Strain Gauge Balance Research Articles

Experimental Validation of the Wing Dihedral Effect Using a Whirling Arm Equipment

By the use of whirling arm equipment, tests have been carried out on a wing model equipped with a suitable strain-gauge balance. The purpose of this work was to verify analogous theoretic results from the dihedral effect trends (C 1β derivative), which were detected on a wing with generic geometry. Full details are put forward concerning the remarkable possibilities that this system offers for the experimental determination of all main aerodynamic derivatives in the state variables within the aircraft dynamics equations. The experimental facility induces a test situation, which calls for careful and in-depth result processing in addition to the normal corrections arising from Reynolds and Mach numbers. The results, presented as theoretical-experimental comparisons, strongly support the theoretic calculation

Further tests validating the adaptation process of the adaptive walls wind tunnel in Naples

The present work reports some experimental results from the adaptive walls wind tunnel in Naples and must be considered a logical step forward in validating the wall adaptation process of this tunnel. Two sets of new tests were made for evaluating the effects of wall adaptation: one on the location of laminar–turbulent transition and turbulent separation points, the other one on the measurement of aerodynamic forces and moments, taken by a strain gage balance. Up to now, aerodynamic forces and moments were evaluated by the integration of pressure distributions on the model surface. All results agree with the theory, and measurement of aerodynamic forces has also been partially validated. Using a balance proved to be a technical shortcoming in the testing device.

The accuracy of internal wind tunnel balances for wind tunnel force measurements, the problem of definition and verification

AbstractFor the definition of the acccuracy of an internal strain gauge balance no general agreement exists. So it is difficult or even impossible, to compare balances from different manufacturers or balances used at different places.Since most wind tunnel tests at least in the aeronautical field are done on a reference basis i.e. the results are compared with earlier results achieved with a more or less different configuration, repeatability is at least as important as absolute accuracy or even more important. So also repeatability, which in a physical sense is quite different from accuracy, must be defined and proven for each balance.With the examples of several balances manufactured for the DNW (Deutsch-Niederländischer Windkanal) and ETW (European transonic tunnel) accuracy and repeatability definitions are discussed.A proposal for a generally binding definition of accuracy and repeatability is given.

A new approach to the third order calibration of internal strain gauge balances used for aerodynamic load measurement

Abstract With respect to wind tunnel aerodynamic load measurement, an internal strain gauge balance (often referred to as a sting balance) is essentially a compact load cell designed to fit within a cavity of the aerodynamic body and form a link between the model and a fixed ground point via a sting support system. The structure of an internal strain gauge balance is designed to incorporate a series of planar surfaces such that the deflection of each surface is predominantly induced by a unique aerodynamic load. Strain gauges, mounted on groups of surfaces in a Wheatstone bridge arrangement produce output signals proportional to the applied aerodynamic loads. A strain gauge balance is calibrated by applying known loads, measuring the bridge outputs and then formulating an equation which relates the two variables together. Although calibration techniques are well established, reservations have been recently expressed concerning the ability of the associated calibration equation to satisfactorily model the response of the balance when subjected to a six component aerodynamic loading. This generally accepted calibration equation (referred to here as the traditional equation) results in a quadratic approximation to the behaviour of the output signals with applied loads, whereas a more appropriate variation would be cubic. Other limitations of the traditional calibration equation are that the behaviour of the balance to two simultaneously applied loads is based upon limited combinations of the two applied loads, and that the acquisition of the required loads from the strain gauge signals is frequently based upon an approximate matrix inversion method. The proposed calibration equation, described within this paper, models the behaviour of the sting balance to the third order, takes account of all possible combinations of two simultaneously applied loads, and avoids the use of an approximate matrix inversion when deriving the desired aerodynamic loading from the signal outputs. It is also shown that the proposed method may be used to determine the interaction of all possible combinations of up to three simultaneously applied loads.

Analytical method for determining the inverse functions of a system of nonlinear equations describing the static characteristics of a wind-tunnel strain-gauge balance

An analytical method is described for inverting functions in a system of nonlinear equations. The method has successfully passed certification for a system of nonlinear equations describing the static performance characteristics of a wind-tunnel strain-gauge balance. It is well suited to multicomponent instruments in which correlation is observed between the output signals.

