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RADIO CONTROL BY BOB YOUNG Model R/C helicopters; Pt.2 Following last month's introduction to flying radio control helicopters, here we look at some of the mechanical aspects. By any standard, model helicopters are extremely complicated mechanisms. In some respects, model helicopters are more complex than real helicopters. They are more difficult to fly than other R/C aircraft too but all of this is part of the attraction; helicopters are a lot of fun. Flying one represents a complete departure from the traditional aspects of R/C model aircraft. In some respects it is almost easier to have no previous R/C aircraft experience when fronting up to these exotic little machines. This at least saves you from having to unlearn heavily conditioned reflexes built up over many years of fixedwing flying. Last month I mentioned the problem of helicopter emergency procedures being the exact opposite to those of fixed wing aircraft; in a fixed wing aircraft we instinctively chop the throttle and pull up elevator when things suddenly go pear-shaped. In a helicopter this would be catastrophic; the correct course is usually to apply full throttle and full forward cyclic pitch. The latter course results in an increase in altitude from the increased power, collective pitch and translational lift component introduced by increasing the forward speed. It also moves the helicopter into clean air, away from any vortexes generated during hovering. These actions are quite contrary to fixed-wing procedure. Add to this the facts that helicopters obey a more complex set of aerodynamic laws and that building a model helicopter is more akin to model engineering than aircraft modelling in the traditional sense. It then becomes obvious that helicopter fliers live in a dramatically different world to the conventional aeromodeller. Fig.1 is an isometric view of the internals of a small modern helicopter, the Robbe Schluter Futura Super Sport .60. The “.60” designation, by the way, refers to the capacity of engine required for the size of the aircraft – in this case, 0.60 cubic inch capacity. The Futura Super Sport .60 is an interesting design featuring some novel mechanical approaches to A typical example of today’s radio controlled model helicopters is the Robbe Schluter Moskito Expert. Learning to fly an aircraft like this will take the average person many, many hours and probably involve a fair number of “hard landings”. 60  Silicon Chip Fig 1: some idea of the complexity of a model helicopter can be gained by this exploded view of the Futura Super Sport .60 from the Robbe Schluter catalog. A more detailed view of the power plant is shown overleaf. February 1999  61 Fig. 2: it is perhaps not surprising that things can, and do, go wrong. This drawing shows more detail of the engine, cooling and starting components. Refer to the text for an explanation of many of the numbered parts. long-standing problems. The main transmission is fully exposed and the designers have utilised a toothed belt drive from the clutch bell to the first driving pulley. The idea of using the Cobb belt is to isolate the vibration from the motor as much as possible. The system works well and reliably. It is interesting to note that Tony Montanari, my old flying mate from my days in the early 1970’s, built his own helicopter back then and used Cobb belts, so the idea is far from new. I certainly prefer them to straight gears. They are quieter, more durable and much easier to replace, being available almost anywhere. Referring back to Fig.1, let us step through the mechanics in logical order. The very first thing that hits you is the overwhelming complexity of the drawing. The machine is a maze of linkages, drive belts and bits of metal, all stuck together with a million nuts and bolts. Where are the balsa, solar 62  Silicon Chip film and plywood? There isn’t any if the machine is fitted with fibreglass rotor blades. If you are an old-time modeller and yearn for gluing bits of wood together with Tarzan’s Grip then this is not the game for you. About the only balsa you will find in these models is on the trailing edges of the composite rotor blades. Instead you must swap your modelling knives and razor planes for some fairly fancy screwdrivers, Allen Key sets, socket sets etc, for you are now in the land of Meccano sets. And here begins the first lesson. Screws, nuts and bolts under constant vibration will all tend to shake loose over a period and extreme care must be exercised in assembly to ensure absolutely nothing ever comes adrift. Believe me, it only takes one loose screw to cause a very serious accident with a model helicopter. In the course of those three years of helicopter flying, I learned an awful lot in the hardest way possible. I once had a throttle linkage come adrift and the throttle stayed set at just on neutral buoyancy, which meant that the model was bouncing up and down and drifting all over the field. I was on my own at the time and the model was the large Schluter Huey-Cobra