Trimming your RC model is an important procedure, and when properly trimmed your model will be so much nicer to fly, making your life at the transmitter sticks that much easier.
The aim of trimming a RC model correctly is to get it flying straight and level at cruising speed, with no input from you at the transmitter sticks. In other words, with the elevator, aileron, and rudder sticks in their central positions the model should fly in a straight line. The throttle stick will typically be halfway to two-thirds full power, depending on the model.
If you’re new to RC flying, it’s likely your trimming will involve nothing more than moving the transmitter trims to compensate for any unwanted tendencies of your model so it will fly in the direction in which you want it to fly.
Moving a transmitter trim will move that particular control surface a very small amount, and that new position will then be the new neutral position. This lets you fine tune your models’s flight path. For example, if your model wants to roll to the left all the time then you would apply right aileron trim until the model flies level on its own, without wanting to roll. This type of trimming is common practice and even the best trimmed model will need the occasional, or even regular, re-tweak of a transmitter trim.
Transmitter trims get very regular use, even by the best RC pilots. However, trimming a RC model actually goes beyond only relying on the transmitter trims; it involves a bit of aerodynamical knowledge and good workshop habits. In other words, you need to build your model correctly and understand the forces that act on it in flight.
If you’ve bought a ready to fly (RTF) or an almost ready to fly (ARF) model then all the construction work has been done for you, but you should still check it over to check for good and correct alignment of wings and tail surfaces. Poorly aligned, bent, or twisted surfaces will only result in a poorly performing model.
If you’re building from a kit or from scratch, you need to take care when building and be sure that there are no warps, bends, or twists where there shouldn’t be. Any of these will have a very negative effect on your model’s flight characteristics.
Assuming that your model is correctly built and has everything aligned correctly, it should fly perfectly well first time out. However, that rarely happens. RC models rarely fly perfectly well first time out and the best we can hope for is that a couple of clicks of transmitter trim will sort things out on its maiden flight. But what if your model flies really badly, yet it’s been built well? Here we come to the true point of this page, and that’s how to trim your model by adjusting its centre of gravity (CG) and/or motor thrust angles.
This all might sound very technical and complicated, but it’s really not, and once you’ve done it once, and seen the results, you’ll want to do it to all your models.
Centre of Gravity
The CG is the point at which your model balances fore-aft (longitudinally). This means that when your model is supported on its CG position and is balanced correctly, it will balance level or perhaps with its nose pointing slightly down.
RC models rarely have one exact point of balance; the acceptable CG usually lies within a narrow range, and the precise location of the CG can be moved within this range, without having any adverse effects on the model’s flight characteristics. It’s only when you move the CG outside of its range that problems begin.
This moving of the CG fore or aft, within its range, is a major part of correctly trimming your model. Any movement of the CG will slightly affect your model’s pitch attitude, elevator sensitivity and how it wants to fly as a result.
If you move the CG forward (i.e., nose down), this increases the model’s stability (up to a point) and generally makes it less sensitive to elevator inputs. Conversely, moving the CG rearwards (i.e., tail down) increases the sensitivity to elevator inputs and makes the model less stable – but better for aerobatics. Moving the CG too far rearwards will make the model completely uncontrollable. Don’t forget, nose too heavy, it flies badly, tail too heavy, it flies once!
Balancing your model correctly in the first place is very important, but once balanced you can tweak the CG position to improve the model’s flight characteristics, and to suit your personal preferences.
Roll, or lateral balance to give it its proper name, is an often-overlooked task and isn’t as critical as longitudinal balance. But a RC model that has one side heavier than the other will have a tendency to naturally roll and turn to the heavier side, making your life on the sticks a bit harder. Also, a heavier side will almost always result in that wing dropping when the model stalls, potentially putting the plane in to a spin.
The usual cause of a model being out of balance laterally is unequal weights of the wings. To roll balance your model simply hang two lots of strong string from something fixed and stable (i.e., a ceiling), each piece being in a large loop. Put one around the propeller shaft, between the prop and the fuselage, and the other around the rear of the fuselage, as close to the tail as you can get. If possible, insert a small-gauge pin or screw into the very rear of the fuselage to hook the string around. This gives a more accurate result since the plane can hang more freely.
If you can’t hang the plane from something solid, like the ceiling, use a rigid pole to hang the strings from. Whatever method you use, let the plane hang freely on the string – see if it wants to roll to one side or another. If it does, then you need to add some small ballast to the wing tip of the lighter (higher) side. Add only enough to make the plane hang so that both wings are level.
Taping the ballast to the wingtip is an easy method, although you might want to take the trouble to set any ballast into the wing tip and cover it over. Bear in mind that adding larger bits of tape will actually add weight in addition to the ballast.
For models with foam wings, pushing a small gauge nail or panel pin into the foam is a great way of adding any necessary weight, and it can be easily hidden with suitable paint or marker pen.
Do take a few minutes to check your roll balance as well as your CG.
So, there you have it… two easy methods of balancing RC models. As already mentioned, the longitudinal balance (CG) is very critical if you want to keep your plane in one piece. Lateral balance isn’t so critical but is worth doing anyway.
A correctly balanced model will always be safer and easier to fly and won’t need as much trimming at the transmitter.