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Question 255-1 : A body is placed in a certain airstream. the density of the airstream decreases to half of the original value. the aerodynamic drag will decrease with a factor ? [ Explanation maintenance ]
2.
.the drag d formula is d = cd 1/2 rho v² s..where cd = drag coefficient.rho = density.v = tas in m/s.s = surface..if rho is divide by 2, drag will decrease with a factor 2.
Question 255-2 : The point, where the aerodynamic lift acts on a wing is ?
The centre of pressure.
.the pressure created by an aerofoil at any point may be represented by a vector at right angles to its surface, whose length is proportional to the difference between absolute pressure at the point and the free stream static pressure..all of them can be represented by a single vector acting at a particular point, called the centre of pressure. 669.the centre of pressure is a theoretical point on the chord line through which the resultant of all forces the total reaction is said to act..its position is usually around 25% of the way from the leading edge, simply because more lift is generated there, but it moves steadily forward as the angle of attack is increased, until just before the stalling angle, when it moves rapidly backwards the centre of pressure's most forward point is just before the stalling angle. this is why an aeroplane's nose drops when the wings stall and the centre of pressure moves behind the cg...thus, when speed is increased in straight and level flight on a positively cambered aerofoil, you have to decrease the angle of attack to keep the the total lift force constant and the point where the resultant of all forces are acting the centre of pressure moves aft.
Question 255-3 : The location of the centre of pressure of a positively cambered aerofoil section at increasing angle of attack will ?
Shift forward.
.the pressure created by an aerofoil at any point may be represented by a vector at right angles to its surface, whose length is proportional to the difference between absolute pressure at the point and the free stream static pressure..all of them can be represented by a single vector acting at a particular point, called the centre of pressure. 669.the centre of pressure is a theoretical point on the chord line through which the resultant of all forces the total reaction is said to act..its position is usually around 25% of the way from the leading edge, simply because more lift is generated there, but it moves steadily forward as the angle of attack is increased, until just before the stalling angle, when it moves rapidly backwards the centre of pressure's most forward point is just before the stalling angle. this is why an aeroplane's nose drops when the wings stall and the centre of pressure moves behind the cg...thus, when speed is increased in straight and level flight on a positively cambered aerofoil, you have to decrease the angle of attack to keep the the total lift force constant and the point where the resultant of all forces are acting the centre of pressure moves aft.
Question 255-4 : The si unit of measurement for density is ?
Kg/m³.
Question 255-5 : The unit of measurement of pressure is ?
Psi
Question 255-6 : A laminar boundary layer is a layer, in which ?
No velocity components exist, normal to the surface.
. vertical axis is also call normal axis. by normal they mean perpendicular to the wing surface..the airflow in the laminar boundary layer is parallel to the wing surface, in a horizontal direction, and hence there are no flows perpendicular to the surface.
Question 255-7 : The subsonic static pressure ?
Decreases in a flow in a tube when the diameter decreases.
. /com en/com080 815.jpg.the static pressure decreases in a venturi a tube where the diameter decreases , and the airflow speed increases.
Question 255-8 : The lift and drag forces, acting on a wing cross section ?
Depend on the pressure distribution about the wing cross section.
.the distribution of velocities, with resulting pressure distribution, is determined by the shape or profile of the surface and the angle of attack cl or cd..the aerodynamic force of lift results from the pressure differential between the top and bottom surfaces of the wing..drag is the product of dynamic pressure, drag coefficient and surface area.
Question 255-9 : The lift force, acting on an aerofoil no flow separation ?
Is mainly caused by suction on the upperside of the aerofoil.
Question 255-10 : The relative thickness of an aerofoil is expressed in ?
% chord.
Question 255-11 : The aerofoil polar is ?
A graph of the relation between the lift coefficient and the drag coefficient.
. /com en/com080 1264.jpg.airfoil and aircraft polars have been used for a long time in aeronautics. the correct german term is lilienthal'sches polardiagramm , indicating that this method of graphing results of airfoil tests has been used first by otto lilienthal. he plotted the lift and drag forces of his experimental results as lift versus drag, as we still use it today using coefficients instead of the actual forces.
Question 255-12 : Aspect ratio of a wing is the ratio between ?
Wing span squared and wing area.
.aspect ratio is the ratio of the square of the wing span to the total wing.planform area..you could have two aerofoils of equal surface area but different aspect ratios, depending on what they were designed for. the higher the aspect ratio of length to width, the more lift you get with less induced drag at the tips..a wing with higher aspect ratio with long wingspan and small chord reaches higher lift coefficient, but stalls at a lower angle of attack than the wing with low aspect ratio.
Question 255-13 : Dihedral of the wing is ?
The angle between the 0.25 chord line of the wing and the lateral axis.
.the dihedral is the angle between the wings and the horizontal, looked at from the front, where the wingtips are higher than the roots..it is an angle that raises the centerline of the wing tip above the centreline of the root. /com en/com080 208.jpg..notice horizon isn't a plane, neither an axis...the angle between the 0.25 chord line of the wing and the lateral axis is the correct answer.
