Automatic FC Slides

8/8/2019 Automatic FC Slides

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Flight Performance, Stability and Control

Dr. Parag MantriAssistant Professor

Department of Aerospace Engineering

June 21, 2010

University of Petroleum and Energy Studies


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Road-Map

Static Stability and Control

Aircraft Equations of Motion

Automatic Control: Classical

Automatic Control: Modern

Application to Aircraft AutoPilot Design

University of Petroleum and Energy Studies Introduction to Automatic Flight Control


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Stability

Aircraft Stability: Following a displacement from an original steady flightpath, an aircraft has stability if it returns to its path without movementsof its flight control surfaces having to be applied.

Static Stability: Immediate reacting of the aircraft

Dynamic Stability: Reacting of aircraft over a period of time

A static or dynamic stability can be divided into stable, unstable andneutral equilibrium

A system can be statically stable but dynamically unstable but it cannot

be dynamically stable unless it is statically stable.



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Aircraft Stability

Just like the disturbances of the aircraft, the stabilizing motion will beonly in three planes; Pitch, roll and yaw

These planes change relative to the ground but are fixed relative to theaircraft

Lateral stability is about longitudinal axis, longitudinal stability is aboutlateral axis and directional stability about normal axis



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Axis of Rotation



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Longitudinal Stability

When aircraft returns to trimmed angle of attack (equilibrium), positivestatic longitudinal stability

Influenced by aircraft center of gravity and horizontal stabilizer

When the elevators are maintained in the neutral or streamlined position,

static stability is referred to as stick-fixed stability

Stick-free stability refers to the condition in which the elevators areallowed to float in the airflow and return to neutral position after thepilot releases the stick

Dynamic neutral longitudinal stability is Phugoid



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Longitudinal Stability Contd.

If the aircraft is displaced pitch-up, angle of attack and hence liftincreases.

In this condition if Aerodynamic center of the wing is ahead of C.G, thenthe net effect is to increasing the pitch-up moment (Not desirable)

However, tails AC is aft of CG and hence it produces restoring moment

For certain velocity and angle of attack, a given airplane is in equilibrium.This angle of attack is called the trim point.

The difference in incidence angle of wing and horizontal stabilizer is

longitudinal dihedral angle.



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Pitching and Restoring Moments



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Center of Gravity Location

Degree of longitudinal stability depends on the relative position of theaerodynamic centers and center of gravity.

Keeping Aerodynamic center Aft of CG, produces stable restoringmoment

As the CG moves rearwards, static stability decreases, becomes neutraland then becomes unstable

The CG location which results in neutral longitudinal static stability isneutral point of the aircraft

The distance between CG at any point and the neutral point is knownas static margin



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Dynamic longitudinal Stability



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Directional Stability: Yaw axis



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Lateral Stability: Longitudinal or roll axis

Unlike longitudinal stability, lateral stability effects directional stability aswell and vice-versa.

This means a rolling motion can create yaw as well and yawing motioncontributes to roll.

The idea of static and dynamic stability nonetheless remains same

Factors contributing to lateral static stability

Dihedral angle Swept back angle Vertical location of the wings relative to the fuselage.



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Other Instability

Directional Divergence: Poor directional stability

Spiral Divergence: High directional stability but poor lateral stability

Dutch Roll: Positive directional stability (lower than spiral) and poor

lateral stability. Rolls and yaws out of phase.



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Directional and Spiral Divergence



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Dutch Roll



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Control Surfaces

Ailerons (Roll)

Rudder (Yaw)

Elevator (Pitch)

Flaps (Increased drag and lift)

Spoilers (additonal drag for landing)

Slats on leading edge (Increased drag and lift)



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Automatic Flight Control System

Closed loop servomechanism technique used for

Overcome any stability deficiency Improving the handling or ride qualities, e.g. holding altitude or

air-speed Carry out maneuvers which pilot is unable to perform due to accuracy

required or any external hindrance



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Typical Control System



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Internal vs. External Conditions

Internal conditions are those derived from sensors within AFCS (Pitch,roll and yaw altitudes and their rates, acceleration, etc.)

External Conditions relate to to airspeed, altitude, track, and othernavigational information derived from sensors external to (but integrated

with) AFCS.

Inner loop handles the internal conditions and the outer loop handles theexternal conditions



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Components of an AFCS

Sensors: These measure the relevant parameters and transmit theinformation in the computation group

Computers: These convert the information from the sensors into thesignals to be fed to output devices

Output devices: Convert computer signals into necessary output actionor control surface movements.



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Attitude Sensing: Gyroscope

Basic Functionality

Components

types


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