El Velocímetro y las Velocidades en Aviación — Transcript

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1. 0:07Today we will talk about the operation of the airspeed indicator as well as the
2. 0:11speeds used in aviation fits emphasize that the general theme of
3. 0:16the airspeed indicator will be divided in two videos, in this first part we will
4. 0:21focus on how an aispeed indicator works, what are the errors and
5. 0:25corrections to which it is subject to same as the different definitions of
6. 0:30speeds used in aviation while in the second part it is
7. 0:35say in the next video we will see the colored arcs of the indicator and
8. 0:40what speeds represent, and how does the react airspeed indicator in case of
9. 0:45a pitot tube or static port blockage
10. 0:49so having said this let's get started with how an airspeed indicator works
11. 0:54the airspeed indicator is one of the six basic instruments that we are going to
12. 0:58find in any aircraft and how we said in the previous video this is
13. 1:02linked to the pitot-static system, is say it depends on the measurement of the
14. 1:07air pressure to give the reading, with that said, let's see what it is then
15. 1:13the related pitot-static system here how we can observe both the
16. 1:18pitot tube as the static port provide information to the indicator
17. 1:23on one hand the pitot tube gives total air pressure information
18. 1:27while the static port provides atmospheric pressure information or in other words static pressure
19. 1:35in order to understand why
20. 1:36the airspeed indicator needs these two pressure readings we must review their
21. 1:41definitions as we will remember from the previous video about the pitot-static system
22. 1:45static pressure is the atmospheric pressure of the air and this is
23. 1:51exerted equally in all directions regardless if the
24. 1:55object is moving or is still on the other hand the dynamic pressure
25. 2:00corresponds to air pressure generated by moving through it and
26. 2:05this is proportional to the speed and air density, that is, the more
27. 2:10fast we go higher dynamic pressure we will have and vice versa
28. 2:15and finally the total pressure corresponds to the sum of the static pressure
29. 2:19and dynamic air pressure this means that when measuring the pressure
30. 2:25from a flow of air we will obtain the total pressure measurement
31. 2:29now the airspeed indicator, also known as ASI for its acronym in English
32. 2:35"Airspeed indicator" corresponds to an instrument that indicates the speed of
33. 2:39the aircraft relative to the air and as we know this indicated speed
34. 2:45is directly proportional to the dynamic air pressure in other
35. 2:49words for example an aircraft that travels at 80 knots will have less
36. 2:53dynamic pressure than one moving at 120 knots
37. 2:58with which the instrument takes advantage of this principle to measure the dynamic pressure
38. 3:03and give an indication of speed in other words the
39. 3:07airspeed indicator is a dynamic pressure gauge that is calibrated to show
40. 3:11speed so that if we experience higher dynamic pressure
41. 3:16instrument is going to interpret it as a higher speed and vice versa
42. 3:21having said this then it is clear that the airspeed indicator needs to receive
43. 3:26information about what the dynamic pressure is, to be able to give the reading
44. 3:30of speed, the problem is that the pitot tube gives total pressure information
45. 3:36not only dynamic pressure as we had previously seen the total pressure
46. 3:40is the dynamic pressure plus the Static pressure
47. 3:45that is, if we want to have only dynamic pressure we have to take
48. 3:49that total pressure and subtract the static pressure which we obtain through
49. 3:55from the static port this is why we need
50. 3:59information from the pitot tube as well as static port, that is to say that in essence the
51. 4:05instrument subtracts static pressure to the total pressure thus obtaining
52. 4:09only the dynamic pressure with which it can indicate speed
53. 4:14that is, in this case we take the total pressure of the pitot tube that in
54. 4:18essence is dynamic pressure plus static pressure and we subtract the static pressure
55. 4:23that comes to us from the static port, that we obtain only the
56. 4:27dynamic pressure which can be used for speed reading
57. 4:33if we observe then how it is designed the instrument we will find
58. 4:37the next, here we have the static port and here we have
59. 4:41the pitot tube inlet the difference is that the pitot tube inlet is
60. 4:47connected to a diaphragm inside the instrument which in turn is connected
61. 4:51to certain gears that allow move the needle on the dial
62. 4:57so, this would work the following way: the static port makes
63. 5:01that the whole instrument is filled with that static pressure while the pitot tube
64. 5:06causes the diaphragm to fill with total pressure this means that the
65. 5:11total pressure inside the diaphragm try to expand it while the
66. 5:15static pressure inside the instrument try to contract that diaphragm
