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Thread: Trim, forces and control devices in FS - explained

  1. Default Trim, forces and control devices in FS - explained

    [font size="1" color="#FF0000"]LAST EDITED ON Jan-02-02 AT 04:18AM (EDT)[/font][p]I've been following the Capt's interesting thread about solving the control force problem in FS. Whilst the Capt and I have not seen eye to eye in the past, I do warmly welcome this discussion as I think it may bring together many people who otherwise did not agree on some fundamental issues about Fs2002.

    At the moment there are two main alternatives when designing a flight model in FS. One is to assign the aircraft absolutely full control potential, thereby allowing theoretically extreme control inputs which in normal circumstances might break off the wing of a real aircraft if attempted at high airspeed.

    The other is to compromise on the above by allowing sufficient control to attain normal, and sometimes marginally extreme manouevres outside the "normal" envelope. Without decent force feedback this in my view currently provides for more acceptable and smoother control in most airliner types going about their "normal" flights, but of course this is not ideal.

    The other problem is trim. Trim on a real aircraft only has one principle function: to relieve stress to the arms of the pilot, by means of a trim tab which nullifies control forces and keeps the control column in the desired position for a given flight envelope.

    FS, and many other sims, are unable to truly emulate the trim tab and instead the trim routines in effect change the Centre of lift, and weight, forward and back according to settings. In effect, trim in most home sims becomes another, extra, powerful elevator.

    The problems this causes are obvious. Trim becomes a way of actually directing pitch, rather than relieving arm pressure. There are other unwanted side-effects, the most serious of which is that aircraft tend to become very twitchy if given generous elevator control authority AND a full trim range.

    The force-feedback mechanisms provided by some manufacturers of control devices have not yet really solved the problem, since their implimentation is crude and rather un-refined.

    If FS sim aircraft are to be given absolute and real-world control surface potential, then we need two vital things:

    1. A joystick which has sufficient total movement to allow the user to control with finesse and subtelty.

    2. Force feedback which progressively introduces resistance, commensurate with actual forces building as airspeed and other G related forces increase.

    The other thing that needs to be fined tuned in flight models, (and many of us involved in this are increasingly finding ways to emulate this), is the vertical inertia of an aircraft when for instance a dive recovery is executed. When an aircraft is pointing down (and here I do not include high performance aircraft with very high G tolerance) and increasing in airspeed, no amount of elevator is going to suddenly get that aircraft to travel upwards. It might succeed in pointing the nose up but the vertical inertia of the aircraft needs *time* to be overcome. In this cases extreme stick movements merely add enormously to the already increasing G force and this is where wings would be in danger of snapping off.

    A well-designed piece of force feedback software, custom made for each type of aircraft, in combination with a joystick which had accurate control and correct forces, would go a long way to discourage the PC pilot from heavy handed control and if that was the case I would completely redesign my flight models to fit.

    So in essence I am all for what Joe is proposing to achieve, and at the same time I'm posting this in a spirit of new year goodwill!


    Rob Young

  2. #2
    Captain_Slarty Guest

    Default RE: Trim, forces and control devices in FS - explained

    Many thanks Rob.. n A Happy one to you n yours.

    Excellent post, and I agree fully.

    There is a problem at the moment with the 'throw' range of the current sticks and yokes, even the much touted (I actually own one :-)) AETI has a pitch travel of about 4 inches - thats all, as opposed to perhaps 12 to 18 in some light aircraft and sometimes more in heavies.

    The only thing that can be done at the mo for the mechanism is to control the forces, centre and 'feel' of the stick. I have actually tested inducing a slight high pitch sinewave (available in ff spec) oscillation into the stick handle that increases with innacurate placing - a sort of 'warning' to the pilot.

    I used to like FU2 with its 'creaking' noises as loads increased .. whislt not necessarily realistic, it is a good thing (imho) due to the boundries of the simulated world.

    If the FF routine works out ok, and from first tests it seems to, then we need to look at perhaps redesigning yokes and sticks with a force mechanism - not necessarily proprietry FF - that are just for fs fans.

    because we dont want to simulate shakes and rattles, explosions etc, a far simpler and more responsive mechanism and electronics can be used.

