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REMOTE CONTROL BY BOB YOUNG The military applications of radio controlled aircraft Not all radio controlled planes are built for fun. Some are built for military surveillance and aerial photography, and some are used for target practice for gunnery and missiles. One of the most interesting periods of my career in R/C modelling came through my involvement in the military applications of R/C models. This began with the development of a system for training the operators of shoulder launched missiles, which subsequently led to the Silvertone Weightlifter (Fig.1) and the De Havilland Enmoth. These mini RPVs ultimately found their way into various Government Departments and could form the basis of a separate story. However, as a result of the shoulder launch project, I became known in military circles which then led to my involvement with the All Arms Air Defence project. This was a very exciting project and forms the basis of this article. As a result of experience gained during the Vietnam War, the West learned from bitter experierice that small bore weapons could inflict seri- ous damage on modern high speed ground attack aircraft. Although they rarely came down as a result -of this damage, it is almost impossible to put a bullet through the modern military aircraft without damaging something -usually a very expensive something. As a result, aircraft were constantly grounded and repair and maintenance problems became a major issue. With these lessons in mind, it was decided that All Arms Air Defence would be introduced into the training curriculum of the Australian Army. The project started out in a small way using the well-known Mini Sticks which we supplied and these were flown by Danny Mazlowitz, a keen modeller and a member of the regular army. During the first shoot, these proved to have serious shortcomings, not the least being that they were far too slow. In any sort of headwind, the closing speed was pathetic (and we had to fly in all conditions regardless of weather, for once a shoot was scheduled that was it) . So we soon decided that we needed a better aircraft. Development of the MAT-DF Fig.1: the Sivertone Weightlifter was developed for the Australian Army as a target aircraft for shoulder launched missiles. 80 SILICON CHIP The MAT-DF (Model Aircraft Target - Delta Foam) concept grew out of the experience gained with All Arms Air Defence Targets of conventional styling and construction. In common with most targets of that time, the original Silvertone target was constructed of balsa and followed the normal MAT layout. Again in common with all targets of that type, the problems of fragility, lack of adequate speed envelope and the high number of man hours (cost) per airframe were painfully obvious. Fig.2: the MAT-DF delta-winged target aircraft. It was driven by a rear-mounted engine & was made from moulded polystyrene. Note the lack of a rudder. As these problems were similar to those encountered by all model flyers, we decided to investigate a type of model utilising simple, low cost construction techniques which would fulfil both military and commercial requirements. From the very beginning, it was decided that an all out attempt at a throw away aircraft was the most suitable approach to the problem of the MAT requirement. Time to repair was a major cost factor in target aircraft. Also, holding up a shoot resulted in 40 impatient soldiers leaning over your shoulder muttering obscenities; definitely a great incentive to reduce repair times at the design stage. Thus, we proposed a completely moulded airframe which would require very little finishing work and if broken could be discarded and replaced at less than repair cost. Without any doubt, the scales are heavily loaded in favour of any proposal which can fulfil the role with a minimum of cost due to the short lifespan of the finished item. Careful analysis of the crash damage in the conventional tractor MAT design revealed that the motor was a major cost item and also a constant source of trouble in subsequent operation. With the tractor layout, the motor is subject to extreme punishment and the problems of bent crankshafts, dirt ingestion and broken components can pass unnoticed in the field and lead to subsequent unreliability and therefore loss of train- ing time. These problems can be largely overcome by using a rearmounted motor. Also, the risk of injury due to out of control targets is much reduced with the motor in the rear. The pusher is also much more efficient and the big plus is that there is no burnt castor oil on the airframe, which results in much better repair conditions. The repair issue was the all important consideration in this whole project. With this in mind, the final design very quickly firmed up as a 3-channel pusher delta (Fig.2 & Fig.3), for this layout lends itself well to moulding and repair techniques. The final design thus resolved itself into a 3-piece expanded polystyrene mould set with a wingspan of 93cm and fitted with an O.S. 40 FSR, retimed to run in reverse, in order to use standard props. The final weight when new (unrepaired) was 1.5kg, but the repaired weight could go as high as 2.5kg. Three-channel radios were used on throttle and elevons, the latter driven via a specially designed electronic mixer. Rudder control was not fitted. The big problem with the pusher layout is the negative stability introduced at high angles of attack. This is particularly true at slow speed and, in particular, during hand launching. This was to be a vulnerable point, for the flying sites used called for hand launching and a much repaired model suffered a dramatic weight increase. In the hands of a tyro pilot, launches became a real health hazard for the model. The problem stems from the fact that at low speed and the subsequent high angles of attack, as experienced just after a hand launch, the centre of drag is well above the