Basic Model Set Up

In addition to the very basic advice on the link to North Anston Society of Aeromodellers web site which you may find of interest, as the cost of models increase, added reliability is sort after, and before the development of 2.4 GHz many competitors use Dual Conversion receivers on PCM (pulse code modulation).  JR and Futaba also supply heavy duty switches in place of the ones supplied with the lower cost Tx/Rx combos.  There are those who fit two 'half size' nicads with separate switches with individual battery state indicators, and others use higher voltage (Lithium ion or Lithium polymer) cells working through a voltage regulator to provide a constant voltage throughout the flying session.

The following information is taken from recent articles in  Radio Control Model Flyer (www.modelflyermagazine.com) and have been included with the kind permission of the editor.

"Before I take any aircraft into the air I feel it is important to spend a couple of hours setting up the model so when the first flight eventually arrives I know the model is prepared to the best of my ability and should fly with no surprises.  I’m sure you can appreciate that nerves are bad enough without having to worry about a poorly set up model. 

With the model completely assembled and ready for flight, except for a full tank of fuel (if IC) (but with batteries installed if electric), the centre of gravity is the first thing to be checked.  The measurement is usually taken from the leading edge and needs to be within an inch of the manufacturers’ recommendations. I find it easiest to do this with the help of a friend by placing one finger under each wing tip and lifting the model.  I find that using a finger under each wing tip is sufficiently accurate for the first flight as final adjustments can only be made after the model has been flown and trimmed.  It is always better to try and shuffle the radio equipment around to alter the centre of gravity rather than adding lead to the nose or tail.  If this is not possible then adding lead as far from the centre of gravity as possible will add the least amount of weight.  Often forgotten by many modellers but it is also important to check lateral balance by lifting from the spinner (with the crank positioned away from the compression stroke so there is no resistance if IC) and the rudder.  It is usually necessary to add a small amount of weight to one wing tip.  This can be done by attaching a small amount of lead or a inserting a nail of the correct weight into the wing tip.

Next to check are the incidences of the wing and the tail plane.  The wing should be set to between 0.5° and 1° positive.  This is usually best done with an incidence meter but can be done with some accurate measuring on a flat bench.  The tail plane is normally set at 0° but refer to the manufacturers’ instructions for the best staring point.  If you have stabiliser incidence adjusters installed, the tailplane will finally be aligned with the elevators after flight trimming".

Here a 'Robart' incidence metre is being used to set the wing at 1º and the tail plane at 0º

During the build it is essential to check the distance from each wing tip to the trailing edge of the elevators, it should be the same one each side.

Not a bad idea to make sure the wing and tail plane alignment is correct as well.

 

Although this is aimed at an F3a pattern aircraft there is no reason why it couldn’t be applied to your latest sports model or scale warbird.

A basic trimming chart can be found here (Link to Trim Chart)  but you may want to look at Bryan Hebert's (USA) thoughts on the triangular method of trimming which has worked well for many international flyers: www.hebertcompetitiondesigns.com/Triangulation.aspx?id=3

It is not often we can see the reasons for some of the trim settings necessary on our aircraft.  The following photograph taken from the excellent www.airliners.net  web site gives some indication of one of the normally unseen forces we have to contend with in trimming.  This photograph is reproduced with the kind permission of Michael Balter, Professional Aviation Photographer. The need for engine side thrust (if it was a single engine aircraft) is clearly visible.  Due to high humidity levels the prop swirl over the top of the engine nacelles can be seen here.  On a single engined aircraft this swirl around the fuselage eventually hits the large area of the fin and rudder on top of the fuselage to create unwanted yaw  which is counteracted on most models with engine side thrust. (I have heard of one successful top pilot who uses electronic mixing, rudder to throttle mix, on their radio gear to counteract this but that is unusual).   

This photograph has been reproduced with the kind permission of Michael Balter (Click here for the link to Michael's web site)      

It has often been said that side thrust is used to counteract engine torque taken from the fact that any force is equalled by the opposite reaction.  (The propeller turns one way the model tries to roll in the opposite direction).  Therefore any change in heading due to engine torque can only be a secondary reaction to the induced roll which may be caused by engine torque in minute amounts under some circumstances. Logic says that any roll to the right (with an engine turning clockwise, viewed from the back) has to be corrected by aileron or be balanced with the size of the fin and rudder.  Yaw cannot be induced by engine torque, however the propeller wash over the top of the fuselage eventually hitting the fin and rudder will produce yaw which can to some extent be corrected by side thrust.  I say to some extent because any side thrust can only fully counteract and match the yaw with a given propeller at one RPM setting. It is likely that the nearest anyone will get to perfection in this area is an F3A aircraft designed and balanced to specifically eliminate these forces, but some rudder to throttle mix may still be necessary.  The swirl created by propeller wash on the fin and rudder may well help to counteract the induced roll caused by engine torque, proving that design and trim of an aircraft is a balance and a compromise of many components. (if anyone would like to add to this statement please email the pro@gbrcaa.org).

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