- •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
7 Stalling
Composition and Resolution of Forces
A force is a vector quantity. It has magnitude and direction, and it can be represented by a straight line passing through the point at which it is applied, its length representing the magnitude of the force, and its direction corresponding to that in which the force is acting.
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FORCE |
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FORCE |
VECTOR |
FORCE |
VECTOR |
FORCE FORCE
VECTOR
FORCE
VECTOR
Figure 7.21 The resolution of a force into two vectors and the addition of vectors to form a resultant
As vector quantities, forces can be added or subtracted to form a resultant force, or they can be resolved - split into two or more component parts by the simple process of drawing the vectors to represent them. Figure 7.21.
Using Trigonometry to Resolve Forces
If one of the angles and the length of one of the sides of a right angled triangle are known, it is possible to calculate the length of the other sides using trigonometry. This technique is used when resolving a force into its horizontal and vertical components.
Hypotenuse
Opposite
Ad j acen t
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Figure 7.22
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Lift Increase in a Level Turn
45º |
LIFT INCREASE |
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L HYPOTENUSE |
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Figure 7.23
Figure 7.23 shows an aircraft in a level 45° bank turn. Weight always acts vertically downwards. For the aircraft to maintain altitude, the UP force must be the same as the DOWN force. Lift is inclined from the horizontal by the bank angle of 45° and can be resolved into two components, or vectors; one vertical and one horizontal. It can be SEEN from the illustration that in a level turn, lift must be increased in order to produce an upwards force vector equal to weight. We know the vertical force must be equal to the weight, so the vertical force can be represented by 1. The relationship between the vertical force and lift can be found using trigonometry, where φ (phi) is the bank angle:
cos φ |
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transposing this formula gives, L = |
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In this case φ = 45 degrees
1
L = 0.707 = 1.41
This shows that:
In a 45° bank, LIFT must be greater than weight by a factor of 1.41
Another way of saying the same thing: in a level 45° bank turn, lift must be increased by 41%.
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7 Stalling
Effect of Load Factor on Stall Speed
It has been demonstrated that to bank an aircraft and maintain altitude, lift has to be greater than weight. And that additional lift in a turn is obtained by increasing the angle of attack. To consider the relationship between lift and weight we use Load Factor.
LOAD FACTOR (n) or ‘g’ = |
LIFT |
WEIGHT |
(a)Increasing lift in a turn, increases the load factor.
(b)As bank angle increases, load factor increases.
In straight and level flight at CLMAX it would be impossible to turn AND maintain altitude. Trying to increase lift would stall the aircraft. If a turn was started at an IAS above the stall speed, at some bank angle CL would reach its maximum and the aircraft would stall at a speed higher than the 1g stall speed.
The increase of lift in a level turn is a function of the bank angle only. Using the following formula, it is possible to calculate stall speed as a function of bank angle or load factor. VSt is the stall speed in a turn
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Load factor does not |
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Using our example aeroplane: the 1g stall speed is 150 knots CAS, so what will be the stall speed in a 45° bank?
VSt = 150 √ |
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= 178 knots CAS |
0.707 |
In a 60° bank the stall speed will be:
VSt = 150 √ |
1 |
= 212 knots CAS |
0.5 |
Stall speed in a 45° bank is 19% greater than VS1g and in a 60° bank the stall speed is 41% greater than VS1g, and since these are ratios, this will be true for any aircraft.
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As bank angle is increased, stall speed will increase at an increasing rate. While operating at high CL, during take-off and landing in particular, only moderate bank angles should be used to manoeuvre the aircraft. For a modern high speed jet transport aircraft, the absolute maximum bank angle which should be used in service is 30° (excluding emergency manoeuvres). The normal maximum would be 25°, but at higher altitude the normal maximum is 10° to 15°.
If the 1g stall speed is 150 kt, calculate the stall speed in a 25° and a 30° bank turn. (Answers on page 191).
If the stall speed in a 15° bank turn is 153 kt CAS and it is necessary to calculate the stall speed in a 45° bank turn, you would need to calculate the 1g stall speed first, as follows:
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Effect of High Lift Devices on Stall Speed
Modern high speed jet transport aircraft have swept wings with relatively low thickness/chord
ratios (e.g. 12% for an A310). The overall value of CLMAX for these wings is fairly low and the clean stalling speed correspondingly high. In order to reduce the landing and take-off speeds,
various devices are used to increase the usable value of CLMAX. In addition to decreasing the stall speed, these high lift devices will usually alter the stalling characteristics. The devices include:
a)leading edge flaps and slats
b)trailing edge flaps
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From the 1g stall formula: |
VS1g |
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it can be seen that an increase in CLMAX will reduce the stall speed. It is possible, with the most modern high lift devices, to increase CLMAX by as much as 100%. High lift devices will be fully described in Chapter 8. High lift devices decrease stall speed, hence minimum flight speed, so
provide a shorter take-off and landing run - this is their sole purpose.
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