fig3 the overall length ol of the test section 18 extends from the outlet end of the nozzle 16 to the inlet at the base of the collector 19 this test section length ol is comprised of a forward clearance length fc a vehicle test length vt a rear clearance length rc and a pressure rise length pr a test vehicle 23 is located in the test section 18 the vehicle test length vt can represent the maximum vehicle length that can be tested while still obtaining accurate and reliable aerodynamic and acoustic test data in general as air flows from the exit end 28 through the test section 18 a front clearance length fc is required to avoid influence of the vehicle on the nozzle flow and to ensure minimal influence of the nozzle 16 on the measurements on the vehicle 23 in general it is expected that the front clearance length fc for most wind tunnels will be about three fourths of the hydraulic diameter of the nozzle 16 opening after the air flow 22 passes the test vehicle 23 a certain distance is needed in the form of the rear clearance length rc to avoid influence of the pressure rise on the measurements on the vehicle 23 following this the pressure begins to increase in the pressure rise length pr area as the air flow 22 approaches the collector 19 in general the combined length of the rear clearance length rc and pressure rise length pr in most wind tunnels is expected to be about one to three times the hydraulic diameter of the exit end 28 within the vehicle test length vt zone it has been determined that accurately and repeatedly positioning the vehicle center at a designated location.