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RADIO CONTROL BY BOB YOUNG Model R/C Helicopters; Pt.3 This month, we will look at some aspects of flying model helicopters. With a modern transmitter and a tail rotor gyro, a lot of the difficulty has been taken out of learning to fly a helicopter but they are still not simple by any means. As with most aspects of radio controlled models, ul­timate success rests in the preparation of the model during setting up. In model helicopters, this applies even more so than with fixed-wing aircraft because you need a clear understanding of the complex interaction between all the angles (or thrust vectors) generated by these exotic machines. Because helicopters are so convenient to operate, many beginners tend to go it alone, never setting foot on a club field and therefore cutting themselves off from a pool of valuable experience. As a result, many do not make the transition to a successful flyer and thus the “Trading Post” abounds with adverts for second­hand model helicopters, many virtually unflown. So if you want to be good at helicopter flying, I cannot stress too strongly the need for help and guidance from a mentor. So what has to be done to set up your new helicopter. First, make sure that all linkages work freely, all screws are fitted with lock nuts, sealed with Locktite or otherwise held in place so that there is no risk of them coming loose under vibra­tion. Next, you need to set up the transmitter controls. There are two recognised modes and Fig.1 shows the correct setup for these. Regardless of the mode you choose, high throttle is always with the stick towards the top of the transmitter case (closest to antenna) and the forward cyclic is to the top of the case. Left and right are natural but we’ll talk more on this point later. Model memory is a two-edged sword to my mind, useful but potentially dangerous. The best place for Fig.1: this diagram shows the two most common modes for setting up the transmitter controls. Regardless of the mode you choose, high throttle is always with the stick towards the top of the transmitter case (closest to antenna) and the forward cyclic is to the top of the case. model memory is in the model itself. That way there is no chance of the wrong memory being loaded. Many a model has crashed because of this problem. If you must insist on using model memory on your transmit­ter, try to give yourself the best chance of surviving a momenta­ry lapse by always maintaining the same servo directions on the main flying controls (at least) wherever possible. And it is no use saying it will not happen because it does, as one of my flying mates found out to his horror during the Nationals this Christmas. Control angle variations During the learning period it is also very important to set the control angle variations a little on the soft side so that the risk of over-controlling is reduced as far as possible. The instruction sheets for your helicopter will guide you in this regard. Fig.2 shows the setup for the collective pitch angles on the main rotor blades. When learning, pitch variations from 0° to 5° positive are usually recommended. Per­sonally, I prefer one to five degrees as this keeps the helicop­ter a little more buoyant on low throttle settings. For advanced flying including auto-rotations, pitch varia­tions of -3° to +10° are more usual. When learning, -3° is enough to drive the helicopter into the ground like a corkscrew if you get excited and chop the throttle in a hurry. Remember here that collective pitch and throttle are both coupled to the throt­tle stick. Incidentally, if you are confused about collective pitch control, we did talk about this briefly in the January 1999 issue of SILICON CHIP. The instruction sheets will also show you the correct loca­tion for the MARCH 1999  53 While I do not like serious flyers using these accessories, when you are learning you need all the help you can get. So my advice is use a gyro until you master the monster and then ex­periment with switching it off. Believe me it adds a whole new dimension to your dexterity and skill! from idle to full throttle is a recipe for disaster. Watch out for signs of the motor overheating or ingesting its own exhaust gas during extended hover in still air. Make sure the correct fuel is used and that you are familiar with the difficulties and dangers of working around a helicopter with the motor running. Remember that spinning rotor blades can be lethal! That should be obvious but it needs to be stated. While you might think that a ready-to-fly helicopter would not need it, you need to pay particular attention to the static and dynamic balancing of the main rotor blades and check that both blades are tracking (ie, set to the same coning angle). The coning angle corresponds to the dihedral on fixed wing aircraft. One blade tip painted a vivid colour is a great help in this regard. A blade not tracking will show up quite clearly if you watch the tips on one side of the disc. If the coloured tip is above the unpainted tip, reduce the pitch angle on the high blade or increase the pitch on the low blade, whichever is most appro­ priate at the time. Fig.3 is a plan view of a helicopter showing the torque reaction which results from the rotation of the main rotor. Torque reaction means that while the main blade rotates in one direction (ie, clockwise), the torque reaction causes the body to rotate in the other direction. The small tail rotor is there to counteract this torque reaction and by varying the thrust (pitch angle) on the tail rotor it is possible to move the nose of the helicopter either left or right. Now there is a tricky little piece of logic involved with setting up the tail rotor and it is important to get this correct from the very beginning. By increasing the thrust (pitch angle), the tail will be pulled left and by decreasing the thrust it will move to the right as result of the torque reaction. Check the motor Flying by the nose Before we get that far however, there are still the basic things to check before you even think about getting into the air. Make sure the motor runs reliably above all else. A motor failure when learning is serious. Pay particular attention to the idle and the transition from idle to full throttle. A motor that sags during the transition Now here is the tricky bit: we want to fly the nose of the helicopter not the tail. This