System, Dynamics and Vibration

Motion of an aircraft

In relation to translational motion and rotational motion around a fixed set of axes.

Translational Motion

By which a vehicle travels from one point to another in space.

Rotational Motion

Related to the motion of the aircraft around three defines axes: pitch, roll, and yaw.

Primary Flight Control Systems

Consist of ailerons, elevator, and rudder and required to control the aircraft safely during flight. This are attached to the airframe on hinges or tracks so that they may move and thus deflect the air stream passing over them.

Secondary Flight Controls

Intended to improve the aircraft performance characteristics of to relieve excessive control loading, and consist of high lift devices such as slats, and flaps as well as flight spoilers and trim systems.

Aircraft engine controls

Provide a means for the pilot to control and monitor the operation of the aircraft’s powerplant

Aircraft with a fixed-pitch propeller

There is no direct control over the propeller rotational speed which depends on the airspeed and loading

Jet aircraft

Employs FADEC consisting of a digital computer, called an Electronic Engine Controller (EEC) or Engine Control Unit (ECU), and its related accessories that control all aspects of aircraft engine performance

Full Authority Digital Engine Control

A computer-managed aircraft ignition and engene control systems used in modern commercial and military aircraft to control all aspects in place of analog electronic controls.

Fuel pressurization

Engine feed

Fuel transfer

Refuel/defuel

Fuel storage

Vent systems

Use of fuel as heat sink

Fuel jettison

In-flight re-fueling

The essential characteristics of a modern aircraft fuel management system may embrace some or all of the following modes of operation

Flaps

Slats

Spoilers

Airbrakes

The secondary flight controls

Undercarriage-gear and doors

Wheel-brakes and anti-skid

Parking brake

Nosewheel steering

In-flight refueling probe

Cargo doors

Loading ramp

Passenger stairs

Bomb bay doors

Gun purging scoop

Canopy actuation

Utility systems

Hydraulics

Plays a role in many areas of commercial airliner

High pressure bleed air

In the engine, it is used as the motive power for many of the valves associated with the bleed air extraction.

Medium-pressure bleed air

Used to start the engine in many cases, either using air from the ground power unit, APU or cross bled from another engine on an aircraft which is already running.

Bleed air

Used to provide anti-ice protection by heating the engine intake cowling and it is also used as the motive power for the engine thrust reversers

Bleed air tapped from the engine

On the aircraft, it is used to provide air to pressurize the cabin and provide the source of air to the cabin conditioning environmental control system.

Bleed air

Also used to provide main wing anti-ice protection and for a number of ancillary functions around the aircraft: pressurizing hydraulic reservoirs, providing hot air for rain dispersal from the aircraft windscreen, pressurizing the water and waste system etc.

Air Driven Pumps (ADPs)

In some aircraft it are used as additional means of providing aircraft hydraulic power

• Air conditioning (ATA Chapter 21)

• Cargo compartment heating (ATA Chapter 21)

• Wing and engine anti-icing (ATA Chapter 30)

• Engine Start (ATA Chapter 80)

• Thrust reverser (ATA Chapter 78)

• Hydraulic reservoir pressurization (ATA Chapter 29)

• Rain repellent nozzles-aircraft windscreen (ATA Chapter 30)

• Water tank pressurization and toilet waste (ATA Chapter 38)

• Air driven hydraulic pump (ADP) (AATA Chapter 29)

Comprehensive list of aircraft systems with which bleed air is associated with civil ATA Chapter classification:

Kinetic heating

Occurs when the aircraft skin heats up due to friction between itself and air molecules. The skin, in turn, heats up the interior of the aircraft such as the cockpit and equipment bays.

100 C

Skin temperatures can reach up to ______ or more in low-level flight at transonic speeds, and even higher temperatures can be reached in supersonic flight at medium and high altitudes.

Ram temperatures

Aircraft leading edges feel the full effect of kinetic heating due to friction and reach what are known as ______________.

Recovery temperature

All other surfaces away from the leading edges are subject to slightly lower temperatures termed __________.

