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REMOTE CONTROL By BOB YOUNG The controls on a model aircraft This month we will begin to look at how a model aircraft manages to fly. This is necessary if we are to understand the amazingly sophisticated computer encoders used in the new generation of PCM/PPM radio control sets. As stated previously the introduction of the microprocessor into R/C transmitters has revolutionised the model control business. However, this revolution has been so far-reaching that the full implications of the uses and technology involved are almost completely outside the understanding of the average newcomer to R/C modelling. Why do we need such sophisticated encoders and what do they really do anyway? For that matter, how does an aeroplane fly? These are all vital questions to the R/C novice. These questions and many more will form the basis of the next three or four columns. In these columns it will be necessary for me to outline the basis of model aerodynamics and the problems which arise that call for sophisticated computerised solutions. 86 SILICON CHIP And for those dedicated electronics buffs who are scratching their heads over why an electronics magazine should have pages of aerodynamics, the following is an interesting practical application of Bernoulli's Theorem of considerable relevance to the electronics industry. Ever wondered why the solder smoke goes up your nose? Simple! The heat of your body causes the surrounding air to warm up and thus reduce density. This causes the warm air to rise. According to Bernoulli's Theorem, this will create a low pressure region close to your body. Below: the SR-71 Blackbird highaltitude spy plane. As with other modern high-speed aircraft, it could not be flown without a computerised flight control system. Inevitably, smoke from the soldering iron will move into this low pressure region and up the front of the body and some will pass right into your nostrils, aided by the low pressure inside the lungs. Considering that this smoke consists of lead, copper and PVC residues, this is an issue affecting the health of all in the industry. We all live at the bottom of an ocean of air. We breath it, we fly in it. Air is of great interest to us all. For those interested in learning more, read on. The next few columns will be centred upon modern aerobatic model aircraft for two reasons. Firstly, they are probably the most demanding of sophisticated solutions and secondly, I am thoroughly familiar with these problems, having flown competitive aerobatics for many years. This does not mean that other types of model do not have sophisticated needs of their own, merely that for me the explanation is much simplified when describing the modern neutrally stable, centreline aerobatic aircraft. This approach will serve to outline the principles involved. The high point of my aerobatic career came when I competed in PORT WING STARBOARD WING ~ ' RADIAL AIR-COOLED ENGINE LOW CANTILEVER WING TAIL PLANE /FUSELAGE STARBOARD AILERON PORT AILERON \ Fig.1: the basic layout for a typical full-size twin-engine aircraft. The three main components are the wing, fuselage and tail section, the latter two carrying the various control surfaces. the 1971 World Aerobatic Championships in Pennsylvania, USA. It was here that I learned just how badly I flew. Spurred on by that jolt, I went on to become a much better flyer, winning several high level local events, but pressure of business forced me to drop out of competition flying in 1976 and I have not competed since then. The amount of time one must devote to practice to remain competitive is now so great that only the truely dedicated can maintain the standard, yet I still see many of the old names in the contest results. Model aerodynamics Model aircraft aerodynamics and engineering bears only a superficial resemblance to full size aerodynamics and engineering and this is an important point to remember. People coming into the R/C movement from full size avia- tion or those that are just aviation buffs are a real problem to teach. They do, however, have one saving grace: a sound knowledge of aeronautics and airmanship. When they eventually master the subtleties of R/C flying they usually go on to make first class R/C pilots. The big problem they face is that most of them want to build models that look like " real aeroplanes". This is fine when you can fly, but the best flying models are not necessarily the prettiest. In fact, some are downright ugly but they fly well, are cheap and easy to build, and make great trainers for this reason. Scale aircraft are difficult and expensive to build and very demanding to fly - in fact some can be downright mean. The Spitfire has a terrible reputation as a scale model. It is an unfortunate fact of life that scale models do not behave at all like their big brothers. There are three main reasons for this: (1). We do not have "scale" air. This problem is of paramount