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Flight Dynamics > Static Stability > Flashcards

Static Stability Flashcards

1
Q

Define static stability for an aircraft.

A

The static stability of an aircraft is the tendency of that aircraft to develop restoring forces and moments when it is perturbed from a steady-state position.

Ref: Yechout Pg 196

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2
Q

For aircraft static stability what is the general evaluation reference?

A

Aircraft stability is generally evaluated relative to a steady-state trimmed flight condition.

Ref: Yechout Pg 196

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3
Q

If an aircraft is perturbed by a gust of wind and it develops a moment that returns it to its original position, this is termed ______ stability.

A

Positive static

Ref: Yechout Pg 196

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4
Q

If an aircraft is perturbed by a gust of wind and it develops no moment to return it to its original position, what type of stability is this?

A

Neutral stability.

Ref: Yechout Pg 196

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5
Q

If an aircraft in steady-state flight is perturbed and the aerodynamic forces that result cause it to diverge further from its original steady-state position, what type of static stability is this?

A

Negative stability

Ref: Yechout Pg 196

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6
Q

For longitudinal static stability, C_m_alpha must be _____.

A

Negative

Ref: Yechout Pg 199

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7
Q

True or False

The higher the absolute value of a control derivative the more moment is generated for a given control deflection.

A

True.

Ref: Yechout Pg 199

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8
Q

What is a positive elevator control surface deflection?

Sign convention – Do not confuse with moment convention

A

Trailing edge down (TED).

Ref: Yechout Pg 199

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9
Q

What is a positive horizontal stabilizer control deflection?

A

A positive incidence angle of the horizontal stabilizer is leading-edge up (LEU).

Ref: Yechout Pg 199

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10
Q

When is aileron deflection considered positive?

A

Trailing Edge Down (TED)

Note: A positive aileron deflection occurs when the trailing edge moves down. This does not include composite ailerons.

Ref: Yechout Pg 199

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11
Q

With respect to static stability, what defines a positive rudder deflection?

A

Tailing Edge Left

Note that this is different from Dr. Smith’s convention.

Ref: Yechout Pg 199

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12
Q

What are cross-control derivatives?

A

Cross control derivatives define the change in moment that results from the change in a control surface that is not the primary control surface for that axis.

For example, how does the moment on the rudder change with the ailerons are deflected by a pilot.

Ref: Yechout Pg 200

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13
Q

What comprises aircraft longitudinal motion?

A

Pitch rotation (rotation about the y-axis), X-axis translation, and Z-axis translation.

Ref: Yechout Pg 200

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14
Q

What comprises aircraft lateral motion?

A

Roll, yaw rotation, Y-axis translation.

Ref: Yechout Pg 200

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15
Q

A general aircraft moment coefficient varies with what three parameters related to control surface deflections?

A
  1. Angle of attack
  2. Elevator deflection
  3. Incidence angle of the horizontal stabilizer

CM = f(alpha, delta_E, i_H)

Ref: Yechout Pg 198

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16
Q

The angle between the horizontal stabilizer chord line and the fuselage reference line (x-body axis) is defined as positive for __________.

A

TEU conditions.

Ref: Yechout Pg 198

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17
Q

For symmetrical aircraft, C_l_0 and C_n_0 are usually _____.

A

Zero

Ref: Yechout Pg 198

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18
Q

A positive elevator deflection will result in a ______ pitching moment.

A

Negative (nose down)

This is different from Dr. Smith’s convention. In his case, a positive elevator deflection is up and the positive pitching moment causes the nose of the aircraft to rise.

Ref: Yechout Pg 199

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19
Q

A positive deflection of the horizontal tail will typically result in a ______________.

A

Negative pitching moment.

Ref: Yechout Pg 199

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20
Q

A positive aileron deflection will typically result in a _________ moment.

A

Positive, right-wing down pitching moment.

Ref: Yechout Pg 199

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21
Q

A positive rudder deflection will typically result in a ______ moment.

A

Negative nose left yawing moment.

Ref: Yechout Pg 199

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22
Q

What are the signs of the primary control derivatives?