    I personally think this is the next major step in flight simulation. Airfile guys can build a superbly accuarate numbers and inertia file within the constraints of the sim and a separate feel file added for the type of controller in use.

    I am currently working on some outline designs for controllers that would be affordable to the masses too, I could do with input from a production engineer as to cost estimates and engineering specs.


  3. #3
    Captain_Slarty Guest

    Default re the vertical inertia.

    Just pondered that one Rob..

    If you saw my post re the flybywire envelope for an airbus, you will see that it SHOULD be possible to control stick input even if the user pushes it all the way. Theoretically, the vertical inertia you talk of could be accomplished in the feel model, ie - the throw is limited for the duration of the desired action.

    this does not get over the lack of throw or forces with current sticks / yokes, but certainly offers an option.

    with a long throw stick yoke, it should be fully possible to load the force to a very high factor and also limit envelope if the deficiencies exist within fs's model. Interesting...



  4. #4
    Pace Guest

    Default RE: re the vertical inertia.


    Im glad we are all blowing "kisses" around at the moment :-)

    I too feel that a "force Simulator" (MicroSoft Force Simulator or MSFS:-)will be a major advancement in flight simulations especially if it is widely available to the ordinary desktop flight sim guy.

    Force feedback associates too much with those vibrator things which bounce around in your hands so Force Simulator is maybe a better name.

    Yes we do need to have a simple trial aircraft and build a set of graphs to work from....
    Just thinking that one out........

    Flower power(whoops! force power and kisses to all :-)


  5. #5
    Captain_Slarty Guest

    Default RE: re the vertical inertia.

  6. Default RE: re the vertical inertia.

    [font size="1" color="#FF0000"]LAST EDITED ON Jan-02-02 AT 06:38AM (EDT)[/font][p]OK, with you Slarty.

    What we need, time allowing, is an aircraft, probably not a jet but a prop twin, to serve as a test bed. The tuning can be done to accomodate the proposed FFB.

    Might be a good idea to have two flight models, one accomodating the Forces and the other not, in order to compare.



  7. #7
    TheFlyingFinn Guest

    Default RE: re the vertical inertia.

    I know I'm really talking things I don't understand, but this trim thing. With the FF2 joystick it is possible to move the center the joystick wants to return to. I don't think FS2002 works that way but with a external program, can't you change the "center" of the joystick, somewhat the way real trim works. I apologize if this is what you've been talking about or impossible or a really bad idea... :-hmmm

  8. #8
    Captain_Slarty Guest

    Default RE: re the vertical inertia.

    Just spoke to Peter.
    He is gonna get loads of readings from the Seneca.

    that should form a great start - we can ignore all 2 engine stuff for the time being, just the force table and reactions.

    yep, sounds good.

    will test it first on a standard ff stick, but as an aside..

    I have just ordered some motor/gearbox assemblies for an FF Yoke. I have a simple design that should work well, also, I have a couple of genuine Piper yokes I can use one for the test rig.


  9. #9
    Captain_Slarty Guest

    Default RE: re the vertical inertia.

    yes it is, I do believe that fs attempts this anyway, ?? it is just not very good at it - ie, there should be resistance to return - even though it is returning to a position, it should not just fly back, it needs to be controlled in some cases as loads are removed from the flying surfaces and applied in an opposite direction.

  10. #10

    Default RE: re the vertical inertia.

    a few comments:

    - In order for a joystick to emulate the real thing it is not very important that the physical movement would be the same. Accurate forces, smoothness and accuracy are much more important.

    - I don't see the importance of taking into account the inertia of an AC as part of joystick emulation considerations. It is really a matter of what the a/c model will do as a result of a stick input. (BTW it is emulated pretty good in the finest AIR files)

    - Trim forces - again, if a simmer uses the trim button as a pitch command rather than stress relief, it is only an indication of flying habits rather than a criteria for a good/bad joystick. I really think that emulating the real forces on the stick is the biggest challenge.

    I addition, it would be very useful if those AIR file experts would deal with one subject that has left behind for a long time - drag due to asymmetric flight (slip)

    To make the initiative above more of a good engineering practice, I would like to recommend that you first agree on a set of requirements you're going to deal with.

    All the best


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