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AERODYNAMICS > AERODYNAMICS TWO EXAM BANK > Flashcards

AERODYNAMICS TWO EXAM BANK Flashcards

(61 cards)

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1
Q

Define Free Stream Mach (Mfs) number

A

The Mach number of the flow sufficiently clear of an aircraft to be unaffected by it.

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2
Q

Define Local Mach (ML) number

A

The speed of air at a specific location around the aircraft

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3
Q

Why can Local Mach be different to Free Stream Mach

A

ML can be the same as, higher or lower than Mfs.

Due to different parts of the a/c experiencing different local pressure and temperature changes

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4
Q

Define Mach Detachment (Mdet)

A

Marks the upper end of the transonic region & is the speed at which bow wave attaches to a zero radius leading edge aerofoil, with all local Machs are at 1.0 or greater.

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5
Q

Define the following speed ranges:

DRAW THE DIAGRAM

  1. Subsonic
  2. Transonic
  3. Supersonic
  4. Hypersonic
A
  1. Subsonic: All ML <1.0
  2. Transonic: Mcrit to Mdet, Some ML >1.0 some <1.0
  3. Supersonic: All ML >1.0, up to Mfs 5.0
  4. Hypersonic: ML >5.0
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6
Q

Describe the downgoing portion of the CL graph in the transonic range
(point B to C ~0.81Mfs - 0.89Mfs)

A

-Weaker shockwave forms on the lower surface of the wing and continues rearward toward the trailing edge
- Top, much stronger, shockwave remains stationary
-Lower shockwave moves aft of the upper shockwave & a small area of negative lift develops (This is why CL drops between B&C)

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7
Q

Describe the peak of the CL graph, before it drops off (point B ~0.81Mfs)

A

-The top shockwave forms & strengthens at about 70% chord
- Px behind shockwave is higher than ahead.
-No shockwave has formed on the lower surface yet

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8
Q

Describe the trough of the CL graph (point C ~0.89Mfs)

A

-Lower shockwave is at its rear most point
-Area of negative lift is at its strongest
- Any further increase in Mfs results in the upper shockwave moving aft & -ive lift reduces. (pt C to D)

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9
Q

Why do shockwaves form on the upper surface first?

A

Px above the wing is lower than px below the wing, which leads to a decrease in temperature, thus lowering the speed of sound in that region, and therefore air above the wing has a greater Mach number and reaches supersonic speeds before the underside

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10
Q

What factor affects the speed of sound and how does the speed of sound change with increasing altitude?

A

Temperature

Local SoS decreases with an increase in alt due to the reduction in temp.

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11
Q

What are the three phases of the spin?

A
  1. Incipient Spin
  2. Fully developed spin
  3. Recovery
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12
Q

What is the fully developed spin recovery?

A

Altitude - Check (eject?)

PCL - Idle

Turn Needle - Identify direction of spin

Rudder - Full opposite to spin direction

Control Column - Centrally forward

Spin Stops - Centralise rudder, recover from unusual attitude

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13
Q

What is the B/A ratio

A

The relationship between moments of inertia in pitch & roll has a profound effect on spin characteristics and recovery

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14
Q

Explain what B/A ratio sum effect has on the inertial moment of yaw?

A

The inertia in the yawing plane is approx equal to the sum of the rolling & pitching moments, so ‘C’ will always be larger than A & B, thus if:

  • B is greater than A the inertial yawing moment is Anti-spin,
  • A is greater than B then the yawing moment is Pro-spin
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15
Q

What is auto rotation wrt spinning?

A
  • Autorotation occurs when the aircraft has departed controlled flight & has stalled beyond the critical AoA.
  • As a result the aircraft enters a state of self-sustaining roll and yaw.
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16
Q

Why are propellor blades twisted along the length?

A
  • Prop rotational speed has to increase with distance from the hub (tips are fastest)
  • Blade sections have different RAF due diff rotational velocity & therefore diff AoAs
  • TR is decided by AoA, so therefore would vary
  • Twist is rqd to maintain a constant AoA over its length. (BA needs to be reduced towards tip)
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17
Q

Describe:

  1. Centrifugal Twisting Moment (CTM)
  2. Aerodynamic Twisting Moment (ATM)
  3. Whether CTM or ATM is greater
A
  1. Centrifugal force acts from the hub spanwise to the tip. (Mass of the blade wants to align itself with the PoR = fine/decrease BA). Prop gov & Pitch change mechanism must overcome this force.
  2. The pitch-change axis is behind the centre of pressure of the blade, Total reaction acting through the CoP acts to coarsen the propellor under normal operating conditions
  3. CTM > ATM (bad)
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18
Q

What are the propellor induced effects which cause yaw?

