Modelling and verification of helicopter multibody dynamics for different rotor configurations

• Modellbildung und Verifikation der Hubschrauber-Mehrkörperdynamik für verschiedene Rotorkonfigurationen

Leitner, Roland Martin; Alles, Wolfgang (Thesis advisor); Moormann, Dieter (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2022

Abstract

The process of developing a new type of helicopter starts with the planning and design phase, in which modern methods such as model-based design are used. By this, conceptual errors in the preliminary design can be detected and eliminated at an early stage. This saves time and effort, which has a positive effect on costs. The basis of model-based design in helicopter construction is typically a parameterisable multibody model with a modular structure, which allows the modelling of any helicopter configuration. In the context of the present thesis, a multibody formalism is presented which automates the modelling of helicopters. The formalism allows the synthesis of all known helicopter configurations in a tree structure while adhering to the generally valid notation of flight mechanics (DIN 9300 and ISO 1151). Due to the tree structure, the algorithm is particularly well suited for model-based design and real-time simulations, as additional calculation steps through kinematic loops are not required. To ensure that the formalism is correct, two helicopter configurations (one standard and one coaxial) containing rotor hubs, rotor blades and four-point landing gear are modelled and subjected to plausibility tests. Techniques based on trim calculation and linearisation as well as numerical simulation of the nonlinear helicopter dynamics are used. For this purpose, an algorithm based on the convolution integral was developed to linearise the periodic and non-minimum-phase helicopter dynamics. The trim calculation is based on the multi-dimensional secant method, which transforms the inherent helicopter dynamics into stationary conditions for the flight dynamic analysis. The conditions include hover, horizontal flight and ground case, where the helicopter is still on the ground with the rotors not turning. Plausibility tests, in which numerics play a role, are performed with desktop or Pilot-in-the-Loop simulations. In the Pilot-in-the-Loop Simulation, the test pilot acts as a quality inspector whose task is to detect and identify abnormal behaviour due to model errors in the controlled helicopter dynamics. This is achieved by the use of validated state controllers and virtual reality techniques. For this purpose, the helicopter model is ported to a real-time computer connected to a radio control unit and virtual reality goggles to provide the test pilot with an extremely realistic test scenario. A total of six tests are carried out to verify the multibody formalism. Among them is an analytical test in which the linear flapping motion of the rotorblade is derived from the non-linear Equations of Motion of the multibody model, which can be seen as the first indication of a correct multibody formalism. The remaining tests are performed using trim calculation and linearisation as well as numerical simulation. These include the drop test, trim and stability tests with varying Neutral Point of the main rotor, and the Pilot-in-the-Loop test, which can be performed by an experienced test pilot. Analogous to the analytical test, the results of all plausibility tests confirm the correctness of the multibody formalism.