Thread: Discussion Two 144" long T-38's
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Old Mar 29, 2012, 02:05 PM
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Originally Posted by gbmoore View Post
Yeh ... it's fun
just finished an evening with the neighbor-kid (now 1st year masters in Aero at Univ. of Colorado) talking about pivot points.....
When CU did the DreamChaser for NASA, the pivot point was placed about 5% in front of MAC (which rang a bell from the thread I mentioned earlier).
When we took the stab from my (lamented) Tam's F-16, the pivot point was about 48% of the root, which calculated to about 20-21% of MAC.
He (of course) was talking about the Beta movements and the ??? factors, of which I only understood the Greek word "Beta"......
Soooo........looks like both of your stabs are in the ball park.
Keep up the PHENOMENAL work!
"Beta" is a generic term for negative.. it could mean angle of attack, or stability with pitching moments, etc depending on the context. I'd guess he was referencing static/dynamic stability of the stabilizer as it relates to MAC, center of lift / center of pressure... where the pivot point is, and how that plays out. This is a generic symmetrical subsonic airfoil so the variables are fewer, that helps.

Most model airplanes are built with massive servo torque relative to the control surface compared with a full scale. This is good enough to over come small build and engineering errors that are hard to really know exactly how to iron out without wind tunnel and extensive flight testing. Besides we need a strong structure to withstand transportation and abuse real planes don't get. So if we put in a "powerful enough" servo and very strong control support and linkage "for a little extra" we are good (experience shows what works in the past for some models of certain types). I think in this case even if there is some argument about where the actual pivot point should "mathematically" be, it's probably safe to say as long as you have a very strong flight control actuator and supporting structure you could probably get away with some considerable setup "deviations" and never really be the wiser.
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Last edited by Eddie P; Mar 29, 2012 at 04:25 PM.
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