Dorta Tomás
email: tomas.dorta@umontreal.ca

Hybridlab

Davide Pierini Hybridlab

Sana Boudhraâ
email: sana.boudhraa@umontreal.ca

Why 360° and VR headsets for movies? Exploratory study of Social VR via Hyve-3D

Keywords: Cinéma RV, RV sociale, Narration, Expérience du cinéma VR cinema, Social VR, Storytelling, Cinema experience. ACM Classification H.5.1 [Multimedia Information Systems] Artificial, augmented, and virtual realities. H.5.2 [User Interfaces] Screen design. J.4 [Social And Behavioral Sciences] Psychology

Le but principal de la Réalité Virtuelle (RV) dans le domaine du cinéma est d'immerger le spectateur dans la scène. Les derniers visiocasques de RV essayent d'atteindre cet objectif en utilisant des vidéos à 360°. Cependant, la narration et l'expérience sociale du cinéma peuvent être affectées car les spectateurs qui utilisent des visiocasques doivent regarder tout autour continuellement pour explorer la scène. De plus, les visiocasques limitent les utilisateurs à une expérience individuelle, dans laquelle la plupart de communications non verbales (expressions faciales, gestes et postures) sont obstruées. Cela perturbe potentiellement la présence sociale et l'expérience partagée de regarder des films. Cet article explore pour la première fois ces problèmes potentiels en comparant un système de RV social (Hyve-3D) avec un visiocasque de papier carton durant le visionnement de films avec ou sans narration. Les résultats suggèrent que le système de RV social est plus adapté pour l'expérience cinéma de type RV.

INTRODUCTION

Human kind is used to listen to stories. Since primitive times, people get together after hunting or picking activities, and share stories around a meal. Restaurants and cinemas are social spaces and when people are alone in those places they feel uncomfortable because those behaviours and habits are usually social and the experience is shared. Nowadays stories are still told around the table or transferred by media like novel books and movies, and they are considered leisure activities.

Watching movies and playing video games are two different leisure actions, compared to work where the users are confronted to achieve specific tasks with different degrees of interaction and effort. In the case of movies, users watch them to enjoy and relax. Focusing principally on the narrative, spectators take a passive role compared to active interaction roles of video games. Even if stories are presented in video games, like in the famous graphic adventure puzzle video game Myst -1993 (Cyan Inc.), the storyline is cut with moments of interaction where users have to interact with the game, resolving enigmas or activating triggers in the 360° surrounded environment. In movies people enjoy the story, getting deeply in it. In videos games people are active and engaged in the interaction they have to achieve to pass to the next level or chapter of the story/game. Standard cinemas experiences make the user watch the movie from the outside of the scene, which is observed through a frame (the shape and the ratio of the display). This setup does not lead to an immersive experience since the audience feels being in the movie theatre or in the living room instead of feeling to be in the scene and the story among characters. The "flat screen" is a window into another reality where the stereoscopic vision provides a display in relief. Nowadays, movie theatres are designed as tunnels where some decorations are present on the sidewalls. Making the field of view or the screen bigger (like IMAX) to make users feel they are inside the action is not the solution: it is not the size of the screen that matters, but the effect of the display on the user to make them feel immersed, like anamorphic images or the trompe-l'oeil effect.

During the last years we witnessed the renaissance of VR thanks to the advent of a new generation Head Mounted Retina Displays (HMrD) or VR headsets using high resolution screens (Oculus Rift™). Even if VR headsets are not designed for cinema, nowadays there is an important trend of using HD small cameras (GoPro™) and stitching software (Kolor Eyes™), that allows 360° videos, and some companies (i.e. Jaunt) start to propose VR cinema content for VR headsets. This new content goes even forward of simple 360° videos, since the storytelling is challenged and the way those films are shot is different. The composition of the scene, the staging as well as the technical and direction support during filming have to be revised for the 360°. Moreover, the appearance and performance of characters need also to be adapted to that. All this because spectators using VR headsets have become able to look at any direction of the scene, since from their individual point of view, they can watch where they desire. But what are the intentions of the directors of these movies? How they can be sure the important facts and details of the story are well perceived? Moreover, users are cut from their bodies, including gestures and facial expressions compromising non-verbal communications during collective viewing. Since the sound of some VR headsets is individual, verbal communication could be further an issue.

The aim of this study is to explore the efficacy of the interaction and the user experience while watching short movies using two different VR systems: a VR headset (HMrD), individual in its nature, and a social VR system Hybrid Virtual Environment 3D (Hyve-3D), allowing a shared immersive visualization. We are not focused on comparing different kinds of VR displays and not even on comparing them to traditional cinema screens; it is rather to explore the impact of the VR headsets 1) on the social dimension of watching a movie with someone else and 2) on the vision of a movie with a storyline. In order to do this, we conducted a comparative study between cardboard VR headsets and Hyve-3D. Hyve-3D is a social VR system implemented for co-design (Figure 1) [START_REF] Tomas | A New Embodied Interface for Immersive Collaborative 3D Sketching[END_REF]. The immersive visualisation of this system consists in a spherical concave screen that is not 360° and produces immersion using anamorphic images without glasses. Results suggest that the freedom to explore the 360° environment disrupt the reception of the narrative and affect the social dimension of the cinema's experience.

RELATED WORK

The history of 360° projections goes back hundreds of years, from the use of trompe-l'oeil effect on the domepaintings of baroque churches to the panoramas in 19th century World Fairs where the creation relied mostly on manual and sometimes photographic methods [START_REF] Stephan | The panorama: history of a mass medium[END_REF]. The anamorphic technique produces distorted projections that look normal when viewed from a particular position, projected on a spherical surface, or viewed through a curved mirror or lens. Spherical recording and projection of video content has also been explored recently [START_REF] Carolina | The renaissance of VR: are we going to do it right this time?[END_REF], in which case the necessary anamorphic deformations were achieved either by employing optical methods or digital post-production of video content [START_REF] Paul | Dome projection: An alternative approach (mirrordome). 219 IHM'16: Contextes et domaines applicatifs[END_REF][START_REF] Courchesne Luc | Where are you?: an immersive experience in the panoscope 360[END_REF]. Using cylindrical and spherical distorted panoramic images, the user can visualize the scene from the inside and feel being present in it. Spherical panoramas can be reproduced from multiple angle images [START_REF] Eric | Quicktime VR: An image-based approach to virtual environment navigation[END_REF][START_REF] Jacobs | Interactive panoramas: techniques for digital panoramic photography[END_REF].

The project xTV (explorative television) [16] generated a series of explorative applications for television (i.e. sport events, documentary, fictional) in 360°. During brainstorming sessions with directors, event organizers and experts in immersive experiences, it emerged that contents in 360° could be intuitively explored without forcing users to participate. However, this project focused more on the production and the diffusion of this new kind of videos rather than studying the user's reception of these contents. In addition, they used traditional computer or TV screens and no innovation was explored about the visualization system.

McGinity et al. [START_REF] Matthew | AVIE: a versatile multi-user stereo 360 interactive VR theatre[END_REF] proposed the AVIE, a cylindrical display theatre for immersive, interactive, and multi-user experience (up to 20 people). The AVIE combines a realtime 360° omnistereo projection with a surround audio and a marker-less motion tracking system. The screen is a 10 meters' diameter cylinder, standing 3.6 meters high. Even though it could be an interesting installation for cultural, educational or artistic institutions (i.e. museum), it seems hardly applicable in the context of cinema and home video especially because of the huge dimensions and because people have to stay standing.