A

Asymmetric blade effect (P factor)
Slipstream Effect
Torque Reaction
Gyroscopic Effect

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19
Q

Describe:

  1. Flying for Range
  2. Flying for Endurance
A
  1. Maximising distance travelled for a given amount of fuel (Vimd)
  2. Maximising flight time for a given amount of fuel (Vmp)
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20
Q

Define Gross Fuel Consumption (GFC)

A

GFC = Fuel quantity used per unit time (lbs/hr)

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21
Q

Define Specific Fuel Consumption (SFC)

A

Fuel used per unit power produced

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22
Q

Define Specific Air Range (SAR)
Dependent on?

A

SAR = Air NM flown / fuel used
Both airframe efficiency & engine performance

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23
Q

Describe Recommended Range Speed (RRS) and the three reasons to fly at this speed

True range speed?

A

RRS (LRC) is a speed slightly faster than Vimd (TRS) (usually 10-20% higher than Vimd), which is used when flying for range.

  1. Provides a faster flight time without undue loss of range.
  2. Greater energy mgmt as you don’t end up behind the back of the drag curve. The variation from the TAS/power relationship & the loss of range are negligible at these speeds
  3. when flying at Vimd, turbulence or manoeuvres will cause a loss of height which can only be regained with more power/fuel use, thus a higher speed ensures a margin
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24
Q

What are the types of stability and their definitions?
Sub categories?

A

Static Stability: The immediate reaction of a body after a disturbance

Dynamic Stability: The subsequent behavior of a body over time after a disturbance

