Aircraft Aerodynamics Flashcards
(157 cards)
1
Q
What is the composition of the earth’s atmosphere?
A
78% Nitrogen
21% Oxygen
1% Other
2
Q
Define Static Pressure:
A
The static pressure of the air at any altitude results from the mass of air supported above that level.
3
Q
At 40,000 feet what is the percentage of:
1. Static Pressure
2. Density
Compared to standard sea level?
A
- 19% (20%) Static Pressure
- 25% Density
Of the sea level volume.
4
Q
What is the standard sea level temperature, pressure and density?
A
15C
14.7psi / 29.92 in.Hg
1.225 kg/m3 or 0.002378 slugs per cubic foot
(A slug (sl) is an Imperial unit of mass that accelerates by one foot (per second) (per second) (1 ft/sec²) upon force of one pound-force (1 lbf))
5
Q
What is the most important property when studying aerodynamics? (Static Pressure, Temperature, Density, Viscosity, etc.)
A
Density (density altitude and temperature)
6
Q
Define the density of the air:
A
The mass of air per cubic foot and a direct measure of the quantity of matter in each cubic foot.
7
Q
How much does air weigh at a standard sea level condition?
A
0.0765 pounds per cubic foot (3/4 of 1/10 of a pound)
8
Q
Define viscosity:
A
Viscosity is a measure of resistance to flow (thickness). viscosity of gases are unusual because the viscosity is generally a function of temperature alone and an INCREASE in temperature increases the viscosity.
9
Q
Why are the viscosity of gases unusual?
A
Because the viscosity is generally a function of temperature alone and an INCREASE in temperature increases the viscosity.
(Low viscosity flows easily because its molecular makeup results in very little friction when it is in motion. Higher temperature increases the frequency of intermolecular collisions at higher temperatures.)
10
Q
Define Pressure Altitude:
A
The altitude that the altimeter reads if the altimeter is set to the standard sea level pressure value of 29.92. It is not corrected for local pressure variations.
11
Q
Define Density Altitude:
What will cause an increase in density altitude?
A
Density altitude is pressure altitude corrected for nonstandard temperature. As temperature and altitude increase, air density decreases. Density altitude is air density given as a height above mean sea level. The more appropriate term for correlating aerodynamic performance in the nonstandard atmosphere.
Both an increase in temperature, decrease in atmospheric pressure, and, to a much lesser degree, increase in humidity will cause an increase in density altitude.
12
Q
In the lower regions of the atmosphere temperature decreases with altitude at a fairly uniform rate up until what altitude and why?
A
Approximately 39,000’ (36,089’ = Geopotential of the Tropopause).
The atmosphere is warmed by convection from Earth’s surface, this lapse or reduction in temperature is normal with increasing distance from the conductive source.
13
Q
A more usual form of viscosity measure is the proportion of absolute viscosity and:
A
Density
14
Q
In Bernoulli’s tube what stays constant at all stations along the length and what changes?
A
Density of the flow remains constant.
Static pressure and velocity are the variable quantities.
15
Q
Airspeed indicators are simply ________ gauges which measure ________ pressure related to various airspeeds.
A
Pressure
Dynamic (Dynamic pressure is the kinetic energy (energy that it possesses due to its motion) per unit volume of a fluid particle.)
16
Q
The difference between ________ and ________ pressure is the pressure indicated by the pitot static system.
A
Total
Static
17
Q
Define indicated airspeed (IAS):
What are some of the causes of errors between instrument indication and actual flight speed.
A
Actual indication of the instrument.
Factors such as an altitude other than standard sea level, errors in the instrument and installation, compressibility may create a great variance.
18
Q
Define calibrated airspeed (CAS):
A
Correcting IAS for errors of the instrument and errors due to position or location of the installation. Position errors are most usually confined to the static source.
19
Q
Define equivalent airspeed (EAS):
A
Correcting the CAS for compressibility effects due to the fact that the airspeed indication is given an erroneous magnification.
20
Q
Define true airspeed (TAS):
A
Speed of the aircraft relative to the airmass in which it is flying. Equivalent airspeed (EAS) corrected for density altitude. True airspeed increases as air density decreases. Warm air is less dense than cold air, all else being equal so as temperature increases, TAS increases.
21
Q
Why do balls with backspin tend to rise and what is that called?
A
The increased local velocity on the upper surface causes an increase in upper surface suction while the decrease local velocity on the lower surface causes a decrease.
The Magnus Effect
22
Q
Chord Line
A
A straight line connecting the leading edge and the trailing edge of the airfoil (like you’re stretching a line from leading to trailing edge).
