Motion ?

Regulatory bodies such as the Federal Aviation Administration (FAA) allow Level D Full Flight Simulators (FFS) to be used for pilot aircraft type conversion and periodic pilot recertification with a zero flight time requirement. Level D simulators are required to provide motion feedback to the crew, which is done through a motion platform on which the simulator cabin is mounted.

The motion platform must produce accelerations in all of the six degrees of freedom (6-DoF) that can be experienced by a body that is free to move in space. To maintain the proper view Out The Window (OTW), it follows that the display system must also be mounted on the motion platform, i.e., the motion platform is going to be moving large structures around with considerable force.

Until recently, the motion platforms used on these commercial simulators were driven by hydraulic actuators. Wikipedia has a neat little animated GIF that shown a Stewart platform being driven with 6 -DoF by 6 hydraulic actuators like the motion platform of a FFS might be.

It should be noted that the motion system does not actually mimic the airplanes attitude in a given scenario, rather, it is the acceleration forces on the body that are simulated. The motion system is designed to deliver an effect called acceleration onset cueing, the effect is more comprehensively described in the Wikipedia article, but in brief, the effect is achieved in three phases :

• The initial acceleration of the cockpit is reproduced by rapid movement of the platform which is detected by the occupant's inner ear balance mechanism
• Because the jacks have finite travel limits, the platform can only move over a short distance, so, after the initial acceleration force has been simulated, the platform motion is slowed at a rate low enough such that it is imperceptible to the occupant, before coming to a stop.
• The platform is then slowly reset to its neutral position, ready for the next motion event.

Operating in tandem with the visual system, the motion system is designed to give the impression of the movements and forces that the pilots would experience when performing the equivalent manoeuvre in a real aircraft. Although the absolute distances moved are relatively small, the motion platform needs to perform fast movement over sufficient distance to fool the body into thinking that is it moving.

The Canadian company CAE is a leading manufacturer of commercial full-flight simulators, their website has some great photos of their simulators that illustrates the size of these things - they are NOT small and neither are the buildings that they put them in! The actuators only move over relatively short distances, say 1-2m before imperceptibly moving back to the neutral point, but even so, considerable space is needed around the simulator cabin to achieve this.

Again, courtesy of Wikipedia, here's an example of a FFS used by Lufthansa that illustrates the space needed to install and operate one.

In addition to the motion features, the ability of FFS to generate realistic simulation of turbulence and other aircraft vibrations should not be forgotten either.

Do I want one? - Of course I do, but it's not realistic aspiration.

Full motion simulation is starting to come within the reach of the enthusiastic amateur though, there are now 6-DoF platforms driven by electric motors that are smaller and more affordable, for example, the system made by CKAS Mechatronics in Australia. They manufacture a range of motion platforms, including the 6-DoF "W" series shown below

This product has small stroke actuators with linear displacements in each axis of 150mm, but in the parked position, the platform still sits up to 1m from the floor. You'd then have to sit the cockpit and visual system on top, so the space requirements are still significant, as is the cost. A motion system of this type would still cost in the region of $50,000.

In my case, there would also be access issues, commercial simulators often have movable access platforms or gantries that can be retracted once the pilots have entered the cockpit, again, meaning more space is needed for the setup.

As well as the cost and space requirement of the hardware, there is also the question of software. If the motion sensation is at odds with the visuals, then this is likely to be a worse case than not having motion at all. I don't know the details, but I imagine that tuning a FFS to get the motion and visuals "in-sync" is not a trivial exercise and therefore is likely to incur significant cost in itself.

Another issue that comes to mind is the impact of movement on projector stability. As I noted earlier, the display or projectors/screen would also need to be mounted on the motion platform. For optimum results from a projected system, alignment of the projectors is critical and likely to be compromised by sudden and frequent movement of the supporting structure.

So, for a whole host of reasons, I won't be going for a full-motion simulator and, like the vast majority of home users, will be going for a "fixed base" simulator. Similar devices are used for training in the airline industry and are used to deliver cost-effective training in scenarios where FFD devices are not required, e.g., normal and abnormal procedures, ground operations and in-flight training, as well as general cockpit layout familiarisation and operation. They are often referred to as Flight Training Devices (FTD) or Fixed Base Procedural Trainers (FBPT).