importance, the ultimate effect being that lifting surfaces whose width is less than 20cm do not work well at all. The main effect of this is that wingtips and tailplanes on models are very inefficient. This leads to scale aircraft being laterally and longitudinally unstable unless some compensation is made to wingtip and tailplane size. This will be explained fully in the discussion on Reynolds numbers. (2). A model aircraft does not carry a pilot. He is firmly planted on the ground and completely out of touch with the model which leads to great difficulties in learning to fly. In fact, the process of learning to fly an R/C model is an extremely subtle one which involves learning to judge speed and propeller loading (climbing or diving) by the sound of the engine. Level flight must be attained even at long distances, at very odd angles and in bad lighting. Finally, and most importantly, the pilot must learn to cope with the apparent reversal of controls when the model changes direction from going away to coming directly towards him. The full size aviator has no previous experience in any of these subtleties. Because of the lack of a pilot though, manouevres generating forces in excess of 10G are quite permissible. This results in vast structural differences between models and full-size aircraft as well. (3). The model aircraft movement has a vastly different set of aims and goals to that of the full size aircraft movement. The most pronounced difference arising from this statement is the question of wing and power loadings. Confused? Fear not, all will be explained in due course. Design compromises To begin, it must be clearly understood that an aircraft is a vehicle whose design is a compromise of hundreds of conflicting factors and which is intended to operate in 3-dimensional space. As MARCH 1990 87 YAW AXIS <p ---ROLL AXIS ~· Fig.2: this diagram shows the three control axes of a model aircraft. The roll axis is controlled by the ailerons, the yaw axis by the rudder, and the pitch axis by the elevator. a result, this vehicle becomes a 3-axis device exhibiting a high level of interaction between all three axes. Here then is reason number one for sophisticated solutions. Carried through to modern high speed aircraft typified by the SR-71 Blackbird - a high speed, high altitude spy and research aircraft - this means that they cannot be flown safely without a computer assisted flight control system. Aircraft components Fig.1 shows the layout of a typical full size twin engine aircraft. The three main components of this aircraft are the wing, fuselage and tail section. Perhaps now would be a good time to have a look at the glossary of terms accompanying this article, so that you'll recognise the various terms as ·they are mentioned. The wing provides the lift and is responsible for supporting the aeroplane and providing roll stabilisation. Setting the two wing halves at a small angle from horizontal provides this roll stabilisation. This angle is termed the "dihederal angle". The fuselage carries the motor, fuel, R/C equipment and any other device that is carried aloft. It also serves as a structural member to locate and hold the wing and tail section. The tail section comprises a horizontal and vertical stabiliser which provide the primary pitch and yaw stability. The wing and tail sections have hinged portions which are used to control the direction of flight by exerting force around the three axes of movement. The three control axes are illustrated in Fig.2. These axes are usually said to act through the centre of gravity but are, in reality, only arbitrary representations. The roll axis The roll axis is controlled by the ailerons which are situated on the trailing edge of the wing. These may take the form of full length strip ailerons or shorter built-in RIGHT All.ERON "UP'' t :::r- _J__ L LEFT AILERON "DOWN" Fig.3: the ailerons are situated on the trailing edge of the wing and travel in opposite directions to each other. If left aileron is down & right aileron is up, the aircraft rolls to the right. 88 SILCON CHIP ailerons at the wing tip. These controls are usually situated on the main control column in a full size aircraft and on the lateral axis of the right hand control stick on a model control Tx. In both cases the movement is from centre neutral to left and right. On the model Tx, neutral is spring loaded. On a full size aircraft, aerodynamic loads provide the centering. The operation of the ailerons is a little tricky and great care must be exercised in setting them up. To turn right, the right aileron must move up and the left aileron must go down (see Fig.3). This increases the lift on the left hand wing and decreases that on the right. The result is a roll to the right. This roll will continue for as long as the aileron deflection is held. Eventually the aircraft will roll through 360°, passing through the inverted position whilst doing so. Learning to turn a model is tricky