Cause: Center of Gravity, (CG), too far aft.
Fix: Move the CG forward.
Comment: Although rarely seen, instability in the pitch axis of a model aircraft can occur if the centre of gravity of the aircraft is sufficiently rearward.
Typically for an aeroplane to have sufficient stability in the pitch axis, we would expect to see about 10 percent Static Margin, (SM), where the percentage referred to is expressed as a percent of wing chord. In the limit, an aeroplane with zero SM. would be where the CG is coincident with the Neutral Point. In such a case the aircraft would probably be un-flyable. Specialist jet aircraft can enter this region and but usually only with computer assist.
When model aircraft reach the point where they are difficult to fly the first awareness is that they seem "twitchy". Beyond this, unexpected and uncommanded rotations about the pitch axis can occur. The video shows this behaviour.
The cause of the behaviour is as follows:
The wing is kept at an angle of attack which is suitable for flight by the tailplane, (stabiliser). The stabilising effect of the tailplane is achieved by moments about the CG. The wing's desire to auto rotate is stopped by the tailplane's counter moment, (a product of tailplane moment arm and tailplane lift force).
As the CG moves rearward towards the Neutral Point, the greater the wing's moment about the CG becomes, and the lesser the tailplane's restoring moment about the CG becomes, until, in the limit, the tailplane fails to do it's job, and auto rotations about the pitch axis begin.
To understand more about these aerodynamic aspects of model aircraft, we would recommend Martin Simons' book, Model Aircraft Aerodynamics.
That said, if these phenomena occur they tend to occur at takeoff and landing where the tailplane's ability to provide the needed restoring moment is least effective because of low airflow. Should a pilot get airborne under these conditions however, the only way forward is to accumulate airspeed, as without it, the tailplane will fail to control the wing. This applies to landing too, i.e., no possibility of gliding gently down.
Interesting though this may be, the answer is simple get the CG location correct in the first place.
Given the level of excellence of model aircraft today, both in terms of ARTFs etc. and plan built aeroplanes, the chances of getting into these conditions are probably limited to the following:
1. Failure to balance the model correctly.
2. Failure to re-balance the model correctly, e.g. after rework which made the tailplane heavier e.g. adding tail mounted servos.
3. Designing your own aeroplane without any knowledge of aerodynamics.
Using the software below, will allow the model pilot to enter his aircraft's data and obtain the Static Margin value. i.e. establish before flying or maidening a new aircraft, whether the CG to be used has adequate SM.
Related Articles:
Neutral Point and Static Margin. 1. Visualisation.
Neutral Point and Static Margin. 2. The Winning Formula.
Neutral Point and Static Margin. 3. Software.
Neutral Point and Static Margin 4. Tucking In
Neutral Point and Static Margin. 5. Stall On Landing Approach
Neutral Point and Static Margin. 6. Aircraft unstable about the Pitch, or lateral Axis.