23
Q
Chord
A
The length of the chord line. All airfoil dimensions are measured in terms of the chord.
24
Q
Mean Camber Line
A
A line drawn halfway between the upper surface and the lower surface.
25
Maximum Camber
The maximum distance between the mean camber line (a line drawn halfway between the upper surface and the lower surface) and the chord line.
26
Maximum Thickness
The maximum distance between the upper and lower surfaces. A typical low speed airfoil may have a thickness of 12% located 30 aft of the leading edge.
27
Leading Edge Radius
A measure of the sharpness of the leading edge. It may vary from zero for a knife-edge supersonic airfoil to about 2% of the chord for rather blunt leading-edge airfoils.
28
Relative Wind (RW)
The speed and direction of the air impinging on a body passing through it. It is equal and opposite in direction to the flight path velocity.
29
Angle of Attack (AOA)
The acute angle between the relative wind and the chord line of an airfoil.
Not to be confused with angle of incidence which is the angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.
30
Lift
The net force that is perpendicular (90 degrees) to the relative wind.
31
Drag
The net force that is parallel to the relative wind.
32
Center of Pressure (CP)
The point on the chord line where the aerodynamic force acts.
33
Laminar Flow
Smooth airflow with little transfer of momentum between parallel layers.
34
Streamlined Flow
Same as Laminar Flow (smooth airflow with little transfer of momentum between parallel layers).
35
Turbulent Flow
Flow where the streamlines break up and there is much mixing of the layers.
36
The coefficient of drag
Corresponds to a decrease in drag which corresponds to an increase in airspeed at which you fly the plane to optimize the laminar flow airfoil and minimize the drag. This yields a high speed cruise with minimum drag.
37
Which identical aircraft will stall at a higher angle of attack: One with higher weight, dynamic pressure, bank angle or the lesser?
The two will stall at the same angle of attack. Stall speed however will be affected.
38
The speed of the aircraft at which it stalls is affected greatly by what?
Weight
| Bank Angle
39
An airplane has a high L/D ratio if it produces a large amount of lift or a small amount of drag. List four things that occur at (L/D)max:
1. Maximum endurance of jet powered airplanes.
2. Maximum range of propeller driven airplanes.
3. Maximum climb angle for jet powered airplanes.
4. Maximum power-off glide range, jet or prop.
(Minimum drag)
40
Considering power-off glide range, gross weight will / won't affect the gliding performance?
Gross weight will NOT affect the gliding performance if the airplane is flown at the angle of attack for (L/D)max.
Glide ratio is numerically equal to the lift-drag ratio. The gross weight would affect the glide airspeed necessary for this particular angle of attack but the glide ratio would be unaffected.
41
The maximum lift coefficient (CLmax) corresponds to the _______ _____ available in flight.
Minimum Speed (how slow can you go before stall)
42
Any steady turn requires that the vertical component of lift be equal to ______ and the horizontal component of lift equal to the ___________ _____.
(Hint: Forces that are opposite of vertical and horizontal)
Any steady turn requires that the vertical component of lift be equal to weight of the airplane (acting toward the center of the earth) and the horizontal component of lift equal to the centrifugal force (acting perpendicular to weight).
43
Above _____ degrees of bank the increase in load factor and stall speed is _____.
Above 45 degrees of bank the increase in load factor and stall speed is quite rapid. This fact emphasizes the need for avoiding steep turns at low airspeeds - a flight condition common to stall-spin accidents.
44
In a climbing turn the ______ wing (with the greatest rate of turn) has the highest angle of attack and will stall ______ the inboard wing. As a result the aircraft will therefore _________. When the aircraft descends the _____ wing has the highest angle of attack, at the stall the wing will drop _______ the bank angle even more.
In a climbing turn the higher wing (with the greatest rate of turn) has the highest angle of attack and will stall before the inboard wing. As a result the aircraft will therefore roll level. When the aircraft descends the lower wing has the highest angle of attack, at the stall the wing will drop increasing the bank angle even more.
45
Why does stall speed increase in a turn?
Load factor increases
46
List a few high lift devices and state the primary purpose of them:
Flaps, slots, slats.
To increase the CLmax (maximum lift coefficient) of the airplane and reduce the stall speed. With the higher lift coefficient available, less dynamic pressure is required to provide the necessary lift.
47
Flaps are usually what percent of the cord?
15 to 25%
48
Describe a plain flap:
A simple hinged portion of the trailing edge. Drag increases greatly.