Question 255-14 : The induced drag ?
Increases as the lift coefficient increases.
Question 255-15 : Flap extension at constant ias whilst maintaining straight and level flight will increase the ?
Maximum lift coefficient clmax and the drag.
.lift = cl x 1/2 rho v² x s..cl = lift coefficient.rho = density.v = tas in m/s.s = surface..clmax available increases with all types of flap. so will drag. the pilot adjusts attitude and power to maintain speed and straight and level flight.
Question 255-16 : When trailing edge flaps are extended whilst maintaining straight and level flight at constant ias ?
The centre of pressure moves aft.
.to maintain straight and level flight at constant ias while deploying trailing edge flaps, you have to lower the nose decreasing angle of attack , so your lift coefficient will remain constant..the centre of pressure moves backward when the angle of attack decreases.
Question 255-17 : Which of the following situations leads to a decreasing stall speed ias ?
Decreasing weight.
Question 255-18 : The difference between ias and tas will ?
Decrease with decreasing altitude.
.true air speed tas is obtained from indicated air speed ias by correcting for instrument, position, compressibility and density errors...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.if altitude increases, difference between ias and tas increases. inversely, if altitude decreases, difference between ias and tas decreases...for rapid calculation.tas = ias + 2% per 1000 ft altitude.
Question 255-19 : Compared with stalling airspeed vs in a given configuration, the airspeed at which stick shaker will be triggered is ?
Greater than vs.
.a stick shaker is a mechanical device to rapidly and noisily vibrate the control yoke the 'stick' of an aircraft to warn the pilot of an imminent stall. it is connected to the control column of most business jets, airliners and military aircraft..when the angle of attack alpha approaches a critical value, the stick is made to shake as a warning...example stalling airspeed vs is 100 kt, stick shaker activation is at 110 kt the airspeed at which stick shaker will be triggered is greater than vs , to give enough time to the pilot to react, by increasing speed and/or reducing the angle of attack.
Question 255-20 : The term angle of attack in a two dimensional flow is defined as ?
The angle between the wing chord line and the direction of the relative wind/airflow.
Question 255-21 : The critical angle of attack ?
Remains unchanged regardless of gross weight.
.the angle at which the stall occurs is called the critical angle of attack..a fixed wing aircraft by definition is stalled at or above the critical angle of attack rather than at or below a particular airspeed. the airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor, bank angle, the center of gravity of the aircraft and other factors. however the aircraft always stalls at the same critical angle of attack.
Question 255-22 : Comparing the lift coefficient and drag coefficient at normal angle of attack ?
Cl is much greater than cd.
.for maximum aerodynamic efficiency it is necessary to generate enough lift to balance the weight, while at the same time generate as little drag as possible. the higher the lift/drag ratio the greater the aerodynamic efficiency. therefore, at normal angles of attack cl is much higher than cd between 10 and 20 times greater.
Question 255-23 : Which statement is correct regarding cl coefficient of lift and the angle of attack ?
For a symmetrical aerofoil section, if the alpha is zero, cl is zero.
. /com en/com080 878.jpg.a symmetrical aerofoil need to have a positive angle of attack to produce lift. it the pitching moment is zero, the lift is zero.. /com en/com080 687.jpg.a positively cambered aerofoil asymmetrical aerofoil section with positive camber produces lift with a zero angle of attack.
Question 255-24 : The mean aerodynamic chord mac for a given wing of any planform is ?
The chord of a rectangular wing with same moment and lift.
.for a rectangular wing of constant aerofoil dimensions and constant chord, the mac is just the chord.. same moment and lift means equilibrium. on a rectangular wing, mac permits to define a point where the lift forces acting this single point is about half way out along the wing and at about 25% back from the leading edge.
Question 255-25 : Induced drag may be reduced by ?
An increase in aspect ratio.
.induced drag varies with lift, speed and aspect ratio, is inversely proportional to aspect ratio and v² so multiply by 1/v² , and directly proportional to lift²/cl²/weight².. /com en/com080 38.jpg.aspect ratio is defined as the square of the wingspan divided by the area of the wing planform.
Question 255-26 : The relationship between induced drag and the aspect ratio is ?
A decrease in the aspect ratio increases the induced drag.
.it is reduced because the effect of wing tip vortices is reduced...coefficient of induced drag cdi = cl² / pi x aspect ratio..aspect ratio is defined as the square of the wingspan divided by the area of the wing planform.. /com en/com080 38.jpg.induced drag varies with lift, speed and aspect ratio, is inversely proportional to aspect ratio and v² so multiply by 1/v² , and directly proportional to lift²/cl²/weight².
Question 255-27 : A high aspect ratio wing produces ?
A decrease in induced drag.
.it is reduced because the effect of wing tip vortices is reduced...coefficient of induced drag cdi = cl² / pi x aspect ratio..aspect ratio is defined as the square of the wingspan divided by the area of the wing planform.. /com en/com080 38.jpg.induced drag varies with lift, speed and aspect ratio, is inversely proportional to aspect ratio and v² so multiply by 1/v² , and directly proportional to lift²/cl²/weight².
