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

[Gordan Sikic]

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 :)

Force on the stick grip may be represented in the following form:

P = K*H

where:
 P is force to be realized,
 K represents characteristics of linkage, that may be presented as 
(dimensional) coefficient, or simply the gain.
 H is hinge moment which is a function of  AD forces acting on a control 
surface.

For elevator, hinge moment might be presented as
H = q*S*c*Ch,

q == dynamic preasure (0.5*rho*V*V)
S == referent area (usually wing area)
c == referent length (usually MAC)
Ch == nondimensional hinge_moment_coefficient,

For elevator,  hinge moment might be expressed  as:

Ch = b1 * alpha_horisontal_tail + b2 * elevator_deflection + b3 * 
trimm_tab_deflection

b1, b2, b3 are mainly functions of control surface geometry, but are 
usually defined as constants.



if everything is  put into one equation, the following results:

P = K * [q*S*c*( b1* alpha_horizontal_tail + b2 * elevator_deflection + 
b3 * trimm_tab_deflection)]


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.



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.

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..

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

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....

After all this is done, there should be reasonably nice force on the 
stick using following algorithm:
while simulation runs
 *gather required data in another program
 *calc (in real time) required stick force, and pass it to the hardware
 *realize the force, using *motors, or pneumatics, or hydro.... anything 
suitable
 

At the end, "simplified" I mentioned at the beginning means that tere 
are no inertial forces acting on the control surfaces, damping effects, 
linkage flexibility..... so on,  but if the calculation program is made 
in software, it is "easy" to add those afterwards.


hope this helped a bit,

ciao,

Gordan