Airspeed measurement using Pitot-Static probe

Air:To measure airspeed using U-tube manometer and Pitot-Static probe.

Equiptments used: Wind tunnel facility, U-tube manometer and pitot-static probe

Theory:Air speed data is needed for navigation and stability of aircraft.Airspeed, v, iscomponent of ‘½ ρ V2‘ which is expression of dynamic pressure. Hence ‘V’ can be found out from dynamic pressure.

Bernoulli’s equation: In a flow, Static pressure + Dynamic Pressure= Total pressure PStat + ½ ρ V2= PT ----(1) ; PT - PStat= ½ ρ V2----(2) ;

The airspeed measuring instruments which are using PT and PStat are called pressure-based instruments and that are directly interacting with dynamic pressure or velocity pressure of the flow are called velocity based instruments. The Velocity based airspeed measuring instruments are also called as ‘Anemometers’.

Pressure-based instrument Velocity based instrument1. Air speed Indicator2. U-tube manometer

1. Vane type anemometer2. Hot-wire anemometer3. Laser Doppler anemometer

Description:

Pitot-static probe is designed for tapping total pressure (as stagnation pressure) and static pressure separately from the flow. When there is no air flow, there is no dynamic pressure (‘½ ρ V2‘) as ‘v’ is zero; and both the ports of the probe will pick up atmosphere only.

Pitot-static probe

Pitot-static probe is constructed out of two concentric tubes. The inner tube functions as total probe. The probe is kept in alignment with the direction flow.

At the tip of the total probe, ‘v’ becomes zero and thus the total pressure is picked up by the inner, total probe.

The static ports are on the external surface of the outer tube and thus are able to pick up Static pressure in a direction normal to the flow direction.

The static ports form a circle in a plane perpendicular to axis of the probe and are minimum ‘6D’ away from the tip of the probe. So, static pressure is picked up reasonably away from tip where flow gets disturbed. ‘D’ is the diameter of the outer tube.

The dynamic pressure as differential of total pressure and static pressure is sensed when connected to U-tube manometer. Velocity is derived from dynamic pressure.

U-tube manometer

P1 > P2

U-tube manometer is made up of glass tube having uniform cross section area, ‘a’ throughout. It contains a suitable fluid, like methyl alcohol, which shall be of non-sticky nature.

Differential pressure between P1& P2 enables fluid to assume different levels resulting in differential head ‘h’, such that :

(P1 – P2 ) = h . ρliquid . g where ’ ρliquid ’ is density of methyl alcohol ( 800 kg / m3) ;

‘g’ is the acceleration due to gravity (9.8m/s2) and ‘h’ is the differential head in meters.

The flow has both the static pressure and the dynamic pressure as given by Bernoulli’s equation, Static pressure + Dynamic Pressure = Total pressure PStat + ½ ρ V2=PT----(1) ; ½ ρ V2 = PT - PStat----(2) ; Eqn. (2) is rewritten as follows:

Bernoulli also states that the dynamic pressure exerts on solids which it comes across. The arrow marks corresponding to duct static pressure shown in the above figure explain the same.Total and static pressures are tapped using pitot-static probe and are connected as inputs to a u-tube manometer, as shown in the following figure.

From the corresponding differential pressure which in this specific case is nothing but dynamic pressure,Airspeed is calculated. The term ‘½ρ v²‘ is related to ‘h’ as follows:PT - PStat = ½ρ v² = h .ρliquid . g , where ‘h’ is the differential head measured from manometer.ρ is 1.1 kg/m³; ρliquidis 800 kg/m³; g is 9.8 m/sec² ; h is in meters and v is in m/s.½ρ v²= h.ρliquid . g ; ½ X 1.1 X v²= h.800. 9.8 ; V = k (√ h ) m/s (where ‘h’ is in meters)

Units of

differential

head‘∆h’mm cm M (meter)

Value of‘k’ 3.78 11.95 119.5

Units of

velocity ‘v’

m/s m/s m/s

Wind tunnel facility could be set to provide uniform flow at required rpm of motor.

Procedure:After confirming that wind tunnel is intact and free from any external objects or materials, fan motor is switched on and set to run at 400 rpm. Manometer readings corresponding to static pressure as well as total pressure are taken down to arrive at corresponding ‘h’ in mm. Airspeed is calculated from the formula, V = k √ h m/s. This is repeated for 600, 800 and 1000 rpm in steps and corresponding ‘v’ is found. RPM Vs. velocity may be plotted to get a view on tunnel performance.

Tabulation:

TUNNEL PERFORMANCE (RPM Vs VELOCITY)

0 200 400 600 800 1000 1200 14000

5

10

15

20

25

30

35

40

45

50

Result / Conclusion:

The Airspeed is measured using the combination of pitot-static probe and u-tube manometer.

V is plotted against different rpm of motor to view the wind tunnel performance.

The combination of “pitot-static probe with U-tube manometer “ is a standard reference by

itself and is used for calibration of pressure sensors/probes or other air speed measuring instruments.

Sl.

No. RPM

Readings in _________ (k= _______ ) ‘V’ in m/s = k √ h

hT hStat h = hT - hStat

1 400

2 600

3 800

4 1000

5 1200