Performed correctly the stall turn is a very pretty manoeuvre and has, perhaps surprisingly, numerous formats. First we’ll take a look at the basic manoeuvre, which is usually performed at one end of your flying space (or ‘box’) with the top of the model facing you throughout.
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As with all aerobatic manoeuvres proper entry is crucial, beginning straight and level at a steady speed (not full chat) with all the controls being used as required to keep the model as straight as an arrow.
Begin to pull back on the elevator, gently at first for the first 5 – 10° radius of the corner, then tighten it up with a little more elevator through the next 80° and soften off again for the last part of the pull to finish in a vertical heading.
The corner needs to be smooth but properly defined. Too soft and you can’t clearly determine the start and end, and the corner might be a bit big. Too hard and fast and the corner looks tight and rushed with the increased probability of a poor vertical line. Perfect this to a nice, straight vertical ascent – it’s a crucial part of the finished manoeuvre. Look to add in some extra power once you’re about 1/2 – 3/4 of the way around the corner to give continued authority in the vertical climb.
Using power, and all of the control surfaces, try to sustain a nice clean, straight, vertical flight path – a line that you can definitely see and that separates the corner from the stall turn itself. Remember that the wings must be perfectly level on entering the corner; use aileron to keep them that way if necessary. If you don’t the vertical line will be offset one way or the other from the very start. Take account of weather conditions too; the flight path relative to the attitude of the model and if you’re doing this manoeuvre against a strong wind, it may be that you need to adjust the corner so that the model has a slight angle against the wind such that it doesn’t look vertical. This doesn’t matter; it’s the vertical trajectory of the C of G point that you’re interested in. Failure to get this some way right causes the model to be blown downwind – particularly when it slows approaching the top.
There are various ways to perform the turn, indeed, the accepted method of execution in F3A comps has changed over the years – ranging from a turn that virtually pivots around the centre of the fuselage to nigh-on a wing-over.
On approaching your planned height for the turn (all the time keeping it straight, particularly on rudder – don’t forget the wind compensation) come off the power. The transition must be quick, smooth, but not all the way to idle. Some people pull up to the vertical and then bang to idle as if flicking a switch. I don’t think this sounds nice, or looks right, and you end up making life more difficult for yourself as far as an accurate turn is concerned.
So, ideally, we’re at about 10% power, and you should be clearly able to see the model coming to a stop in the vertical – neither too quickly nor too slowly. A second or two before it stops introduce your chosen rudder direction, gently at first. This will prepare the model for a nice turn. If it’s not been kept straight properly on the way up, then make the rudder direction the same as the bias that already exists on the flight path to avoid it going bad. Properly entered this vertical section might show up side-thrust inaccuracies in your model, too.
Bring in the rest of the rudder smoothly and quickly. The model should rotate at the top, and as it does so, bleed off that last 10% of power down to idle. As the model rotates through 90° (9 o’clock) start to reduce the rudder input and, depending on the model and it’s fin size, potentially take off all the rudder by the time you are at 120 – 150° – (8 – 7 o’clock), possibly adding a little amount of opposite rudder and a few blips of power before getting to 180° (6 o’clock). This pre-emptive correction helps to avoid oscillation and tail wagging at the end of the manoeuvre, and gives the model a clear downward direction. It also helps to finish the turn as smoothly as it started. The fact is, all models behave a little differently in this manoeuvre and you should do what works for you and your particular aeroplane, but all the time aim towards smooth and wobble-free rotation between perfectly straight, vertical lines. Now, the size of that turn…
As a guide I would say that the width of a stall turn should be about half the wingspan, i.e. so the fuselage comes down the line made by the wing tip on the way up (Fig. 1). As mentioned earlier, some rotate the model through 180° and come straight back down the line they went up… this is more difficult to achieve unless you have a short-coupled model (i.e. short distance between centre of gravity and tail). If you rotate wide around one wing tip it’s arguably a wing-over rather than a stall turn, and to achieve that you must have either held power on or started the turn too early. My preferred method is as mentioned – half a wingspan.
You might need to add a little aileron or elevator to keep the model vertical and straight throughout the manoeuvre; with the motor at 10% power instead of idle there’ll be a little more authority in control without there being enough power to pull the aircraft off course, and there’s also less difference to make up if you need to blip the throttle (gently) to encourage it over.
Remember the wind problem? Taken to extremes, and in order to make a point, you may find yourself pushing the nose against the oncoming wind as you point skyward, indeed, at the top when the power drops, the model could have an attitude relative to the vertical ascent of getting on for 45°. If you then use only rudder to rotate it, the model will end up heading back towards the ground at 45° from the vertical, but down-wind. In order to avoid this situation, you need to juggle the aileron and elevator through the turn to ensure that the nose is facing into wind at 45° from vertical, only this time, of course, your pride and joy will be heading back to mother earth. Hopefully Fig. 2 will clarify this somewhat!
Side-thrust is the offset built into the fuselage at the firewall, or induced with spacers behind the engine mount, to tilt the motor to the right (viewed standing behind the model). This is done to counteract the tendencies the model has to pull left as a result of torque from the motor. There are lots of ways to test whether or not you have too much or too little but one of the most obvious areas that this shows up is in vertical lines, i.e. during a stall turn. You might notice that without any rudder correction the model tends to track left or right up that vertical line. This may be an indication that you have too much or too little right thrust. If it’s very noticeable then it’s probably worth making adjustments, otherwise (remember you’re not competing) don’t worry too much, you’ll just have to use rudder on the way up to correct the situation. Be sure that you’ve entered the line with wings level when doing this test – if you don’t the model will head off in the direction of the low wing… not the same as a side-thrust problem.
As mentioned earlier, the stall turn can be interpreted in a number of ways. Experiment with your chosen style and what your model is happiest doing. A turn induced as the model begins to tail slide can be nice, achieved by delaying the introduction of the rudder.
A wing-over can be used where you don’t allow the speed to decay so far and then confidently power the model over the turn with rudder and increased throttle. Or, of course, you can have a go at trying to get it to pivot around its centre.
You can start to use the stall turn as a centre manoeuvre, pulling up as before but this time so that the vertical line is in front, dead centre. Add a 1/4 roll on the way up to make the top face you, stall turn, then 1/4 roll on the way down to pull out on the original heading.
To perform this manoeuvre, as intended by the FAI rules, the 1/4 rolls need to be in the middle of the vertical lines. You can make it a little harder by 1/4 rolling the other way so the underside faces you, or perhaps a full 1/2 roll up and down. Do two of these in a row and you have what’s known as a ‘Stall M’.