Aircraft Design Questions and Answers – Lateral-Directional Static Stability and Control

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This set of Aircraft Design Objective Questions & Answers focuses on “Lateral-Directional Static Stability and Control”.

1. Stability about yawing axis is called as __________
a) directional stability
b) lateral stability
c) longitudinal stability
d) pitching moment stability
View Answer

Answer: a
Explanation: Directional stability of the aircraft is defined as the stability about the yawing axis. In typical aircraft yaw and rolling both will be produced by deflecting the rudder. Stability about rolling moment is called lateral stability.
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2. Yawing moment is positive if __________
a) right wing goes back
b) right wing comes forward
c) if nose pitches up
d) if nose pitches down
View Answer

Answer: a
Explanation: Yawing moment is said to be positive by sign convention if right wing goes back and vice versa. If nose pitches up then it is a positive pitching moment and similarly if nose pitches down it is called negative pitching moment as per the sign convention is considered.

3. Stability about roll axis is called _____________
a) lateral stability
b) directional stability
c) longitudinal stability
d) elevator control
View Answer

Answer: a
Explanation: Stability about the rolling is termed as lateral stability. Lateral and directional stability are closely coupled. When we deflect only aileron it will generate rolling as well as it will also cause the aircraft to yaw. Longitudinal stability is Stability about pitching moment. Elevator is used for such purposes.

4. If aircraft is in straight flight (cruise with lift of 100N) then, what will be the value of net rolling moment? Consider ideal conditions.
a) 0 unit
b) 12 Nm
c) 23 unit
d) 25 N/m
View Answer

Answer: a
Explanation: Given, aircraft is in cruise, lift = 100N.
At cruise condition, net rolling moment is zero. As at cruise all the forces and moments are in equilibrium or in balanced.

5. Which is the minimum requirement for pure directional stability?
a) Slope of yawing moment curve positive
b) Negative lift curve slope
c) Negative pitching moment coefficient curve slope
d) Positive zero lift pitching moment coefficient
View Answer

Answer: a
Explanation: An aircraft is said to be in directional stability if the yawing moment curve slope is positive. Negative pitching moment coefficient curve slope is minimum criteria for longitudinal static stability. Positive value of zero lift pitching moment coefficient will be used to design an aircraft to trim at positive AOA.

6. Rolling moment will influence _______
a) aircraft lateral stability
b) longitudinal stability
c) pitch axis stability
d) pitching stability only
View Answer

Answer: a
Explanation: Rolling moment will influence the aircraft lateral stability. It also affects the yawing and therefore directional stability of the aircraft. Longitudinal stability is used for pitching moment.
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7. Following diagram represents ___________
aircraft-design-questions-answers-lateral-directional-static-stability-control-q7
a) typical roll stability concept
b) typical longitudinal stability
c) lift curve
d) drag polar
View Answer

Answer: a
Explanation: Above diagram is illustrating the concept of roll stability. Typical lateral stability criteria can be observed in the diagram. Longitudinal stability is represented by using pitching moment curve. Lift curve will be used to provide relationship between lift and angle of attack. Drag polar will correlate drag and lift.

8. An aircraft experiences sideslip of 4° and side wash at vertical tail is 1.2°. What will be the AOA at vertical tail?
a) 5.2°
b) 6.89°
c) 1.2°
d) 21.3°
View Answer

Answer: a
Explanation: AOA = sideslip + side wash = 4°+1.2° = 5.2°.

9. Aircraft can suffer from adverse yaw during rolling.
a) True
b) False
View Answer

Answer: a
Explanation: Yes, it is true that aircraft can suffer from the adverse yaw phenomenon due to rolling motion. When an aircraft is banked to execute turning operation, the aileron may produce a yawing motion that opposes the turn. This is called adverse yaw.

10. An aircraft is flying in the north direction at a velocity of 60.5m/s under cross wind from the east to west of 5m/s. If the value of Cnβ=0.02/deg, where. Find sideslip angle β.
a) -4.72°
b) -5°
c) 4.7°
d) 3.18°
View Answer

Answer: a
Explanation: Given, North velocity n = 60.5m/s, East to west velocity e = 5m/s.
For given cross wind condition sideslip angle β is given by,
β = -arctan (e/n) = -arctan (5/60.5) = -4.72°.

11. Determine the value of rudder deflection δr for an aircraft which is flying in north with the velocity of 60.5 m/s under the crosswind of 5m/s from east to west with Cnβ=0.02/deg and Cnδr = -0.045/deg, where sideslip angle β is -4.72°.
a) -2.09°
b) 3.5°
c) -4.5°
d) -4.74°
View Answer

Answer: a
Explanation: Given, North velocity n = 60.5m/s, East to west velocity e = 5m/s.
Now, rudder deflection δr = – [Cnβ/ Cnδr]*β = -[0.02/-0.045]*(-4.72) = -2.09°.

12. Find sideslip angle if [u, v, w] = [100, 5, 2.5]. Consider steady level flight.
a) 2.86°
b) 4.5°
c) 4.7°
d) 2.1°
View Answer

Answer: a
Explanation: Given, v = 5, V = [u2+v2+w2] 0.5 = [100*100+5*5+2.5*2.5] 0.5 = 100.156.
Sideslip angle = arcsine (v/V) = arcsine (5/100.156) = 2.86°.
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13. Find resultant velocity if [u, v, w] = [80, 2, 4.5]. Consider steady level flight.
a) 80.151
b) 90
c) 10.52
d) 100.159
View Answer

Answer: a
Explanation: Resultant Velocity V = [u2+v2+w2] 0.5 = 80.1521.

14. Determine sideslip angle for a steady level unaccelerated flight with [u, v, w] = [80, 2, 4.5].
a) 1.43
b) 5.4
c) 5
d) 12.32
View Answer

Answer: a
Explanation: Given, v = 2, V = [u2+v2+w2] 0.5 = [80*80+2*2+4.5*4.5] 0.5 = 80.1521.
Sideslip angle = arcsine (v/V) = arcsine (2/80.1521) = 1.43°.

15. Determine the net velocity if sideslip angle is 3.8 degree and [v] = 4.2 unit.
a) 63.23 unit
b) 100 unit
c) 100.23 unit
d) 102.623 unit
View Answer

Answer: a
Explanation: Net velocity V = v*sine (sideslip) = 4.2*sine (3.8°) = 63.23 unit.

Sanfoundry Global Education & Learning Series – Aircraft Design.

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Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He is Linux Kernel Developer & SAN Architect and is passionate about competency developments in these areas. He lives in Bangalore and delivers focused training sessions to IT professionals in Linux Kernel, Linux Debugging, Linux Device Drivers, Linux Networking, Linux Storage, Advanced C Programming, SAN Storage Technologies, SCSI Internals & Storage Protocols such as iSCSI & Fiber Channel. Stay connected with him @ LinkedIn