67. 5:20Now, as we will remember the total pressure is static pressure plus dynamic pressure
68. 5:26that is, we can divide it into those two
69. 5:29part of the pressure inside the diaphragm is static and part is dynamic
70. 5:35and since we have the same amount of static pressure both inside and outside the diaphram
71. 5:40we can then cancel that pressure
72. 5:44so now, there´s only the dynamic pressure. In other words the diaphragm only
73. 5:49will expand or contract depending on dynamic pressure
74. 5:54for example, if the dynamic pressure increases, the diaphragm is going to expand making the
75. 5:59gears move the needle to indicate a higher speed
76. 6:04while if the dynamic pressure decreases the opposite happens
77. 6:08in this case the diaphragm contracts and the gears move the needle to
78. 6:12indicate a lower speed in essence then this is the way
79. 6:17which an airspeed indicator works
80. 6:21now here's something we must keep in mind and is that the instrument is not
81. 6:25totally perfect with which the speed reading has some
82. 6:29errors that is why in aviation we use different types of
83. 6:33speeds that are corrected by certain errors, now we will
84. 6:38see a definition of each of them, let´s start with the simplest of all
85. 6:43which is the indicated speed that has the acronym IAS from the English
86. 6:49"indicated speed" this is basically the speed indicated on the instrument
87. 6:55and it´s not corrected for any type of error
88. 7:00so, it's the speed we read directly from there this speed
89. 7:04can be expressed either in knots or in miles per hour depending on the
90. 7:08instrument for example here we have one which is indicating 70 knots and here
91. 7:14we have another that is indicating 70 miles per hour
92. 7:17now apart from this some instruments include both scales of
93. 7:21speed both miles per hour and knots as is the example we see in the
94. 7:26screen having already seen what speed is the indicated airspeed
95. 7:30let's see some errors that are produced the first is the installation error
96. 7:35and it´s produced because during the installation and manufacture of
97. 7:39instrument may have slight damage as well as wear of
98. 7:42instrument gears etc. with which there will be a slight error
99. 7:47on the speed indication, and second type of error we have here
100. 7:51is the position error that we already saw more in detail in the previous video
101. 7:55about the pitot-static system this is due to changes in maneuvering, angles of
102. 8:01attack or aircraft configuration and directly affects the reading of both
103. 8:06the static port as from the pitot tube in such a way that for this part also
104. 8:11we have a little error in the pressure measurement this then brings us to the
105. 8:16next definition of speed which is the calibrated speed of the acronym CAS
106. 8:22from the english "calibrated airspeed" this corresponds to the indicated speed
107. 8:27IAS that we saw previously, corrected for these installation and position errors
108. 8:32that is if we take the speed indicated by the instrument and
109. 8:36we correct it for these errors we will get the calibrated speed
110. 8:41the question is how is this calibrated airspeed determined, well for this
111. 8:46manufacturers build tables of speed calibration which
112. 8:51are based on certain configurations and speeds as we can see in
113. 8:56this example in this case this is a speed calibration table for
114. 9:00a cessna 172 type aircraft with this one we can do a little example
115. 9:05suppose they ask us what it would be the calibrated speed if we fly with
116. 9:10an indicated speed of 100 knots with flaps extended to 10 degrees then
117. 9:16well in this case we refer to the row of flaps 10 degrees and we see that
118. 9:21we have both the indicated speed and calibrated here as a side note
119. 9:26when speeds are expressed in knots it is common to include a K
120. 9:31before the speed as we can see in this example now they are asking us for
121. 9:36an indicated speed of 100 knots with which we then see 100 knots of
122. 9:41indicated corresponding to 98 knots of calibrated speed and this would be the
123. 9:46answer as we can see in this case the difference between the
124. 9:50indicated and calibrated speed is quite small with which almost always in the
125. 9:56practice it is assumed that the indicated and calibrated speed are the same
126. 10:01having said this we can move on with the next type of error which is
127. 10:06known as compressibility error. When the pitot-static system is designed
128. 10:11the air is assumed to be an ideal gas and one of the characteristics of an
129. 10:15ideal gas is that it is incompressible is say you can't compress it
130. 10:20However in practice the reality is that the air when flying at high speed
131. 10:25is compressed and therefore it generates certain errors in the measurement of the
132. 10:29system when I say high speed I mean speeds greater than
133. 10:33300 knots with which this error is almost nil in small aircraft