for this reason as the average beginner forgets that the model will continue to roll unless he takes off the aileron once the correct angle of bank is attained. Before he has time to realise what is happening the novice has his aircraft inverted and is in terrible trouble. The correct sequence for turning is aileron on until the correct angle of bank is attained, . aileron off, elevator up and hold. The elevator is then held until the turn is complete. You then apply elevator off, opposite aileron on, until the wings are level, then aileron off. The correct angles of deflection are dependent upon the airframe control and stability factors and can only be learned by experience. It is very easy to get the ailerons hooked up in the wrong sense. Even the great Bob Young has arrived at the field only to find his ailerons were hooked up in reverse. It only happened once, but that was enough. Fortunately, I found it on the ground but I have seen models crashed because of this error. To emphasise the point, the prototype Avro Tudor II G-AGSU crashed in 1947 due to the ailerons being reassembled in reverse, killing Avro's chief designer Roy Chadwick and-the pilot. Glossary of Terms Mainplane: the wing or primary lifting surface. Wing root: junction of the wing and fuselage. Wing tip: the outer end of each wing. Wing section: the cross section of the wing taken along the chord line; commonly called the aerofoil section. Wing chord: width of the wing from leading edge to trailing edge. Chord line: the straight line through the extreme leading and trailing edge of the wing aerofoil. Dihedral: the angle each wing half is lifted from the horizontal. Aspect ratio: the relationship between wing span and wing cord . Aileron: hinged portion of the wing trailing edge used for control over the roll axis. Flap: hinged portion of the trailing edge used to improve aerofoil shape, to increase the angle of attack and therefore generate more lift. Downthrust: the angle at which the thrustline is offset to the fuselage centreline. Firewall: the bulkhead between the motor compartment and the fuselage proper. Usually used to attach the motor. Longitudinal dihedral: the angle The moral is always check the direction of controls before the first flight of the day, particularly if you have servo reversing switches on your Tx. These switches are very dangerous and need constant attention. I check the operation and direction of the controls before every takeoff. Aerodynamically, the ailerons have a very complex and sometimes peculiar effect and we will examine the sophisticated computer and aerodynamic solutions to these problems shortly. The pitch axis The pitch axis is under control of the elevator or hinged portion of the between the chord line of the wing and the chord line of the tailplane. Angle of incidence: the rigging angle of the lifting surfaces relative to some datum line, usually the fuselage centreline. Undercarriage: the landing gear, usually two main wheels and a tail wheel or skid, or a tricycle, comprising a nosewheel and two mains. May be fixed or retractable. Fin: vertical stabiliser. Rudder: hinged rear portion of the vertical stabiliser and used for control over the yaw axis. Tailplane: the horizontal stabiliser Elevator: the hinged rear section of the horizontal stabiliser; used for control over the pitching axis. Stability: the ability to return to some particular condition of trimmed flight after a disturbance, without any effort on the part of the pilot. Instability: the tendency to diverge farther away from the trimmed position when disturbed. Neutral stability: the ability to maintain the new position after disturbance until disturbed again. Essential for aerobatic aircraft. Centre of gravity: the gravitational balance point of the aircraft. Must be carefully positioned in relation to the aerodynamic centre of pressure of the aircraft. tailplane. Sometimes the entire tailplane is pivoted on a centre axle and this arrangement is termed an "all moving tailplane". This arrangement does have some aerodynamic advantages which are balanced by some mechanical disadvantages. It is not used as often as the conventional hinged elevator arrangement. The entire tailplane also provides the longitudinal stability of the aircraft. The angle between the wing and tailplane chord lines is termed the "longitudinal dihedral". This angle, the tailplane area, wing and tailplane section and centre of gravity location determine the overall longitudinal stability and manoeuvreability of the aircraft. As stated previously, compromise is the name of the game in aircraft design, and the correct balance of the foregoing factors determines how well an aircraft will perform. In the neutrally stable, centreline aerobatic model, the wing and tailplane are rigged at zero-zero incidence, with the thrustline, symmetrical wing and symmetrical tailplane section on the fuselage centreline. The result is an aircraft that will go