Elements of Aerospace Engineering
Content
Elements of Aerospace Engineering
Lecture 10
Measurement of Airspeed
Elements of Aerospace Engineering
Airspeed
Measurement of Airspeed AE602 Elements of Aerospace Engineering
Manoj T. Nair IIST 10.1
Agenda
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
1
Measurement of Airspeed
2
Airspeed
10.2
Measurement of Airspeed I
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Airflow velocity in the test section of a low-speed wind tunnel can be obtained by measuring p1 − p2 We assumed that the flow properties are constant over a cross section This is not true in reality - the velocities in the middle of the test section would be higher than near the walls These changes can only be obtained if we are able to measure velocities at a point This measurement can be done by a Pitot static tube
10.3
Measurement of Airspeed II Static pressure at a point is the pressure we would feel if we were moving along with the flow Whenever the word pressure is used, it means the static pressure
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The second type of pressure is the total pressure
Consider a fluid element moving along a streamline The pressure of the gas in this element is the static pressure Imagine we slow down this element to zero velocity isentropically 10.4
Measurement of Airspeed III
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The p, T , and ρ would increase above their original values when the element was moving along the streamline The value of p, T , and ρ of the fluid element after it has been brought to rest are called total values These are denoted at p0 , T0 and ρ0 Therefore, total pressure at a given point in a flow is the pressure that would exist if the flow were slowed down isentropically to zero velocity When the gas is not moving, i.e. the fluid element has no velocity in the first place, then p0 = p
10.5
Measurement of Airspeed IV There is an aerodynamic device that measures the total pressure at a point in the flow: Pitot tube
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This consists of a tube placed parallel to the flow and open to the flow at one end (point A) The other end of the tube is closed (point B) 10.6
Measurement of Airspeed V Now we start the flow Gas fills inside the tube and, after a few moments there would be no motion inside the tube
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The gas would be stagnant everywhere inside the tube including at point A Due to this flow field would see the point A as an obstruction The fluid element moving along the streamline C would stop when it arrives at A Since friction and heat transfer are absent, the process would be isentropic 10.7
Measurement of Airspeed VI
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
i.e., due to the presence of the pitot tube, the fluid element moving along C would be brought to rest So, the pressure at A must be the total pressure p0 Then if a pressure gauge is kept at point B, we would be able to measure this total pressure By definition, any point in the flow where V = 0 is called a stagnation point Therefore, A is a stagnation point 10.8
Measurement of Airspeed VII Now consider the following arrangement
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
There is uniform velocity with velocity V1 moving over a flat surface parallel to the flow There is small hole in the surface at point A Point A is called the static pressure orifice
10.9
Measurement of Airspeed VIII
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Since the surface is parallel to the flow, only the random motion of the molecules is felt at the plate The surface pressure is the static pressure, p This would be the pressure at the orifice at point A The pitot at point B would feel the total pressure p0 If the static pressure orifice at A and Pitot tube at B are connected across a pressure gauge, it will measure the difference p0 − p This pressure difference gives a measure of the velocity V1
10.10
Measurement of Airspeed IX A combination of total and static pressure measurement allows us to measure the velocity These to measurements can be combined in a single instrument call the Pitot-static probe
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
