Jet Characteristics

Sir George Cayley

"Father of Aerodynamics"

• first to scientifically define 4 flight forces

• proposed fixed-wing machines with separate systems for lift, propulsion, and control.

• 1st successful manned glider model which featured cambered wings, establishing basic blueprint for all future A/C

The Wright Brothers (1903)

developed a lightweight 12-horsepower engine and propellers acting as rotating airfoils.

Golden Age of Aviation (1918-1939)

Period of rapid transformation from slow, wooden biplanes to faster, all-metal Monoplanes.

World War II Aviation Impact

introduction of Radar, paving way for Jet age.

The Jet Age Manufacturers Focus

Aerodynamics

Propulsion

Flight Controls.

de Havilland Comet (1952)

World’s 1st commercial jet airliner

de Havilland Comet Crew

Required a crew of 4:

2 pilots

1 flight engineer

1 navigator.

Modern Jetliner Performance

A/C routinely cruise at

• over 500 mph (Mach 0.85)

• altitudes above 40,000’

Aerodynamics

Branch of Fluid Mechanics studying

• Air in Motion

• Forces Acting on Bodies moving Through it

(Main science that Aviation is Based on)

Pressure (Atmospheric or Barometric)

Force per Unit Area Exerted on a body, Above Atmospheric Column (F/A)

• Decreases as Altitude Increases

Pressure Altitude

Altitude in ISA where; pressure = observed pressure; indicated when

altimeter is set to 1013.25 hPa (29.92 inHg)

Density

Mass per unit Volume (M/V)

when Density↑, Performance↑

Typical Density Values by Altitude:

@ 20,000’ →D=50% of Surface value.

@ 40,000’ →D=25% of Surface value.

@ 60,000’ →D=10% of Surface value.

Density Altitude (densityعكس ال)

Pressure Alt. Corrected for Non-Standard Temp. ;

Critical performance parameter for

• Engine Output

• Aerodynamic Efficiency

High Density Altitude

Hot days or high elevation airports; results in

• ⬇Density

• ⬇Performance.

Low Density Altitude

Cold days or sea level conditions; results in

• ⬆Density

• ⬆Performance.

Temperature and Pressure Relationship

Inversely related at a given altitude;

as temperature increases, density and pressure decrease.

Law of Conservation of Mass

Matter (Mass) cannot be Created nor Destroyed;

in Subsonic Steady flow; Entering = Exiting

Continuity Equation

Product of AXV (areaXvelocity) is Constant

‘Smaller area makes air Speed up’

‘Larger area makes air SLow down’

because Same Mass must Pass Through

A1 × V1 = A2 × V2

Leading to Bernoulli’s Equation.

Bernoulli’s Principle

As Velocity⬆ Increase Press. ⬇ Decreases

Airfoil shape (curved upper surface) causes air over wing to accelerate,

creating lower pressure above wing.

Differential Pressure Creating Lift.

Above (⬆ Velocity, ⬇Pressure)

Below (⬇ Velocity,⬆Pressure) the Wing.

Air naturally flows from high to low pressure.

• Higher pressure below the wing + lower pressure above creates lift.

• The total aerodynamic force from pressure distribution acts through the Center of Pressure (CP). ✈

Newton’s First Law (Law of Inertia)

“Inertia”

-Body in Rest Remains in Rest

-Body in Motion Remains in Motion @

• Constant Speed

• Same Direction

Unless acted upon by an External Force,

such as Engine Thrust Overcoming Inertia

Newton’s Second Law (Law of Acceleration)

F = M x A

Acceleration is:∝ to Net Force & 1/∝ to Mass

Heavier A/C accelerate Slowler @ same thrust as lighter one.

Newton’s Third Law (Action-Reaction)

Actions have Reactions,

Equal in Force & Opposite in Direction

Jet engine Pushes air Backward (Action) to

Propel A/C Forward (Reaction)

Velocity

rate of change of position with time in one direction

Lift

Upward Force created by Differential Pressure (Above & Below Wing)

L = ½ × ρ × V² × S × CL

Directly Proportional to;

• Air Density (ρ)

• Velocity Squared (V²)

• Wing Surface Area (S)

• Coefficient of Lift (CL).

