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Prepare for Slope Aerobatics | Back to Article Index

Radio Control Model World - Nov '95

by Stan Yeo

INTRODUCTION

During the non-flying periods of my trips to the slopes I regularly lapse into my 'Desmond Morris' mode (you know, him of the Naked Ape and Human Zoo fame) and observe the way people fly their models. This is quite enlightening as it is often possible to highlight areas of weakness in a modeller's flying skill and predict the type of accident that will befall them (watch out Stans about!). This is not always the case as some models demand so little of the pilot, except on landing, that it is difficult to form an opinion. As a result of these observations I have come to the conclusion that most modellers fly very conservatively, or at least the ones I meet. Very few flyers 'throw' their models around as if there were no tomorrow. Most are content with stooging around and doing the occasional loop and the odd roll. This is in no way meant as a criticism of this style of flying, each to his own. The object of the exercise is to enjoy ones-self and I have spent many a happy hour cruising up and down a slope in light lift on a balmy summer's evening. But when the lift is good I often ask myself why is it that modellers are not a little more adventurous in their flying. Is it because they have no interest in aerobatics or is it, as I suspect, a lack of confidence to have a go, after all nobody likes to take their model home in bits. Whenever I bend a model two thoughts race through my mind. One, how am I going to repair the model so that it is still presentable, and two, how am I going to find the time to carry out the repair?

Unfortunately 'conservative' flying does little to improve overall flying skill or increase confidence in your flying ability. This was brought home to me a few years ago when I took up full size gliding again after a lapse of almost 30 years (I learnt to fly in the ATC on T31s and Sedburghs). I was never a confident flyer and in fact I gave it up originally because someone scared the living daylights out of me on a check flight, consequently the things that frightened me then came back to haunt me again. Spins in the 1960s were not a compulsory part of the ab initio training but they are now so the closer I got to re-soloing the more apprehensive I became until one day when I could put it off no longer I bit the bullet so to speak. After doing a couple I was left wondering what was all the fuss about. Now at the end of a good soaring flight, if I have height to burn off, I will do a few spins, not only does it keep me in current practice but it gets the blood circulating before I attempt a landing! The same applies to performing aerobatics with a model, it develops flying skill and builds confidence. It could be the difference between staying up in marginal conditions to ending up with a pile of bits or not flying at all.

WHERE TO START

The first point to consider before attempting to fly aerobatics is the safety aspects of models, people and property. If the sky is crowded or there are a lot of people wandering around or the conditions are marginal it would be prudent to postpone the 'adventure' until conditions are more favourable. After all you do not want to 'bend' your model let alone hit someone with all the consequences that may bring.

Points to remember to remember before carrying out aerobatics are:

1. Have you got sufficient height?

2. Is the model structurally sound and capable of carrying out the manoeuvre?

3. Where are the other models?

4. Have you made sufficient allowance for the model being blown back on the wind? i.e. is the model far enough forward.

5. Have you learnt the sequence of control movements required to carry out the manoeuvre? Practice them at home with a friend holding the model and simulating the manoeuvre in response to your control inputs.

6. Plan the manoeuvre so that the model exits the manoeuvre pointing into wind. If the model does not exit the manoeuvre into wind it is likely there will be a panic control input to turn it into wind away from the slope.

7. Where possible have an experienced aerobatic flyer on hand to talk you through the manoeuvre and highlight areas of weakness.

8. Always remember what manoeuvre you are performing and the stage the model has reached. This may sound a bit fundamental but you would be surprised how many flyers forget when things start to go wrong and let 'instinct' take-over with often disastrous results.

CARRYING OUT THE MANOEUVRE

there are four distinct stages to performing an aerobatic manoeuvre:

1. The preparation.

2. The dive.

3. The manoeuvre.

4. The recovery.

Preparation

This consists of carrying out all the necessary safety checks, some of which are mentioned above and positioning the model in the right place in the sky to carry out the manoeuvre. Positioning is the most important part of any aerobatic manoeuvre. If it is not right at the start of a manoeuvre then it will get progressively worse during the manoeuvre. Most aerobatic competitions, even at club level require that the manoeuvres are carried out crosswind i.e. along the face of the slope. Into wind loops do not score points! Initially flyers will find this difficult so until you are confident in performing a manoeuvre into wind do not attempt it crosswind. Some may disagree with this advice but the object of the exercise is to practice the manoeuvre and keep the model in one piece. Into wind manoeuvres do not look as pretty as those carried out crosswind but they do reduce the risk of a serious accident.

