Sistema Pitot-Estático — Transcript

Full transcript

1. 0:06Today we will talk about the pitot static system which is a system that powers some of the basic flight instruments
2. 0:14The basic flight instruments are these six instruments that we will find in any aircraft since they provide information
3. 0:22about the most important parameters to consider when flying an airplane
4. 0:27precisely because of what there are six instruments and they are in this arrangement they are sometimes referred to as the six pack
5. 0:34in this we find the speedometer
6. 0:37attitude indicator altimeter turn coordinator heading indicator and variometer or
7. 0:44vertical speed indicator
8. 0:47However, it is not enough just to know what information these instruments give us and how to interpret it
9. 0:53but also to know how they work, that is, their principle of operation
10. 0:58in this case three of these instruments
11. 1:01specifically the velocimeter the altimeter and the variometer use air pressure to give the information
12. 1:07while on the other hand the attitude indicator the heading indicator and the turn coordinator use the gyroscopic principles
13. 1:16with which the operation of these instruments is quite different
14. 1:20In this particular video we are going to focus on the system that allows us to measure the air pressure to power these three instruments
15. 1:28which is known precisely as the pitot-static system
16. 1:33this then measures the air pressure and provides the information to the relevant flight instruments
17. 1:39however it is not so simple since there are different types of air pressure
18. 1:44we have the static pressure the dynamic pressure and the total pressure
19. 1:49So in order to understand how the pitot-static system works, we must see the definition of each of these pressures
20. 1:57then we start with the static pressure
21. 2:00This is also known as atmospheric pressure and it is the pressure that the air exerts on the objects that are inside the atmosphere
22. 2:09this pressure is distributed evenly
23. 2:12around all objects as we can see in this image
24. 2:16and we must also say that this pressure is present at all times
25. 2:21regardless of whether the object is stationary or moving
26. 2:26having seen these basic characteristics we could ask ourselves on what does the static pressure depend
27. 2:32Well the atmospheric pressure or static pressure varies mainly with altitude but let's see why
28. 2:39here we have the
29. 2:41surface that in this case we are going to assume is at sea level and here we have a column of air that is
30. 2:46spreads up into the atmosphere
31. 2:49As we can see the air molecules are much closer together at the bottom near the surface
32. 2:57while as we go up these they are separating more and more
33. 3:02and this has an explanation
34. 3:04quite logical air is made of matter and matter has weight
35. 3:09with which the gravity of the earth makes the column of air that is above us exert a certain weight
36. 3:17which causes the particles underneath to compress more and more and stay closer together
37. 3:24This certain weight that we feel from the air column that we have above is precisely the static pressure
38. 3:30and at sea level this static pressure is approximately 30 inches of mercury
39. 3:36to be exact under standard conditions this is 29,92 inches of mercury
40. 3:43however this value may vary slightly
41. 3:46depending on the weather conditions
42. 3:49in this order of ideas as we climb in the atmosphere we experience less static pressure
43. 3:54for example if we climb a mountain in this case the column of air that we have above is smaller
44. 4:01with which there is less air that is exerting weight on us and therefore the static pressure is reduced
45. 4:08in this example in this part of the mountain we measure a pressure of 24 inches of mercury
46. 4:14obviously less than sea level pressure
47. 4:17and if we continue climbing the mountain we can see that the static pressure decreases more and more
48. 4:23at the top for example we can measure a pressure of 15 inches of mercury
49. 4:28since we just have very little air above us that is pressing us
50. 4:33in general terms we can say that the static pressure is reduced by one inch of mercury for every 1000 feet that we increase
51. 4:41our altitude
52. 4:43which means that an aircraft that is flying at low altitudes will experience a higher static pressure than one that is
53. 4:50flying higher
54. 4:52and this phenomenon not only occurs with air but also with any other fluid such as water
55. 4:59here we have the example of a swimming pool
56. 5:02as we can observe the way in which the molecules behave is quite similar
57. 5:07with which a person who is swimming on the surface will experience less pressure than one who is
58. 5:14swimming in the depth
59. 5:16and we can actually feel that change in the static pressure of the water as we submerge
60. 5:23especially in the ears
61. 5:26since we have seen and understood the definition of static pressure, let's move on to the dynamic pressure
62. 5:32this is the pressure that air exerts on an object moving through it
63. 5:37that is, when the air impacts at a certain speed, that air exerts a certain pressure on us, that is, the dynamic pressure
64. 5:45in this case it is logical that this pressure is exerted in the opposite direction to the trajectory of the object as we can see in this example
65. 5:53This dynamic pressure depends directly on the speed at which the aircraft is moving and also on the density of the air
66. 5:59which means that as we fly faster we will have a greater dynamic pressure and vice versa
67. 6:07For example, if we fly at 80 knots through the air, we will have less dynamic pressure than if we are flying at 120 knots.