Center of Pressure (CP)

Point On MAC where Aerodynamic Force Acts

resulting from Pressure Distribution

Coefficient of Lift (CL)

Dimensionless Number representing;

• Lift Generation Efficiency

depending on: Shape & Inclination of airfoil.

Relative Wind

Air moving in;

• Opposite Direction

• Parallel to Airfoil.

Angle of Attack (AOA or α)

Angle° between:

Chord line & Relative Wind Opposing it

Maximum Lift Coefficient (CLmax)

where Highest Efficiency is Achieved.

As AoA ⇡, CL ​⇡ (Linearly)

Until reaching Critical Angle of Attack. before Stall occurs.

Stall

Rapid Decrease in Lift caused by

Separation of Airflow from wings surface,

Leading to a Rapid decrease in CL.

Basic Elements of the Wing

-Leading Edge; meets the Oncoming air,

-Trailing Edge; where airflow Reunites.

-Chord Line; Straight Line between them,

-Mean Camber Line: Halfway between Upper & Lower surfaces.

-Camber; Max. Dis. between them, Measures Curvature of Wing

Swept-Wing

Effective velocity component;

• Perpendicular to Leading edge (chordwise flow)

• Less (<) True Airspeed (TAS).

Indicated Airspeed (IAS)

speed Observed on airspeed Indicator, Reference for ;

• Aerodynamic Forces (Crew Usage)

Calibrated Airspeed (CAS)

IAS corrected for Positioning errors→closer to true dynamic pressure.

Used for;

• Performance Calc.

• Determining regulatory speed limits (V-speeds)

Position Error

Error in Airspeed/Mach indications caused by:

• location of the pitot-static ports on the aircraft fuselage.

Causes:

1. Static ports measure pressure on the aircraft skin, not the true free-stream atmospheric pressure.

2. Airflow around the fuselage distorts pressure.

Therefore: Measured static pressure ≠ True ambient static pressure.

Main Factors Affecting It

• Aircraft attitude (Angle of Attack)

• Airflow direction

• Aircraft speed

Effect on Indications:

• Causes difference between Indicated & Actual values, especially:

  • Airspeed

  • Mach number

Mach Number Effects

• Indicated Mach ≈ True Mach at normal speeds.

• Near Maximum Demonstrated Mach (Mmo) → position error increases due to pressure changes around pitot/static ports.

• Some modern jet Mach indicators intentionally over-read at high Mach to compensate.

Correction

• Correction charts in Flight Manual if the error is significant.

• Modern aircraft: Air Data Computer (ADC) automatically applies the correction.

Equivalent Airspeed (EAS)

CAS corrected for Compressibility error, represents

• Actual Dynamic Pressure (1/2pXv²) for Structural Analysis.

True Airspeed (TAS)

EAS corrected for Density variation;

• Actual speed relative to Undisturbed Air.

TAS Rule of Thumb

TAS ⇡ compared to IAS

as Altitude ⇡

Ground Speed (GS)

TAS corrected for Wind variation

• Actual speed over Ground.

Used in: Navigation

Drag

Any force Opposing airfoil in its Forward Motion.

acts Parallel to Relative Wind.

2 main types:

-Induced

-Parasite

Induced Drag

By-product of Lift

Due to (Downwash): Downward Deflection of air

Creating: Wing Tip Vortices (Wake Turbulence).

Highest @

• Low ⇣Speeds

• High ⇡AoA°

Parasite Drag

Drag Not Associated with Lift

• (Present @ Zero Lift).

• ⬆with Square of Airspeed (V²),

• Dominant @ High Speeds.

This includes

-Form Drag (shape),

-Skin Friction (surface roughness),

-Interference Drag (interaction between adjacent parts)

Form Drag

Parasite drag: Resulted from Turbulent wake,

Caused by Shape (form) of A/C.

Skin Friction Drag

Parasite drag; due to A/C’s Surfaces Roughness (e.g., rivet heads, paint)

as Air Molecules Pass Over Surface.

Interference Drag

Parasite drag Generated by Interaction of airflow

between 2 Adjacent parts (e.g., wing & fuselage).

Total Drag

• Total Drag = Induced Drag + Parasite Drag.

• The Total Drag curve has a minimum point.

• This minimum occurs at L/Dmax (Maximum Lift-to-Drag Ratio) speed.

