Free-flight force measurement technique in a shock tunnel

Abstract

A novel force measurement technique has been developed at the impulsive facility HIEST, in which the test model is completely non-restrained for the duration of the test, so it experiences completely free-flight conditions for a period on the order of milliseconds. This technique was demonstrated with a three-component aerodynamic force measurement with a blunted cone of total length 318 mm and a total mass of 22 kg in hypervelocity test flow. A miniature modelonboard data-logger, which was a key technology for this technique, was also developed in order to store the measured data. The data-logger was designed to be small enough to be instrumented in test models, with an overall size of 100 mm x 100 mm x 70 mm, including batteries. Since the logger was designed to measure force and pressure, it includes six piezoelectric amplifiers and four piezoresistive amplifiers, as well as high-speed analogue-digital converters, which digitize the measured data with 16-bit resolution. The logger’s sampling rate and sample size are 500 kHz and 400 ms, respectively. For the autonomous operation, the logger waits for a trigger signal (accelerometer output) and then starts to take measurements with arbitrary adjustable trigger threshold level and pre-trigger delay time. Measured data is stored to static memory for transfer to a PC via a USB interface after a wind tunnel test. To demonstrate the entire measurement system, wind tunnel experiments were conducted in HIEST. In the present wind tunnel test campaign, records of pressure, axial force, nominal force and pitching moment were obtained under conditions of H0 = 4 MJ/kg, P0 = 14 MPa. This demonstrated that the system worked correctly in the short test duration and harsh conditions typical of HIEST. Use of this data-logger allows the elimination of a large-diameter sting, ending concerns about the sting’s interference with the base flow of the model, which could cause serious errors in measurement in wind tunnel tests.