Through a human-centred design approach Bleumers et al. [START_REF] Lizzy | Seeing the bigger picture: a user perspective on 360 tv[END_REF] conducted a study aiming at understanding which characteristics make a TV-program suitable for the 360° video recording. In this study participants watched a fragment of a 360° video, then researchers collected data about user experience. In addition, by using the "laddering" technique they prompted users to envision possible opportunities for future usage of the new technology. Users pointed out both problems and opportunities of 360° videos: this type of visualization was thought to be suitable for videos that trigger the desire of exploration such as documentaries, touristic videos, programs featuring houses or interiors; moreover, 360° visualization was perceived as an opportunity when the spectator wants to experience the scene in the firstperson perspective (i.e. watching a sport action from the perspective of our favourite player) or during social programs in order to perceive non-verbal communication cues (i.e. frowning, specific postures, gestures, etc.). On the other hand, the main issues concerned the information load (i.e. too many things to look at in the scene), the disruption of the social dimension of watching TV (i.e. watching movies alone instead of together), and the risk of missing parts of the narrative sequence in programs having an elaborate narrative (i.e. looking elsewhere when something crucial happens).

Other authors also acknowledge this last issue and propose to solve this question with an interface able to separate spatial and temporal navigation. Using head movement, users can explore the environment and with hand gestures they can control the temporal navigation [START_REF] Benjamin | Towards effective interaction with omnidirectional videos using immersive virtual reality headsets[END_REF]. Yet, "freezing" time in order to explore the ambient while watching a movie is likely to disturb the continuity of the narration and therefore, the whole experience.

SOCIAL VR

We argued that people prefer to watch movies together because they want (whether consciously or not) to share the experience generated by the story told, as mentioned in other studies [START_REF] Isaac | Erving Goffman et la microsociologie[END_REF][START_REF] Suresh | Consuming with Others: Social Influences on Moment-to-Moment and Retrospective Evaluations of an Experience[END_REF].

At the most fundamental level, the social interaction is based on the actual perception of some sort of information "emitted" by other people. At the level of verbal communication, we could, for example, listen to their words; as for non-verbal communication we need on one hand to visually perceive some features of the other people's body, such as postures, quality of movements, facial expressions; on the other hand, the auditory channel allows us to detect the non-verbal communication features of the voice.

We think that the use of the VR headsets in the context of cinema is an inappropriate use of an existing technology because, by disconnecting the visual and auditory systems from the environment, they disrupt the perceptual processes at the base of the interpersonal communication. Therefore, natural social interactions during the watching of a movie will be at least difficult when all people wear a VR headset.

From this stand the Hyve-3D preserves the natural perceptual channels upon which communication is based, providing an "interaction framework" spontaneously enabling verbal and non-verbal communication among spectators. We named such VR systems as Social VR.

VR HEADSETS ISSUES

In this section we summarize the VR headsets issues according to the above mentioned related work. VR headsets introduce two main changes: first, the users are confronted to a new display closer to their eyes individualizing the experience of watching movies; second, the 360° movies paradigm changes the way people watch movies and how moviemakers conceive and shoot movies. We think these two changes are too many variations for spectators and cinema industry. In regard to the related works presented above, we formulated two main working hypothesis concerning the impact of VR headsets:

VR headsets are not the best media for the social experience of VR cinema, because of the following considerations:

People prefer to experience stories together, pointing facts into the movie and express their opinions verbally and through facial expressions. When users are disconnected from reality they cannot see and hear each other, they cannot see their gestures and interact with their near context (their seats, meals, drinks, etc.).

360° is irrelevant for VR movies considering the following:

The focus of the action related to the main characters can be watched from one specific angle of view. If users are looking around in 360° the chances they miss details or even main events the director wants to communicate increases. In addition, since VR headsets prevent users from seeing each other and therefore their hands pointing something in the screen, this risk is further increased. This issue has also to be considered during the design of the storytelling because shooting in 360° requires an action in several parts of the scene, and this could confuse users.

Shooting the scene in 360° is also an issue for actors and technical staff. In fact, the direction and the technical staff, including sound, and all supporting personnel, has to be hidden from the 360° scene. In addition, actors are far from the direction and this eventually could affect their performance. Finally, the scene has to be designed considering a total surround view, requiring more resources, including more synchronized figurants, becoming consequently more expensive.

From an ergonomics stand, the 360° view has potential issues. In order to locate where the main action is going on, users have to explore the whole scene by turning around their heads and even their bodies. In the context of cinema or home video, the fact that people are seated on a chair or on a sofa, constraints these movements, and therefore the exploration of the scene.

Current dome theatres and planetariums provide a collective and immersive experience to the users. However, the spectators have to look to the ceiling (like looking to the stars) but this is not the natural position we look at the world (frontal viewing) like in movies. Yet, finding the main action in 360° remains an issue. For more casual visual and acoustic explorations (e.g. watching a panorama) these systems could be suitable but in the context of following the storytelling of a movie we think they could be inadequate.

VR SYSTEMS USED IN THE STUDY

Hyve-3D: is an innovative VR system that allows creating objects and environments collaboratively (local or remote) by 3D sketches and a natural interaction using tablets. Via a unique spherical-concave screen and an original projection (not 360°) based on anamorphic images, the system enables participants' immersion in life-sized representations without the need of VR headsets. The system was designed for collaboration between different co-localized users, or between several remotely interconnected systems. Hyve-3D is designed for creative fields like architecture, design, engineering, but also for visualisation in medicine, chemistry, sciences and cinema (Figure 1). Hyve-3D is an immersive system where a complete immersive (real-time) 3D scene is displayed via a panoramic non-stereoscopic projection. Using a single projector, the spherically distorted image is reflected off a dome mirror and displayed on a 2.5m high, 5m-diameter concave spherical screen made of an opaque fabric material. The screen allows the projection of the distorted image around an audience (see Figure 1). Thanks to the trompe l'oeil effect, users watching the movie surrounded by the spherical concave screen perceive the deformed image as a corrected perspective. This concave spherical screen is not 360° like dome planetariums in order to place the image principally in front of the spectators and avoid the above-mentioned VR headsets issues. We chose this system because at the same time it enables a non 360° immersive visualisation and a collective experience without wearing VR headsets. VR headsets: we chose the cardboard VR headsets (HMrD) because they are less intrusive (i.e. not to be worn head-mounted using straps), allowing people to easily taking them off, in the context of evaluating social impact of the VR headset while watching movies. We agree that this kind of HMrD is the minimum expression of a VR headset (no tracked, etc.), but they offer us the possibility to control the impact of the sound which was external in both study's conditions. This also offers to the participants the affordance of listening and talking to each other. Even though more sophisticated VR headsets can offer parallax and head tracking, they would heavily affect the social interaction considering the total isolation (visual and auditory) from reality, in addition of the difficulty to take them off. In our study, we used two iPhones 5s with retina display inside the cardboards, displaying 4K resolution 360°, non-stereo movies.

Concerning digital representations of spectators, at this time exist some implementations of having point cloud 3D scan of users inside virtual environments (Dassault Systèmes, Dream Sketcher, etc.). However, in the context of 360° movies these techniques could be an issue, because of resolution issues concerning face expressions since these representations are in 3D while the movie is spherical and the fact of including several spectators in the scene. Concerning 3D avatars, we think that using 3D trackers in the context of cinema to allow social interaction is irrelevant and disturbing.

