wingspan modification design question

[robbie]

i have noticed several times in these forums that scratch builders have modified airplane designs by extending wingspan and apparently leaving the rest of the design the same. i can see how this would lower overall wing loading by increasing surface area, but how much is too much? doesn't this mess up the proportions of the plane?
aren't there some design parameters that must be obeyed?
the wingspan is some percentage of the chord, and the surface area of the tail group and the control surfaces are all supposed to be in proportion?
(btw, i did this mod on a plane before and it flew perfect, i just want to know "why" i guess)
can somebody point me to an article or thread on this subject?
thanks

[Sparky Paul]

The effect on changing wing area is mostly asorbed by the very lax requirements for the stabilizing tail surfaces.
Only if the plane's proportiongs are close to or at the minimum safe limits re the tail surface area would much effect be noticeable in flight.
And if the performance requirements are changed to accomodate the change in proportions, there might be no change in what the plane is doing.
I've changed the tail surface area proportions on Kadets, either by adding wing area or reducing the tail area, and for the purposes intended, nothing of note happens.

[Ollie]

From a stability point of view, increasing the wing span and area moves the neutral point (aerodynamic center of the whole plane) forward. If the CG is moved forward a corresponding amount, then the static margin will be unchanged. The damping factor will be reduced so that the plane will take longer to settle down from a disturbance but that might not be noticed. Some rudder control power will be lost but that might not be noticed if there was excess to begin with. In planes with lots of dihedral or equivalent polyhedral (yaw to roll coupling) a dutch roll may develop because of the reduction in yaw stability associated with a longer span.

Up to the point of instability, how much stability is required or desired is dependent on the pilot's skill and the purpose of the plane. Whether enough control authority remains after modification depends on how much there was to start with and also on the flying style of the pilot. So, one of the contributing factors to the looseness of criteria for stability and control power, it is the adapability of the pilot.

Planes that are intended for aerobatic competition are designed and adjusted for lots of control power and minimum stability. Such planes may be near the limits of controlability during some extreme maneuvers. If such a plane had the wing area increased substantially, without making any other compensating changes, then reduction of stability and control power would likely be noticable.

[robbie]

wow,
thanks for the detailed answers.