Mach Flashcards
What is a sound wave?
What type of wave is it?
How fast is the speed of sound?
Is a pressure wave (disturbance) travelling through a medium.
Is a longitudinal wave.
343m/s in air, 331m/s in 0*c air
What equations make up the equation to solve the speed of sound? So what does this mean for conditions? What is the equation?
Adibatic gas law, IGL, Eulers equation & continuity equation
The equations assume ideal gas.
a= SQRT(RmTy)=39SQRT(T)
What is the only thing that effects the speed of sound? Is this necessarily true?
Temperature
Y is constant
Rm = p/p …….. p/p=c so is a fixed ratio
T can change (in kelvin)
No, it depends on the question. Speed of sound is only affected by temperature in air, but is not solely based off temperature, it is also affected by Rm & y which are constants.
Pressure and density DO NOT directly affect the speed of sound, it have an affect on temperature which affects the speed of sound (p/p=RmT)
What is Mach number?
A dimensionless quantity that is a property of air, which is a function of temperature.
Is the ratio to the airspeed (TAS)at a location/speed of sound at that location in the same atmospheric conditions
What are the other Mach numbers?
Free stream Mach (Mfs)= TAS/speed of sound in the freestream
Local Mach number (Ml)= speed of flow at a location/local speed of sound (ie over the wing)
Critical Mach number (Mcrit)= Mach number of Mfs when any Ml=1.0 as this is the value when there is first evidence of local sonic flow and a shockwave can be formed on the a/c.
When calculating the local speed of sound for Mcrit, be sure to use energy equation as the change in velocity results in a change in temperature which will affect the local speed of sound.
When is an a/c in flight considered sub/tans/super sonic?
How is this different to simple airflow?
An a/c in flight has the following values as at a given TAS there are multiple different airflow velocities over the aircraft.
Subsonic Mfs1.2
However, in simple airflow, subsonic is VVs.
Where Vs is speed of sound
What happens to the pressure wave in supersonic flow?
Air particles travel faster than the pressure wave so there is a shock wave that forms
What is the mathematical principal behind the machmeter?
(P-S)/S proportional M^2
Compares relationship between dynamic pressure and static pressure.
Draw the machmeter & explain its principal of operation
For diagram and in detail explanation refer to notes, basics are as follows…
-Static px is in chamber
-Pitot px is in ASI capsule
-Px in ALT capsule is set
They expand and contract due to pressure differences which are geared to show relationship between Pd & Ps which is Mach number
What is the relationship between Mach number and an increase in altitude?
As ALT increases temperature decreases, so the speed of sound decreases. This means due to M=v/a Mach number will increase
What is Vmo?
Is Vne as beyond this point local shockwaves may be created and is indicated by red/white pointer on machmeter.
Does density and compressibility error affect the machmeter?
No.
As it measure ratio of Pd to Ps and changes to density and temperature affect both sides of the pressure ratio equation so it remains free of density issues.
Is calibrated for compressibility error to be compensated for.
So what errors is the machmeter susceptible to?
Instrument error
Blockages/leaks
Position error
Explain instrument and position error
I: is due to errors in manufacturing and mechanical losses from friction. Is approx 0.01M, but doesn’t exists in ADC machmeter.
P: due to position of machmeter on a/c can affect readings. Must be reduced where possible as they need very accurate static pressure readings as used twice in calculation
Where do blockages & leaks occur?
Blockages occur in the pilot and static ports.
Leaks occur in the ASI/ALT capsule