is a mistake that many beginners make; they concentrate on the tail when learning to hover. Quite often they even set up the transmitter controls in the correct sense to control the tail, whereby moving the transmitter stick to the left moves the tail to Fig.2: the setup for the collective pitch angles on the main rotor blades. When learning, pitch variations from 0° to 5° positive are usually recommended. Fig.3: this plan view of a helicopter shows the torque reaction which results from the rotation of the main rotor. Torque reac­tion means that while the main blade rotates in one direction (ie, clockwise), the torque reaction causes the body to rotate in the other direction. The small tail rotor is there to compensate for this. centre of gravity of the helicopter, a most import­ant point. Incorrect CG locations can cause serious problems, particularly when you are learning. The usual CG location is just in front of the main rotor shaft. Should you use a gyro or not? Gyros are a wonderful devel­opment which make all the difference for the tyro flyer. Before we go any further I should briefly mention what a gyro does, although you could write a whole chapter on this subject alone. There are two types, gyroscopic and piezoelectric, but they both do the same job –they sense sudden tail rotor movements and apply an appropriate correction to the servo which controls the tail rotor pitch. 54  Silicon Chip the left. Then when they move into forward flight all hell breaks loose because their perceptions of direction are suddenly reversed. Thus we want to set up the tail rotor control on the trans­mitter (Fig.1: horizontal axis of the lefthand stick) so that when the stick is moved left, the nose moves to the left! This is a most important point. Tail rotor pitch Now how much pitch do we apply to the tail rotor blades during setup? Too much and the helicopter will spin to the right immediately it breaks ground and too little will see it spin left. The instruction sheets will serve as a guide but unfor­ tunately they do not necessarily give the correct answer. Fortunately, gyros now help take the sting out of any tail rotor setting error. It is in trimming the model that a helpful friend who is an experienced helicopter pilot comes into his own. Fig.4 shows the forces acting on a helicopter in hover. While you may think that the rotor disc would remain horizontal to produce lift and no horizontal thrust, it doesn’t work out that way. In fact, because the tail rotor acts to stop the heli­copter spinning out of control, it also applies horizontal thrust and this means the helicopter will move sideways. So if you are to hover in the one spot, you must have side­ ways tilt applied to the main rotor to counteract the sideways thrust from the tail rotor. This is achieved by increasing the cyclic pitch on the lefthand side of the main rotor disc and reducing it on the right. Now here is the point. Every time we change the throttle/collective pitch control, the tail rotor requires a change in pitch. Consider now the situation in landing where the helicopter is in equilibrium, hovering just centimetres above the ground and about to touch down. The main rotor is applying torque to the left which is countered by the thrust from the tail rotor. And the sideways thrust of the tail rotor is countered by the sideways tilt of the main rotor. Suddenly power is reduced to allow the helicopter to settle onto the ground. That means we reduce the thrust from the main and tail rotors but the sideways tilt of the main rotor Fig.4: if you are to hover in the one spot, you must have side­ways tilt applied to the main rotor to counteract the sideways thrust from the tail rotor. This is achieved by increasing the cyclic pitch on the lefthand side of the main rotor disc and reducing it on the right. is still there. Unless we correct this, the helicopter will move to the right at a most critical moment, just as the skids touch the ground. So the tilt must be reduced at the same time as we reduce the power. The mixing in a modern transmitter solves these prob­lems to a very large degree. Without a modern transmitter and a tail rotor gyro, landing a helicopter on narrow skids is difficult indeed. It requires a very definite sequence of powerfully executed, reflexive com­ mands. You must not dribble a helicopter onto the ground. Like horses, they require a firm hand at all times and it takes hours of practice to acquire this. Every action must be precise, calcu­lated and well executed. A helicopter taking off suffers from the same complex interaction of forces. The throttle is being constantly increased as it leaves the ground and conditions are changing rapidly as it passes through ground effect into clean air. Again good solid reflexive actions built up over an extended period of practice will see the take off look smooth and well executed. The modern radio and gyro make this a breeze. Practising the hover Try to practice the hover out of ground effect whenever possible, particularly when first using narrow skids. Letting the helicopter dribble around the field an inch from the ground is just setting the model up for a serious accident. A tuft of grass, a rock or any similar projection can catch a skid at any time Now you can see why helicopters are fitting with training wheels (or at least large, wide spaced outriggers). Helicopter blades are expensive and the risk of a bent main shaft is ever present. One very popular form of training undercarriage is in the form of a pair of crossed dowels strapped to the skids with ping-pong balls glued to each of the four ends of the dowels. The aim is to prevent the helicopter tipping on landing. The advice given to me was to learn to “hover out the tank” (ie, use up a tank of fuel) out of ground effect (at a height exceeding one main rotor disc span above the ground) and exactly over a designated spot on the ground. When you can do this with the nose pointing away from you at first and later with the nose pointing towards you, you are getting somewhere. Then and only then, are you ready to undertake out and return flights. That advice was given to me 27 years ago and it is as true today as it was then. Keep practising. So there you have it: a rudimentary guide into some of the complexities of getting a helicopter into hover and more impor­tantly, out of hover and SC safely back onto the ground. MARCH 1999  55