49
Describe a split flap:
Consists of a plate deflected from the lower surface and produces a greater change in the maximum lift coefficient than the plain flap. A much larger change in drag results but that may be an advantage to accomplish a steeper landing approach or require a higher power from an engine to minimize engine acceleration time for go around.
50
Describe a slotted flap:
Similar to the plain flap but the gap between the main section and flap leading edge is given specific contours. High energy air from the lower surface is ducted to the flap upper surface. Slotted flap can cause a much greater maximum lift coefficient than the plain or split flap and sections of drag are much lower.
51
Describe a fowler flap:
Similar to the slotted flap except the deflected flap segment is moved aft along a set of tracks which increase the chord and effects an increase in wing area. This causes large increases in the maximum lift coefficient and minimum changes in drag.
52
Talk about compressibility effect at higher speeds:
As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. If the aircraft passes at a low speed, typically less than 250 mph, the density of the air remains constant. But for higher speeds, some of the energy of the aircraft goes into compressing the air and locally changing the density of the air. This compressibility effect alters the amount of resulting force on the aircraft.
53
What is the speed of sound at sea level on a standard day?
661 knots
54
What affects the speed of sound and why?
In our atmosphere the speed of sound is only affected by temperature. As the temperature increases the speed of sound increases. Molecules at higher temperatures have more energy, thus they can vibrate faster. Since the molecules vibrate faster, sound waves can travel more quickly.
55
What is the critical mach number?
The critical Mach number (Mcr) of an aircraft is the lowest Mach number at which the airflow over some point of the aircraft reaches the speed of sound, but does not exceed it.
56
Explain mach tuck:
In aircraft not designed to fly at or above the critical Mach number, shock waves in the flow over the wing and tailplane are sufficient to stall the wing, make control surfaces ineffective or lead to loss of control.
57
Talk about laminar flow:
The flow near the leading edge flows smoothly, giving it the name laminar boundary layer. As the flow continues back friction forces dissipate energy of the airstream and the laminar boundary layer increases in thickness. As it does so it begins to oscillate (Example: smoke from a cigarette in still air). Particles are now traveling from layer to another producing energy change. This should not be confused with the large scale turbulence associated with airflow separation. This airflow now has a turbulent boundary layer with a laminar sub-layer.
58
What are vortex generators and why do they work?
A vortex generator (VG) is an aerodynamic device, consisting of a small vane usually attached to a lifting surface. When the aerofoil or the body is in motion relative to the air, the VG creates a vortex, which, by removing some part of the slow-moving boundary layer in contact with the aerofoil surface, delays local flow separation, thereby improving the effectiveness of wings and control surfaces.
59
As an object moves through an air mass, velocity and pressure changes occur which create pressure disturbances in the airflow surrounding the object. At what speed are these changes occurring?
The speed of sound
60
What speed is subsonic classified as?
Mach numbers below 0.75
61
What speed is transonic classified as?
Mach numbers from 0.75 to 1.20
62
What speed is supersonic classified as?
Mach numbers from 1.20 to 5.00
63
What speed is hypersonic classified as?
Mach numbers above 5.00
64
The Mach number at which an aircraft is flying can be calculated by:
```
M = v / vsound
where:
M is the Mach number
v is velocity of the moving aircraft and
vsound is the speed of sound at the given altitude
```
65
What is a Mach number?
Mach number is a dimensionless quantity representing the ratio of speed of an object moving through a fluid and the local speed of sound.
66
Talk about the speed of sound in relation to temperature and altitude:
The speed of sound has nothing to do with altitude but everything to do with temperature. As temperature decreases speed of sound decreases.
67
Aspect ratio
The aspect ratio of a wing is the ratio of its length to its average chord. A high aspect ratio indicates long, narrow wings, whereas a low aspect ratio indicates short, stubby wings.
68
How can you define center of gravity?
The center of gravity is the average location of the weight of an object. In flight, both airplanes and rockets rotate about their centers of gravity.
69
Relative wind and flight path:
The relative wind is the direction of movement of the atmosphere relative to an aircraft or an airfoil. It is opposite to the direction of movement of the aircraft or airfoil relative to the atmosphere.
70
List the four forces of flight and their relationship:
When an airplane flies, the wing is designed to provide enough Lift to overcome the airplane’s Weight, while the engine provides enough Thrust to overcome Drag and move the airplane forward. An aircraft is in steady flight when all is equal.
71
What is an arm in regards to weight and balance:
The arm is the chordwise (fore-and-aft) distance from the datum (a fixed starting point) to any point within the aircraft.
72
What is moment in regards to weight and balance?
| How do you calculate moment?
