Fullscreen HTML5Med Res (??MB)HTML4

REMOTE CONTROL BY BOB YOUNG The mysteries of mixing This article has nothing to do with the making of alco­holic drinks, about how to behave at parties or even the design of radio receivers. It is about mixing the control signals to servos in models. Mixing makes difficult models easier to fly. Now that my Mk.22 transmitter design is close to realisa­ tion, it is appropriate to consider the mysteries of mixing. Why? – because one of the most powerful features of the proposed Mk.22 transmitter is the provision for mixing any or all channels from 1 to 24. Now mixing is a little understood subject and so we will spend some time examining the interaction between elec­ tronic theory and practical application. It is the ability to mix controls on the modern transmitter that has contributed greatly to the vastly improved standards of performance and skills of the operators. Without mixing, some flying manoeuvres would be virtually impossible, particularly in helicopters and high performance gliders. Mixing is best defined as the modification of one or more control positions by inputs from one or more different control channels. In its simplest form, it consists merely of a small shift in neutral on one channel controlled by the full excursion of another channel. In its most complex form, it may require inputs from three or four channels, some with add-subtract (dif­ ferential or inversion) inputs. There are many practical reasons for using mixing and mostly they fall into the category of making it easier on the driver. A good example is the tail rotor control on a model helicopter. The prime function of the tail rotor is to hold the tail boom in the desired location against the torque of the main rotor blades. If the throttle/collective pitch control (these are usually coupled on model helicopters) is increased, there will be more torque and the tail rotor will therefore require more pitch to Fig.1: a simple mixing circuit which could be useful for easier control of a helicopter. Some of the throttle control input (CH1) is fed to the tail rotor control channel (CH2) to introduce automatic compensation for torque changes in the main rotor. VR1 is used to set the mix ratio of the feedback voltage. compensate. Likewise, if the throttle is reduced, the tail rotor will require a reduction in pitch. Now flying helicopters is a real handful at any time because all four primary controls are constantly in motion and the level of manual dexterity required from the pilot to co-ordinate all four controls simultaneously is very high. Here then is a prime application for mixing. If we take some of the throttle control input and feed it across to the tail rotor control channel, then we can effectively introduce automat­ic compensation for torque changes in the main rotor. Fig.1 shows a representative circuit for a mixer of this style. It is the most simple of the mixing circuits in that a small percentage of the main control channel is used modify the neutral of the second channel. The direction of the feedback remains constant with no inversions required. I must point out that the circuits presented here are representative of the type of mixer for use on voltage driven encoders (they will not work on the old 1/2 shot encoders). These encoders use a reference which is 1/2 of the regulated supply rails. In this manner, the control pots can be inverted for servo reversing without any neutral shift in the servos. Thus, the REF input is connected to the 1/2 regulated supply rail of the trans­mitter. Referring to Fig.1 the main control pot of Channel 1 (CP1) supplies a feedback voltage to the control input of Channel 2 (CP2) via the gain set control pot VR1. This pot is used to set the mix ratio of the feedback voltage. The values will vary depending on which encoder you are hooking the mixer into. Typi­cally, pots CP1 and CP2 are 5kΩ, VR1 is 50kΩ and all fixed resis­tors are about 100kΩ. In the practical example of our helicopter model, CP1 is the control December 1995  81 Fig.2(a): mixed elevators/flaps are used for aerobatics or com­ pensation for trim shift induced by large angles of takeoff/landing flaps. It is desirable to arrange for the mixing to be switched in or out very quickly and easily during normal flight. Fig.2(b) shows elevator trim compensation for the pitch change that takes place when the takeoff or landing flaps are selected. The direction of compen­sation will depend on the configuration of the aircraft Fig.3: the plan and end elevation of a typical glider wing. The outboard trailing edge panels are the ailerons and perform some unusual functions. The inboard panels are the variable camber panels and they also perform multiple tasks. pot (stick) for the Throttle/Collective pitch and is thus the primary control. CP2 is the stick control pot for the tail rotor. To set the system up, you would place the Throttle and Tail Rotor control sticks in neutral and set VR1 for an approximation of the desired mix ratio. Moving the Throttle stick will now induce a neutral shift on the Tail Rotor pitch. The amount of Tail Rotor pitch change is adjustable via potentiometer VR1 and is found by experimentation. This will vary from model to model 82  Silicon Chip due to aerodynamic influences. Model aircraft Another application is the mixing of flaps and elevators in a model aircraft. There are two basic scenarios here: (1) the use of mixed elevators/flaps for aerobatics; and (2) compensation for trim shift induced by large angles of takeoff/landing flaps. In both cases, unlike the helicopter scenario, it may be desirable to arrange for the mixing to be quickly switched in or out during flight. To do this, a switch inserted in the feedback line from VR1 is all that is required. This switch is best mounted on the front of the Tx case. In this case, the flaps work in reverse to the elevators but deflect equally about neutral (Fig.2a). The ratio of elevator movement to flap movement is again set via VR1. This is an old control-line trick and the effect of this arrangement is to tighten the radius of inside and outside loops to the point where square loops are possible. A further extension of this circuit is used for elevator trim compensation of the pitch change that takes place when the takeoff or landing flaps are selected (Fig.2b). Putting the flaps down can result in violent trim changes on full size and model aircraft. This is brought about by the large change in angle of attack on the wing and the sudden shift in the centre of drag in relation to the thrust line of the aircraft. As a result, large control inputs may be required on the elevators. The direction of compensation will depend on the configu­ration of the aircraft. As a general rule, high wing aircraft will require down elevator trim and low wing aircraft, up eleva­tor trim. Further variations are possible in that the flaps may be proportional or switched. In the first case, a further complica­ tion is introduced in