Lateral Stability Of Aircraft | Aircraft Lateral Stability | Lecture 41 — Transcript

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1. 0:00latroll's stability involves the study
2. 0:03of moments
3. 0:04about the longitudinal axis
4. 0:07if the aircraft is displaced away from
5. 0:09wings level
6. 0:10the inclination of the lift vector will
7. 0:13generate a side force and the aircraft
8. 0:16will start to move sideways through the
9. 0:18air
10. 0:19this is known as sideslip
11. 0:22from our study of directional stability
12. 0:25it is obvious
13. 0:27that a side slip will generate a yuring
14. 0:29moment
15. 0:32if we freeze the animation during the
16. 0:33side slip we can see that a side slip
17. 0:36angle exists
18. 0:38and the resultant fin force will
19. 0:39generate a yearing moment
20. 0:41towards the direction from which the
21. 0:43airflow is coming
22. 0:44and the aircraft will weather into
23. 0:46the wind
24. 0:50we are seeing a sample of the
25. 0:51interaction between lateral stability
26. 0:54and directional stability which will be
27. 0:56fully explained at the end of this
28. 0:58lesson
29. 1:01for the study of lateral stability we
30. 1:03will concentrate on the rolling moments
31. 1:05generated by a side slip
32. 1:08the type of lateral stability is
33. 1:10determined by the direction of the
34. 1:12rolling moment
35. 1:13generated by the side slip
36. 1:16if the side slip generates an
37. 1:18unfavorable rolling moment
38. 1:20the aircraft will roll further away from
39. 1:22wings level
40. 1:24this is unstable due to side slip
41. 1:28if the side slip generates no rolling
42. 1:30moments
43. 1:31the aircraft will neither roll further
44. 1:33away from nor back towards wings level
45. 1:37this is neutral if the side slip
46. 1:40generates a favorable rolling moment
47. 1:43the aircraft will roll back towards
48. 1:44wings level
49. 1:46this is stable due to side slip
50. 1:53the degree of lateral stability an
51. 1:54aircraft has
52. 1:56is the net result of the contribution of
53. 1:58its component parts
54. 2:01first we will look at the contribution
55. 2:03of the wing
56. 2:05as the aircraft side slips the relative
57. 2:08airflow is no longer parallel to the
58. 2:10longitudinal axis
59. 2:13and the forward speed vector and the
60. 2:15side slip vector
61. 2:16give a resultant relative airflow from
62. 2:18the side
63. 2:20it will be easier to see what is
64. 2:22happening if a two-dimensional view is
65. 2:24used
66. 2:26as the aircraft side slips
67. 2:30the relative airflow is from the side
68. 2:36which we can replace with a view of an
69. 2:38airflow stream
70. 2:41we can see that the airflow path is
71. 2:43partly over the top
72. 2:46and partly beneath the fuselage
73. 2:52this airflow is around the fuselage
74. 2:54where the wing is attached
75. 2:58first we'll look at a wing mounted on
76. 3:00the bottom of the fuselage
77. 3:03the wing into wind has a decreased
78. 3:06effective angle of attack
79. 3:10and less lift
80. 3:14whereas the wing out of the wind has an
81. 3:16increased angle of attack
82. 3:21and more lift
83. 3:25the difference in lift on each half of
84. 3:27the wing will generate a rolling moment
85. 3:30that rolls the aircraft further away
86. 3:32from wings level
87. 3:34the aircraft has negative lateral static
88. 3:37stability
89. 3:42the designers can reduce the unstable
90. 3:45contribution of a low-mounted wing
91. 3:47by fitting the wing on the fuselage at
92. 3:49an angle
93. 3:51this is called dihedral
94. 3:55the upward inclination of the wing from
95. 3:57the horizontal
96. 4:00let's see how dihedral reduces the
97. 4:02unstable contribution
98. 4:04of a low-mounted wing
99. 4:07it is clear that dihedral removes the
100. 4:09wing from the influence of the airflow
101. 4:12around the bottom of the wing fuselage
102. 4:14junction
103. 4:17and the airflow over the top gives more
104. 4:19lift on the wing into wind
105. 4:21and less lift on the wing out of the
106. 4:23wind
107. 4:27which generates a rolling moment back
108. 4:29towards wings level
109. 4:31dihedral gives an aircraft with a low
110. 4:34mounted wing
111. 4:35positive lateral static stability
112. 4:42we will now take a look at the high
113. 4:44mounted wing
114. 4:47the airflow over the top of the fuselage
115. 4:50wing junction
116. 4:51gives more lift on the wing into wind
