Of course there are guidelines. For sailplanes, speedsters, duration and range, minimize drag. For aerobatic models, speedsters, duration, range, load carriers and sailplanes, minimize weight. For aerobatic models add drag to limit acceleration on the down line. For durability maximize the strength to weight ratio. For beauty don't employ a design team (a camel being a horse that was engineered by a committee). For aerobats, speedsters, and load carriers, maximise thrust. For an all around sports model kit provide low landing speed, provide lots of maneuverability, provide adequate stability, provide good vertical performance and do it at a minimum cost in materials and labor.

The general sorts of guidelines then have to be quantified and the quantities fed into equations, mathematical models or computer programs to generate a range of possible solutions. For example, the structural equations and the aerodynamic equations don't solve the problem of creating something that satisfies both perfectly. How those solutions are integrated and allocated is not a deterministic activity. In other words, the component solutions do not converge on a "best" solution but have to be synthesized or integrated by the designer using his talents of judgement, creativity and artistry including those needs that were not originally specified but are necessary for practicality. Practicality is a moving target that evolves as new materials and processes are developed.