Comfort is a pivotal construct in human-centered design, connecting technical functionality with well-being across products, services, and systems. The 2023 International Comfort Congress (ICC2023) brought together researchers and practitioners to explore how new contexts of use, predictive approaches, and user diversity are reshaping this field. Contributions in this special issue address comfort in automated driving and turboprop aviation, where physical, psychological, and environmental factors converge. Advances in predictive comfort science demonstrate how objective measures, physiological sensing, and modeling complement traditional self-reports. At the same time, inclusive design approaches—ranging from XR-based co-creation to analyses of posture and movement variability—highlight the importance of accounting for heterogeneous user needs and comfort trade-offs. Together, these studies illustrate a shift toward more adaptive, accessible, and sustainable systems, underscoring comfort as a multidimensional construct that evolves with technological innovation and societal change.

Background
A quantitative comfort model will aid in evaluating comfort levels of various target groups before the actual flight of an airplane. However, constructing the model is always a challenge due to the complexity of the phenomenon.

Objectives
In this paper, we present quantitative comfort models to predict the (dis)comfort of passengers flying with turboprops based on objective measures.

Methods
Ninety-seven participants took part in two experiments conducted during real flights, during which forty of them had environmental and personal factors recorded using (self-developed) measurement tools. The collected data were analyzed to model the relations between objective measures and subjective feelings.

Results
Two preliminary models based on gradient boosting regression were developed. The models were able to predict the changes in comfort and discomfort of individual passengers with an accuracy of 0.12±0.01 and 0.21±0.01 regarding normalized comfort and discomfort scores, respectively. Additionally, contributions of different factors were highlighted.

Conclusion
The outcomes of the models show that we took a step forward in modeling the human comfort experience using objective measurements. Anthropometry (including age), seat positions, time duration, and row (noise) emerged as leading factors influencing the feeling of (dis)comfort in turboprop planes.

Background
Turboprop aircraft offer the possibility of lower emissions for regional travel in comparison to jets. Future low-carbon aircraft concepts include propeller-generated thrust powered from fuel cells, hydrogen, biofuel, battery or hybrid power. The noise and vibration experienced in a turboprop cabin is different to that experienced in a jet, with signals characterised by tonal components related to the propeller blade pass frequency. These components have been associated with noise and vibration discomfort. There are few published studies of aircraft cabin vibration measured on the seat surface according to ISO2631-1; none report data for the whole flight.

Objective
The objective was to measure and evaluate the vibration experienced by passengers for complete turboprop flights and compare vibration data with standards associated with vibration comfort.

Methods
Vibration data was measured on the surface of three occupied seats during two turboprop aircraft flights. Measurements were made on full flights, and covered the entire duration from gate-to-gate.

Results
Data showed that the vibration is highly tonal, and is affected by position and flight phase. Frequency-weighted vibration showed magnitudes below thresholds for health risk. The highest magnitudes of vibration occurred at the blade pass frequency and its harmonics. These frequencies are rejected by standard comfort assessment methods that use frequency weightings.

Conclusions
Whole-body vibration exposure in the turboprop tested in this study did not approach health risk thresholds using ISO2631-1. Analysis of the vibrational comfort requires use of band-limited vibration assessment methods to include the dominant vibration components in analysis.

The comparatively environmentally friendly turboprop aircraft should be used more often, but still need some improvements, especially regarding noise. To facilitate research into these improvements, an On-Ground Regional Passenger Cabin Demonstrator was built and validated through a comparison with passengers’ reactions to real turboprop flights. Seventy-three subjects answered questions on various environmental factors during a simulated flight in the Cabin Demonstrator. Subject testing revealed that the Cabin Demonstrator was overall perceived as realistic compared to real flights, and the comfort level was comparable to the previously conducted in-flight subject test. Thus, the Cabin Demonstrator can be used for multiple future tests.

Turboprop aircraft are a useful alternative to reduce CO2 emissions, but they have a higher noise and vibration level compared to turbojets. We investigated how passengers perceived noise and vibration during two turboprop flights in an ATR72.In general, noise as assessed as slightly loud and slightly annoying, but became less noisy and more comfortable over time. Subjects evaluated the vibration as rather comfortable, but the perception of vibration varied for different body areas. Discomfort of vibration decreased. If noise sensitivity is added to these analyses, changes over time largely disappeared showing the complex interaction between characteristics of the person and the environment. In summary, the rather negative evaluation of noise and vibration could be the factors why people decide not to fly with turboprop aircrafts. The development of turboprop aircraft with less noise and vibration remains important so that this more sustainable aircraft type is competitive.