thrustline and any increase in power tends to lift the nose even higher, thereby increasing the drag even further. At this point, the model starts to sink towards the ground at which point the tyro pulls in up elevator and lifts the nose higher still. This results in more drag and more sink, and so the situation very quickly gets out of hand for the inexperienced flyer. The answer is simple - a touch of down elevator cures the problem immediately. But here is the rub: inexperienced pilots do not like giving down elevator when three feet off the ground. The problem also manifested itself in go-arounds where the aircraft seemed to absorb all the power you could give it without any increase in airspeed. It was a peculiar feeling but again any increase in throttle had to be accompanied by a touch of down elevator. Once the thing got on the step, it flew as straight as an arrow and was very pleasant to fly. However, this problem was to give us some grief when the less experienced army pilots took over the project. Flying sequence As stated previously, after several sessions with conventional airframes, the shortcomings soon made themselves felt. As a result we approached the Army with the view of developing a more suitable airframe at our own expense. I placed this stipulation upon the project in order that I could sell the finished item commercially without obligation if it proved successful. This was agreed to and unfortunately introduced an unexpected side effect into the project. A prototype was produced and test flown in an actual shoot. This prototype model flew absolutely brilliantly for it weighed only 1.5kg. Very fast (180km/h plus), it was voted by gunners who had trained on Mirages in combined operational exercises as the most realistic target they had put their sights on. It was also easy to launch because of the light weight and very smooth to fly. It could not however stand the wear OCT0BER1991 81 Fig.4: a rear mounted motor is an advantage in a target aircraft since it is not as susceptible to impact damage when the aircraft is shot down. and tear of operational flying, particularly emergency landings in scrub brought about by the fact that the gunners would do nasty things like shooting the motor clean off the airframe, or putting a bullet straight through the fuel tank. This was to be a constant source of worry duri_ng the whole project. You never knew what was coming next and although the aircraft flew very well with the motor shot off and could be landed safely (all that happened was that the centre of gravity moved forward), a bullet through the battery pack was a pretty tedious affair. Nor did you ever get used to that awful feeling that came with an abrupt loss of control - no warning, just instant emergency. You got pretty good at nursing home wounded birds. As a result, we did many, many outlandings, most completely out of visual contact, behind trees and the like. As you can imagine, wear and tear was a very real problem. I made a serious error in beefing up the mouldings with fibreglass and heavier and stronger wood in order to prolong the life as much as possible. In so doing, I completely destroyed the project, for we lost the throwaway concept. The heavier airframes were more expensive and took much longer to build, hence there was a much greater reluctance to throw them away (especially since I was paying for them), and so they were repaired and just got heavier and heavier and more and more difficult to launch. This was the real trick in this project 82 SILICON CHIP and that involved striking a balance between wear and tear and the throwaway concept. The new models did however withstand crashes and wear and tear infinitely better. Returning to the prototype, we flew with that model for about six hours on the first day, nursing it along through various trials and tribulations until the inevitable - a bullet straight through the Rx . Those little bits of steel used to do terrible things to the radio reception. Now I must point out here that most models did not go this long before being brought down . We were using 1 in 3 tracer which is pretty hard on the gun barrels (Brens and M60's) but did give the instructors a very clear picture of the pupil's progress. Also, eight guns firing simultaneously gave a very interesting and somewhat spooky visual effect. As I was flying from behind the guns and flying head on into the guns, I could thus view the position of the aircraft in relation to the tracer stream and therefore adjust my position to stay out of the worst of it. In the actual shoots, I was ordered to fly a straight line and this increased the hit rate dramatically. In the test flying when I was flying evasively, we fired about 10,000 rounds before we brought the model down. Flying nonevasively, we averaged one hit in 600 rounds and it took six hits on an average to bring the model down. I found the experience of flying into a stream of tracer quite eerie and thought back on airmen who had to actually face this from inside the air- craft. I am glad I missed the experience for it is one I can live without. One interesting phenomena that I did notice was that the shock wave from the nose of the bullet would jolt the airframe in near misses. This effect is used in sonic miss detectors and it is surprisingly strong. When the aircraft started to rock you knew someone was getting too close and a quick application of control would ease the model out of harm's way; usually! Returning now to the prototype, when it did finally crash, that was it. The lightweight construction just allowed it to disintegrate and it was not repairable on the field. However, it did fly again in the next session. So ended day one of the Delta project. Our next session was with the six beefed up airframes. From the outset, we had difficulty in launching. That extra 500 grams made all of the difference for it was only supported by 511 square inches of wing (the small