134. 10:40in case we're going to fly at high speed we must then add a
135. 10:44compensation for the air compressibility factor giving as
136. 10:48result the following definition of speed which is the equivalent speed
137. 10:53with the acronym EAS of the English "equivalent speed"
138. 10:57this is the calibrated speed CAS corrected for compressibility errors
139. 11:03of the air that is if we take the calibrated speed and we apply the
140. 11:08compressibility errors corrections we get the equivalent speed
141. 11:14remember that this correction only it's done if we're going to fly high
142. 11:18speed and the truth is that doing this correction manually may be a bit
143. 11:22complex, that´s why normally this correction is calculated in real time
144. 11:26by the avionics computers of the plane with which the pilot does not have
145. 11:31to worry about this correction
146. 11:35having seen this let´s continue with the other type of error which is the error produced by
147. 11:40density of the air and is that as we said previously dynamic pressure does not
148. 11:44only depends on the speed but also of the air density in other
149. 11:49words the indication of our instrument will not only depend
150. 11:52of our speed with respect to air but also its density let's see
151. 11:57this by means of an example here we have two pitot tubes that correspond
152. 12:02to two aircraft in this case the tube on top is going to be moving
153. 12:07in a low density air that is to say that air particles are more
154. 12:11separated from each other while the tube on the bottom will be
155. 12:15moving in higher density air that is, the particles are more
156. 12:20close to each other assuming then that both tubes are
157. 12:24moving at the same speed through the air, let's see what happens with the
158. 12:28speed indication, in the case of the tube on the top we can see that
159. 12:33as it moves in low air density it captures few air particles which
160. 12:39means that it experience less dynamic pressure with which the airspeed indicator
161. 12:43will show a lower speed on the other hand the tube that is
162. 12:48moving in denser air we can see you get a greater amount of
163. 12:53particles so then it will measure a greater dynamic pressure and therefore the
164. 12:58indicator will give a higher speed reading. we can see then that
165. 13:04even though these tubes were moving at the same speed through the air the
166. 13:08simple fact of moving in different densities result in different
167. 13:12indicated speeds and that is why speed must be corrected by
168. 13:17density of the air which gives us as result true airspeed
169. 13:22also known as TAS from english "TRUE AIRSPEED" this is the speed corrected by density error
170. 13:29and that mainly depends on the air temperature and static pressure
171. 13:35we can also say that this is the actual speed of the aircraft
172. 13:38with respect to the air, since we´ve already applied all nessesary corrections
173. 13:44so in order to understand how to apply
174. 13:47this correction we must know how does the air density behaves in our planet
175. 13:52in this case at higher altitude we will have less pressure and
176. 13:55then we will have lower air density, this means that if we take two samples of
177. 14:01air one at sea level and another at 10,000 feet we will have a lower density at
178. 14:0610,000 feet and since there is less density an aircraft flying at that altitude will
179. 14:11experiment lower dynamic pressure, so the airspeed indicator reading will not
180. 14:16correspond to the true airspeed, something important to note here
181. 14:21is that the instrument is calibrated so it shows the true airspeed only at sea level
182. 14:27with standard conditions, so
183. 14:30in other words the airspeed indicator assumes that the density of air is always that of sea level
184. 14:34with which we can say that at sea level
185. 14:38the indicated airspeed and true airspeed will be
186. 14:42approximately equal, however to As we climb and the density starts
187. 14:47reducing then this relationship will change, let's see an example
188. 14:52here we have a column of air from 0 to 10,000 feet and we can see how
189. 14:57the density changes with altitude in this case we are going to assume that
190. 15:01the conditions are standard so at sea ​​level we have a pressure of 29,92 InHg
191. 15:08and the temperature is 15 degrees celcius
192. 15:11assuming this, let´s suppose that an aircraft flies very close to the sea level
193. 15:15​and in this case its airspeed indicator shows you 100 knots
194. 15:20since near the sea level the indicated and true airspeed are
195. 15:24roughly equal the TAS speed is also 100 knots
196. 15:30However, if the aircraft flies higher in lower air density and its
197. 15:34indicator gives 100 knots that would mean that the aircraft must be flying at a
198. 15:39higher true airspeed in this case the TAS is 120
199. 15:44knots and in this way the TAS continues to increase as we climb
200. 15:50the question is how do we determine that true airspeed?