The moment is the moment of force, or torque, that results from an object’s weight acting through an arc that is centered on the zero point of the reference datum distance. Moment is also referred to as the tendency of an object to rotate or pivot about a point (the zero point of the datum, in this case). The further an object is from this point, the greater the force it exerts. Moment is calculated by multiplying the weight of an object by its arm.
73
What are some characteristics of an aft CG?
* The airplane will cruise faster with an aft CG location because of reduced drag. The drag is reduced because a smaller angle of attack and less downward deflection of the stabilizer are required to support the airplane and overcome the nose-down pitching tendency.
* The airplane becomes less stable as the CG is moved rearward. This is because when the CG is moved rearward it causes an increase in the angle of attack. Therefore, the wing contribution to the airplane’s stability is now decreased, while the tail contribution is still stabilizing. When the point is reached that the wing and tail contributions balance, then neutral stability exists. Any CG movement further aft will result in an unstable airplane.
* Increase in VMC (due to decreased moment arm from rudder)
* Unstable along the longitudinal axis
74
What are some characteristics of an forward CG and stall?
* The airplane will stall at a higher speed with a forward CG location. This is because the stalling angle of attack is reached at a higher speed due to increased wing loading.
* Higher elevator control forces normally exist with a forward CG location due to the increased stabilizer deflection required to balance the airplane. Slower cruise.
* A forward CG location increases the need for greater back elevator pressure. The elevator may no longer be able to oppose any increase in nose-down pitching. Adequate elevator control is needed to control the airplane throughout the airspeed range down to the stall.
75
Forward CG
Stall Speed:
Cruise Speed:
Stability:
Stall Speed: Faster IAS
Cruise Speed: Slower
Stability: More
76
Aft CG
Stall Speed:
Cruise Speed:
Stability:
Stall Speed: Slower IAS
Cruise Speed: Faster
Stability: Less
77
Angle of incidence
The angle at which the wing or horizontal tail of an airplane is installed on the fuselage, measured relative to the axis of the fuselage.
Not to be confused with angle of attack which is the acute angle between the relative wind and the chord line of an airfoil.
78
Where is the generation of net lift force of an object?:
Lift acts through the center of pressure of the object and is directed perpendicular to the flow direction.
79
If airspeed doubles, lift will increase by how much?
Four times.
80
Induced Drag:
Induced drag is a component of total drag that arises whenever a three-dimensional wing generates lift. At low speeds an aircraft has to generate lift with a higher angle of attack, thereby leading to greater induced drag.
81
Form drag / Profile drag
Form drag is caused by movement of the aircraft through the air. This type of drag, also known as air resistance or profile drag varies with the square of speed. For this reason profile drag is more pronounced at higher speeds.
82
A symmetrical wing has ______% lift at 0 degrees angle of attack.
A symmetrical wing has zero lift at 0 degrees angle of attack.
83
What is the camber of an airfoil?
Camber is the asymmetry between the top and the bottom surfaces of an aerofoil. An aerofoil that is not cambered is called a symmetric aerofoil.
84
What is a refluxed camber airfoil and why would you use it?
An aerofoil where the camber line curves back up near the trailing edge is called a reflexed camber aerofoil. Such an aerofoil is useful in certain situations, such as with tailless aircraft, because the moment about the aerodynamic center of the aerofoil can be 0.
85
Where is the center of pressure and aerodynamic center on a cambered airfoil during large and small angles of attack?
For a conventionally cambered airfoil, the center of pressure (average of all the pressures acting on the airfoil) lies a little behind the quarter-chord point at maximum lift coefficient (large angle of attack), but as lift coefficient reduces (angle of attack reduces) the center of pressure moves toward the rear.
The aerodynamic center (the pitching moment of a wing) lies exactly one quarter of the cord behind the leading edge.
An increase in lift moves the center of pressure toward the aerodynamic center.
86
Most foil shapes require a positive angle of attack to generate lift, but ________ airfoils can generate lift at zero angle of attack.
Most foil shapes require a positive angle of attack to generate lift, but cambered airfoils can generate lift at zero angle of attack.
87
Where is the center of pressure and aerodynamic center lie on a symmetrical airfoil during large and small angles of attack?
On a symmetric airfoil, the center of pressure (average of all the pressures acting on the airfoil) and aerodynamic center (the pitching moment of a wing) lie exactly one quarter of the chord behind the leading edge. The aerodynamic moment about the aircraft is zero for all angles of attack.
88
What is the center of pressure?
