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Setting up your model

Article by Andrew Gibbs


This part of the guide is particularly important, and offers a few pointers designed to maximise your chance of success. Depending on how they’re set up, two apparently identical training models can fly very differently, one being very easy to control yet the other very difficult. Let’s have a look now at some of the most important issues which can affect how easy a training model is to fly:

Set up issue: Balance point
Correctly balancing your model is vital, and plenty of models have been wrecked on their first flight because this issue was overlooked. You should ensure your model assumes a slightly nose down attitude when supported at exactly the position indicated in the instructions. This will usually be somewhere near 25 - 33 % chord position, i.e. about one quarter to one third of the way back from the wing’s leading edge.

Often, the flight battery can be moved to achieve the correct balance position (sometimes called centre of gravity, C of G or CG), but if this doesn't do the trick, weight will have to be added to nose or tail as appropriate. Add as much weight as needed to achieve the required balance position, even if you don’t like it! If the required additional weight seems excessive, you can always consider moving other items such as servos to new positions.

As the saying goes, heavy models have to fly a little faster, but tail heavy models won't fly for long at all! A model that has too rearward a balance point may be unstable and will be more responsive in pitch (elevator), even with limited control throws. If the balance point is sufficiently far rearward it will be extremely difficult to fly, even for an expert pilot. Conversely if the balance point is too far forward the model will again be difficult to fly and may lack sufficient elevator response.

Set up issue: Lateral Balance
It’s also a good idea to check the model’s lateral balance. To do this, ideally the model should be checked when fully assembled. As an alternative, the wings can be removed from the model and checked to ensure one of them is not heavier than the other. This method doesn't work so well with i.c. powered models which can have the relatively heavy engine installed with the cylinder well off to one side. If required, add weight to the light wing tip. Firmly secured small nails can work well for this purpose. Batteries can also be positioned off-centre to reduce the need for any additional tip weight.

Set up issue: Straight and true
A 'bent' model will never fly properly and this can add enormously to the difficulty in leaning to fly, so it’s really important that your model is assembled with its flying surfaces straight and true. Time invested to ensure your model is well built, especially in this regard will be time well spent.

The wing must not be accidentally warped. You can check for warps by sighting along the wing from each tip to the root (centre). No twisting should be seen, in other words the wing tip should be at the same angle as the wing root all the way along.

However, there’s one possible exception to this, and this involves a type of warp that is acceptable and even beneficial. This is known as ‘wash out’ and is present when the wing’s trailing edge can be seen to be slightly twisted upwards towards the tip. This gives the wing tip a lower angle of attack than the root, which can improve the model’s stall behaviour. The design of some models deliberately incorporates wash out.

Unintentional wash out, provided it’s not too severe can usually be accepted. In either case, both wings must have an equal degree of wash out. The opposite, ‘wash in’ is totally unacceptable and must be corrected before the model is flown.

Tail surfaces
Tail surfaces should also be unwarped. These are generally made from sheet material and usually don’t warp significantly. Surfaces should be at right angles, both to each other and to the fuselage. Small errors here are undesirable but they can be tolerated.

Set up issue: Control surface centering
Make sure surfaces are set truly at neutral when the transmitter sticks are centered, with the trims at neutral. This can be checked using a ruler or with straight lengths of balsa placed between the tail and the control surface in question.

Set up issue: Control surface movement
One of the classic ‘gotchas’ with model flying is to provide too much movement of the control surfaces, the usual reasoning being that more control must be a good thing. The reality is that training models need very little control surface movement, and that crashes, especially at the early stages of learning are much more likely to be a result of over-controlling than from insufficient control authority. In over 30 years of flying RC models I’ve never experienced or even heard of a crash resulting from insufficient control movement. Remember that free flight models fly perfectly well with no pilot at all controlling them! This is actually an important point to appreciate – a correctly trimmed model (or full size) aeroplane will almost fly itself, and will require very little pilot input.

The aim of early flights is simply to gain flight time without breaking the model, so tight manoeuvres requiring large control surface deflections have absolutely no part to play. As a guide, set the control surface movement exactly as the instruction manual for your model suggests. That said, in my opinion, sometimes the suggested control throws are substantially more than actually required. You’ll have to be the judge, but for a correctly set up model probably only 10-15 degrees of movement will be plenty for some models.

Set up issue: Control surface direction of movement
Make absolutely sure that the controls operate the correct way. Many full size aircraft have been lost because of this seemingly obvious matter.

With the elevator stick pulled towards you, the elevator should move up, and vice versa.

When right rudder is commanded at the transmitter, the rudder must move to the right, as observed from the tail of the model.

When right roll is demanded, the right aileron must move up, and the left aileron downwards. It can be said that when standing behind the model (imagine yourself in the cockpit), 'the aileron must travel up to meet the stick'. The ailerons are particularly easy to get wrong, so take extra care.

Set up issue: Exponential
If your RC outfit has an exponential function, this can be usefully used to soften control responses about the neutral point, for example by adding perhaps 30-50% exponential to rudder, elevator and aileron throws. Make certain that you have set these set up in the correct sense. If you get this the wrong way around the model may be very hard to control indeed!

For example, softened responses around the neutral stick position result from using positive (+ve) exponential with JR and Spectrum, but to get the same result with Futaba negative (–ve) exponential must be used. In both cases the 'expo' function works in the same way. The differences result only because each manufacturer uses their own convention. For those transmitters with a graphical representation of control response, simply make sure the flat part of the graph corresponds to the neutral stick position. In any case, its well worth carefully examining the control response by observing how the surfaces move in relation to the stick commands - do this before flying!