Effect of turbulence on a plano-convex circular disk

The two mean force components and the mean torsional moment acting on a plano-convex circular disk have been measured on a strain gauge balance in uniform wind flows of various turbulence intensities and scales. The thickness-to-diameter aspect ratio of the disk ranges from 0.15 to 0.30. The results are compared with those for a flat circular disk with varying porosity and a sphere as well as some limited amount of data for dish shaped bluff bodies. The data presented are relevant to the design of wind loading on dish-type aerial antennae and those with a front shield in particular. The effects of incident turbulence intensity and longitudinal integral scale on wind loads for codification are discussed.

Experimental study of the drag of the forebody and the cylindrical part of the HB-1 model with an upstream injection of a fluid in a supersonic flow

Values of the gas-dynamic drag of the forebody and the cylindrical part of the HB-1 test model in a supersonic flow have been separately measured using an internal strain-gauge balance. It is shown that an upstream injection of a fluid jet from the model forebody decreases the drag of both the forebody and the cylindrical part.

Forebody Flow Control for Extended High-Angle-of-Attack Maneuvers

Tangential forebody blowing for the purpose of yaw control has been investigated on a 6% generic e ghter model in the 2.1 3 1.5-m low-speed wind tunnel at the University of Bath. Force and moment data were obtained from a sting-mounted, six-component strain gauge balance over an angle-of-attack range of 210 to 120 deg. The tangential blowing slots were incorporated into a vacuum-formed, plastic forebody, which contained two independent plenum chambers. The geometry of the slots was optimized for high-angle-of-attack operation based on results from previous research. Three different forebodies were used to investigate the effects of different azimuthal slot positions. At angles of attack beyond 60 deg, all of the slot locations were shown to produce signie cant yawing moments, even when compared to conventional yaw control at low angles. This control moment did not decay at the maximum angle of attack tested, which suggests that the envelope of control could be extended farther. The concept remained effective even at conditions where the e ow along the longitudinal body axis was reversed. At angles of attack appropriate for normal cruise operation, takeoff, or landing, all slot locations proved effective in producing yawing moments. Consequently, tangential forebody blowing may prove capable of replacing conventional e n and rudder combinations in establishing weathercock stability.

Force Measurements in Hypersonic Impulse Facilities

A six-component strain gauge balance is used to measure aerodynamic loads of different models in hypersonic wind tunnels with running times down to I ms. Three models are employed, a pointed cone, an Apollo CM capsule, and a delta wing configuration, ELAC I. All models are tested in the Aachen shock tunnel TH2; the capsule model was additionally tested in the von Karman Institute Longshot facility. Two different methods are applied to compensate for low-frequency inertia forces caused by the model support system for the capsule tests

Combined effect of longitudinal riblets and LEBU-devices on turbulent friction on a plate

The possibility of reducing turbulent friction with the help of large-eddy-breakup devices (LEBUs) and riblets is studied experimentally. The tests were conducted in a low-turbulence wind tunnel on a flat plate for 2·106 ≤ Re ≤ 7·106. The local friction coefficient was measured using internal strain-gauge balances, and the total drag was estimated by the momentum-transfer method. It is shown that a combination of LEBUs and riblets makes it possible to reduce the total turbulent friction drag of a flat plate 1800 mm long by 16%. The effects of the length of a ribbed surface on the efficiency of friction reduction and of LEBUs and riblets on the structure of a turbulent boundary layer are analyzed.

Wind flows and forces in a model spruce forest

Wind tunnel tests have been conducted on a 1:75 scale model of a Sitka spruce forest in a correctly scaled turbulent boundary-layer flow. 12000 tree models were manufactured with mass, flexibility and aerodynamic drag characteristics chosen to give dynamical similarity with typical 15 m trees in a 30ms−1 gale. To measure the dynamic response of a sample tree, set within this model forest, a miniature, fast response strain-gauge balance was designed and built. Linked to a computer for on-line data sampling, this balance provided measurements of the fluctuating along-wind and acrosswind components of the overturning moment at ground level, leading to values of mean and extreme moments and the frequency spectrum of the sway motion. Associated measurements of local wind flow characteristics were made with hot-wire anemometers and a laser anemometer. The response of the tree has the characteristics of classical lightly damped vibration and there is evidence that resonant sway motion increases the extreme overturning moments significantly above the values produced by wind gust forces alone.

