- •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
The Atmosphere 2
Relationships between Airspeeds
Indicated Airspeed: (IAS). The speed registered on the Airspeed Indicator.
Calibrated Airspeed: (CAS). An accurate measure of dynamic pressure when the aircraft is flying slowly. The position of the pitot tube(s) and static vent(s), together with the aircraft’s configuration (flaps, landing gear etc.) and attitude to the airflow (angle of attack and sideslip) will affect the pressures sensed, particularly the pressures sensed at the static vent(s).
Under the influence of the above conditions a false dynamic pressure (IAS) will be displayed. When IAS is corrected for this ‘position’ or ‘pressure’ error, as it is called, the resultant is Calibrated Airspeed. (The airspeed corrections to be applied may be displayed on a placard on the flight-deck, or in the Flight Manual, and will include any instrument error).
Equivalent Airspeed: (EAS). An accurate measure of dynamic pressure when the aircraft is flying fast. Air entering the pitot tube(s) is compressed, which gives a false dynamic pressure (IAS) reading, but only becomes significant at higher speeds.
At a given air density, the amount of compression depends on the speed of the aircraft through the air. When the IAS is corrected for ‘position’ AND ‘compressibility’ error, the resultant is Equivalent Airspeed.
True Airspeed: (TAS) or (V). The speed of the aircraft through the air. THE ONLY SPEED THERE IS - All the other, so called, speeds are pressures.
TAS = |
EAS |
Where, |
б is Relative Density |
|
√ б |
||||
|
|
|
The Airspeed Indicator is calibrated for ‘standard’ sea level density, so it will only read TAS if the density of the air through which the aircraft is flying is 1.225 kg/m3. Thus at 40 000 ft where the ‘standard’ density is one quarter of the sea-level value, to maintain the same EAS the aircraft will have to move through the air twice as fast!
The Speed of Sound: (a) Sound is ‘weak’ pressure waves which propagate spherically through the atmosphere from their source. The speed at which pressure waves propagate is proportional to the square root of the absolute temperature of the air. The lower the temperature, the lower the speed of propagation. On a ‘standard’ day at sea level the speed of sound is approximately 340 m/s (660 kt TAS).
At higher aircraft True Airspeeds (TAS) and/or higher altitudes, it is essential to know the speed of the aircraft in relation to the local speed of sound. This speed relationship is known as the
Mach Number (M).
M = |
TAS |
Where (a) is the Local Speed of Sound |
|
(a) |
|||
|
|
The Atmosphere 2
If the True Airspeed of the aircraft is four tenths the speed at which pressure waves propagate through the air mass surrounding the aircraft, the Mach meter will register M 0.4
Critical Mach Number: (MCRIT) The critical Mach number is the Mach number of the aircraft when the speed of the airflow over some part of the aircraft (usually the point of maximum
thickness on the aerofoil) first reaches the speed of sound.
31
2 The Atmosphere
Atmosphere The 2
Airspeed
This information is to reinforce that contained in the preceding paragraphs.
The airspeed indicator is really a pressure gauge, the ‘needle’ of which responds to changes in dynamic pressure (½ ρ V2 ).
The Airspeed Indicator
is a Pressure Gauge
Calibration of the airspeed indicator is based on standard sea level density (1.225 kg/m3). The “airspeed” recorded will be different from the actual speed of the aircraft through the air unless operating under standard sea level conditions (unlikely). The actual speed of the aircraft relative to the free stream is called true airspeed (TAS), and denoted by (V). The ‘speed’ recorded by the airspeed indicator calibrated as above, if there are no other errors, is called equivalent airspeed (EAS).
It may seem to be a drawback that the instrument records equivalent rather than true airspeed, but the true airspeed may always be determined from it. Also, many of the handling characteristics of an aircraft depend mainly on the dynamic pressure, i.e. on the equivalent airspeed, so it is often more useful to have a direct reading of EAS than TAS.
Errors and Corrections
An airspeed indicator is, however, also subject to errors other than that due to the difference between the density of the air through which it is flying and standard sea level density.
•Instrument Error: This error may arise from the imperfections in the design and manufacture of the instrument, and varies from one instrument to another. Nowadays this type of error is usually very small and for all practical purposes can be disregarded. Where any instrument error does exist, it is incorporated in the calibrated airspeed correction chart for the particular aeroplane.
•Position Error (Pressure Error): This error is of two kinds, one relating to the static pressure measurement, the other to the pitot (total) pressure measurement. The pitot tube(s) and static port(s) may be mounted in a position on the aircraft where the flow is affected by the presence of the aircraft, changes in configuration (flaps and maybe gear) and proximity to the ground (ground effect). If so, the static pressure recorded will be the local and not the free stream value. The pitot pressure may be under-recorded because of incorrect alignment - the tube(s) may be inclined to the airstream instead of facing directly into it (changes in angle of attack, particularly at low speeds). The magnitude of the consequent errors will generally depend on the angle of attack and, hence, the speed of the aircraft.
•Compressibility Error: At high speeds, the dynamic pressure is not simply ½ ρV2, but exceeds it by a factor determined by Mach number. Thus the airspeed indicator will over-read.
Because of the errors listed, the ‘speed’ recorded on the airspeed indicator is generally not the equivalent airspeed. It is called instead the indicated airspeed. Corrections to rectify the instrument and position errors are determined experimentally. In flight, using special instruments, measurements are taken over the whole range of speeds and configurations, from which a calibration curve is obtained which gives the corrections appropriate to each indicated airspeed. The compressibility error correction may be obtained by calculation.
32
The Atmosphere 2
The indicated airspeed, after correction for instrument, position (pressure) and compressibility errors, gives the equivalent airspeed ½ ρ V2.
V Speeds
These include: VS , V1 , VR , V2 , VMD , VMC , VYSE and many others - these are all Calibrated Airspeeds because they relate to aircraft operations at low speed. However, the appropriate
corrections are made and these speeds are supplied to the pilot in the Flight Manual as IAS.
VMO - The maximum operating IAS is, however, an EAS because it is a high speed, but again it is supplied to the pilot in the Flight Manual as an IAS.
The Atmosphere 2
33
2 The Atmosphere
Atmosphere The 2
Summary
Dynamic pressure (Q) is affected by changes in air density.
Q = ½ ρ V2
Air density decreases if atmospheric pressure decreases.
Air density decreases if air temperature increases.
Air density decreases if humidity increases.
With the aircraft on the ground:
Taking off from an airfield with low atmospheric pressure and/or high air temperature and/or high humidity will require a higher TAS to achieve the same dynamic pressure (IAS).
For the purpose of general understanding:
A constant IAS will give constant dynamic pressure.
Increasing altitude decreases air density because of decreasing static pressure.
With the aircraft airborne:
As altitude increases, a higher TAS is required to maintain a constant dynamic pressure. Maintaining a constant IAS will compensate for changes in air density.
There is only one speed, the speed of the aircraft through the air, the TAS. All the other, so called, speeds are pressures.
The Airspeed Indicator is a pressure gauge.
Aircraft ‘V’ speeds are CAS, except VMO which is an EAS, but all are presented to the pilot in the Flight Manual as IAS.
34