The ideal position to start most manoeuvres is with the model straight in front of you, at an angle of approximately 60 degrees to the horizon and at a height 150 - 200 feet (50 - 65 metres). This is of course a subjective measurement as it is very difficult to judge a models height without some form of external reference. The 60 degree angle to the horizon is the critical yardstick as it helps to ensure the model is a sufficient distance away from the slope to carry out the manoeuvre.

The Dive

Once the model is in the correct position it can be dived to build up speed to carry out the manoeuvre. Diving was discussed at some length in an earlier article ( July '94 RCMW) but essentially the most efficient way to build up speed with the minimum height loss is to dive at an angle of 15 - 30 degrees. The more efficient the model the shallower the dive angle. Diving at an angle greater than 45 degrees is definitely not a good way to build up speed as so much energy is lost during the recovery that often there is insufficient left to carry out the manoeuvre properly. Most models need to be dived for at least 4 seconds before they have built up sufficient speed to carry out the manoeuvre. The dive angle and dive time is very much dependant on the type of model and the prevailing conditions and can only be determined by experience.

Some manoeuvres such as the roll require that the model be returned to level flight before the manoeuvre is carried out. On some models returning the elevator to neutral is sufficient whilst on others a small amount of up elevator may be required. Again experience will dictate the amount of control required.

The Manoeuvre

At the start of an aerobatic manoeuvre the model has a certain amount of energy in excess of that which it has in normal flight. It is this excess energy that is used to carry out the manoeuvre. If too much energy is used at the beginning of the manoeuvre by erratic or excessive control movements then it is likely that the model will fail to complete the manoeuvre successfully. It will either 'flick' roll out of it in the beginning or fall out of it at the end through lack of speed. A successful aerobatic manoeuvre is one that is performed smoothly with energy used at a uniform rate throughout. Being able to fly smoothly is a pre-requisite of a good aerobatic flyer. It also pays dividends in marginal lift conditions when the slightest hint of over-control would mean a premature arrival.

The Recovery

On completion of the manoeuvre the model must be returned to normal level flight either flying in the same direction or in the opposite direction as in the case of a stall turn. Ideally, for novice aerobatic pilots, the model should finish up pointing into wind, hopefully above slope height! This means that before carrying out the manoeuvre the exit from it should be planned in advance i.e. the model should be correctly positioned at the start of the manoeuvre.

THE MANOEUVRES

The basic aerobatic manoeuvres most pilots would like to master are the Loop, Roll, Outside Loop (bunt), Inverted Flying, Stall Turn, Cuban Eight and the Spin. There are others of course but they are not within the scope of this article (it is long enough already!) As mentioned before my approach to performing some of the manoeuvres may offend some dedicated aerobatic flyers but please remember the purpose of this article is to encourage the inexperienced flyers to have a go at slope aerobatics by minimising the risks.

The Loop

This is the most basic of aerobatic manoeuvres and one that I recommend is initially practised into wind. Position the model as previously described i.e. straight in front, at an angle of 60 degrees and approximately 150 to 200 feet high. Dive the model, straight, wings level, to build up speed, again as previously outlined and then slowly apply approximately 25 to 30% of the available up elevator to smoothly enter the loop. If the model has insufficient speed or not enough up elevator has been applied the model will stall. Recover and try again, this time either building up more speed or using more elevator control as the situation dictates. This time, assuming the model has sufficient speed and you have applied the right amount of up elevator continue with the loop until the model is upside down. When the model is upside down and has just started the recovery dive slowly release some of the applied up elevator. This will help to prevent an inverted stall and to allow the model to build up speed for the recovery to level flight. For the recovery slowly re-apply the up elevator until the model is again in level flight. To prevent the model zooming up into a stall it may be necessary to apply a small amount of down elevator until the excess speed has been 'burnt off'.

