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Question 261-1 : A commercial jet aeroplane is performing a straight descent at a constant mach number with constant mass. the operational speed limit that may be exceeded is ? [ Theoretical lift off ]
Vmo.
Vmo is defined as the maximum operating limit velocity. vmo is expressed in knots calibrated airspeed kcas , while mmo is expressed in mach number. the vmo limit is usually associated with operations at lower altitudes and deals with structural loads and flutter. the mmo limit is associated with operations at higher altitudes and is usually more concerned with compressibility effects and flutter. at lower altitudes, structural loads and flutter are of concern at higher altitudes, compressibility effects and flutter are of concern...in descent and at low altitude vmo..in climb and at high altitude mmo.
Question 261-2 : 'tuck under' is the ?
Tendency to nose down when speed is increased into the transonic flight regime.
Img /com en/com080 418.jpg..
Question 261-3 : Tuck under may happen at ?
High mach numbers.
Tuck under is the tendency to nose down when speed is increased into the transonic flight regime. it may happen at high mach numbers when the center of pressure moves rearward.. /com en/com080 418.jpg..
Question 261-4 : Which of these statements about 'tuck under' are correct or incorrect. i. 'tuck under' is caused by an aft movement of the centre of pressure of the wing.. ii. 'tuck under' is caused by a reduction in the downwash angle at the location of the horizontal stabiliser. ?
I is correct, ii is correct.
.'tuck under' is the tendency to nose down when speed is increased into the transonic flight regime.. /com en/com080 418.jpg.'tuck under' or mach tuck is caused principally by two basic factors. 1 shock wave induced flow separation, which normally begins near the wing root, causes a decrease in the downwash velocity over the elevator and produces a tendency for the aircraft to nose down.. 2 aftward movement of the center of pressure, which tends to unbalance the equilibrium of the aircraft in relation to its center of gravity cg in subsonic flight.
Question 261-5 : An a 310 aeroplane weighing 100 tons is turning at fl 350 at constant altitude with a bank of 50 degrees. its flight mach range between low speed buffeting and high speed buffeting goes from.. err a 081 100 ?
M= 0.69 to m higher than 0.84
. /com en/com080 100.jpg..
Question 261-6 : With increasing altitude and constant ias the static lateral stability 1 and the dynamic lateral/directional stability 2 of an aeroplane with swept back wing will ?
1 increase 2 decrease.
First statement.static stability refers to the initial tendency, or direction of movement, back to equilibrium. in aviation, it refers to the aircraft's initial response when disturbed from a given aoa, slip, or bank...use the very simple ertm diagram ertm for e as/ r as rectified air speed or cas / t as/ m ach.. /com en/com080 103a.jpg.the ias eas line is vertical because the question states increasing altitude and constant ias...what can we see.tas increase with altitude at constant ias...so the dihedral effect will roll the wings level faster with a faster tas..dihedral effect. /com en/com080 103b.jpg.roll moment tends to return wing to level condition...second statement.dynamic stability refers to the aircraft response over time when disturbed from a given aoa, slip, or bank...when turbulence or rudder application causes the aircraft to yaw to one side, the right wing presents a longer leading edge perpendicular to the relative airflow. the airspeed of the right wing increases and it acquires more drag than the left wing. the additional drag on the right wing pulls it back, turning the aircraft back to its original path..but the roll rate to level the wings will also increased with an increasing tas the cycle is repeated..so swept back wing will decrease the dynamic lateral/directional stability.
Question 261-7 : A jet transport aeroplane is in a straight climb at a constant ias and constant weight. the operational limit that may be exceeded is ?
Mmo.
Vmo/mmo is defined as the maximum operating limit speed. vmo is expressed in knots calibrated airspeed kcas , while mmo is expressed in mach number. the vmo limit is usually associated with operations at lower altitudes and deals with structural loads and flutter. the mmo limit is associated with operations at higher altitudes and is usually more concerned with compressibility effects and flutter. at lower altitudes, structural loads and flutter are of concern at higher altitudes, compressibility effects and flutter are of concern...in descent and at low altitude vmo..in climb and at high altitude mmo.
Question 261-8 : What data may be obtained from the buffet onset boundary chart ?
The values of the mach number at which low speed and mach buffet occur at different weights and altitudes.
.here is a buffet boundary graph for an a330.. /com en/com080 151.jpg..
Question 261-9 : Mcrit is the free stream mach number at which ?
Somewhere about the airframe mach 1 is reached locally.
.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..
Question 261-10 : Which of the following 1 aerofoils and 2 angles of attack will produce the lowest mcrit values ?
1 thick and 2 large.
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.
Question 261-11 : The consequences of exceeding mcrit in a swept wing aeroplane may be assume no corrective devices, straight and level flight ?
Buffeting of the aeroplane and a tendency to pitch down.
