Basic Aerodynamics Ii Stability Large

04/18/23 Author: Harry L. Whitehead 1

Basic Aerodynamics

III. Basic Aerodynamics A. The AtmosphereB. PhysicsC. The AirfoilD. Lift & DragE. StabilityF. Large Aircraft Flight

ControlsAircraft Stability•Stability: General

•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•STABILITY is the characteristic of an airplane in flight that causes it to return to a condition of equilibrium, or steady flight, after it is disturbed.

Basic Aerodynamics

•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•MANEUVERABLILITY is the characteristic that permits the pilot to easily move the airplane about its axes and to withstand the stress from these moves

Basic Aerodynamics

•To insure an airplane has good handling qualities in all flight regimes, we need it to be STABLE, MANEUVERABLE, and CONTROLLABLE•CONTROLLABILITY is the capability to respond to the pilot’s control inputs

Basic Aerodynamics

•Unfortunately, these characteristics are at odds with each other

•Increase in one leads to a decrease in another• = all airplane designs are compromises

•If make it too stable = it’s hard to control

Basic Aerodynamics

ControlsAircraft Stability•Stability: General Types

•2 TYPES OF STABILITY

•STATIC•The ability of an object to return to its equilibrium state after being disturbed

•DYNAMIC•The way the object moves after being disturbed

Basic Aerodynamics

•3 CONDITIONS OF STABILITY:

•POSITIVE•The disruption of an object gets less over time

•NEGATIVE •The disruption gets greater over time

•NUETRAL•The disruption neither increases or decreases over time

Basic Aerodynamics

•Positive Stability:

•The tendency to return to the original equilibrium•Example: Ball in a trough•Generally desirable in an airplane but does decrease maneuverability

Basic Aerodynamics

•Negative Stability:

•The tendency to move away from the equilibrium•Example: Ball on a hill•Undesirable in an airplane

Basic Aerodynamics

•Nuetral Stability:

•The tendency for the correcting forces to neither increase or decrease over time•Example: Ball on a flat surface•Somewhat OK in an airplane

Basic Aerodynamics

ControlsAircraft Stability•Stability: About the Aircraft Axes

•Since we have 3 main axes of an aircraft, we also have 3 main types of Stability:

•LONGITUDINAL•LATERAL•DIRECTIONAL

Basic Aerodynamics

•LONGITUDINAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE LATERAL AXIS•Is PITCH STABILITY•Or keeping the Longitudinal Axis stable

Basic Aerodynamics

•LONGITUDINAL STABILITY•Since the CENTER OF PRESSURE (or CENTER OF LIFT) moves with Angle of Attack changes

Basic Aerodynamics

•LONGITUDINAL STABILITY•We need to be sure the Center of Gravity doesn’t get behind the Center of Pressure or severe flight problems will occur (such as can’t lower the nose)

Basic Aerodynamics

•LONGITUDINAL STABILITY•Airplanes are designed so the Center of Pressure or Lift is behind of the Center of Gravity = a downward pitching moment on the nose of the aircraft at all times

Basic Aerodynamics

•LONGITUDINAL STABILITY•This, coupled with the downward TAIL LOAD created by the HORIZONTAL STABILIZER, create a balanced set of conditions to keep the Longitudinal Axis stable

Basic Aerodynamics

•LONGITUDINAL STABILITY•If the aircraft gets disturbed so the nose goes up, the Horizontal Stabilizer’s Angle of Attack is decreased and creates less Down Load to compensate

•Hor. Stabs. are usually symmetrical airfoils

Basic Aerodynamics

•LONGITUDINAL STABILITY•If the aircraft gets disturbed so the nose goes down, the Horizontal Stabilizer’s Angle of Attack is increased and creates more Down Load to compensate•This is Positive Longitudinal Stability

Basic Aerodynamics

•LONGITUDINAL STABILITY•The Horizontal Stabilizer will be installed at some particular Angle of Incidence so it can do its job correctly

•This may be negative, positive, or zero

Basic Aerodynamics

•LATERAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE LONGITUDINAL AXIS•Is ROLL STABILITY•Or keeping the Lateral Axis stable

Basic Aerodynamics

•LATERAL STABILITY•Provided mostly by wing DIHEDRAL

•This is the upward angle between the wing and the Lateral Axis

Basic Aerodynamics

•As an airplane is upset so a wing drops, it starts to SIDESLIP toward the low wing

Basic Aerodynamics

•This slipping motion plus the downward movement of the wing add downward vectors to the Angle of Attack production and lead to an increase in on the lower wing = more lift on that wing

Basic Aerodynamics

•On the wing moving up, the upward vector reduces the = less lift

Basic Aerodynamics

•These two changes to lift create a rolling force in the direction to restore the wings to level = Positive Lateral Stability

Author: Harry L. Whitehead 25

Basic Aerodynamics

•LATERAL STABILITY•A HIGH-WING Airplane will not need as much Dihedral as a LOW-WING Airplane •since the Center of Gravity is below the Center of Lift it tends to right itself naturally (it’s inherently more Laterally Stable)