VR MOVIES

Many software solutions like Kolor Eyes™ (by Kolor) can deform stitched cylindrical panoramas (Figure 2) into spherical panoramas videos (Figure 3). This kind of 360° videos use a 6 or more camera-rig like those developed by Jaunt (http://www.jauntvr.com), placed within the scene, pointing the cameras at orthogonal axis directions. Other spherical lens like Gopano Pro™ (by EyeSee360) can be used to film directly in spherical panoramas videos although with less resolution because only one camera is used. The cylindrical spherical panoramas coming from stitched cameras can be produced at the ratio of 2:1 from resolution attending for example 3840x1920 and 4096x2048 pixels (4k), that can be handled by the graphic cards of current laptops and smart phones (Figure 2). Both Hyve-3D and VR headsets using iPhones can display effectively these spherical panoramic movies.

METHOD

We compared Hyve-3D to cardboard VR headsets. In order to better explore the social experience and the effectiveness of the two VR systems for movies we collected different kind of data. Specifically, we collected data about subjective experience through a questionnaire; in addition, we obtained data from a behavioural analysis of the video recordings; finally, the study was enriched with qualitative data of an exit interview aimed at 214 214 IHM'16: Contextes et domaines applicatifs 25-28 oct. 2016, Fribourg, Suisse obtaining a deeper understanding of the quantitative results [START_REF] John | Designing and conducting mixed methods research[END_REF].

Participants

Twenty participants (4 women and 16 men) were recruited from design courses and people from the building of our laboratory. The educational level of the group ranged from under to graduated students and the age from 21 to 44 years (mean=26.8; standard deviation=5.1). All participants were new to Hyve-3D and just one of them had a previous experience with VR headsets. We ask each participant to bring a student friend in order to watch a VR movie together. Participants knowing each other were coupled in 10 dyads and this request intended to prevent a bias in the social interaction caused by a poor relationship between the two participants.

Procedure

We conducted a within-subjects study. We asked participants to watch two sequences (1 & 2) each one composed by two different comparable short videos (around 5-minute length in total). Short videos were selected instead of long films since the latter are hard to find, and because their implementation and analysis are time-consuming considering the exploratory character of this study. The first video of each sequence was a video without a storyline (i.e. panoramic rollercoasters), while the second one was a thrilling act presenting a short story.

In both conditions participants were in standing position, in order to avoid constraints due to seats. The videos were 360°, and in Hyve-3D, we oriented the portion of the video containing the main action in front of the spectators, since the main story action happens in a particular portion of the scene. With Hyve-3D only around 60° of the scene were hidden.

6 dyads watched first the sequence 1 (3 Hyve-3D, 3 VR headsets) and 4 dyads started watching sequence 2 (2 Hyve-3D, 2 VR headsets). The two videos were started at the same time by two researchers and the audio coming from one of the VR headset was shared using a high quality Bluetooth speaker. The same speaker was used for Hyve-3D. After watching each sequence, they filled a questionnaire and then they participated in an exit interview in groups of four. Movie watching and exit interviews were video recorded for the subsequent behavioural analysis.

Measures

Questionnaire.

We administered a questionnaire (Figure 4) asking questions about: sense of presence, user experience, social interaction, and personal preferences. We used a 7point Osgood-type scale presenting opposite-meaning adjectives at each end (i.e. close/far, inside/outside, easydifficult) for questions measuring the degree of a sensation, an attitude, or a personal preference. The rest of the items (2) required a yes/no answer. Here is a description of each variable measured:

Presence. According to Lombard et al. [START_REF] Matthew | Measuring presence: the temple presence inventory[END_REF] we measured the sense of presence by using two items: the first item intended to measure presence as sense of transportation into the VR (how much participants felt themselves "there in the scene" -inside-outside); the second item intended to measure presence as psychological immersion (how much objects and characters have been perceived close to the user -close-far). We also asked about the need of a stereoscopic vision in order to have a better experience. Storytelling. In order to evaluate the ability to follow the narrative of the videos presented, we asked participants if they noticed to have missed something in the scene (yes/no).

Social interaction.

One question asked participants if they tried to interact with their mate during the movie viewing (easy-difficult).

Disturbing sensations. We asked participants to write the most disturbing sensation they experienced (if any) and to rate it (intense-mild).

Moreover, we asked a question whether they like to watch a movie with someone or alone and which VR systems seems more suitable for watching long movies.

User behaviour

In order to obtain also additional objective information, we analysed user's behaviour within the Hyve-3D and while using the VR headsets. We focused on two types of behaviours in video recordings: visual exploration of the immersive space (head turning, body re-orientation) and social interactions (talking and pointing at the scene).

Data analysis

For each condition, we analysed the mean values obtained from each Osgood-type scale of the questionnaire using the Wilcoxon signed-rank test. This statistical test is applied for median comparison in repeated measures design (such the present study). In addition, it is recommended with ordinal variables, with small samples, and when data are not normally distributed [START_REF] Alan | The art and science of learning from data[END_REF]. The significance level was set a priori for a p-value<.05.

As for behavioural analysis, we used the computerassisted qualitative data analysis software Atlas.ti. By looking at the video recording, each visible movement of the head was coded as a "head movement", while when the movement involved a movement of the hips it was coded as "body re-orientation". Concerning the social interactions, we counted the number of times the participants talked or pointed something with their hands.

We then considered the number of behaviour for each condition. In addition, we compared the behaviour during the vision of the videos with and without storyline. Finally, in the analysis of the exit interview we inductively identified the main themes. These different types of data were then triangulated in order to draw our conclusions.

Results

Most of the variables showed a skewed distribution justifying the use of the Wilcoxon signed-rank test [START_REF] Alan | The art and science of learning from data[END_REF].

Outliers were discarded from the analysis.

Using both Hyve-3D and the VR headsets participants experienced a good sense of presence: they felt themselves "transported" into the scene and they perceived objects and characters as if they were close to them. However, VR headsets induced a higher sense of transportation and immersion than Hyve-3D (Figure 5). The Wilcoxon signed-rank test showed both differences (transportation and immersion) to be statistically significant (z=1.965; p=0.0494; z=2.577 p=0.0100).

Participants also reported a general positive experience with both systems, the more positive while using Hyve-3D (Figure 5). Again, the Wilcoxon signed-rank test showed this difference to be statistically significant (z=-2.442; p=0.0146). Regarding the ability to follow the storytelling, 80% of participants using VR headsets reported having missed some content while only 15% using Hyve-3D (Figure 6). Concerning the length of the movies, Hyve-3D was perceived as more suitable for watching long movies (Figure 7). About the preference to share the experience of watching movies, 83% of participants declared that they usually watch movies with friends. Using Hyve-3D, 50% of participants tried to interact with the partner, while 55% using VR headsets. Social interaction using Hyve-3D has been perceived far easier than with the VR headsets (Figure 8). The Wilcoxon signed-rank test showed this difference to be statistically significant (z=1.965; p=0.0494).

Around 55% of the group reported some kind of disturbing sensation using both displays. We didn't find a significant difference between intensity of these sensations between the two VR systems.

Behaviour analysis

Behaviour analysis of video recordings showed that people using VR headsets changed their body orientation far more than in Hyve-3D (253 body re-orientations vs. 5) (Figure 9 A).

They often looked around, up, and down turning their body of 180° and sometimes also of 360°. In addition, they often displayed multiple rapid movements of the head back and forth (e.g. right-left). Interestingly, the use of VR headsets produced more body re-orientations and head movements while watching the video with a storyline than the one without a storyline (Figure 9 B).

In Hyve-3D, small head movements seemed enough to watch the videos and to follow the story, while the body remained almost always oriented in the same direction. In addition, in Hyve-3D we observed the same pattern of exploration behaviour watching videos with as well as without storyline (Figure 9 B).

