- •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
FLOW INDUCED BY
JETFAN EXHAUST
Figure 10.30
Essentially the same destabilizing effect is produced by the flow induced at the exhaust of turbo-jet/fan engines, Figure 10.30. Ordinarily, the induced flow at the horizontal tail of a jet aeroplane is slight and is destabilizing when the jet passes underneath the horizontal tail. The magnitude of the indirect power effects on stability tends to be greatest at high CL, high power and low flight speeds.
Conclusions to the Effects of Power
The combined direct and indirect power effects contribute to a general reduction of static stability at high power, high CL and low dynamic pressure. It is generally true that any aeroplane will experience the lowest level of static longitudinal stability under these conditions. Because of the greater magnitude of both direct and indirect power effects, the propeller powered aeroplane usually experiences a greater effect than the jet powered aeroplane.
High Lift Devices
An additional effect on stability can be from the extension of high lift devices. High lift devices tend to increase downwash at the tail and reduce the dynamic pressure at the tail, both of which are destabilizing. However, high lift devices may prevent an unstable contribution of the wing at high CL. While the effect of high lift devices depends on the aeroplane configuration, the usual effect is destabilizing. Hence, the aeroplane may experience the most critical forward neutral point during the power approach or overshoot/missed approach. During this condition of flight, the static stability is usually the weakest and particular attention must be given to precise control of the aeroplane.
The power-on neutral point may set the most aft limit of CG position.
268
Stability and Control 10
Control Force Stability
The static longitudinal stability of an aeroplane is defined by the tendency to return to equilibrium upon displacement. In other words, a stable aeroplane will resist displacement from trim or equilibrium. The control forces of the aeroplane should reflect the stability of the aeroplane and provide suitable reference to the pilot for precise control of the aeroplane.
EFFECT OF ELEVATOR DEFLECTION |
|
||
CM |
ELEVATOR |
10 |
|
DEFLECTION |
|||
+ |
TRIM FOR |
Control |
|
and |
|||
|
|||
|
10º UP |
||
TRIM FOR 0º |
Stability |
||
|
|||
- |
|
CL |
|
|
|
||
|
CG @ 20% MAC |
|
Figure 10.31
The effect of elevator deflection on pitching moments is illustrated by the graph of Figure 10.31. If the elevators of the aeroplane are held at zero deflection, the resulting line of CM versus CL for 0° depicts the static stability and trim lift coefficient. If the elevators are held at a deflection of 10° up (aircraft trimmed at a lower speed), the aeroplane static stability is unchanged but the trim lift coefficient is increased.
269
10 Stability and Control
As the elevator is held in various positions, equilibrium (trim) will occur at various lift coefficients, and the trim CL can be correlated with elevator deflection as shown in Figure 10.32.
|
TRIM CL VERSUS ELEVATOR DEFLECTION |
|
|
CG LOCATION |
|
|
10% MAC |
|
|
20% MAC |
|
|
UP |
|
10 |
30% MAC |
|
CL |
||
Stability |
||
40% MAC |
||
|
||
Control and |
(NEUTRAL POINT) |
|
DOWN |
||
|
Figure 10.32
When the CG position of the aeroplane is fixed, each elevator position corresponds to a particular trim lift coefficient. As the CG is moved aft, the slope of this line decreases, and the decrease in stability is evident by a given control displacement causing a greater change in trim lift coefficient. This is evidence that decreasing stability causes increased controllability and, of course, increasing stability decreases controllability.
If the CG is moved aft until the line of trim CL versus elevator deflection has zero slope, neutral static stability is obtained.
A change in elevator position does not alter the tail contribution to stability
270
Stability and Control 10
TRIM AIRSPEED VERSUS ELEVATOR DEFLECTION
|
STABLE |
UP |
UNSTABLE |
|
EQUIVALENT |
|
AIRSPEED |
DOWN |
|
Figure 10.33
Since each value of lift coefficient corresponds to a particular value of dynamic pressure required to support an aeroplane in level flight, trim airspeed can be correlated with elevator deflection as in the graph of Figure 10.33.
If the CG location is ahead of the neutral point and control position is directly related to surface deflection, the aeroplane will give evidence of stick position stability. In other words, the aeroplane will require the stick to be moved aft to increase the angle of attack and trim at a lower airspeed and to be moved forward to decrease the angle of attack and trim at a higher airspeed.
It is highly desirable to have an aeroplane demonstrate this feature. If the aeroplane were to have stick position instability, the aeroplane would require the stick to be moved aft to trim at a higher airspeed or to be moved forward to trim at a lower airspeed.
Stability and Control 10
271
10 Stability and Control
Control and Stability 10
There is an increment of force dependent on the trim tab setting which varies with the dynamic pressure or the square of equivalent airspeed. Figure 10.34 indicates the variation of stick force with airspeed and illustrates the effect of tab setting on stick force.
|
EFFECT OF TRIM TAB SETTING |
|||
PULL |
|
|
|
|
0 |
|
|
EAS |
|
1 |
2 |
3 |
||
CG @ 20% MAC |
||||
PUSH |
|
|
|
Figure 10.34
In order to trim the aeroplane at point (1) a certain amount of up elevator is required and zero stick force is obtained with the use of the trim tab. To trim the aeroplane for higher speeds corresponding to points (2) and (3), less and less aircraft nose-up tab is required.
Note that when the aeroplane is properly trimmed, a push force is required to increase airspeed and a pull force is required to decrease airspeed. In this manner, the aeroplane would have positive stick force stability with a stable “feel” for airspeed.
272
Stability and Control 10
EFFECT OF CG POSITION
|
|
CG POSITION |
|
|
10% MAC |
TRIM |
|
PULL |
|
|
|
20% MAC |
SPEED |
||
|
30% |
MAC |
|
0 |
40% |
MAC |
EAS |
|
|
|
|
|
50% |
MAC |
|
PUSH |
|
|
|
Figure 10.35
If the CG of the aeroplane were varied while maintaining trim at a constant airspeed, the effect of CG position on stick force stability could be appreciated. As illustrated in Figure 10.35, moving the CG aft decreases the slope of the line of stick force through the trim speed. Thus, on decreasing stick-force stability it is evident that smaller stick forces are necessary to displace the aeroplane from the trim speed. When the stick force gradient (or slope) becomes zero, the CG is at the neutral point and neutral stability exists. If the CG is aft of the neutral point, stick force instability will exist, e.g. the aeroplane will require a push force at a lower speed or a pull force at a higher speed. It should be noted that the stick force gradient is low at low airspeeds, and when the aeroplane is at low speeds and high power and has a CG position near the aft limit, the “feel” for airspeed will be weak.
|
EFFECT OF CONTROL SYSTEM FRICTION |
PULL |
TRIM SPEED |
BAND |
|
0 |
EAS |
|
|
PUSH |
FRICTION FORCE |
|
BAND |
Figure 10.36
Control system friction can create very undesirable effects on control forces. Figure 10.36 illustrates that the control force versus airspeed relationship is a band rather than a line. A wide friction force band can completely mask the stick force stability when the stick force stability is low. Modern flight control systems require precise maintenance to minimize the friction force band and preserve proper feel to the aeroplane.
Stability and Control 10
273