Longitudional & Lateral Static Stability Review

Question 1

Static equilibrium occurs whenever there is no ________ on the aircraft.

Answer 1

Acceleration

Acceleration occurs when there are unbalanced forces and/or moments on the airplane.

Mechanics of Flight, W. Phillips, Pg. 377

Question 2

Aircraft symmetry will often result in zero rolling and yawing moments, but the pitching moment is usually zeroed with ___________.

Answer 2

Control inputs sent to a horizontal control surface.

Mechanics of Flight, W. Phillips, Pg. 377

Question 3

Define “trim.”

Answer 3

A state of flight in which the controls are set so that there are no unbalanced forces and the net moment on the aircraft is zero.

Mechanics of Flight, W. Phillips, Pg. 377

Question 4

For static stability, what is the relation between Cm and alpha?

Answer 4

The rate of change of Cm with respect to alpha (dCm/dalpha) must be negative.

Mechanics of Flight, W. Phillips, Pg. 383

Question 5

True or False
A simple cambered wing is statically unstable in free flight.

Answer 5

True.

Mechanics of Flight, W. Phillips, Pg. 383

Question 6

What two specific purposes does the horizontal tail fulfill?

Answer 6

Stability and control in pitch (trim)

Mechanics of Flight, W. Phillips, Pg. 384

Question 7

What is a stabilator?

Answer 7

An empennage design in which the front stabilizer and aft stabilizer rotate independently of one another to provide pitch control.

Mechanics of Flight, W. Phillips, Pg. 385

Question 8

True or False
If the tail is located in the wake of the wing or the fuselage, the dynamic pressure on the tail will be less than the freestream value.

Answer 8

True.

Mechanics of Flight, W. Phillips, Pg. 387

Question 9

Define the “tail efficiency.”

Answer 9

The ratio of dynamic pressures between the tail and the free stream (q_tail / q_infinity).

Mechanics of Flight, W. Phillips, Pg. 388

Question 10

True or False
A symmetric airfoil immersed in subsonic ideal flow produces a non-zero moment about its aerodynamic center.

Answer 10

False. For the case of a symmetric airfoil in ideal flow, there is no moment about the aerodynamic center.

Mechanics of Flight, W. Phillips, Pg. 388

Question 11

Why does negative lift on the horizontal tail impede aerodynamic efficiency?

Is negative lift on the horizontal tail a requirement for static stability?

Answer 11

If the lift on the horizontal tail is negative, the main wing must support the added load, along with the aircraft weight. This increases induced drag and reduces the effective performance of the wing.

No, it is not a requirement for static stability.

Mechanics of Flight, W. Phillips, Pg. 389

Question 12

Explain how the bound vortex on the wing affects the horizontal tail.

Answer 12

The bound vortex creates an upwash ahead of the wing and a net downwash aft of the wing (see Fig. 4.3.4 in the reference). This creates a net downwash on the horizontal tail, which reduces the effective angle of attack.

Mechanics of Flight, W. Phillips, Pg. 390-391

Question 13

From lifting line theory, it can be shown that the downwash at any point in the proximity of a finite lifting wing is proportional to the _________________.

Answer 13

The freestream velocity, multiplied by the lift coefficient for the wing and divided by the aspect ratio for the wing.

Vy = -K(V*Cl/AR) Eq. (4.3.17)

Extra Notes: The proportionality constant “K” depends on the planform shape of the wing and the position of the tail relative to the wing.

Mechanics of Flight, W. Phillips, Pg. 391

Question 14

A stable airplane in free flight will always seek the angle of attack that makes the pitching moment about the C.G. equal to ___________.

Answer 14

Zero.

Mechanics of Flight, W. Phillips, Pg. 392

Question 15

True or False
Increasing the tail volume for an aft tail will always increase the stability of the airplane.

Answer 15

True.

