- •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
10 Stability and Control
Control and Stability 10
Key Facts 1
Self Study (Insert the missing words, with reference to the preceding paragraphs).
Stability is the ________ of an aircraft to return to a _____ state of flight, after being disturbed by an external ______, without any help from the _____.
There are two broad categories of stability: ________ and ________ .
An aircraft is in a state of __________ (trim) when the sum of all forces is ____ and the sum of all ________ is zero.
The type of static stability an aircraft possesses is defined by its ______ tendency, following the removal of some disturbing force.
The three different types of static stability are:
a)_________ static stability exists if an aircraft is disturbed from equilibrium and has the tendency to return to equilibrium.
b)______ static stability exists if an aircraft is subject to a disturbance and has neither the tendency to return nor the tendency to continue in the displacement direction.
c)_________ static stability exists if an aircraft has a tendency to continue in the direction of disturbance.
The longitudinal axis passes through the ____ from _____ to _____.
The normal axis passes “vertically” through the ___ at __° to the ___________ axis.
The lateral axis is a line passing through the ___, parallel to a line passing through the ____ tips. The three reference axes all pass through the _______ ___ ________.
Lateral stability involves motion about the __________ axis (_______). Longitudinal stability involves motion about the ______ axis (_______). Directional stability involves motion about the _______ axis (_______).
We consider the changes in __________ of lift force due to changes in angle of ________, acting through a __________ point; the ___________ ______.
The aerodynamic centre (AC) is located at the ___% chord position.
The _________ pitching moment about the AC remains ________ at normal angles of attack. A wing on its own is statically ________ because the ___ is in front of the ___.
An upward vertical gust will momentarily ________ the angle of attack of the wing. The
________ lift force magnitude acting through the ___ will increase the ______ pitching moment about the ___. This is an ________ pitching moment.
The ________ is positioned to generate a _________ pitching moment about the aircraft ___.
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Stability and Control
If the tail moment is greater than the wing moment, the sum of the moments will not be ____
and the resultant nose _____ moment will give an angular _________ about the ____.
The ______ the tail moment relative to the wing moment, the _______ the rate of return
_______ the original __________ position.
The tail moment is increased by moving the aircraft ___ forwards, which _________ the tail arm and decreases the _____ arm.
If the nose-down (_______) tail moment is greater than the nose-up (_______) wing moment, the aircraft will have _______ __________ stability.
The position of the CG when changes in the sum of the tail moment and wing moment due to a disturbance is zero is known as the ______ _____.
The further forward the ___, the ______ the nose-down angular __________ about the ___ - the ______ the degree of _____ __________ stability.
The _______ the ___ is forward of the ________ point will give a measure of the _____
longitudinal stability; this distance is called the static ______.
The greater the static margin, the ______ the _______ ___________ stability.
The ____ CG limit will be positioned some distance _______ of the _____ _____.
The distance between the ___ ___ limit and the neutral point gives the required _________
static stability ________.
An aircraft is said to be _______ if all ________ in pitch, roll, and yaw are equal to _____.
Trim ( __________ ) is the function of the _______ and may be accomplished by:
a)______ effort
b)trim _____,
c)moving _____ between the wing ______ and an aft located _____ tank, or
d)bias of a surface _______ ( ________ flying controls).
The term ____________ refers to the ability of the aircraft to respond to control surface displacement and achieve the desired ________ of flight.
A high degree of stability tends to reduce the ____________ of the aircraft.
The stable tendency of an aircraft resists displacement from ___ equally, whether by ____ effort on the controls ( ______ force) or _____.
If the CG moves forward, static longitudinal stability ________ and controllability _________
(stick force ________).
If the CG moves aft, static longitudinal stability __________ and controllability ________ (stick force ________ ).
10
Stability and Control 10
255
10
Control and Stability 10
Stability and Control
With the CG on the forward limit, static longitudinal stability is _______, controllability is ____
and stick force is _____.
With the CG on the aft limit, static longitudinal stability is _____, controllability is _______ and stick force is ____.
The aft CG limit is set to ensure a _________ degree of static longitudinal stability.
The fwd CG limit is set to ensure a _________ degree of controllability under the worst circumstance.
