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RADIO CONTROL BY BOB YOUNG Operating model R/C helicopters This month, we will take a look at some of the technical aspects related to the operation and flying of model R/C helicop­ters. They are not easy to fly, as we will find out. September 19th, 1971. Place, Doylestown Pennsylvania, USA and a very much younger Bob Young was standing engrossed, contem­plating the gruelling events of the last four days. Today was the last day of the 1971 World Aerobatic Championships and scheduled for demonstration flying, which simply meant fun and lots of it. Suddenly all pain was forgotten as a magical sight suddenly commanded complete attention. Here was a sight that made the entire trip worthwhile. Gone were all thoughts of the winter months of early morning practice sessions, the long nights of preparation and the strain of competing in a contest alongside some of the best R/C fliers in the world. There in front of my eyes, drifting inches above the ground, was not one but two quite large model helicopters. I was about to witness what was billed as the first public demonstra­tion of a model helicopter. Looking back at the flying from a 1998 viewpoint, Dieter Schluter (the designer) and his friend put on a quite tame demonstration that day, with coordinated stall turns as the high­light of the aerobatic routine. But we were all stunned. In 1971 this was an amazing feat of model aerodynamic engineering. Dieter had gone where no other modeller had been before and not content to demonstrate one machine, floored us with a synchronised display featuring two helicopters. The display brought the sky down. Flown with great authority, Dieter and his mate gave us a never-to-be-forgotten show. The difficulties facing the engineers developing the model helicopter were enormous. Not only were they faced with scale effect and Reynolds numbers, they faced problems with inadequate engines, incredibly involved mechanical linkages and finally, keeping this untested mechanical nightmare in one piece while learning to fly at the same time. They had no teachers because they were entirely on their own. It was a truly difficult task and the modern modeller owes a great debt to the people who made it all happen. The fact that these models didn’t make an ap­pearance until 1971 Fig.1: the major components of a model helicopter. (Diagram courtesy of Max Tandy R/C Helicopters Australia). 80  Silicon Chip is a measure of the scope and difficulty of the task. My first helicopter I knew of all of these difficulties but I was hooked! I had to have one of these machines and when I returned to Sydney arrangements were made to procure one of the Kalt (45 powered) Huey Cobras, a smaller Japanese licence built version of the Schluter (60 powered) Huey. By modern standards they were a primitive machine. Fitted with a fixed pitch Hiller type head with swash plate for pitch and roll, throttle for climb/descend and tail rotor pitch control for yaw/torque compensation, they were simple indeed. There were no gyros or computer radios in those days! But they flew and they flew well. Cooling was a major prob­lem with the motor buried deep inside a slab-sided fuselage. Very large extra air vents had to be cut into the sides and covered with fine mesh and air ducting from the dummy air scoop brought in cool ram air once the helicopter started to move forward which it rarely did for the first two months. For those first two months of learning to hover, the motors sat inside that fuselage bathed in their own hot, oily exhaust fumes, and in the Huey they sometimes choked on these stale gases. Fresh clean air is a must and lots of it. Because the motors ran at a higher temperature, there was a much denser smoke haze generated. I remember one dead still, cool evening right at dusk. The local baseball team was practising in the park where I was flying and I heard “Strike, talk about pollution!” I looked up and found the whole park covered in what looked like stage smoke. It was an eerie sight. For one hour every day after work I religiously toiled at mastering the hover. It was all very new and very difficult; made even more difficult by the fact that I had no-one to turn to for help. I was one of the first in Australia and very much on my own in Sydney at least. However I was fortunate to have as my teacher from time to time, Yuri Oki, the man who built the models under licence in Japan. Oki insisted that before an out-and-return flight could be attempted I had to be able to hover at eye level and over the same spot, for an entire tank of fuel, about 15-20 minutes. This Hughes 300 model helicopter was built by Mike Zimmerman. (Photo courtesy “Airborne” magazine). This was advice that I was very grateful for when I did eventually undertake my first out-and-return flight. Now Oki was a wild man and he loved to fly helicopters. He flew one inside my factory when it was a bare shell just after I moved into those premises in February 1972. We nearly gassed ourselves that night and in the end we were all hanging out of windows gasping for breath. We flew the model in the street outside my factory and again over the factory from the local park about 500 metres away. I shudder when I think of all of this now, for just after that flight I had my first motor cut out and with no auto-rotation there was only one way to go and that was down and not very nicely at that. Fortunately, I was quite low in the park at the time and little damage was done. Oki gave demonstration flights at the Royal Easter Show on several occasions and on one such occasion he asked me to call for him while he did a flight around the clock tower at the far end of the main arena. It