FMS .par file questions - Started Discussion at Ezone

[ggunners]

I've been trying to gain and share additional information on tweaking the .par file for my models. The discussion I started on Ezone (rcgroups) is located .

In an effort to not duplicate this thread on Ezone, I've linked to it here. If everyone would prefer I generate the same post on the FMS Forum, please let me know.

-- ggunners

[lego]

ggunners,

just be patient for a while. The next version of FMS does have a new par-file with real aerodynamic parameters... It will be much more easier to understand.

With the actual par-file you cannot create models that behave realistic, because the fligtmodel parameters are too rough to describe a plane's behavior.

The lift gradients are for the behavior at larger angle of attack. E.g. if you enter a minimum gradient of 0 you cant fly inverted because there will be no lift upwards (if the bottom side is upwards).
So if you have a thick wing profile you have a big max.gradient and a low min.gradient.
If you hava thin profile both values a smaller but more equal.
If you have a symmetrically profile both values are the same except the algebraic sign (+/-).

Then of course the position of the center of gravity is important together with the main wing's angle (of attack).
The angle of the stabilizers are always 0 in the actual FMS version (that will be changed in future).

If you want to "feel" the plane's weight you should use larger values for the inertia moments. These values influesnce how quickly e.g. a roll stops when you return the stick into neutral position.

The dihedral value is just the V-angle of the wing. It influences how the plane stabilises itself about the flightaxis.

The tail moment arm is very important because it is the distance between the main wing and the elevators/rudders.
It has a big influence on how the planes reacts in relationship to the elevator/rudder-angles.
With a small tail moment arm you only need small angles for elevators/rudders.

[ggunners]

Thanks for the update lego, I'm just trying to work with the tool I have now. FMS is very cool, just a little short of documented understanding on the .par file and it's taking a bit of trial and error to adjust flight characteristics to be as close to real as possible.

-- ggunners

[flyingelephant]

Hi lego and ggunners,
thanks for that really useful information.
I did notice that the posts are old but I didn't find a better topic to put my question into.
I was wondering about what sort of approach you guys take when modelling a new airplane.
I imagine you start by getting the geometrical data (wing area, length and so on...) and then move on to lift and drag coefficients, changing parameters iteratively, so as to make the simulation behave like the real model. But: how do you estimate lift and drag? How do I find "reasonable" values to start that iteration? There are quite a few degrees of freedom to take into account when you're trying to model the behaviour.

I'd find it excellent if we could discuss these matters here, as you seem to have a lot of experience in modelling for FMS.

[ggunners]

I start with the closest known equivalent .par file. Just borrow someone else's.

At a minimum, you then adjust the collision and wheel points to be actual.

If that's all you want, you're done!

How about some questions and answers.

Q: The airplane doesn't climb fast enough, what do I change?

A: Wow, that's a tough one. You could change the angle of incidence for the horizontal stabilizer or the main wings. Usually the stab is at a slight negative number and the wings at a slightly positive number. Increase the wing number and decrease the stab number for an increased angle of attack. You can also add more motor power. The easiest hop up is to increase battery cell count. Also, reduce the weight, that'll help performance on all models. Of course the goal is to try to approach reality, so keep it real.

-- ggunners

[flyingelephant]

[sneezy]

Guy's

I'd like to tweak one of my favorite models rudder strength parameter. I don't want to make new models or fiddle anything else.
Is there a single parameter for a 2.8 par that will control the amount of rudder available ? I want less rudder by about 50%.

thanks, if you help me with my quest.

The model is the Seduction glow model found on RC-SIM.de which flies great apart from an unfeasable amount of rudder power.


Martin

[ggunners]

So far I've been unable to accurately model an extremely light airplane with FMS. I've tried some 14 gram airplanes and found the control authority seemed insignificant. So let us know if you get an accurate light model flying in FMS and tell us your secrets. <smile>

To reduce the rudder throw you need to adjust the throw in degrees of the angle for the rudder wing (flachen). Which one is the rudder? Good question. If it's labeled, you'll know. If not, check for an entry that looks something like this.

//Wing4 <FIN>
-0.36 0 -0.05 //—ƒˆÊ’uX,Y,Z [m]
1 0 0 //Ex X,Y,Z
0 0 -1 //Ey X,Y,Z
2 //polar number
0.18 //span [m]
4 //number of elements
AUTOELEMENT
-1 //AttackAngle [deg]
0.16 //chord length at the first end [m]
0.16 //chord length at the second end [m]
0.08 //chord length of the flap [m]
0 //start position of the flap [m]
0.18 //end position of the flap [m]
-30 //Maximum Angle [deg]
3 //channel

Change the maximum angle to -15 and you should have roughly half the rudder authority. As an alternative, you can reduce the chord length of the flap to half. In this case, flap chord from 0.08m to 0.04m.

All the best!

-- ggunners

[sneezy]

Hi, thanks that bit of info helped out. I also deduced some other stuff like changing elevator throws as well. Great stuff.

However one thing still escapes me completely in the 2.8 par.
Motor thrust. I want to tame down the Seduction, but changing the engine capasity does not seem to do it.
Altering the prop diameter did a bit. Is that the 'real' way to drop the thrust ?

cheers,
martin

[ggunners]

[ggunners]

Well after a bit of playing around, we found that the motor size does not control the thrust for the COMBUSTION MOTOR in FMS. The fuel burn is key to increasing and decreasing thrust. I've always adjusted several parameters at once so it seemed they were related.

So besides adjusting prop diameter and pitch, also adjust the prop moment which can be calculating using Mr. Masuoka's ParEdit program by selecting the [calc propeller] tab.

You should also adjust motor items in tandem if you want to keep the sim realistic. Just adjusting the motor capacity doesn't cause the change.

So adjust the cylinder capacity (ml), the fuel consumption (g/s) and the tank size (ml) if you've got fuel simulation turned on.

//MOTOR1 <MOTOR>
0.926 0 0 //Position (X,Y,Z) [m]
1 0 0 //Motor Tilt
0.73 change to 0.6 //propeller diameter [m]
0.29 change to 0.2 //pitch, upward gradient of propeller [m]
2 //number of propeller blades
0.033 //mean chord length of propeller blades [m]
RIGHT //Rotation(RIGHT/LEFT)
0.0032 change to 0.0018 //moment of inertia [kg*m^2]
4 //channel
COMBUSTION //COMBUSTION/ELECTRO
9.0 change to 3.0 //cylinder capacity [ml]
2.7 change to 0.9 //fuel consumption at full throttle [g/s]
1.5E5 //friction constant of engine [pA]
0.05 //friction constant of engine [Pa/s^2]
400 change to 100 //tank size [ml]

Hope this helps others to tweak their models for their personal enjoyment.

-- ggunners