117. 4:56and less lift on the wing out of wind
118. 5:00which generates a rolling moment back
119. 5:03towards wings level
120. 5:05an aircraft with a high mounted wing has
121. 5:07positive lateral static stability
122. 5:10merely because of the wing position on
123. 5:11the fuselage
124. 5:15because a high mounted wing gives the
125. 5:17same positive contribution to lateral
126. 5:19stability as does dihedral
127. 5:22a high mounted wing is known as a
128. 5:24dihedral effect
129. 5:27a designer may decide on a high wing
130. 5:29configuration
131. 5:30because a low fuselage position with the
132. 5:33aircraft on the ground
133. 5:34is desirable for ease of loading
134. 5:38potentially the dihedral effect of the
135. 5:40high wing
136. 5:41may give too much lateral stability
137. 5:46if so the designer can reduce the stable
138. 5:49contribution of a high wing
139. 5:51by fitting the wing at a downwards angle
140. 5:54this is called anhedral
141. 5:58the downward inclination of the wing
142. 6:00from the horizontal
143. 6:08most jet transport aircraft have a swept
144. 6:11wing
145. 6:11to increase the efficiency of high-speed
146. 6:13flight
147. 6:16in a right-side slip the wing into the
148. 6:18wind has less effective sweep angle
149. 6:22and the wing out of the wind has more
150. 6:23effective sweep angle
151. 6:27this gives more lift on the right wing
152. 6:29and less lift on the left wing
153. 6:32which generates a stabilizing left
154. 6:34rolling moment
155. 6:37the swept wing provides a stable
156. 6:39contribution to lateral stability
157. 6:41so is also classified as a dihedral
158. 6:44effect
159. 6:47if an aircraft has a high mounted wing
160. 6:49that is also swept
161. 6:50it will have two dihedral effects and
162. 6:53may have too much lateral stability
163. 6:57if this is the case the wing will
164. 6:59require anhedral
165. 7:01to ensure the proper amount of lateral
166. 7:03stability
167. 7:05the fin has already been identified as
168. 7:08the aerofoil that provides directional
169. 7:10stability
170. 7:13but in the side slip
171. 7:17the aerodynamic force on the fin acts
172. 7:19above the cg
173. 7:23which generates a small stabilizing
174. 7:25moment
175. 7:28the fin therefore is also a dihedral
176. 7:30effect
177. 7:31but a small one the contribution of a
178. 7:35ventral fin
179. 7:36is very small indeed as you can see
180. 7:40the aerodynamic force is in the same
181. 7:42direction but is acting below the cg
182. 7:48and is destabilizing
183. 7:51for all practical purposes any
184. 7:53contribution of the ventral fin to
185. 7:55lateral stability
186. 7:56is insignificant
187. 8:00the illustration shows the low pressure
188. 8:02area on the top of the wing
189. 8:06each half of the wing generates a share
190. 8:08of the lift
191. 8:09and each component of lift will act at a
192. 8:12certain distance from the cg
193. 8:15any change in lift in a side slip will
194. 8:18act through a given arm length
195. 8:19to generate a rolling moment
196. 8:24when the flaps are down the inboard
197. 8:26portion of the wing
198. 8:27generates a larger percentage of the
199. 8:29lift and each component of lift
200. 8:32will be located further in board
201. 8:36with the flaps down any change in lift
202. 8:38in a side slip will act through a
203. 8:40shorter arm
204. 8:41and generate a smaller rolling moment
205. 8:45hence with flaps down lateral stability
206. 8:48is reduced
207. 8:55as has become increasingly obvious in a
208. 8:58sideslip
209. 8:59the lateral and directional response of
210. 9:01an aircraft will be coupled
211. 9:04sideslip will simultaneously produce a
212. 9:07rolling
213. 9:08and a yearing moment
214. 9:11the type of lateral dynamic stability an
215. 9:14aircraft will exhibit
216. 9:15is determined by which type of static
217. 9:18stability
218. 9:18is dominant directional or lateral
219. 9:23if directional static stability is
220. 9:26dominant
221. 9:27the aircraft will suffer from spiral
222. 9:29instability
223. 9:32if lateral static stability is dominant
224. 9:35the aircraft will suffer from dutch role
225. 9:38a combination of rolling and yuring
226. 9:43because jet transport aircraft have a
227. 9:45swept wing
228. 9:46they have a tendency to dutch role and
229. 9:49are fitted with
230. 9:50a device called a your damper to prevent
231. 10:02it