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Center of Gravity Basics

After checking your engine opening and a radio operation, always make certain that your indication is scrupulously offset before your initial flight. It happens too often, unfortunately: Someone with a code new aeroplane shows adult during a margin and has all operative perfectly. The radio checks out, and all a controls pierce in a scold directions; a engine runs reliably and provides copiousness of power. The indication taxis out and runs down a margin and becomes airborne. Right away, it starts to stone a wings; a nose points adult sharply; and after several frightening seconds, it hits a ground—hard! The commander did all right solely that he forgot to check a model’s balance.

To successfully test-fly a new model, be positively certain that a indication is offset during a scold core of sobriety (CG). Nothing can spoil your day some-more than perplexing to arrange out a tail-heavy indication after it has left a ground. It is a satisfactory gamble that crude CG plcae is obliged for some-more damaged airplanes than engine and radio issues combined. Let’s demeanour during some methods for last a CG.

Better than regulating your fingers, a accurate balancing device, such as a Great Planes CG Machine, will assistance we get a change indicate correct.

Please note that these techniques are good for tighten estimations and will get your indication offset in a ballpark good within a scold CG operation so that it is protected to test-fly. We’re not holding into comment surprising configurations or lifting tail surfaces. If we wish to see a math and some-more precise, minute aerodynamic principles, check out Andy Lennon’s book Basics of R/C Model Aircraft Design, available from


While balancing a monoplane, use a plcae indicated in your model’s instructions or skeleton totalled behind from a heading edge. If we don’t have this information, it isn’t tough to figure it out yourself. For many models, a change indicate falls between 25 and 30 percent of a meant aerodynamic chord (MAC); 27 to 28 percent seems to be a average. With a consistent chord wing, like on a Piper Cub, a MAC line is a wing’s chord line, or a stretch from a heading corner to a trailing edge, and it can be totalled anywhere along a wing’s camber (Figure 1). With a slim or swept wing, we have to establish a accurate plcae of a MAC line (Figures 2 and 3).


With a consistent chord wing, a MAC can be totalled anywhere along a span. You can change a craft anywhere along a dimensions line.


For a slim wing, after we lay out a base and tip prolongation lines, bond their ends with projection lines to form a X pattern. Where these lines join is a plcae of a MAC, that is together to a base chord line.


Finding a MAC for a swept wing is accurately a same as with a slim wing. Note, however, that a dimensions lines for a 25 and 30 percent MAC positions are over behind along a base chord.

Using a wing’s tip view, a chord length of a tip rib is combined to possibly side of a base rib, and a base chord length is combined to possibly side of a tip rib. The ends of these prolongation lines are afterwards connected with projected lines combining an X. Where these dual lines join is a plcae for a MAC line. The MAC line is together to a base chord. The MAC line is afterwards divided by 4 to get a 25 percent MAC plcae and by 3 to find a 33 percent MAC location. These dual points yield a wing’s CG range. You can afterwards extend magnitude lines from a dual MAC locations, that are 90 degrees to a base chord, and we can afterwards change your indication anywhere along these lines.

Figure 4 shows Andy Lennon’s movement for anticipating a MAC line of a slim wing row where a MAC plcae is during a intersection of a singular erratic projected line and a line joining a midpoints of a base and tip rib chords. The outcome for a dual techniques is intensely tighten and good within a altogether CG range.


Andy Lennon recommends a reduction concerned approach to find a MAC. Instead of regulating a X pattern, he simply connects a midpoints of a base and tip chords and afterwards uses a singular erratic line to bond a ends of a prolongation lines. Where it and a ½ chord line join is a MAC.


When it comes to biplanes, a technique is really similar. Figure 5 shows a side perspective of a biplane with dual true consistent chord wings. The MAC is estimated as a craft stretch from a tip wing’s heading corner to a bottom wing’s trailing edge. You afterwards order a MAC line to find a CG change range. There are other ways to find a biplane’s CG location, though this is a simplest. If, however, your biplane has a swept tip wing and a true bottom wing, like with a Pitts Special, we use a same technique as for a slim wing shown in Figure 6. The stretch from a tip wing’s base heading corner to a bottom wing’s base trailing corner becomes a base chord line, and a stretch from a tip tip’s heading corner to a bottom tip’s trailing corner becomes a tip chord line.

After last a position of a MAC and anticipating a 25 percent and 33 percent MAC locations, supplement nose weight or change a onboard apparatus around so that a model’s change indicate falls somewhere within a CG range. Use a stout balancing mount that’s sized according to your model, afterwards make certain that your counterbalance weight is scrupulously cumulative so that it can’t pierce or tumble out of a airplane. we like to start in a core of a operation since a customarily approach to truly figure out either a indication is somewhat nose- or tail-heavy is to test-fly it.


It is easy to find a MAC for a biplane with dual consistent chord wings. It is simply a craft stretch from a tip wing’s heading corner to a bottom wing’s trailing edge.


With a biplane that has a swept tip wing, simply provide a planform as a slim wing. Then use a prolongation lines and a projected X lines to find a MAC.



An glorious approach to check your model’s CG is to use a dive exam process (Figure 7). We schooled this technique from Keith Shaw, an consultant electric-airplane engineer and a MAN contributor, and nonetheless it competence seem backward, a exam works really well. According to Keith, “An improper core of gravity, customarily too nose-heavy, also increases neglected drag. A nose-heavy craft has to lift up-elevator trim to say longitudinal stability. This means that a stabilizer is ‘lifting’ downward, and that creates some prompted trim drag. It also means that a wing contingency now rise even some-more lift to say turn flight, producing even some-more drag!

“Fly your craft during half throttle, and adjust a conveyor trim until it can say hands-off turn flight. Check this by creation several passes but changing a stifle setting. You should be drifting during an altitude of 100 to 150 feet. When a craft is impending core stage, kindly lift it into a 30-degree dive, and reason it until a atmosphere speed has increasing noticeably. At this point, take your palm off a hang and observe what happens. If a craft pulls adult sharply, it’s really nose-heavy. If it continues in a dive or pulls adult slightly, a CG is only right. If it tries to tuck under, it’s tail-heavy. This happens since a boost in speed amplifies a trim corrections. If a indication was carrying some up-trim to scold a nose-heavy condition, a increasing dive speed creates a indication lift adult and clamp versa.

“A garland of additional advantages comes with carrying a scold CG location.

The volume of conveyor chuck required for any scheme will decrease, and that will meant reduction control drag. There will be probably no need for down thrust, that is an unhandy try to ‘fix’ a bent of an overstabilized aircraft to nose adult as energy is increased.”


With a model’s CG is in a protected change range, we can take off and arrange things out on a initial flight. Fly a indication during about ½ to ¾ throttle, and set a trims for true and turn flight. After alighting and checking a trim-lever locations, adjust a clevises so that a radio’s trim levers can be set behind to neutral. Make another flight, retrim, and see how a indication performs. If it flies normally—not climbing or diving with teenager energy changes—this is a good CG indicator and we are really close. If there is still a lot of up- or down-trim, afterwards a CG still needs to be adjusted. A lot of up-trim tells we that a CG is too distant brazen (nose-heavy); a lot of down-trim indicates an abaft CG (tail-heavy).


The best word process for any indication aeroplane is to have it offset as tighten as probable to a scold CG before flying. Feeling what a indication tells we when test-flown will assistance we fine-tune a balance. Tail pressure increases maneuverability, while nose-heaviness increases stability. A nose-heavy airplane, however, tends to land during a faster speed, while intensely tail-heavy airplanes can turn uncontrollable. Some simple calculations before that initial moody will assistance keep your indication whole. Get a change right.

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