Originally Posted by rudderrudderrat
Thanks for your explanation for the reluctant introduction of LM2.
You say “Note: a typical person will feel the cab rolling when it is rolling at 2 degrees/second or more.”
Similarly, I would have thought a typical pilot could also sense the cab pitching at 2 degs/second or more. During take off, the cab rotates backwards quite steeply and quickly to simulate the acceleration on the runway. (The converse is true during braking.) With strong visual clues, we pilots interpret that as an acceleration and not as a rotation.
Actually, the acceleration and rotation for takeoff cueing typically works as follows (and there are some alterations, substitutions, and variances from manufacturer to manufacturer and operator to operator): the initial acceleration of the airplane is replicated closely by the platform moving forward and the movement is at such a rate that it is recognized by the on-board crew. However, the platform jacks cannot go on moving without reaching their “limit stops” and a technique is used that prevents the stops from being reached without being discernible to the crew. This technique is to gradually decrease the displacement rate (known as a “washout”) where that rate of change is below the sensory threshold of the simulator occupants (typically called the “recognition threshold”), eventually reaching the zero, and begins “resetting” the simulator to the “neutral” position fore/aft (again, below the recognition threshold rate).
Sometimes, a simulator may be adjusted in "pitch" - again, below that threshold rate, to provide a cue that the simulator is continuing to accelerate. Depending on the rotation angle for the takeoff/climb, the
rotational angle of the simulator cab, may or may not be gradually lowered. If the cab remains tilted up, it will provide a sensation of continuing to accelerate, but if the attitude is not significant, the acceleration rate will not be “felt” as significant either, although it will be present and very likely noticable. At the point where the simulated airplane is to be rotated to achieve a climb angle, the simulator cab is rotated, this time at a rate that is noticed by the on-board crew, and the rate of rotation is gradually reduced, again below the threshold rate. At some logical point during the first portion of the climb, the rotation angle is eventually reduced all the way to zero – removing all acceleration and rotation cues.
Originally Posted by rudderrudderrat
Similarly prolonged sideways accelerations can be simulated with cab roll and provided we have strong visual clues, we would interpret the roll as a sideways acceleration.
Unless you are describing something unusual, I think you might want to re-examine that conclusion. In an airplane, a sideways displacement – either to the right or to the left, is normally not a long-lived experience. Flat, long term, level movements are not normally experienced, and when they are, they are almost always short lived, brief duration excursions, usually caused by disrupted airflows, and are typically recognized in the airplane as little more than “turbulence,” and turbulence models are typically available in a simulator.
If you’re describing a level turn, either right or left, in the airplane, the occupants would physically recognize the left or right bank, and from that point forward the prominent force recognized is more of an increase in “g-forces” to maintain a constant altitude and is the reason your coffee doesn’t spill in your lap. To have the simulator accurately replicate an airplane level turn, either left or right, you would want the simulator occupants to recognize the on-set cue of the initiation of the airplane bank, but as the desired bank angle is approached, the bank angle should be removed, and return the simulator cab back to a level attitude ... all accomplished below the recognition threshold ... at the same time an increase in simulator nose-up attitude is also achieved ... also below that same threshold. By doing this the force of gravity would provide a recognition by the simulator occupants as an increased “g-load,” as would be recognized in an airplane under the same circumstances.
Rolling out of that turn would essentially be achieved in a simulator by reversing the sequence of the adjustments used to achieve the simulation of the turn in the first place; i.e., reduce the pitch attitude of the simulator (again, below threshold), provide a simulator cab roll in the opposite direction (simulating a roll-out of the bank angle), and again after initiation of the on-set cue, remove that input, again below recognition.
Originally Posted by rudderrudderrat
One of your reasons for not accepting LM2 is “However, this is achieved by limiting the roll capability of the simulator, resulting in a situation where the simulator cab cannot roll much to represent the aircraft's roll, effectively decreasing roll fidelity”
Please understand, it’s not that I don’t “accept” LM2 – it’s just that the technical folks with whom I’ve worked regularly, tell me that today's existing algorithms
can provide a better mix of roll and lateral cueing than LM2 can provide … and doing this would achieve the same, or better, lateral cueing as provided by LM2, and do so without compromising the rolling cues that are present all the time with existing programming; where by using LM2, the roll rate cueing is likely to be compromised in all situations where a roll is accomplished. However, the fact is that one has to know what they are doing to achieve that “better” mix, and most people do not have the experience in doing that. It is for these reasons that the FAA allowed LM2 to be used, but refused to mandate its use.
Originally Posted by rudderrudderrat
I was not aware that the cab rolled at all during a balanced banked turn. Please could you expand some more.
Each time the flight crew initiates a bank angle, the simulator is actually rotated (banked) in the direction of the airplane’s bank (and very likely also tilted up, but at a rate below threshold) to indicate a slight increase in “g-force,” with the simulator cab rate of roll and bank angle being washed out, again, below the recognition threshold, and returned to neutral. When the simulated airplane is rolled to a wings level attitude, the simulator cab is rotated in the opposite direction, to provide that rolling on-set cue, and then both the roll rate and roll angle is washed out and the position is reset to neutral – both, again, below the recognition threshold … as I indicated above.