Basic Aerodynamics

•DIRECTIONAL STABILITY

•Is the ability of an aircraft to remain stable ABOUT (OR AROUND) THE VERTICAL AXIS•Is YAW STABILITY•Or keeping the Vertical Axis stable

Basic Aerodynamics

•DIRECTIONAL STABILITY•Provided by the VERTICAL STABILIZER and Fuselage

•To be Directionally Stable, an aircraft must have more surface area behind the CG than in front so it acts like a Weather Vane

Basic Aerodynamics

•DIRECTIONAL STABILITY•Provided by the VERTICAL STABILIZER and Fuselage

•When the aircraft yaws (= SIDESLIP), the Vert. Stab. creates lift in the restoring direction and the sides of the fuselage offer a surface for the wind to push against

Basic Aerodynamics

•DIRECTIONAL STABILITY•Is also improved by SWEEPBACK of the wings

Basic Aerodynamics

•DIRECTIONAL STABILITY•Is also improved by SWEEPBACK of the wings

•When yawing (SIDESLIP), the wing which is moving forward has a larger effective wing area = more drag to push it back

Basic Aerodynamics

•DIRECTIONAL STABILITY•But sweepback can cause a small problem: Dutch Roll

•If the aircraft’s wing drops it will tend to yaw into the low wing and the dihedral and sweepback will combine to return the wings to level quickly

Basic Aerodynamics

•As the low wing moves up the Lateral Stability will return the aircraft to straight flight = the low wing will be moving faster than the high wing = more lift

Basic Aerodynamics

•= that wing will now rise and the process will repeat in the opposite direction

Basic Aerodynamics

•The resulting low level oscillation (“Dutch Roll”) doesn’t affect aircraft flight safety but is uncomfortable for passengers

Basic Aerodynamics

•Aircraft susceptible to this usually have YAW DAMPERS connected to the rudder controls to automatically apply corrective rudder action

Basic Aerodynamics

ControlsLarge Aircraft Controls•Large aircraft, like small, control the aircraft about the same 3 axes: Lateral, Longitudinal, and Vertical•Major differences:

•More control surfaces •Hydraulic actuated

•Power-boosted•Hydraulic cylinder in parallel with control rods•Pilot moves surface and valve to actuate hydraulic cylinder to help

Basic Aerodynamics

•Power-boosted•Boosting is typically about 14:1 ratio•Disadvantage: in transonic speed range shock waves form on controls and cause buffeting which is fed back into cockpit

Basic Aerodynamics

•Irreversibles•Used to keep buffet from reaching cockpit•Hyd. Cylinders in series with control rods•Also need “feedback” system to give pilot the feel of the controls

Basic Aerodynamics

ControlsLarge Aircraft Controls

•Example: Boeing 727

Basic Aerodynamics

•Uses Irreversible system with 2 separate hydraulic systems, Standby system, and manual backup of Primary Controls (servo tabs)

Basic Aerodynamics

•Example: Boeing 727•Primary Controls: Roll

•Ailerons and Spoilers•4 ailerons and 14 spoilers

Basic Aerodynamics

•Inboard ailerons and 10 flight spoilers do high speed flight with outboard ailerons locked in neutral position

Basic Aerodynamics

•When trailing edge flaps are deployed, outboard ailerons unlocked = all ailerons and flight spoilers work

Basic Aerodynamics

•Example: Boeing 727•Primary Controls: Pitch

•Elevators for normal pitch action•Movable Horizontal Stab. for trim action

Basic Aerodynamics

•Example: Boeing 727•Primary Controls: Yaw

•2 independent rudders with anti-balance (anti-servo) tabs

Basic Aerodynamics

•Example: Boeing 727•Primary Controls: Yaw

•Also receives input from the Yaw Dampers to counteract Dutch Roll

Basic Aerodynamics

•Example: Boeing 727•Auxiliary Lift Devices: Trailing Edge Flaps

•Triple-slotted Fowler Flaps•Take-off = only back, Landing = back and down

Basic Aerodynamics

•Example: Boeing 727•Auxiliary Lift Devices: Leading Edge Flaps

•Krueger-type•Increase area and camber

Basic Aerodynamics

•Example: Boeing 727•Auxiliary Lift Devices: Leading Edge Slats

•Increase camber•Inboard flaps stall first = retain aileron control

Basic Aerodynamics

•Example: Boeing 727•Secondary Controls (Tabs):

•Ailerons have Balance Tabs which also act as Trim Tabs

Basic Aerodynamics

•Elevators have Servo Tabs which also act as Trim Tabs

Basic Aerodynamics

•Rudders have Anti-balance (anti-servo) Tabs which also act as Trim Tabs

Basic Aerodynamics

•These Tabs also serve as manual backups in case of total hydraulic failure

Basic Aerodynamics

•Example: Lockheed L-1011

Basic Aerodynamics

•Example: Airbus A320

Basic Aerodynamics

Controls

Basic Aerodynamics Ii Stability Large

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