p0 is measured at the nose of the probe and p is measured at a point downstream of the nose p0 − p yields V1 , but the formulation differs whether the flow is incompressible, high-speed subsonic, or supersonic 10.11
Measurement of Airspeed X
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.12
Elements of Aerospace Engineering
Measurement of Airspeed XI Incompressible flow
Measurement of Airspeed Airspeed
At point A, the pressure is p and the velocity is V1 At point B, the pressure is p0 and the velocity is zero Applying Bernoulli’s equation between points A and B p+ q =
1 2 2 ρV
1 2 ρV = p0 2 1
is called the dynamic pressure 10.13
Measurement of Airspeed XII
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
For incompressible flow p0 = p + q The total pressure is the sum of static plus dynamic pressure r p0 − p 2 V1 = ρ This allows the calculation of flow velocity from the measurement of p0 − p obtained from the Pitot-static tube
10.14
Measurement of Airspeed XIII A Pitot tube can be used to measure the flow velocity at various points in the test section
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The static pressure is assumed to be constant throughout the test section This constant static pressure assumption is fairly good for subsonic wind tunnel test sections 10.15
Measurement of Airspeed XIV
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
If the test section is open to the room, the static pressure at all pint in the test section is p = 1atm Density is constant The velocity can be obtained from the expression 10.16
Measurement of Airspeed XV Either Pitot tube or Pitot-static tube can be used to measure the airspeed of airplanes Such tubes extend from airplane wingtips, with the tube oriented in the flight direction
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
If a Pitot tube is used, then the ambient static pressure in the atmosphere is obtained from a static pressure orifice 10.17
Measurement of Airspeed XVI
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This static pressure orifice is strategically placed on the airplane surface such that the surface pressure is nearly same as that of the surrounding atmosphere Such a location is found by experience It is generally on the fuselage somewhere between the nose and the wing The measurements of p0 and p are joined across a differential pressure gauge which is calibrated in terms of airspeed This airspeed indicator is in a dial in the cockpit
10.18
Elements of Aerospace Engineering
Measurement of Airspeed XVII V =
r p0 − p 2 ρ
However, one must decide which value of ρ to be used If ρ is the true value in the actual air around the airplane, then we get the true airspeed r p0 − p Vtrue = 2 ρ
Measurement of Airspeed Airspeed
However, this measurement of true air density at the airplane’s location is difficult Therefore, airspeed indicators on low-speed airplanes are calibrated using standard sea-level value of ρs This gives the equivalent air speed r p0 − p 2 Ve = ρs The equivalent air speed Ve differs slightly from Vtrue by a factor (ρ/ρs )1/2
10.19
Measurement of Airspeed XVIII Problem: The altimeter of a low-speed Cessna 150 private reads 1500 meters. The outside air temperature is 7.5◦ C. If a pitot tube mounted on the wing tip measures a pressure of 8.705 × 104 Pascals. From the standard atmosphere table at 1500 meters p = 8.456 × 104 Pascals. What is the true and equivalent airspeed of the airplane? Ans: 68.85 m/s
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.20
Measurement of Airspeed XIX
Elements of Aerospace Engineering
Subsonic compressible flow Measurement of Airspeed
For high speed flows, where the Mach number is still less that 1, other equations must be used
Airspeed
This is the flight regime of commercial transports like Boeing 747 and some military aircrafts cp − cv = R R 1 = 1− cp /cv cp γ−1 R 1 = = 1− γ γ cp γR cp = γ−1 Again consider the Pitot tube Assume the velocity V1 is high enough that compressibility must be taken into account 10.21
Measurement of Airspeed XX The flow is isentropically compressed to zero velocity at the stagnation point on the nose of the probe Let the stagnation values of pressure and temperature be denoted as p0 and T0
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
From the energy equation cp T1 +
Substituting cp =
1 2 V = cp T0 2 1 V12 T0 = 1+ T1 2cp T1
γR γ−1
