FSX Aircraft geometry; affect pitch.

[capt_x]

Didn't know there was a CG Simulator variable. Thanks, Roy. Let me check mine...

edit. Yup. Fueled aircraft shows CoG at 28.2%, same as Airwrench. But empty CoG disagrees: FS says 30.0%, AirWrench says 34.3%...
edit2. Missed to unload one station before. Now FS shows empty CoG at 34.3 too. In total agreement with Airwrench!

 

[Roy Holmes]

Empty CG is what you get with no fuel and no loads of any kind. If AirWrench has 3 values, Fueled, loaded and empty and they are all different, you must have some unloadable load.
Roy

 

[capt_x]

The TCDS states that CoG range at 11,353 lbs (which is almost empty weight) is from 36% MAC (forward limit) to 42% MAC (aft limit). At MTOW (17,968 lbs), CoG range goes from 19% MAC (forward limit) to 32% MAC (aft limit). If I average these two ranges, I get:
empty CoG: 39.0% MAC
loaded CoG: 25.5% MAC

 

[Roy Holmes]

Interesting contrast to the two planes I flew the most.
Hawker Hunter, empty weight 14530 lbs, one pilot 180, ammo 582, pylons 200, drop tanks 808, fuel 8100, MTOW 24500.
McDonnell Phantom F-4M, empty 31140, pilot and navigator 360, drop tanks 924, fuel 21930, MTOW 58000.
Pretty inefficient passenger airplanes!
Roy

 

[jx_]

assuming your elevator deflection moments are correct (they are probably close enough), and assuming your CG and center of lift are correct.... the culprit is most likely Moment of inertia... i.e. I don't see them in your [WEIGHT_AND_BALANCE] section.

add:

;Moments of Inertia
empty_weight_pitch_MOI=250000
empty_weight_roll_MOI=200000
empty_weight_yaw_MOI=100000
empty_weight_coupled_MOI=0


You can use Microsoft's formula to get the exact numbers to use but they are generally a little low (enough to notice the rates in the sim) because of generic weight placement. To tweak, increase a given number will slow down that axis (more sluggish to start moving and more overshoot when control pressure is reverse [more momentum].)

• MOI = EmptyWeight * (D^2 / K)

Where:
Pitch Roll Yaw
D = Length (feet) Wingspan (feet) 0.5* (Length+Wingspan)
K = 810 1870 770

 

[jx_]

Ed.


LEMAC and wing_pos_apex_lon only coincide on a wing with an unswept leading edge. When the wing has sweep the LEMAC position is aft of wing_pos_apex_lon by a distance equal to the sweep offset. Sweep offset is the distance from the leading edge of the root chord to the leading edge of MAC

For example, I'm working on a jet with 45 degree leading edge sweep. MAC is 8.51. CG and COL coincide at 0,0,0 which is 24.95% MAC. CG readout from A:CG PERCENT, percent is 24.94.
Wing_pos_apex_lon is 10.74. LEMAC is at 2.127.
The MAC, CG and COL data are from Airwrench. CG percent is from the A:Var. FS and AirWrench are in agreement.
Roy

yes, the wing apex is the imaginary point the two leading edges meet at the airplane's centerline...

The LEMAC and MAC can be found easily in the Type Certification Data Sheet.

The Center of Lift (AC, aerodynamic center) however is not usually known without some calculations...but it is very important to good modelling.

To find the right AC, we need to know a CG and trim setting for that CG.

if CG = AC then trim = 0. As CG moves forward, trim moves nose up to maintain level flight. I can explain the math behind this if you need, but it's basic torque arm math of weight x distance from CG to AC of the wing; versus AC of the horizontal stabilizer.


I've attached a drawing for you to illustrate Roy's points. Notice how far back the AC is on a swept wing aircraft.