Machmeter
A machmeter is a pitot-static flight instrument that indicates the ratio of the true airspeed TAS (English True Airspeed ) to the speed of sound . This ratio is a dimensionless quantity , called the Mach number . On a Mach meter, the Mach number is displayed as a decimal number with decimal places. An airplane flying at the speed of sound flies with a Mach number of one (Mach 1.00).
When the airspeed approaches the speed of sound, the critical Mach number is first reached, at which the surrounding air initially flows at a part of the aircraft at the speed of sound - usually this is the upper side of the wing . This is where boundary layer detachments and shock waves form , which in aircraft that are not designed for this speed range severely impair flight stability due to flow stall and can lead to a crash. The airspeed determined by the airspeed indicator in this situation relative to the ambient air ( true airspeed ) depends on the ambient pressure, which in turn is dependent on the air pressure, and would therefore be less suitable for reliably displaying the approximation to the speed of sound, especially since the Mach number is not a fixed value , because the speed of sound depends on the altitude and the resulting ambient air temperature. The flown Mach number would therefore first have to be calculated by the pilot taking into account the current flight altitude and ambient temperature or read from a table. This is done for him by the Machmeter.
For many aircraft a maximum operating Mach number M MO (English maximum operating Mach number ) is shown. For example, the maximum permissible speed for an aircraft with an M MO of Mach 0.83 at an altitude of 30,000 ft (9,144 m), at which the speed of sound is 1093 km / h under standard conditions , is 907 km / h.
Older mechanical Machmeters use an aneroid that converts the pitot static pressure into the Mach number. Modern electronic machmeter calculate the conversion to the machmeter value from air pressure values.
The Machmeter is named after the Czech - Austrian physicist Ernst Mach .
literature
- Instrument Flying Handbook . US Federal Aviation Administration, United States Government Printing Office , Washington DC ( Instrument Flying Handbook [PDF]).