We are not flying at low speed, this is not a stall condition eg tendency to pitch up for a swept wing aeroplane..at high speed, the center of pressure will move aft from its low transonic position at about 25% of the chord to a value at 45% of the chord. this will result in a tendency to pitch down..the mach trim system will prevent this phenomenon tuck under. /com en/com080 418.jpg.mach trim is automatically accomplished above mach.615 by adjusting the elevators with respect to the stabilizer as speed increases.
Question 261-12 : A jet aeroplane cruises buffet free at constant high altitude. which type of stall is most likely to occur if this aeroplane decelerates during an inadvertent increase in load factor ?
Accelerated stall.
An accelerated stall is a stall that occurs while the aircraft is experiencing a load factor higher than 1 g, for example while turning or pulling up from a dive. in these conditions, the aircraft stalls at higher speeds than the normal stall speed which always refers to straight and level flight. accelerated stalls are often caused by abrupt or excessive control inputs. if you're in a dive and pull back with enough suddenness and force to load the airplane to a typical design load factor of 2.5 g's, you'll enter an accelerated stall if the airspeed drops below 1.58 times the stall speed at 1 g loading the square root of 2.5 is approximately 1.58..a stall at a load factor greater than 1, even if the aircraft is decelerating, is an accelerated stall.
Question 261-13 : As the mach number increases from subsonic to supersonic, the centre of pressure moves ?
To the mid chord position.
Question 261-14 : The aft movement of the centre of pressure during the acceleration through the transonic flight regime will ?
Increase the static longitudinal stability.
.transonic flight regime between sonic and supersonic speeds, only part of the airflow over the wing is above the speed of sound...the aft movement of the centre of pressure will increase the initial tendency to return to the original state of equilibrium after being disturbed on the lateral axis longitudinal stability because more down force on the tail is required to maintain level flight thus a more important force will be necessary for disturbing this equilibium... 686..notice lateral stability decreases in the transonic range but it is not caused by the centre of pressure shift. it is caused by asymmetric shock wave generation on either wing.
Question 261-15 : In supersonic flight aerofoil pressure distribution is ?
Rectangular.
The pressure distribution top and bottom on a subsonic aerofoil is curved with a peak adjacent to the centre of pressure. /com en/com080 164a.jpg..in supersonic flight there is no peak in the pressure, distribution it being almost equal over the chord with the centre of pressure at 50%. this makes it retangular. /com en/com080 164b.jpg..
Question 261-16 : Shock stall ?
Occurs when the lift coefficient, as a function of mach number, reaches its maximum value.
.when explaining the basic principles of flight, we consider air to be incompressible at speedsless than 0.4 mach. that is, pressure is considered to have no effect on air density. at speeds higher than 0.4m it is no longer practical to make that assumption because density changes in the airflow around the aircraft begin to make differences to the behaviour of the aircraft...at high altitude, a large high speed jet transport aircraft will be cruising at a speed marginally above its critical mach number, and will have a small shock wave on the wing. if such an aircraft increases its lift by pitching up = lift coefficient increases the shock wave will rapidly grow larger, causing the static pressure to increase sharply in the immediate vicinity of the shock wave. the locally increased adverse pressure gradient will cause the boundary layer to separate immediately behind the shock wave. this is called a 'shock stall'..'shock stall' occurs when the lift coefficient, as a function of mach number, reaches its maximum value.
Question 261-17 : Which of the following flight phenomena can only occur at mach numbers above the critical mach number ?
Mach buffet.
.mach buffet is a condition in which the wings begin to vibrate at supersonic speeds over the wing. you will have to fly at a speed above mcrit to encounter this phenomenon.
Question 261-18 : Which of the following flight phenomena can occur at mach numbers below the critical mach number ?
Dutch roll.
.you have to fly at a speed above mcrit the critical mach number of an aerofoil is the free stream mach number at which sonic speed is first reached on the upper surface to encounter. tuck under the tendency to nose down when speed is increased into the transonic flight regime. mach buffet mach buffet is a condition in which the wings begin to vibrate at supersonic speeds over the wing. 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 mach numbers above the critical mach number..dutch roll may happen when static lateral stability is much more pronounced than static directional stability, this can occur at mach numbers below the critical mach number.
Question 261-19 : The mach trim system will ?
Adjust the stabiliser, depending on the mach number.
.as speed increases beyond the critical mach number mcrit , shock wave formation at the root of a swept back wing will. reduce lift forward of the cg, and. reduce downwash at the tailplane...together, these factors will generate a nose down pitching moment. at high mach numbers, an aircraft will become unstable with respect to speed instead of an increasing push force being required as speed increases, a pull force becomes necessary to prevent the aircraft accelerating further. this is potentially dangerous..a small increase in mach number will give a nose down pitch which will further increase the mach number. this in turn leads to a further increase in the nose down pitching moment. this unfavourable high speed characteristic, known as 'tuck under' would restrict the maximum operating speed of a modern high speed jet transport aircraft...to maintain the required stick force gradient at high mach numbers, a mach trim system must be fitted. this device, sensitive to mach number, may.. deflect the elevator up. decrease the incidence of the variable incidence trimming tailplane or. move the cg rearwards by transferring fuel from the wings to a rear trim tank..by an amount greater than that required merely to compensate for the trim change. this ensures the required stick force gradient is maintained in the cruise at high mach numbers.