Concerning the social interaction, "talking" turned out to be the dominant behaviour during movie watching. People talked more while using VR headsets than Hyve-3D (Figure 10). Also, in both display systems they talked more during videos with a storyline. In both displays some people pointed contents in the scene, obviously their communication failed when using VR headsets (they took the VR headset off 3 times) (Figure 11). While watching a movie with a storyline using the VR headset, participants mainly asked questions about their own point of view "where are you looking? Are you looking at ...?" and checked if the mate did see some element in the scene "Did you see it?". In the Hyve-3D they commented detail of the scene, made conjecture about the coming events or asked questions about details.

Exit interviews

At the end of the experience and after having filled the questionnaire we conducted a collective exit interview. general, all of them appreciated both experiences with Hyve-3D and VR headsets. The main topics of the discussions were the sense of immersion, the quality and nature of the visualization and the social interaction during videos.

Immersion was indeed present in both cases, but in different ways. Hyve-3D, unlike VR headsets, kept an anchor with the external environment since it was still possible to see the limits of the screen and enabled to see the other person. Even if VR headsets gave freedom to explore in all directions and a sense of deeper immersion, the natural field of view was limited because peripheral vision was cut by the VR headsets' frame. In fact, users reported they had to move their head in every direction to be able to follow the action, which was perceived as tiring and disturbing for some. Only two participants reported that the seams of the fabric screen disturbed the immersion.

Besides the discomfort of the eyes, using VR headsets participants declared that they were not able to understand the story and they often missed some key elements of the storyline. In fact, when we asked them about some important details related to the story in the videos (i.e. a character that appeared) they confirmed they missed objects or actions. But, as the vision is wider in Hyve-3D, it gives more comfort and better observation capacity through peripheral vision. One participant says: "In Hyve-3D, we have a capacity of vision which is very wide without the VR headsets. As humans, we naturally see wide [...] while with the glasses you can only see what you have in front [...] so you have to look everywhere all the time." This requires more physical and mental effort because of increased movements and the fact they need to search in the scene. Moreover, this seems to prevent passive relaxation experienced in the cinema. All participants expressed their concern about the use of VR headsets to watch a long film. This is due to the fact that there are visualization issues, difficulty in following the story, and also because VR headsets isolated them from the external environment, blocking communication with the other spectator. Since participants confirmed that cinema is considered a "social experience", the presence of the other was an important factor to enjoy the movie, and Hyve-3D was far favoured. Sharing the experience matters above of all. "We feel more alone with glasses, so we feel more the need to talk. In Hyve-3D, I can see, I know what the other spectator sees, so I did not feel the need to talk to him. I think it is important for humans to see the experience of the other."

The issue of social presence repeatedly came in the discussion as one of the characteristics of the cinema experience. With VR headsets the presence of the other is missed, and even if participants make the effort to talk about something in the scene, it was often difficult to easily know in which direction to look. Also, the absence of the body in VR headsets was an issue for some participants, and when they noticed that, they often disconnected from immersion. Since, many people understand 360° vision as an opportunity to explore the virtual space, participants got disappointed because they were not able to touch the objects around them.

Videos were not stereoscopic. Participants declared that they did not expect it in Hyve-3D. However, they would prefer to have it in VR headsets. The 3D illusion (anamorphic and trompe l'oeil effect) was much appreciated in Hyve-3D view, thanks to the extended field of vision, the scale of the objects and the curvy shape of the screen.

Discussion

The main goal of VR in cinema, is to immerge spectators in the scene and improve their experience. However, it is unfortunate that VR headsets are proposed as the only hardware for VR in cinema.

According to our study, user's experience while watching short movies using a VR headset seems an issue, because the cinema experience seems to be preferably social. Our results indicate that social interaction was perceived to be quite difficult using the VR headsets. Despite this, participants talked more using the VR headsets. Because from the interviews emerged that to be able to feel the mate's presence was an important aspect of the whole experience, we interpreted this result as the attempt to reach their friends verbally: since they lost all visual clues about their presence, their position, and their actions, verbal communication occurred to compensate the lack of other's presence. Also, in both display systems participants talked more during videos with a storyline. As demonstrated in other studies [START_REF] Suresh | Consuming with Others: Social Influences on Moment-to-Moment and Retrospective Evaluations of an Experience[END_REF], there is more enjoyment and appreciation of the experience when this one is shared and the persons are mutually conscious of other's reactions. This consciousness enhances emotions and contagion of emotional expressions between spectators even with a simple "out of the corner of one's eye" [START_REF] Suresh | Consuming with Others: Social Influences on Moment-to-Moment and Retrospective Evaluations of an Experience[END_REF], which is a characteristic that Hyve-3D offers with ease to spectators.

Even if this exploratory study had several limitations (small and uneven sample, short movies, low quality VR headsets), this research pinpoint a bunch of possible issues in transferring the VR headsets in the context of cinema and it opens the door to further studies. The impact of the gender sharing the cinema experience, remains to be analysed in a more equilibrated sample. And even if participants suggested they prefer to watch long movies in Hyve-3D, the impact of VR in long films deserve further investigation.

The practical contribution of this study is to explore new VR displays for cinema that fulfil the needs of spectators about VR, in terms of immersion and the social experience and sharing. In this study Hyve-3D is presented as an alternative that really addresses these characteristics going beyond of headsets for VR cinema.

Conclusions

In order to succeed in the context of cinema, VR has to consider the perspective of spectators. In this article we present a comparative study regarding social interaction of spectators using Hyve-3D and VR headsets. Even though VR headsets seem to be suitable for some genre of videos (e.g. panoramic, touristic videos), we argue that this kind of display is intrusive in the context of cinema, affecting the shared experience and storytelling.

Defining VR in terms of technology only rather than an experience, it brings us to the particular situation we are describing in this work: the hardware is imposed to users instead of being adapted to them for an optimal experience which seems to require a social dimension.

Our results, consistently with those of Bleumers et al. [START_REF] Lizzy | Seeing the bigger picture: a user perspective on 360 tv[END_REF], showed that people do prefer to interact with each other while watching movies. Increased talking while watching a video with a storyline using VR headsets seemed an attempt to keep the contact with the other person in order to ensure the sharing of the experience. Moreover, the increased exploratory behaviour during videos with a storyline using VR headsets, and the high percentage of people that missed contents using those glasses, suggest that the "freedom" given by the 360° scene is potentially confusing for the users.

Hyve-3D seems to be more suitable than VR headsets in the context of collective watching of movies. Because the peripheral vision and the particular spherical concave shape ensure an adequate immersive experience and at the same time, preserve the social interaction during visioning an immersive movie. Hyve-3D unobtrusively enables users to easily scan the scene and to perceive, also through the peripheral vision, the important components of a story.

VR headsets generate the perception of a frame of vision around the eyes giving the perception to the user that they are wearing blinders affecting the peripheral vision (moving objects coming from the side, etc.). This in our opinion is still maintained with solutions to improve the peripheral images [START_REF] Robert | Augmenting the Field-of-View of Head-Mounted Displays with Sparse Peripheral Displays[END_REF] since the illusion of an extended image cannot replace the fact of be immersed collectively in a social VR display as in the anamorphic image of Hyve-3D. Moreover, VR headsets bring a hygienic issue. They have to be worn for a long period of time in the context of a feature film during which they are too close with people's skin, eyes and hair. Therefore, they are perceived as private items rather than public. Finally, there are some technical and economic implications concerning the use of VR headsets since each user needs his/her own system that have to be synchronized with others in order to provide a collective experience.

