Aero Mail
"If you have a letter to the editor, please submit it to MA Editor-in-Chief Jay Smith at 5161 E. Memorial Dr., Muncie IN 47302, or email to [email protected].
"Airplane Setup Methodologies"
Dave Scott made a serious mistake in his "Airplane Setup Methodologies" article in the August 2013 issue. Dave Scott wrote, "A neutral CG location at the wing's thickest point provides the best overall handling without restricting maneuverability."
This is dangerous! He should have said a CG at 1/4 chord point. The aerodynamic center of most airfoil sections is at 25% to 26%. On the old nonlaminar sections, four-digit and five-digit NACA sections, the thickest point is at 25% to 27% chord, but the newer NACA laminar sections go back to 45%!
Dave is partly right. My Aerostar (last built by Piper) uses a laminar 64 series with the thick point at 37%, the Piper Cherokee uses a laminar series 65 with the thick point at 40%, and the P-51 uses a 66 series with the thick point at 45%! If you put the CG at the thick point of a laminar section, bring a box to gather the pieces! Use 25% chord, period!
He also said, "A nose-heavy airplane tends to be less stable and less maneuverable, and ... it will be less maneuverable, but more stable." Aft of 25% is less stable, and forward of 25% is more stable.
—Stan Burak
When the article was rewritten for the magazine, the wording somehow got jumbled. I double-checked the article that I submitted to Model Aviation and my original did state that a nose-heavy airplane tends to be more stable, etc. I originally wrote: "A nose-heavy airplane tends to be very stable, less maneuverable, and will behave differently depending on the speed. All things considered, a neutral CG location at the wing's thickest point provides the best overall handling without restricting maneuverability."
So, good catch!
The reason why I'm able to achieve flight characteristics representative of neither nose heavy nor tail heavy on 99.9% of the countless models I've set up for our students adhering to the wing's thickest point rule of thumb is because nearly all models use very similar "cookie cutter" airfoils with the center pressure points 25–30% of the chord (indeed, I've yet to see a laminar airfoil on a commercially built model).
Of course, you know that the FAA requires most certified airplanes to ensure the CG is ahead of the center of pressure, e.g., to ensure the nose-lowering tendency during stall. Or as I often put it, if one is going to err, nose heavy is definitely better than tail heavy.
Thanks for your input, Stan!
—Dave Scott 1st US RC Flight School
Dave is indeed correct that the wording was jumbled during editing. MA regrets the error.
—MA staff
Transcribed from original scans by AI. Minor OCR errors may remain.
