How does an aircraft move?
To understand how a flight simulator motion system works we need to first consider how an aircraft moves.
Aircraft move through the air in 6 degrees of freedom: pitch, (tilting up and down) roll (rolling left and right), and yaw (rotating left and right) they also move through the air forwards and (very rarely) backwards, and can be buffeted left and right by turbulence, and of course they move up and down in the sky.
A six degree of freedom (6DOF) motion base moves, as the name suggests, in six degrees of freedom, three rotational movements, pitch, (tilting up and down) roll (rolling left and right), and yaw (rotating left and right) and three linear movements: heave (up and down) surge (fore and aft) and sway (left and right) here's a demonstration video.
Obviously a real airplane moves a great deal more than any full motion simulator can, yet the simulator motion system can still create a very convincing illusion of continuous movement in all six degrees of freedom, how does it achieve this?
We sense changes in acceleration and don’t perceive continuous movement, that's why we can walk around in an airplane even though we're flying along at five hundred miles an hour,- we don't feel the forward motion through the air if we’re at a steady speed, but if the plane speeds up or slows down we will feel that change, because we are experiencing acceleration or deceleration.
A correctly programmed motion base actually simulates changes in acceleration along the 6 rotational and linear axis, and does not tilt following aircraft body angles as you might expect. This is very important for realistic motion cueing, if the motion base simply follows the pitch, bank and yaw of the aircraft it will feel wrong.
For example, when an aircraft tilts sideways to turn (banks) there are other acceleration effects that cancel out the sensation of banking, this does not mean that you feel nothing when an aircraft banks, you feel the onset of the bank, but as the aircraft starts to turn as a result of being banked in the air, a centrifugal force starts to act on the pilot balancing out the effect of gravity in the banked aircraft, similar to riding a bicycle around a corner, you don't feel the banked angle of your bike as you cycle around a corner because the centrifugal force of the turn is acting against the pull of gravity on your banked bicycle.
But how does a motion platform simulate a sustained acceleration, when it can only move a small distance?
Let's look at takeoff acceleration as an example. When you release the brakes in the sim with the engines at full power you experience a forward movement of the sim (surge) but the motion base can’t keep surging as the aircraft keeps accelerating, the motion base actuators quickly reach their end stops, so instead, while surging forwards, a full motion simulator also sneakily tilts back causing you to be pushed into your seat, although the sim is now tilted up, the visual out the window shows you still level and still buzzing along the runway, this creates a very strong sense of continued acceleration.
This illusion has a name, it’s called the Somatogravic illusion and it’s been the cause of some real world plane crashes, it’s defined as: “the tendency – in the absence of visual references – to incorrectly perceive acceleration as a change in pitch attitude, a perception that can lead pilots to instinctively make pitch inputs even if the aircraft is flying level.” Pilots are taught to be aware of this illusion in the Human Factors element of flight training.
A full motion flight simulator takes advantage of the Somatogravic illusion to generate sustained and very convincing acceleration cues on all linear axes.
How do we simulate sustained acceleration?
Some flight simulator experience providers offer sims with motion that is instructor controlled which means that the motion in the simulator is controlled manually by the instructor using switches. In these simulators the motion base is completely ignorant of the aircraft movement and is being told to move when a switch or dial is operated by the instructor. This offers only the most basic movement, and is not responsive to control inputs, turbulence, or anything else that the aircraft is actually doing.
The key difference between motion sims like the one described above, and a true full motion flight simulator like the Jetex simulator lies in the use of motion base software which translates the aircraft movement into motion base movement in real time.
At Jetex we’ve written our own software to interface with Moog’s motion base host software. We are constantly tuning it with input from our 737NG pilot partners. We’ve measured motion accelerations in a real 737 and have tuned our motion cueing model (and lots of other things) to match, which means our 737 simulator gets extremely close to the feeling of the real thing.
In addition to the large simulator movements that our motion base can do, we've also added high frequency/low amplitude (fast and small) effects to the motion software. These generate things like runway rumble/bumps, and other vibration effects, for example when the landing gear is lowered or the speed brake is extended, you will feel a noticeable vibration through the airframe.
The motion base has a movement frequency limit of about 20HZ meaning the actuators can change direction about twenty times a second. While this is very fast, considering the motion base is shifting a payload greater than three tons, it isn’t fast enough for other very high frequency vibrations felt in an aircraft, like vibration coming from the engines. These effects are provided by a sonic transducer, controlled by separate software that listens in to other simulator parameters and generates an appropriate dynamic vibration response.
How does the motion base know what the plane is doing?
Moog Motion Base
Our full motion flight simulator, replicates the feeling of aircraft movement very accurately using a Moog motion platform. Moog is the aviation industry leader in professional Level D motion systems.
Our motion system consists of six electric actuators and three pneumatic balance rams. Giving a total payload capacity in excess of three tons. At full payload the motion base can accelerate at +/- one G meaning that it can make you weightless for a split second.
Real motion systems work in quite unexpected ways, and installing and commissioning ours has been quite fascinating, if you’re interested in what we've learned read on.