- •Textbook Series
- •Contents
- •1 Overview and Definitions
- •Overview
- •General Definitions
- •Glossary
- •List of Symbols
- •Greek Symbols
- •Others
- •Self-assessment Questions
- •Answers
- •2 The Atmosphere
- •Introduction
- •The Physical Properties of Air
- •Static Pressure
- •Temperature
- •Air Density
- •International Standard Atmosphere (ISA)
- •Dynamic Pressure
- •Key Facts
- •Measuring Dynamic Pressure
- •Relationships between Airspeeds
- •Airspeed
- •Errors and Corrections
- •V Speeds
- •Summary
- •Questions
- •Answers
- •3 Basic Aerodynamic Theory
- •The Principle of Continuity
- •Bernoulli’s Theorem
- •Streamlines and the Streamtube
- •Summary
- •Questions
- •Answers
- •4 Subsonic Airflow
- •Aerofoil Terminology
- •Basics about Airflow
- •Two Dimensional Airflow
- •Summary
- •Questions
- •Answers
- •5 Lift
- •Aerodynamic Force Coefficient
- •The Basic Lift Equation
- •Review:
- •The Lift Curve
- •Interpretation of the Lift Curve
- •Density Altitude
- •Aerofoil Section Lift Characteristics
- •Introduction to Drag Characteristics
- •Lift/Drag Ratio
- •Effect of Aircraft Weight on Minimum Flight Speed
- •Condition of the Surface
- •Flight at High Lift Conditions
- •Three Dimensional Airflow
- •Wing Terminology
- •Wing Tip Vortices
- •Wake Turbulence: (Ref: AIC P 072/2010)
- •Ground Effect
- •Conclusion
- •Summary
- •Answers from page 77
- •Answers from page 78
- •Questions
- •Answers
- •6 Drag
- •Introduction
- •Parasite Drag
- •Induced Drag
- •Methods of Reducing Induced Drag
- •Effect of Lift on Parasite Drag
- •Aeroplane Total Drag
- •The Effect of Aircraft Gross Weight on Total Drag
- •The Effect of Altitude on Total Drag
- •The Effect of Configuration on Total Drag
- •Speed Stability
- •Power Required (Introduction)
- •Summary
- •Questions
- •Annex C
- •Answers
- •7 Stalling
- •Introduction
- •Cause of the Stall
- •The Lift Curve
- •Stall Recovery
- •Aircraft Behaviour Close to the Stall
- •Use of Flight Controls Close to the Stall
- •Stall Recognition
- •Stall Speed
- •Stall Warning
- •Artificial Stall Warning Devices
- •Basic Stall Requirements (EASA and FAR)
- •Wing Design Characteristics
- •The Effect of Aerofoil Section
- •The Effect of Wing Planform
- •Key Facts 1
- •Super Stall (Deep Stall)
- •Factors that Affect Stall Speed
- •1g Stall Speed
- •Effect of Weight Change on Stall Speed
- •Composition and Resolution of Forces
- •Using Trigonometry to Resolve Forces
- •Lift Increase in a Level Turn
- •Effect of Load Factor on Stall Speed
- •Effect of High Lift Devices on Stall Speed
- •Effect of CG Position on Stall Speed
- •Effect of Landing Gear on the Stall Speed
- •Effect of Engine Power on Stall Speed
- •Effect of Mach Number (Compressibility) on Stall Speed
- •Effect of Wing Contamination on Stall Speed
- •Warning to the Pilot of Icing-induced Stalls
- •Stabilizer Stall Due to Ice
- •Effect of Heavy Rain on Stall Speed
- •Stall and Recovery Characteristics of Canards
- •Spinning
- •Primary Causes of a Spin
- •Phases of a Spin
- •The Effect of Mass and Balance on Spins