The center of pressure is the average of all the pressures acting on the aerofoil and as such it will change in magnitude and position as the pressure patterns around the airfoil changes. The biggest change will be because of changes in angle of attack.
89
What is the aerodynamic center?
The aerodynamic center is a point on the aerofoil that does not move as you change angle of attack. It is a point found at about 25% of the chord between the leading edge and the trailing edge where there will be no change in the pitching moment of the aerofoil as the pressure pattern on the airfoil changes.
The aerodynamic center is defined as a point on the chord line that :
(1) does not move as angle of attack is changed
(2) the pitching moment of a wing is always constant about it
That is why aerodynamicists say it is effectively the point about which all changes in lift act.
90
What is dihedral and why is it important?
Wing Dihedral is the upward angle of an aircraft's wing, from the wing root to the wing tip. The amount of dihedral determines the amount of inherent stability along the roll axis.
On low-wing aircraft, the center of gravity is above the wing and roll stability is less. This factor requires the use of greater dihedral angles in low-wing airplanes. When a wing rolls downward, the relative wind on the descending wing becomes a component of the forward motion of the airplane and the downward motion of the wing. This produces a higher angle of attack on the descending wing and consequently more lift.
91
What is the datum in reference to weight and balance?
The reference datum is a reference plane that allows accurate, and uniform, measurements to any point on the aircraft. The location of the reference datum is established by the manufacturer and is defined in the aircraft flight manual.
92
What does the horizontal stabilizer do for an aircraft?
The tailplane provides stability and control. In many modern conventional aircraft, the center of gravity is placed ahead of the center of pressure of the main wing. The wing lift then exerts a pitch-down moment around the centre of gravity, which must be balanced by a pitch-up moment (implying negative lift) from the tailplane. A disadvantage is that it generates trim drag.
93
What does an increase in angle of attack do to the center of pressure in relation to the aerodynamic center?
An increase in angle of attack moves the center of pressure towards the aerodynamic center which reduces the pitching moment till the stall occurs at which time the center of pressure moves back.
94
What does increasing true airspeed do to the center of pressure and what is its result?
Increasing true airspeed moves the center of pressure back which increases pitching moment.
95
Lift and drag are dependant on: (7)
1. Velocity
2. Density
3. Shape
4. Angle of attack
5. Surface area
6. Compressibility
7. Viscosity
96
Stall speed is influenced by: (4)
1. Weight
2. Center of lift
3. Density
4. Wing Area
The airspeed at which the aircraft stalls varies with the weight of the aircraft, the load factor, the center of gravity of the aircraft and other factors. However the aircraft always stalls at the same critical angle of attack.
97
Principle of ground effect:
When an aircraft is flying at an altitude that is approximately at or below the same distance as the aircraft's wingspan, there is, depending on airfoil and aircraft design, an often noticeable ground effect. This is caused primarily by the ground interrupting the wingtip vortices and downwash behind the wing. When a wing is flown very close to the ground, wingtip vortices are unable to form effectively due to the obstruction of the ground. The result is lower induced drag, which increases the speed and lift of the aircraft.
98
While in ground effect you will need:
A _____ angle of attack:
______ thrust:
While in the ground effect, the wing will require a lower angle of attack to produce the same amount of lift. If the angle of attack and velocity remain constant, an increase in the lift coefficient will result, which accounts for the "floating" effect. Ground effect will also alter thrust versus velocity, in that reducing induced drag will require less thrust to maintain the same velocity.
Low winged aircraft are more affected by ground effect than high wing aircraft.
99
Types of drag:
Parasite: moving a solid object through a fluid
Induced: the result of the creation of lift
Total drag = Parasite + Induced
100
Parasitic drag is a combination of what 3 things?
| Greatest when?
Parasitic drag is a combination of form drag, skin friction drag and interference drag (the presence of multiple bodies in relative proximity).
Increases with speed.
101
Induced drag:
| Greatest at:
Drag resulting in the redirecting of air to cause lift. Greatest at high angles of attack and slow airspeeds.
102
When is total drag the least?
L/D Max is where parasite drag and induced drag are equal and total drag is the least.
103
The back side of the power curve is called:
| What is happening?
The back side of the power curve is called the region of reversed command. To maintain altitude while decreasing your speed below the maximum endurance speed (L/D Max), the amount of power required actually increases, hence the name. As airspeed decreases, the power required to maintain altitude increases. This leads to an extremely important point—a climb is initiated by adding more power, not necessarily by pitching the nose up.
The greater the angle of attack, the greater the drag. By pitching the nose up, all you’re doing is increasing the power required to overcome all of the induced drag you’re creating. It also slows you down. By lowering the nose, you increase your airspeed, decrease the drag, and decrease the power required to overcome it.
