This is a perfectly nice rule of thumb, but is far too simplistic to use across the flight envelope of the airplane. This technique involves simply forcing the nose to return to a centered position along the flight path with a certain acceleration for each degree of offset from straight-ahead flight of the airplane-for every degree of angle of attack the nose is raised, the nose should return to center with a certain acceleration. Most other simulators use something called “stability derivatives” to compute how an airplane flies. This method of computing the forces on the airplane is much more detailed, flexible, and advanced than the flight model that is used by most other flight simulators. Aren’t computers great? Advantages of Blade Element Simulation The process is repeated from step 2, and the whole thing is run over again at least 15 times per second. Forces are then divided by the aircraft mass for linear accelerations, and moments of inertia for angular accelerations. Using the coefficients just determined in step 3, areas determined during step 1, and dynamic pressures (determined separately for each element based on aircraft speed, altitude, temperature, propwash and wing sweep), the forces are found and summed for the entire aircraft. In supersonic flight, the airfoil is considered to be a diamond shape with the appropriate thickness ratio pressures behind the shock waves are found on each of the plates in the diamond-shaped airfoil and summed to give the total pressures on the foil element. Compressible flow effects are considered using Prandtl-Glauert, but transonic effects are not simulated other than an empirical mach-divergent drag increase. The airfoil data entered in Part-Maker is 2-dimensional, so X-Plane applies finite wing lift-slope reduction, finite-wing CLmax reduction, finite-wing induced drag, and finite-wing moment reduction appropriate to the aspect ratio, taper ratio, and sweep of the wing, horizontal stabilizer, vertical stabilizer, or propeller blade in question. Using local air density, X-Plane determines the propwash required for momentum to be conserved.ĭownwash is found by looking at the aspect ratio, taper ratio, and sweep of the wing, and the horizontal and vertical distance of the “washed surface” (normally the horizontal stabilizer) from the “washing surface” (normally the wing), and then going to an empirical look-up table to get the degrees of downwash generated per coefficient of lift. Propwash is found by looking at the area of each propeller disk, and the thrust of each propeller. Downwash, propwash, and induced angle of attack from lift-augmentation devices are all considered when finding the velocity vector of each element. The aircraft linear and angular velocities, along with the longitudinal, lateral, and vertical arms of each element are considered to find the velocity vector of each element. Ten elements per side per wing or stabilizer is the maximum, and studies have shown that this provides roll rates and accelerations that are very close to the values that would be found with a much larger number of elements. The number of elements is decided by the user in Plane-Maker. Element Break-Downĭone only once during initialization, X-Plane breaks the wing(s), horizontal stabilizer, vertical stabilizer(s), and propeller(s) (if equipped) down into a finite number of elements. X-Plane goes through the following steps to propagate the flight: 1. These forces are then converted into accelerations, which are then integrated to velocities and positions… Of course, all of this technical theory is completely transparent to the end user… you just fly! It’s fun! It does this by an engineering process called “blade element theory”, which involves breaking the aircraft down into many small elements and then finding the forces on each little element many times per second. X-Plane works by reading in the geometric shape of any aircraft and then figuring out how that aircraft will fly.
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