• Kinetic Heating

• Solar Heating

• Avionics Heat Loads

• Airframe System Heat Loads

• Need for Cabin Conditioning

• Need for Avionics Conditioning

Controlled Environment

• Ram Air Cooling

• Fuel Cooling

• Engine Bleed

• Bleed Flow and Temperature Control

Environmental Control Systems

Air Cycle Refrigeration Systems

Turbofan Systems

Bootstrap Systems

Cooling Systems

Aircraft systems

Designed to operate once and only when there is an emergency. Because of this it is not possible to test the systems at the beginning of each flight-the systems are essentially “dormant”.

Reliance

Placed on sound design to ensure that the system will work when is is needed, and on periodic or sample testing. Example of such systems are parachutes, passenger escape slides and ejection seat

Commercial Aircraft List of Cockpit and Cabin Emergency Equipment

90 seconds

With additional research FAA required _________ to totally evacuate a commercial aircraft during cabin fire or at any emergency situation.

Flight Data Recorder (FDR)

Cockpit Voice Recorder (CVR)

Two types of blackbox

Advisory Circular (AC)

Describes an acceptable means for showing compliance with the requirements of Title 14, Code of Federal Regulations (CFR), Part 25, s. 25.795 ( c) “Least risk bomb location”

landing gear extension and retraction, extension of speed brakes, free play in movable surfaces, and system malfunctions.

Causes of airplane vibrations

Flight Crews

Need to understand the causes and effects of airplane vibration so that they can take appropriate action to maintain flight safety and prevent excessive wear or airframe damage.

Vibration

Buffet

Flutter

Noise

Types of Vibration and Noise

Vibration

Oscillating, reciprocating, or any other periodic motion of a rigid or elastic body forced from a position or state of equilibrium.

Harmonic

If the frequency and magnitude of vibration are constant

Random

When the frequency and magnitude vary with time, the vibration is ____________.

Buffet

A form of vibration usually caused by aerodynamic excitation. It usually is random and associated with separated airflow.

Flutter

Un unstable condition in which unsteady aerodynamics excite the natural frequencies of structure over which the air flows. The resulting vibrations can grow to a magnitude that causes the structures to fail

Noise

A vibration that excites the air and can be heard.

Frequencies and magnitudes

All vibrations have associated _____________ that may be readily detected or barely perceptible to the flight crew and passengers.

Normal Vibration

This is a consequence of mass distribution and structural stiffness that result in vibration modes at certain frequencies.

Background noise

When external forces act on the airplane, such as normal airflow over the surfaces, very-low-level vibrations result. Typically, this is perceived as ____________.

Abnormal Vibration

Usually is related to one or more of the following causes; engine rotor imbalance, malfunction of mechanical equipment, and airflow disturbance acting over doors or control surfaces that are mis-rigged or mis-faired or that have excessive wear or free play. It is rarely is caused by structural failure or an unstable power control system

Aeroelastic instability or flutter

Very rarely causes abnormal vibration. Through design, extensive analysis, and certification tests, all configurations of commercial jet airplanes are free from flutter for all design conditions within the aeroelastic stability envelope.

Flutter

Can be differentiated from buffet in that it can occur in smooth air; the vibration originates from the airplane rather than from the atmosphere.

Limited Cycle Oscillation (LCO)

During ________, the vibration is self-excited, but non-linear effects such as friction, clearances, and free play (or backlash) limit the amplitude. It is most often is caused by excessive free play within the flight control surfaces and associated components

Detection of airplane vibration

Depends almost entirely on crew sensibility.

Sight, sound, feel

All other airplane vibration is detected by the crew through (1), (2) and (3).

Vibration and buffet

Both can shake the whole airplane, so it can be difficult for crews to distinguish between them

The response of flight crews to vibration

Fundamentally an exercise in airmanship.

experience

The best tool for gauging the severity of an airplane vibration is the __________ of the flight crew.

airspeed

If performance considerations do not override the severity of the vibration, the flight crew should reduce ________ and engine speed.

High-frequency tactile vibration

This type of vibration can be constant during all phases of flight, but it may vary with airspeed. It is felt in either the hands or feet.

Lower frequency

This type of vibration usually relates to a large-mass component acting on the airframe, such as the rudder, horizontal stabilizer, or elevator. This can be felt by the entire body.

In-flight observations

Can provide essential clues to the source of a vibration.

Vibration

Concerned with the oscillatory motions of bodies and the forces associated with them.

Linear and Nonlinear

Oscillatory systems characterized by

Linear Systems

The principle of superposition holds, and the mathematical techniques available for their treatment are well developed.