Question 261-20 : The mach trim system will prevent ?
Tuck under.
Img /com en/com080 418.jpg.. mach trim system.a mach trim system provides speed stability at the higher mach numbers. mach.trim is automatically accomplished above mach.615 by adjusting the elevators with respect to the stabilizer as speed increases. the flight control computers use mach information from the adiru to compute a mach trim actuator position. the mach trim actuator repositions the elevator feel and centering unit which adjusts the control column neutral position.
Question 261-21 : When air has passed an expansion wave, the static pressure is ?
Decreased.
Lynx11.expansion waves, like shockwaves, occur in supersonic flow only. in a shockwave the flow may be reduced to subsonic, but through an expansion wave the flow accelerates to a higher supersonic value and so if speed increases pressure decreases.
Question 261-22 : The critical mach number of an aeroplane is the free stream mach number that produces the first sign of ?
Local sonic flow.
Abratinok.why shockwave is not a valid answer.thanks... jomargra.is not related the local sonic flow with the shock wave can we have local sonic flow in the wing without shock wave...critical mach number is the free stream mach number at which local sonic flow exists on any part of the aeroplane. sonic flow not supersonic, therefore not more than m1.0, this is the reason why we don't have a shock wave.
Question 261-23 : In transonic flight the ailerons will be less effective than in subsonic flight because ?
Aileron deflection only partly affects the pressure distribution around the wing.
.because the effect of the aileron on pressure distribution will not be felt ahead of the shockwave.
Question 261-24 : Compressibility effects depend on ?
Mach number.
Compressible flow is the area of fluid mechanics that deals with fluids in which the fluid density varies significantly in response to a change in pressure. compressibility effects are typically considered significant if the mach number the ratio of the flow velocity to the local speed of sound of the flow exceeds 0.3. the most distinct differences between the compressible and incompressible flow models are that the compressible flow model allows for the existence of shock waves.
Question 261-25 : The formula for the mach number is a= speed of sound ?
M= tas / a
.mach number formula is.m = tas / a..tas is true airspeed expressed in knots..'a' is 'local speed of sound' expressed in knots.
Question 261-26 : If the altitude is increased and the tas remains constant in the troposphere under standard atmospheric conditions, the mach number will ?
Increase.
Question 261-27 : Assuming isa conditions, climbing at a constant mach number up the tropopause the tas will ?
Decrease.
Question 261-28 : The speed of sound is determined only by ?
Temperature.
Question 261-29 : In case of supersonic flow retarded by a normal shock wave a high efficiency low loss in total pressure can be obtained if the mach number in front of the shock is ?
Small but still supersonic.
To obtain a high efficiency, the mach number in front of the shock wave should remain supersonic. if mach number is too slow, or equal to the supersonic speed, the shock wave would not appear. if the mach number is too fast, we will obtain a low mach number after the shock wave and a too weak efficiency.
Question 261-30 : The regime of flight from the critical mach number up to approximately m = 1.3 is called the ?
Transonic range.
.subsonic range is from 0 and ends at mcrit..transonic range starts at mcrit up to mach 1.3..supersonic range is from mach 1.3 to approximately mach 5..above mach 5 the hypersonic speed range.
Question 261-31 : Just above the critical mach number the first evidence of a shock wave will appear at the ?
Upper side of the wing.
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 at leading edge...the first evidence of a shock wave will appear at the upper side of the wing where sonic speed is reached.
Question 261-32 : Shock induced separation results in ?
Decreasing lift.
.for larger mach number, shock induced separation is encountered and the flow becomes unsteady, with unsteady shock motions on the upper and lower surface that are out of phase with each other. as the mach number is increased, a steady shock reappears. it is sufficiently strong to induce flow separation..the shock induced separation results in a loss of lift on the upper surface of the airfoil along with increased drag.
Question 261-33 : In the transonic range lift will decrease at the shock stall due to the ?
Separation of the boundary layer at the shock waves.
. /com en/com080 188.jpg.when shock wave interacts with boundary layer flow, flow separation occurs..you will have an increase in aerodynamic drag, and therefore a loss of lift.
Question 261-34 : Is a transport aeroplane allowed to fly at a higher mach number than the 'buffet onset' mach number in 1g flight ?
No, this is not acceptable.
Question 261-35 : The mach number is the ratio between the ?
Tas of the aeroplane and the speed of sound of the undisturbed flow.
.mach number formula is.mach number = tas / lss..tas is true airspeed expressed in knots..lss is local speed of sound expressed in knots.
Question 261-36 : A normal shock wave ?
Can occur at different points on the aeroplane in transonic flight.
Question 261-37 : 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 261-38 : 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 261-39 : 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 261-40 : The critical mach number of an aeroplane is the mach number ?
Above which, locally, supersonic flow exists somewhere over the aeroplane.
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