Applications of Physics

Introduction

During the twentieth century, more progress has been made in Technology and Science than was made in all the previous centuries since the beginning of time. The Apollo program program has established the reality of travel between the earth and the moon and the possibility of establishing manned research stations on the surface of the moon. Interplanetary communication and exploration has been accomplished. Although less spectacular than space travel, great advances have been made in air transportation and the automatic control of aircraft. The Jumbo jets, Airbuses and Supersonic transport aircraft have brought air travel throughout the world to millions of people who could not have had this advantage a few years ago. All these accomplishments are the result of the application of the laws of physics. In our present age the person concerned with the operation, maintenance, or design of any of the thousands of mechanical and electronics devices in regular use is constantly faced with the application of scientific law.

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A very simple example applicable to millions of people is the driving of an automobile. If the driver understands the physical laws governing the operation of the engine and the driving mechanism of the car and also understands the forces affecting the car as it moves on the street or highway, that driver can operate the automobile more safely and in a manner that will prolong its life. The world in which we live and work is almost completely dependent upon ‘modern conveniences’ such as electrical appliances, automobiles, airplanes, steamships, radio, television, spacecraft and others too numerous to mention in our limited space. Most of the jobs or positions at which we work involve some technical ‘know-how’ and technical know how involves an understanding of basic scientific law. To provide technicians interested in aircraft and space vehicles with basic technical knowledge pertaining to these devices, it is desirable that we examine briefly of the common laws of physics. In our study here we shall explore the areas of mechanics, heat, sound, electricity and magnetism. While studying these principles that the trainee will recognize their many applications to familiar devices.

Wing Construction

Conventional wings are of three general, monospar, two spar and multiple star. True stressed skin wings may have shear webs but no true “spars”. The monospar wing has only one spar, the two spar wing has two spars as the name indicates, and the multispar wing has more than two spars. A wing spar, sometimes called wing beam, is a principle spanwise member of the wing structure. In the metal wing all principle parts are made of aluminum alloy, and the tie rods or brace wires are made of steel. In the wood wing, the spars may be the only members made of wood or both the spars and the compression struts may be of wood. The wires carrying drag loads are called drag wires, and those carrying the loads opposite drag are called antidrag wires. The wing spar for the wood wing must be made of aircraft quality spruce meeting the requirements set forth Federal Aviation Administration Advisory Circular 43.13-1A.

Wing for a Jet Airliner.

The construction of the wing for a modern Jet Airliner such as the Douglas DC-8 provides an understanding of how the great strength required for such a wing is attained. When we consider the weight of the wing itself plus the two jet engines hung under the wing on each side of the airplane plus the weight of the fuel carried in the wing. It seems almost impossible that the wing could be made strong enough to carry the weight alone, much less the additional loads imposed upon it in flight and upon landing. The basic structure of the wing consists of three spars with conventional sheet-aluminum alloy webs and vertical stiffeners. Between the spars are ribs and bulkheads to provide additional strength and form separations between the tank sections. A wing designed and constructed so it is also a fuel tank called a “wet wing”.

Types of Fuselages

In general, we can say that fuselages are classified in three principle types, depending upon the method by which stresses are transmitted to the structure. The three types according to this classification are truss, semimonocoque, and monocoque. A truss is an assemblage of members forming a rigid framework, which may consists of bars, beams, rods, tubes, wires, etc. The truss type fuselage may be subclassified as the Prat truss and Warren trusses. The primary strength members of both Pratt truss and Warren trusses are the four longerons. The longeron is a principle longitudinal member of the airplane fuselage. In the truss type fuselage, lateral bracing is placed at intervals. The lateral structures may be classified as bulkheads, although this is not strictly true from the technical standpoints and the spaces between the bulkheads are called bays. In the original Pratt truss the longerons were connected with rigid vertical and lateral members called struts, but the diagonal members were made of strong steel wire and were designed to carry tension only.

General Construction of fuselages.

Fuselages are designed with a variety of structural components. Truss type fuselages consists of the rigid framework covered with fabric and dope, plywood, fiberglass or metal. The great majority of fuselages are all-metal, semimonocoque in construction. This applies to small, medium and large aircrafts. The Interior structure to which the skin or plating is attached consists of longerons, frames, bulkheads, stringers, gussets and probably intercostal members riveted and bolted together to form a rigid structure which shapes the fuselage. The skin or plating is riveted or bonded to the structure to form the complete unit. It can be noted that the thickness of the skin varies according to position on the fuselage. The required thickness of material for a given section of the fuselage is determined by Engineers during the design and stress analysis of the aircraft.

Aircraft Tabs

Trim Tabs

Trim tabs trims the aircraft in flight. To trim means to correct any tendency of the aircraft to move towards an undesirable flight attitude. Trim tabs control the balance of an aircraft so that it maintains straight and level flight without pressure an the control column, control wheel, or rudder pedal. Note that the tab has a variable linkage which is adjustable from cockpit. Movement of the tab in one direction causes a deflection of the control surface in the opposite direction. Most of the trim tabs installed on an aircraft are mechanically operated from the cockpit trough an individual cable system. However, some aircraft have trim tabs that are operated by an electrical actuator. Trim tabs are either controlled from the cockpit or adjusted on the ground before taking off. Trim tabs are installed on elevators, rudder and ailerons.

