Question 262-1 : The critical mach number of an aeroplane is the mach number ? [ Theoretical lift off ]

Question 262-2 : The mach trim function is installed on most commercial jets in order to minimize the adverse effects of ?

Changes in the position of centre of pressure.

As the mach increases the centre of pressure moves aft and the nose of the aircraft will tend to drop mach tuck mach trim is automatically applied above m06 on most commercial jets to the elevators to counteract this and to provide speed stability

Question 262-3 : Which statement is correct about a normal shock wave ?

The airflow changes from supersonic to subsonic.

com ennormal and oblic shock wavejpg

Question 262-4 : When the air is passing through a shock wave the static temperature will ?

Increase.

com ennormal and oblic shock wavejpg

Question 262-5 : When the air is passing through a shock wave the density will ?

Increase.

com ennormal and oblic shock wavejpg

Question 262-6 : When air has passed through a shock wave the local speed of sound is ?

Increased.

com ennormal and oblic shock wavejpg

Question 262-7 : If the mach number of an aeroplane in supersonic flight is increased the mach cone angle will ?

Decrease.

com encom080 602jpg com encom080 308jpgthe higher the mach number is the narrower the cone angle will be

Question 262-8 : The loss of total pressure in a shock wave is due to the fact that ?

Kinetic energy in the flow is converted into heat energy.

Question 262-9 : When the air is passing through an expansion wave the local speed of sound will ?

Decrease.

The effect on pressure temperature density and velocity is the opposite in expansion waves to obliquenormal shock waves expansion wave also known as a prandtl meyer expansion fan when a supersonic airflow passes over a surface that suddenly drops away or rounds corner an expansion fan of an infinite number of shock waves forms at the apex of the corner and diverts the airflow around the cornerairflowing through an expansion fan goes through the following changes 1the airsteam accelerates and the air behind the shock wave has higher supersonic velocity2the airflow direction follows the surface contour3the static pressure behind the expansion wave decreases4the density behind the expansion wave decreases5there is no loss of energy com encom080 369jpg obliquenormal wave com ennormal and oblic shock wavejpg

Question 262-10 : When the air is passing through an expansion wave the mach number will ?

Increase.

The effect on pressure temperature density and velocity is the opposite in expansion waves to obliquenormal shock waves expansion wave also known as a prandtl meyer expansion fan when a supersonic airflow passes over a surface that suddenly drops away or rounds corner an expansion fan of an infinite number of shock waves forms at the apex of the corner and diverts the airflow around the cornerairflowing through an expansion fan goes through the following changes 1the airsteam accelerates and the air behind the shock wave has higher supersonic velocity2the airflow direction follows the surface contour3the static pressure behind the expansion wave decreases4the density behind the expansion wave decreases5there is no loss of energy com encom080 369jpg obliquenormal wave com ennormal and oblic shock wavejpg

Question 262-11 : When the air is passing through an expansion wave the static temperature will ?

Decrease.

The effect on pressure temperature density and velocity is the opposite in expansion waves to obliquenormal shock waves expansion wave also known as a prandtl meyer expansion fan when a supersonic airflow passes over a surface that suddenly drops away or rounds corner an expansion fan of an infinite number of shock waves forms at the apex of the corner and diverts the airflow around the cornerairflowing through an expansion fan goes through the following changes 1the airsteam accelerates and the air behind the shock wave has higher supersonic velocity2the airflow direction follows the surface contour3the static pressure behind the expansion wave decreases4the density behind the expansion wave decreases5there is no loss of energy com encom080 369jpg obliquenormal wave com ennormal and oblic shock wavejpg

Question 262-12 : Compared with an oblique shock wave at the same mach number a normal shock wave has a ?

Higher compression.

Normal at 90° perpendicular to the shock medium's flow directionoblique at an angle to the direction of flowa normal shock wave has a higher loss in total pressure and a higher compression

Question 262-13 : Compared with an oblique shock wave at the same mach number a normal shock wave has a ?

Higher loss in total pressure.

Normal at 90° perpendicular to the shock medium's flow directionoblique at an angle to the direction of flowa normal shock wave has a higher loss in total pressure and a higher compression

Question 262-14 : To increase the critical mach number a conventional aerofoil should ?

Have a low thickness to chord ratio.

Question 262-15 : The critical mach number can be increased by ?

Sweepback of the wings.

Question 262-16 : What is the influence of decreasing aeroplane weight on mcrit at constant ias ?