L/Dmax (best Lift to Drag ratio)

Total Drag Curve Minimum; Point where Total Drag is at its Minimum.

Providing Best:

-Glide Range

-Cruise Performance Efficiency.

Thrust

Force generated by engines that;

• Opposes drag

• acts Parallel to Thrust-Line

Steady Unaccelerated Flight

Condition where is an Equilibrium in

Lift, Weight, Thrust, Drag

High-Speed Wing Design

Minimize ⇣ Drag & Delay Compressibility, Require Wings to be;

• Thin (little camber)

• Swept-back

Low-Speed Wing Design

Generate Sufficient ⇡Lift @ low ⇣Speeds for T/O & landing, Requires:

• Thicker wings (more camber)

• high-lift Devices (flaps/slats)

Wing Planform

• Rectangular

• Elliptical

• Tapered

• Delta

Rectangular Wing

Simple to Manufacture→Good Stall Characteristics (Root stall 1st),

but Aerodynamically Inefficient→High Induced ⇡ Drag

Common @ Light Low speed A/C

Elliptical Wing

Minimizes ⇣ Induced Drag,

but Complex & Expensive to Manufacture (e.g., Spitfire).

Tapered Wing

Practical Compromise Reducing ⇣ Weight & Drag;

Common for Jet transports.

Delta Wing

Low ⇣Aspect Ratio;

Excellent for→High-speed/supersonic flight

but has Poor→Low-speed Performance.

Factors Affecting Aerodynamic Forces

of the Wing

(3eS,@,MAD)

Divided into 3 parts,

1st; Wing's

1. Surface Area (S)

2. Span

3. Sweep Back Angle

Then

1. Aspect ratio

2. Taper ratio

3. Thickness to chord ratio

Lastly,

1. Mean aerodynamic chord (MAC)

2. Angle of incidence

3. Diherdral angle

Wing Area (S)

∝ to Lift

if increased can

1. Better ⇡Stability @ Lower ⇣Speeds

Wingspan (b)

Tip-to-Tip Dimension of A/C wing,

⇡Wingspan= ⇣induced Drag (by weakening wingtip vortices)

Wing Sweep (Λ - Lambda)

Angle° between a line Perpendicular to A/C’s Centerline

and Wing’s Quarter-chord;

• Delays Compressibility Effects.

• Allows Higher TAS before airflow over wing reaches Sonic-speeds

Mean Aerodynamic Chord (MAC)

imaginary rectangular wing Having actual Wing’s Same;

• Aerodynamic Characteristics

• Pitching Moment

1. CP moves along MAC

2. CG expressed as % of MAC length.

3. MAC is important for weight & balance Calculations and Longitudinal Stability. ✈

Aspect Ratio (AR)

Ratio Between Wing’s Span / Average Chord
AR = b² / S

High Aspect Ratio Characteristics

Long & Narrow; produces

• Less induced Drag;

• Efficient for Slow flight & Endurance.

Low Aspect Ratio Characteristics

Short and Stubby;

• generates More induced drag.

• Stronger & Suited for High speeds

Taper Ratio

Ratio of Tip chord to Root chord;

• helps Approximate Elliptical Lift-Distribution.

• Reduce induced ⇣ Drag & Weight

Thickness-to-Chord Ratio

Ratio of maximum thickness to chord length;

Thin wings are essential for high-speed flight.

Dihedral (Γ - Gamma)

Upward° angle of wings from horizon;

Contributes to Lateral Stability.

If banked, Lower wing presents Higher AOA, generating more Lift to Restore Level Flight

Angle of Incidence

Angle between Wing’s Chord Line &

a line Parallel to plane's Longitudinal axis.

Slightly ⬆Increasing AoA during Cruise

Compressibility

Change in the Volume of matter

when External Force is Applied

Speed of Sound

Distance traveled per Unit time as sound wave Propagates

• Directly proportional ∝ to Temp.

• Depends on Medium (Compressibility & Density)

Speed of sound Equation

= 39 ×√273 +SAT °C

Shock Wave 🍩

Boundry Layer between

Compressed & Undisturbed air

Formed when air is compressed as object moves at or beyond SoS

Sudden changes in temp, pressure, and density occur.