Yaw control by tangential forebody blowing

SummaryAircraft yaw control at high angles of attack by tangential forebody blowing has been investigated experimentally. Tests were performed in the University of Bath 21 m x 1.5 m low speed wind tunnel using a 6% scale generic combat aircraft model fitted with blowing slots in the nose cone. Six component strain gauge balance force and moment data were measured for angles of attack up to 90° for various slot geometries and locations. The effect of slot azimuthal location is demonstrated and a slot stall phenomenon described. A geometry dependent forebody/wing flowfield coupling has been identified which can lead to unexpected yawing and rolling moments. The primary source of yawing moment is shown to be the enhanced area of attached flow on the blown side of the forebody rather than direct vortex influence.The optimum slot extent and location depend on the angle of attack range over which control is required. For regions where steady vortex asymmetry is present, slots near the apex of the forebody produce severe control reversals at low blowing rates. These reversals can be minimised by placing the slots away from the apex. For control in regions where the flow is dominated by periodic vortex shedding, long slots offer efficient control to 90° angle of attack. The most suitable compromise for wide range control would appear to be a short slot placed away from the apex of the forebody.

Interference Effects of Very High Bypass Ratio Nacelle Installations on a Low-Wing Transport

A twin-engine, low-wing transport model, with a supercritical wing designed for a cruise Mach number of0.77 and a lift coefficient of 0.55, was tested in the 16-Foot Transonic Tunnel at NASA Langley Research Center. The purpose of this test was to compare the wing/nacelle interference effects of superfans (very high bypass ratio turbofans, BPR ≈ 18) with the interference effects of advanced turbofans (BPR ≈ 6). Flow-through nacelles were used in this study. Forces and moments on the complete model were measured using a strain gage balance and extensive surface static pressure measuements (383 orifice locations) were made on the model’s wing, nacelles, and pylons. Data were taken at Mach numbers from 0.50 to 0.80 and model angle-of-attack was varied from −4 to +8 deg. Results of the investigation indicate that superfan nacelles can be installed with approximately the same drag penalty as conventional turbofan nacelles.

Strain-Gauge Applications in Wind Tunnel Balances

Aerodynamic forces and moments acting on scaled models of aircraft and missiles are generally measured by a multi-component transducer called the ‘Internal Strain Gauge Balance.’ This multi-component transducer which fits inside the model, incorporates six specially machined elements to measure three components of forces and three components of moments on the model. The air loads on the elements appear as strain, which is measured by resistance strain gauge transducers. The paper describes two variants of six-component balance configurations, strain gauge installation procedure, thermal performance, typical calibration and operational of strain-gauge balance in wind tunnel test applications.

A review of rigid body response on sting supported models at high angles of incidence

The new requirement to test wind tunnel models of combat aircraft at high angles of incidence and high kinetic pressures has led to a review of the factors controlling the model stability. The review suggested that dangerous motions might occur (possibly without prior warning) on models at high angles of incidence unless special preventive measures were taken. An internal tuned damper and balance bump stops were recommended to limit the responses. The bump stops would also prevent the moment limits of the strain gauge balance from being exceeded. The effectiveness of both devices was confirmed by tests on a swept wing model which experienced dangerous bending oscillations in a vertical plane at a Mach number of 0.50 in the incidence range from about 27–29° together with dangerous yawing oscillations in a horizontal plane above an incidence of about 35°. Further research is recommended to ensure the safety of other models. For sting supported models in a conventional wind tunnel, it is shown by analysis that the structural damping in the sting bending mode needs to be about 4 to 6% critical damping. In a cryogenic wind tunnel corresponding levels would need to be 7 to 10% critical damping because of the possibility of increased negative aerodynamic damping relative to ambient conditions.

Measuring Aerodynamic Interference Drag Between a Bird Body and the Mounting Strut of a Drag Balance

