buck_fdm_notes.xml

<cossack> 
  1. The plane doesn't hold wings straight, it spins around fast, but hold a hdg when set (autopilot) Note: i deleted all autopilot references
  2. The plane bounce on high speed, just like tu-134 did before 
</cossack>
    
<buck>  
I'm not sure about #1, I'd have to try flying the aircraft first. 
#2 is typical of many low-effort YASim FDMs. It happens for a number of reasons, often because a 
wing develops too much lift at cruise speed, or an elevator is assigned too much lift to get a YASim 
solution, often compensating for other FDM problems, etc.

I had a very quick look at the FDM. It will need a lot of work. Some thoughts that may help:

The YASim solver shows a Lift Ratio of 525. This is much too high. Numbers like that mean the aircraft is /very/ slick-- it will not fly realistically 
and will be difficult to slow down. Lift Ratio will need to be brought down to the 100-150 range before the FDM will be reasonable for this sort of jet. 
Lower numbers will generally give better results.

Aircraft mass: This appears to be set for zero-fuel weight of a 145XR. It would be better if the aircraft had <weight> and <solve-weight> settings, and 
the aircraft mass represented an empty mass. This would allow users to adjust the payload and experience the aircraft at various weights.

Approach settings: An Approach AoA of 8 is probably 3 or 4 degrees too high. Airliners have fairly low approach angles for good runway visibility and 
other reasons. A YASim "approach" actually represents a stall, the aircraft's low-speed performance bracket, so flaps and slats are best deployed at 
their full range (1.0). Slats should be set to 1 for landing-- this is imporant for proper stall characteristics. I'd set approach fuel to something 
close to typical for landing, maybe 0.2 or 0.3.

Cruise setttings work best when they represent the aircraft's typical top-end peformance. I would set engines to max throttle and make sure cruise 
speed is the max level speed at altitude. Fuel is OK at 0.7. Flaps, slats, and gear should all be set to 0.

Wing (and primary mstab) settings: For an regional airliner, try a stall AoA of 12 or so. Slat AoA of 2 is OK. With no wing incidence or twist, this 
means with slats deployed the aircraft will stall at 14 degrees alpha, 12 without slats deployed. Slat control settings appear to be tied to flap output 
via flap-pos-norm-- I'm not sure YASim will like this, maybe it will, but in my opinion slats should have their own unique control parameter, with 
that paramter slaved to flap settings outside of the FDM. Stall widths should not be so extreme (1?)-- try a value in the 4-6 range, maybe 5. 
No twist is shown in the FDM, but there will always be some aerodynamic twist so that the aircraft begins to stall before the ailerons lose control. 
A twist of -2 in the <wing> makes a good guess. Also, aileron lift looks too great at 1.3; that will make the aircraft much too responsive. 
I suggest a value of 1.15. I'd reduce flap lift slightly and increase flap lift. Flaps should mostly generate drag, if we ignore camber and chord 
effects, which YASim unfortunately doesn't yet simulate.

I would eliminate the second mstab (the one that has a length of 1.1). This is someone's idea of representing winglets, 
but many people don't understand what winglets actually do. The point is to reduce wing inefficiences drag due to vortex effets. 
This is best done by tweaking other wing parameters rather than adding new wing surfaces which just makes YASim perform more 
calculations for no benefit. It's best to keep the YASim FDM simple and get the fundamental aspects right.

hstab and vstab: Again, a stall AoA of 20 is much too high. The stabilizer should stall after the wing, so a value of 15 or 16 should work. 
Stall widths of about 4 are reasonable. Leave peaks at 1.5. Also, unless you know otherwise, drag values should match or exceed lift values 
for any control surface.

The engine nacelle fuselage elements should generally be smaller than actual size-- by design these have very little drag or mass, and their 
lateral effects aren't that significant.

I don't know much about the engines used (except I think they're made not far from where I live!), but a specific fuel consumption of 0.65 
is too high for a turbofan. Try something like 0.55 or 0.54 for more reasonable economy. The action point is a little too far aft at -9.77. 
Thrust is applied not at the very rear of the engine, but at the forward portion of the thrust chamber in turbojets, and much more forward 
than that in turbofans (at the fan thrust bearings). Try the midpoint of the engine nacelle for more reasonable results. YASim doesn't 
simulate turbofans very well, so you may have to play with "exhaust-speed" to get more reasonable cruise performance without having huge 
numbers for Lift Ratio.

I have no idea if the CG is correctly placed. This is important to get right. It's also important to place fuel tanks reasonable. Airliner 
fuel tanks are usually positioned in the forward position of the CG range. In the FDM, the center tank looks incorrectly positioned, being 
too far forward. Since the fuel capacity of a 145XR is at least 5187 kg/11435 lbs (probably more with an additional central tank), the fuel 
settings appear to be wrong. I'm guessing the fuel tanks haven't been researced for this FDM.

Hope these thoughts give you some ideas.

-Buck
</buck>