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  • Control and Stability

    Gary B

    It has been a while since I wrote an article for the website and I’m in the mood so here goes!

    Although I’m not a BMFA instructor I do teach club members occasionally using a buddy lead and give a limited amount of theory to help them understand how models fly (too much glazes their eyes over!).

    It is understandable that beginners have very little knowledge of the theory of flight but it is sometimes surprising that experienced model flyers also have a limited knowledge.

    In a former life I was a full-time gliding instructor as part of the Joint Services Adventurous Training organisation, we took soldiers, sailors and airmen with no prior flying experience and aimed to send them solo on aerotow after four or five days intensive coaching.


    If I was instructing (as opposed to running the course or flying the tugs and motor-gliders) I would be given a group of four people and ‘nurture’ them through the week. On the first day of the course they would all be flown in the motor-glider (Grob 109B) and given an hour instruction purely on the effects of controls with most of the course members being able to fly straight and level and carry out medium banked turns by the end of the lesson.

    The motor-glider lesson was often preceded by a short blackboard briefing and use of the hands to simulate aircraft movements!

    What we taught was quite basic but fundamental, particularly the standard terminology that we used, it also applies to model aircraft.

    So…what did we talk about? The very first lesson was ‘effects of controls’, quite simple and most people got this straight away. Text in speech marks was the standard ‘patter’ (in-flight commentary) that we used, key words are in bold.


    Primary effects:


    ‘The effect of the elevator is to control the aircraft in pitch, it pitches the nose up or down, it changes the attitude and the speed.’ Demonstrated by moving the stick forward and seeing the horizon position change and the speed increasing then stick back to slow down to the stall which involved a ‘nose drop’ showing that the elevator won’t always raise the nose.


    ‘The primary effect of the ailerons is to roll the aircraft to a bank angle’. Demonstrated by moving the stick to left or right and seeing the response.


    ‘The primary effect of rudder is to yaw the aircraft’. Demonstrated by applying rudder one way or the other and seeing the response.

    Secondary effects:

    There were always a few puzzled looks when we explained that two of the controls had secondary effects!


    ‘The secondary effect of ailerons is to yaw the aircraft’. Demonstrated by holding the rudder central and moving the stick from side to side which produced a yaw in the opposite direction to the roll (adverse yaw). Explained very briefly by stating that the down going aileron produces more lift but also more drag, like having a little man on the wing tip pulling it back! We cheated in the first aileron demo by applying rudder to mask the effect.


    ‘The secondary effect of rudder is to roll the aircraft.’ Demonstrated by holding the stick central and applying rudder, the glider would yaw then roll. Again we cheated in the first rudder demo by using opposite aileron to stop the roll. Some students did notice this cheating going on!

    By the end of the first day the ‘studes’ had a good grounding of the effects of controls and the standard terminology (pitch, roll and yaw).


    On these basic courses we did not go much further with theory but for advanced courses (Bronze ‘C’ badge level) the knowledge requirement was more in depth but still simple enough to understand.

    The axes were introduced (normal, lateral and longitudinal) and stability features were explained. Other concepts such as differential aileron design and washout were also taught.

    The answers were often squeezed out of the students by firing questions at them, i.e. ‘what gives stability in pitch?’

    We would fill out a table on the blackboard which I have reproduced below:

    Control Axis Primary Effect Secondary Effect Stability
    Elevator Lateral Pitch None Tailplane
    Ailerons Longitudinal Roll Yaw Dihedral
    Rudder Normal Yaw Roll Fin

    I always thought that time spent on this subject was important, it can certainly help us modellers to understand how our gliders behave.

    The ultimate use of this is understanding what is happening in a side slip manoeuvre, yaw is applied with the rudder to change the angle of the fuselage for more drag, opposite aileron is applied to stop the secondary effect (roll) of the rudder to keep the wings level  and elevator is used to maintain a constant pitch attitude. I've always wanted to try this with a radio-controlled model but I would have the disadvantage of not sitting in the cockpit to see what is going on!


    Gary Binnie


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