – The ailerons form a part of the wing and are located in

the trailing edge of the wing towards the tips. The control stick is

connected by means of wires or hydraulics to the wings’ ailerons. By

turning the stick, the pilot can change the positions of the ailerons.

ROLLING

Control around the Longitudinal Axis

Rudder – The rudder is a

moveable control surface

attached to the trailing edge of the

vertical stabilizer. The foot pedals

are connected by means of wires

or hydraulics to the rudder of the

tail section. The rudder can also

be used in controlling a bank or

turn in flight.

YAWING

Control around the Vertical Axis

Moving rudder to the right forces tail to the left, nose to the right

Moving rudder to the left forces tail to the right, nose to the left.

Elevators – Elevators are the

movable control surfaces hinged to

the trailing edge of the horizontal

stabilizer. The control stick is

connected by means of wires or

hydraulics to the tail section’s

elevators.

- Stabilator

- Ruddervator

PITCHING

Control around the Lateral Axis