Mechanics of Flight, W Phillips, Pg. 394

Question 16

True or False
The stick-fixed neutral point is the aerodynamic center of the complete airplane.

Answer 16

True.

Mechanics of Flight, W. Phillips, Pg. 400

Question 17

True or False
The position of the neutral point depends on the location of the C.G.

Answer 17

False. The position of the neutral point does NOT depend on the location of the C.G., the definition of the reference chord length, or the choice of datum.

Mechanics of Flight, W. Phillips, Pg. 401

Question 18

For an airplane to be statically stable in pitch, where must the center of gravity be placed along the fuselage with respect to the stick-fixed neutral point?

Answer 18

The C.G. MUST be forward of the stick fixed neutral point for static pitch stability.

Question 19

The minimum pitch stability constraints should always be set based on _________________.

Answer 19

Dynamic considerations (not static margin).

Mechanics of Flight, W. Phillips, Pg. 402

Question 20

True or False
The moment about the stick-fixed neutral point changes drastically with any variation in angle of attack.

Answer 20

False.
The stick-fixed neutral point is the aerodynamic center of the aircraft. By definition, the aerodynamic center is the point about which the change in the resultant moment with respect to the angle of attack is zero.

Mechanics of Flight, W. Phillips, Pg. 5, & 403

Question 21

With respect to longitudinal static stability and general controllability (trim) of airplanes; if the C.G. is moved too far aft, the airplane _____________. If the C.G. is moved too far forward, the airplane _____________.

Answer 21

> Can become unstable in pitch

> Is difficult or impossible to trim at low airspeeds (such as landing)

Mechanics of Flight, W. Phillips, Pg. 407

Question 22

Define “trim drag.”

Answer 22

The total additional induced drag resulting from the deflection of horizontal tail surfaces to trim the aircraft.

Mechanics of Flight, W. Phillips, Pg. 409

Question 23

For static stability in pitch, the canard must be mounted __________ of the C.G. to maintain trim.

Answer 23

forward

Mechanics of Flight, W. Phillips, Pg. 424

Question 24

With respect to longitudional static stability and trim; a canard will always have a _________ effect on the airplane.

Answer 24

destabalizing

Mechanics of Flight, W. Phillips, Pg. 425

Question 25

For a stable canard configuration, where must the aerodynamic center of the wing be placed?

Answer 25

Behind the C.G., and far enough back to counter the destabilizing effects of the canard. Mechanics of Flight, W. Phillips, Pg. 425

Question 26

When using a wing-canard configuration, the pitching moment provided by the canard must be ________ to overcome the C.G.--wing couple moment.

Answer 26

positive Extra Notes: In the provided reference, a positive pitching moment is defined as a moment that would cause the nose to move upward from level flight. Mechanics of Flight, W. Phillips, Pg. 5, & 425

Question 27

When using a wing-canard configuration, the lift on the canard must be _______ for static stability.

Answer 27

positive Mechanics of Flight, W. Phillips, Pg. 425

Question 28

What are some of the advantages of using a canard configuration?

Answer 28

1. Both lifting surfaces (the wing & canard) provide positive lift over the entire velocity range. 2. Since the canard also produces positive lift, the lift requirement of the main wing is reduced, resulting in a reduction of the total induced drag. 3. It is very difficult to stall the main wing under routine maneuvers. Mechanics of Flight, W. Phillips, Pg. 425, 427

Question 29

To maintain both trim and static stability with a canard configuration, the lift coefficient for the canard must be ______ than that of the wing.

Answer 29

greater Mechanics of Flight, W. Phillips, Pg. 426

Question 30

The contribution of the fuselage, & nacelles to static longitudinal stability is almost always ____________.

Answer 30

Destabilizing. Mechanics of Flight, W. Phillips, Pg. 473

Question 31

True or False Yaw stability, directional stability, and weathercock stability are all analogous terms.

Answer 31

True. Mechanics of Flight, W. Phillips, Pg. 473

Question 32

What is the general criterion for static yaw stability?

