Airspeed.pdf

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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.

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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.

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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      

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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      

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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

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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 =

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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

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