APPLICATION OF SEMICONDUCTOR STRAIN GAUGES IN MEASUREMENTS OF DYNAMIC STABILITY DERIVATIVE
Abstract
Low amplitude of signals in measurements ofstability derivatives is a known problem.Solution by applying semiconductor straingauges is presented. Foil strain gauge balancewas replaced with a semiconductor fivecomponentstrain gauge balance, andsemiconductor excitation moment sensor wasapplied. Wind tunnel test data are presented
1 Introduction
In the first dynamic stability tests performed inthe T-38 wind tunnel [1], the problem of smallamplitudes of output signals from forces andmoments sensors, common in measurements ofdynamic stability derivatives, was encountered.A solution of this problem by application ofsemiconductor strain gauges is presented in thispaper, with emphasis on the effects onmeasurements of dynamic stability derivativesin roll.Semiconductor strain gauges provide highlevel of output signals even at low levels ofstrain, which makes them very useful inmeasuring small loads with accuracy andprecision. Introduction of these gauges inmeasurements of stability derivatives in the T-38 wind tunnel required some modifications onthe already built apparatuses. One modificationwas the replacement of the foil strain gaugeforce balance with a semiconductor fivecomponentstrain gauge balance. Usage ofsemiconductors strain gauges insured increasedbalance sensitivity as well as increased stiffnessof the balance in all degrees of freedom. Thesecond modification is made on the rollapparatus. The excitation moment sensor madewith foil strain gage was substituted by onemade with semiconductor gauges.These modifications were followed byseries of wind tunnel measurements of stabilityderivatives. One of them was roll-dampingmeasurement of the Modified Basic FinnerModel, MBFM, a standard calibration model formeasurements of stability derivatives.
2 Description of Measurement Equipmentand Technique
The T-38 test facility of VTI is a blowdowntypepressurized wind tunnel with a 1.5m x1.5m square test section, Figure 1 [2]. Forsubsonic and supersonic tests, the test section iswith solid walls, while for transonic tests, asection with porous walls is inserted in thetunnel configuration. Mach number in the range0.2 to 4.0 can be achieved in the test section,with Reynolds numbers up to 110 million permeter. Run time is in the range 6s to 60s,depending on Mach number and stagnationpressure. Model is supported in the test sectionby a tail sting mounted on a pitch-and-rollmechanism by which desired aerodynamicangles can be achieved. The facility supportsboth step-by-step model movement andcontinuous movement of model (“sweep”)during measurements.The technique for measurements ofstability derivatives applied in the T-38 windtunnel is forced oscillation technique.to this technique model is forced tooscillate at constant amplitude within a singledegree of freedom, which implies that anyaerodynamic reaction coherent with suchmotion, donated as “the primary motion”, canonly be due to such motion.
All the experiments are based onapplication of small/amplitude oscillatorymotion to a model in the primary degree offreedom and measurement of aerodynamicreactions produced by such motion in thatparticular and in other (secondary) degrees offreedom. Those reactions, in turn, yield relevantdirect and cross as well as cross-couplingderivatives due the motion considered herein.
Atypical wind-tunnel run includes the following stages:
• Tare run, when the model is oscillatedbut the tunnel is not running. Thismeasurement enables determination ofthe inertial forces.
• Wind-on run, when the model isoscillated at the frequency same asduring the tare run but with the windtunnel running.Dynamic stability derivatives are obtainingby subtracting data from tare run and wind-onrun. For a primary degree of freedom for modeloscillatory motion in rolling plane,determination of direct static and dampingstability derivatives is expressed in equation (1).In equation (1) all values with index ‘0’ aremeasured in tare run

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