- •Spin Recovery
- •Special Phenomena of Stall
- •High Speed Buffet (Shock Stall)
- •Answers to Questions on Page 173
- •Key Facts 2
- •Questions
- •Key Facts 1 (Completed)
- •Key Facts 2 (Completed)
- •Answers
- •8 High Lift Devices
- •Purpose of High Lift Devices
- •Take-off and Landing Speeds
- •Augmentation
- •Flaps
- •Trailing Edge Flaps
- •Plain Flap
- •Split Flap
- •Slotted and Multiple Slotted Flaps
- •The Fowler Flap
- •Comparison of Trailing Edge Flaps
- •and Stalling Angle
- •Drag
- •Lift / Drag Ratio
- •Pitching Moment
- •Centre of Pressure Movement
- •Change of Downwash
- •Overall Pitch Change
- •Aircraft Attitude with Flaps Lowered
- •Leading Edge High Lift Devices
- •Leading Edge Flaps
- •Effect of Leading Edge Flaps on Lift
- •Leading Edge Slots
- •Leading Edge Slat
- •Automatic Slots
- •Disadvantages of the Slot
- •Drag and Pitching Moment of Leading Edge Devices
- •Trailing Edge Plus Leading Edge Devices
- •Sequence of Operation
- •Asymmetry of High Lift Devices
- •Flap Load Relief System
- •Choice of Flap Setting for Take-off, Climb and Landing
- •Management of High Lift Devices
- •Flap Extension Prior to Landing
- •Questions
- •Annexes
- •Answers
- •9 Airframe Contamination
- •Introduction
- •Types of Contamination
- •Effect of Frost and Ice on the Aircraft
- •Effect on Instruments
- •Effect on Controls
- •Water Contamination
- •Airframe Aging
- •Questions
- •Answers
- •10 Stability and Control
- •Introduction
- •Static Stability
- •Aeroplane Reference Axes
- •Static Longitudinal Stability
- •Neutral Point
- •Static Margin
- •Trim and Controllability
- •Key Facts 1
- •Graphic Presentation of Static Longitudinal Stability
- •Contribution of the Component Surfaces
- •Power-off Stability
- •Effect of CG Position
- •Power Effects
- •High Lift Devices
- •Control Force Stability
- •Manoeuvre Stability
- •Stick Force Per ‘g’
- •Tailoring Control Forces
- •Longitudinal Control
- •Manoeuvring Control Requirement
- •Take-off Control Requirement
- •Landing Control Requirement
- •Dynamic Stability
- •Longitudinal Dynamic Stability
- •Long Period Oscillation (Phugoid)
- •Short Period Oscillation
- •Directional Stability and Control
- •Sideslip Angle
- •Static Directional Stability
- •Contribution of the Aeroplane Components.
- •Lateral Stability and Control
- •Static Lateral Stability
- •Contribution of the Aeroplane Components
- •Lateral Dynamic Effects
- •Spiral Divergence
- •Dutch Roll
- •Pilot Induced Oscillation (PIO)
- •High Mach Numbers
- •Mach Trim
- •Key Facts 2
- •Summary
- •Questions
- •Key Facts 1 (Completed)
- •Key Facts 2 (Completed)
- •Answers
- •11 Controls
- •Introduction
- •Hinge Moments
- •Control Balancing
- •Mass Balance
- •Longitudinal Control
- •Lateral Control
- •Speed Brakes
- •Directional Control
- •Secondary Effects of Controls
- •Trimming
- •Questions
- •Answers
- •12 Flight Mechanics
- •Introduction
- •Straight Horizontal Steady Flight
- •Tailplane and Elevator
- •Balance of Forces
- •Straight Steady Climb
- •Climb Angle
- •Effect of Weight, Altitude and Temperature.
