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What are the factors affecting rigidity?
- The mass of the rotor
- The speed of rotation
- The distance at which the mass acts from the centre of rotation (ie moment arm)
How can you compensate for gyroscopic wander?
- Tying gyroscopes: Using precession to apply torques to the gyro to keep the spin axis aligned with a chosen reference datum.
- Correction of information: Allowing the gyro to wander freely and correcting the information. This has its practical limitations and is used almost exclusively for azimuth gyros.
What are the main properties of wave motion?
- Consist of oscillating electric and magnetic fields at right angles to each other and the direction of propagation.
- Require no medium.
- May be reflected, refracted, diffracted and are subject to interference and the doppler effect.
- EM waves pass through many opaque bodies, although will suffer attenuation when doing so.
- Higher EM frequencies are normally attenuated most.
How are sound waves propagated?
They are propagated as longitudinal waves where the particles oscillate about a fixed point in the direction of travel.
What are the factors affecting the velocity of sound?
- The nature of the medium in which it is travelling. Sound travels faster in:
- Water than air due to greater number of particles per unit area.
- Salt water than fresh water
- Warm water than cold water
- Increasing depth of water
What determines the loudness of sound?
The greater the amplitude the louder the sound
Describe reflection of light wrt even and uneven surfaces.
On an even surface the angle of incidence equals the angle of reflection from the normal. On an uneven surface, there will be many normals and therefore scattering of lights in many directions.
The term used to describe the bending of light rays as they travel from one medium to another. If the light travels from a less dense to a more dense medium, the light will bend towards the normal. If from more dense to less dense, it will bend away from the normal.
The bending of light around an obstruction. Light rays will change the direction of propagation when part of the wave-front is cut off by an obstruction.
What is the critical angle?
The angle at which the refraction of light is along the surface of the boundary and where beyond this angle all the light is internally reflected.
Where two sources of coherent waves coincide. If two troughs or peaks coincide, they will reinforce and give increased amplitude whereas if a peak and a trough meet, the resultant would give 0 amplitude.
At what temperature do objects stop generating Infrared radiation?
0 Kelvin (-273 degrees C). Above this temp, all objects have motion which releases IR radiation.
What is the relationship between temp and particle motion?
As temp increases, particle motion increases.
Can you use IR imagery to fly through clouds?
What is attenuation and what affects it?
Attenuation is the gradual loss in intensity of any kind of flux through a medium. The intensity of waves decreases with the inverse square of the distance.
What must you have to have current flow in a circuit?
- An electrical circuit capable of conductiong electricity
- Free electrons
- 2 different electrical charges
What are lines of flux?
- They represent the attractive force in a magnetic field. They also:
- Are concentrated near the poles
- Are continuous
- Lines do not cross
- Tend to repel each other
- Closer the lines of flux, the stronger the field
What is resistance?
Measure of how well a current will flow through a piece of material. The more resistance, the less current a material will conduct.
What is impedance?
It is the total opposition to current flow and is a combination of resistance, inductive reactance and capacitive reatance. Can be thought of as AC resistance of a circuit.
What is capacitance?
The ability of a body to hold charge. Capacitors therefore store and release energy in the form of an electrostatic field by the seperation of two conductors by an insulating material.
What is rectification?
Changes AC into pulsating DC. Half wave rectification uses only the positive half whereas the full wave rectifier uses both positive and negative half cycles.
What are LEDs, CRTs and LCDs?
- LEDs: are light emitting diodes which produces luminescence when current flows in one direction
- CRTs: Cathode Ray Tubes: An electron tube in which electrons emitted from a cathode are formed into a narrow beam and accelerated onto a screen
- LCDs: Liquid Crystal Display: electronically modulated optical device shaped into a thin, flat panel.
What are the different layers of the ionosphere?
- D layer: 12 - 50 miles of Earth's surface. Ionisation not very dense has little effect on the path of radio waves through it. Freq up to 500kHz generally reflected.
- E layer: 50 - 90 miles above the earth. Ionisation fairly dense and up to 3500khz may be reflected with higher freq slight refraction.
- F layer: 120-220 miles. Freq up to 40MHz reflected.
What is a skywave?
A sky wave is a radio transmission that is reflected by ionised gases in the atmosphere.
What is the relationship between freq and wavelength in the radio spectrum?
As freq increases, wavelength decreases.
How do radio waves travel?
Radio waves travel by the oscillations of perpendicular electric and magnetic fields with the electric field parallel to the aerial.
What is polarisation?
Where light rays are made to pass through narrow slits which only allows light parallel to the slights to pass through. Any light perpendicular to these slits will be totally blocked.
What is a secondary radar?
Allows transmission of discrete data from an aircraft to the ground station. Sends an interrogation signal to the a/c and a seprate signal is sent back by the transponder.
What are the types of radar displays?
- PPI: Plan Position Indicator: most common, used by ATC
- RHI: Range Height Indicator used in precision radar approaches for glidepath
- B scope: Used in Strike aircraft to display airbourne threats.
What are the advantages/ disadvantages of continuous wave radar
- Ground clutter and moving targets are easily distinguished
- High mean power can be transmitted giving greater range
- Equipment is less complex than pulse radar
- no range facility with basic system
- seperate aerials necessary
- direct reception by reciever possible
- can only handle a small number of targets
What are the definitions of Apparent Solar time and Mean Solar time?
- AST: The time between successive transit of the real sun over a given meridian.
- MST: The time between successive transits of a non-existent mean sun over a given meridian. A day is always 24 hours.
What is the relationship between GMT and UTC?
UTC= GMT plus/minus 0.7s
What is a solar eclipse and a new moon?
- Solar eclipse: when the sun's disc is blocked by the disc of the moon.
- New moon: Where the moon and the sun are over the observer's meridian at the same time.
What is the period of the Earth's orbit?
One orbit around the sun in 365.25 days.