The plywood insert Jim McEwen uses to establish c.g. (above). The center hole is the measured c.g. Jim’s elegant method in use (at right). This makes taping items to the outside prior to final installation very easy, thereby minimizing the amount of ballast needed.

line of the fuselage, but the wing doesn’t go all the way there; so we extend the leading and trailing edges of our drawings until they do. On an F-15 it is amazing how large the root is given the width of the fuselage. Now trans- fer this number to a scale drawing of your wing. Likewise, measure the chord of your wing tip—ignoring any weird or strangely shaped tips—and add this measurement to your drawing. We now add the tip chord to the root, and the root chord to the tip—it doesn’t matter which direction (front or back) as long as they are opposite each other. Draw a line to connect the two additions

you just did and you will get a diagonal line cutting across the wing. Next, draw a line connecting the mid-points of the root and tip chords, and where the diagonal line crosses it, draw a line directly fore and aft. This is the Mean Aerodynamic Chord (MAC) for this particular wing. Measure the length of this line and do the very simple math (.25 × the length) and you now know where the 25% MAC point is. Remember: this means we are balancing one-quarter of the way back from the leading edge at the MAC, not at the root. All right, we’ve drawn, measured and cal- culated, how do we determine if our plane balances there or not … oh yeah! we use our fingertips! That is a perfectly acceptable way to balance smaller airframes, and your flying buddy’s fingertip on the left wing and yours on the right is just an extension of that, but how about bigger planes? How

about figuring out balance prior to installing all the equipment? … unless you have a fondness for lead. Several companies sell a cantilevered bal-

ancing rig that works well, but can be pricey! The very first rig I was exposed to was a pair of sharpened 3⁄8-inch dowels glued into the cross arms of an elongated “+” sym- bol. A pair of plywood plates was taped to the bottom of the wing with a line noting the measured c.g. and the plane then set on top of the dowels. When the plane sat level, all was done. Craig Gottschang took this to another

level (and I think easier and safer) to bal- ance his large (50-pound range) Mibo A-10. The “V” shape of the uprights gives a larg- er surface area for the ply plates to rest on. An extension on this idea is to tape a large and long piece of triangle stock (spruce would probably be the best) to the bottom of the wing at the c.g., and rest them on saw horses. My most used method over the last 15

years has been a simple sling. I make a loop in a length of parachute cord (blind cord would work well too) and then anoth- er loop further down the length. One loop goes under the nose (usually stopped by the main gear) and the other loop as far back as I can get it to stay. The center of the looped cord goes over a large nail ham- mered into one of the joists over my work- bench. Once the airplane is hanging level, a plumb-bob is hung from the nail, which

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CD: Frank Tiano 863-607-6611 www.franktiano.com

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gives a very accurate assessment of the c.g. The only downside of this method is that

the rope can slide, so care must be taken when taping things to the outside of the fuselage to determine where they can go prior to installing them, so as to minimize ballast, and minimize damage to the air- plane bouncing the tail on the workbench as the balance shifts. Lastly, the most elegant of the hanging-

style balancing systems comes from Jim McEwen. He takes a length of plywood that will fit between the wing and fuselage, with appropriate holes for the spars cut and an extension that extends above the wing. Into the extension (think in terms of a box shape) he drills a series of holes with one on the c.g., and several both in front, and aft, of the c.g. A line is cut to go over his garage door open- er hanger (my nail will be my target) and goes through loops in pieces of music wire, which fit through the holes in the ply. Very easy, very stable and no need to worry about the line slipping on the nail like in the sling method! The most elegant, and fiddly (to my mind)

is balancing by weight … just like they do it on full-scale planes. A complete write up can be found on the Jet Pilots Organization web- site at: www.jetpilots.org/weight_balance/ WB%20instructions.pdf, though I’m very happy hanging things above my workbench! Well, I’m out of space, so until next time:

Stay warm, keep busy with the building pro- jects, fly safe, and trust in thrust!

CD: Frank Tiano 863-607-6611 www.franktiano.com

www.tucsonwarbirds.com (registration)

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