and with a full tank. My main worry was that the throttle would gradually vibrate to full throttle and the model become airborne. With no collective and no autorotation, when the fuel finally ran out there was going to be a messy result! I had no alternative but to grab the tail boom and get under the model with the rotors whizzing inches from my head. I finally managed to remove the fuel line and shut down the engine but that is the sort of thing that can result in serious personal injury. There are many ways to lock screws and nuts, Loktite being one of them. Loktite will let go under fairly intense heat but on the field it can be awkward to make adjustments with Loktited screws and nuts. My favourite method is to use contact cement. It peels off readily and can be dissolved with methylated spirits if required. But it holds those nuts and bolts under all conditions. The opening photograph shows a complete Robbe Schluter Moskito Expert as flown by Melbourne helicopter whiz, Nick Csabafy. This helicopter uses the more traditional fully enclosed reduction gearbox, with a reduction of around 1:9 or 1:10. Amongst the millions of problems facing the model helicopter pioneers, gearbox reduction ratios were one of the big ones. Taking their lead from full-size helicopters, they were running the main rotor too slowly. They had forgotten the problems introduced by scale effect. Once again Reynolds numbers reared their ugly heads. Over and over in model development we encounter this problem. Once rotor speeds were increased, things started to move in the right direction. Thus reduction ratios are a very important factor in model helicopter design. Reduction ratios of 1:10 result in a main rotor speed of approximately 1,000 RPM. Referring to Fig.2 we can see that the Futura is built around a pair of cleverly designed “U” shaped plates (4100). These provide the mounting for the engine, transmission, main rotor bearing blocks (4102, 4103), tail boom (4135) and servo mounts. In short, everything hangs off these plates. The fuselage for this model is similar to the Moskito shown in Photo 1. The beauty of this type of fuselage shell is that all exhaust gases are blown clear of the motor. Nick has even fitted a tuned pipe exhaust to his Moskito which pushes the exhaust gases even further away from the carburettor. Now the point here is that running a motor without a propeller inside a completely enclosed fuselage gives rise to several very serious problems. First and most obvious is that without the stream of air provided by the propeller, the motor is going to run very hot. Thus helicopters use a cooling fan (item 4124) fitted inside a streamlined housing to provide adequate cooling. While on this point, the correct type of fuel is also a very important issue in helicopters. Incorrect oil types and mix ratios will result in the engine overheating and plenty of auto-rotation and engine overhaul practice. Secondly, it is most important to ensure that the exhaust gases are pumped outside the fuselage and that they are not sucked back in during extended hover in still air. These gases are very hot and depleted of oxygen. As the carburettor is gulping great quantities of air it can draw in these hot, oxygen-depleted gases, further overheating the engine and degrading the engine performance markedly. Watch for exhaust leaks after each flight and for telltale signs of the exhaust gases being drawn back into the fuselage during operation. This was the most serious problem we faced with the Huey Cobras. The engines drowned in their own exhaust effluent. Before we modified the cooling arrangement the motors ran hot and sagged badly, particularly in hover. Large cooling gills cut in the fuselage sides and covered with fine mesh plus a ram air-scoop from the dummy jet intake cured the problem completely. The airflows around hovering helicopters are very complex and can do some very strange things, so stay alert to these types of problem, particularly in still air. Attached to the cooling fan is the main clutch (4123) and the clutch housing (4105) is integral with the tooth belt pinion. In operation, the clutch engages when the engine RPM reach a pre-determined level. It’s not all fun and games: model R/C helicopters have practical business uses too! Here an X-cell .60, built and flown by Bob Haines from Brisbane, carries aloft a specially mounted video camera for aerial filming. Still cameras can also be mounted in this way – they're especially popular with real estate agents. Sure beats $1000 an hour or more to hire a full size helicopter! February 1999  63 This allows the main rotor drive to be disengaged for starting and to ease the load on the engine when in idle. The motors are started with an electric starter and a boss is usually provided to allow ready access for the starter cone. The belt drives the first reduction gear, a Nylon-toothed pulley (3099) which is fitted with a second reduction pinion (4114). This drives the second reduction gear (3099), an internal straight cut gear. From here the drive goes