Common faults when performing a loop are:

1. Incorrect initial positioning i.e. model off to one side, too low or too close to the hill.

2. The dive is either too steep or not held on long enough to build up sufficient speed.

3. The loop is not entered with the wings level and the model corkscrews off to one side during the loop.

4. Too much up elevator at the start of the loop resulting in either a very tight loop that could overstress the model, a flick roll, or the speed being 'scrubbed' off leaving the model with insufficient speed to complete the manoeuvre.

5. Too much up elevator at the top of the loop resulting in an inverted stall or poor recovery.

6. Too much elevator during the recovery resulting in a zoom climb and stall.

7. Inadvertent introduction of rudder or aileron control when applying up elevator resulting in the model 'screwing out' of the loop.

Once you have successfully looped the model a few times try a loop with a little more speed and less elevator. This will open the loop out and should the model start to screw off to one side there will be more time to apply control correction, that is once you have worked out which way to apply it! Experiment with the manoeuvre until you have got the feel as to how much speed and elevator is required. This will take a few sessions and when you are confident that you have mastered into wind loops try crosswind loops. These are often easier because you can see what the model is doing and there is less wind variation during the manoeuvre. Take care though that you leave enough space between the hill and the model because all the time the model is performing the manoeuvre it is drifting back towards the hill. Remember also, which direction you need to turn to get back into wind after you have completed the manoeuvre.

The Roll

The roll is another manoeuvre, the rudiments of which are best learnt flying into wind. the secret to performing a good roll is to:

1. Have sufficient speed.

2. Recover the model to a level attitude before commencing the manoeuvre.

3. Good co-ordination between the aileron and elevator controls.

To perform a roll first position the model as for a loop but a little lower. A roll does not require quite as much speed as a loop consequently the dive is shorter and can be started closer in. Dive as before to build up speed, recover to a level attitude and apply full aileron. As the model starts to roll inverted progressively apply a small amount of down elevator to keep the nose up during the inverted phase of the manoeuvre. The amount of down elevator required will vary widely from model to model; the less symmetrical the section the more down elevator. Once the model is fully inverted and starts to recover to the upright position so the elevator is returned to the neutral position. Sometimes it may be necessary to apply a small amount of up elevator to stop the nose dropping at the end of the roll but be careful as too much up applied at this point will cause the tail to 'wag'. For initial attempts it is often easier if you start the manoeuvre with a slightly nose-up attitude this way there is often no need to apply down elevator correction.

As you have probably realised the most difficult part in performing this manoeuvre is the synchronisation of the elevator control input with the lateral rotation of the model and this is one reason full aileron control is used during early attempts at rolling. Another reason is a large proportion of models do actually require full aileron to successfully complete a roll.

Minor control input errors will manifest themselves as rear end waggling. More severe errors will result in loss of control. If this happens let the controls go and after a suitable pause recover the model from the ensuing dive. Novice pilots will find it virtually impossible to fly their way out of these situations. It does require a certain amount of height and it may sound a bit drastic but, a common failing among tyro pilots is not to admit defeat in these circumstances and try to fly their way out of trouble, often with disastrous results. Most models when out of control will, if left to their own devices, end up in a dive, recovery from which is usually straightforward. Try it. Position the model at a safe altitude and briskly apply full control on all control surfaces. Hold for a second or so and then let the sticks go. The model may flick roll and start to spin but when the controls are neutralised a dive will result, honest!!

Inverted Flying

Very few slope flyers that I meet fly inverted for any length of time, possibly because very few have aeroplanes the symmetrical sections. It is possible to fly inverted with non symmetrical sections but it requires much better lift conditions. Once again initial attempts at inverted flying are best practised into wind. To enter the inverted position build up a small amount of speed and perform a half roll. As with the roll, slowly apply down elevator to prevent the nose dropping when the model is upside down. The amount of down elevator required will be slightly more than that for a complete roll. Once inverted keep the wings and nose level. This will require a steady hand and the suppression of your natural instincts! Developing the ability to suppress these natural instincts i.e. not to over control is invaluable in tight situations and could mean the difference between an undamaged model and a bag of bits. Incidentally whilst the Rudder and Elevator controls are reversed upside down the Ailerons are not (fortunately).

