- •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
Limitations 14
For a given gust speed and aircraft TAS, the increment in the load factor depends on the increase in CL per change in angle of attack due to the gust (the slope of the lift curve). If the lift curve has a steep slope, the ‘g’ increment will be greater. Factors which affect the lift curve are aspect ratio and wing sweep.
C L |
HIGH ASPECT |
RATIO |
LOW ASPECT RATIO
(or sweepback)
Figure 14.8
Wings having a low aspect ratio, or sweep, will have a lower lift curve slope, and so will give a smaller increase in ‘g’ when meeting a given gust at a given TAS.
High wing loading reduces the ‘g’ increment in a gust. This is because the lift increment produced is a smaller proportion of the original lift force for the more heavily loaded aircraft.
For a given TAS and gust speed, the increase of lift will be proportional to the wing area. Therefore, the increase in load factor is inversely proportional to the wing loading.
Wing Loading = |
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For a given aircraft the only variables for load factor increment in a gust are the aircraft TAS and the gust speed.
Effect of the Gust on Stalling
If an aerofoil encounters an upgust, it will experience an increase in angle of attack. For a given gust velocity the increment in angle increases as the aircraft TAS decreases. If the angle of attack is already large (low speed), the increment due to the gust could cause the wing to stall. There is thus a minimum speed at which it is safe to fly if a gust is likely to be met so as not to stall in the gust.
Limitations 14
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14
Limitations 14
Limitations
Operational Rough-air Speed (VRA / MRA)
For flight in turbulence an airspeed must be chosen to give protection against two possibilities: stalling and overstressing the aircraft structure. Turbulence is defined by a gust of a defined value. If this defined gust is encountered, the aircraft speed must be:
•high enough to avoid stalling.
•low enough to avoid damage to the structure.
These requirements are fulfilled by calculating the stall speed in the gust and then building in sufficient strength for this speed.
The key is the chosen value of the gust, as this will dictatethe strength required and therefore the aircraft weight. The gust velocity is associated with the design speed, VB, and the vertical value of the gust is 66 ft per second. Encountering a gust before the pilot is able to slow the aircraft, plus the possibility of hitting a gust if the aircraft is ‘upset’ at high speed, must also be taken into consideration. Because these probabilities are lower however, progressively lower values of gust velocity are chosen at the higher speeds. These values are 50 ft per second at the design cruise speed VC and 25 ft per second at the design dive speed VD.
The design gust values of 66, 50 and 25 ft per second for gusts at the design speeds of VB, VC and VD have existed since the early 1940s. In the UK they were established as a result of the earliest “Flight Data Recorder” results. Modern flight recorder results and sophisticated design analyses continue to support the original boundaries of the design gust envelope.
Generally, design for strength is based on calculating the increase in load on the aircraft as a function of an instantaneous increase in angle of attack on the wing page 469.
On large aircraft, additional allowances have to be made for several reasons:
•The greater dynamic response due to increased structural flexibility.
•The possible implications of the smaller margin between actual cruise speed and design cruise speed.
•The significance, in the more advanced designs, of the effects of build-up of gusts and unsteady flow generally.
•The frequency of storm penetrations.
•The implications of the limited slow-down capabilities.
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Limitations 14
All design speeds, and design gust values, are EAS. But, remember: the increase in angle of attack due to a gust is a function of the TAS of the aircraft and the TAS of the gust.
The choice of rough-air speed to be used operationally must be consistent with the strength of the aircraft. At the same time the aircraft must comply with both minimum stability and control criteria. There is also the important consideration of what maximum speed reduction can be achieved in a slow-down technique. A typical chart of the speeds to which the roughair speed is related, is shown below in Figure 14.9. The illustration is drawn for a single (mid) weight. Line AB is the 1g stall speed.
50 |
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Limitations |
0 |
A |
C |
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M |
G |
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SPEED - KNOTS EAS |
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Figure 14.9
Line CE is the stall speed in a 66 ft per sec gust.
(This assumes the 66 ft per sec. gust up to maximum altitude. Note that point E would represent an extremely high true airspeed gust value).
Line GHI is the VMO/MMO line.
Line JKL is the VDF/MDF line.
Line MN is an example of a maximum strength speed line for a 66 ft per sec gust.
Line RS is the 1.3g altitude.
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14 Limitations
Limitations 14
At all speeds above the line CE the aeroplane will sustain a 66 fps gust without stalling and at all speeds below the line MN the aeroplane is strong enough to withstand a 66 fps gust. The rough-air speed therefore should lie somewhere between these two speeds, and the line OP gives equal protection between accidentally stalling and overstressing the aircraft.
The line MN is a curious shape because different parts of the structure become critical at different altitudes. This line is actually the lowest speed boundary of a collection of curves at the higher speed end of the chart.
Because of the obvious attraction of a single speed at all altitudes up to that at which the rough-air speed becomes a rough-air Mach number, the line could be adjusted slightly so as to avoid any variations with altitude. As turbulence is generally completely random, this halfway speed would give equal protection against the 50-50 probability of being forced too fast or too slow.
It has been stated that the diagram is drawn for a mid weight. The effect of weight change in terms of the lower and upper limits to rough-air speed is, of course, significant, but selfcancelling. At low weights the stall line for a 66 ft per sec gust falls to lower speeds and the maximum strength speed line increases to higher speeds. There is therefore no point in attempting a sophisticated variation of VRA with weight.
The maximum altitude limit does, however, vary significantly with weight, and also varies for the level of manoeuvre capability chosen. A 0.3g increment to buffet is not too much protection in severe turbulence. A lower altitude will therefore be required for a higher level of protection, and, for a given level of protection, a lower altitude will be required for higher weights.
Landing Gear Speed Limitations
The landing gear will normally be retracted as soon as possible after take-off to reduce drag and increase the climb gradient. There is no normal requirement for the gear to be operated at high IAS so the retract and extend mechanism together with the attachment points to the structure are sized for the required task. To design the gear for operation at high IAS would unnecessarily increase structural weight.
VLO: the landing gear operating speed is the speed at which it is safe both to extend and to retract the landing gear. If the extension speed is not the same as the retraction speed, the two speeds must be designated as VLO (EXT) and VLO (RET).
When the gear is retracted or extended the doors must open first. The doors merely streamline the undercarriage bay and are not designed to take the aerodynamic loads which would be placed on them at high IAS. Consequently VLO is usually lower than VLE.
VLE: the landing gear extended speed. There may be occasions when it is necessary to ferry the aircraft with the gear down, and to do this a higher permissible speed would be convenient. VLE is the speed at which it is safe to fly the aircraft with the landing gear secured in the fully extended position. Because the undercarriage doors are closed, VLE is normally higher than VLO.
472