FanFacts H
ello again, and welcome back! Hopefully your building projects are well underway, and major problems have been avoided!
Since we are getting much closer to flyable weather, for most of us, I thought I’d spend a little time on measuring the c.g. and bal- ancing … even though I know a refresher isn’t needed! Last fall, I watched a plane fall from the
sky for several reasons. First was a radio is- sue in roll (the offending item subsequently replaced by the manufacturer) and the sec- ond was an aft c.g. The pilot stated that the c.g. was exactly where it had been prior to a repair (which added close to 3⁄4 pound), de- termined by propping up the main wheels and checking the weight on the nose gear. Unfortunately, the weight on the nose gear as a percentage of the total had decreased and the plane was tail heavy. So how do we avoid losing a perfectly good airplane? The first is always checking the plans c.g. against a calculated one. I re- member the late Ivan Munninghoff talking about being at an event and helping balance a large bomber whose plans c.g. was off by several inches (yes, inches!) as the designer happened to walk by and congratulate him for finding the wrong c.g. prior to the maid- en flight, which would have also been its final flight! I have also found an incorrect c.g. on several review models … so it does happen! Okay then, how do we find the correct c.g.?
On a “normal” model, the drawing below works around 99 percent of the time. The difficulty is in knowing what percentage of the Mean Aerodynamic Chord (MAC) to use. For most jets and performance airplanes, 25% of MAC is a good starting point, though some aerobatic 3D types will end up at the 33–35% range after flight-testing. When we
by greg moore You can reach Greg Moore via e-mail at
jetflyr@comcast.net
PHOTOGRAPHY: GREG MOORE
Greg’s favored method is a sling with plumb-bob. Caution must be taken since the parachute cord holding the plane can slip around the hanging point and ding the tail on the workbench. When hanging level, the plumb-bob points at the c.g., and equipment moved/weight added to establish level at the correct c.g.
start adding leading edge extensions/ strakes, like on the F-18, I am uncertain of measuring the correct spot, though lots of engineering types will come up with the cor- rect answer on any of the forums. I know, the formula doesn’t include the
area of the stabilizers, but for a convention- al airframe, be it the swept wings of a MiG- 15 or the “Hershey bar” constant-chord wing of a high-winged trainer, it gets you in the ball park and on the field around 99 percent of the time, and then fine tuning the balance and feel is a very individual thing. I’ll presume we have all run to the shop
and checked the c.g. on our new pride-and- joy (let’s face it: if it is a production kit or ARF that has flown many thousands of suc- cessful flights—BVM whatevers, Ziroli plans whatevers, CARF whatevers and so- on) we really don’t need to go crazy other than as a good exercise … unless we need to repair them and have lost the instructions. If it is a Chinese foamy, I’m going to mea- sure every time and if it is a set of plans, I’m definitely measuring them also! Looking at the drawing, we see that the
root measurement (the front to back mea- surement of the wing) is taken at the center-
A simple scale drawing of your wing (above left), combined with three simple measurements and three straight lines makes finding the Mean Aerodynamic Chord (MAC) of almost any wing simple, and thus the 25% MAC point easy. Needing a device that was robust and stable to balance his almost 50-pound
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Mibo A-10, Craig Gottschang fabricated this simple framework (above right) from 2×4s. The wedge shaped tips on the uprights rest on plywood plates taped to the A-10’s wings at the measured c.g. While the finger tip method works well with the smaller models, this stand makes it easier with the much larger models.
APRIL 2012
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