201. 15:55how the TAS is calculated? well There are two main methods the most
202. 16:01precise is by means of a flight computer either manual or electronic this
203. 16:05procedure we already saw how to do it in the video about the flight computer
204. 16:09however there is another method which is by means of an approximate formula and
205. 16:15it is based on the concept that the TAS speed increases by
206. 16:18about 2 percent for each 1000 feet of altitude gain
207. 16:23in such a way that knowing this we can make a simple rule of 3 to
208. 16:28determine what the increment of the TAS at a certain altitude is, let's see
209. 16:33these two methods by means of an example, suppose they ask us to
210. 16:37calculate true airspeed with an indicated airspeed of 110 knots
211. 16:41under the following conditions: a pressure altitude of 10,000 feet and a
212. 16:465°C outside temperature
213. 16:49initially let's use the formula, in this case if for every 1000
214. 16:54feet the speed increases by 2% how much will it increase in 10,000 feet then
215. 17:00here then we solve this equation doing the math so
216. 17:05that it gives us 20% of increment here we can do two things
217. 17:10we can multiply 110 by 0.20 which is going to give us the 20 percent of 110
218. 17:16which is 22 knots and then add those 22 knots to the original airspeed which was 110
219. 17:23which gives us 132 knots of TAS
220. 17:27or the other simpler way is multiplying directly 110 by 1.20
221. 17:34say 110 multiplied by 120% which gives us as a result
222. 17:39directly the 132 knots of TAS either of these two
223. 17:43procedures can be done 132 knots is then the result of the
224. 17:49approximate formula let's see what is the result in case of using the
225. 17:53flight computer, well, first we align in this part the
226. 17:58temperature of 5 degrees centigrade with 10,000 feet pressure altitude
227. 18:02then we look at the middle scale 110 indicated airspeed and we observe in
228. 18:09the outer scale 130 knots of TAS
229. 18:13apart from these two methods there is another one that we can use
230. 18:17being on the plane and it is through the TAS speed scale of the
231. 18:22airspeed indicator and is that some instruments incorporates a sliding scale of TAS
232. 18:28let's see how it´s used we must first use this knob
233. 18:32in the bottom right corner to adjust altitude with the corresponding temperature
234. 18:37on this scale of the top, once that is done
235. 18:42the scale below will be adjusted automatically to display the
236. 18:45speed you are against the indicated airspeed
237. 18:50for example in this case this instrument has an IAS of
238. 18:54175 knots and then on the scale of TAS it shows 202 knots in such a way
239. 19:01that we can determine directly and in real time the TAS
240. 19:07finally the last speed figure
241. 19:10what we are going to see is the speed with with respect to the terrain also known
242. 19:14as GS or ground speed this is the actual speed of the aircraft
243. 19:19with respect to the terrain and this corresponds to the true airspeed (TAS)
244. 19:26corrected by wind
245. 19:28first let´s see how an aircraft is affected by wind when an aircraft experiences
246. 19:33tailwind is literally going to to be pushed is to say that its speed
247. 19:38with respect to the terrain will increase while if the aircraft flies
248. 19:43against the wind that is to say with a headwind this is going to reduce it´s speed
249. 19:48so we are going to have a lower speed with respect to the terrain
250. 19:53in order to then determine the ground speed
251. 19:56we must apply the correction for wind we have to start from the idea that
252. 20:01when there is no wind the speed with respect to the air (TAS) , is going to be equal to
253. 20:06the speed with respect to the ground is say ground speed (GS) let's see it by means of
254. 20:11an example here an aircraft is then moving in a calm air that is
255. 20:16that there is no wind, if this aircrafts travels at a speed of 100 knots TAS
256. 20:22since the air is still, that means that the aircraft is also moving