A

C_m_delta_e < 0
C_l_delta_a >0
C_n_delta_r <0

Ref: Yechout Pg 199

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23
Q

What are the three stability derivatives?

A

C_m_alpha
C_l_beta
C_n_beta

Ref: Yechout Pg 198

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24
Q

What are the four primary control derivatives?

A

C_m_delta_e
C_m_i_h
C_l_delta_a
C_n_delta_r

Ref: Yechout Pg 199

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25
Q

What are the two cross-control derivatives?

A

C_l_delta_r
C_n_delta_a

Ref: Yechout Pg 200

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26
Q

What three parameters affect the drag coefficient?

A

Alpha
i_h
delta_e

Ref: Yechout Pg 201

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27
Q

What three parameters affect the drag coefficient?

A

Alpha
i_h
delta_e

Ref: Yechout Pg 203

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28
Q

Give the equation for the angle of attack experienced by a wing.

A

alpha_wing = alpha + i_w

Where i_w is the incidence angle of the wing.

Ref: Yechout Pg 204

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29
Q

To calculate the angle of attack of the horizontal tail ____ must be taken into consideration.

A

Downwash.

Ref: Yechout Pg 204

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30
Q

True or False
downwash decreases the effective incidence angle of the relative wind at the horizontal tail from that experienced at the nose of the aircraft.

A

True

Ref: Yechout Pg 204

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31
Q

What is elevator effectiveness (tau_e)?

A

It is a ratio that relates a change in the angle of attack to a change in the incidence angle of the elevator (delta_e).

Ref: Yechout Pg 205

32
Q

True or False
the larger the cord of the elevator relative to the overall cord of the airfoil the larger the elevator effectiveness will be.

A

True

Ref: Yechout Pg 205

33
Q

If the aircraft has a symmetric tail, the value C_L_0_h will be _____.

A

Zero.

Ref: Yechout Pg 206

34
Q

A positive i_h deflection produces a ____ increment on pitching moment.

A

Negative.

Ref: Yechout Pg 209

35
Q

A positive delta_e deflection produces a _____ increment on pitching moment.

A

Negative.

Ref: Yechout Pg 209

36
Q

Define the aerodynamic center.

A

The aircraft aerodynamics center is the point on the aircraft where the variation of aircraft pitching moment coefficient with angle of attack is equal to 0.

Note: the location of the aircraft’s aerodynamic center is normally defined with respect to the mean court of the wing.

Ref: Yechout Pg 211

37
Q

If C_m_alpha is zero, where is the aerodynamic center?

A

At the CG.

Ref: Yechout Pg 211

38
Q

What are some ways to move the aerodynamic center aft?

A
  1. Increase X_AC_h
  2. Increase S_h
    3 Increase C_L_alpha_h

Ref: Yechout Pg 211

39
Q

True or False
moving the aircraft aerodynamics center aft will increase static longitudinal stability and moving it forward decreases static stability for a given CG location.

A

True.

Ref: Yechout Pg 211

40
Q

For longitudinal trim, the overall pitching moment acting on the aircraft must be _____

A

Zero

Ref: Yechout Pg 214

41
Q

For a tail aft airplane x_AC_wf – x_CG is normally ______.

A

Positive.

Ref: Yechout Pg 215

42
Q

True or False

With x_CG less than or equal to x_AC_wf, a negative L_h is needed for trim.

A

True

Ref: Yechout Pg 215

43
Q

If x_cg > x_AC_wf, an ______ wing fuselage combination will occur.

A

Unstable.

Ref: Yechout Pg 215

44
Q

How does delta_e_trim vary with C_L?

A

It decreases with increasing C_L (negative slope).

Ref: Yechout Pg 216

45
Q

What is the primary requirement for longitudinal static stability?

A

C_M_alpha < 0

Ref: Yechout Pg 216

46
Q

For a general aircraft, what is the easiest way to ensure that C_m_alpha < 0?

A

Put the CG in front of the AC.

Ref: Yechout Pg 217

47
Q

True or False
x_CG must be less than x_AC (as measured from the front of the aircraft) in order to assure longitudinal static stability.

A

True

Ref: Yechout Pg 218

48
Q

For neutral static stability, C_m_alpha will be equal to _____.

