Question 239-1 : A normal shock wave ? [ Formation assignment ]
Can occur at different points on the aeroplane in transonic flight
Question 239-2 : 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
.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 en/com080 152 jpg. in this example mcrit = 0 72 .on transonic passenger jet you must flight below mcrit since if you encounter turbulence the air flow around the wing may reach mcrit on the upper surface and buffeting can appear furthermore the center of pressure will move aft from its low transonic position at about 25% of the chord to its high transonic value at 45% of the chord
Question 239-3 : An aeroplane is descending at a constant mach number from fl 350 what is the effect on true airspeed ?
It increases as temperature increases
.mach number is a dimensionless quantity representing the ratio of speed and the local speed of sound .the speed of sound is function of the temperature .speed of sound in kt = 39 x sqrt temperature in k°
Question 239-4 : A jet aeroplane is cruising at high altitude with a mach number that provides a buffet margin of 0 3g incremental in order to increase the buffet margin to 0 4g incremental the pilot must ?
Fly at a lower altitude and the same mach number
.by flying at a lower altitude with the same mach number you will decrease the low speed buffet the stall margin is increased
Question 239-5 : The critical mach number of an aeroplane is the mach number ?
Above which locally supersonic flow exists somewhere over the aeroplane
Question 239-6 : 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 m0 6 on most commercial jets to the elevators to counteract this and to provide speed stability
Question 239-7 : Which statement is correct about a normal shock wave ?
The airflow changes from supersonic to subsonic
. /com en/normal and oblic shock wave jpg.
Question 239-8 : When the air is passing through a shock wave the static temperature will ?
Increase
. /com en/normal and oblic shock wave jpg.
Question 239-9 : When the air is passing through a shock wave the density will ?
Increase
. /com en/normal and oblic shock wave jpg.
Question 239-10 : When air has passed through a shock wave the local speed of sound is ?
Increased
. /com en/normal and oblic shock wave jpg.
Question 239-11 : If the mach number of an aeroplane in supersonic flight is increased the mach cone angle will ?
Decrease
. /com en/com080 602 jpg. /com en/com080 308 jpg.the higher the mach number is the narrower the cone angle will be
Question 239-12 : 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 239-13 : 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 oblique/normal 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 corner .airflowing through an expansion fan goes through the following changes .1 the airsteam accelerates and the air behind the shock wave has higher supersonic velocity.2 the airflow direction follows the surface contour.3 the static pressure behind the expansion wave decreases.4 the density behind the expansion wave decreases.5 there is no loss of energy . /com en/com080 369 jpg. oblique/normal wave . /com en/normal and oblic shock wave jpg.
Question 239-14 : 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 oblique/normal 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 corner .airflowing through an expansion fan goes through the following changes .1 the airsteam accelerates and the air behind the shock wave has higher supersonic velocity.2 the airflow direction follows the surface contour.3 the static pressure behind the expansion wave decreases.4 the density behind the expansion wave decreases.5 there is no loss of energy . /com en/com080 369 jpg. oblique/normal wave . /com en/normal and oblic shock wave jpg.
Question 239-15 : 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 oblique/normal 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 corner .airflowing through an expansion fan goes through the following changes .1 the airsteam accelerates and the air behind the shock wave has higher supersonic velocity.2 the airflow direction follows the surface contour.3 the static pressure behind the expansion wave decreases.4 the density behind the expansion wave decreases.5 there is no loss of energy . /com en/com080 369 jpg. oblique/normal wave . /com en/normal and oblic shock wave jpg.
Question 239-16 : 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 direction .oblique at an angle to the direction of flow .a normal shock wave has a higher loss in total pressure and a higher compression
Question 239-17 : 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 direction .oblique at an angle to the direction of flow .a normal shock wave has a higher loss in total pressure and a higher compression
Question 239-18 : To increase the critical mach number a conventional aerofoil should ?
Have a low thickness to chord ratio
Question 239-19 : The critical mach number can be increased by ?