Figure 1 :

 1 Figure 1: Hyve-3D.

Figure 2 :

 2 Figure 2: Cylindrical panoramic movie, ratio 2:1. Hollywood Rip Ride Rockit video (by Universal 360°, from Youtube.com).

Figure 3 :

 3 Figure 3: Spherical panoramic movie, deformed by Kolor Eyes™ (by Kolor). Hollywood Rip Ride Rockit video (by Universal 360°, from Youtube.com).

Figure 4 .

 4 Figure 4. Questionnaire including all the research aspects User experience. We measured the valence of the experience by 1-item question asking how much they enjoyed the experience (positive-negative).

Figure 5 .

 5 Figure 5. Perception of presence (sense of transportation and psychological immersion) and user experience. *Difference statistically significant.

Figure 6 .

 6 Figure 6. Percentage of people that missed at least one content using each VR system.

Figure 7 .

 7 Figure 7. How long participants thought they could watch a movie using each VR system.

Figure 8 .

 8 Figure 8. Perceived easiness of social interaction during movie watching using each VR system. *Difference statistically significant.

Figure 9 .

 9 Figure 9. A -"Head movements" and "Body re-orientation" using Hyve-3D and the VR Headset. B -Comparison of "Head movements" and "Body re-orientation" each VR system during movies with and without storyline.

Figure 10 .

 10 Figure 10. Verbal and non-verbal communication usingHyve-3D and the VR Headset.

Figure 11 .

 11 Figure 11. Verbal and non-verbal communication using each VR system during movies with and without storyline.

IHM'16: Contextes et domaines applicatifs 25-28 oct. 2016, Fribourg, Suisse

IHM'16: Contextes et domaines applicatifs 25-28 oct. 2016, Fribourg, Suisse

IHM'16: Contextes et domaines applicatifs 25-28 oct. 2016, Fribourg, Suisse

ACKNOWEGMENTS

We would like to acknowledge all the participants of this study, the reviewers of this paper and the support of Canada SSHRC-Insight research grant and the CFI-Leaders Opportunity Fund research grant.


Pierre-Marie Basset
email: basset@onera.fr

Philippe Beaumier

Thomas Rakotomamonjy

CREATION: a numerical workshop for rotorcraft concepts generation and evaluation

Keywords: rotorcraft pre-sizing, flight performance, environmental impact, multidisciplinary optimization, multi-objectives optimization

A.T.I.O.N. for "Concepts of Rotorcraft Enhanced Assessment Through Integrated Optimization Network", is a multidisciplinary computational workshop for the evaluation of rotorcraft concepts. The evaluation concerns both flight performance and environmental impacts (acoustics, air pollution/fuel consumption, etc.). The CREATION workshop must allow the evaluation of any rotorcraft concept whatever the level of details of the description data initially available. Therefore the tool must cope with the preliminary conception problems. The paper describes the tool providing examples of application for each of the three main milestones stepping its building. Besides the multidisciplinary models which are the core of the workshop, some original methods have been developed. Among them, a creation process of rotorcraft configurations (not prescribed by engineers) will be presented as well as a multi-objective impartial optimization process.

INTRODUCTION

Rotary wings aircraft are more complex to simulate than fixed wings. Since the sixties engineers have been working on building computational simulation tools. Still nowadays, engineers are improving further their highest fidelity models in their respective fields: flight dynamics, aerodynamics, aeroelasticity, aeroacoustics … These high-fidelity expert tools tend towards the most accurate representation of the physics. They are generally very demanding in terms of both computational time and input data for describing the rotorcraft and its environment. This kind of models is therefore not adapted for the conceptual and pre-sizing studies. Indeed, when a new rotorcraft must be predesigned from a set of mission requirements, the cost in simulation time must be low enough for performing a lot of parametric sensitivity studies and pre-sizing optimization loops. Moreover at this early stage, starting from scratch, the data needed by the most sophisticated models are not available yet. Thinking "the more a model is complex, the higher is its fidelity", is not true. A better paradigm is the more a model is adapted to the degree of description given by the available data, the more it can provide the most valid results.

Many rotorcraft concepts exist and this variety will be further enriched by the worldwide strong interest on Rotary Wing Uninhabited Aerial Vehicles. What is the best concept for a set of missions? A simulation tool able to model any kind of rotorcraft architecture, combining rotary wings as lifting and/or propulsion devices as well as fixed wings, will be useful at a very early stage of development for selecting one or some best candidates. It requires to model each element with the same accuracy (open rotor, ducted fan, contra-rotating coaxial rotors, dual tandem rotors, tiltrotors, wings, etc.) and to take into account their interactional aerodynamics. Moreover their relative comparison needs the use of relevant metrics especially in terms of flight performance and environmental impact. CREATION, which stands for "Concepts of Rotorcraft Enhanced Assessment Through Integrated Optimization Network", is a numerical workshop built by ONERA for this purpose.

Beyond the presentation of the numerical workshop, in terms of models and methods, the paper provides concrete results by illustrating the approach on practical application cases.

CREATION has been built during a federative research project from January 2011 to December 2014 on ONERA own funding. It involves five departments related to the different disciplines required for building the workshop: rotorcraft flight performance and dynamics, aerodynamics, acoustics, materials and structure, aircraft multidisciplinary predesign.

For building this tool, three main milestones have been defined for developing incrementally its capabilities.

•

Milestone 1 -evaluation capability: the flight performance and environmental impact (external noise and air pollution) of a known helicopter can be assessed whatever its degree of description (from a minimal set of about ten main data) ; • Milestone 2 -predesign capability: the pre-sizing of a helicopter suited for a set of mission requirements can be addressed starting from scratch ; • Milestone 3 -innovation capability: the computational workshop provides means for the investigation of alternate concepts (with respect to the conventional single main rotor -single tail rotor helicopter).

These capabilities aim at evaluations for quantifying objectively (as unbiased as possible) performance and environmental metrics of any rotorcraft known or to be invented. These evaluations rely on models which are organized in multidisciplinary modules and in multi modeling levels within each disciplinary module. A 3-Dimensional view of this framework representing the CREATION workshop is shown in Fig. 1. The models are organized in a matrix structure by disciplinary modules and complexity levels. With the tools (models and methods) available in this workshop, the engineer is able to assess on any mission profile the "Flight performance" and "Environmental impact" of any rotorcraft configuration.

Around these two core pillars, five "means modules" provide the required data: Missions & Specifications, Architecture & Geometry, Weights & Structures, Aerodynamics, Power Generation.

This "horizontal organization" in disciplines is stratified in a "vertical structuring" in modeling levels in order to adapt the computation effort to the available data and to the requested fineness of the analysis. Four main levels of modeling have been implemented in most of the modules:

• Level 0: Response Surface Models based on databases or simulations, • Level 1: simple analytical models based on physics, • Level 2: more comprehensive analytical models, • Level 3: numerical models (like blade element model, finite element models, etc.).

More details about the models can be found in previous papers [START_REF] Basset | project for rotorcraft concepts evaluation: the first steps[END_REF][START_REF] Basset | the Onera multi-level rotorcraft concepts evaluation tool: the foundations[END_REF]. The models have been implemented in a well suited environment (ModelCenter©) able: to deal with executables in different languages (C, C++, Fortran, Python, Excel, etc.), to manage their connection and workflows and to apply different techniques for sensitivity analysis, meta-modeling (reduced models), optimization algorithms … About the methods, besides known ones in Multidisciplinary Design Optimization (MDO), such as methods for generating surrogate models (e.g. [START_REF] Sacks | Design and analysis of computer experiments[END_REF]) or evolutionary algorithms for exploring widely the design space (e.g. [START_REF] Welch | Expensive global optimization of expensive black box functions[END_REF][START_REF] Murata | MOGA : multi-objective genetic algorithms[END_REF]), some original methods have been developed. Among them, a multi-objective impartial optimization process will be presented for the milestone 2 as well as a creation process of rotorcraft configurations (not prescribed by engineers "a priori" ideas, yet with safeguards) for milestone 3.

