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|For a long life span, a model must be well built, well flown and well maintained (click small images to enlarge & get more information).||To minimise slop, it's important that the hole in the servo output arm is no larger than necessary (click small images to enlarge & get more information).|
The instructions should specify the amount of control surface movement necessary. Some modelers increase these throws, reasoning that more control is better than less. However, in my view this is a mistake. Increased throws may well mean an over-sensitive model which is twitchy and difficult to control accurately, especially when landing. I have been reading model reviews for many decades, and I do not ever recall hearing about a model that crashed due to insufficient control authority. However, many, many models have been crashed due to having too much control movement!
RC system wiring should be arranged so that it is as far away as possible from the wiring of the electric power system. It's also worth making sure wires from servos, UBEC units and so on are not loose and able to move around in flight, because eventually loose wires can break.
|Excessive control surface movement makes models more difficult to fly accurately and harder to land with precision.||Wires from ESCs etc must not be allowed to flop about. This UBEC wire is safely secured at both ends.|
Total servo movement
The maximum amount servo movement i.e. the amount of angular rotation can be adjusted from the usual 100% at the transmitter. For example, my JR transmitter allows me to set anything from 0% up to 150% of normal servo movement.
For controls which have too much movement it can be very tempting to reduce the overall servo throw using this function of the transmitter. However, there are two disadvantages to doing this:
1. Servo power is effectively reduced when the total servo movement (termed travel volume by Futaba) is reduced. Also, for a given control input, the servo has to work harder.
2. As the total servo throw is decreased, the proportion of play, or slop, in the control system is effectively increased.
If the amount of control throw needs to be reduced, it is better by far to accomplish this by adjusting the physical geometry of the model’s control system. If a control has too much movement, reposition the linkage so that it attaches to the control arm of the control surface a hole further from the surface itself. The same servo movement will now translate to a smaller control surface movement.
Alternatively (or even as well), you can reposition the control linkage to connect to the servo's output arm using a hole nearer to the centre of the servo. This will provide less overall travel for the surface.
Applying this practice reduces control surface slop and maintains effective servo power. Ideally, the travel volume function should only be used to make small adjustments that cannot be made by using different holes on the servo arm of control horn.
Make sure that with the transmitter stick at full deflection, the linkage is not binding on the servo. A buzzing servo is a give away that this is happening.
|The amount of control throw should be as much as is required, but no more. A common beginner mistake is setting controls up with too much travel.||Differential throw is often a great idea for ailerons. A 2:1 ratio (more up than down) is a good starting point.|
It is usual to set elevators up with the same amount of up and down movement. Make sure the total movement is not excessive, as this can make an accurate landing flare difficult to execute. Adding some exponential (explanation below) may be useful to desensitise the control response around neutral, which may help with landing.
Plenty of rudder movement is often necessary to allow sufficient control authority both for taxiing and also for stall turns. However, the control must not be so sensitive that smooth, straight take offs become difficult. The addition of expo can be of particular assistance for the rudder control in this regard.
|The clevises on this model have yet to have keepers added.||All hardware must be fit for purpose. Replace any items which are not of good quality.|
When any control is deflected, it will generate drag. This is of particular importance for the ailerons because the down going aileron is a particularly prone to generating drag, and this can cause a phenomena known as adverse yaw. For example, suppose we want to roll the model to the right. A right aileron command will mean the left aileron is lowered and the right one is raised. Because the left hand aileron is going down, it will generate more drag. If the left hand (down-going) aileron produces a large increase in drag compared to the up-going one, an airplane could have a tendency to yaw (nose swinging to one side) to the left - this is the opposite direction to the intended direction of turn.
Often, an effective solution for this is to adjust the aileron travel so that the down going aileron has a significantly reduced travel compared to the up going one. This is known as differential aileron and it works because the additional drag caused by the down-going aileron is reduced.
Adverse yaw affects full size aircraft as well, and differential aileron is more likely to be necessary for models (and full size airplanes) with high aspect ratio wings, such as gliders and certain light aircraft like the Piper Cub. Setting up ailerons in this way carries an additional bonus in that tip stalling becomes less likely in low speed situations such as take off and approach, making for safer flying. I almost always set my own models up with differential aileron travel. A ratio of 2:1 is a good starting point. To maintain servo power, it's best to arrange for differential aileron to be provided geometrically rather than by electronically limiting travel in one direction.
It’s worth noting that for good quality balanced turns, all aircraft including models need a combination of aileron and rudder. Generally, the higher the aspect ratio the greater the proportion of rudder that is required.
Many full size aircraft are designed with differential aileron movement, so the value of this is well established. For example, the Tiger Moth has much less down-going aileron than up.
Differential movement reduces the likelihood of a model suffering from tip stalling during flight, especially on take-off when airspeed is low. The only disadvantage to setting up ailerons with differential movement is that the risk of tip stalling is increased during inverted flight. However, the model is very unlikely to be traveling slowly enough while inverted for this to be a problem. As long as inverted flight is not carried out slowly or with large amounts of aileron movement, there should be no adverse effects. In any case, the amount of differential aileron should not be any more than is needed.
One final point on ailerons - if you have a model with more down aileron than up, this is likely to pose a serious risk to your model's health. This condition should definitely be corrected before trying to fly the model.
|Exponential can be used
to soften or desensitise the model's control response
around the neutral point.
||Check your model carefully - it's easy to miss a small detail. Can you see the missing parts?|
Modern computer radios have many features which make radio set up much easier than it used to be. Exponential, or expo for short, is a particularly useful function, and this is usually used to soften or desensitise or soften a control around the neutral point. I usually start with around 25% expo on all surfaces.
Note that expo can be set positive (+ve) or negative (–ve), and if this variable is set up in the incorrect sense, the control will become more sensitive around the neutral point, not less, making the model much harder to fly accurately.
Checking the sense of expo
The expo setting to produce a softer neutral is termed +ve by some manufacturers (e.g. JR) and –ve by others (e.g. Futaba). To check the sense of the expo I have set up, I like to temporarily ramp it up to a very high value e.g. +95 % as a check. If the control then becomes very insensitive around the neutral point, this confirms that + is the correct sense for a softer neutral. The expo is then returned back to a sensible value, perhaps +25%.
However if the control became very sensitive around the neutral point and less so as the stick was moved further away from neutral, this would tell me that +ve expo was incorrect and I would know to select –ve expo for softer neutrals. I would repeat the test going all the way to -95% to confirm my conclusion. I would then assign a suitable –ve value of expo.
It's worth spending a little time test flying a model and making adjustments to the controls so that the response of the elevator and aileron controls feel similar. A model will be unpleasant to fly if for example the elevator is much more sensitive than the ailerons.
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