How aircraft fly – Bernard Foot; Horse & steam trams – John Perkin

Bernard Foot gave a fascinating talk on “How aircraft fly”, illustrated by slides, video clips, and demonstrations using a model Spitfire aircraft provided by Graham Spencer, and his own slightly simpler wooden model.

We learned about the two pairs of opposing forces – lift vs. weight, and drag vs. thrust. The weight of an aircraft can vary by a ratio of up to 20:1 during the course of a flight. Thrust, which is needed to deliver lift in a powered aircraft, is provided by 4 types of aero-engine: piston, turbo-jet, turbo-prop, and turbo-fan. They all work by accelerating the oncoming air backwards, resulting in a reactionary force on the engine forwards. There was a discussion on the principle of lift generation, and the traditional explanation based on the Bernoulli effect being discarded in favour of reaction to the deflection of air. The pilot has a number of options to adjust lift to meet varying needs during the flight: changing the shape and area of wings, and changing the Angle of Attack. Thrust can also provide lift more directly than just through its role of driving the aircraft forwards.

Looking at drag there are two types: “parasite” drag arising from air friction and “induced” drag which is a by-product of the lift generation process. Stability in flight is provided by the fin, tailplane, and wing features such as dihedral and high-wing design. Control is provided by movable control surfaces at the ends of the wings (ailerons), on the fin (rudder), and on the tailplane (elevator). Finally we looked at the flight envelope, a chart defining possible combinations of altitude and speed for a particular aircraft.

[Post-meeting editorial note by Garth Wilkinson: there has been much controversy during the decades since the invention of powered flight over the relative roles of the Bernoulli effect, & Newton’s reaction force due to air deflection, in contributing to aerofoil lift. The Bernoulli effect can be derived from Newton’s laws of motion (https://en.wikipedia.org/wiki/Bernoulli%27s_principle) – the problem seems to be assumptions about the way it is applied to the actual airflow over an aerofoil. A general consensus seems to be that both approaches are needed to fully explain the complex theoretical problem of solving the Navier-Stokes aerodynamic equations, e.g. see https://www.grc.nasa.gov/WWW/K-12/airplane/bernnew.html,https://en.wikipedia.org/wiki/Lift_(force)]


John Perkin gave a short talk on Horse & Steam Trams, using a slideshow of the Oxford Tramways (1881-1914) & Wantage Tramways (1875-1946)

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