Scale counts

When thinking about aerodynamics, many people go first to proven full-scale designs with a human in the cockpit and assume that everything can be scaled down. The truth is, it will rarely perform similarly, and the optimal designs are simply different for different size craft.

2x Scale, 4x area, 8x weight

In structural engineering of an airframe, one has to consider the square to cube ratio: when you double the size of something that relies on a stressed structural member in two dimensions, you may quadruple the strength but make the mass 8x bigger. At the same time, stiffness is typically proportional to the third power of thickness.

Engine is Everything at full scale

• Real-world airframes are usually optimized around a particular type of internal combustion engine, and that engine may be engineered to only perform efficiently in a particular thickness of air (say, at 40,000 feet for a turbofan).

Aerodynamics

Aside from this, aerodynamics has two of its own scaling laws:

Reynolds Number

the Reynolds number for scale airfoils is much, much smaller than full-size versions of the exact same airfoil. This is an due to emergent properties of subsonic air pressure waves, and it usually translates into a significant loss of efficiency given the same airfoil.

The Sound Barrier

Most full-scale aircraft are heavily concerned with avoiding the transonic zone, where drag stresses suddenly rise from a smooth curve to almost an asymptote. The phrase 'breaking the sound barrier' comes from the fact that it took a decade of test pilot deaths before planes could be redesigned to operate this fast routinely without shaking themselves to pieces. Even the design of slow planes is heavily oriented around preventing propeller/rotor tips from going supersonic and cracking to pieces. This is not something RC aircraft have to worry about.