KEY FACTS 1 WITH THE MISSING WORDS INSERTED CAN BE FOUND AT THE END OF THIS CHAPTER.
Graphic Presentation of Static Longitudinal Stability
Static longitudinal stability depends upon the relationship of angle of attack and pitching moment. It is necessary to study the pitching moment contribution of each component of the aircraft. In a manner similar to all other aerodynamic forces, the pitching moment about the lateral axis is studied in the coefficient form.
M = CM Q S (MAC)
or
M
CM = Q S (MAC)
where:
M= pitching moment about the CG (positive if in a nose-up direction)
Q |
= |
dynamic pressure |
S |
= |
wing area |
MAC |
= |
mean aerodynamic chord |
CM |
= |
pitching moment coefficient |
The pitching moment coefficients contributed by all the various components of the aircraft are summed up and plotted versus lift coefficient (angle of attack).
Study of the plots of CM versus CL is a convenient way to relate the static longitudinal stability of an aeroplane.
256
Stability and Control 10
+A
|
|
TRIM |
x |
|
CM = 0 |
|
LIFT COEFFICIENT
y |
CL |
Figure 10.16 Graph A
Graph A illustrates the variation of pitching moment coefficient (CM) with lift coefficient (CL) for an aeroplane with positive static longitudinal stability. Evidence of static stability is shown by a tendency to return to equilibrium, or “trim”, upon displacement. The aeroplane described by graph A is in trim or equilibrium when CM = 0, and if the aeroplane is disturbed to some different CL, the pitching moment change tends to return the aircraft to the point of trim. If the aeroplane were disturbed to some higher CL (point y), a negative or nose-down pitching moment is developed which tends to decrease angle of attack back to the trim point. If the aeroplane were disturbed to some lower CL (point x), a positive or nose-up pitching moment is developed which tends to increase the angle of attack back to the trim point. Thus, positive static longitudinal stability is indicated by a negative slope of CM versus CL. The degree of static longitudinal stability is indicated by the slope of the curve (red line).
+ B STABLE
TRIM
NEUTRAL
CM
CL
UNSTABLE
Figure 10.17 Graph B
Graph B provides comparison of a stable and an unstable condition. Positive static stability is indicated by the red curve with negative slope. Neutral static stability would be the result if the curve had zero slope. If neutral stability existed, the aeroplane could be disturbed to some higher or lower lift coefficient without change in pitching moment coefficient.
Stability and Control 10
257
10 Stability and Control
Control and Stability 10
Such a condition would indicate that the aeroplane would have no tendency to return to some original equilibrium and would not hold trim. An aeroplane which demonstrates a positive slope of the CM versus CL curve (blue line) would be unstable. If the unstable aeroplane were subject to any disturbance from equilibrium at the trim point, the changes in pitching moment would only magnify the disturbance. When the unstable aeroplane is disturbed to some higher CL a positive change in CM occurs which would illustrate a tendency for continued, greater displacement. When the unstable aeroplane is disturbed to some lower CL a negative change in CM takes place which tends to create continued displacement.
|
+ |
C |
|
STABLE |
|
|
|
|
|
|
UNSTABLE |
CM |
|
CL |
|
|
|
|
|
LESS STABLE |
|
|
NEUTRAL |
Figure 10.18 Graph C
Ordinarily, the static longitudinal stability of a conventional aeroplane configuration does not vary with lift coefficient. In other words, the slope of CM versus CL does not change with CL. However, if:
•the aeroplane has sweepback,
•there is a large contribution of “power effect” on stability, or
•there are significant changes in downwash at the horizontal tail,
noticeable changes in static stability can occur at high lift coefficients (low speed). This condition is illustrated by graph C. The curve of CM versus CL of this illustration shows a good stable slope at low values of CL (high speed). Increasing CL gives a slight decrease in the negative slope hence a decrease in stability occurs. With continued increase in CL, the slope becomes zero and neutral stability exists. Eventually, the slope becomes positive and the aeroplane becomes unstable or “pitch-up” results.
Remember, at any lift coefficient, the static stability of the aeroplane is depicted by the slope of the curve of CM versus CL.
258