wlbragg wrote in Sat Feb 25, 2017 7:14 pm:I adjusted the empty weight CG to 8.5" and what a difference it makes. I feel way more comfortable with this number and the handling and feel of the aircraft in all configurations. It may very well be the sweet spot.
I think it is not good because it is based on the AERORP at x=0 (leading edge). See below.
But you can get the same handling by shifting the AERORP and CG aft by the same quantity.
dany93 wrote:On SVN, I see that the AERORP is at x=0. Weird for the geometry and not compatible with CG at 12" aft ... (?)
wlbragg wrote in Sat Feb 25, 2017 7:14 pm:I don't quite understand this, could you please explain "Weird for the geometry" and "not compatible with CG at 12" aft ".
dany93 wrote:The AERORP location is a difficult subject. For a single wing, the aerodynamic center is about 25% the mean aerodynamic chordbut for the entire aircraft it is a more or less aft.
"Weird for the geometry" because the AERORP should not be at the leading edge. As a first indication, 25% of MAC gives 63"/4 = 15.75" aft from the L.E..
Those data for the entire aircraft are very hard (if not impossible) to find. To start, I would take it between 25% and 30%, that is between 15.75" and 18.9".
With this AERORP aft from the Leading Edge it will be easier to move the empty CG more aft, in the (+10.6) to (+22.7) or (+8.5) to (+20.3) range.
More complicated, the AERORP changes with the AoA. At my knowledge not taken into account in FG, but this can be circumvented (see below).
Moreover (see my previous post) a general rule is that the CG must be reasonably before the AERORP (for stability).
To start, I would locate the empty CG at about 16.5" or 13.8" (if this one with empty wt 766.5 lbs can be trusted). Or this Weight and Balance doc for 1216 lbs empty.
Still, you should also check that, with these AERORP and empty CG locations, you do not get the CG aft from the AERORP with some acceptable loading configurations.
However, there is some margin for the CG even if placed aft from the AERORP because you wrote that 8.5" aft from the AERORP (with AERORP at the LE) was still pleasant to fly (in FG ! Dangerous IRL).
And you test it in flight.
As the CG location seems more reliable than the AERORP location, you can fiddle firstly with the AERORP. I dislike that cheating but as long as we have nothing else...
There is also some margin for the CG slightly before from my figures.
If the aircraft is permanently (but not excessively) nose heavy, it can be corrected by some (preset) elevator trim or, better, some "pitch moment at zero alpha" with zero trim.
Still better, an alpha-dependent "pitch moment due to alpha", which also comes to take into account the AERORP variation with AoA (not perfect, but that's simulation).
I hope that I have responded above, those values are for the AERORP at X=0 (Leading Edge).If 12 " is not compatible, would 8.5" also not really be compatible?
Are you basically saying that if the AERORP is at x=0 and 3D X-origin is at the datum, then the CG should be close to 0".
If the above is so, is there anything wrong with moving the empty weight to 8.5", so long as the aircraft feels correct and performs well (or better) and allows for the recommended CG load-outs?
Those considerations for the AERORP location are mainly based on feeling (flight tests) by lack of documentation.
Most often, the empty CG for an aircraft can be found in the POH.
Generally, I try as much as possible to base my settings on published data. Easier for many like lift, drag, glide ratio. Not here.
When introducing a new feature, I also insist (at least try) to rely on physics or / and aerodynamics laws tu justify it. JSBSim is a wonderful tool for that.