Over recent articles we’ve covered a number of basic aerobatic manoeuvres in detail, and before moving on I thought it a good idea to recap on some of the key points and look a little more at a subject that's equally important – model set up.
When taking a closer look at the loop we talked a little more about accurate positioning and compensation for the wind, as well as effective use of throttle and introduction of the rudder control. Similarly, when moving on to consecutive rolls we broke the manoeuvre into manageable zones, making more use of the rudder control and realising the importance of mastering a particular manoeuvre such that it can be performed comfortably in any direction. Hopefully, the advice given thus far has helped improve your aerobatic capability.
MODEL SET UP
Having top-notch equipment is not the be-all and end-all, but a well-built, true model is key. So, even if we’re flying an average Sunday sport model, there’s a lot that can be done to improve it. Let’s take a look at some basics, which apply as much to ARTFs as they do to kits and plan built aircraft.
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Hinge Lines: All too often big gaps are evident along hinge lines. This is either the result of inferior build / assembly, the selection / provision of inappropriate hinges or, in some ARTF cases, poor manufacture. The bottom line is simple – we’re looking for bind-free, easy moving surfaces with zero gaps. If you have gaps along the hinge line then fill them using film or diamond tape on the underside with the surface at maximum deflection (away from you) as you stick it in place. If the gaps were quite large before you taped them up, be prepared for a significant improvement in control response the next time you fly. In this respect, it would be a wise precaution to set the rate switches, just in case!
Geometry: No matter how clever your radio or how expensive your model, there’s no substitute for proper mechanical geometry. In other words, a computer radio is not a substitute for alignment and build accuracy. The ‘trickery’ of such radio is used to enhance the performance of a good model, not create it from a bad one.
For each control surface on an aerobatic design (rudder, elevator and aileron, where you want equal travel in either direction) aim for a collection of 90° angles on the control rod set-up, the most important of which is at the servo. Let me explain…
With the servo and control surface centred, the angle of the servo output arm to the control rod should be 90° when viewed from above. Equally, for an aileron (viewed in cross-section), the control rod should be at 90° to an imaginary line drawn perpendicular to the centreline of the wing section, though this is a little less critical unless you get into 3D, where huge control throws are the order of the day.
Another key point is the provision of straight control runs. Ultimately snakes, or similar, are out and any kind of bend in the rods is a no-no; also you shouldn’t be able to waggle the surfaces by hand to any great extent without moving the servo in the process. Get the basic geometry right and achieve the required throw with a combination of output arm size, position of the control rod on the control horn and ATV settings. Then, you should be close to equal control deflection without differential.
FLIGHT TRIMMING
With all the control surfaces properly centred and no significant twists in the flying surfaces a model can still experience side effects to given control inputs in the air. This can be down to a design problem, balance, weight, thrust lines, incidence and a whole lot more. However, there are some easy checks and remedies that will make a difference; let’s take a look at a couple now and come back to others later in the series.
Centre of gravity: How do you know what’s right? Next time you fly, position the model carefully and roll inverted for a straight run along the patch. There will be variations in the response depending on whether or not you have a symmetrical or semi-symmetrical wing section, but you’ll generally find this rule of thumb works: If a good amount of down elevator is required then the C of G is probably too far forward – add some lead to the tail. If you don’t need much elevator at all or the model wants to climb away inverted then the C of G is too far aft – add some lead to the nose.
Tip weight: Most models will have a heavier side, if for no other reason than the fact that the cylinder head or silencer is bias to one side. It’s good practice to balance the model in the workshop before flying but this isn’t always the complete solution as a statically balanced model isn’t necessarily dynamically balanced, i.e. when in flight there may be other forces at work.
Carefully trim the model for smooth and straight upright flight, fine tuning the aileron trim if required. Roll the aircraft inverted, wings level, and let everything go again. If the left wing drops add weight to the right tip, and vice versa. If it stays on track then you’re in pretty good shape. Another test for fine-tuning is to pull a tight loop and a tight bunt. It’s important to enter either manoeuvre level and use elevator only to perform the figure. If the model comes out of the loop with the right wing low you may need to add weight to the left tip, and vice versa. The same applies to the bunt. Perform this several times to be sure and, of course, if you have lead on the wrong tip the first thing is to remove that before adding it to the other.
SPIN
Finally to the spin then. The spin is a simple manoeuvre that, despite its simplicity, can scare pilots a little. The ‘ordinary’ spin requires the use of all four controls at the same time; in essence the model should be spinning when both transmitter sticks are in the bottom left or bottom right corners of the box. Starting the spin is supposed to be done from a stall, but how?
Flying along at a good height, reduce the throttle to idle and keep steering on track – you’ll need to steadily bring elevator into play to maintain the height with the absence of power, causing the model to slow up to a stop. As it does, introduce a little rudder in the direction that you want to spin. This should influence the direction in which the model finally stalls, and it should drop that wing. When this happens, throw the Tx sticks into the relevant corner that you chose by early rudder input and watch. When you’re done, centre the sticks, leaving throttle at idle. The model should undo itself within a turn (within a quarter turn if the C of G is right), and all you need do then, is simply pull out.
When you’re happy, try to perfect the entry and cause it to occur right on the centreline. Decide on how many spins you want to perform and practice stopping at the right time so that you end on a whole turn with the top of the plane facing the direction it entered. Finish the manoeuvre with a short vertical descent and pull out neatly onto a base line of your choice (e.g. at the height for starting a loop) before re-applying power.