wing was forced on us by the limitations in the size of the foam moulding equipment) . It also took me a while to wake up to the trick of applying down elevator to stop the power absorption. Once this was mastered, it was no problem. Furious shooting Shooting started at a fast and furi ous pace. The mission profile called for a racecourse track starting 600-700 metres out in front of the guns and flying directly towards the guns which were eight abreast on the firing point. I was to cross over the guns at about 20 feet altitude. I felt this was a little dangerous and pointed this out to the officer in charge. I do not think he realised what I meant for he just asked me to carry on. All proceeded smoothly with the usual bullet holes being patched and fuel tanks being replaced etc, until on one pass a bullet passed through the bottom of the left hand wing and struck the elevon pushrod smack in the middle. As this was only 16 gauge wire, it immediately kinked into a "V" and applied full "up" on left elevon. Normally this would not be too serious and the model would have just spun into the ground. As it happened however, it was the last round out of one of the guns and the model was directly in front of the firing point and at an altitude of only 20 feet. Fig.4: the Falcon "A" target aircraft was a fibreglass & foam version of the Senior Falcon & was very popular with the Army pilots. long as everything was working, the It completed one half roll and plowed into the ground 20 feet in rough patch of ground available for a front of the officers ' table and skidded landing area was more or less satisstraight into their feet, amid a great factory. It was small but reasonably cloud of dust. With that, the O.C. clear. The belly skid stopped the jumped up and demanded to know model in a very short distance ana why I crashed into their feet? What mostly with no damage. could I say? The incident impressed It was when the aircraft was damme however, for I realised that had aged in flight that the problems bethe bullet passed 3mm either side of gan. The most common faults were the pushrod, the model would not strikes to the engine or fuel system, have crashed. In such a random, horwhich meant an engine loss over impossible terrain, mostly out of sight to ribly indeterminate way do men die or become maimed in times of war. the pilot. The aircraft suffered badly With that little example behind us, here and the weight gradually spiralled after constant repair, with the conwe had no more head on approaches sequent increase in problems at hand and all profiles from that point on were oblique. There was still that ele- launch. Danny and I seldom had problems ment of danger however and one had to be constantly alert for any type of here but the less experienced Army pilots who flew the last missions had emergency. After weeks of this , the great difficulty and the project was pilots got very good at emergency procedures. ended at that point. You never knew where the hits took However, the more robust airframes place and if a part weakened by a hit were staggeringly strong. The later went unnoticed, it could let go unexversions, with their fibreglass cladpectedly. On one occasion, Danny was ding, were 40% recoverable even afabout to start the motor when someter a full throttle dive into the ground, one pointed to the prop and there which they did, time after time (reright in the middle of one blade was a · coverable being defined as field reperfectly neat 7.62mm hole, clean as pairable). The Delta configuration gave a whistle. Post flight inspection had immense strength due to the thickto be thorough. ness of the wing and the shape of the However, it was the outlandings fuselage. The nose used to crumple, that took their toll of the models. As but that wa·s it. Splice on a new nose and away you went. The motors were never marked and we needed nothing in the way of motor spares except in the case of bullet strikes. All in all the project was a success. The target was very realistic and the repairability of the airframes good. However the hand launch problem plagued us to the end and subsequently turned the Army pilots off the Delta configuration. Yet had we stuck to the original concept of a true lightweight throwaway model or even been able to increase the original wing area, the project would have been a complete success. As it was, once the moulds were made it was too late to change. The problems came about by allowing outside factors to intrude on the original design parameters. I also wondered at a much later date about more downthrust. Would this have helped? Would it have upset the flying characteristics. We never knew, for once the project lapsed I never flew another of these aircraft. This was the thing that I liked most about the military work. It was so demanding. You had to be able to develop a system for all types of pilots. The army pilots were not modellers and did not have that wide background that comes with years of flying. They had attended the official courses and whilst well trained and quite capable as flyers , they just lacked that extra something which comes with years of experience. The Falcon We subsequently went on to develop the Falcon "A", a fibreglass and foam version of the famous Senior Falcon. This model was very popular with the Army pilots and we deliveredmany of these models which were used for All Arms Air Defence Targets, amongst other things (Fig.4) . However to me, they just did not look right after flying what to my mind was the ultimate All Arms Target. Through the ring sight, the MAT-DF looked for all the world like a full size Mirage as it came boring in on us from virtually out of sight. The gunners loved it even though it was hard to hit - unless you were a crack shot, that is. I remember one gunner who was pulled off the guns after bringing down two aircraft in two bursts. I guess they figured he needed no further training! SC OCT0BER1991 83