History of Flight Stability, Control & Flying Qualities

History of Flight Stability, Control & Flying Qualities How Did It Become Obvious That An Airplane Should Be Inherently Stable? When the Wright brothers made their historic record in 1903 of the first powered flight, there were no solid theories of flight. The Wrights had built their airplane by trial-and-error. The emergence of this new machine (the airplane) posed danger to countries in political tension. Whoever reveals the mystery of flight first and arrives at a constructive way of designing airplanes will be able to build better ones in shorter times, and hence dominate the sky. So, during the Great War, there was a parallel mathematical war between Britain and Germany to develop two theories: one on lift and another on stability and control. The Cambridge school of mathematical physics, led by Bryan and Bairstow, had won the mathematical war on stability, while the Gottingen school of technical mechanics, led by Prandtl, had won the war on lift. In this talk, we will focus on Bryan’s story. Remarkably, his stability analysis became standard routine in the aviation industry, which points to his lasting impact. Nevertheless, his work was mocked and declined twice for publication. Having learnt how to assess stability, the question comes: How much stability do we need for our airplanes if any? Recall that the only successful airplane which made the historic record of the first powered flight was unstable, both longitudinally and laterally. In fact, most successful airplanes of WWI were unstable. Also, the aeronautical community learnt (the hard way) that stability is in conflict with controllability, and that one of the main factors behind the successful flight of the Wrights is their focus on the latter, on the expense of the former. There was no single event that caused a shift in the mindset of the aeronautical community about stability. It was a slow gradual process. However, the harder question is: If stability is needed, how stable our airplanes should be? Clearly too much stability is not desirable as this would undermine controllability. The main issue lies in the fact that the pilot interaction with the aircraft alters its stability characteristics. The inherent stability characteristics of the aircraft are not necessarily felt by the pilot. Indeed, the handling quality of the airplane = airplane dynamics + pilot. While the former can be modeled mathematically, the latter is too complicated to model. So, the aeronautical community had to rely on flight testing. Extensive flight testing was performed between the 1920s until 1960s and afterwards to correlate the inherent (open loop) (in)stability characteristics with pilot ratings, culminating in standard guidelines for airplane designers and research engineers.