Nonlinear systems

Techniques for the analysis of this are less well known, and difficult to apply. However, some knowledge desirable, because all systems tend to become nonlinear with increasing amplitude of oscillation.

Vibratory Motion

Linear Motion

Curvilinear Motion

Oscillatory Motion

Circular Motion

Rotatory Motion

Types of Motion

Free Vibration

Forced Vibration

Two general classes of vibration

Free Vibration

Takes place when a system oscillates under the action of forces inherent in the system itself, and when internal impressed forces are absent.

Forced Vibration

Takes place under the excitation of external forces.

damping

Vibrating systems are all subject to _____________ to some degree because energy is dissipated by friction and other resistance.

Degree of Freedom (DOF)

The number of independent coordinates required to describe the motion of a system.

Periodic Motion

When the motion is repeated in equal intervals of time (t).

Harmonic Motion

The simplest for of periodic motion, often represented as a projection on a straight line of a point that is moving on a circle at a constant speed.

Harmonic Motion

The acceleration is proportional to the displacement and is directed toward the origin.

J. Fourier (1768-1830)

French mathematician who showed that any periodic motion can be represented by a series of sines and cosines that are harmonically related.

Fast Fourier Transform (FFT)

A computer algorithm commonly used to minimize the computation time.

Peak Value

Generally indicates the maximum stress that the vibrating part is undergoing. It also places a limitation on the “rattle space” requirement.

Average Value

Indicates the steady or static value, somewhat like dc level of an electric current.

Mean square value

Found from the average of the squared values, integrated over some time interval T

Root mean square (rms)

The square root of the mean square value.

Decibel

A unit of measurement that is frequently used in vibration measurements.

octave

When the upper limit of a frequency range is twice its lower limit, the frequency span is said to be an _________.

Rayleigh

Showed that with a reasonable assumption for the shape of the vibration amplitude., it is possible to take into account previously ignored masses and arrive at a better estimate for the fundamental frequency

Unbalance

In rotating machines, it is a common source of vibration excitation.

Static unbalance

When the unbalanced masses all lie in a single plane, the resultant unbalance can be detected by a static test in which the wheel-axle assembly is placed on a pair of horizontal rails.

Dynamic unbalance

When the unbalance appears in more than one plane, the resultant is a force and a rocking moment.

Rotor balancing machine

Machines to detect and correct the rotor unbalance, essentially, the balancing machine consists of supporting bearings that are spring-mounted so as to detect the unbalanced forces by their motion.

Whirling

The rotation of the plane made by the bent shaft and the line of centers of the bearings.

Shaft whirling

Inertial forces from rotating eccentric mass will tend to push the shaft away from its axis of rotation

Vibration

A mechanical phenomenon in which the oscillations occur about an equilibrium point.

Critical Speed

If the speed of rotating shaft reaches its Natural Frequency, Resonance occurs in the shaft which causes the shaft to vibrate violently in transverse vibration with very high amplitude and this ultimately leads to failure of shaft. That is why the Resonance speed of the shaft is called as _____________.

Modes of vibration

The different types in which the system tries to oscillate naturally, i.e. without any excitation force in vibrational analysis.

modal frequency (or natural frequency)

The frequency of oscillation is termed as

Mode Shape

The shape made by the system in vibrational analysis.

Modal analysis

A technique to study the dynamic characteristics of a structure under vibration excitation.

Modal

The simplest analysis and the only thing it does is finding out what are the “resonance frequencies” of the geometry.

Synchronous whirl

The whirling speed θ is equal to the rotation speed ω

Viscous damping

Damping that is proportional to the velocity of the system.

Under earthquake excitation

The pistons of the viscous damper installed in the structure generates a reciprocating motion, which will force the viscous fluid to flow through the orifices, and then the damping force is generated and the structural dynamic responses are reduced.

Isolation system

Attempts either to protect a delicate object from excessive vibration transmitted to it from its supporting structure or to prevent vibratory forces generated by machines from being transmitted to its surroundings.

Transmissibility (TR)

The ratio of the transmitted force to that of the disturbing force

Conservation of energy

The term used to describe the physics phenomenon that energy cannot be created or destroyed. It can only be converted from one form into another.

Energy dissipation

Helps us to understand more about physical interactions.