68. 6:15Having already seen then the definition of dynamic pressure, let's now turn to the last one, which is the total pressure.
69. 6:22also known as pitot pressure
70. 6:25this corresponds to the sum of the static pressure plus the dynamic pressure
71. 6:29let's see this through an example
72. 6:31Let´s suppose we are traveling on the highway and put one hand out the window in such a way that the palm of the hand is
73. 6:38exposed to air flow
74. 6:40in this case the hand will experience two pressures on the one hand we have the static pressure that is always present
75. 6:48regardless if we are moving or not. And we will also experience
76. 6:52dynamic pressure of the air flow against which we are moving
77. 6:57so in this case in the palm of your hand you will experience the sums of both
78. 7:01pressures that is the sum of the static pressure plus the dynamic pressure
79. 7:06and this is exactly what happens with an aircraft that is moving through the air. It will experience both pressures
80. 7:14having already understood all these definitions, let us now see what the pitot-static system consists of.
81. 7:20This in its simplest form is composed of a pitot tube and a static port that feed the three instruments that we had mentioned previously.
82. 7:29in this case the static socket sends information to the three instruments, to the velocimeter
83. 7:35altimeter and variometer while the pitot tube only sends information to the velocimeter
84. 7:41then we will see in more detail each of these sensors, let's start with the pitot tube
85. 7:47This is a small tube that allows us to measure the impact pressure of the air as the plane moves through it
86. 7:54we can find it typically under the wings on small aircraft or in the front of the fuselage on larger aircraft
87. 8:03The idea of ​​the pitot tube then is to measure precisely the pressure with which the air impacts the aircraft
88. 8:09and as we had already said with the example of the hand in this case the air pressure will combine the static pressure and
89. 8:16the dynamic pressure with which the pitot tube measures total pressure
90. 8:22Now in order to ensure that this pitot tube
91. 8:25make an adequate measurement it is important that it is aligned with the longitudinal axis of the aircraft as we can see in this example
92. 8:33and apart from this that it is located in a part where the air flow is free from interference
93. 8:40that is, when an aircraft moves through the air, precisely this flow is altered by the parts of the plane
94. 8:46like for example the propeller, the wings etc.
95. 8:50and this interfering flow is not suitable for measuring total pressure
96. 8:55therefore, the pitot tube should be located in such a way that it is exposed to the proper air flow
97. 9:01in other words the free air flow
98. 9:04However, a problem with this design is that being an object
99. 9:08exposed to air flow is also susceptible to icing which can block the main orifice
100. 9:14and thus produce errors in pressure measurement
101. 9:19With which to solve this the pitot tube incorporates a heating system
102. 9:24consisting of electrical resistances that when they are turned on from the cabin make the tube heat up and
103. 9:31melt any ice that may clog the tube
104. 9:34the switch by which this heating system is turned on
105. 9:38it is usually marked as pitot heat and is located near the light switches
106. 9:45Now another component of the pitot tube is the drain hole
107. 9:49this is a small hole in the back of the tube that allows water to be removed from the system
108. 9:55this in case of flying in rainy conditions or when thawing ice using the pitot tube heating system
109. 10:03As we can see, the design of this pitot tube is designed exactly to eliminate any possible error in the pressure measurement.