Positive, negative & neutral for both

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25
What are the 3 types of STATIC stability and draw diagrams?
Positive: Once body is displaced, its initial reaction is to tend to return to its original pstn. Neutral: Body will come to rest and neither return nor move away from original pstn. Negative: Body will continue to move away from original state
26
Describe: Positive Dynamic Stability Diagram?
After an initial reaction of the body, the amplitude of each overshoot is dampened as it returns to its original position (picture refs damped phugoid)
27
What are the two types of positive dynamic stability?
-Damped Phugoid: after initial reaction each amplitude is dampened as it returns to OG pstn. -Deadbeat convergence: oscillations heavily dampened so body never overshoots OG rest pstn.
28
How does centre of gravity movement rel to CoP affect DIRECTIONAL stability?
A/C viewed from above; disturbed in yaw, vert stab & keel exposed, restoring force pivots a/c around CoG = +ive directional stability. The distance b/t CoG and TP CoP affects directional stab in that a longer moment arm provides a greater restoring force for a given control SFC deflection in comparison to a smaller moment arm.
29
Explain with a diagram, how does roll damping work? Proportional to?
-During roll, the down-going wing AoA is increased and the up-going wing AoA is decreased. -The difference in AoA produces a lift increase on the down-going wing and a lift decrease on the up-going wing Combined effect is that a neutrally static restoring moment in roll is created against the direction of the sideslip & is proportional to the rate of roll.
30
What effect does a dihedral wing shape have on an aircraft and and what is its effect on lateral stability? (Draw Diagram)
- Wingtips are higher than the wingroot. - The wing in direction of side slip (downgoing) has a higher AoA (and therefore increase in lift) - Upgoing wing lower AoA and lift relatively less providing increased lateral stability & countering unwanted bank. (roll damping effect proportional to roll rate) = NUETRAL STATIC STABILITY
31
Explain Dutch Roll
- A/C disturbed in yaw, results in one wing further fwd than the other - Fwd wing travels faster due to yaw & generates more lift as a result = roll created toward retreating wing. - Fwd wing also generates more drag = yaw away from retreating wing, now opposite wing obtains more lift & roll direction reverses. Results in undulating oscillatory motion in directional & lateral planes
32
What effect does flap deployment have on lateral stability?
Lift increase near the wing root moves the CoP inwards towards the fuselage, causing the moment arm to reduce and therefore decreasing lateral stability
33
Describe the effect of increasing altitude at constant EAS on: 1. Longitudinal (pitch) stability 2. Lateral (roll) stability
Overall: stability decreases with altitude. 1. Tail plane restoring moment is reduced due to the shallowing of the AoA with an increase in TAS. (longitudinal stab is reduced) 2. Damping in roll effect is reduced, increasing roll rate due to same effect above (lateral stab decreases)
34
How is the aircraft flown for endurance?
Flown at Vmp (min fuel flow), plus about 10kts for control, at a low to medium altitude
35
What is the equation for Load Factor?
Load factor = Lift/Weight
36
Effect of sweepback on lateral stability when slipping (diagram?)
When an a/c slips, the lower wing presents more span to the RAF & generates more lift that upper wing. This increases the effective dihedral of the wing, increasing the lift it produces, creating a restoring moment. (stable)
37
SGR?
Specific ground range = ground nms flown per unit fuel used
38
Describe incipient phase of spin?
The period b/t departing controlled flight to the point where spin behavior has established consistent pattern. A/C is stalled and is autorotating. It is experiencing an unsteady attitude in pitch & roll, and has not yet established around a vertical axis.
39
Describe a fully developed spin?
A condition of stalled flight, where the a/c has established around a vertical axis and is descending at a high but steady RoD in a helical/spiral motion while experiencing a rapid & regular cycle of roll, pitch and yaw, with fluctuations in airspeed. (self sustained autorotation) Side-slipping is also occurring & nil forward velocity
40
Describe recovery phase of spin?
Pilot inputs must be made to unbalance the forces to stop the autorotation, wings must be unstalled & the unusual attitude recovered from.
41
Incipient spin recovery
PCL - IDLE CONTROLS - NUETRAL ALTITUDE - CHECK RECOVER - FROM UA
42
Describe why auto rotation occurs with reference to Lift & drag Draw the diagram?
Occurs due to a/c entering a wing-drop situation (both wings stall at different points due to being unbalanced & having diff AoAs - one wing stalls more than the other) - Higher AoA for down going wing means it enters a deeper stall (drag increased, lift decreased) - Upgoing wing AoA comparatively less = greater CL than downgoing wing. Drag is increased but is less than downgoing wing. -The result is increase in roll rate due asymmetric lift & PRO roll yaw increases due asymmetric drag. (towards dropping wing)
43
Describe How prop/engine TQ effect occurs?
With prop & eng turning in the same direction, a reaction is created in the opposite direction (Newtons law). This tends to rotate the a/c body around the longitudinal axis & applies more apparent weight on one wheel than the other. (LH wheel on T6) = Yaw to the left for T6 (CW rotation)
44
Describe How slipstream effect occurs?
Prop rotation causes slipstream to have a corkscrew effect, causing it to strike a side of the aft fuselage & vertical fin (LH side for T6 = LH yaw)
45
Describe How gyroscopic precession effect on T/O occurs?
Taildraggers only Prop acts as gyro, when tail is raised, this has effect of applying force to top of prop disc. precession results in yaw away from direction of rotation (LH for CW rotation)
46
Describe How asymmetric blade effect occurs?
- A/C in tail down attitude (increase in AoA) - Downgoing blade has higher AoA to RAF & therefore "bites" more air in comparison to up going blade. - Downgoing therefore produces more thrust - Thrustline shifts right (for CW rotation) - A/C yaws left
47
Define: Plane of rotation Blade angle Pitch Draw these & forces on the prop
Plane of rotation: perpendicular to prop shaft (if prop replaced by plate, plate would represent PoR) Blade angle: Angle b/t chord line of prop blade & the PoR (nothing to do with airflow or AoA) Pitch: A linear measure of blade angle. Low blade angle = fine, high = coarse.
48
When do compressibility effects become noticeable?
0.4M
49
Define the speed of sound
Distance travelled per unit time by a sound wave; The speed at which a very small px disturbance is propagated in a fluid under specified conditions.
50
Define Mach number
Ratio of object speed to to local speed of sound
51
Mcrit? Drag rise?
The lowest free stream Mach number at which the airflow over any part of the a/c reaches the Speed of sound (M1.0). It is also the speed at which the aerodynamic drag begins to increase rapidly due to the formation of a shock wave development
52
Effect of alt on TAS?
TAS ^ as alt ^ for a given EAS
53
Effect of alt on turning? T/F: Roll rate for a given aileron deflection increases with alt due reduction of roll damping
^ Alt = decrease in turn rate & increase in turn radius (due increase in TAS) True
54
Draw the drag/TAS & PWR/TAS graphs - label: Vimd, Vmp & connection. What do we fly Vimd vs. Vmp for? Affect of wind on range?
Vimd: Max range Vmp: Max endurance TWC: To maximise range, fly slower HWC: fly faster
55
What are the 3 moments of inertia & their axis ? Pro spin vs anti-spin controls?
A = roll, anti-spin (longitudonal axis) B= Pitch, pro-spin (lateral axis) C = yaw, can be either (Normal axis) Pro spin controls: elements of a/c design of ctrl inputs that assist/contribute to the a/c remaining in the spin & vice versa for anti.
56
Prop windmilling - what happens to ATM?
It is reversed, may be too much for pitch chg mechanism. (as both CTM & ATM will be acting against it)
57
How to achieve max range for both airframe efficiency & engine performance? How is it affected with alt?
By maximizing TAS/PWR ratio = tangent to zero on TAS/PWR curve, which is in line with Vimd on TAS/Drag curve. TAS/PWR ratio unaffected with ^alt, but curve moves up & right because power increases & TAS increases for a given IAS.
58
What are shockwaves? Characterised by?
Pressure waves that move faster than the speed of sound. Characterised by abrupt changes in px, temp & density.
59
T/F: Stability generally reduces with altitude in all 3 planes?
True
60
What provides STATIC DIRECTIONAL stability? What provides STATIC LONGITUDONAL stability What provides STATIC LATERAL stability
Vert stab & keel surface (Rudder/yaw) (normal axis) Horizontal stabiliser (elevator/pitch) (lateral axis) Wings (aileron/roll) (longitudinal)
61
When would we end up with spiral instability? What can occur?
If directional stability overcomes lateral stability. If no pilot input, can enter a spiral dive (a/c with +ive spiral stab is likely to suffer from dutch roll)

AERODYNAMICS (3 decks)

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