How to trim your model for rock&roll (part 2)

Step 3: The CG. The best CG location depends in part of your likes. After testing the model with the user manual forest CG location, it’s time to move it aft. This is usually very simple and only need to move the pack aft. Always make little changes (1/8 inch in the pack position it’s ok), and always make a mark to point out exactly where the pack was placed. For most of the 3D maneuvers an aft CG is a great helper. Only one advice is needed here, after some point a little movement of the CG aft means a huge change in stability, then be very careful with the latest adjustments you made because changes are not proportional near the optimal point for 3D.

Step 4: Motor incidence. To change the motor incidence can be as simple as to add or remove some washers in the motor mounting, and it pays the effort twice. Try this, flying in line and levered put the motor at full power without moving elevator letting the model to gain speed. After a few seconds cut the power and see what happens. Our goal is to have no changes in the altitude during all the test. If the model goes down at full power and up after cutting it, the motor has too much negative incidence (it is “pointing” down) and needs some washers in the bottom mounting screws, and vice versa if the model goes up when the power is at the top and down when is cut.

Step 5: Motor incidence 2. Now is time to set the right incidence, to do this we need to climb the model in the vertical as long as we can and see if the model goes right or left at the end of the climb. No rudder correction should be done during this test. If the model goes right it means that it has too much incidence, and then we need to add some washers in the right mounting screws. We need to do the contrary if the model goes left. Please consider that the nut that hold the propeller should be always in the center line. A motor pointing to the right but with an offset to the right, it is the same that nothing, or worse. Then if you need to add a lot of right, consider too to move the back of the motor to the left, keeping the propeller centered in the same line that the rudder is. 

Steps 4 and 5 must be repeated if you change either the motor or the propeller, that’s why you need first to select the proper ones.

 Step 6: only now we are ready to apply some mixing in order to improve performance a little. May be some down elevator at full power, may be some left rudder at idle, or some elevator (up or down) with rudder to mitigate coupling in knife edges. Anyway, mixers do not solve problems, just mitigate them. Furthermore, big amounts of mixing probably ends in new and worse problems.

The order of the steps is important too, as well as to try all this at a safe altitude. To ask an experience buddy for helping is a good idea too.

All this process take time, then take it easy, mix some fun sessions between the trimming ones, and most important enjoy the flight. Once the model is trimmed, you will appreciate the effort.

Nice landings,

Adrian Muiño


Acerca de Adrián Muiño Ing. Adrián Muiño Educación Ingeniero en Electrónica (UTN, Argentina) Maestría en Dirección de Empresas (EOI, Madrid) Postgrado en Marketing (UCA, Bs. As.) Project Manager Professional, PMP del PMI (USA) Actividad académica (10+ años) Profesor de Postgrado en la Univ. Católica Argentina Instructor en gestión de proyectos Actividad profesional (10+ años) Program and Project Manager (BGH SA, Neoris, Petrobras, Ericsson) Escritor de artículos (PM y aeromodelismo) Beta tester
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