EXAMPLES OF RESULTS THROUGH THE THREE MILESTONES Milestone 1: case of a known helicopter

As a first validation exercise, the chain of models was tested for evaluating the flight performance of an existing helicopter. The Dauphin SA365N was chosen because of the availability of reliable and numerous flight test data.

The performance in steady flights results from the balance between the required power to maintain the considered flight condition (defined mainly by weight, altitude and temperature) and the available power as provided by the engine(s) taking into account losses, power withdrawal and limits. Therefore the assessment of the power required by the rotors is a key for performance evaluation.

The following two figures (Figs. 2 and3) present the comparisons between the total power required (main rotor plus tail rotor) computed with CREATION (modeling level 1 in red, level 2 in green) with respect to the flight test data in blue. In Fig. 2, the agreements with both kinds of models are quite good above 10 m/s of translation speed. Near hover, the calculations done with the level 1 model without interaction modeling underestimate the required power whereas the level 3 model provides good correlation. By taking into account the interference between the main rotor and the airframe, the simple analytical level 1 model succeeds providing a good estimation of the required power even in hover and very low speeds as shown in Fig. 3.

Fig. 2: CREATION computation done with level 1 and level 3 models compared with SA365N flight

test data (σ is the ratio of air density with respect to the air density at sea level in ISA condition) Fig. 3: Same as Fig. 2 but with level 1 model including main rotor / airframe interference compared with SA365N flight test data.

In descent and climb flights, the calculation with the level 1 model (Power Balance analytical model) are also in good agreement with the flight test data as can be seen for example in Fig. 4 for advancing speeds around 30 m/s and gross weight between 3886 kg and 4052 kg. For assessing the generality (or "universality") of the models, other comparisons were performed for different existing helicopters: EC225, EC135, EC145 … These comparisons demonstrated the ability of the CREATION models to calculate the steady flight performance of known helicopters validating the evaluation capability.

Milestone 2: pre-sizing of a new helicopter

Different options were envisaged for this milestone. For example, from the mission requirements or performance of a known helicopter, an exercise would have been to find back its main characteristics by doing the pre-sizing. Yet the inter relationship between on the one hand the sizing characteristics of a helicopter (dimensions, number of blades, rotational speed, engine, weights etc.) and on the other hand the requirements or performance, is not bijective in the sense that different helicopters may answer more or less to the same requirements. For example, the time period at which the helicopter was designed (with the available technologies on that time) has of course a significant impact on the weights, the engine, the blade geometry …

Besides, ONERA and NASA decided to collaborate on the topic of environmentally friendly rotorcraft (see [START_REF] Russell | Conceptual Design of Environmentally Friendly Rotorcraft -A Comparison of NASA and ONERA Approaches[END_REF]). The purpose was to study the impact on predesign of taking into account environmental metrics (mainly related to air pollution and greenhouse gas emissions, "GHG"). Nowadays helicopters have a marginal contribution to GHG emissions, as they represent only 1% of aviation emissions which produces between 3~5% of the total anthropogenic GHG emissions (depending on the year and considered scale of states, EU or worldwide). But in the prospective assumption of using rotorcraft for contributing to unblock the airports (thanks to their VTOL capability), so by transporting 90~120 passengers over a range of about 1000 km, there would be much more rotorcrafts and bigger ones than nowadays, hence in this context their environmental impact must be considered.

Therefore for the milestone 2, the pre-sizing capability was studied by considering first the transportation mission defined within this collaboration: a 90 passenger transport helicopter with a range of 1000 km, (more detailed requirements are given in [START_REF] Russell | Conceptual Design of Environmentally Friendly Rotorcraft -A Comparison of NASA and ONERA Approaches[END_REF]).

The chain of models is of course different with respect to the one used for milestone 1. Indeed, it must include an internal loop of convergence on the gross weight. The pre-sizing requires the assessment of weights since it influences the sizes, the number of blades, the fuel weight, the engine weight … Within CREATION the weight models combine existing models based on statistics and newly developed models for example for the blades and for the main gear box.

Beyond the models, a significant step forward was performed in terms of methods for dealing with helicopter pre-sizing. Multidisciplinary Design Optimization (MDO) includes both meta-modeling and multi-objectives/multi-design parameters optimization techniques. An overview has been given in [START_REF] Tremolet | Some meta-modeling and optimization techniques for helicopter pre-sizing[END_REF]. A methodology has been set up and applied on the concrete case of the 90-passenger transport helicopter mission (see [START_REF] Basset | Helicopter pre-sizing by multidisciplinary -multi objective optimization[END_REF] for more details). A brief synthesis of some key aspects is given here.

Objectives or cost functions

In most cases, the pre-sizing of such a complex system as a rotorcraft is a multi-objective problem. For example, for this transport mission three main objectives can be considered:

• The fuel weight must be minimized (for reducing the operation cost, Wfuel),

• The empty weight must be minimized (more or less in direct relationship with the acquisition cost, Wempty), • The noise footprint in the landing approach (mean noise level on the ground footprint noted Facou hereafter).

They are not independent but not equivalent and have significant effects on flight performance and environmental impact. They are many techniques for addressing multi-objectives optimization. One family of methods consists in prioritizing them and solving the problem in a hierarchical process (e.g. [START_REF] Miettinen | Nonlinear Multiobjective Optimization[END_REF][START_REF] Osyczak | An approach to multicriterion optimization problems for engineering design[END_REF]). Other methods consist in aggregating the different cost functions (or objectives) into one global criterion which could incorporate ponderation for putting more or less weight on the different objectives (e.g. [START_REF] Marler | Survey of multi-objective optimization methods for engineering[END_REF]). In both approaches, prioritizing or weighting the objectives is often delicate and dependent on the point of view: builders, operators, customers, surrounding population very often do not share the same priority.

One approach proposed in [START_REF] Basset | Helicopter pre-sizing by multidisciplinary -multi objective optimization[END_REF] consists in searching first for an "impartial multi-objectives optimization" before any arbitrary weighting (which can be considered later in the predesign process).

Utopian Point

This approach is based on selecting the optimal solution as the closest to the "Utopian Point". Before defining this last term (UP), it is worthwhile to recall what a Pareto front is (it has for dimension the number of objectives minus one in the dimensional space of the objectives). On this frontier all the points represent optimal solutions in the meaning that it is not possible to improve further one objective without degrading one or more of the other objectives. Therefore in most or all the multiobjectives problems, the point gathering all the best values of the objectives is beyond the Pareto front and is so called Utopian Point as being not achievable, here for example:

UP= (Wfuel min , Wempty min , Facou min )

Normalization

This peculiar point being defined, a distance with respect to it must be settled (in the space of objectives). That supposes to normalize the objectives which are different in nature and unit (fuel and empty weights, noise, air pollution, etc.). Some builders translate all objectives in a money unit. Here each objective is normalized by the range between its maximum and minimum values.

A special attention must be paid to the normalization of each objective criterion. Different options are possible for determining the minimum and maximum values of each objective.