Should the nose of the model start to 'nod' or should the model sink rapidly release some of the 'down' (up actually!) elevator and allow the speed to increase slightly. The model is at the point of stalling and in a high drag situation. The most common fault when flying inverted is over control, particularly the elevator. To overcome this try flying with reduced control throws i.e. use rates. Recover from the inverted position by completing the other half of the roll, not forgetting to use up elevator on the recovery.

As your confidence increases practice gentle 45 degree. turns left and right, always recovering into wind. During these turns increase the 'down' elevator to prevent the nose dropping i.e. just as you would apply up elevator in a turn with the model the right way up. It is my experience though that slightly more 'up' elevator is required when inverted. Once you have mastered these turns try a complete 360.

If you find the model close to the slope, inverted and sinking rapidly DO NOT PANIC. Gently turn the model into wind and inch it forward away from the hill, avoiding any sudden control movements. DO NOT attempt a panic recovery, it is a recipe for disaster. If you keep your nerve the worst that will happen is the model will land inverted. So what, the forward speed will be low because the model is flying into wind, sink will also be low because the model is still in lift albeit insufficient to keep it airborne. I have been in this predicament a number of times and the most damage the model has sustained is a broken fin. The alternative, a panic recovery attempt, invariably results in a much more severely damaged model.

The Stall Turn

The stall turn is a simple manoeuvre that is sometimes difficult to get right because timing, speed and positioning are all important. A stall turn looks similar to a skate board zooming up a ramp, performing a 180 degree turn at the top before zooming back down again. The manoeuvre must be performed crosswind with the 180 degree turn INTO wind i.e. a stall turn performed from left to right would require LEFT rudder.

Start the manoeuvre a little way out from the slope, off to one side at a height of 50 to 100 feet (15 - 30 metres). Enter a shallow dive to build up a little speed. Pull up into a wings level almost vertical climb, and as the speed drops off apply FULL rudder. The model should, with the aid of the wind (hence the into wind turn) perform a 180 degree rudder turn!! The difficulty in performing this manoeuvre is judging:

1. The speed on entry (not too fast or too slow).

2. The angle of climb (approximately 80 degrees).

3 When to apply full rudder.

It is possible to cheat a little if the manoeuvre is not working out as desired by applying a small amount of aileron at the start of the turn and a 'dab' of down on the exit. The stall turn is a very pretty manoeuvre when performed properly but it does require good judgement and a model with an effective rudder.

The Cuban Eight

We are now getting into the more advanced aerobatics by combining two or more manoeuvres into one. If viewed from the side a Cuban Eight looks like a figure eight laid on its side with a twist in the middle. Hence the eight in the name, I do not know where the Cuban came from perhaps someone could enlighten me. It consist of two loops sharing the same entry and exit that are joined together at the crossover by two half rolls.

Sounds complicated, so how do you do it? Firstly, perfect your cross-wind looping technique because this manoeuvre must be performed cross-wind. Next position the model off to one side, a good distance out from the slope (the model will get blown back during the manoeuvre) at a reasonable height. Dive to build up speed and start the first loop. When the model is coming down vertically during the second half of the loop, perform a half roll away from the slope and start the second loop. Again, when the model is coming down vertically, carry out another half roll into wind and recover, hopefully in the same position as when you started the manoeuvre.

When performing a Cuban Eight there are three main problem areas:

1. The model was incorrectly positioned and or aligned at the start of the manoeuvre resulting in the model getting too close to the ridge.

2. Incorrect speed on entry (normally too slow).

3. Insufficient speed for the second loop because of a poor first loop.

Alignment is probably more of a problem than positioning. The model should enter the manoeuvre wings level and flying parallel to the slope. The speed on entry into the first loop should be a shade faster than that for a normal loop. After completing the first half roll do not 'snatch' at the up elevator on the start of the second loop as this will 'kill' the speed making it virtually impossible to complete it satisfactorily. Likewise the up elevator applied during the recovery should be gradual to allow the model to regain normal flying speed on exit. It is possible to cheat a little by stretching the manoeuvre at the crossover. This will allow the model to regain the speed lost during the first loop making the second loop easier to perform.

