T
I
C
G
Airspeed Measurements
ICeT
E
Airspeed Fun (part 1)
In his book, Sled Driver, SR-71 Blackbird pilot
Brian Shul writes: "I'll always remember a certain
radio exchange that occurred one day as Walt (my
back-seater) and I were screaming across
Southern California 13 miles high. We were
monitoring various radio transmissions from other
aircraft as we entered Los Angeles airspace.
Though they didn't really control us, they did
monitor our movement across their scope.
Airspeed Fun (part 2)
I heard a Cessna ask for a readout of its ground speed. "90 knots"
Center replied.
1
Airspeed Fun (part 3)
"Moments later, a Twin Beech required the same." "120 knots," Center answered.
Airspeed Fun (part 4)
We weren't the only ones proud of our ground speed that day as
almost instantly an F-18 smugly transmitted, "Center, Dusty 52
requests ground speed readout." There was a slight pause, then the
response, "525 knots on the ground, Dusty." Another silent pause.
Airspeed Fun (part 5)
As I was thinking to myself how ripe a situation this was, I heard a
familiar click of a radio transmission coming from my back-seater. It
was at that precise moment I realized Walt and I had become a real
crew, for we were both thinking in unison." "Center, Aspen 20, you
got a ground speed readout for us?"
There was a longer than normal pause “Aspen, I show 1,742
knots" No further inquiries were heard on that frequency.
2
Airspeed Indicators
A simple mechanical device attempting to “solve” a very complex equation.
Static Pressure Port
Total Pressure Port
Output calibrated to measure
indicated airspeed, VI.
Total Pressure
Static Pressure
Indicated Airspeed
Simply put, it’s what the pilot reads on the airspeed indicator is
not the actual speed of the aircraft with respect to the air - Why
not?
In manufacturing the airspeed indicator we have assumed the
airplane is flying at sea level, standard day conditions and is
perfectly installed on the aircraft.
Must convert to True Airspeed through a series of corrections:
VI : Indicated airspeed
VC : Calibrated airspeed
Ve : Equivalent airspeed
Correct for individual
installations
Correct for actual pressure
Correct for actual density
V : True airspeed
True Airspeed Equation
1



 ( Po − P∞ )  3.5 !  
 1 

e
V = V∞ =
+ 1m − 1 
 7 P∞ 

P∞ t ho

 ρ ∞ 
a

 
 hi s

t
ry
This
requires
true static tpressure,
density, ρ ,
Correct
for density
pressure
∞
ot CorrectP∞for, and
Do
n
but
the True
airspeed indicator is calibrated to measure:
through
a formula
through a table
Vc
=
1



 ( Po − P∞ )  3.5  
 1 

+ 1 − 1 
 7 PSL 


 PSL

 ρ SL 

 


3
Calibrated Airspeed
Calibrated airspeed is the first step in correcting
indicated airspeed back to “true” airspeed - the
aircraft’s actual velocity through the air
It is indicated airspeed corrected for position error
VC = VI + ∆VP
Vc : Calibrated airspeed
V I : Indicated airspeed
∆VP : Position error
Equivalent Airspeed
Equivalent airspeed is calibrated airspeed corrected for
non-standard pressure
We make the correction using a pressure correction or
“f” factor:
Ve = f Vc
Ve : Equivalent airspeed
f : Pressure correction factor
True Airspeed Equation
1



 ( Po − P∞ )  3.5  
 1 

V = V∞ =
+ 1 − 1 
 7 P∞ 

P∞


 ρ ∞ 

 


This requires true static pressure, P∞ , and density, ρ ,
Correct for pressure
∞
but the True airspeed indicator is calibrated to measure:
through a table
Vc
=
1



 ( Po − P∞ )  3.5  
 1 

+ 1 − 1 
 7 PSL 


 PSL

 ρ SL 

 


4
Compressibility Correction f Factors
Density Correction
1



3.5


P
−
P
(
)
 1 

o
∞

V = V∞ =
+ 1 − 1 
 7 P∞ 

P∞


 ρ ∞ 

 

This
requires
true static pressure, P∞ , and density, ρ ,
Correct
for density
∞
but
the True
airspeed indicator is calibrated to measure:
through
a formula
Ve
=
1



 ( Po − P∞ )  3.5  
 1 

+ 1 − 1 
 7 P∞ 


P∞


 ρ SL 

 

True Airspeed
True airspeed (V) is equivalent airspeed corrected for nonstandard density
It is the aircraft’s actual speed with respect to the air mass
Note: q =
1
1
ρV 2 = ρ SLVe 2
2
2
5
Groundspeed
Groundspeed is the aircraft’s actual velocity with
respect to the surface of the Earth
It is true airspeed corrected for wind
Groundspeed is found from a vector sum:
VG = V + Vwind
Airspeed Summary
ρ
V =V
V =V +∆V
C
I
SL
ρ
e
T
I ndicated airspeed
C alibrated airspeed
e quivalent airspeed
T rue airspeed
→
→
VG = V + V wind
P
I
G
→
C
e
V = fV
e
C
Example
You are flying your F-117 with an indicated airspeed of 304 knots. Your
altimeter reads 20,000 feet PA (pressure altitude), and your stealthy
thermometer reads -33o F (burrr!). You have a 40 knot tailwind and
know that the position error for the aircraft is -4 knots. Calculate the
aircraft’s groundspeed.
VI = 304 KIAS
VC =
VE =
V=
VG =
6
Manometers
Devices for measuring pressure differences
Assume dh is small - implies g is constant
Assume the fluid is liquid - ρ is constant
P2
dP = − ρ gdh
2
2
1
1
h2
∫ dP = − ρg ∫ dh
P1
Manometry Equation
∆h
+
P 2 − P 1 = − ρ g ( h 2 − h 1)
h1
ρ liquid = constant
Low Speed Wind Tunnel
For our conditions:
1
2
ƒ ρS = ρroom = 1.0066 kg/m3
(SA @ 2 km)
Settling
Chamber
ƒ VS = Vroom = 0
Test Section
PS P
V
VSA
1
1
ρS ρ
1
Nozzle
Diffuser
PT P
V
VTA
Therefore Bernoulli’s equation:
2
2
ρT ρ
2
ƒ PS = Proom = PO
ƒ PO = PT +
1
ρVT
2
2
And Velocity:
∆h
VT =
2 (P O − P T
ρ
)
=
2 ρ liquid g ∆ h
ρ
air
7