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Senior Design > Longitudional & Lateral Static Stability Review > Flashcards

Longitudional & Lateral Static Stability Review Flashcards

1
Q

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

A

Acceleration

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

Mechanics of Flight, W. Phillips, Pg. 377

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2
Q

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

A

Control inputs sent to a horizontal control surface.

Mechanics of Flight, W. Phillips, Pg. 377

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3
Q

Define “trim.”

A

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

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4
Q

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

A

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

Mechanics of Flight, W. Phillips, Pg. 383

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5
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 383

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6
Q

What two specific purposes does the horizontal tail fulfill?

A

Stability and control in pitch (trim)

Mechanics of Flight, W. Phillips, Pg. 384

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7
Q

What is a stabilator?

A

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

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8
Q

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.

A

True.

Mechanics of Flight, W. Phillips, Pg. 387

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9
Q

Define the “tail efficiency.”

A

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

Mechanics of Flight, W. Phillips, Pg. 388

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10
Q

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

A

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

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11
Q

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

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

A

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

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12
Q

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

A

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

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13
Q

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 _________________.

A

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

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14
Q

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

A

Zero.

Mechanics of Flight, W. Phillips, Pg. 392

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15
Q

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

A

True.

Mechanics of Flight, W Phillips, Pg. 394

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16
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 400

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17
Q

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

A

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

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18
Q

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?

A

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

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19
Q

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

A

Dynamic considerations (not static margin).

Mechanics of Flight, W. Phillips, Pg. 402

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20
Q

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

A

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

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21
Q

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 _____________.

A

> Can become unstable in pitch

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

Mechanics of Flight, W. Phillips, Pg. 407

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22
Q

Define “trim drag.”

A

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

Mechanics of Flight, W. Phillips, Pg. 409

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23
Q

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

A

forward

Mechanics of Flight, W. Phillips, Pg. 424

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24
Q

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

A

destabalizing

Mechanics of Flight, W. Phillips, Pg. 425

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25
Q

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

A

Behind the C.G., and far enough back to counter the destabilizing effects of the canard.

Mechanics of Flight, W. Phillips, Pg. 425

26
Q

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

A

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

27
Q

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

A

positive

Mechanics of Flight, W. Phillips, Pg. 425

28
Q

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

A
  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

29
Q

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

A

greater

Mechanics of Flight, W. Phillips, Pg. 426

30
Q

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

A

Destabilizing.

Mechanics of Flight, W. Phillips, Pg. 473

31
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 473

32
Q

What is the general criterion for static yaw stability?

A

dCn/dbeta >0

Mechanics of Flight, W. Phillips, Pg. 500

33
Q

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

A

left to right.

Mechanics of Flight, W. Phillips, Pg. 502

34
Q

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

A

Leftward

Mechanics of Flight, W. Phillips, Pg. 503

35
Q

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..

A
  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

36
Q

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

A

Stabilizing.

Mechanics of Flight, W. Phillips, Pg. 504

37
Q

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

A

Destabilizing.

Mechanics of Flight, W. Phillips, Pg. 505

38
Q

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.

A

False. It affects both.

Mechanics of Flight, W. Phillips, Pg. 506

39
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 509

40
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 509

41
Q

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 _________.

A

Destabilizing.
Stabilizing

Extra Notes: See Eq. 5.2.28 in the reference.

Mechanics of Flight, W. Phillips, Pg. 509

42
Q

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

A

Stabilizing.

Extra Notes: See Eq. 5.2.28 in the reference.

Mechanics of Flight, W. Phillips, Pg. 509

43
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 510

44
Q

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

A

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

45
Q

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

A

increases

Mechanics of Flight, W. Phillips, Pg. 515

46
Q

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

A

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

47
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 517

48
Q

Explain why swept wings can provide some yaw stability.

A

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

49
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 517

50
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 517

51
Q

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

A

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

52
Q

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.

A

positive
negative
reduces

Extra Notes: See Fig. 5.5.3 on page 530 of the reference.

Mechanics of Flight, W. Phillips, Pg. 530 & 531

53
Q

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

A

True.

Mechanics of Flight, W. Phillips, Pg. 531

54
Q

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

A

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

55
Q

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.

A

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

56
Q

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

A

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

57
Q

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

A

Negative.

Mechanics of Flight, W. Phillips, Pg. 549

58
Q

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

A

Stabilizing
Destabilizing

Mechanics of Flight, W. Phillips, Pg. 557

59
Q

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

A

A high wing is stabilizing, and a low wing is destabilizing.

Mechanics of Flight, W. Phillips, Pg. 558

60
Q

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

A

By giving the wings a dihedral (usually around 4 degrees)

Mechanics of Flight, W. Phillips, Pg. 558, 559

Senior Design (13 decks)

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