[simpits-tech] May the Force Feedback be with you...

[Matt Bailey]

On Monday 10 February 2003 13:00, you wrote:
> Hi Matt,
>
> It's me again.
>
> >If you *really* want to do it right, I'd think you'd need a force and
> > moment from the control surface. But I don't know of any flight sim that
> > actually calculates this. You might be able to arrive at a reasonable
> > guestimate with something like X-Plane that handles each airfoil
> > seperately, and divides an airfoil into up to 10 sections. There are at
> > least two ways you might go about this: by using airfoil speed, AoA,
> > control surface span/chord/travel, you could perhaps calculate control
> > surface pressure reasonably.
>
> If you're willing to do "simplified" design, it is not so hard :)
>
> Let me explain briefly how to calculate forces on the stick:
>
> (some math follows :)
<snip>

	I dunno exactly what math is required although from the math you are doing 
it looks like you're thinking the same method I was, but you know more of the 
math required. :) This was actually my second choice, however......I was 
thinking a much simpler way would be to use airfoil lift. Perhaps not as 
accurate as the previous method but much easier to handle for a non-math dude 
like me. :)

> Knowing all this, one needs AoA of horizontal tail, elevator
> deflection,  trim_tab_deflection, dynamic preassure, S and c, in order
> to calculate hinge moment. When the moment is known, calculating force
> is not a problem.

	By dynamic pressure, are you referring to the pressure caused by the plabe 
moving through the air, or the actual pressure on a control surface?

>
>
> possible problems:
>
> Be awared of the dimensions (sqft, or m^2), reference areas, and
> lengths... do check the program manual for dimensions of the aoutput data.
>
> One of the possible problems is that b1, b2, b3 values are needed, and
> those are specific for every aircraft. For the first approximation,
> those values might be taken form some textbook, given the geometry data.

	FWIW, X-Plane does not model trim tabs. I've heard that Fly has a really 
good trim system but I don't know if it models the physical trim tab.

> Another problem is AoA of horizontal tail, which includes downwash
> gradient (epsilon_alpha), rotational speeds, so.... If one is willing to
> sacrifise more, AoA of the wing might be used (but don't tell anyone I
> suggest this :), otherwise, if anyboy is interested in more elaborate
> solution, I'll be happy to help..

	I believe X-Plane already considers downwash of the wing on the horizontal 
stabilizer, although I don't know how well (it's not doing fluid dynamics, I 
know that).

> Yet another problem is gathering required values of AoA, deflections,
> areas, preassure.... I have checked docs of X-plane, and it is capable
> of exporting all required data.  All calculations might be done using
> hardware, but I preffer another program which might run on same computer
> (or another as well), which will gather data, and calculate required
> force on the stick

	You can spit the data out locally and send it to a third-party program. Or 
you could wait for the plug-in SDK, it supposedly will allow a lot of this 
type of stuff to be extracted.

> Yet yet another problem is finding suitable K. Other than analysing
> complete linkage system of the aircraft (which invoves quite a bit of
> mathematics), try_and_error method might be applied, with following:
> 1 choose K
> 2 compare forces (subjectivelly in this case) to the forces in the real
> AC in the same flight conditions
> 3 if forces == OK, finish
> 4 correct K
> 5 goto 2
>
>
> Pls take a note that there is no hardware connection of the trim wheel
> to the assembly producing the force on the stick. It is done using
> output of the program only.
> and the rest is done on hardware....

	This is good as long as your sim supports it. The main difference I can 
think of between having an actual simulated trim tab compared to just moving 
the force feedback assembly is that changes in trim would affect how much 
control authority you have........not by much, but theoretically it should 
change. For instance, if you have full nose-down trim (trim tab deflected 
upwards), you will have less pitch-down control possible than with the tab 
neutral, and more pitch-up control. But I would think the difference would be 
pretty small. But it would be nice to have it done right.

	-Matt Bailey