PURPOSE The purpose of this experiment is to study an d calibrate a pitot static tube. A Pitot tube is an integral component of a Pitot Static System (PSS) used for measur measuring ing the airspe airspeed ed of an aircra aircraft. ft. Proper Properly ly calibr calibrati ating ng it determ determine iness the resultant accuracy of the airspeed.
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T'EORY
igure 1 Pitot Static System (igure !ourtesy http"##$$$.grc.nasa.go%) ig.1 sho$s a schematic dra$ing of a pitot&static tube. Pitot&Static tubes' $hich are also called Prandtl tubes' are used on aircraft as speedometers. The actual tube on the aircraft is around 1 inches (2 centimeters) long $ith a 1#2 inch (1 centimeter) diameter. Se%eral small holes are drilled around the outside of the ________________ _______________________ _______________ ________________ _______________ _______________ ________________ _______________ __________ ___
tube and a center hole is drilled do$n the axis of the tube. The outside holes are connected to one side of a de%ice called a pressure transducer. The center hole in the tube is +ept separate from the outside holes and is connected to the other side of the transducer. The transducer measures the difference in pressure in the t$o groups of tubes by measuring the strain in a thin element using an electronic strain gauge. The pitot&static tube is mounted on the aircraft' or in a $ind tunnel ' so that the center tube is al$ays pointed in the direction of the flo$ and the outside holes are perpendicular to the center tube. ,n some airplanes the pitot&static tube is put on a longer boom stic+ing out of the nose of the plane or the $ing. -ifference in Static and Total Pressure
Since the outside holes are perpendicular to the direction of flo$' these tubes are pressuried by the local random component of the air %elocity. The pressure in these tubes is the static pressure ( P S ) discussed in /ernoulli0s euation. The center tube' ho$e%er' ho$e%er' is pointed pointed in the direction direction of tra%el tra%el and is pressuried pressuried by both the random and the ordered air %elocity. The pressure in this tube is the total pressure ( P T ) discus discussed sed in /ernou /ernoulli lli0s 0s euati euation. on. The pressu pressure re transd transducer ucer measur measures es the difference in total and static pressure $hich is the dynamic pressure .
Measurement = q = PT
−
P S
S56e 2r )e5/i7
ith the difference in pressures measured and +no$ing the local %alue of air density from pressure and temperature measurements' $e can use /ernoulli0s euation to gi%e us the %elocity. /ernoulli0s euation states that that the static pressure plus one half the density times the %elocity 3 suared is eual to the total pressure.
5f the %elocity is lo$' the difference in pressures is %ery small and hard to accurately measure $ith the transducer. 6rrors in the instrument could be greater than the measurement So pitot&static tubes don0t $or+ %ery $ell for %ery lo$ %elocities.
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5f the %elocity is %ery high (supersonic)' $e0%e %iolated the assumptions of /ernoulli0 /ernoulli0ss euation euation and the measurement measurement is $rong again. At the front of the tube' a shoc+ $a%e appears that $ill change the total pressure. There are corrections for the shoc+ $a%e that can be applied to allo$ us to use pitot&static tubes for high speed aircraft. aircraft.
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5f the tubes become clogged or pinched' the resulting pressures at the transducer are not the total and static pressures of the external flo$. The transducer transducer output is then used to calculate calculate a %elocity %elocity that is not the actual %elo %eloci city ty of the the flo$ flo$.. Se%er Se%eral al year yearss ago' ago' ther theree $ere $ere repo report rtss of icin icing g problems occuring on airliner pitot&static probes. ,utput from the probes $as used as part of the auto&pilot and flight control system. The solution to the icing problem $as to install heaters on the probes to insure that the probe $as not clogged by ice build&up.
Properly mount the pitot tube in the $ind tunnel !onnect the t$o tubes of the pitot tube to the manometer Start the $ind tunnel Slo$ly increase the speed of the $ind tunnel from m#s to 19 m#s :ecord the %alues of differential pressure on the manometer against $ind speed Plot the graph of differential pressure against $ind speed in 8AT;A/ Perform regression analysis on 8AT;A/ and obtain a mathematical relationship bet$een the independent and dependent %ariables. 5deally' you should get a second order polynomial relationship. ( PT
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P S
=
ρ V
2
2
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Plot the ideal relationship assuming air density to be 1. 22 +g#m* Plot the experimentally obtained relationships onto the ideal relationship graph. Also plot the error plot i.e. the difference bet$een the ideal and empirical results. hat are the possible reasons for the error<