257. 20:25with respect to the terrain at 100 knots
258. 20:29with which we can show that the ground speed is equal to the TAS
259. 20:35now, in the event that we have a headwind the
260. 20:38speed with respect to the ground will be less than the speed with respect to the air
261. 20:43let's see why, in this case this aircraft is going to move
262. 20:46in an air mass that moves at 20 knots against it, therefore we have a 20 knots headwind
263. 20:53in this case if the aircraft is moving
264. 20:56through the air at 100 knots we will see that the speed of that aircraft
265. 20:59with respect to the ground will be 20 knots lower in this case 80 knots with which
266. 21:06we can show that the groundspeed is going to be less than the TAS
267. 21:11and the opposite happens when we have a tailwind in this
268. 21:15case the speed with respect to the terrain is going to be greater than the
269. 21:19speed with respect to air for example here we have an aircraft that moves with
270. 21:24a tailwind of 20 knots if this aircraft travels through the air at
271. 21:28100 knots and the air travels in the same direction at 20 knots, that means
272. 21:33the aircraft is going to be moving with with respect to the terrain at 120 knots
273. 21:38so we can show that the groundspeed is greater than the TAS in this case
274. 21:43with this we then end the explanations of the different speeds
275. 21:49as a summary if we take the indicated airspeed and we correct it for installation and position errors
276. 21:54it will give us the calibrated speed, if then
277. 21:57we apply compressibility errors, that gives the equivalent airspeed, if then
278. 22:02we apply density error, it gives us the true airspeed, and finally if
279. 22:07we apply error due to wind, it gives us as result the ground speed or
280. 22:11with respect to the terrain. Now some aircraft perform these calculations
281. 22:16automatically in real time via their avionics computers on board for
282. 22:20example air data computers, However if the aircraft is not equipped with
283. 22:25this system the pilot will have to do these corrections manually
284. 22:30another important thing to note is that in practice it can be assumed that the indicated
285. 22:34airspeed is approximately the same as the calibrated speed and apart from that if
286. 22:40we fly at low speeds we will have a little influence from the
287. 22:43equivalent airspeed with which we can summarize this in these speeds
288. 22:48indicated airspeed corrected by
289. 22:51density gives us true airspeed and this corrected for wind
290. 22:56gives us ground speed
291. 22:59and this what is applied in day to day operations. another important thing to mention
292. 23:04is that it is important to understand the definition of each of the
293. 23:08speeds especially these three that are the most important since each one
294. 23:13is used for different purposes, it is false that only one of these is used
295. 23:20let´s see some practical applications of
296. 23:22these speeds, the indicated speed directly depends on dynamic pressure
297. 23:28and that is why it is useful for establishing structural limits,
298. 23:31take-off and landing speeds, stall speeds and
299. 23:35some performance speeds like
300. 23:39glide speed, Vx and Vy speed
301. 23:44on the other hand the TAS is the true speed at which the
302. 23:48aircraft moves through the air which is useful for the ATS flightplan
303. 23:54calculation of MACH speed and some endurance calculations
304. 23:57the wind correction angle calculation just like it is
305. 24:01used in some aspects of flight planning
306. 24:05and finally the ground speed is the actual speed of the aircraft
307. 24:10with respect to the terrain and therefore it´s used for calculating flight time
308. 24:14calculating estimates, calculating fuel as well as climb and
309. 24:19descent parametters as for the general flight planning
310. 24:24I hope this video has served you well to understand how an
311. 24:28airspeed indicator works and what are the different speeds used in aviation
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