This question explains that an aircraft can be statically stable (it will seek to return to equilibrium) but dynamically unstable (the amplitude of the oscillations increase) if there isn't enough damping in the stability equation.

That's fine from a mathematical point of view, but what practical change would increase the damping and dynamic stability?

I've seen quite a few radio-control trainers that are dynamically unstable, and would love to know how to fix the problem so that they're easier for the student pilot to fly.

These planes typically pull out of a dive on their own, but then climb excessively and stall, leading to another dive. Each subsequent stall and dive is more dramatic than the last.

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    $\begingroup$ The old saw is to move weight forward. The ASE explanation generally involves torque around the CG. Keeping wing CP aft of CG would help control rise of nose as lift increases (because the wing actually torque the nose down). Additionally, more of the wing now functions as a "tail" (stabilizing area behind the CG). From work with model gliders, essentially the wing is too large/strong for its "tail". One must be wary of excessive stability, which makes it harder to pull out of a dive. $\endgroup$ – Robert DiGiovanni Jan 8 at 10:53
  • $\begingroup$ @RobertDiGiovanni - Isn't that just static stability? I'm talking about planes that pull out of dive on their own, but then climb excessively and stall, leading to another dive. $\endgroup$ – Robin Bennett Jan 8 at 11:26
  • $\begingroup$ Not really, it depends whether or not the wing is lifting the nose or the tail is pushing down. I can see how swept wings really drive engineers batty, particularly when slats are deployed. Moving CP too far can make control difficult, especially with a smaller tail. $\endgroup$ – Robert DiGiovanni Jan 8 at 11:30
  • $\begingroup$ @RobertDiGiovanni - Can you explain how that is different from static stability? $\endgroup$ – Robin Bennett Jan 8 at 11:53
  • $\begingroup$ I've seen in claimed that moving the CG forward can actually make dynamic instability worse, because more decalage (download on tail) is needed. I don't know the truth of that. A good place to seek further discussion would be this on-line forum -- rcgroups.com/modeling-science-136 $\endgroup$ – quiet flyer Jan 8 at 14:06

Damping is produced by drag and by large induced speeds at the tail surfaces from a given disturbance. This can be caused by long lever arms of these surfaces or by high air density.

More on the topic can be found here:

These planes typically pull out of a dive on their own, but then climb excessively and stall …

This is the classic long period mode in longitudinal stability. Since rotation rates are low, pitch damping also is low and the most important damping contribution is from drag. A low L/D reduces the tendency to overshoot, a high trim speed reduces the tendency to stall (and shifts the motion to higher speeds with lower L/D). Reducing static stability will make the period longer such that it becomes easier for the pilot to react. However, lower stability will make the pitch response more sensitive which increases the risk of too large control inputs.

  • $\begingroup$ That certainly matches the planes I've seen. The ones that were dynamically unstable were efficient glider style planes, while basic boxy designs with fixed gear were much more stable. Also, learning to fly in this sort of trainer usually involves the student's response time improving with practice until they no longer have POI - usually only an hour or two of flight time but it would be nice to skip that stage! $\endgroup$ – Robin Bennett Jan 8 at 15:48
  • $\begingroup$ You mean Pilot Induced Oscillation? $\endgroup$ – skipper44 Jan 8 at 17:20
  • $\begingroup$ @skipper44 Yes, that's why I linked to the Wikipedia page for pilot induced oscillation. $\endgroup$ – Peter Kämpf Jan 8 at 18:16
  • $\begingroup$ @PeterKämpf - you and I both miss-typed PIO as POI $\endgroup$ – Robin Bennett Jan 9 at 10:36
  • $\begingroup$ @RobinBennett Thank you for spotting this! Corrected. I wonder how that happened. $\endgroup$ – Peter Kämpf Jan 9 at 12:52

These planes typically pull out of a dive on their own, but then climb excessively and stall, leading to another dive.

R/C flying can be started at an early age and gives any potential pilot a huge head start in gaining experience in the all important fundamentals of flight.

A lesson in the importance of keeping CG within the specified range (and the consequences of not) is better learned at model, rather than full scale.

Not paying attention to aft CG limits (weight too far back) will reduce directional stability in pitch and yaw. It will roll 360 beautifully, but only because the tail will constantly try to drop throughout the roll, always raising the nose. In general the aircraft will be more maneuverable, but harder to control. (This is why modern military aircraft use computers to assist stability).

Among the plethora of bad things (such as stalling low and slow) that can happen, dynamic instability is another consequence of out of range CG. Especially with models, a fraction of an inch can matter.

But if you build from scratch, it is important to properly match the tail and wing. Amazingly, a horizontal "stabilizer" destabilizes pitch when a plane rises or sinks vertically. This is a very important aspect of static stability.

Rising or sinking vertically is a function of lift. Therefor, excessive lift can cause a plane to "overshoot" its correction to original flight path. An extreme example of this is a loop.

So we generally design the wing Center of Pressure to be aft of the Center of Gravity so the torque of the wing lift around the center of gravity helps control the pitching tendency when lift is increased.

The further back the center of gravity, the greater the pitching tendency will be.

For custom scratch builders, a larger tail or longer tail moment is an option, but remember, if the tail supports weight, you are essentially building a bi-plane.


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