V12 T0 γ − 1 V12 = 1+ = 1+ T1 2 [γR/(γ − 1)] T1 2 γRT1 From speed of sound a12 = γRT1 T0 γ − 1 V12 = 1+ T1 2 a12
10.22
Elements of Aerospace Engineering
Measurement of Airspeed XXI Since Mach number M1 = V1 /a1
Measurement of Airspeed
γ−1 2 T0 = 1+ M1 T1 2
Airspeed
Since the gas is isentropically compressed at the nose, we can use γ γ/(γ−1) ρ0 T0 p0 = = p1 ρ1 T1 Then p0 = p1
ρ0 = ρ1
γ−1 2 1+ M1 2
γ/(γ−1)
γ−1 2 M1 2
1/(γ−1)
1+
The temperature relation holds for adiabatic flow, while the pressure and density relation holds for isentropic flow 10.23
Elements of Aerospace Engineering
Measurement of Airspeed XXII
Measurement of Airspeed
Rearranging the following relation
Airspeed
p0 = p1
1+
γ−1 2 M1 2
γ/(γ−1)
We get M12
2 = γ−1
"
p0 p1
(γ−1)/γ
# −1
Hence for subsonic compressible flow, the ratio of total to static pressure p0 /p1 , is a direct measure of Mach number Individual measurement of p0 and p1 can be used to calibrate an instrument called Mach meter The instrument directly reads the flight Mach number of the airplane
10.24
Measurement of Airspeed XXIII The flight velocity can be obtained as " # (γ−1)/γ 2a12 p0 2 V1 = −1 γ−1 p1 " # (γ−1)/γ 2a12 p0 − p1 2 V1 = +1 −1 γ−1 p1
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This gives the true air speed However, it requires a1 and hence T1 The static temperature in the air surrounding the airplane is difficult to measure Hence a1 is assumed to be equal to the standard sea-level value of a = 340.3m/s The calibrated airspeed is given as " # (γ−1)/γ 2as2 p0 − p1 2 +1 −1 V1 = γ−1 ps where as and ps are the standard sea-level values of speed of sound and static pressure
10.25
Measurement of Airspeed XXIV Problem: A high speed subsonic McDonnell-Douglas DC-10 airliner is flying at a pressure altitute of 10 km. A Pitot tube on the wing tip measures a pressure of 4.24 × 104 N/m2 . Calculate the Mach number at which the airplane is flying. If the ambient air temperature is 230 K, calculate the true air speed and the calibrated air speed. (At 10,000 m, p = 2.65 × 104 N/m2
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.26
Elements of Aerospace Engineering
Airspeed I Airspeed is the velocity of an aircraft along its flight path It is an important parameter, as aerodynamic forces are proportional to square of airspeed
Measurement of Airspeed Airspeed
F α V2 Aircraft maneuvers during take-off, turns and landing are initiated only at specified airspeeds Different airspeeds are used in flight mechanics literature Indicated airspeed (IAS) Equivalent airspeed (EAS) Calibrated airspeed (CAS) True airspeed (TAS)
There are also terms like Ground speed, Stalling speed, etc For calculating aircraft performance, TAS is used For the pilot, airspeed is the value shown on the airspeed indicator (ASI) - IAS 10.27
Airspeed II
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Ground Speed (Flight Speed)
The ground speed of an aircraft is its speed along its flight path w.r.t. a fixed location on the ground This is denoted by Vg The ground speed is used when referring to mileage or endurance of the aircraft Presence of winds causes differences between the ground speed and TAS Ground speed is also TAS in case of no wind
10.28
Elements of Aerospace Engineering
Airspeed III
Measurement of Airspeed
True Airspeed
Airspeed
TAS is defined as the velocity of the c.g. of the aircraft w.r.t. the wind velocity If Vw is the wind velocity parallel to the flight path and Vg is the ground speed of the aircraft, TAS velocity is V = Vg ± Vw The +ve sign is for headwind The -ve sign is for tailwind
10.29
Airspeed IV
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Headwind is an oncoming wind that moves in the direction from head to tail The reverse is tailwind TAS is defined w.r.t. the wind due to 1
2 3
The speed sensing devices (pitot-static tube) sense p0 and p w.r.t the wind It is the speed the aircraft configuration feels Aerodynamic forces and moments are proportional to the square of relative speed
10.30
Airspeed V
Elements of Aerospace Engineering
Measurement of Airspeed
Indicated Airspeed
Airspeed
Indicated airspeed is the airspeed shown by the airspeed indicator which a pilot reads in his cockpit It is denoted by Vi Readings of ASI generally has following errors instrument error lag error position error
IAS is designed to read EAS in incompressible flow IAS reads CAS in compressible flow regions
