THE WEIGHT IN YOUR PLANE -- WHERE'S IT AT?

-by Clay Ramskill

Every kit we buy will show where the point of balance is supposed to be, and it's up to us to see that it gets there. The point of balance is the front to back measurement of where the center of gravity (CG) should be.

The CG placement is a VERY important part of how any plane is going to perform, essentially adjusting the plane's pitch stability. If the CG is too far to the rear, the plane will have marginal stability. That is, it will take very little elevator movement to effect a significant pitch (and angle of attack) change. A plane with an aft CG will be very quick to respond in pitch and will not be inclined to seek out a trim position.

With a forward CG, a plane may require some "up" elevator to fly -- and will require a lot of elevator movement to make any pitch changes. However, if upset from a trim attitude, it will very quickly try to regain that attitude.

In short, the farther forward the CG, the more stable the plane, pitchwise. A plane that is too stable will seem "sluggish" when maneuvering -- with a more aft CG, and a less stable plane, the terms might be "sensitive," or even "skittish." In some more extreme cases, the term has been "unflyable"!!

While on the subject of balance, let's not forget LATERAL balance. The plane should be balanced laterally, as well as longitudinally. Just an ounce of weight in a wingtip for lateral balance can make a big difference in whether your wings stay where you want them while manuvering!

But there's more to weight placement than CG. Weight DISTRIBUTION also can affect a plane's performance. The farther out in "the ends" (of fuseleage and wings) weight is located, the slower a plane moves initially to control inputs. Imagine, for instance, putting a half-pound weight in each wing tip of a plane. That plane will eventually have the same rate of roll -- BUT, due to the inertia of the weights, it will take a while longer to achieve that rate.

The same goes for pitch -- if we have a heavy engine, well forward of the CG, we will have to compensate by moving servos and batteries (and maybe add lead) aft in the fuselage. Inertia from that weight distribution will cause a slower initial reaction to elevator control. The same reasoning concerning weight in the ends of the plane also applies to directional control, slowing down reaction to rudder inputs.

Note that overcoming inertia is applicable both when we begin or end a maneuver. The plane with weight "in the ends" won't be as "quick," but will tend to be "smooth." This isn't all bad -- in a trainer or pattern plane, for instance. But a plane that is too heavy out in the wingtips is another matter - we may be able to cope with having the plane respond slowly to start rolling; it's really tough to contend with a tendency to keep rolling when we neutralize aileron controls!

The same goes for spin entry and recovery; our "heavy ended" plane won't be nearly as quick to enter either a snap or spin - but more importantly, it won't STOP snapping or spinning as promptly, either!

So, by all means, keep your plane light, if you can - but remember, perhaps even more important is - where's that weight AT?

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Last Update: 10/12/97