104
During a steady climb the rate of climb is based on:
The rate of climb is based on excess power (Vy).
| The angle of climb is based on excess thrust (Vx).
105
During a steady climb the angle of climb is based on:
The angle of climb is based on excess thrust (Vx).
| The rate of climb is based on excess power (Vy).
106
How does air density effect thrust produced?
Air density has a profound effect on the thrust produced. The volume of the air flowing through the engine is relatively fixed for any particular rpm by the size and geometry of the inlet duct system. Since the thrust is determined by mass, not the volume of air, any increases in its density increases the mass and thus the thrust.
107
How does altitude affect thrust produced?
Altitude has a double effect on thrust. As the altitude increases, the air becomes colder (denser), up to the beginning of the stratosphere. This causes the thrust to increase. At the same time, the increase in altitude causes a decrease in pressure, thus a decrease in density and corresponding decrease in thrust. Since the loss of thrust caused by decreasing pressure is greater than the increase caused by decreasing temperature. Thus the thrust decreases as the aircraft ascends.
At the beginning of stratosphere (approximately 36,000') temperature stabilizes at -56.5C and remains at this temperature up to around 85,000'. The pressure continues to fall above the 36,000 feet and the thrust therefore drops off at a faster rate than it does at the lower altitudes. This increased drop off in thrust makes 36,000' a good altitude for a long range cruise in jet powered aircraft.
108
Explain ram effect in jet engines:
The compression of air in an inlet duct arising from forward motion is called ram pressure or ram effect. The faster the aircraft moves the greater the mass flow through the engine.
109
Longitudinal stability is:
| What could improve this?
Longitudinal stability is the stability of an aircraft in the longitudinal plane (lateral axis (pitch)) under steady-flight conditions.
It is greatest with a large horizontal stabilizer and a forward cg.
110
Lateral stability is:
| What could improve this?
Stability about the airplane's longitudinal axis (rolling) which extends from nose to tail, is called lateral stability. This helps to stabilize the lateral or rolling effect when one wing gets lower than the wing on the opposite side of the airplane.
There are four main design factors which make an airplane stable laterally - dihedral, keel effect, sweepback, and weight distribution.
111
Keel effect:
Keel effect is the result of the side force-generating surfaces being above (or below) the center of mass (which coincides with the center of gravity) in any aircraft. Examples of such surfaces are the vertical stabilizer, rudder, and parts of the fuselage. In a slight slip, the fuselage provides a broad area upon which the relative wind will strike, forcing the fuselage to parallel the relative wind. This aids in producing lateral stability.
112
Directional stability:
| What could improve this?
Directional stability is associated with the realigning of the longitudinal axis with the flight path after a disturbance causes the aircraft to yaw out of alignment and produce slip; remember yaw is a rotation about the normal (vertical) axis. The restoring moment – the static stability.
It is greatest with a large vertical stabilizer and forward CG.
113
Trim tab movement is what in relation to the primary control surface?
Movement of the trim tab (like the servo tab) is opposite that of the primary control surface.
114
Explain servo tabs:
They are on the trailing edge of the control surface and are mechanically linked to move opposite the direction of the surface. This "servo" movement helps move the control surface.
115
Explain anti-servo tabs:
They move in the same direction as the primary control surface. This means that as the control surface deflects, the aerodynamic load is increased by movement of the anti-servo tab. This helps to prevent the control surface from moving to a full deflection. It also makes a hydraulically-boosted flight control more aerodynamically effective than it would otherwise be.
116
Why do jet aircraft have control tabs?
For use in the event of loss of all hydraulic pressure. Movement of the control wheel moves the control tab which causes the aerodynamic movement of the control surface. The control tab is used only during manual reversion; that is, with the loss of hydraulic pressure. They work the same as a servo tab but only in the manual mode.
117
What is a mass balance in a control surface?
A weight placed forward of the hinge line to bring CG of surface up to the hinge line to prevent flutter or vibration.
118
Relation of control and stability in an aircraft:
The more controllable the aircraft the less stable and the more stable the aircraft the less controllable.
119
Positive static stability:
After a disturbance if the aircraft tends to restore itself to its original speed and orientation, without human or machine input, the vehicle is said to be statically stable. The aircraft has positive stability. For a vehicle to possess positive static stability it is not necessary for its speed and orientation to return to exactly the speed and orientation that existed before the minor change that caused the upset. It is sufficient that the speed and orientation do not continue to diverge but undergo at least a small change back towards the original speed and orientation.