1-⬆increase in TPD

"Temp., Press, Denisty"

1- Sudden ⬇decrease in ‘Velocity'

Wave Drag

Due to Shockwave Formation, Only in Transonic & Supersonic

Mach Number (M)

Ratio between True Air Speed (TAS) / Actual Speed of Sound.

Mach Indicator Error at High Speeds

Position error increases;

Fixed by; over-reading Mach indicators

aircraft speed Regimes

1. Subsonic:Below 0.75

2. Transonic: From 0.75 To 1.20

3. Supersonic: From 1.20 To 5.00

4. Hypersonic: Above 5.00

Subsonic Speed Range

Mach numbers below 0.75.

Transonic Speed Range

Mach numbers from 0.75 to 1.20;

greatest effects of compressibility occur here.

Supersonic Speed Range

Mach numbers from 1.20 to 5.00.

Beyond Mach 1.2, the decrease in drag rise enables the aircraft to fly normally

Hypersonic Speed Range

Mach numbers above 5.00.

Shock Wave Formation Location

Form when speed is reduced from sonic to subsonic (airflow decelerating over less curved part of wing).

High-Speed Buffet

Caused by wake turbulence found after shock wave.

Shock Wave Development Flow

As speed increases, shock waves move aft toward the trailing edge.

Bow Wave

New shock-wave that forms in Front of Leading edge

as air piles up at supersonic speeds (>1 Mach)

Sound Barrier

Sudden and massive ⇡⇡⇡Drag

during shock wave Formation

Requiring ⇡⇡⇡Thrust to Overcome

Critical Mach Number

Speed @ which Coefficient of DRAG

⬆\by 0.002%

when Airflow over Any Part of wing Reaches 1 MACH

Stalling Speed

a function of weight & CG, Occuring at fairly Constant EAS, which is;

Min. speed required to keep air attached to A/C’s wings

• At very high altitudes, stalling speeds tend to slightly increase.

VMO / MMO

Indicated airspeed to be Intentionally flown,

in knots or in Mach number, Ensuring safe Operation of A/C

• MMO is always below Critical Mach number

VDF / MDF

Highest airspeed

Planned to be Flown

in Knots and Mach number

• used during Certification Testing

• Handling qualities are reduced here.

Factors Increasing Critical Mach

1- Swept Back wing

2- Vortex Generator

3-Less thickness to chord ratio

Swept Wing Critical Mach Benefit

Chordwise airspeed is lower than TAS,

allowing higher speeds before sonic velocity is reached.

Thin Wing Critical Mach Benefit

Produces limited acceleration,

preventing airflow from reaching sonic speed at a given TAS.

Vortex Generator

Small, low-aspect-ratio vertical airfoil that;

• Injects energy into boundary-layer to Delay Separation.

• Designed to Penetrate fully through boundary-layer.

⬆Critical Mach, but produce ⬆Drag, making overall Benefit Questionable.

Mach Tuck (Tuck Under)

Nose-down Pitching Moment Due to Shock wave Formation.

caused by AFT movement of CP

Dutch Roll

Coupled yaw-rolling Oscillation

caused by Strong rolling moment of swept wings by;

• sudden Gust

• Uncoordinated Rudder input,

due to Reduced Inherent Damping

Dutch Roll Recovery

Vertical Fin & Rudder work

with Inertia to Restore Stability

through Damped Oscillations

“Yaw Damper”

Jet Propulsion

Reactive force resulting from Acceleration of a Mass of air.

2 types;

• Reciprocating Engines (Piston)

• Jet Turbine Engines

Advanced Piston Engine Limit

Maximum of Approx.3,400hp thrust @ sea level.

20 engines needed to provide necessary thrust for a jumbo jet at its maximum T/O weight.

Jet Engine Advantages

• High Power Output per engine

• better Power-to-Weight Ratio (High power, Low weight)

• Small Frontal Area (⬇Drag)

• Low Fuel Consumption at high altitudes

• High overall Efficiency & Reliability

Reciprocating & Jet Similarities

• produce power through Gaseous Expansion (burning fuel)

• Utilize Ambient Oxygen to support Combustion

• Generate thrust by applying Basic Laws of Motion

• Operate on a 4-stage cycle:

1. Intake

2. Compression

3. Combustion

4. Exhaust