Sweepback of the wings
Question 239-20 : 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 en/com080 152 jpg. in this example mcrit = 0 72 .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 reduced .if 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 earlier .therefore if weight is reduced the angle of attack is reduced this increasing the mcrit speed limit
Question 239-21 : 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 considered .for those questions use the very simple ertm diagram . /com en/com080 336 jpg.the eas/ias line is vertical because the question states climb at constant ias . ertm for e as/ r as rectified air speed or cas / t as/ m ach .ias corrected for instrumental and static error = cas .cas = 1/2 rho v².density will decrease during climb and since we want to maintain constant ias tas will increase .mach = tas / local speed of sound.local 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 239-22 : 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 layer .at low speed these vortices essentially sweep away the boundary layer and excited air close to the wing and the trailing control surfaces .vortex generators decrease shock wave induced flow separation by transfer energy from the free airflow into the boundary layer .the 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 en/com080 162 jpg
Question 239-23 : Tuck under will happen ?
Only above the critical mach number
. /com en/com080 418 jpg
Question 239-24 : High speed buffet is induced by ?
Boundary layer separation due to shock waves
Question 239-25 : If a symmetrical aerofoil is accelerated from subsonic to supersonic speed the aerodynamic centre will move ?
Aft to the mid chord
Question 239-26 : At what speed does the front of a shock wave move across the earth's surface ?
The ground speed of the aeroplane
. /com en/com080 352 jpg.the front of a shock wave move across the earth's surface at the ground speed of the aeroplane
Question 239-27 : 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 tuck .by 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 speed .to 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 gradient .because 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 239-28 : 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 239-29 : 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 it .2 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 oblique/normal 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 corner .airflowing through an expansion fan goes through the following changes .1 the airsteam accelerates and the air behind the shock wave has higher supersonic velocity.2 the airflow direction follows the surface contour.3 the static pressure behind the expansion wave decreases.4 the density behind the expansion wave decreases.5 there is no loss of energy . /com en/com080 369 jpg
Question 239-30 : 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 239-31 : In order to provide an adequate 'buffet boundary' at the commencement of the cruise a speed of 1 3vs is used .at a mass of 120000 kg this is a cas of 180 knots .if the mass of the aeroplane is increased to 135000 kg the value of 1 3vs will be ?
Increased to 191 knots drag will increase and air distance per kg of fuel will decrease
.value of 1 3vs at 135000 kg = 180 x sqrt 135000 / 120000 ..value of 1 3vs at 135000 kg = 180 x 1 0607..value of 1 3vs at 135000 kg = 190 926 kt .'sqrt' means 'square root'
Question 239-32 : The speed range between high and low speed buffet ?
Increases during a descent at a constant ias
Mach buffet boundaries .mach buffet is a function of the speed of the airflow over the wing not necessarily the speed of the aircraft any time that too great a lift demand is made on the wing whether from too fast an airspeed or from too high an aoa near the mmo the 'high speed' buffet occurs there are also occasions when the buffet can be experienced at much lower speeds known as the 'low speed mach buffet' .an aircraft flown at a speed too slow for its weight and altitude necessitating a high aoa is the most likely situation to cause a low speed mach buffet this very high aoa has the effect of increasing airflow velocity over the upper surface of the wing until the same effects of the shock waves and buffet occur as in the high speed buffet situation the aoa of the wing has the greatest effect on inducing the mach buffet at either the high speed or low speed boundaries for the aircraft .if the ias remains constant during a descent the mach number will also steadily reduce this is because tas reduces and the local speed of sound increases low speed buffet will therefore occur at a lower mach number the speed range between high and low speed buffet will increase
Question 239-33 : 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 en/com080 418 jpg
Question 239-34 : What is the significance of the maximum allowed cruising altitude based on the 1 3 g margin at this altitude ?
A manoeuvre with a load factor of 1 3 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 1 3 this value is an operating limitation but not a regulatory one the corresponding altitude is called the '1 3g buffet limited altitude' or 'buffet ceiling'
Question 239-35 : 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 239-36 : A transonic mach number is a mach number ?
At which both subsonic and supersonic local speeds occur
Question 239-37 : During a climb at a constant ias the mach number will ?
Increase
Question 239-38 : 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 239-39 : 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 number .shock 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 thanks ..an 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 239-40 : A normal shock wave is a discontinuity plane ?
That is always normal to the local flow
~
Exclusive rights reserved. Reproduction prohibited under penalty of prosecution.