If such a change causes further changes that tend to drive the vehicle away from its original speed and orientation, the vehicle is said to be statically unstable. The aircraft has negative stability.
If such a change causes no tendency for the vehicle to be restored to its original speed and orientation, and no tendency for the vehicle to be driven away from its original speed and orientation, the vehicle is said to be neutrally stable. The aircraft has zero stability.
120
Negative static stability:
After a disturbance, if such a change causes further changes that tend to drive the vehicle away from its original speed and orientation, the vehicle is said to be statically unstable. The aircraft has negative stability.
If the aircraft tends to restore itself to its original speed and orientation, the vehicle is said to be statically stable. The aircraft has positive stability. For a vehicle to possess positive static stability it is not necessary for its speed and orientation to return to exactly the speed and orientation that existed before the minor change that caused the upset. It is sufficient that the speed and orientation do not continue to diverge but undergo at least a small change back towards the original speed and orientation.
If such a change causes no tendency for the vehicle to be restored to its original speed and orientation, and no tendency for the vehicle to be driven away from its original speed and orientation, the vehicle is said to be neutrally stable. The aircraft has zero stability.
121
Dynamic stability:
The dynamic stability of an aircraft refers to how the aircraft behaves after it has been disturbed following steady non-oscillating flight.
122
Proverse roll:
Proverse roll is the tendency of an airplane to roll in the same direction as its yawing due to forward moving wing producing more lift.
123
Adverse yaw:
Adverse yaw is the undesirable tendency for an aircraft to yaw in the opposite direction of a roll. It is caused by the difference in profile drag between the upward and downward deflected ailerons, the difference in lift and thus induced drag between left and right wings, as well as an opposite rotation of each wing's lift vector about the pitch axis due to the rolling trajectory of the aircraft.
124
Dutch roll:
Dutch roll is a type of aircraft motion, consisting of an out-of-phase combination of "tail-wagging" and rocking from side to side. Dutch roll stability can be artificially increased by the installation of a yaw damper.
As a swept-wing aircraft yaws (to the right, for instance), the left wing becomes less-swept than the right wing in reference to the relative wind. Because of this, the left wing develops more lift than the right wing causing the aircraft to roll to the right. This motion continues until the yaw angle of the aircraft reaches the point where the vertical stabilizer effectively becomes a wind vane and reverses the yawing motion. As the aircraft yaws back to the left, the right wing then becomes less swept than the left resulting in the right wing developing more lift than the left. The aircraft then rolls to the left as the yaw angle again reaches the point where the aircraft wind-vanes back the other direction and the whole process repeats itself.
125
Wake turbulence:
Wake turbulence is turbulence that forms behind an aircraft as it passes through the air. This turbulence includes wingtip vortices and jetwash. Jetwash refers simply to the rapidly moving gases expelled from a jet engine; it is extremely turbulent, but of short duration. Wingtip vortices, on the other hand, are much more stable and can remain in the air for up to three minutes after the passage of an aircraft.
126
Mach varies with?
Temperature: Decreases with a decrease in temperature.
127
For a constant Mach number as altitude increases, TAS will?
TAS will decrease because temperature decreases with increasing altitude.
128
Mcrt:
What does this stand for?
Will it increase or decrease with a higher altitude?
Mcrt Critical Mach is the speed at which airflow first reaches the speed of sound but doesn't exceed it. It is lower at higher temperature.
129
What are some advantages and disadvantages of a swept wing aircraft?
Swept wing increases Mcrt but poor slow speed characteristics (wing tips stall first).
Moves CL further aft, increasing longitudinal stability.
Increases the effect of dihedral to achieve greater lateral stability.
130
Shock Stall Mach Tuck:
As an aircraft exceeds Mcrt, a shock wave forms on the wing root that can cause a shock stall. Local sonic flow creates a shock wave symmetrically near the wing root and a loss of down wash on the elevator results.
131
What is happening in a slip?
| Why is this happening and what are some results?
During a slip the yawning moment is to the outside of the turn. This is a result from not applying enough rudder.
Turn rate decreases and turn radius increases. Horizontal component of lift is greater than centrifugal force.
132
What is happening in a skid?
| Why is this happening and what are some results?
During a skid the yawning moment is to the inside of the turn. This is a result from applying too much rudder.
Turn rate increases and turn radius decreases. Horizontal component of lift is less than centrifugal force. This is more dangerous than a slipping turn if close to stall because in a skid the lower wing will stall first.
133
For the same AOA, TAS _________ with altitude.
For the same AOA, TAS increases with altitude.