Mcrit increases as a result of flying at a smaller angle of attack.

The critical mach number of an aerofoil is the free stream mach number at which sonic speed m=1 is first reached on the upper surface com encom080 152jpg in this example mcrit = 072 if the aerofoil is thicker the airflow will have to travel a larger distance in a same time mach 1 will be reached earlier mcrit the speed in front of the aerofoil is reducedif you increase angle of attack the speed of the airflow over the wing is increased during lift generation the airflow is accelerated on the upper side of the wing mach 1 over the wing will be reached earliertherefore if weight is reduced the angle of attack is reduced this increasing the mcrit speed limit

Question 262-17 : Assuming isa conditions which statement with respect to the climb is correct ?

At constant ias the mach number increases.

The mach number is the ratio of the aircraft true airspeed to the sonic velocity at the altitude consideredfor those questions use the very simple ertm diagram com encom080 336jpgthe easias line is vertical because the question states climbat constant ias ertm for e as r as rectified air speed or cas t as m achias corrected for instrumental and static error = cascas = 12 rho v²density will decrease during climb and since we want to maintain constant ias tas will increasemach = tas local speed of soundlocal speed of sound varies only with temperature and the question states assuming isa conditions temperature decreases by 2° per 1000 ft thus if tas increases and lss decreases the mach number increases

Question 262-18 : Vortex generators mounted on the upper wing surface will ?

Decrease the shock wave induced separation.

Many swept wings were found to suffer from separation at transonic speeds because shocks formed on the wing create an increasing pressure that slows the air suddenly and causes flow separation laminar airflow will separate from the airfoil stall easier than turbulent airflow vortex generators cause turbulent high energy little tornado airflow in the boundary layerat low speed these vortices essentially sweep away the boundary layer and excited air close to the wing and the trailing control surfacesvortex generators decrease shock wave induced flow separation by transfer energy from the free airflow into the boundary layerthe advantage of wing devices that create vortices is that a vortex adds energy to the airflow and increases its forward momentum this momentum encourages the airflow to remain attached to the surface of the wing at higher angles of attack than it would otherwise as a result the wing is able to continue generating lift in conditions where it would have stalled this behavior is particularly advantageous on high performance military aircraft that need to be extremely maneuverable at high angles of attack in combat the advantage for commercial airliners is increased safety since the plane is less likely to experience a wing stall during critical stages of flight like takeoff and landing com encom080 162jpg

Question 262-19 : Tuck under will happen ?

Only above the critical mach number.

com encom080 418jpg

Question 262-20 : High speed buffet is induced by ?

Boundary layer separation due to shock waves.

Question 262-21 : If a symmetrical aerofoil is accelerated from subsonic to supersonic speed the aerodynamic centre will move ?

Aft to the mid chord.

Question 262-22 : At what speed does the front of a shock wave move across the earth's surface ?

The ground speed of the aeroplane.

com encom080 352jpgthe front of a shock wave move across the earth's surface at the ground speed of the aeroplane

Question 262-23 : A machtrimmer ?

Corrects insufficient stick force stability at high mach numbers.

As soon as mcrit is exceeded shockwave formation on the top of the wing moves the centre of pressure cp rearwards this generates the nose down pitching moment known as mach tuckby design all aeroplanes must require an increasing push force from the pilot with increasing speed but the illustration shows the effect of the cp moving rearwards is to decrease the basic stick force at speeds above mcrit this is unacceptable and would eventually require an increasing pull force with increasing speedto maintain the required stick force gradient a mach trim system is fitted the mach trim system input gives a resultant stick force to maintain the required stick force gradientbecause the mach trim system maintains the required stick force gradient the tempting answer of increases the stick force per g at high mach numbers is clearly incorrect

Question 262-24 : Whilst flying at a constant ias and at n = 1 as the aeroplane mass decreases the value of mcrit ?

Increases.

At a lower mass at the same speed you have to reduce your angle of attack to maintain the value of lift at a reduced angle of attack the air is less accelerated over the upper wing surface the critical mach number of an aeroplane is the highest speed possible without supersonic flow over the wing consequently as the aeroplane mass decreases the value of mcrit increases

Question 262-25 : Which statement is correct about an expansion wave in a supersonic flow 1 the density in front of an expansion wave is higher than behind it2 the static pressure in front of an expansion wave is higher than behind it ?

1 and 2 are correct.