10.31
Lecture 10
Measurement of Airspeed
Elements of Aerospace Engineering
Airspeed
Measurement of Airspeed AE602 Elements of Aerospace Engineering
Manoj T. Nair IIST 10.1
Agenda
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
1
Measurement of Airspeed
2
Airspeed
10.2
Measurement of Airspeed I
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Airflow velocity in the test section of a low-speed wind tunnel can be obtained by measuring p1 − p2 We assumed that the flow properties are constant over a cross section This is not true in reality - the velocities in the middle of the test section would be higher than near the walls These changes can only be obtained if we are able to measure velocities at a point This measurement can be done by a Pitot static tube
10.3
Measurement of Airspeed II Static pressure at a point is the pressure we would feel if we were moving along with the flow Whenever the word pressure is used, it means the static pressure
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The second type of pressure is the total pressure
Consider a fluid element moving along a streamline The pressure of the gas in this element is the static pressure Imagine we slow down this element to zero velocity isentropically 10.4
Measurement of Airspeed III
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The p, T , and ρ would increase above their original values when the element was moving along the streamline The value of p, T , and ρ of the fluid element after it has been brought to rest are called total values These are denoted at p0 , T0 and ρ0 Therefore, total pressure at a given point in a flow is the pressure that would exist if the flow were slowed down isentropically to zero velocity When the gas is not moving, i.e. the fluid element has no velocity in the first place, then p0 = p
10.5
Measurement of Airspeed IV There is an aerodynamic device that measures the total pressure at a point in the flow: Pitot tube
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This consists of a tube placed parallel to the flow and open to the flow at one end (point A) The other end of the tube is closed (point B) 10.6
Measurement of Airspeed V Now we start the flow Gas fills inside the tube and, after a few moments there would be no motion inside the tube
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The gas would be stagnant everywhere inside the tube including at point A Due to this flow field would see the point A as an obstruction The fluid element moving along the streamline C would stop when it arrives at A Since friction and heat transfer are absent, the process would be isentropic 10.7
Measurement of Airspeed VI
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
i.e., due to the presence of the pitot tube, the fluid element moving along C would be brought to rest So, the pressure at A must be the total pressure p0 Then if a pressure gauge is kept at point B, we would be able to measure this total pressure By definition, any point in the flow where V = 0 is called a stagnation point Therefore, A is a stagnation point 10.8
Measurement of Airspeed VII Now consider the following arrangement
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
There is uniform velocity with velocity V1 moving over a flat surface parallel to the flow There is small hole in the surface at point A Point A is called the static pressure orifice
10.9
Measurement of Airspeed VIII
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Since the surface is parallel to the flow, only the random motion of the molecules is felt at the plate The surface pressure is the static pressure, p This would be the pressure at the orifice at point A The pitot at point B would feel the total pressure p0 If the static pressure orifice at A and Pitot tube at B are connected across a pressure gauge, it will measure the difference p0 − p This pressure difference gives a measure of the velocity V1
10.10
Measurement of Airspeed IX A combination of total and static pressure measurement allows us to measure the velocity These to measurements can be combined in a single instrument call the Pitot-static probe
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
p0 is measured at the nose of the probe and p is measured at a point downstream of the nose p0 − p yields V1 , but the formulation differs whether the flow is incompressible, high-speed subsonic, or supersonic 10.11