134
Lift equation:
AOA x Velocity = Approximate Lift
135
For the same AOA what will TAS do with an increase in altitude?
Increase with altitude
136
What two factors affect lift in a turn?
AOA and airspeed
137
What increases turn radius?
Less angle of bank
Increased airspeed
Decrease load (G)
138
What affects turn rate?
Velocity and load
139
What is a major advantage of a swept wing and why?
Generally have a higher cruising speed due to a higher critical Mach number.
140
What are some swept wind disadvantages? (3)
1. Reduced lift requiring the need for high lift flaps and slats.
2. At low airspeed the wingtips tend to stall first, resulting in a loss of aileron effectiveness.
3. Poor yaw tendencies which could developes into Dutch Roll (As a swept-wing aircraft yaws (to the right, for instance), the left wing becomes less-swept than the right wing in reference to the relative wind. Because of this, the left wing develops more lift than the right wing causing the aircraft to roll to the right. This motion continues until the yaw angle of the aircraft reaches the point where the vertical stabilizer effectively becomes a wind vane and reverses the yawing motion. As the aircraft yaws back to the left, the right wing then becomes less swept than the left resulting in the right wing developing more lift than the left. The aircraft then rolls to the left as the yaw angle again reaches the point where the aircraft wind-vanes back the other direction and the whole process repeats itself).
141
Stall speed doesn't change with altitude except at high altitudes it _________ due to _________.
Stall speed doesn't change with altitude except at high altitudes it increases due to compressibility and viscosity but it is negligible.
Stall speed is directly related to AOA and it remains the same, but the TAS where the stall occurs increases with altitude because of the lower air density.
142
What is a deep stall? What types of aircraft are more likely to get into one?
A deep stall (or super-stall) is a dangerous type of stall that affects certain aircraft designs, notably those with a T-tail configuration. In these designs, the turbulent wake of a stalled main wing "blankets" the horizontal stabilizer, rendering the elevators ineffective and preventing the aircraft from recovering from the stall.
143
A higher the gross weight will have what effect on stall speed and why?
Increase stall speed.
Stalls depend only on angle of attack, not airspeed. However, the slower an airplane goes, the more angle of attack it needs to produce lift equal to the aircraft's weight. Greater weight takes more lift takes more angle of attack.
144
A higher load factor will have what effect on stall speed and why?
The normal stall speed, specified by the VS values, always refers to straight and level flight, where the load factor is equal to 1g. However, if the aircraft is turning or pulling up from a dive, additional lift is required to provide the vertical or lateral acceleration, and so the stall speed is higher. An accelerated stall is a stall that occurs under such conditions.
145
What is an accelerated stall?
The normal stall speed, specified by the VS values, always refers to straight and level flight, where the load factor is equal to 1g. However, if the aircraft is turning or pulling up from a dive, additional lift is required to provide the vertical or lateral acceleration, and so the stall speed is higher. An accelerated stall is a stall that occurs under such conditions.
146
What is the effect of flap extension?
Flaps increase camber and lifting ability which allows the wings to support the same load at a lower airspeed.
147
What do slats do?
Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack (increase camber). A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances.
148
What do slots do?
A leading edge slot is a fixed aerodynamic feature of the wing of some aircraft to reduce the stall speed and promote good low-speed handling qualities. A leading edge slot is a spanwise gap in each wing, allowing air to flow from below the wing to its upper surface. In this manner they allow flight at higher angles of attack and thus reduce the stall speed.
149
Maneuvering load factor: effect of velocity and gross weight?
At high gross weights the limit and ultimate load factors are reduced. It is easier to exceed the limits with a heavier aircraft. Same with a faster aircraft. It is easier to exceed the limit load factor.
150
Specifically, parasite drag increases:
Increases with the square of its speed
151
Induced drag is proportional to:
The angle of attack
152
What factors affect V1?
```
Higher gross weight increases V1
Flap settings
Density altitude
Temperature
Contamination will decrease V1
```
153
What is the relationship between AOA and TAS as altitude increases?
As altitude increases, TAS increases for a given IAS and AOA. TAS increases with an increase in altitude and increase in temperature.
154
A +10% in speed causes how much more in landing distance?
+20%
155
When are wingtip vortices the strongest?
Heavy, Clean, Slow
156
What effect does altitude have on a certain indicated airspeeds mach number?
Mach number goes up as altitude increases. The speed of sound decreases in colder air.
157
What effect does altitude have on TAS stall speed?
The TAS stall speed increases. Increased density decreases stall speed so the higher the altitude the higher the true stall speed.