ABSTRACT The drag of a bird body mounted on the strut of a drag balance in a wind tunnel is more than the sum of the drags of the isolated strut and the isolated body. The strut changes the air flow around the body and generates additional drag, known as interference drag. This paper describes practical methods for measuring the drag of bird bodies: a strain-gauge drag balance, dimensions for struts made with machine or hand tools, and a procedure for correcting drag measurements for interference drag. Interference drag can be measured by extrapolating a relationship between the drag of isolated struts with different cross-sectional sizes and shapes and the drag of a body mounted on those struts. The interference length -the length of an isolated strut that produces drag equal to the interference drag -is a useful quantity for predicting interference drag. The relationship mentioned above is a straight line for a model peregrine falcon (Falco peregrinus L.) body mounted on smooth struts -struts with convex cross-sectional shapes ranging from streamlined to circular. This finding simplifies the determination of interference drag in three ways: (i) the line can be found from measurements with just two struts -a standard strut with low drag and a calibration strut with high drag; (ii) the two struts need not have the same shape -for example, the standard strut can be changed to a calibration strut by attaching a spoiler without disturbing the body mounted on the strut -and (iii) a single value of interference length (33.1mm) describes smooth struts with a range of shapes and sizes. These struts had drag coefficients between 0.33 and 0.91 at Reynolds numbers between 2100 and 10800. The interference length of a strut supporting the actual falcon body with a feathered surface is not significantly different from that of the strut supporting the model body with a rigid surface. As a hypothesis, interference length (h1, in metres) of a smooth strut varies with the size of the body mounted on it: where m is the body mass (in kg) of the intact bird. 95 % of the interference drag appears to arise from a 15 mm length of the strut nearest the body.

Control of asymmetric vortical flows over delta wings at high anglesof attack

Results from a wind tunnel experiment will be used to confirm that Tangential Leading Edge Blowing is capable of controlling the vortical flow over a delta wing to very high angles of attack. The production of significant rolling moments and the ability to unburst a vortex has been demonstrated to 55 angle of attack. Strain gauge balance measurements and upper surface pressure distributions will be presented which illustrate the various modes of operation and which, in particular, identify the uncoupled/coupled nature of the vortex control at pre- and post-stall angles of attack. At angles of attack beyond 40°, rolling moments are produced that exceed those produced by 20 of aileron deflection at 0 angle of attack.

Measurements on an oscillating 70-deg delta wing in subsonic flow

A series of low-speed wind tunnel tests on a 70-deg sharp leading-edged delta wing at both static and dynamic conditions were performed to investigate the aerodynamic forces and moments. Forces and moments were obtained from a six-component internal strain-gauge balance. Large amplitude dynamic motion was produced by sinusoidally oscillating the model over a range of reduced frequencies. Static results compared well with previous experimental findings. Significant Reynolds number effects were present in the experimental measurements. Reynolds number effects are reduced, but still present when a sharper leading-edge delta wing was tested. Large hysteresis loops and a delay in dynamic stall were seen in the dynamic data. Dynamic forces and moments were a strong function of reduced frequency. Nonzero sideslip created complex rolling moment and lift behavior due to asymmetric vortex bursting.

A new principle for a short-duration six component balance

The presented principle of a sting balance applying semiconductor strain gauges bears a number of advantages in comparison with classical strain gauge balances. Simultaneous, highly time resolved measurement of all six aerodynamic force components is possible. The manufacturing is comparatively easy, a milling machine is sufficient to achieve reasonable accuracy. The balance needs only little space, which makes it suitable for small and especially for short models. The disadvantages of semiconductor strain gauges like zero drift and temperature effects do not play a significant role during the test time of five milliseconds. However, they have to be taken into account during the long lasting calibration process. The attachment of the model to the cross is critical with regard to preloadings and hysteresis. Therefore a connecting part is used between balance and model, which should not be removed once the balance is calibrated. The model should be attached to the balance so that the centre of mass is located at the origin of the coordinate system shown in Fig. 2. Then, the moment of inertia with respect to the balance fixed coordinate system is minimal, whereas the natural frequencies of the rotating degrees of freedom are maximized. If the flow over a model is studied the coordinate system only has to be rotated to obtain the aerodynamic forces. Furthermore, gauging and wiring of the strain gauges is uncomplicated, all locations of the gauges are easy to reach. As the construction is compact and of high stiffness it may withstand high loads. The existing balance has a design load of 1,000 N. Strain gauges with a gauge factor of k=130 were used. With an amplification factor of 10,000 even forces below 1 N can be observed, using an appropriate amplifier (e.g. “2311 Signal Conditioning Amplifier”, Vishay Measurements Group). The accuracy of the measurement mainly depends on the efforts invested in the calibration and data processing. With a suitable algorithm as proposed by Schnabl (1987), an accuracy in the order of one percent should be possible. So far the balance has not been applied for measurements on a model, but a number of tests is being made to investigate the natural frequencies and short time response. The results of these tests will lead to the final geometry of the balance and its attachment to the model. It is hoped to have the next prototype manufactured, gauged and calibrated by the end of this year. First results of the investigation of a model with the balance might be presented in 1990.

Improving noise immunity in strain-gauge balances

Improving noise immunity in strain-gauge balances