The effect on pressure temperature density and velocity is the opposite in expansion waves to obliquenormal shock waves expansion wave also known as a prandtl meyer expansion fan when a supersonic airflow passes over a surface that suddenly drops away or rounds corner an expansion fan of an infinite number of shock waves forms at the apex of the corner and diverts the airflow around the cornerairflowing through an expansion fan goes through the following changes 1the airsteam accelerates and the air behind the shock wave has higher supersonic velocity2the airflow direction follows the surface contour3the static pressure behind the expansion wave decreases4the density behind the expansion wave decreases5there is no loss of energy com encom080 369jpg

Question 262-26 : How will the density and static temperature change in a supersonic flow from a position in front of a shock wave to behind it ?

Density will increase static temperature will increase.

Question 262-27 : In order to provide an adequate 'buffet boundary' at the commencement of the cruise a speed of 13vs is usedat a mass of 120000 kg this is a cas of 180 knotsif the mass of the aeroplane is increased to 135000 kg the value of 13vs will be ?

Increased to 191 knots drag will increase and air distance per kg of fuel will decrease.

Value of 13vs at 135000 kg = 180 x sqrt 135000 120000 value of 13vs at 135000 kg = 180 x 10607value of 13vs at 135000 kg = 190926 kt'sqrt' means 'square root'

Question 262-28 : The speed range between high and low speed buffet ?

Question 262-29 : Tuck under is caused by i movement of the centre of pressure of the wing and ii which change of the downwash angle at the location of the stabiliser ?

I aft ii decreasing.

com encom080 418jpg

Question 262-30 : What is the significance of the maximum allowed cruising altitude based on the 13 g margin at this altitude ?

A manoeuvre with a load factor of 13 will cause buffet onset.

In order to maintain a minimum margin against buffeting and ensure good aircraft maneuverability it is necessary to determine an acceptable load factor limit below which buffeting shall never occur this load factor limit is generally fixed to 13 this value is an operating limitation but not a regulatory one the corresponding altitude is called the '13g buffet limited altitude' or 'buffet ceiling'

Question 262-31 : When the mach number is slowly increased in straight and level flight the first shock waves will occur ?

On the upper surface at the wing root.

Question 262-32 : A transonic mach number is a mach number ?

At which both subsonic and supersonic local speeds occur.

Question 262-33 : During a climb at a constant ias the mach number will ?

Increase.

Question 262-34 : As the mach number increases in straight and level flight a shock wave on the upper surface of the wing will ?

Move towards the trailing edge.

Question 262-35 : During which type of stall does the angle of attack have the smallest value ?

Shock stall.

A shock stall is a stall caused by the airflow over an aircraft's wings being disturbed by shock waves when flying at or near to the aircraft's critical mach numbershock stall is the separation of the boundary layer behind the shock wave bcg160 i would like to request a brief explanation or definition about accelerated stall thanksan accelerated stall is caused by abrupt or excessive control movement an accelerated stall can occur during a sudden change in the flight path during manoeuvres such as steep turns or a rapid recovery from a dive it is called an accelerated stall because it occurs at a load factor greater than 1g an accelerated stall is usually more violent than a low speed stall 'normal' stall at 1g and is often unexpected because of the relatively high airspeed

Question 262-36 : A normal shock wave is a discontinuity plane ?

That is always normal to the local flow.

Question 262-37 : An aeroplane is flying through the transonic range whilst maintaining straight and level flight as the mach number increases the centre of pressure of the wing will move aft this movement requires ?

A pitch up input of the stabiliser.

As the mach number increases in the transonic range the centre of pressure will move afta corrective pitch up input on the stabiliser is required 685to maintain equilibrium an increase in downward force is required the blue arrow vector of the total aerodynamic lift vertical to the centre of pressure moves aft red point is the centre of gravitytransonic range starts at mcrit up to mach 13

Question 262-38 : If the mach number is 08 and the tas is 400 kt what is the speed of sound ?

500 kt.

Mach no formula mach no = tas lssmach no is mach numbertas is true airspeed expressed in knotslss is local speed of sound expressed in knotsso in this question 08 = 400lss08 lss = 400lss = 400 08lss = 500 kt

Question 262-39 : In supersonic flight all disturbances produced by an aeroplane are ?

Within a conical zone dependent on the mach number.

Question 262-40 : Mach buffet occurs ?

At the mach number at which shock wave induced boundary layer separation occurs.