12. Speed Flashcards
What is speed?
Speed is the rate at which distance is covered or,
the rate of change of position.
Speed is expressed as what units?
Mph = Miles per hour
Knots (kts) = Nautical miles per hour
fps or ft/sec = Feet per second
Kph = Kilometres per hour
MACH = Relative to local speed of sound
How do we measure speed on an aircraft?
Pitot tube
Pitot Pressure = Static Pressure + Dynamic Pressure
How does an ASI work?
The simple Airspeed Indicator removes Static Pressure by allowing it to impinge on the outside of a flexible metal capsule that is fed with Pitot Pressure air
What errors does an Airspeed Indicator suffers from?
- Due to small manufacturing imperfections and the need to generate a large needle movement from a small capsule movement.
- Any error is noted when the instrument is tested and calibrated prior to use.
An error card is mounted in the aircraft cockpit near the indicator.
What errors does the ASI suffer from? (3)
Pressure Error
- Position error - static vent can’t read properly
- Manoeuvre Error - AC movement
Density Error - due to calibration with ISA
Compressibility Error - air compression at high speed
Types of Airspeed? (5)
- Indicated Airspeed - directly from ASI
- Rectified Airspeed - IAS after correction for pressure & instrument errors
- True Airspeed - RAS corrected for density error
- Ground Speed - actual speed over the earth
- Mach Number - fraction of the local speed of sound
How is Mach number calculated?
True Air Speed of aircraft (TAS) divided by the local Speed of Sound ( approx. 660kts at sea level) = Mach Number (given as a decimal point of 1)
Levels of Mach flight?
Subsonic - Mach number is less than one (M < 1 )
Transonic - some part of the aircraft is above Mach 1.
Supersonic - all of the airframe is above Mach 1.
Classification of Speeds?
Subsonic Flight - less than M 0.8
Transonic Flight - M 0.8 to M1.2
Supersonic Flight - M1.2 to M 5.0
Hypersonic Flight - above M 5.0
What is mCrit?
If the air flowing over a wing reaches Mach 1 when the wing is only moving at Mach 0.8, then the wing’s critical Mach number is 0.8. At this point a shock wave is generated on the wing’s upper and lower surfaces.
What happens when mCrit is reached?
- Lift is lost, the centre of pressure moves
- Wave Drag is generated.
Result: buffeting and change of pitch angle and need for extra thrust.
Regulations for supersonic flight
Aircraft heading directly out to sea may accelerate to supersonic speed when at least 10 nautical miles out to sea on a flight path diverging by at least 20 degrees from the mean line of the coast.
Supersonic flights with the aircraft pointing
towards the land, turning or flying parallel to
the coast are to take place at least 35 nm from
the nearest coastline.