- •Power-on Descent
- •Emergency Descent
- •Glide
- •Rate of Descent in the Glide
- •Turning
- •Flight with Asymmetric Thrust
- •Summary of Minimum Control Speeds
- •Questions
- •Answers
- •13 High Speed Flight
- •Introduction
- •Speed of Sound
- •Mach Number
- •Effect on Mach Number of Climbing at a Constant IAS
- •Variation of TAS with Altitude at a Constant Mach Number
- •Influence of Temperature on Mach Number at a Constant Flight Level and IAS
- •Subdivisions of Aerodynamic Flow
- •Propagation of Pressure Waves
- •Normal Shock Waves
- •Critical Mach Number
- •Pressure Distribution at Transonic Mach Numbers
- •Properties of a Normal Shock Wave
- •Oblique Shock Waves
- •Effects of Shock Wave Formation
- •Buffet
- •Factors Which Affect the Buffet Boundaries
- •The Buffet Margin
- •Use of the Buffet Onset Chart
- •Delaying or Reducing the Effects of Compressibility
- •Aerodynamic Heating
- •Mach Angle
- •Mach Cone
- •Area (Zone) of Influence
- •Bow Wave
- •Expansion Waves
- •Sonic Bang
- •Methods of Improving Control at Transonic Speeds
- •Questions
- •Answers
- •14 Limitations
- •Operating Limit Speeds
- •Loads and Safety Factors
- •Loads on the Structure
- •Load Factor
- •Boundary
- •Design Manoeuvring Speed, V
- •Effect of Altitude on V
- •Effect of Aircraft Weight on V
- •Design Cruising Speed V
- •Design Dive Speed V
- •Negative Load Factors
- •The Negative Stall
- •Manoeuvre Boundaries
- •Operational Speed Limits
- •Gust Loads
- •Effect of a Vertical Gust on the Load Factor
- •Effect of the Gust on Stalling
- •Operational Rough-air Speed (V
- •Landing Gear Speed Limitations
- •Flap Speed Limit
- •Aeroelasticity (Aeroelastic Coupling)
- •Flutter
- •Control Surface Flutter
- •Aileron Reversal
- •Questions
- •Answers
- •15 Windshear
- •Introduction (Ref: AIC 84/2008)
- •Microburst
- •Windshear Encounter during Approach
- •Effects of Windshear
- •“Typical” Recovery from Windshear
- •Windshear Reporting
- •Visual Clues
- •Conclusions
- •Questions
- •Answers
- •16 Propellers
- •Introduction
- •Definitions
- •Aerodynamic Forces on the Propeller
- •Thrust
- •Centrifugal Twisting Moment (CTM)
- •Propeller Efficiency
- •Variable Pitch Propellers
- •Power Absorption
- •Moments and Forces Generated by a Propeller
- •Effect of Atmospheric Conditions
- •Questions
- •Answers
- •17 Revision Questions
- •Questions
- •Answers
- •Explanations to Specimen Questions
- •Specimen Examination Paper
- •Answers to Specimen Exam Paper
- •Explanations to Specimen Exam Paper
- •18 Index
6 Drag
Drag 6
Power Required (Introduction)
We will now consider the relationship between Thrust, Drag and Power. These sound like engine considerations which might be better studied in Book 4, but it has already been shown that Drag can also be referred to as ‘Thrust Required’ and you will now see that a similar relationship exists with ‘Power Required’ - they are both important airframe considerations.
• Thrust is a FORCE (a push or a pull), used to oppose Drag,
but Power is the RATE of doing WORK, or POWER = |
WORK |
||
|
|
|
TIME |
and |
WORK = FORCE × DISTANCE |
|
|
so |
POWER must be |
FORCE × DISTANCE |
|
TIME |
|
||
|
|
|
For Power Required:
Which Force?
Drag.
Distance divided by time is speed.
Which speed?
The only speed there is - the speed of the aircraft through the air, True Airspeed (TAS).
Therefore: POWER REQUIRED = DRAG × TAS
•If an aircraft climbs at a constant IAS, Drag will remain constant, but TAS must be increased - so power required will increase.
It is necessary to consider power required when studying Principles of Flight because Work must be done on the aircraft to “raise” it to a higher altitude when climbing. Logically, maximum work can be done on the aircraft in the minimum time when the power available from the engine(s) is greatest and the power required by the airframe is least.
For easy reference, associate the word POWER with the word RATE. e.g. minimum rate of descent is achieved in a steady glide when the aircraft is flown at the minimum power required speed (VMP ).
