Force and moment measurement

Abstract

Measurement of steady and fluctuating forces acting on a body in a flow is one of the main tasks in wind tunnel experiments. In aerodynamic testing, strain gauge balances will usually be applied for this task as, particularly in the past, the main focus was directed on the measurement of steady forces. In many applications, however, balances based on piezoelectric multicomponent force transducers are a recommended alternative solution. Contrary to conventional strain gauge balances, a piezo balance features high rigidity and low interference between the individual force components. High rigidity leads to very high natural frequencies of the balance itself, which is a prerequisite for applications in unsteady aerodynamics, particularly in aeroelasticity. Moreover for measurement of extremely small fluctuations, the possibility exists to exploit the full resolution independently from the preload. Concerning the measurement of small, steady forces, the application of piezo balances is restricted due to a drift of the signal at constant load. However, this problem is not as critical as generally believed since simple corrections are possible. The aim of this chapter is to give an impression of the possibilities, advantages and limitations offered by the use of piezoelectric balances. Several types of external balances are discussed for wall-mounted models, which can be suspended one-sided or twin-sided. Additionally an internal sting balance is described, which is usually applied inside the model. Reports are given on selected measurements performed in very different wind tunnels, ranging from low-speed to transonic, from short- to continuous running time and encompassing cryogenic and high pressure principles. The latter indicates that special versions of our piezo balances were applied down to temperatures of −150 °C and at pressures of up to 100 bar. The projects span from a wing/engine combination in a low-speed wind tunnel to flutter tests with a swept-wing performed in a transonic wind tunnel, and include bluff bodies in a high pressure and cryogenic wind tunnel, as well. These tests serve as examples for discussing the fundamental aspects that are essential in developing and applying piezo balances. The principle differences between strain gauge balances and piezo balances will also be discussed.