Spring Tabs

Spring tabs are similar in appearance to trim tabs, but serve an entirely different purpose. Spring tabs are used for the same purpose as hydraulic actuators, that is to aid in moving a primary control surface. There are various spring arrangements used in the linkage of the spring tabs. On some aircraft, a spring tab is hinged to the trailing edge of each aileron and is actuated by spring loaded push-pull rod assembly which is also linked to the aileron control linkage. The linkage is connected in such a way that movement of the aileron in one direction causes the spring tab to be deflected in the opposite direction. This provides a balanced condition, thus reducing the amount of force required to move the ailerons. The deflection of the spring tabs is directly proportional to the aerodynamic load imposed upon the aileron, therefore, at low speeds the spring tab remains in a neutral position and the aileron is a direct manually controlled surface. At high speeds, however, where the aerodynamic load is great, the tab functions as an aid in moving the primary control surface.

Rigging Checks

Although the dihedral and incidence angle of conventional modern aircraft cannot be adjusted . They should be checked at specified periods and after heavy landings or abnormal flight loads to ensure that the components are not distorted and that the angle within permitted limits. The relevant figures together with permitted tolerances are specified in the appropriate manual for the aircraft concerned, but the actual figures relevant to an individual aircraft are recorded in the aircraft log book.

The usual method of checking rigging angles is by the use of special boards in which are incorporated or on which can be placed an instrument for determine the angle, that a spirit level or clinometer as appropriate. On a number of modern aircraft the rigging can be checked by means of sighting rods and a theodolite.

Verticality of Fin.

After the rigging of the tail planes has been checked, the verticality of the fin relative to a lateral datum can be checked from a given point on the either side of the top of the fin to a given point on the port and starboard tail planes respectively, the measurements should be similar within prescribed limits. When the verticality of the fin stern post has to be checked, it may be necessary to remove the rudder and drop a plumb bob through the rudder hinge attachment holes, when the cord should pass centrally through all holes. It should be noted that some aircraft have the fin offset to the longitudinal centerline to counteract engine torque.

Engine Mountings

Engines attached to the wings are usually mounted with the thrust line parallel to the horizontal longitudinal plane since, due to their disposition along the wing, the outboard engines are often offset a degree or so to enable the slipstream from the propellers to converge on the tail-plane. The check to ensure that the position of the engine, including the degree of offset, is correct depends largely on the type of the mounting, but usually entails a measurement from the centerline of the mounting to the longitudinal center line of the fuselage at a point specified in the relevant manual.

Classes of Landing gear

Types of Landing Gear can be further divided up into
  • Cantilever
  • Semi cantilever
  • Braced structure
Cantilever

Usually employed on the small aircraft, although in clean design it has been used in larger aircraft. It consists of shock absorber attached at its upper end to the aircraft structure and its lower and carried the wheel and the axle. These types do not generally retract and are ‘faired off’ to reduce aerodynamic drag and are satisfactory up to speed of 150 knots.

Semi Cantilever and Braced

These are similar and used commonly on aircraft with a retracting landing gear. In this design the shock absorber has a pivoted attachment at its upper end to the aircraft structure, and is braced sideways by a folding strut anchored to a point on the leg, and connected to the aircraft structure at its upper end. To effect retraction the strut can be broken by means of hydraulic jack, which also drives the leg upwards to full retraction. This jack may also be used to operate the locks which are mandatory at the up and down positions.

Maintenance

Oleo-pneumatic undercarriage should be subjected to inspections similar to those recommended for spring and rubber chord types, such as examinations for cracks or damage to mounting structure, corrosion and wear at pivot points. In addition following maintenance is necessary.

  • Machined surfaces of the strut inner cylinder should be wiped free of dust or dirt at frequent intervals, to prevent damage to the lower cylinder seals. A lint free cloth, soaked in the fluid used in the strut, should be used for this purpose.
  • The extension of the inner cylinder, that is the length of the visible portion of the inner cylinder, should be checked frequently against the center of gravity/loading graphs provided in the approved Maintenance Manual.
  • The strut should be inspected frequently for fluid leaks. If leaks are due to faulty glands the glands may be replaced, but if they are due to a scored inner cylinder, the strut should be changed.
  • Torque links, steering arms and damper attachments should be checked for security and for cracks, wear or any other damage.
  • All moving parts of the undercarriage should be lubricated on assembly, and at the intervals specified in the approved Maintenance Manual.

Bogie Undercarriages

On heavy aircraft, the need to spread the weight over a large area has resulted in the use of multiple wheel undercarriages. The undercarriage unit normally consists of shock absorber strut, at the lower end of which a bogie beam is pivoted, and the axels are attached to each end of the beam. On some aircraft the rear pair of wheels swivels on the bogie beam, and castors when the nose wheel is turned through a large angle, on others the upper torque link member is replaced by a pair of hydraulic jacks which, when nose wheel steering is applied, rotates the whole bogie. Castor or steering prevents excessive torque on the undercarriage leg and minimizes tire scrubbing during turns. For normal operation the swiveling pair of wheels is locked in line with the fixed pair. Break torque at each wheel is transmitted through compensating rods to the shock absorber strut, thus preventing excessive loads on the boogie beam. On retractable landing gear a levelling strut or ‘hop damper’ provides a means of positioning the bogie beam at suitable angles for retraction and landing, this strut is usually connected into the hydraulic system to prevent retraction if the bogie is not at a suitable angle, and combines the functions of hydraulic ram and damper unit.