straight to the main rotor via an elaborate set of bearings, the most important of which is item 4448, the Sprague clutch. This is a special type of bearing that free-wheels in one direction and locks up in the other direction. Its function is to allow the main rotor to be driven from the motor but when the motor stops the main rotor can free-wheel to allow auto-rotative decent. This is the heart of the modern helicopter. I once fitted one of these bearings to an early Kavan Jet-Ranger that was not designed for auto-rotation. However, I thought I would get smart and separate the collective and throttle con­trols at the same time, in order to make practicing auto-rotations easier. What a mistake! I got excited on the first flight and reduced the pitch without reducing the throttle. The rotor RPM shot up and I could literally see the blades stretching in front of my eyes. I thought the blades were going to come off. We had all heard horror story of blades coming off and I thought this was it. I chopped the throttle and the Sprague clutch disengaged and the blades kept flying around at the same speed. It seemed to take forever for those blades to slow down but at least they stayed on the helicopter. The most amazing thing however was that all I had to do to stop the blades was gradually increase the pitch. I could have done that without increasing the engine RPM and re-engaging the main clutch. Instead I just stood there mesmerised by the whirling rotor blades. It was a classic case of inadequate training in emergency procedures. You just cannot approach any aviation-related activity with a half-baked mental attitude. You are in boots and all, right from the moment that aircraft leaves the ground, because you only get one go 64  Silicon Chip TOP/SIDE VIEW Fig. 3: gyroscopic precession means that an action expected to occur at one point will actually occur about 90 degrees of blade rotation later. Thus to raise the rear of the helicopter (the action at point B) the control must be exerted at point A, which would normally be expected to give forward/aft control. and it has to be right the first time. I went straight back to the factory and re-coupled the collective and throttle servos. All went well after that. Item 4418 is the bevel gear drive for the tail rotor. The tail rotor is fitted with a pitch control mechanism and provides the anti-torque stabilisation as well as the yaw control. Because the motor is driving the main rotor in one direction, the fuselage will attempt to rotate in the opposite direction. The tail rotor prevents this from occurring, however it does introduce a complication. There is a reaction set up that pushes the helicopter sideways and this must be offset by some tilt in the main rotor disc. We will look at this next month in the flying section. The main rotor assembly is made up of the two main blades and two smaller paddles. The action of the paddles is quite complex but essentially they are the equivalent of trim tabs on fixed-wing aircraft. The cyclic pitch controls are fed into the paddles and the paddles move the main blades. There is an added complication here in the form of gyroscopic precession. This means that any control variation must be introduced 90° out of phase with the main rotor location. The action occurs 90° later (in the direction of the rotor rotation). Thus to raise the rear of the rotor disc to move the helicopter forward, the correct blade must be increased in pitch on the forward (rotational) side of the helicopter – see Fig.3 Is it any wonder that the early pioneers had so much trouble getting these things to work? They are a brilliant piece of engineering and are now commonplace and quite manageable, even for tyro modellers. The human mind never ceases to amaze me. In technology nothing seems impossible. Sadly in sociology, nothing seems possible! The swash plate is the rotor head control centre. This plate is tilted for cyclic control and raised and lowered for collective pitch control. This is the plate in Fig.1 at the bottom of the maze of linkages just below the rotor and paddle junction. A single screw is used to anchor the main rotor blades in the modern helicopter. This allows self-alignment of the blades plus it largely eliminates the danger of a blade splitting between multiple bolt holes, especially if the tip strikes the ground. This was a major cause of blades flying off in the early days. The rest of the helicopter is largely made up of brackets for mounting the servos, receiver, battery pack, switch harness, gyro and fuel tank. Anchor points are also provided for mounting the fuselage shell. All in all, it is a very impressive package. The second photograph shows an interesting twist: a helicopter fitted with a video camera. The model is an X-cell heli by Bob Haines in Brisbane (photo courtesy Max Tandy). Next month we will look at flying one of these little devils. SC Acknowledgments:        My thanks to: (1) Nick Csabafy, N. C. Helicopter Services, Vic. (2) Max Tandy Helicopters, Qld. (3) Drawings; Robbe Schluter, Germany.