Answer 32

dCn/dbeta >0 Mechanics of Flight, W. Phillips, Pg. 500

Question 33

Sidewash is usually defined to be positive from ______ to ________.

Answer 33

left to right. Mechanics of Flight, W. Phillips, Pg. 502

Question 34

A _______ deflection of the rudder is defined to be positive.

Answer 34

Leftward Mechanics of Flight, W. Phillips, Pg. 503

Question 35

What are some natural aerodynamic phenomena that reduce the sidewash angle on the vertical tail in flight? Assume a tractor prop that rotates counterclockwise when viewed from the front of the airplane. The vertical tail is located aft of the C.G..

Answer 35

1. The rotating component of the prop wash will create a positive sidewash which reduces the sidewash angle. 2. Wing tip vortices also induce a positive sidewash which reduces the sidewash angle. Mechanics of Flight, W. Phillips, Pg. 502

Question 36

An aft-mounted vertical tail is always __________ with respect to lateral static stability and trim.

Answer 36

Stabilizing. Mechanics of Flight, W. Phillips, Pg. 504

Question 37

The contribution of the fuselage, nacelles and external stores to the static yaw stability of an airplane is typically __________.

Answer 37

Destabilizing. Mechanics of Flight, W. Phillips, Pg. 505

Question 38

True or False With respect to lateral static stability and trim; a prop can affect the trim, but NOT the static stability of an airplane.

Answer 38

False. It affects both. Mechanics of Flight, W. Phillips, Pg. 506

Question 39

True or False For a prop that is mounted forward of the wing, the sidewash gradient is small and can usually be ignored.

Answer 39

True. Mechanics of Flight, W. Phillips, Pg. 509

Question 40

True or False With respect to yaw stability, when a prop is mounted aft of the wing, the effects of sidewash cannot be ignored.

Answer 40

True. Mechanics of Flight, W. Phillips, Pg. 509

Question 41

With respect to yaw static stability; if the prop is forward of the center of gravity, the prop contribution to the yaw stability derivative is _________. If the propeller is aft of the center of gravity, the prop contribution to the yaw stability derivative is _________.

Answer 41

Destabilizing. Stabilizing Extra Notes: See Eq. 5.2.28 in the reference. Mechanics of Flight, W. Phillips, Pg. 509

Question 42

For yaw static stability, if the prop is aft of the center of gravity, the propeller contribution to dCn/dbeta is _______________.

Answer 42

Stabilizing. Extra Notes: See Eq. 5.2.28 in the reference. Mechanics of Flight, W. Phillips, Pg. 509

Question 43

True or False The use of counter-rotating props does not affect yaw stability.

Answer 43

True. Mechanics of Flight, W. Phillips, Pg. 510

Question 44

Why would a tail dragger be limited in terms of the angle of attack of the prop during the takeoff roll?

Answer 44

The slipstream effect on the vertical tail at high prop angles of attack can exceed the ability of the rudder to provide directional control. Mechanics of Flight, W. Phillips, Pg. 513 & 514

Question 45

For any single prop airplane, the yawing moment produced by the propeller _________ with angle of attack.

Answer 45

increases Mechanics of Flight, W. Phillips, Pg. 515

Question 46

True or False A lightly swept wing can have a large effect on yaw static stability.

Answer 46

False. Extra Notes: The wing in general (swept or unswept) has little effect on yaw static stability and can usually be ignored for preliminary design. However, if both the wing and tail are highly swept, they can contribute to the yaw stability derivative. Mechanics of Flight, W. Phillips, Pg. 515

Question 47

True or False For tailess airplanes, the wings must be swept back to provide passive yaw stability.

Answer 47

True. Mechanics of Flight, W. Phillips, Pg. 517

Question 48

Explain why swept wings can provide some yaw stability.