that there will be a full excursion of the flap channel from the up or closed position which will be the neutral position for the elevator feedback, hence the flaps only supply a one-way correction. A more simple system is the fitting of a 3-position switch as the flap control instead of the pot. This would provide closed (0°), takeoff (15-20°) and landing (60-90°) flap positions. The same circuit could be used to control the cavitation plates on high speed model boats. Here, they could be coupled to the throttle and possibly even with some rudder mixed in to help control the turns. All of the above come under the heading of operator aids – nice touches, designed to make life easier for the driver. Glider controls A more complex situation arises in the class F5B and F5J gliders. These are required to perform a variety of tasks which include endurance, distance and pure speed runs. These tasks virtually call for three separate airframes and the design of a single airframe to achieve the best compromise is a very flap movement. Also, during the speed run, a small amount of up flap deflection may improve the aerofoil, again depending on the aerofoil selected for the model. All of the above only requires a simple mixer. Getting complicated Fig.4: this mixer provides add-subtract outputs. Thus, the two channels controlling the aileron servos are coupled together, with a reversal on one channel for normal aileron control. It may also be desirable to mix some aileron control into the flap panels to help improve turns. demand­ing exercise indeed. To get the results they require, the glider operators make extensive use of mixing. Here we find mixing being used to actually reconfigure the physical properties of the entire wing and this application falls well and truly outside the bounds of mere operator comfort. For the competition glider pilot, this is life and death stuff. One of the big problems they face is getting the model back on the ground due to the cleanness of the airframe. These models are capable of very high speeds and most enter the speed trap at speeds around 220km/h. (Yes I did say they were gliders. You know, no motor). Once these models hit ground effect, they can glide on forever and so very effective spoilers are a must. In addition, the endurance run requires a different camber on the wing aerofoil to that required for the speed run. Thus, the entire trailing edge of the wing is given over to variable camber devices which are required to carry out a variety of functions. Fig.3 shows the plan outline of a typical glider wing. The outboard trailing edge panels are the ailerons and perform some unusual functions. The inboard panels are the variable camber panels and they also perform multiple tasks. In addition to the complex wing functions, these models need aileron/ rudder coupling for the entire flight. This is largely due to the reduction in drag on the inboard wing tip and the increase in drag on the outboard wing tip screwing the aircraft in the opposite direction to the turn. The long, high aspect ratio wing (typically 13-15:1) makes this effect more pronounced on gliders, particularly during the slow speed endurance flights. To discuss the mixing required for contest gliders, we need to understand that each control surface on the wing requires a separate servo and thus four servos and four separate channels are used, all with mixing applied. In addition, there is the usual config­uration of a separate elevator and rudder servos, the only unusual feature being that the rudder and elevator servos may be buried in the fin or rear fuselage for balance. So we are talking about a very sophisticated little aeroplane capable of a wide range of tasks. To begin, let’s put the simple mixer of Fig.1 in place for a coupled aileron/ rudder. This is usually switched out during the speed run. During the high speed runs, very snappy turns are required and here the old control line trick discussed previously is of great benefit. Thus, we must add another mixer for coupled flaps/elevators, only this time we mix in both flap ser­vos. So, when the elevators go up both flap servos go down, the mix again being determined by experimentation. This must be capable of being switched in and out, as it is not desirable to use this feature in the endurance run, for example. The typical maximum deflection of the flap is about 5°. It is also desirable to use variable camber on the trailing edge of the wing to provide the best lift/drag ratio on the aerofoil for each task, so we must have normal flap control. Hence, we select a bit of flap to increase the camber during the endurance run, to improve the lift/ drag ratio of the wing. Thus, both flaps need to be able to be moved down as a normal flap, the angle of deflection depending on the aerofoil section used. In addition, during winch launch, the wing camber is in­creased for maximum lift and thus line tension. This calls for approximately 20° of down Now we get to the really complicated bit. The ailerons which control the roll axis require opposite rotation from each servo, thus any mixing applied to these controls will require an inverter with a gain of -1. The mixer in Fig.4 is typical and provides Add-Subtract outputs. Thus, the two channels controlling the aileron servos are coupled together with a reversal on one channel for normal aileron control. It may also be desirable to mix some aileron control into the flap panels to help improve the turns. The landing configuration calls for the lift to be dumped and the drag to be increased as much as possible. Here we see a remarkable configuration used on the wing which is known as “crow”. In this configuration, the ailerons which usually work in opposition are both raised up 20°. This reduces the lift across this portion of the wing and also ensures that the wing tips do not stall before the centre section. Conversely, the centre section flaps are deflected down by approximately 60° to provide the drag necessary to slow these missiles down for landing. All of this requires very complex mixing facilities and a great deal of experience on the pilot’s behalf to set up and master. All of the above combinations must be capable of being switched in and out instantly and in the heat of a turn at 220km/h, initiated up to 1km from the operator and sometimes close to the ground. This is definitely not for the fainthearted. So there you have it. It only takes a moment’s reflection to see that the development of a commercial computerised transmitter with the flexibility to handle all of the above scenarios is a serious undertaking. You can also see why the modern computer radio has become so complex and why in many instances it has outgrown the requirements of the average club modeller. The proposed Mk.22 transmitter will have a simple system which can be tailored to your own requirements. You add only the features you need. Only a handful of people require a system as SC complex as described earlier. December 1995  83