was the most January 1999  81 Fig.1: the main rotor blades in a helicopter are arranged so that the pitch can be changed in each quadrant of the main rotor disc. This is called the “cyclic pitch control” and is used for the main pitch (fore and aft) and roll (lateral) control functions. Fig.2: when a helicopter hovers close to the ground in still air, the air is forced down from the rotor, hits the ground and rebounds. This upward moving air is then drawn back down into the rotor disc and accelerated further, hitting the ground and rebounding with even more energy than before to create a dangerous ground effect. Fig.3 another dangerous situation. Air moving down through the disc on the cliff side will reduce the lift on that side of the disc and the helicopter will gradually begin to bank towards the cliff. Any attempt on the part of the pilot to increase the lift on that side of the disc will only serve to increase the velocity of the vortex, further exacerbating the problem. The only answer once this situation arises is to move for­ward into clear air and come around again after the vortex has died away. difficult pylon call I have ever made. I still have visions of that model disappearing out of sight behind the tower. It only took moments to reappear but it seemed like an eternity. They were fun days and we could not get enough of it. Rumour has it that Oki was asked to leave his hotel one night after he flew a helicopter 82  Silicon Chip in his room. As I said, he was a wild man and loved flying. He was also very good at it. It is typical of the man that these days he is knee-deep into model turbines. I subsequently flew helicopters for about three years after that and eventually gave it away to return to my first love, aerobatic flying. One interesting aside here: when I returned to aerobatics I was a far better pilot because I had gained complete mastery of my left thumb as a result of flying with no gyro on the tail rotor. Helicopters demand constant attention to the tail rotor, hence the modern helicopter with tail-rotor gyro. With no gyro you become very adept with your left thumb, a most important movement in multi-point rolls on fixed wing aircraft. I do not agree with all of the modern gadgets. It is like a concert pianist using an electronic piano. However, the modern crop of gadgets has made life much easier for the tyro helicopter pilot and it does not take anywhere near as long to learn to fly now as it took us. How they work So how do these fabulous machines work? A helicopter is classified as a rotary winged aircraft and the aerodynamics of this type of machine are quite different to that of a fixed-wing aircraft. Helicopters, both full size and models, are very difficult to learn to fly, as they require a great deal of dexterity and coordination. Basically, the controls are as follows. The main rotor blades are arranged so that pitch can be changed in each quadrant of the main rotor disc – see Fig.1. This is called the “cyclic pitch control” and is used for the main pitch (fore and aft) and roll (lateral) control functions. The lateral cyclic pitch control corresponds to the aileron stick in a fixed wing aircraft and the forward and aft cyclic pitch control corresponds roughly to elevator. “Cyclic pitch”, as the name suggests, alters the pitch of the main rotor blades on a cyclic basis. Thus to bank left, the pitch on each rotor blade is reduced in the left quadrant and increased in the right on each cycle of the main rotor blades. To move forward the pitch is reduced in the forward quadrant and in­creased in the aft. Collective pitch control is used to increase or decrease the pitch angle of all blades over the entire cycle and serves as the climb or descend control in conjunction with the throttle. The linkages required to achieve all these pitch variations are very elaborate and took a long time to develop. There is also the problem of the This Eurocopter “Tigre” model helicopter is 1.9 metres long and weighs just 7kg. (Photo courtesy “Airborne” magazine). increase and decrease of lift on the advancing and retreating blade in forward flight. This creates an unbalanced lift distribution across the transla­ tional lift disc and was one of the biggest problems facing the pioneers of model helicopters. The solutions to this problem are outside the scope of this article and we may deal with this one later. The torque of the main rotor is counteracted by the small tail rotor. By increasing or decreasing the collective pitch on this small propeller, yaw control can be effected. Loss of tail rotor control is a serious business and many helicopters have crashed as a result, so routine maintenance on this seemingly insignificant item is very important. The Americans lost over 5000 helicopters in Vietnam and one of the favourite tricks of the Viet Cong was to shoot at the tail rotor. There was an interesting exhibit in the Canberra War Museum of a tail rotor assembly of an Australian helicopter that was riddled with bullet holes. The controls in a helicopter are highly interactive and learning to fly one of these models may become a long drawn out affair. Great strides have been made in transmitter and gyro design and mixers and gyros have simplified learning significant­ly. Flying hazards Flying helicopters is difficult and fraught with hazards unknown in fixed wing aircraft. To begin with, there are two forms of lift equations; one for hover and one for forward move­ment. In the hover, lift is a function of blade area, rotor speed and angle of attack of the blades. In forward motion, the blade area becomes the total swept area of the blades; in other words, the total rotor disc. This is referred to as translational lift and