In the design certifications for big transport jets, there are two factors that must be met in order for a new aircraft to be certified; manoeuvring loads, and gust loads. Why are these two separate requirements, rather than simply specifying a minimum gravitational load factor?
Manoeuvring loads are well defined, controlled and regulated. The (auto-)pilot induces them, and the quasi-static loads can therefore be fully controlled.
Gust loads are sudden impulse loads, producing aeroelastic effects such as high speed buffet. Aero-elasticity, Mach number, wing sweep angle all play a role:
- the faster the plane flies through the gust, the less affected it is;
- the swept wing does not fly into the buffet all at once;
- lower wing loading accelerates the aircraft with the gust earlier, and provides gust load relief;
- torsion and bending stiffnesses of the wing must be such that no eigen-frequencies are induced from entering the gust.
The above is similar to what happens with a car during cornering on a smooth freeway, compared to driving at high speed over a bumpy surface.