- •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
Chapter
14
Limitations
Operating Limit Speeds |
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Loads and Safety Factors |
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Loads on the Structure . . . . . . . . . . . . . . . . |
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. 459 |
Load Factor . . . . . . . . . . . . . . . . . . . . |
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. 460 |
The Manoeuvre Envelope (V - n Diagram) . . . . . . . . |
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The CLMAX Boundary |
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Design Manoeuvring Speed, VA . . . . . . . . . . . . |
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. 462 |
Effect of Altitude on VA . . . . . . . . . . . . . . . |
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Effect of Aircraft Weight on VA . . . . . . . . . . . . . . . . . . . . . . |
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. 463 |
Design Cruising Speed VC . . . . . . . . . . . . . . |
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464 |
Design Dive Speed VD . . . . . . . . . . . . . . . . |
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. 464 |
Negative Load Factors . . . . . . . . . . . . . . . . |
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. 464 |
The Negative Stall |
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Manoeuvre Boundaries |
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Operational Speed Limits |
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466 |
Gust Loads |
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467 |
Effect of a Vertical Gust on the Load Factor . . . . . . . . |
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. 468 |
Effect of the Gust on Stalling |
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Operational Rough-air Speed (VRA / MRA) . . . . . . . . . |
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470 |
Landing Gear Speed Limitations |
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472 |
Flap Speed Limit |
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473 |
Aeroelasticity (Aeroelastic Coupling) . . . . . . . . . . |
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474 |
Flutter . . . . . . . . . . . . . . . . . . . . . . |
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. 477 |
Control Surface Flutter . . . . . . . . . . . . . . . . |
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. 478 |
Aileron Reversal |
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480 |
Questions . . . . . . . . . . . . . . . . . . . . |
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482 |
Answers . . . . . . . . . . . . . . . . . . . . . |
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486 |
457
14 Limitations
Limitations 14
458
Limitations 14
Operating Limit Speeds
In service an aircraft must observe certain speed limitations. These may be maximum speeds or minimum speeds, but in each case they are set to give safe operation in the prevailing conditions. The limits may be set by various considerations, the main ones being:
•strength of the aircraft structure.
•stiffness of the aircraft structure.
•adequate control of the aircraft.
Strength is the ability of the structure to withstand a load, and stiffness is the ability to withstand deformation.
Loads and Safety Factors
•Limit load: The maximum load to be expected in service.
•Ultimate load: The failing load of the structure.
•Factor of safety: The ratio of ultimate load to limit load.
For aircraft structures the factor of safety is 1.5.
The safety factor on aircraft structures is much lower than the safety factors used in other forms of engineering because of the extreme importance of minimum weight in aircraft structures. To keep the weight as low as possible, the safety factor must be kept to a minimum. Because of this it is extremely important not to exceed the limitations set on the operation of the aircraft, as the safety margin can easily be exceeded and structural damage may occur.
Loads on the Structure
The airframe structure must obviously be strong enough to take the loads acting upon it in normal level flight, that is the forces due to lift, drag, thrust and weight. However, the aircraft is also required to manoeuvre and to fly in turbulent air. Under these conditions the loads on the aircraft will be increased, so it must also be strong enough to withstand whatever manoeuvres are specified for the aircraft and the gusts which are required to be considered.
The structure should also have sufficient stiffness to ensure that phenomena such as aileron reversal, flutter and divergence do not occur within the permitted speed range of the aircraft.
Limitations 14
459
14 Limitations
Load Factor
The loads which must be considered are given in the design requirements of an aircraft. They are given in terms of load factor (n), colloquially known as ‘g’.
Load Factor (n) = |
Lift |
Weight |
In level flight, since lift equals weight, the load factor is 1.0 (1g). If the aircraft is performing a manoeuvre such that, for example, the lift is twice the weight, the load factor is 2.0 (2g).
The limit load is given in terms of load factor to make the requirement general to all aircraft. However, it should be appreciated that failure of the structure will occur at some particular applied load. For example, if the structure fails at 10 000 lb load, an aircraft weighing 4000 lb will reach this load at a load factor of 2.5. However, if the aircraft weighs 5000 lb, the failing load is reached at a load factor of 2.0, i.e. it takes less ‘g’ to overstress a heavy aircraft than a light one.
Limit load factors are based on the maximum weight of the aircraft.
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CLMAX |
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Limitations |
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SPEED |
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VS |
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NEGATIVE |
CLMAX |
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Figure 14.1 The manoeuvre envelope
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