Trim

Introduction

In the days of Free Flight Trimming a model was vital. A badly trimmed model did one of two things. It either crashed or flew very badly. Without "real time" correction once the model was launched it was literally in the hands of the gods. Radio Control has made modellers lazy in so much as the model can be trimmed in flight with a bit of elevator, a bit of rudder etc. That's it, job done. As long as the Center of Gravity is more or less correct most problems can be sorted on your first flight, if you are fast on the trims. Free flight models could usually be test glided into long grass and trimmed out more or less correctly before a real flight. Radio Models usually have a high wing loading and test glides are out. So what is the answer. Read on.

Building and Design

The performance of any model starts with the design, a badly designed model will never fly well. Modern ARTF models are produced in such volumes that a bad design is a rarity. They will not sell and the considerable investment will be lost. The same applies to plans or kits as these are usually tested before release to the public. There is still a lot that can go wrong and often does. For any model to perform well it MUST be built true and square. If wings are warped or the fin skewed it will never fly well. Weak structures that twist in flight are also a problem and can lead to a crash. Build the model true, make sure it is strong enough to withstand aerodynamic stresses, flutter or warping and get the C of G in the correct position. Is one wing heavy, balance if badly out with a bit of lead one one tip. Once your model is ready to fly spend 20 mins checking out the build. Are the wings true and set on the fuselage at the correct incidence. Same with the tail. Is the fin square to the fuselage center line. Does it all balance. Are their any warps. Correct ay problems before you fly. Make sure ALL the controls move in the required direction. Set the rates to LOW with plenty in reserve if required.

Side Thrust and Downthrust

All engines, Electric and Glow, with the exception of Gas Turbines and Contra Rotating Props induce torque to the model. Generally a prop driven model will tend to roll to the left and climb under power. This is corrected by adding DOWNTHRUST and SIDETHRUST to the engine mounting. As a start with most engines a couple of degrees right and down will suffice. Very large engines in small airframes will need more, but one assumes if you are going down this road you do not need to read this article in the first place. To check the degree of downthrust and sidethrust fly the model straight and level on as low a power as possible. Open up to FULL power and see what happens. If the model climbs, add more down thrust, if it rolls to the left add more sidethrust. It should continue to fly straight and level, very hard to get spot on.

Aileron and Rudder Trim

A model that will only fly level with a few degrees of aileron trim has a problem. This is either a heavy wing on one side, warps or poor build. Rudder offset or a bend in the fuselage. One wing is producing more lift and this must be corrected. The same applies to the rudder.

Elevators

If your model needs UP Elevator to fly level first check the C of G. Try increasing the wings incidence to provide more lift. Moving the C of G rearwards may make the model very sensitive. If the model needs loads of lead in the nose just to get the C of G in the correct location there is not a lot you can do. At all costs the C of G must be located in the correct position. With the C of G in the correct position and down trim still required try reducing the wing incidence or increasing the tailplane incidence.

There are many books on this subject that go into trimming in greater detail See Chapter 12 Model Aircraft Aerodynamics by Martin Simons ISBN 0-85242-915-0 Motrorbooks International.

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