Helicopter Gyro's


Gyro stabilising systems are fundamental in achieving control as without it a RC helicopter is uncontrollable because its relative low weight makes it react too quick for human reflexes.  Though generally not considered is that the rotor head itself is one big gyro and it was the development of the flybar, again a form of gyro control, that allowed for success.

Following on from control of the main rotors, control of the 'tail' was to prove the most difficult aspect of learning to master a helicopter.  Because of the relative light weight and thus low inertia, damping of rotation around the axis of the main rotor shaft was near enough non existent.  Additionally, as lift was altered for height control the required counter torque to prevent rotation also changed and thus the tail could only be 'trimmed' for one lift condition.  Fixed pitch control did remove the unbalance aspect because as the main rotor speeded up, so did the tail and thus 'automatic' torque balance was achieved.  Though what did remain was torque 'twisting' due to the change in power required to accelerate (or decelerate) the rotor head, to counter this it was necessary to anticipate the swing and apply opposite tail when changing power setting.

Unfortunately when considering the use of collective pitch the rotor system stayed at a constant speed so there was no 'automatic' balancing from the tail rotor when changing pitch.  At anything other than the trim set position the helicopter would rotate and to counter this, 'holding on' opposite tail control was required.  Various means of mechanical connection from rotor pitch to tail pitch were used to overcome this aspect but these were not fully effective and again had to be 'tuned' around a set condition i.e. generally the hover for learners or forward flight for more experienced pilots who could 'hold in' a little tail rotor control for hovering.  Even with reasonable mixing, as with the fixed pitch system, rapid changes in main rotor pitch resulted in 'torque' twisting and if carried out quickly, in calm conditions could pirouette the helicopter before stability was achieved.

The rate gyro was a means of slowing down this power induced rotation as it reacted to movement; thus if the tail swung it would automatically apply the opposite 'stick' control to try and prevent the movement and thus relieve the pilot of a lot of 'work' when hovering especially in gusty conditions.  The only down side with this method of control is that the gyro would also tend to 'fight' the control of the tail even if the pilot wanted it to move and thus if too much gain (opposite movement as a result of the input movement) was used the helicopter would be very difficult to rotate and whilst good for hovering would seriously degrade general flying; the setting was therefore a compromise between stability and controllability.

So how does it work?  As stated above with the same principle as the rotor head in that if you spin a fixed weight, like a wheel, around a spindle then press on on side, it actually moves 90 degrees from where you press it.  Same goes for the force of a movement and this can be shown by spinning a bicycle wheel and holding the axle bolt horizontal.  If you move it backwards and forwards then nothing happens, it just moves however, try rotating the spindle horizontally and it will try and pull out of your hands very strongly in a vertical movement.  Same thing applies for the model gyro which has a motor with a flywheel at each end and supported on a spindle; when the motor spins the motor rotates on the spindle.  Now resist this movement with a spring so that the amount it can move is directly related to the speed of rotation and you have a control system.  Connect the spindle to a variable resistor and with a bit of electronic 'gubbins' connected up, low and behold a tail servo rate gyro.

In order to allow for this 'double' requirement the gain circuit of the gyro could be fitted with a trimmer moved by an arm and therefore vary the gain from low to high.  With the arm connected to a servo on a 'spare' channel, the gain could be changed in flight and adjusted to whatever setting the pilot required.  However, in reality it was found that only the two setting for low gain whilst in the circuit and high gain whilst in the hover was really necessary and so a 'switched' function was provided with a trimmer adjustment for each position thus ensuring they could be matched to the pilots requirement.

An alternative solution would be to have a method of reducing the gain when the pilot wanted the helicopter to rotate but ensure it stayed high when the pilot did not want it to rotate, this method of control was known as 'Pilot authority'.  Basically the gain control monitored the tail rotor servo signal and if it moved either way from the neutral position the gain was reduced until at maximum throw the gyro gain was zero.  This meant that very high pirouette speeds could be achieved but releasing the tail rotor stick meant a return to maximum gyro gain which swiftly and dramatically brought the tail under control.  Not as popular as the two gain switched gyro's because rotation rate was not constant with stick movement and so moving the stick twice as far would give four times the rotation speed not twice the speed as in a fixed rate gyro; useful for aerobatic's etc but not really for nice smooth scale or sport flying.

Up to this point a gyro worked on the principle of the gyroscopic action of a quickly rotating, relatively heavy flywheel and inertia played a big factor in the speed of reaction.  For faster reactions the flywheel could be lightened or the spring rate reduced but then smooth operation suffered and so there was a limit to what could be achieved with a 'mechanical' gyro.  However, it was found that some semi-conductors reacted to changes in pressure be it static air pressure or just movement inertia.  They were very sensitive and could react to extremely small movement and that reaction could be built into a circuit to monitor the movement and provide a correcting signal to operate a servo.  This new technology was called a piezo sensor and was capable of very quick tail control even for slight movement or extremely quick rotations.  Now this new technology did not utilise the principle of a spinning gyroscopic mass so it should not have been called a gyro.  However, by this time the term 'gyro' had come to also mean the action that was synonymous with a spinning gyro and thus anything that provided a controlling influence on rotational movement tended to be called a gyro; so 'Piezo' gyro it was...!!!

Gyro technology had taken a significant leap forward and they were now capable of almost (but not quite) holding a tail steady; reason being it still required movement (albeit now only very small movement) before the gyro would react.  As stated above, the piezo gyro was first and foremost a very accurate movement sensor and coupled with the technology of the time it could accurately measure how many degrees the tail was moving and the rate of movement.  It was thus only a matter of time before a PLC (mini computer chip) was incorporated into the circuitry that plotted the movement and thus knew exactly where the tail was from an arbitrary starting point.  With this information it was then possible to develop a program that could return the tail to this starting point and thus the 'Heading Hold' piezo gyro was born.  Simple to say, but not that simple to achieve and even when brought out on the market the programming updates were to continue in refining the control parameters for different applications.

Gyro's had been for quite a while finding their way onto multi-blade models in order to tame some of the characteristics of 'Flybarless' helicopters.  A Heading hold facility as first glance might appear ideal however, it would try and hold the body in the same attitude which is not always the same as controlling the wayward operation of the rotor head.  What was needed was a combination of the two depending upon what the head was doing and how much it was moving around.  Computers again, and it was only a matter of time before the software for specific 'scale' head gyro's were being written.  The first examples had two sensors for roll and pitching movement of the head and they worked after a fashion but occasionally did weird and wonderful things if the gain was too high.  Eventually they incorporated three sensors for full 3 axis control and the programming had improved to such a point that it was possible to make a 'Flybarless' model behave exactly as if it was controlled with a Flybar and thus a 'Tyro' could start with one if he so wishes.  However, again there are downsides to everything, in Heading Hold mode the sticks do NOT actually control the servo's but instead initiate a command for a set movement.  If for some reason the model cannot move then the servo's will continue to maximum throw in an attempt to 'obey' the last command, this has tipped over many a 'Flybarless' model when spooling up or spooling down and is particularly nasty if carryout out an autorotation landing once the skids touch down.  Ok this last bit is probably well in the future for covering in a Vintage web site so without further ado;