Determining TH-1H Tailboom Loads from Measured Strain Gage Data

Abstract

This work develops tailboom aerodynamic loading for a TH-1H helicopter in hover by integrating a finite element model (FEM) and in-service strain time histories in accordance with structural mechanics and aerodynamics principles. The FEM is a whole aircraft model used to establish stress spectra at critical aircraft components for fatigue and fracture analyses from main rotor and tail rotor forces. The in-service time histories are the responses from sixteen uniaxial strain gages attached to the tailboom primary longerons and the corresponding structure inside the main cabin. Five separate loading modalities are used as FEM static load cases. Published experimental drag coefficients are used to develop two aerodynamic pressure load distributions for the tailboom as well as separate left and right elevator pressure loads. The fifth case is a lateral tail rotor force. Weighting factors are determined for these five modalities so that the weighted sums of the FEM strains best-fits the measured strains at the sixteen gage locations. This fitting process is executed for each time step of each strain gage in a given hover regime, as well as for the average gage values for the duration of the regime. Weighting factors are evaluated for admissibility (i.e. non-negative values, bounded magnitudes that do not produce unrealistically high stresses). The results are compared against the measured strains. The mechanics of the tailboom structure is also evaluated with respect to strains, longeron loads, and netsection bending moments. The findings highlight that the longerons (where the strain is measured) account for approximately two-thirds of the tail boom bending moments; the external skins and stiffeners provide the balance of the moments. Two load modality combinations emerged as best-fits: tail boom aerodynamic pressure loading plus tail rotor force, and elevator pressure loading plus tail rotor force. Both show varying levels of fidelity to the measured data, which suggests that additional load modalities should be considered, and that additional instrumentation of the skins should be implemented for future strain surveys.