The rudder and elevator surfaces seen here have been carefully checked to make sure they line up exactly with the corresponding fixed surfaces.

Reduce 'slop'
To keep control movements reasonable without introducing excessive ‘slop’, make sure you use the outermost hole on the control surface horns, and one of the inner holes (as required) on the servo arm. For the same reason, take care not to make the holes in the servo arms and control horns any bigger than absolutely necessary.

Only after limiting overall movement in this way should you use the transmitter's travel volume function (ATV for Futaba) to reduce it down further if required.

Set up issue: Appropriate power
Training models need to be appropriately powered – too little power will make the model hard to keep airborne, while too much will make learning to fly much more difficult for a beginner than it needs to be. Training models are often fitted with much more power than they actually need. The usual answer to this criticism is that the engine can always be throttled back, but timely and appropriate throttle management is a skill that only experience can provide, which is something the beginner is very short of. Excess power in a model produces a number of side effects that the beginner will simply not be well equipped to deal with and which will make control much harder compared to an ideal model. These include the following, any of which could allow a model to 'get ahead' of the pilot flying it:

1. The model will be faster.
2. The rate of climb will be higher (perhaps much higher).
3. The control responses will be sharper due to higher airspeed and because of the high speed prop wash over the tail.
4. The asymmetric forces resulting from prop rotation will be higher. Engine side thrust may not be enough and the model may difficult to control during take off.
5. The model may have a strong tendency to pitch up sharply under full power, possibly leading to a stall.

In short, excess power can transform a pleasant handling training model into a high performance machine requiring a high level of expertise and experience to handle successfully. Full sized basic training machines are always modestly powered and in my opinion RC training models should be as well.

Pleasingly, the issue of power is an area where electric power has an advantage – simply by fitting a smaller prop, the maximum output power of an electric motor can be reduced at will. So, choosing a larger than necessary motor for an electric training model can indeed be a sensible decision, provided that the model still remains sensibly powered by an appropriate choice of prop. Depending on the model type and power system efficiency, as little as 40 - 50 Watts per pound of model weight may be sufficient, and less may actually be better for the trainee pilot.

I’ll conclude this section with a specific recommendation - a training model is properly powered for a beginner when it will climb at a moderate rate at full power with the model in an appropriate state of trim, i.e. at not too fast an airspeed – just like a full size training aircraft.

Set up issue: Undercarriage (gear) and wheels

Whether trainer or otherwise, the undercarriage must be adjusted if necessary to ensure good ground handling.

It’s important that the ground handling of your model is good, otherwise every take off will be fraught with difficulty. All the wheels on your model must turn freely otherwise it may tend to veer towards the sticky wheel. You can expect to have to add a little right rudder on take off under high power conditions due to prop effects. However if the model won’t roll straight on the ground even at low power then the undercarriage will probably need adjusting. Don’t adjust the rudder control for this purpose, otherwise it will be permanently deflected in flight and cause other problems.

This model has been fitted with large
'Tundra' tyres.

Trainers sometimes benefit from having oversized wheels fitted. These will allow the model to handle bumpy ground more easily. Some full size light aircraft have enormous ‘tundra tyres’ fitted for this very reason. The extra aerodynamic drag of large wheels can be considerable and this will also help to keep the speed of the model down in flight.

Set up issue: Installing the RC gear correctly
This subject is far too large to cover in detail in this short article, but a few of the most important areas can be covered.

Ideally the receiver will be wrapped in foam to protect it from vibration. This precaution particularly applies to non - 2.4 GHz equipment. In any case, make sure the receiver is not too close to the ESC and any associated power system wiring. Any UBEC unit (separate BEC) if fitted should also be distant from the receiver and RC system wiring.

Receiver aerial
The receiver aerial must be undamaged and stretched out to its full length, otherwise range may be compromised. If your receiver has a long aerial (i.e. non - 2.4 GHz systems), try to incorporate a vertical element, for example routing it to the top of the fin as this will help improve reception. Never run any aerial along or near to a metal or carbon fibre component of significant size.

Wiring must be arranged so that plugs and sockets cannot come apart in flight. Include any extension leads in your checks and remember that vibration and G forces will be present.

Keep RC wiring physically separate from the wiring or the power system – at least 50mm (2 inches) is ideal. This minimizes the chance of the power system causing interference to the RC system.

This servo has been glued in position using a dab of hot glue.

Where possible, servos should be installed with their rubber grommets and screws. Some models call for servos to be installed directly into moulded foam cavities, in which case this recommendation can be ignored.

Set up issue: Installing the power system correctly
As with the subject of RC installation, this subject is far too large to cover in detail in this short article, but a few of the most important areas can be covered.

Motor installation
The motor should be securely mounted. It must receive a plentiful supply of cooling air. Ensure the prop driver is a firm, secure fit on the motor shaft and that the prop nut is secure. The prop itself must be balanced.

ESC installation
The ESC should also be assured a plentiful supply of cooling air. Don’t wrap it in foam as it needs airflow on all sides. The ESC will emit radio interference, so Keep it spaced well away from the receiver if at all possible. ESCs can be successfully mounted on the outside of a fuselage – an ugly but practical solution.

This LiPo has been securely retained using a strap.

Battery installation
Make sure the battery is securely retained so it cannot move in flight. You'll need to arrange for a supply of cooling air to reach the battery. LiPo batteries are physically a little delicate, so another idea worth considering is to position a chunk of stiff foam such as polystyrene in front of the battery to absorb some of the impact of a heavy landing.



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