The Outside Loop or Bunt

The outside loop or bunt is similar to the conventional loop except of course it is performed below the entry flight line as opposed to above it. The other main differences are the type of model that will perform a bunt and the speed on entry to the manoeuvre. To perform an outside loop satisfactorily a model with a fully symmetrical section is required. Semi-symmetrical or asymmetrical sectioned models will perform outside loops but with varying degrees of difficulty depending on how 'clean' and efficient they are. With flat bottomed sectioned models it is almost impossible. Another factor which affects the quality of manoeuvres is the drag / volume to weight ratio of the model. With a light model it is often difficult to build up enough speed / energy to complete the manoeuvre satisfactorily.

Start the bunt as per initial attempts at looping i.e. high and into wind but a little further out. The main reason for carrying out the first attempts into wind is that the wind will assist the model during the recovery phase. Start the manoeuvre with a dive to build up speed and then level out. Gradually re-apply down elevator to enter the manoeuvre taking care to keep the wings level. As the manoeuvre progresses continue to gradually increase the down elevator until the model is again in level flight where upon the elevator control is returned to neutral. Too much down elevator at the bottom of the bunt could result in the model rolling out of the manoeuvre. Most embarrassing if the advice high and wide has not been followed. Fully symmetrically sectioned models will require less down elevator than less symmetrically sectioned models. The main problems encountered when performing a bunt are similar to those for a loop i.e. not entering the manoeuvre wings level and as a consequence screwing out.

The Spin

The spin is best described as a stalled spiral dive. It requires plenty of height and for the model to be a good distance out from the slope. A lot of height is lost in a spin and the model gets blown back towards the slope during it. For a model to spin satisfactorily it must have a minimum of rudder elevator control. Aileron elevator controlled models generally do not spin very well as it is difficult to establish the required spin speed necessary to keep the model in the fully stalled condition. Spins with aileron elevator controlled models generally end up with the model in a spiral dive and an even greater loss of height.

To enter the spin point the model into wind and slowly ease the elevator back to gently enter a stall. At the point of the stall apply FULL up elevator, FULL left or right aileron and FULL rudder in the same direction as the aileron. To exit the spin return the controls to neutral and recover from the ensuing dive. Most models will come out of a spin without needing to use anti-spin rudder or aileron. If however nothing is happening after half turn of returning the controls to neutral apply opposite rudder until the spin stops and then use up elevator to recover from the resulting dive. The secret to a good recovery is knowing how many turns or fractions of a turn the model takes to stop spinning so that the model can be recovered into wind. This angular movement is then used to determine when the controls are neutralised i.e. if a model takes half a turn to recover then the controls are neutralised when you can see the top of the model. This is very important as a downwind recovery will often require a panic turn into wind to avoid colliding with the hill!

Inverted spins are similar to upright spins except that the rudder and aileron controls are crossed i.e. applied in opposite directions as the rudder control is reversed when upside down. Recovering inverted is quite difficult and will require considerable practice as the normal control neutrals cannot be used as a reference points.

SUMMARY

The essence of good aerobatic flying is planning, positioning and empathy with the model. Smooth control movements and efficient use of the available energy are also fundamental ingredients. I am not a competition aerobatic pilot so my methods may differ from those of the experts. Aerobatics are an important part of flying, they build confidence and increase flying skill. The purpose of this article is to remove some of the mystique of aerobatic flying and encourage you, the average modeller, to broaden your aerobatic horizons and have a go. The manoeuvres described hear are not the only ones there are many more. Try inventing some of your own and stringing them together to form a montage. It goes without saying however that when performing aerobatics safety must be uppermost in your mind particularly if there are other flyers and spectators about.

One final point. Aerobatic flying will show up any inaccuracies in a model's state of trim such as warps of the flying surfaces, asymmetric control response and out of balance wings etc. so it is worth spending a little time checking for these problems and sorting them out if you want the model is to perform at its best.

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