These and other considerations will be examined more fully during the study of Aircraft Performance in Book 6 and Flight Mechanics in Chapter 12 of this Book.
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Drag 6
POWER |
|
REQUIRED (kW) |
6 |
(DRAG × TAS) |
Drag |
|
VMP |
|
THRUST REQUIRED |
L / DMAX |
or |
|
DRAG (kN) |
TAS (kt) |
|
|
V MD |
Figure 6.21
Figure 6.21 is drawn for sea level conditions where TAS = IAS and is valid for one particular aircraft, for one weight, only in level flight, and shows how a graph of TAS against ‘Power Required’ has been constructed from a TAS/Drag curve by multiplying each value of drag by the appropriate TAS and converting it to kilowatts.
The speed for minimum power required is known as VMP and is an Indicated Airspeed (IAS).
Note that the speed corresponding to minimum power required (VMP), is slower than the speed for minimum drag (VMD).
Effect of Altitude
An aircraft flying at VMD will experience constant drag at any altitude because VMD is an IAS. At altitude the TAS for a given IAS is higher, but the power required also increases because Power Required = Drag × TAS. So the ratio of TAS to Power Required is unaffected and VMP will remain slower than VMD.
This information primarily concerns aircraft performance, but the relationship of speed for minimum power required (VMP) and speed for minimum drag (VMD) is important for the study of rate and angle of descent in a steady glide, outlined in Chapter 12.
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6 Drag
Drag 6
Summary
Parasite Drag is made up of:
Skin friction drag.
Form (Pressure) drag.
(Skin friction drag plus Form drag is known as Profile drag.)
Interference drag.
Parasite Drag varies directly as the square of the Indicated Airspeed (IAS) - Double the speed, four times the parasite drag. Halve the speed, one quarter the parasite drag.
The designer can minimize parasite drag by:
Streamlining.
Filleting.
The use of laminar flow wing sections.
Flight crews must ensure the airframe, and the wing in particular, is not contaminated by ice, snow, mud or slush.
Induced Drag
Spanwise airflow generates wing tip vortices.
The higher the CL (the lower the IAS), the stronger the wing tip vortices.
Wing tip vortices strengthen downwash.
Strengthened downwash inclines wing lift rearwards.
The greater the rearward inclination of wing lift, the greater the induced drag.
Induced Drag varies inversely as the square of the Indicated Airspeed (IAS) - Halve the speed, 16 times the induced drag coefficient (CDi) and four times the induced drag (Di). Double the speed, one sixteenth the CDi and one quarter the Di.
The designer can minimize induced drag by:
Using a high aspect ratio wing planform.
Using a tapered wing planform with wing twist and/or spanwise camber variation, or incorporation of wing end plates, tip tanks, winglets or various wing tip shapes.
132
Drag 6
Total Drag
Total drag is the sum of Parasite drag and Induced drag.
Total drag is a minimum when Parasite drag and Induced drag are equal.
At low IAS Induced drag is dominant.
At high IAS Parasite drag dominates.
The IAS at which Parasite and Induced drags are equal is called minimum drag speed (VMD).
As gross weight decreases in flight, Induced drag decreases, Total drag decreases and VMD decreases.
At a constant IAS, altitude has no effect on Total drag, but TAS will increase as density decreases with increasing altitude.
Configuration changes which increase the “Parasite Area”, such as undercarriage, flaps or speed brakes, increase Parasite drag, increase Total drag and decrease VMD.
Speed Stability
An aircraft flying at a steady IAS higher than VMD with a fixed throttle setting will have speed stability.
An aircraft flying at a steady IAS at VMD or slower with a fixed throttle setting will usually NOT have speed stability.
If an aircraft flying at a steady IAS and a fixed throttle setting within the non-stable IAS region encounters a disturbance which slows the aircraft, the aircraft will tend to slow further; IAS will tend to continue to decrease and Total drag increase.
Power Required
VMP the Indicated Airspeed for minimum ‘Power Required’ is slower than the minimum drag speed (VMD).
Maximum TAS/Power ratio (1.32VMP) occurs at VMD .
Drag 6
133