View Full Version : moments of inertia problem.

October 11th, 2011, 09:52
i I have a little problem with setting the moments of interia on the XB-35.
the calculated moments are.


It seems that FSX doesnt like MOI's over a million, or am i incorrect. The aircraft keeps yawing around in a wide circle so i imagine its my yaw moment thats being rejected.. Glad to answer questions, but could really use advice because i'd like to keep this realistic if possible..


October 12th, 2011, 08:12
R U getting similar values when using the formula provided in the SDK?
I always it and I just adapt the values for significant differences like engine weight/position.
Once this has been set things like roll damping etc. I change via the air file.

October 15th, 2011, 04:52
It turned out to be a CG issue. Tim exported the plane with the exact center of the model, were the reference datum should have been. being sometimes apparently blind as a bat, I didnt pick up on it, so, my CG was in the wrong place for the model.. Once i discovered my error i was able to adjust for the displaced CG and everything seems fine now..
Thanks for answering though .. Its appreciated..

October 17th, 2011, 11:24
The dear departed ACM was great to see where the "actual" plane was and how closely it coincided with the visual one. Sometimes they were not very close. Recent example include having the zero points on the ground and at the nose. Surprisingly some payware aircraft have the CLand CG well outside the wing....

Cheers: T

December 3rd, 2011, 11:30
That might be because of the time crunches some payware developers place on their engineers. Its far easier to move the cg than it is to balance the weight and lift between the main wing and horizontal tail. Quicker too.. I dont know if its because when you start getting into the balance of a plane your actually getting into some faily realistic aerodynamics engineering ( and therefore scary ), or if its just plain convenience ( and blame it on the old flight sim axiom that its an FS limitation )... I've even met some very real pilots who dont understand that the center of lift moves around a wing.
Working on the XB-35 has been extremely educational. I dont have the luxury of a separate tail section, and the CL/CG balance is far more critical.

December 15th, 2011, 11:38
If possible I like to have the CG range of the virtual aircraft closely match the R/W aircraft CG range, if the info is available. Generally the pitch section in the primary aerodynamics tab of the .Air file can manage the proper pitch trim with the CG in the proper range. Additionally thrust, flap and gear pitch can be used to get proper takeoff and landing manners. In aircraft that we do not have a detailed weight and ballance vrs trim setting for tko, I prefer to have a fairly neutral trim for takeoff. Some military (especially WWII fighter and tactical) aircraft may had a suggested takeoff trim for rudder, elevator and aileron. It is good to try to match those settings. Set the CG somewhere in the 20-30% range such that it stays there throught the flight envelope and then set the pitch parameters (plus numbers move the nose down).

The more speed stable an aircraft, the more trimming that will be necessary! Some airfoils may achieve zero lift at some small negative AOA, these aircraft may fly a bit nose down at max speed!

Cheers: T