View Full Version : Has anyone sen this bfore?? What can i do??

October 16th, 2010, 15:31
I'm running into this problem that keeps popping its head back up and whatever i try, the problem does not change..
When i turn the rudder to the left ( as example ) the nose turns left, but the plane rolls to the right..
I've gone so far as to eliminate all of the roll entries in the air file, and the problem persists.. I've compared it to other flight models and all the direction markers ( + - ) match, but still we have backwards roll..
Can someone please give me a clue here?? I'd really appreciate it..

October 17th, 2010, 09:29
Two possible coefficients to look at are dihedral effect (Cl_beta) and roll due to yaw velocity (Cl_r). Both are cross coefficients that cause rolling when the aircraft yaws.

Milton Shupe
October 18th, 2010, 10:01
You might also check the aircraft.cfg Geometry parameters just to ensure they are not over-riding anything in the air file:

htail_pos_vert=0 (zero or negative)

October 19th, 2010, 05:13
Check not only the roll section but the yaw section Cn_p yaw moment - roll rate.

Milton Shupe
October 19th, 2010, 16:06
Jerry once had this to say:

"The 400-series tables, like 452, are for modeling non-linear mach effects on the stability coefficients. As mach number increases, the value of many stability coefficients change, and these tables were intended to model those changes.

As far as whether Cl_dr is positive or negative, it depends on how much of the rudder is above or below the centerline. It's usually a fairly small number.

Most 'roll due to rudder' is actually caused by a combination of other things - the first being yaw due to rudder and the second being roll due to yaw - otherwise known as the dihedral effect.


I never forgot this one.

October 19th, 2010, 16:19
Thanks guys.. That was a big help.. Turns out it was roll due to yaw which was exacerbated by too much force being provided by the rudder.. You all helped a lot and its much appreciated..

November 3rd, 2010, 10:41
In the primary aerodynamics section, a positive value under roll/rudder will cause an opposite roll to the rudder application. A negative value will cause a plane to roll in the same direction as the ruddder is applied. A typical value might be -50 for an average aircraft. Some aircraft such as many jet fighters which have large rudder roll effects might have a figure closer to -100.