The term aero engine can, strictly speaking, be applied to any aircraft power unit, but it is normally applied to the specialized piston engines used in airplanes until they were largely succeeded by the JET engine.

The history of the development of the aero engine has been a struggle to combine high power, lightness and reliability. These qualities were also required in the automobile engine, which was developed at much the same time. But even the earliest aircraft demanded more power than the early automobile industry could produce.

Sound is a pressure wave of small magnitude and its speed of propagation in the fluid is called the speed of sound. The airflow around a body creates higher air pressures in the vicinity which travel upstream, giving advance warning of the presence of the body.

Because of this pressure wave the air moves in a curved path ahead of the body, passing around it with the minimum of disturbance.

If the airspeed is greater than the speed of sound, these warning signals cannot propagate upstream at all and no warning is given – this is called supersonic flow. In this situation the air must change direction suddenly when it encounters the body. If the deviation asked of it is small it does so, producing a small-amplitude shock wave attached to the body. If the deviation is large, a large-amplitude shock wave can move ahead of the body (large amplitude waves travel faster) and behind this the air is slowed to subsonic speed. These shock waves are commonly called sonic booms.

At the beginning of last century, aerodynamics began to attract more attention than hydrodynamics with the possibility of flight in air. Because of the concentration of effort, aerodynamics – building upon the theories of hydrodynamics -soon outstripped its parent. It started with the same assumptions as hydrodynamics but with the assumption of incompressibility replacing what was a fact for water. Prandtl showed that the effect of VISCOSITY for flow around streamlined (smooth) bodies was confined to a thin layer immediately adjacent to the body. This region is called the boundary layer. Outside the boundary layer, viscous forces are negligible and consequently potential flow theories apply. The analysis of streamlined bodies enabled airfoil design to advance rapidly.

Most modern proprietary adhesives are based on synthetic rubber/resin formulations. Synthetic adhesives are generally called synthetic resins because the natural adhesives they most resemble are the resin types. There are many variations, including one not found in natural adhesives: the two-part adhesive, where the adhesive is mixed with a separate hardener or catalyst to make it set. Synthetic resins are normally classed as thermoplastic (melting when heated) and thermosetting (heat speeds hardening).

Thermoplastic adhesives include the vinyl resins, a versatile group that stick well to glass and metal, but are also used in many other applications. Polyvinyl acetate (PVA) based adhesive is water-soluble and used for woodwork, ceramic tiling, flooring and general purpose bonding agents.

Acoustics is the science of sound: its various branches deal with the production, transmission, reproduction and recording of sound, as well as the way it propagates and the effect enclosed spaces have upon the way we experience it.

Being a wave motion of air molecules in the atmosphere, sound obeys the rules of reflection, diffraction and dispersion in a similar fashion to the far shorter wavelength electromagnetic waves which we call light, but obviously from, and through, different materials. The wave length difference is, in fact, crucial. Sound waves are generally long enough to be diffracted quite severely by everyday objects, because their wave lengths are usually similar to the dimensions of the objects.

In any auditorium the whole audience must be able to hear clearly the performer or musician on the stage, without undue echoes. For some types of music, notably choral and organ music, a degree of echo or reverberation actually makes the sound more pleasant, but to hear speech clearly this reverberation must be kept to a minimum. The reverberation time (RT) of a concert hall – the time taken for an initial burst of sound to fall to half of its original level – depends on its shape and the materials of which it is made. Auditoria designed for large scale orchestral works are generally oblong-shaped, so there are plenty of reflections from the sides of the hall into the audience. This reverberant field combines with the directly radiated sound from the musicians and lends a pleasing air of ambiance to the music. For speech, the reverberation time of a hall should be less than one second; for chamber music around one to two seconds and for full orchestras over two seconds.

An achromatic lens is a combination of two lenses made of different types of glass, and has considerably less chromatic aberration (false color) than a single lens. All high quality lens systems in modern binoculars, cameras and other optical instruments use achromatic lenses.

A single lens refracts (bends) parallel light and focuses it to a point. The distance of this point from the lens depends upon both the curvature of the lens and the refractive index (light-bending power) of the lens material.

A slightly different refractive index applies to each color of light. Consequently when white light, which is a mixture of all colors, passes through a simple lens, the various colors are dispersed and are focused at different points. This produces an image with the rainbow-colored fringes characteristic of chromatic aberration.

An accelerometer is an instrument for measuring acceleration. There are two types, one for measuring linear (straight-line) accelerations, the other for measuring angular (twisting) accelerations.

Accelerometers may also be used to measure deceleration, such as the braking of a car. They are much used in the motor industry both to measure the forward acceleration and deceleration of a car, and to measure the sideways and up-and-down accelerations caused by cornering and bumpy roads.

Other uses include testing the strength of safety belts (by measuring the deceleration force at which they break when carrying a known load) and studying vibrations in the hulls of ships and the wingtips of aircraft. They are also important components of inertial guidance systems.