exactly where you point it - just what the doctor ordered for accurate aerobatics. It will not increase or decrease the angle of climb or dive. It has no natural inbuilt stability. Any such aircraft fitted with a failsafe that neutralises the controls is doomed if control is lost. The elevator control is usually located on the main control column in full size aircraft. In R/C transmitters, elevator is fitted to the fore and aft axis with spring loaded neutral [left hand stick in Mode 1 and the right hand stick in Mode 2). In all cases, pushing forwards applies DOWN elevator and pulling back applies UP. This corresponds to the trailing edge of the elevator going up for UP elevator. The action of the elevators is complex but essentially they alter the direction of the thrust vector and the direction and magnitude of the lift vector. This vector is, in turn, related to engine rpm and aircraft weight. Small elevator deflections will alter the trim from climb to dive. With sufficient thrust and up elevator deflection, the aircraft will complete a loop and will continue to loop for as long as that power setting and control deflection is held. The yaw axis The yaw axis is under control of the rudder, or hinged portion of the vertical stabiliser. This stabiliser provides what may be best described as "weather cock" stability and is used for directional stability and control. The rudder controls on a full size aircraft may be located on the rudder bar or foot pedals on the continued on page 95 MARCH 1990 89 Five-in-one soldering tool kit These butane soldering irons keep getting better and cheaper all the time. This Vulcan 30 model comes with a catalytic flameless burner a nd four separate soldering iron tips plus a number of torch accessories: a pencil flame torch which is useful for brazing, a hot knife which is handy for cutting thermoplastic materials, and a hot blower which is almost essential these days for high temperature shrink sleeving. There is also a wide flame. torch which would be ideal for paint removal or perhaps removing stubborn nuts. All of this is packed into a handy carrying case together with a sponge pad for cleaning the soldering tip and a quantity of 1mm resin cored solder. There is also space inside the carrying case for a butane gas lighter which would be convenient for lighting the torch it is on the left hand lateral stick for the Commodore Amiga. It uses the same DB-25 plug and 36 pin Centronics plug but the wiring is different to that for the IBM PC printer cable. Fortunately, Sheridan Electronics has a printer cable for the Commodore Amiga at the very reasonable price of $6.50. Sheridan's has moved recently by the way, and they're now at 286 Cleveland Street, Surry Hills, NSW 2010. They also plan to open a new store in Blacktown soon, so stay posted. Phone (02) 699 6912 for further information. Remote Control ctd from page 89 cockpit floor or on a steering wheel mounted on the main control column. On both modes in a model Tx, axis with spring loaded neutral and left or right deflections. Operation of the rudder is straightforward with the trailing edge of the rudder moving left for LEFT rudder, when viewed from behind the aircraft. Aerodynamically, the action of the rudder is extremely complex, particularly in aircraft with dihedral, and this will be explained in the control and stability section. Essentially it begins by creating a yaw, which in turn, develops into a roll which in turn develops into a spiral dive. Prolonged application of rudder will result in a crash. In the "good old days", stuck rudders led to the demise of many fine models - usually caused by running out of turns on the elastic band driving the rudder actuator. Sounds quaint, doesn't it'? The three foregoing controls, along with the throttle control, form the primary controls required for a model aircraft. In fact, I prefer to think of an aircraft as a 4-axis device as the throttle is a vital control and is used as much as any of the three primary controls in performing manoeuvres. The throttle is a non-return stick fitted with a friction device or ratchet. On Mode 1 Txs it is on the accessories. Our sample unit came · from David Reid Electronics who have the unit in stock at $59.95. See David Reid's at 127 York Street, Sydney, NSW 2000. right hand fore and aft axis and on Mode 2 Txs it is on the left hand fore and aft axis. ~ ACTIVE SHORT WAVE ANTENNA TECHNIKIT AT4SW (SEE SC JAN '90) COMPLETE KIT $59 BUILT and TESTED $119 (BATTERIES INC) CASE $10 LOOP ANTENNA Q TECHNIKIT PX1 COMPLETE KIT $44 BUILT and TESTED $69 (SEE SC JUNE 89) Improved signal strength & signal quality in a portabl e tunable antenna. PACKING & POSTAGE IN AUSTRALIA INCLUDED IN PRICES QUOTED. WRITE OR RING FOR BROCHURES ORDERS ACCEPTED ANYTI ME PAYMENTS BY BANKCARD. VISA, MASTERCARD, CHEQUE or MONEYORDER TRADE ENQUIRIES WELCOME JILOA PTY LTD (TECHNIKIT DIVISION) P.O. BOX 73, GLENHUNTLY, VIC 3163 Phone (03) 571 6303 M ARCH 1990 95