Stacks Image 100
Guy Montagu-Pollock

Flight Model

X-Plane simulates how an aircraft behaves by calculating the aerodynamic forces (lift and drag) plus weight, and thrust. A lot of information is available for the Comet, dimensions, weights, aerofoils, and engine specifications, but drag had to be adjusted by other figures have had to be reverse-engineered from the real aircraft’s performance data. Like a mathematical equation, it is a matter of deriving unknowns from knowns.

No computer program can simulate flight with complete accuracy, therefore the objective for the modeller is to find a compromise between critical phases of flight, where the flight model must be as accurate as possible, and less critical aspects, where performance must at least be in reasonable bounds. An airliner will have different priorities compared with a fighter: it is not necessary for it to be aerobatic, but it is important that it “feels” right during periods of hands-on flying, especially for take-off and landing. It is particularly important that its performance allows accurate flight planning; in other words, the simulator pilot must be confident that they can follow a real route without arriving late or running out of fuel.

Lift and drag for the control surfaces was calculated using JavaFoil, by Martin Hepperle. The Comet 4 used a NACA 63A011 (near symmetrical)

aerofoil, with the addition of “leading edge droop”. A 7.5% increase in lift slope gradient and stall angle was added in Airfoil Maker, according to the Ministry of Aviation report 3455: “Wind Tunnel Studies of Leading Edge Separation Phenomena on a Quarter Scale Model of the Outer Panel of the Handley Page “Victor” Wing, with and without Nose Droop” by R. A. Wallis.

Default values were used for the drag from cylindrical bodies: the fuselage and nacelle tanks. The additional drag caused by having engines ducted into the wing root is unknown. Initially, all control surfaces were treated as ordinary aerofoils. Performance data was recorded in this form to establish the gap between target performance and actual performance. As expected, the results indicated a lack of drag, and this was made up by adding extra drag to the part of the wing that contained the engines. All other control surfaces remained unaltered.

David Plunkett (x-plane.dendra.net) supplied a plugin for the Rolls-Royce Avon engine, using performance data from brochures and manuals for the Avon family of engines.