Answer 48

When an aircraft with swept wings has a positive sideslip angle (see Fig. 5.1.1 for the definition of positive sideslip angle), the normal component of the freestream is increased on the right wing. This increases the lift and, by proxy, the drag on the right wing. The drag on the right wing essentially pulls the fuselage back to center. Mechanics of Flight, W. Phillips, Pg. 517

Question 49

True or False Dihedral effects on yaw stability are small, and dihedral has no effect on airplane trim in yaw.

Answer 49

True. Mechanics of Flight, W. Phillips, Pg. 517

Question 50

True or False Dihedral effects on roll stability are much more pronounced than yaw stability.

Answer 50

True. Mechanics of Flight, W. Phillips, Pg. 517

Question 51

True or False When properly designed, either positive or negative dihedral in an aft tail can increase the yaw stability of the airplane.

Answer 51

True. Extra notes: If sufficient dihedral or anhedral is added to a conventional horizontal stabilizer, the vertical stabilizer can be eliminated completely (see Fig. 5.5.1 in the reference). Such a configuration is commonly called a V-Tail. Mechanics of Flight, W. Phillips, Pg. 529

Question 52

Looking from back to front, if a V-tail aircraft has a positive sideslip angle, the lift distribution on the right surface will be __________, and the lift distribution on the left surface will be _______. The net result is a yawing moment the _______ sideslip.

Answer 52

positive negative reduces Extra Notes: See Fig. 5.5.3 on page 530 of the reference. Mechanics of Flight, W. Phillips, Pg. 530 & 531

Question 53

True or False When a V-Tail is used, it provides stabilizing contributions to both pitching and yawing moments for the complete aircraft.

Answer 53

True. Mechanics of Flight, W. Phillips, Pg. 531

Question 54

Explain why roll stability is fundamentally different from yaw and pitch stability.

Answer 54

Yaw and pitch stability depend on the sideslip angle and angle of attack respectively. Both angles are defined with respect to the relative wind. Roll stability is dependent upon bank angle which is defined with respect to the orientation of the airplane to the Earth. Mechanics of Flight, W. Phillips, Pg. 548

Question 55

True or False A disturbance in pure roll, with no change in angle of attack, sideslip angle, or forward airspeed, produces no DIRECT change in the aerodynamic forces and moments relative to the airplane.

Answer 55

True. Extra Notes: See the last paragraph on Page 548 and Fig. 5.6.1. All aerodynamic forces on the aircraft are a product of the sideslip angle and angle of attack. Since roll is defined in accordance with the aircraft's orientation relative to the earth, a change in bank angle only produces indirect aerodynamic effects. Mechanics of Flight, W. Phillips, Pg. 548

Question 56

Static roll stability requires that a ______ sideslip angle must produce a _________ rolling moment.

Answer 56

Positive Negative Extra Notes: Recall dCl/dbeta <0 for static stability in roll. In the reference cited, a positive rolling moment is defined as a moment that causes the right wing (from the pilot's perspective) to move downward. See also Fig. 5.1.1 for the sign convention for a positive sideslip angle. Mechanics of Flight, W. Phillips, Pg. 5, 498, & 548

Question 57

For static stabililty, the roll stability derivative (C_l_beta) must be __________.

Answer 57

Negative. Mechanics of Flight, W. Phillips, Pg. 549

Question 58

A positive sweep-back angle is ________ for roll stability. Negative sweep-back angles are _______ with respect to roll stability.

Answer 58

Stabilizing Destabilizing Mechanics of Flight, W. Phillips, Pg. 557

Question 59

With respect to roll stability, state the effects of a high wing vs a low wing.

Answer 59

A high wing is stabilizing, and a low wing is destabilizing. Mechanics of Flight, W. Phillips, Pg. 558

Question 60

How do most airplane designers account for the destabilizing effects of a low-wing configuration?

Answer 60

By giving the wings a dihedral (usually around 4 degrees) Mechanics of Flight, W. Phillips, Pg. 558, 559