Measurement of Airspeed X
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.12
Elements of Aerospace Engineering
Measurement of Airspeed XI Incompressible flow
Measurement of Airspeed Airspeed
At point A, the pressure is p and the velocity is V1 At point B, the pressure is p0 and the velocity is zero Applying Bernoulli’s equation between points A and B p+ q =
1 2 2 ρV
1 2 ρV = p0 2 1
is called the dynamic pressure 10.13
Measurement of Airspeed XII
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
For incompressible flow p0 = p + q The total pressure is the sum of static plus dynamic pressure r p0 − p 2 V1 = ρ This allows the calculation of flow velocity from the measurement of p0 − p obtained from the Pitot-static tube
10.14
Measurement of Airspeed XIII A Pitot tube can be used to measure the flow velocity at various points in the test section
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
The static pressure is assumed to be constant throughout the test section This constant static pressure assumption is fairly good for subsonic wind tunnel test sections 10.15
Measurement of Airspeed XIV
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
If the test section is open to the room, the static pressure at all pint in the test section is p = 1atm Density is constant The velocity can be obtained from the expression 10.16
Measurement of Airspeed XV Either Pitot tube or Pitot-static tube can be used to measure the airspeed of airplanes Such tubes extend from airplane wingtips, with the tube oriented in the flight direction
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
If a Pitot tube is used, then the ambient static pressure in the atmosphere is obtained from a static pressure orifice 10.17
Measurement of Airspeed XVI
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This static pressure orifice is strategically placed on the airplane surface such that the surface pressure is nearly same as that of the surrounding atmosphere Such a location is found by experience It is generally on the fuselage somewhere between the nose and the wing The measurements of p0 and p are joined across a differential pressure gauge which is calibrated in terms of airspeed This airspeed indicator is in a dial in the cockpit
10.18
Elements of Aerospace Engineering
Measurement of Airspeed XVII V =
r p0 − p 2 ρ
However, one must decide which value of ρ to be used If ρ is the true value in the actual air around the airplane, then we get the true airspeed r p0 − p Vtrue = 2 ρ
Measurement of Airspeed Airspeed
However, this measurement of true air density at the airplane’s location is difficult Therefore, airspeed indicators on low-speed airplanes are calibrated using standard sea-level value of ρs This gives the equivalent air speed r p0 − p 2 Ve = ρs The equivalent air speed Ve differs slightly from Vtrue by a factor (ρ/ρs )1/2
10.19
Measurement of Airspeed XVIII Problem: The altimeter of a low-speed Cessna 150 private reads 1500 meters. The outside air temperature is 7.5◦ C. If a pitot tube mounted on the wing tip measures a pressure of 8.705 × 104 Pascals. From the standard atmosphere table at 1500 meters p = 8.456 × 104 Pascals. What is the true and equivalent airspeed of the airplane? Ans: 68.85 m/s
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.20
Measurement of Airspeed XIX
Elements of Aerospace Engineering
Subsonic compressible flow Measurement of Airspeed
For high speed flows, where the Mach number is still less that 1, other equations must be used
Airspeed
This is the flight regime of commercial transports like Boeing 747 and some military aircrafts cp − cv = R R 1 = 1− cp /cv cp γ−1 R 1 = = 1− γ γ cp γR cp = γ−1 Again consider the Pitot tube Assume the velocity V1 is high enough that compressibility must be taken into account 10.21
Measurement of Airspeed XX The flow is isentropically compressed to zero velocity at the stagnation point on the nose of the probe Let the stagnation values of pressure and temperature be denoted as p0 and T0
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
From the energy equation cp T1 +
Substituting cp =
1 2 V = cp T0 2 1 V12 T0 = 1+ T1 2cp T1
γR γ−1
V12 T0 γ − 1 V12 = 1+ = 1+ T1 2 [γR/(γ − 1)] T1 2 γRT1 From speed of sound a12 = γRT1 T0 γ − 1 V12 = 1+ T1 2 a12