Display Technology

Right, now we've sorted out what aircraft we want to simulate, it's time to get down to the nitty gritty!

Once we've decided on the aircraft type, the cockpit footprint is likely going to be similar for any of the fixed base simulator manufacturers and the required room size is therefore going to be governed by the choice of display system used. Ideally, the display system chosen should completely fill the Out The Window (OTW) view to provide a fully immersive experience for the pilot(s).

The simplest display option is to use a number of monitors or TVs to display portions of the OTW view. One limitation of using this type of display is that if the OTW image was projected in front of the pilots on a screen and set up for one pilot's view, the other pilot would observe objects that appeared to be at incorrect angles, that is, parallax error would occur. Professional flight simulators use collimated displays to ensure that the OTW view seen by two pilots sitting side by side is the same, with no discrepancy between the views.

To quote Wikipedia, "To avoid [parallax error], collimated optics are used in the simulator visual display system so that the OTW scene is seen by both pilots at a distant focus rather than at the focal distance of a projection screen. This is achieved through an optical system that allows the imagery to be seen by the pilots in a mirror that has a vertical curvature, the curvature enabling the image to be seen at a distant focus by both pilots, who then see essentially the same OTW scene without any distortions."

The principle is illustrated in this image from Glass Mountain Optics (GMO), a manufacturer of collimated displays systems. GMO are now part of simulator manufacturer Flight Safety International

These collimated displays allow, for example, both pilots to see the same view of the runway when landing - an obvious advantage! However, as well as being large, collimated displays are very expensive, making them impractical for the hobbyist or even entertainment level simulators.

Sitting somewhere between a bank of discrete monitors and a collimated display is the option to use one or more image projectors. To get the best field of view available, a bank of three projectors is typically used, with some clever blending required to seamlessly join the images and project them onto a horizontally curved screen which ideally encompasses all of the cockpit windows. (Non-collimated projected screens cannot overcome the parallax issues though.)

When I first used Microsoft Flight Simulator, I used a varying number of PC monitors , but the most I ever managed to fit on my desk was 3. Although, with suitable hardware, it is possible to add more and more monitors to a flight simulator installation, that will never give the same quality of display as a projected screen. It would require a very large number of monitors to achieve that goal and in addition, the monitor edges would compromise the "believability" of the image.


One company that makes projection systems (Natvis in Australia) has a really good section on the pros and cons of monitors vs projected screens. The image below illustrates the viewing area available with 3 x 65" TVs compared with that of one of their 200+ FoV projected systems.

Another image from the Natvis website provides an even more powerful justification for this type of display system when compared to traditional monitors.

Obviously, this site is designed to highlight the advantages of the Natvis solution but I think that it provides a good comparison between the two techniques that would apply equally well to other projection system vendors.

In addition to the cost of the projectors and the display screen, setup and blending of the images requires additional software and the skills of an expert to take get the best results from the image system. Natvis for example use proprietary TruView Warp software and most users chose to have Nat set up the system for them.

I was on holiday in Australia in December 2018 and Nat Crea of Natvis arranged for me to visit one of his customers (Melbourne 737 Project) to see one of his display systems at first hand. Thanks to Nat and Nathan Mizzi I was able to see how well this system works. Nathan's 737 simulator is fantastic - he has spent a lot of time and effort building a faithful reproduction of a fully enclosed Boeing 737 cockpit and the 220 degree FoV display is just stunning. Nathan was kind enough to let me have a go "flying" his sim while I was there which was much appreciated and obviously great fun!

During my visit with Nat and subsequent email conversations we discussed the space required to install such a system in a domestic setting. As the Natvis website advises, "Every NatVIS Installation is unique and tailored to the customers needs. There is no “one size fits all” answer for Screen size. A screen diameter and FOV is dependent on room size and ceiling height, simulator size, shadows, projector placement, throw ratios etc."

Typically though, a 737 with 200 FoV projected screen would need a room width of no less than 4m with a minimum ceiling height of 2.8m. This was going to be a challenge for me in the UK where domestic ceiling heights are typically 2.4m. (Details of how we resolved that issue will be described in a later blog entry.)

Despite the additional cost and space required for a projected screen system, the stunning visuals that are possible means that this is the option that I have chosen for my setup.


For some interesting background reading, you might like to review this article titled Wide-View Visual Systems for Flight Simulation.