Static Stability Flashcards by Bryan Durham

Define static stability for an aircraft.

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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For aircraft static stability what is the general evaluation reference?

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

Ref: Yechout Pg 196

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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.

Positive static

Ref: Yechout Pg 196

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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?

Neutral stability.

Ref: Yechout Pg 196

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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?

Negative stability

Ref: Yechout Pg 196

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For longitudinal static stability, C_m_alpha must be _____.

Negative

Ref: Yechout Pg 199

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True or False

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

True.

Ref: Yechout Pg 199

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What is a positive elevator control surface deflection?

Sign convention – Do not confuse with moment convention

Trailing edge down (TED).

Ref: Yechout Pg 199

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What is a positive horizontal stabilizer control deflection?

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

Ref: Yechout Pg 199

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When is aileron deflection considered positive?

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

Tailing Edge Left

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

Ref: Yechout Pg 199

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What are cross-control derivatives?

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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What comprises aircraft longitudinal motion?

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

Ref: Yechout Pg 200

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What comprises aircraft lateral motion?

Roll, yaw rotation, Y-axis translation.

Ref: Yechout Pg 200

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

Angle of attack
Elevator deflection
Incidence angle of the horizontal stabilizer

CM = f(alpha, delta_E, i_H)

Ref: Yechout Pg 198

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

TEU conditions.

Ref: Yechout Pg 198

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For symmetrical aircraft, C_l_0 and C_n_0 are usually _____.

Zero

Ref: Yechout Pg 198

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

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

Negative pitching moment.

Ref: Yechout Pg 199

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

Positive, right-wing down pitching moment.

Ref: Yechout Pg 199

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

Negative nose left yawing moment.

Ref: Yechout Pg 199

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What are the signs of the primary control derivatives?

C_m_delta_e < 0
C_l_delta_a >0
C_n_delta_r <0

Ref: Yechout Pg 199

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What are the three stability derivatives?

C_m_alpha
C_l_beta
C_n_beta

Ref: Yechout Pg 198

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What are the four primary control derivatives?

C_m_delta_e
C_m_i_h
C_l_delta_a
C_n_delta_r

Ref: Yechout Pg 199

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What are the two cross-control derivatives?

C_l_delta_r C_n_delta_a Ref: Yechout Pg 200

What three parameters affect the drag coefficient?

Alpha i_h delta_e Ref: Yechout Pg 201

What three parameters affect the drag coefficient?

Alpha i_h delta_e Ref: Yechout Pg 203

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

alpha_wing = alpha + i_w Where i_w is the incidence angle of the wing. Ref: Yechout Pg 204

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

Downwash. Ref: Yechout Pg 204

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.

True Ref: Yechout Pg 204

What is elevator effectiveness (tau_e)?

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

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.

True Ref: Yechout Pg 205

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

Zero. Ref: Yechout Pg 206

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

Negative. Ref: Yechout Pg 209

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

Negative. Ref: Yechout Pg 209

Define the aerodynamic center.

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

If C_m_alpha is zero, where is the aerodynamic center?

At the CG. Ref: Yechout Pg 211

What are some ways to move the aerodynamic center aft?

1. Increase X_AC_h 2. Increase S_h 3 Increase C_L_alpha_h Ref: Yechout Pg 211

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.

True. Ref: Yechout Pg 211

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

Zero Ref: Yechout Pg 214

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

Positive. Ref: Yechout Pg 215

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

True Ref: Yechout Pg 215

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

Unstable. Ref: Yechout Pg 215

How does delta_e_trim vary with C_L?

It decreases with increasing C_L (negative slope). Ref: Yechout Pg 216

What is the primary requirement for longitudinal static stability?

C_M_alpha < 0 Ref: Yechout Pg 216

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

Put the CG in front of the AC. Ref: Yechout Pg 217

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.

True Ref: Yechout Pg 218

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

Zero. Ref: Yechout Pg 218

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

A horizontal line. Ref: Yechout Pg 218

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

x_AC Ref: Yechout Pg 219

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

Neutral Point. Ref: Yechout Pg 219

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

Unstable Positive Ref: Yechout Pg 219

What is the definition of static margin?

SM = x_AC -- x_CG Ref: Yechout Pg 219

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

Negative of the static margin. Ref: Yechout Pg 220

The value of C_y_0 is usually zero for ____ aircraft.

Symmetric. Ref: Yechout Pg 226

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

True Ref: Yechout Pg 226

C_y_beta is very important when analyzing ____ dynamics.

Dutch Roll Ref: Yechout Pg 226

What primarily influences C_y_beta?

The vertical tail. Ref: Yechout Pg 226

What is the normal sign of C_y_beta? Why?

Negative Because positive sideslip will typically result in a side force along the negative y-axis. Ref: Yechout Pg 226

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

lateral roll static dihedral effect Ref: Yechout Pg 228

The sign of C_l_beta must be _______ for static stability.

Negative Ref: Yechout Pg 228

A negative C_l_beta implies that _____________.

the aircraft generates a rolling moment that rolls the aircraft away from the direction of sideslip. Ref: Yechout Pg 228

What four aspects of aircraft design influence C_l_beta?

1. Geometric dihedral 2. Wing position 3. Wing Sweep 4. Contribution of the vertical tail Ref: Yechout Pg 228

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

Negative Ref: Yechout Pg 229

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

Negative. Ref: Yechout Pg 229

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

Positive Ref: Yechout Pg 229

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

Neutral. Ref: Yechout Pg 229

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

Toward. Ref: Yechout Pg 230

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.

Larger More lift Rolling Ref: Yechout Pg 230

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

Ailerons. Ref: Yechout Pg 232

C_n_beta is the directional ______derivative.

yaw static stability Ref: Yechout Pg 235

C_n_beta is sometimes called the _______ stability derivative.

Weathercock. Ref: Yechout Pg 235

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

Positive. Ref: Yechout Pg 235

What aircraft component primarily influences C_l_beta?

The vertical tail. Ref: Yechout Pg 235

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

1. C_l = C_n = 0 (trim) 2. C_l_beta < 0 3. C_n_beta >0 Ref: Yechout Pg 239-242