Mach Meter calculates a Ratio between Indicated Air Speed and Speed of Sound.
With increase in altitude, Speed of sound decreases therefore Mach no. increases.
With decrease in temperature, Speed of sound decreases therefore Mach No. increases.
With decrease in Static pressure Speed of sound decreases therefore Mach No. increases.
Mach Meter gives ratio between Dynamic pressure and static pressure.
Air speed capsule is fed with pilot press.
Altimeter capsule is fed with no press. It is a partially evacuated capsule.
If airspeed increases, airspeed capsule expands.
If altitude increases, altimeter capsule expands.
The movement of both capsules is picked by Ratio Arms and Ranging Arms takes it to the indicator.
Mechanical linkage is in such a way that if airspeed increases or altitude increases or both increases the indicated Mach No. increases.
It suffers from the following errors-
- Instrument
- Pressure
- Maneuver induced error.
In the Machmeter, the effect of position error is maximum, because Static is used twice.
In this position error is also called pressure error.
But in terms of Mach No. these error are very small and are ignored therefore Indicated Mach No. = True Mach No.
No Density error- Mach meter uses Altimeter capsule which caters for change in density.
No Compressibility error- Compressibility of air is a ratio between Dynamic pressure ÷ Static pressure and Mach Meter has been calibrated for this error.
We need Mach meter so that we don’t exceed Critical Mach No. while flying at high altitude, and at a higher TAS.
Limiting Mach No.- Mach no. above which Aircraft Stability and Controllability are adversely effected.
Limiting Mach no. is much higher than Critical Mach No.
Flying above limiting Mach No./Critical Mach No following occurs-
- CL decreases
- C.P. moves back
- Co-efficient of Drag increases.
Force Divergence Mach No (Mcdr) is that Mach No. above Critical mach No. above which there is an exponential increasing in Coefficient of Drag.
MCDR is less than Limiting Mach no.
Usually Indicated Mach number = True Mach number
Air Speed Indicator is used for Aerodynamic handling of the aircraft at low speed.
Mach meter is used for High speed handling of the aircraft.
It is used to prevent the aircraft from exceeding a pre-determined mach number.
Below 0.8 Mach number- No shock waves form
0.8 to 1.2 Mach number- In this region shock waves form and effect the controllability of the a/c and decreases stability of a/c.
In shock waves velocity decreases, pressure increases, temperature increases, density increases.
For example to remember- At red light when traffic stops the traffic speed decreases, (Velocity decreases), due to exhaust from the engine the air pressure increases (Pressure increases), due to high temperature of the engines the temperature of the air increases (Air Temperature increases), due to large number of traffic standing at the red light the density increases (Air Density increases).
Therefore in shock waves- Velocity decreases, Pressure Increases, Temperature increases and density increases.
1.2 to 5 Mach number flow is all Super Sonic- Shock waves are left behind
Hypersonic is above 5 Mach number.