A

Zero.

Ref: Yechout Pg 218

49
Q

What will the alpha vs C_m_alpha graph look like for neutral static stability?

A

A horizontal line.

Ref: Yechout Pg 218

50
Q

C_m_alpha is = 0, if x_CG is equal to _____.

A

x_AC

Ref: Yechout Pg 219

51
Q

The aircraft’s aerodynamic center is also referred to as the _____.

A

Neutral Point.

Ref: Yechout Pg 219

52
Q

If the CG is located aft of the AC, the aircraft will be statically ______, and C_m_alpha will be ____.

A

Unstable
Positive

Ref: Yechout Pg 219

53
Q

What is the definition of static margin?

A

SM = x_AC – x_CG

Ref: Yechout Pg 219

54
Q

The slope of the C_L vs C_m graph gives the ____.

A

Negative of the static margin.

Ref: Yechout Pg 220

55
Q

The value of C_y_0 is usually zero for ____ aircraft.

A

Symmetric.

Ref: Yechout Pg 226

56
Q

True or False

Very long slender aircraft can have a non-zero C_y_0 due to the shedding of non-symmetric vortices.

A

True

Ref: Yechout Pg 226

57
Q

C_y_beta is very important when analyzing ____ dynamics.

A

Dutch Roll

Ref: Yechout Pg 226

58
Q

What primarily influences C_y_beta?

A

The vertical tail.

Ref: Yechout Pg 226

59
Q

What is the normal sign of C_y_beta? Why?

A

Negative
Because positive sideslip will typically result in a side force along the negative y-axis.

Ref: Yechout Pg 226

60
Q

C_l_beta is the __________ stability derivative. It is also called the ______ effect.

A

lateral roll static
dihedral effect

Ref: Yechout Pg 228

61
Q

The sign of C_l_beta must be _______ for static stability.

A

Negative

Ref: Yechout Pg 228

62
Q

A negative C_l_beta implies that _____________.

A

the aircraft generates a rolling moment that rolls the aircraft away from the direction of sideslip.

Ref: Yechout Pg 228

63
Q

What four aspects of aircraft design influence C_l_beta?

A
  1. Geometric dihedral
  2. Wing position
  3. Wing Sweep
  4. Contribution of the vertical tail

Ref: Yechout Pg 228

64
Q

The larger the dihedral angle (Gamma) the more _____ rolling moment.

A

Negative

Ref: Yechout Pg 229

65
Q

A high wing position will provide a _____ contribution to C_l_beta.

A

Negative.

Ref: Yechout Pg 229

66
Q

A low wing position will provide a ______ contribution to C_l_beta.

A

Positive

Ref: Yechout Pg 229

67
Q

A mid-wing position will provide a ______ contribution to C_l_beta.

A

Neutral.

Ref: Yechout Pg 229

68
Q

A low wing will tend to roll the aircraft _____ the direction of sideslip.

A

Toward.

Ref: Yechout Pg 230

69
Q

With aft wing sweep, the wing toward the velocity vector has a _____ normal velocity component than the opposite wing. As a result, the upstream wing will produce ______ than the downstream wing. This causes a ________ moment.

A

Larger
More lift
Rolling

Ref: Yechout Pg 230

70
Q

________ are typically the primary control surface for producing rolling moments in response to pilot command.

A

Ailerons.

Ref: Yechout Pg 232

71
Q

C_n_beta is the directional ______derivative.

A

yaw static stability

Ref: Yechout Pg 235

72
Q

C_n_beta is sometimes called the _______ stability derivative.

A

Weathercock.

Ref: Yechout Pg 235

73
Q

C_n_beta must be ______ for the aircraft to have static stability.

A

Positive.

Ref: Yechout Pg 235

74
Q

What aircraft component primarily influences C_l_beta?

A

The vertical tail.

Ref: Yechout Pg 235

75
Q

What are all the requirements for Lateral-Directional static stability?

A
  1. C_l = C_n = 0 (trim)
  2. C_l_beta < 0
  3. C_n_beta >0

Ref: Yechout Pg 239-242

Flight Dynamics (7 decks)

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