The ranges of the design parameters are taken large enough in order to catch the global extremum (minimum or maximum) searched for each objective. Let us say that for all objectives, the considered metrics must be minimized (as it is the case in the given example above, otherwise for the cost function for which the maximum is wanted, it can be replaced by its opposite). It consists in taking the maximum from the exploration of the design space. In that case the user fixes the ranges of the design parameters and the maximum and minimum values of each objective are given by the calculation other this design space as a result of the chain of models. Another option consists in fixing a priori the maximum value acceptable for each objective. The minimum of each objective results from the computation as described hereafter.

Multi-objectives optimization

In [START_REF] Basset | Helicopter pre-sizing by multidisciplinary -multi objective optimization[END_REF] two examples of methods have been proposed and compared.

The first method combined a Genetic Algorithm (GA) and a Deterministic Algorithm (DA). Metaheuristics, among which are found population methods like evolutionary or genetic algorithms, are able to cope with multi objectives problems [START_REF] Murata | MOGA : multi-objective genetic algorithms[END_REF]. They are interesting for exploring widely the design space without focusing on a local extremum. They provide a set of "near optimal solutions" approximately on the Pareto Front. Yet they require a high number of runs, therefore even with the level 1 models of CREATION, the computational time is too long. That is why the complete chain of models (including the convergence loop on the gross weight, the engine pre-sizing and the loop on the entire mission profile for assessing the fuel weight), is replaced by a surrogate model (Response Surface Model RSM as sketched in Fig. 5) through the following steps:

1) Definition of a Design of Experiment (DoE), i.e. a set of representative points of the design space (table of values for the main design parameters such as main rotor characteristics radius R, mean chord c, number of blades b, blade tip speed due to rot ation U, etc.), 2) Computation of the objectives with the complete chain of models for each point of the DoE, 3) Generation of the corresponding RSM (for example with Kriging techniques), 4) Exploration of the design space with a GA for approaching the Pareto front.

Fig. 5: Meta-modelling of the complete chain of models for pre-sizing and evaluation over the mission by a surrogate model (RSM).

After this first multi-objectives optimization with a GA, the calculation of a best compromise solution is done by a DA. From the Pareto front estimation, the minimum and maximum values of each objective is extracted and used for its normalization. Then a global optimum is computed with a DA as being the optimal solution minimizing the distance with respect to the UP (for the distance different norms can be used as the Euclidean norm or the sum of the squares of the objectives for a set of design parameters). This final optimization with a DA can be performed by using the complete chain of models (instead of the RSM).

The second kind of methods uses only DA for the sake of quickness. In a first main step, the UP is estimated by performing separated optimizations with a DA for each of the objectives. Once the UP is assessed, the global optimal design values are computed as the set of design parameters minimizing the distance with respect to the UP. By doing so the multi-objectives problem is reduced as before into a unique criterion and thus a DA can be applied.

The two methods provide very close solutions validating each other as shown in [START_REF] Murata | MOGA : multi-objective genetic algorithms[END_REF]. The advantage of the first one ("GA+DA") is that it is richer than the second one as it gives the overall trade-offs between objectives by approaching the Pareto front. The second one (using only DA) is faster and thus well adapted for fast pre-sizing.

Examples of results

In the practical case of the 90-passengers transport mission, the results obtained respectively by ONERA and NASA are in very good agreement. The resulting design parameters are consistent, for example considering the main rotor:

for ONERA: R=16.93 m, c=1.02 m, b=7, U=187m/s for NASA: R=17.77 m, c= 0.92m, b=7, U=198 m/s

The main difference is on the blade tip speed U due to rotation, but on the ONERA side, reducing the noise was taken as an objective whereas on the NASA side, the noise is considered as a constraint imposing U=650ft/s.

The weight breakdown is also consistent as shown in Fig. 6. The resulting flight performance assessments match very well (Fig. 7). .helicopters [START_REF] Russell | Conceptual Design of Environmentally Friendly Rotorcraft -A Comparison of NASA and ONERA Approaches[END_REF].

DoE

Fig. 7: Flight performance comparisons for the 90-passengers helicopters pre-sized by ONERA (in red) and by NASA (in blue).

Milestone 3: exploring alternate rotorcraft configurations

In this last milestone, two goals are pursued:

• first, generalizing the workshop for dealing with alternate rotorcraft concepts with respect to single main rotor -single tail rotor helicopter, • second, building the capacity to generate not-predefined rotorcraft architecture solutions.

In this purpose other models have been developed for representing the other potential components of a rotorcraft: propellers, ducted fan, wings … One important issue is that beyond the fact to be able to combine them (with rotors and fuselage), a main difficulty is to take into account their interaction mainly from the aerodynamic point of view: rotor/rotor, rotor/wing, rotor/propeller, propeller/wing, …

The Aero-Multi-Body method (AMB)

The AMB code has been developed in order to gather aerodynamic numerical models to be used in pre-design phases of a novel rotorcraft concept. The driving idea of its conception is that any new concept made of fixed components (non-lifting, lifting: wings) or rotating ones (rotors, propellers) must be easily modeled by the code. Therefore, the relative position of any component with respect to the other ones (and with respect to a fixed Galilean frame of reference, at any time step t) must be as far as possible arbitrary.

The code is made of two totally independent parts: a kinematics module and an aerodynamics one.

The kinematics module comprises a set of so-called "rigid segments" respecting the simple following rules:

-A segment is defined by its length and its position with respect to the preceding segment (by 2 fixed Euler angles); -A segment can optionally rotate with uniform RPM around an arbitrary axis with respect to its preceding segment; -The axis of a segment can optionally rotate by a time varying angle with respect to the preceding segment (to simulate pitch angle variations for a blade); -A segment can be connected to an arbitrary number of following segments, but preceded by only one segment.

In order to make easier the construction of a complex model, macro elements gathering several segments have been defined: Wings and Rotors (made of Blades). Figure 8 gives an example of the modeling of a compound similar to the X 3 developed by Airbus Helicopters.

The aerodynamics module comprises:

-A Blade Element Method (BEM): the aerodynamic sectional coefficients (Cl, Cd, Cm) of each (aerodynamic) segment are read in 2D polar curves based on local incidence and Mach number; -The induced velocities on the aerodynamic segments of a blade are computed using 1D momentum theory (a vortex lattice method assuming either prescribed wake or free-wake has been added recently but was not used in the pre-design studies presented in this paper); -An interaction model, especially developed in order to simulate the effect of the interactions between elements of a complex new rotorcraft concept, and that is detailed below.

The interaction model provides the velocities induced by an arbitrary set of rotors at any point of the computational space (in practice, on any aerodynamic segment). It can therefore account for the influence of rotors on other rotors or on fixed components such as wings. It is based on 1D momentum theory, where a stream tube is defined around each rotor (see Fig. 9). This tube is defined by: -A mean-line, the geometry of which is defined assuming that the velocity at the upstream infinity of the line is the free-stream velocity V 0 , the velocity on the rotor disk plane is V 0 +V i (V i being the averaged induced velocity given by 1D momentum theory) and the velocity at the downstream infinity of the line is V 0 +2V i (as a result of 1D momentum theory); -The boundaries of the tube, which has a circular basis, the area of which is computed at each location of the mean line using mass flow conservation equation and assuming an empirical contraction law. The induced velocity V i is computed using as an input the rotor thrust force (axial component) and torque (swirl). For any point T inside or at the edge of the tube (Fig. 9), the velocity induced by the rotor is the one of the corresponding point M on the mean line. Outside the tube, the induced velocity is assumed to be zero. This AMB model as well as the ones developed in the other disciplinary modules (acoustics, weights and structures, power generation etc.) can be used either for pre-sizing and /or evaluating prescribed configurations (tilt-rotors, tandem, compounds etc.) or for rotorcraft configurations generated by the tool itself as explained hereafter.

