Aircraft Active Inceptor Dynamics under Vibration Loads

Lead Research Organisation: University of Bristol
Department Name: Aerospace Engineering

Abstract

Aircraft with fly-by-wire control systems often incorporate so-called active inceptors (stick, throttle, pedals, etc.) that provide the interface between the pilot and the aircraft control surfaces. These devices are complex electro-mechanical gimbal systems utilising motors, servo-actuators, torsional springs, linear springs, ball bearings, spherical bearings and displacement and force transducers - all interconnected via a series of mechanical linkages and a pilot's grip assembly. The system controls the grip force-to-displacement relationship to allow real time variation in feel characteristics to support aircraft operational modes. The dynamic response of the individual elements in an inceptor, and of the system as a whole, is critical in maintaining the level of performance required throughout its service life. Whilst the fatigue life is partly driven by the pilot-applied cyclic loads, aircraft vibration loads can be the primary design driver. The effects of aircraft harmonic vibration loads, as in a helicopter, are difficult to assess: some of the resulting inceptor system resonances can fall within aircraft frequency ranges that must be avoided. This PhD project involves the derivation, development and study of a suitable mathematical model of an active inceptor, which can be used for assessing its dynamic characteristics. BAE Systems will provide a development helicopter collective stick unit and the associated CAD models for this investigation. The mathematical modelling, at least initially, be focused on this collective stick. Any experimental testing of this unit needed to support the modelling will be conducted either by BAE Systems or within the University. The objective of the study is to enhance the understanding of the dynamics of this candidate inceptor system, including under the influence of harmonic excitation representing helicopter vibration, and to explore methods for ensuring that resonant-frequency responses are avoided. Related topics such as friction modelling, force sensing and pilot arm interactions could potentially be incorporated at a later stage.

Studentship Projects

Project Reference Relationship Related To Start End Student Name
EP/R511857/1 01/10/2017 31/12/2022
1953109 Studentship EP/R511857/1 01/10/2017 28/02/2022 Edward Yap
 
Description Inceptors are the controls that pilots use to orientate and manoeuvre an aircraft. They are inherent Multi-Body Dynamic (MBD) systems and in preliminary design stages, challenges often arise in adequately assessing and predicting their dynamic characteristics. High fidelity models may be unavailable or inefficient in assessing the inceptor's dynamic characteristics until detailed design stages. However, the inceptor's resonances may be found to occur at or in the proximity of the target aircraft's forcing frequencies, which is to be avoided. The prospect of an undefined number of inceptor design iterations to ensure sufficient clearance, highlights the need and desirability of a mathematical inceptor model, and an associated design process, that can be used to provide necessary insights into an inceptor's dynamic characteristics during preliminary design stages.

The work funded through this award involves a study into the mathematical modelling of a candidate active inceptor to provide early low-cost means of predicting its dynamic characteristics, namely natural frequencies, at preliminary design stages. Focus is placed on frequencies due to the inceptor's aforementioned adverse vibration issues. The primary modelling approach proposed in this work is the Udwadia-Kalaba (U-K) formulation. It is firstly demonstrated for modelling
generic constrained nonlinear multibody systems through a case study of a planar crank-slider mechanism. A novel framework is developed showing how a U-K formulated model can be used to shift and thereby tune a modelled system's dynamic characteristics, namely natural frequencies, to desired levels by recommending adjustments in design parameters. Tuning studies demonstrated this framework on the U-K crank-slider mechanism model;
the parameter outputs were validated by a model produced in MATLAB's MBD toolkit
Simscape.

The U-K formulation was then extended to model flexible multibody systems using a lumped parameter approach. A flexible planar crank-slider mechanism is modelled
and its predicted natural frequencies agreed well with those from a Simscape model. The modelling of the candidate inceptor was then addressed and a three-dimensional, configurable and low-order U-K candidate inceptor model developed. The model includes the capacity to predict the inceptor's flexible modes arising from the planar bending flexibility of the control stick. This model is validated, and its predicted natural frequencies shown to agree strongly with those from a Simscape model. A finite element (FE) inceptor model was provided by BAE Systems, serving as a high fidelity representation of the inceptor from which the low-order U-K model could be compared. The U-K model revealed a satisfactory correlation with the FE model's predicted modes, verifying the U-K model's representativeness. A physical candidate inceptor unit was also made available by BAE Systems; experimental vibration surveys validated that the U-K model adequately represented the dynamics of the physical inceptor.

Conducted conceptual design studies demonstrated how the U-K inceptor model can shift the inceptor's natural frequencies to desired levels by recommending adjustments in
design parameters. The application of the U-K modelling approach to an inceptor, and the demonstration of its ability to contribute to preliminary design studies and provide powerful insights into the inceptor's dynamics, is to our knowledge, a new contribution to the field.
Exploitation Route The research outcomes of this funding highlighted the benefits that a configurable and low-order U-K inceptor model can provide in the conceptual design of an inceptor, notably in providing an early low-cost means of predicting the inceptor's dynamics, and pre-empting the occurrence of adverse vibration issues. Additionally, the work showed how the U-K inceptor model can be used in its capacity to recommend adjustments in model design parameters, to shift the inceptor's natural frequencies to desired levels, to aid preliminary design stages, in order to satisfy the target aircraft's frequency avoidance requirements.

The developed concept inceptor design framework additionally outlined how a low-order U-K inceptor model can be incorporated into an inceptor's preliminary design
stage, to aid the inceptor's development from conceptualisation to initial design. It is shown how the U-K inceptor model can be used to facilitate the
assessment and tuning of an inceptor design's dynamic characteristics, to arrive at its initial design. The design framework and associated modelling approach is readily transferrable to generic inceptor designs and systems.
Sectors Aerospace, Defence and Marine