10.22
Elements of Aerospace Engineering
Measurement of Airspeed XXI Since Mach number M1 = V1 /a1
Measurement of Airspeed
γ−1 2 T0 = 1+ M1 T1 2
Airspeed
Since the gas is isentropically compressed at the nose, we can use γ γ/(γ−1) ρ0 T0 p0 = = p1 ρ1 T1 Then p0 = p1
ρ0 = ρ1
γ−1 2 1+ M1 2
γ/(γ−1)
γ−1 2 M1 2
1/(γ−1)
1+
The temperature relation holds for adiabatic flow, while the pressure and density relation holds for isentropic flow 10.23
Elements of Aerospace Engineering
Measurement of Airspeed XXII
Measurement of Airspeed
Rearranging the following relation
Airspeed
p0 = p1
1+
γ−1 2 M1 2
γ/(γ−1)
We get M12
2 = γ−1
"
p0 p1
(γ−1)/γ
# −1
Hence for subsonic compressible flow, the ratio of total to static pressure p0 /p1 , is a direct measure of Mach number Individual measurement of p0 and p1 can be used to calibrate an instrument called Mach meter The instrument directly reads the flight Mach number of the airplane
10.24
Measurement of Airspeed XXIII The flight velocity can be obtained as " # (γ−1)/γ 2a12 p0 2 V1 = −1 γ−1 p1 " # (γ−1)/γ 2a12 p0 − p1 2 V1 = +1 −1 γ−1 p1
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
This gives the true air speed However, it requires a1 and hence T1 The static temperature in the air surrounding the airplane is difficult to measure Hence a1 is assumed to be equal to the standard sea-level value of a = 340.3m/s The calibrated airspeed is given as " # (γ−1)/γ 2as2 p0 − p1 2 +1 −1 V1 = γ−1 ps where as and ps are the standard sea-level values of speed of sound and static pressure
10.25
Measurement of Airspeed XXIV Problem: A high speed subsonic McDonnell-Douglas DC-10 airliner is flying at a pressure altitute of 10 km. A Pitot tube on the wing tip measures a pressure of 4.24 × 104 N/m2 . Calculate the Mach number at which the airplane is flying. If the ambient air temperature is 230 K, calculate the true air speed and the calibrated air speed. (At 10,000 m, p = 2.65 × 104 N/m2
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
10.26
Elements of Aerospace Engineering
Airspeed I Airspeed is the velocity of an aircraft along its flight path It is an important parameter, as aerodynamic forces are proportional to square of airspeed
Measurement of Airspeed Airspeed
F α V2 Aircraft maneuvers during take-off, turns and landing are initiated only at specified airspeeds Different airspeeds are used in flight mechanics literature Indicated airspeed (IAS) Equivalent airspeed (EAS) Calibrated airspeed (CAS) True airspeed (TAS)
There are also terms like Ground speed, Stalling speed, etc For calculating aircraft performance, TAS is used For the pilot, airspeed is the value shown on the airspeed indicator (ASI) - IAS 10.27
Airspeed II
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Ground Speed (Flight Speed)
The ground speed of an aircraft is its speed along its flight path w.r.t. a fixed location on the ground This is denoted by Vg The ground speed is used when referring to mileage or endurance of the aircraft Presence of winds causes differences between the ground speed and TAS Ground speed is also TAS in case of no wind
10.28
Elements of Aerospace Engineering
Airspeed III
Measurement of Airspeed
True Airspeed
Airspeed
TAS is defined as the velocity of the c.g. of the aircraft w.r.t. the wind velocity If Vw is the wind velocity parallel to the flight path and Vg is the ground speed of the aircraft, TAS velocity is V = Vg ± Vw The +ve sign is for headwind The -ve sign is for tailwind
10.29
Airspeed IV
Elements of Aerospace Engineering
Measurement of Airspeed Airspeed
Headwind is an oncoming wind that moves in the direction from head to tail The reverse is tailwind TAS is defined w.r.t. the wind due to 1
2 3
The speed sensing devices (pitot-static tube) sense p0 and p w.r.t the wind It is the speed the aircraft configuration feels Aerodynamic forces and moments are proportional to the square of relative speed
10.30
Airspeed V
Elements of Aerospace Engineering
Measurement of Airspeed
Indicated Airspeed
Airspeed
Indicated airspeed is the airspeed shown by the airspeed indicator which a pilot reads in his cockpit It is denoted by Vi Readings of ASI generally has following errors instrument error lag error position error
IAS is designed to read EAS in incompressible flow IAS reads CAS in compressible flow regions
10.31
