Gust Loads

The selection of design loads and requirements is defining the structural weight of airplanes and their
safety. Therefore the definition of requirements should be performed very critically by the customer
and structural weight should be assessed based on sensitivity analysis of the total aircraft which
includes flight manoeuvre simulation, flight control system, aerodynamics and elastic effects
introduced by finite elements. To produce these analyses is the job of the aircraft companies.
After selection of most load critical flight manoeuvres (pull up manoeuvres, initiation of roll
manoeuvres etc.) the calculation of airloads and inertia loads must include the flight control system and
its failure cases because it affects the motion of the control surfaces and therefore the aircraft.
With the advent of carbon fibre composite structures discrete loads are the predominant limiting design
conditions but it should be emphasised that most structures are of a hybrid nature with metal frame
which are still susceptible to fatigue loads. For airplanes designed to civil requirements such as
transport airplanes, tankers etc. the definition of continuous turbulence and inclusion of FCS failure
cases and nonlinearities such as control surface angles is extremely important.
There was a long way from load assumptions used by the Wright Brothers who designed their Flyer to
a 5g limit to the load limiting capabilities of the care free handling flight control system of the
Eurofighter. Also the US-Airforce Mil-Specifications which were used to design NATO airplanes such
as Tornado, F16 and F18 in the 1970’s are obsolete today and the MIL-A-87221 (USAF) is only a
frame without the essential quantitative material. All these issues are addressed in this manual
including comparisons of regulations and descriptions of new specifications. Complete procedures how
to establish design loads are presented which should help for the design of new airplanes.
The importance of dynamic phenomena which produce design loads for various aircraft parts such as
intakes, leading edges etc. is also highlighted. Loads monitoring systems are necessary to prove
calculated loads and monitor fatigue loads to establish the remaining structural life. There is a
description of a modern system.
For transport type aircraft gust load cases are the most critical for strength design and they are also the
main fatigue loading source for the major part of the structure. Methods for discrete and continuous
gust loading cases are presented together with nonlinear example calculations.
In the appendix there is a description of failure cases and their effect on loads for transport aircraft and
a specification of a landing gear which could be used as an example how to specify the whole structure
as a system. The military use of this manual is to establish procedures to build the lightest structure for
the military requirements. Agreement on requirements and design loads within the NATO countries
could standardise pilot training, aircraft usage, increase aircraft life and reduce maintenance. Since the
search of the best usage of the aircraft for its military purpose will continue to integrate structure and
avionics such as fire and flight control systems as an example there will be a continuous need for
future work.