110. 10:11however there are some that cannot be eliminated, the so-called position errors
111. 10:16and it is that when flying at high angles of attack the air flow that hits the aircraft is not parallel to the longitudinal axis
112. 10:23of the plane that is to say that it is not parallel to the pitot tube
113. 10:27let's see this through an example when we fly at low angles of attack we can see that the air flow is practically
114. 10:35aligned with pitot tube
115. 10:36which allows obtaining a fairly adequate total pressure measurement
116. 10:41however if we fly at a high angle of attack
117. 10:44we can see that the air flow is now no longer aligned with the pitot tube
118. 10:49such that slight errors occur in the measurement of the total pressure
119. 10:54these errors are taken into account by the manufacturer in different configurations
120. 10:58and speeds to publish a correction table in the aircraft manual
121. 11:03in this case this calibration table corresponds to the airspeed indicator since it is the instrument to which the pitot tube is connected
122. 11:12however we will see this a little more in detail in the specific video about the airspeed indicator
123. 11:18We must then bear in mind that we are going to have slight errors in the measurement of total pressure when performing maneuvers such as slow flight
124. 11:25gusts and turbulence or the use of flaps and slats
125. 11:30Having said this, we finish with the cover of the pitot tube. It is important to keep the system free of dirt
126. 11:37insects, water etc.
127. 11:39so on the ground a cover is used for this tube that is normally in a striking color and has the inscription
128. 11:46remove before flight
129. 11:48just to remind the pilot to remove this cover before flying
130. 11:52since otherwise you will not have a reading on your airspeed indicator
131. 11:58with this we then end the pitot tube
132. 12:01now let's move on with the other component of the system, the static port
133. 12:06This consists of a small hole normally located on the side of the fuselage and it measures the air static pressure
134. 12:14in other words atmospheric pressure
135. 12:17This static port is located in such a way that only static pressure enters and not dynamic pressure
136. 12:23let's see how this is achieved normally this is located on one side of the fuselage where a precise value can be obtained
137. 12:31suppose that this plane has it in this part if we make a zoom we can see the following
138. 12:37as we know the static pressure is exerted in all directions
139. 12:41while the dynamic pressure is exerted only in the opposite direction to the plane's path
140. 12:47With this design we can see that the static intake only enters the static pressure and not the dynamic
141. 12:55thus obtaining an adequate measurement
142. 12:58However, this design also has some position errors as happened with the pitot tube.
143. 13:04and it is that if we make a skid or a slide
144. 13:07part of the dynamic pressure will enter the static intake thus producing errors in the measurement
145. 13:14This occurs precisely because in a skid or slide the air flow not only hits the air from the front
146. 13:21but slightly on one side
147. 13:24if we look at the previous example but now making a skid we can see the following in the static port
148. 13:30the static pressure will not change, however the dynamic pressure will now be exerted
149. 13:36slightly against the fuselage where the static port is located
150. 13:39which results in that part of that dynamic pressure will enter the static intake
151. 13:46and apart from this effect the use of flaps slats landing gear or other parts of the plane
152. 13:52can also cause changes in the local static pressure, so this effect must also be taken into account in the correction tables.
153. 14:00in this case we have a correction table for the airspeed indicator and another for the altimeter
154. 14:05since, as we will remember, the static port provides information to the three instruments
155. 14:11as we will remember the pitot tube had a heating system that prevented it from being blocked by ice
156. 14:16the question is what happens if this static port gets blocked?
157. 14:20Well for this we have an alternate static port
158. 14:24which in essence is basically an additional static port
159. 14:28that is connected to the system and can be activated by means of a switch in the cockpit
160. 14:34However, this alternate static port has a peculiarity which is that it is normally located inside the cabin
161. 14:41this to avoid a potential blockage by external conditions
162. 14:45which in principle is a pretty good design
163. 14:48however the problem is that the static pressure measured inside the cabin is slightly different from the exterior static pressure
164. 14:56with which we will have a slight error in the measurement
165. 15:00This error is taken into account in the calibration tables, however, make sure that the calibration table
166. 15:05is for alternate static port
167. 15:09with this we have already seen all the information about the static port
168. 15:14one thing to finalize is that in multi-engine aircraft or larger aircraft it is common to find two independent pitot-static systems
169. 15:22one for the pilot's instruments and the other for the copilot's
170. 15:27This design is quite beneficial as it reduces measurement errors and provides redundancy to the system.
171. 15:35since instruments on one side can be compared with those on the other side
172. 15:39to check how much difference they have or if some kind of problem is occurring
173. 15:45This procedure is known as a cross check.
174. 15:50I hope this video has helped you understand what a pitot-static system is
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