Indeed in the acronym CREATION there is the underlying idea of being able to generate solutions not predefined by the engineer. In order to reach this ambitious goal, a code for generating rotorcraft architectures not prescribed "a priori" by the user is developed and described briefly here in its first principles.

Rotorcraft new configurations generator

In order to explore alternate or new rotorcraft architectures and to evaluate the performance of innovative concepts (in the sense that no previous rotorcraft has ever been built around it), a semirandom generation algorithm has been created in a consistent way with the AMB model.

A rotorcraft is defined as a set of different elements, arranged along a specific order, thus defining a tree-like structure of connected objects of different types. A configuration can be generated by associating elements through connections following specific constraints in order to allow only "viable" configurations to be generated. First, the elements are picked among the following sets: is the set of fuselage elements = , -≤ ≤ . In this case the origin (or construction reference point) is chosen as the central element of the fuselage (hence the relative numbering). is the set of wing type elements = , 0 ≤ ≤ 2 . Several wing elements can be assembled together to define a bigger wing. is the set of rotor type elements = , 1 ≤ ≤ (since the objective is focused on rotorcraft, at least 1 rotor is required).

is the set of propulsive propeller type elements = , 0 ≤ ≤ . is the set of lifting propeller type elements = , 0 ≤ ≤ (here, a lifting propeller differs from a rotor by its radius, rotation regime and control mode).

is the set of branch type elements = , 0 ≤ ≤ . A branch is a nonaerodynamic element (generating neither lift nor drag) acting as a connector between two elements, for example between fuselage and wing, or used to represent a rotor/propeller mast.

Each instance of the above elements has specific attributes according to its type (e.g. a wing element has a chord length, aerodynamic characteristics, horizontal or vertical orientation, etc.), which may be chosen a priori or tuned in a further optimization procedure. Moreover, the value of some attributes can be shared by all instances (in the present case, all horizontal wing sub-elements have the same span).

For setting a structural link between two elements, a connection function is specified as follows: : , , , , , → 0; 1 ! " , # $ = % 1 if elements ! " and # are connected 0 otherwise Given an element ! , the set of all connected elements can be formally defined by: # = 6# 7 8 ! , # 7 $9 = 1 Finally, a global set of constraints is defined, in order to ensure that there is no incompatibility between elements. The main difficulty is to impose "safeguard rules" not too restricting for exploring widely the field of potential solutions and not too loose for avoiding "unflyable" chimera. Not all constraints can be listed here, but as example the following rules, among others, should be ensured:

Rule 1: at least four controls must be available for controlling the six degree of freedom of motion in the 3D space. Knowing that a rotor has 3 controls (one collective and two cyclic pitch angles) and a propeller has 1 control pitch angle. Rule 2: if the configuration has only rotors or only propellers, then an even number of contra rotating rotors or propellers is required. Rule 3: two rotors or two propellers cannot be connected together except contra rotating ones:

∀ , 7 , , 7 $ = , 7 $ = , 7 $ = 0
With rules 1 and 2, the following configurations can be obtained covering most of the existing architectures and more (see table 1), even if the number of rotors is limited here (for the sake of brevity) to a maximum of four and the number of propellers to a maximum of twelve. In a final step (still in development), these different codes available within the CREATION workshop are chained together in the same MDO framework (ModelCenter©) according to the following logic.

As illustrated in Fig. 14, this logical chain is composed of four main loops fitting together. The outermost loop is the one on the configurations (or rotorcraft overall architectures), the configuration semi-free (pseudo-random) generator is first called. Then, for each rotorcraft architecture, a pre-sizing loop is performed as the one set up for the helicopter in milestone 2. It includes a convergence loop on the rotorcraft gross weight (as it depends on the sizes of the different components, number of blades, engine weight and fuel weight). The innermost loop is the one required for trimming (for the considered design flight point) the pitch angle controls of the rotors and propellers in order to cancel the six accelerations related to the aircraft "single body" motions (three translations and three rotations). In the case where only four controls are available, the pitch and bank angles are used as the two other degrees of freedom for solving the six equations (as in the helicopter case). The control trimming loop is required when using the AMB model or any flight mechanics model, but not with some simple level 1 models like the ones based on the Power Balance method. Indeed the tool includes other loops, for example on the engine pre-sizing and on the different design flight points or the points of a typical mission profile.

CONCLUSION

This computational workshop is not a pre-project industrial tool, but it is very useful for prospective research and expertise studies allowing to compare different concepts in terms of flight performance and environmental impact. It has already been successfully applied for the ONERA -NASA collaboration, for MoD studies, for Rotary Wings UAVs investigation …

Obviously it still requires developments in terms of models and methods. For example the pseudorandom configuration generator should include other constraints in order to ensure more thoroughly the controllability of the rotorcraft. Today, a collaboration work between different ONERA departments aims at developing the capability to evaluate the environmental impact of rotorcraft.

Fig. 1 :

 1 Fig. 1: 3 dimensional view of the CREATION computational workshop (links are given as examples).

Fig. 4 :

 4 Fig. 4: CREATION level 1 model compared with SA365N flight test data for an advancing speed about 30 m/s and different vertical rates of descent or climb.

Fig. 6 :

 6 Fig. 6: Weight breakdown, ONERA CREATION HO-90 (in red) and NASA NDARC H90 (in blue).helicopters[START_REF] Russell | Conceptual Design of Environmentally Friendly Rotorcraft -A Comparison of NASA and ONERA Approaches[END_REF].

Fig. 8 :

 8 Fig. 8: Modeling of a X 3 -like configuration with the AMB code.

Fig. 9 :

 9 Fig. 9: Principle of the interaction model.

  Figures 10 and 11 give examples of computed induced velocities for different lifting systems made of one or several rotors.

Fig. 10 :

 10 Fig. 10: Applications of interaction model -Single rotor in hover.

Fig. 11 :

 11 Fig. 11: Applications of interaction model -Tandem configuration for different advancing velocities.

Fig. 13 :

 13 Fig. 13: a) Contra rotating lifting rotors and propulsive propellers fully symmetrical configuration, b) Compound rotorcraft with a pair of wings and five propellers on each side.

Fig. 14 :

 14 Fig. 14: Logical chain for RC generation -pre-sizing and evaluation.

  required, sizes, weights, noises, air pollution … (MB: Multi-Body)

Acknowledgements

The authors thank the ONERA Scientific Directorate for the funding in this study and all the colleagues contributing to CREATION. Thanks also to our NASA colleague Carl Russell for our fruitful collaboration.

ROTOR(S) PROPELLER(s) Pitch

An overview of a generated rotorcraft is illustrated in Fig. 12 below as example.

Fig. 12: An example overview of tree structure adopted by the configuration generator.

A complete configuration is obtained through the following steps:

i. Create fuselage: find . ii. Find the number of rotors:

= rand 1, max , where rand ;, < returns a random integer between ; and <. Place them on the fuselage or pairwise on a lateral branch (created at the same time). iii. Find the number of lifting propellers = rand 0, max , place them on a fuselage element or pairwise on a lateral branch. iv. Find the number of propulsive propellers = rand 0, max , place them on an extremal fuselage element ± or pairwise on a lateral branch. v. Find the number of wing elements, = rand 0, max , place them on existing lateral or vertical (e.g. case of fin) branches, or create them if necessary.

Between each steps, the constraints set is verified, and the connections are updated accordingly. The resulting configuration can be written as a data file, and then exported to an aerodynamic performance calculation code like the AeroMultiBody model (AMB).

Two examples of rotorcraft architectures which can be obtained from the configuration generator are presented in Fig. 13 a) and b).