As I’ve said regularly here, in my sordid past I’ve had the pleasure of working with some of the brightest and best thinkers in this industry – why they allowed me to be regular part of their business may be open for a lot of heavy criticism or ridicule, but that is a story/discussion for a later time. Because this LM2 issue seems to be provocative, I’ve talked with a very good friend of mine who happens to be the FAA’s Chief Scientific and Technical Advisor for Flight Simulation Systems (I hope I got that title correct) and the result of my recent conversation with him … and his providing some correction of my thinking and confirming other portions of what I thought, is contained below … and I think many (most?) of you might find the following to be of some interest.
Basically, adding the “LM2” algorithm into a simulator does several things, and probably the primary aspect is that the lateral motion cue (side-to-side movement) is considered primary. With understanding this position, the LM2 forces the lateral movement cue provided by the aero-model (determined from airplane flight testing) to be “in phase” with that model. Note, it is the “phase” of the movement that is being kept the same, not the “magnitude.” The method used to achieve this is to combine the lateral motion (left and right) of the simulator WITH the rolling motion capability of the simulator. When these motions are forced to be “in phase” the resulting lateral motion “feel” in the simulator is quite often better than it would have been previously. However, the rolling motion that is used is NOT the rolling motion of the airplane, it is the rolling motion of the simulator.
It is these 2 motions that are essentially forced to be "in phase." At the same time, this additional LM2 programming also adjusts the rolling movement when the pilot inputs a control wheel movement (either left or right) so that a spurious, or false, side-force in the lateral (side-to-side) direction is not generated – which, by itself, is probably a good thing. 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 – and, some would say, critically decreasing that fidelity, meaning that the potential for experiencing a false roll cue has been increased.
Of course, there are some who would say that there is a darn good reason for having “good lateral cueing.” Certainly, I don’t argue with that premise. However, providing this “good lateral cueing” is accompanied with both, the lessening of the quality of roll cueing and establishing a potential for introducing a false roll cue.
It might be interesting to know that according to the data gathered by a myriad of research involving flight simulation there is an understanding that it isn’t absolutely necessary for the lateral cueing of the airplane and the lateral cueing of the simulator to be exactly at the identical phase (again, we’ve never attempted to match magnitudes - the size and actual movement of the simulator would be prohibitive). This research, I’m told, indicates several things:
1) These two motion values can be as much as 30 degrees out of phase and still be providing a good cue;
2) There is nothing wrong with having some side-force due to roll. In fact, the indications are that up to 0.05g's is acceptable before that lateral movement begins to infringe on the rolling cue recognition by the simulator occupants - primarily the pilots;
3) At the same time, rolling the simulator to produce a good lateral (side-to-side) cue such as is provided by this LM2 algorithm can make the occupants “feel” that the cab is rolling in addition to moving laterally, which would be at least confusing. (Note: a typical person will feel the cab rolling when it is rolling at 2 degrees/second or more); and
4) “Over filtering” the roll cue can, and usually does, provide poor roll cueing, and the method the LM2 algorithm uses to increase lateral fidelity, actually reduces the roll fidelity.
There shouldn’t have to be an “either/or” decision that has to be made. The fact is that “the best of both worlds” can be achieved by using the algorithms typically installed in simulators today by improving the lateral cueing at the expense of the roll cueing … but that does not mean that one should have to go as far as what LM2 does.
I understand, and it has now been confirmed, that some time ago there was a petition filed with the US FAA to add a requirement to include the LM2 approach to the US Simulator Qualification Regulations. This was carefully reviewed and the conclusion was that as long as adding LM2 into the programming of a modern simulator did not negatively affect the required objective testing results and did not negatively affect the required subjective test results, the FAA would not object to its being included as a part of the simulator’s programming … but the FAA also decided that they would not require that LM2 be incorporated into any simulator.
Finally, and according to all the available data and professional and objective analysis, what LM2 provides, is probably somewhat better than where most folks are today, but logically, the question should be one of finding the “happy medium” and not necessarily compromising one cue for providing an increase for another cue. After all, we are attempting to make the simulator as “real” as possible … right?