is a very important factor in helicopter operations. Heavy-lift helicopters are almost always fitted with wheels and a fully loaded takeoff is usually carried out in much the same manner as a fixed wing aircraft, with the machine running along the ground to gain flying speed before lifting off. In this manner the extra lift obtained from translational effects can be fully utilised. Hover and vertical takeoff are an inefficient and somewhat risky pair of manoeuvres and used only when circumstances dic­tate. Great care must be exercised at all times in hovering flight because of the problems arising from vortex generation. Because they shift such huge volumes of air, strange STEPDOWN TRANSFORMERS 60VA to 3KVA encased toroids Harbuch Electronics Pty Ltd 9/40 Leighton Pl. HORNSBY 2077 Ph (02) 9476-5854 Fx (02) 9476-3231 January 1999  83 to the ground. Too late and the speed will not be washed off sufficiently to effect a safe land­ing. Helicopter pilots are an intrepid lot. Dangerous situations A close up view of the Eurocopter “Tigre” model helicopter pictured on the previous page. (Photo courtesy “Airborne” magazine). things can happen when hovering around obstacles. Refer now to Fig.2 which shows a helicopter hovering in still air and in ground effect. Air is forced down from the rotor, hits the ground and rebounds. This upward moving air is then drawn back down into the rotor disc and accelerated further, hitting the ground and rebounding with even more energy than before. In time, this doughnut-shaped ring of air may obtain such a velocity that the speed of the downgoing air entering the rotor disc may exceed the climb rate of the helicopter and the helicop­ter will gradually sink to the ground, even with full power ap­plied. Now you will notice that I particularly stated that this happens in still air. In a strong wind, the aircraft is actually travelling forward relative to the airstream to maintain hover over a fixed spot. As a result, the dirty air is swept away behind the helicopter and it is almost impossible for vortexes to form in strong winds. Which leads us to an interesting observation. One of the things that make learning to fly a helicopter so difficult for an experienced fixed-wing pilot are the radically different emergen­cy procedures. In a model fixed-wing aircraft, in an emergency, more often than not the best procedure is to cut the throttle 84  Silicon Chip and pull full up. This lifts the nose, slows the model and settles it into a glide, giving time for the pilot to stabilise the model and see what should be done next. By contrast, in a helicopter the procedure is usually to go straight to full power and give down elevator (full forward cyclic). This lifts the model away from the ground and moves the model into clean air (away from vortexes) and increases transla­tional lift – all of which gains the pilot height and time to think. The two reactions are exactly opposite. Chopping the throttle on a helicopter is catastrophic because they come down like bricks, especially in the days before auto-rotation. Auto-rotation, by the way, is the ability of the helicopter to convert height into rotor RPM. In an auto-rotative descent, the main rotor blades are put into free wheeling mode and the pitch moved to a slightly negative angle of attack. The downward motion of the helicopter is used to spin up the main rotor and this stored energy is converted to lift at the last moment before touch down. The pilot must gauge the correct moment to engage positive angle of attack on the main rotor and this is a very delicate operation. Too early and the rotor will slow below minimum lift RPM and the helicopter will crash Moving back now to vortexes, Fig.3 shows an interesting variation on the theme. Here we have a typical rescue scenario, where someone has fallen down a cliff into a difficult to reach crevice. The air on the cliff side of the chopper is trapped and will vortex readily. By contrast, the air on the open side is free to move away and now we have a really dangerous situation on our hands. Air moving down through the disc on the cliff side will reduce the lift on that side of the disc and the helicopter will gradually begin to bank towards the cliff. Any attempt on the part of the pilot to increase the lift on that side of the disc will only serve to increase the velocity of the vortex, further exacerbating the problem and if the situation gets out of hand the helicopter could ultimately crash into the cliff face. The only answer once this situation arises is to move for­ward into clear air and come around again after the vortex has died away. Hovering in still air near trees, buildings and cliffs is fraught with danger and must be undertaken with great care. I once got caught with a tail rotor vortex in the early days, after hovering for a long time in still air at about 100 feet. I gradually lost tail rotor control until even full op­posite tail rotor control would not stop the tail from spinning around. I thought the tail rotor servo had packed it in so I had no alternative (or so I thought) but to gradually bring the model down and plonk it unceremoniously on the ground with the fuselage slowly rotating around the main rotor axis. Fortunately, Oki was there that day and he recognised it for what it was and told me how to deal with it correctly. The answer: full throttle and full forward cyclic, thus moving the chopper into clear air and establishing a weather vane effect on the side area of the fuselage until the tail rotor control re-established itself. It never occurred again so I never had the opportunity to put his instructions into practice. So there you have it: a look at the SC art of flying model helicopters.