Question 255-28 : What will happen in ground effect ?
The induced angle of attack and induced drag decreases.
.ground effect changes airflow. /com en/com080 821.jpg.when entering the ground effect, it requires a decrease in angle of attack to maintain the same cl. the induced angle decreases.
Question 255-29 : Which statement is correct about the laminar and turbulent boundary layer ?
Friction drag is lower in the laminar layer
Question 255-30 : Behind the transition point in a boundary layer ?
The mean speed and friction drag increases.
. /com en/com080 247.jpg.the laminar boundary layer, the one at the front of the wing is smooth and thin. inside it the speed of the air reduces smoothly from the free stream value to near zero at the surface. because of this regular reduction in speed the average speed inside the layer is about half the free stream value, and this fact, and the fact that the layer is very thin mean that the total kinetic energy inside the layer is relatively small...the turbulent layer is thicker and the turbulence pulls down free stream air deep into the layer. this has two results. one is that the turbulent layer has more kinetic energy and the other is that near the surface the airflow slows down very rapidly, causing more drag. drag is bad, but kinetic energy is good for the turbulent layer punches through problems and keeps the airflow attached to the wing as you get nearer the stall...in summary, the turbulent layer is thicker, has a higher mean speed within the layer, more drag but more internal energy.
Question 255-31 : When trailing edge flaps are extended in level flight, the change in pitching moment, ignoring any effects on the tailplane, will be ?
Nose down.
.pitch behaviour depends on flap type, wing position, and horizontal tail location. the increased camber from flap deflection produces lift primarily on the rear portion of the wing. this produces a nosedown pitching moment. /com en/com080 253.jpg
Question 255-32 : Extension of leading edge flaps will ?
Increase critical angle of attack.
Question 255-33 : Slat extension will ?
Increase the critical angle of attack.
Question 255-34 : What factors determine the distance travelled over the ground of an aeroplane in a glide ?
The wind and the lift/drag ratio, which changes with angle of attack.
.a headwind will decrease the distance travelled over the ground..a tailwind will increase the distance travelled over the ground...the angle of attack for the best ratio varies with the design of the wing, but is around a third to a quarter of the size of the stalling angle. flying at the best lift/drag ratio is flying at the maximum range speed most lift for the least drag, for the most economy and the most distance for altitude lost..the wind and lift/drag ratio determine the distance travelled over the ground of an aeroplane in a glide.
Question 255-35 : The following unit of measurement kgm/s² is expressed in the si system as ?
Newton.
.si unit kg = mass, and m/s² = acceleration..mass x acceleration = force, and si unit for force is newton.
Question 255-36 : Excluding constants, the coefficient of induced drag cdi is the ratio of ?
Cl² and ar aspect ratio.
.coefficient of induced drag cdi = cl² / pi x aspect ratio...the constant is pi 3.14159... , so excluding the constant, the coefficient of induced drag cdi is the ratio of cl² and ar aspect ratio.
Question 255-37 : One important advantage the turbulent boundary layer has over the laminar type is that ?
It has less tendency to separate from the surface.
.the laminar boundary layer is a very smooth flow, while the turbulent boundary layer contains swirls. the laminar flow creates less skin friction drag than the turbulent flow, but is less stable. boundary layer flow over a wing surface begins as a smooth laminar flow. as the flow continues back from the leading edge, the laminar boundary layer increases in thickness. at some distance back from the leading edge, the smooth laminar flow breaks down and transitions to a turbulent flow. from a drag standpoint, it is advisable to have the transition from laminar to turbulent flow as far aft on the wing as possible, or have a large amount of the wing surface within the laminar portion of the boundary layer. the low energy laminar flow, however, tends to break down more suddenly than the turbulent layer.. /com en/com080 266.jpg
Question 255-38 : When comparing a rectangular wing and a swept back wing of the same wing area and wing loading assume all other factors of importance remain constant , the swept back wing has the advantage of ?
Higher critical mach number.
Question 255-39 : High aspect ratio, as compared with low aspect ratio, has the effect of ?
Decreasing induced drag and critical angle of attack.
.high aspect ratio little downwash, low aspect ratio great downwash. that means that at the same angle of attack, the low aspect ratio wing flies at a lower effective angle of attack than the high aspect ratio wing...suppose..low aspect ratio wing.angle of attack 10.effective angle of attack 5..high aspect ratio wing.angle of attack 10.effective angle of attack 8..at any given angle of attack the high aspect ratio wing will have a higher effective angle of attack and therefore be closer to stall critical alpha than low aspect ratio wing...the effective airflow phenomena explains why high aspect ratio is favorable at low speeds, why elliptical wing is superior drag wise/horrible stall wise, and why ground effect wont let you down.
Question 255-40 : What wing shape or wing characteristic is the least sensitive to turbulence ?
Swept wings.
.turbulence up and down gusts produce transient changes in the angle of attack. different wing planforms give different increases in lift coefficient for each degree of angle of attack. the swept wing is the least sensitive to turbulence.
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