Many new future sustainable aircraft concepts have (electric) propeller propulsion. However, the noise and vibration generated by propeller propulsion might create discomfort for passengers. In the Clean Sky 2 project ‘ComfDemo’, all factors influencing comfort and discomfort are studied, with the aim to improve comfort experience for eco airplanes. The results show that noise, vibration and seat are the most mentioned factors influencing discomfort. The flights without passengers showed different noise characteristics, which makes test flights with passengers inevitable. Noise certainly needs attention, but anthropometrics as well as for instance elbow-elbow width is greater than the 18” seat width for many passengers.

Turboprop aircraft should be improved as they are more environmentally friendly aircraft compared to turbojet aircraft but noise and vibration are often too high for passengers. A simple and uncomplicated way to carry out experiments is using a demonstrator. To determine whether the demonstrator represents the reality, it must be validated. In this project, real flights were first conducted in a turboprop aircraft. During two 70-minute flights, 94 subjects answered questions about symptoms, mood or comfort levels related to noise and vibration, among other things. In the next step, investigations will be carried out in the demonstrator under the same conditions as the real flights. Both results will be compared with each other. If the data from the demonstrator corresponds to that of the real flights, the demonstrator is considered to have been successfully validated. The requirement for this is that the demonstrator data lies within the confidence intervals of the results from the real flights. The aim is to validate a full-scale on-ground demonstrator of a regional turboprop aircraft cabin that will be used for multiple tests like subject tests and comfort evaluation, composite materials and structures, systems and energy consumption.

Comfort is an important factor for passengers in the selection of airlines, and electric propeller aircraft will be an important element of future sustainable aviation. In this paper, we studied the order of importance of different (dis)comfort factors regarding traveling with propeller aircraft. Two experiments were conducted, one was a simulation flight on the ground with 33 participants and the other were two real flights with 97 participants. All participants were asked to rank the importance of different (dis)comfort factors in different phases of flights. Results indicated that though there are differences between the simulation and the real flights, noise, vibration and the seat are among the most important factors regarding discomfort, and space, lighting, temperature and seat are the most important factors of comfort. The results are different to those reported from previous studies on travelling by jet, where anthropometry is the most important factor. This finding suggests a difference in passenger perception between travelling by propeller propulsion and jet engines, and casts new requirements on the aircraft interior and service design for future sustainable aviation.

BACKGROUND: Selecting the most suitable questionnaire(s) in comfort research for product design is always a challenge, even for experienced researchers. OBJECTIVE: The objective of this research is to create a list of Preferred Comfort Questionnaires (PCQ) for product design to help researchers in the selection of questionnaires for comfort research. METHODS: Fifteen questionnaires that are often used in comfort research for product design were selected as candidate questionnaires. During the Second International Comfort Congress (ICC 2019), 55 researchers and practitioners working in the field of comfort joined together in a workshop to rate these questionnaires individually as well as rank them in groups based on their experience. The criteria of rating and ranking included easiness to answer, easiness for data interpretation, time needed to complete, the need for prior training, as well as mapping the applicable design phases and field of application. RESULTS: The elicited responses related to each questionnaire were analyzed. For comfort research in five proposed application fields and four design phases, the preferred questionnaires were highlighted and categorized into four categories: preferred questionnaire, suitable for less prior training, suitable for fast completion and generally applicable, which led to a list of PCQ for product design. CONCLUSION: We expect that the PCQ list can be used as a useful instrument to help researchers in selecting questionnaires for comfort research in product design.

Various aspects related to comfort are described in this special issue. Some papers are focused on the environment, like smell, temperature, light, acoustics, space and some on an artefact touching the human, like the floor angle, the seat, a bed and light. However, in this special issue also topics like modelling and behavior get more attention.

This paper gives an overview of the relevance of the comfort concept, its definitions, boundary conditions, and stakeholders. Current comfort theories are presented and reflected on, both in their applicability and testing methodology. Questionnaires commonly used to study comfort and discomfort are also reviewed. An example of a comfort lab is introduced in its functionality and tools, which can be useful as a benchmark for others studying comfort.