Maintenance.

In addition to the lubrication, testing and maintenance of landing gear described in previous paragraphs, particular care and higher standards of workmanship are necessary with bogie undercarriages. Since this type of undercarriage is fitted to heavy aircraft, the materials used are of very high strength and great care is taken in the manufacture, heat treatment and finishing of the components. However, these materials are usually more susceptible to failure from scratches, indentations or corrosion, then materials of lower strength. All servicing functions should therefore be carried out with special care, particularly with regard to lubrication, the lack of which could result in corrosion or hydrogen embrittlement. If any surface damage is found during inspection, it should be repaired strictly in accordance with the instructions and limitations specified in the manufacturer’s manuals. Or if no adequate guidance is given, in accordance with an approved repair scheme.

Retraction Tests

Retraction tests should be carried out following replacement of a faulty component, whenever incorrect operation is reported or suspected, and after a hard or over weight landing. The sequence of operation will depend on the particular installation and type of retraction system concerned, and full details should be obtained from the relevant Maintenance Manual. The following procedure is applicable to most retraction landing gears.

  • Raise the aircraft so that the wheels are clear of the ground, and lock lifting Jacks. Ensure that no ground equipment or personnel are in the vicinity of the undercarriages and doors.
  • Configure aircraft for proper operation and pulling necessary CB’s or the software’s should be reset.
  • Connect electrical power and external hydraulic or pneumatic servicing equipment as appropriate.
  • Carry out several retractions and extensions, initially at low power to ensure slow operation, and using both the normal and emergency systems.

Fixed-Gear Inspection.

Fixed landing gear should be examined regularly for wear, deterioration, corrosion, alignment and other factors which may cause failure or unsatisfactory operation. During 100 hour period or annual inspection of fixed gear, the airplane should be jacked up so the gear does not bear the weight of the aircraft. The mechanic should then attempt to move the gear struts and wheels to test for play in the mounting. If any looseness is found, the cause should be determined and corrected.

When landing gear which employs rubber shock(bungee) cord for shock absorption is inspected, the shock chord should be inspected for age, fraying of the braided sheath, narrowing (necking) of the cord and wear at points of contact with the structure. If the age of the shock cord near five years or more, it is advisable to replace it with new cord, regardless of other factors. Cord which shows other defects should be replaced, regardless of age.

Shock absorber of the spring-oleo type should be examined for leakage, smoothness of operation, looseness between the moving parts, and play at the attaching points. The extension of the strut should be checked to make sure that the springs are not worn or broken. The piston section of the strut must be free of nicks, cuts and corrosion.

Aircraft Brake Systems

Brakes are fitted to an Aircraft for the following reasons.
  • To enable the Kinetic energy of the moving aircraft to be converted into heat through the medium of friction, so reducing the forward motion of the aircraft during a landing run.
  • To permit the aircraft to be steered while taxying on the ground.
  • To restrict the forward speed when taxying, residual thrust from the engine often being appreciable.
  • To hold the aircraft stationary against full thrust when the engines are run up for testing purposes.
  • To enable the aircraft to be parked safely.
The brake unit must be light, have good retardation effects, and must be able to dissipate the heat generated whist retarding the aircraft. There are several types in use, and they can be placed in the following groups.
  • Shoe – for light aircraft
  • Drum – for light aircraft
  • Disc, predominantly used on modern aircraft.

Maintenance.

Contamination of the friction surfaces of a brake unit by fluids used in aircraft servicing operations is highly detrimental to brake operation. It is essential therefore, to protect brakes from contamination by fuel, oil, grease, paint remover, de-icing fluid, etc., when operations involving their use are undertaken and the condition of the brake units should subsequently be confirmed by inspection. Installed disc brakes may be inspected for signs of fluid leakage, external damage, corrosion, disc pack wear and overheating, and the associated hydraulic pipes for security, distortion, chafing or leaks. Brake disc pack wear can be checked by measuring wear pin protrusion, the limits being specified in the approved Maintenance Manual.

In some installations a worn disc pack may be exchanged after removing the wheel and thrust or back plate, and without disconnecting the hydraulic system, but in order to carryout a detailed inspection the brake unit must be removed from the axle.

At the periods specified in the approved Maintenance Schedule the brake unit should be removed for inspection and overhaul. The wheel should first be removed and the hydraulic pipe couplings should be disconnected at the brake and fitted with suitable blanks. In some cases fluid will drain from these pipes and bleeding will be necessary

after re-connection, but in other cases connection is by self sealing couplings which isolate the hydraulic system from the brake unit. The brake unit attachment bolts should then be removed and the unit carefully withdrawn.