A Little Bit About X-Plane

X-Plane is a proprietary flight simulator for personal computers produced by Laminar Research. It runs on Linux, Mac, and Windows. X-Plane is packaged with other software to build and customize aircraft and scenery. The packages are X-Plane (the actual flight simulator), Airfoil-Maker (to make airfoils for your aircraft if you would like to make your own planes), Plane-Maker (to make your own planes), World-Maker (to make your own scenery to fly in if you like), and Weather-Briefer (to get a weather-briefing before your flight if desired). X-Plane also has a plugin architecture that allows users to create their own modules, extending the functionality of the software.

Traditional flight simulators try to emulate the real-world performance of an aircraft by using lookup tables to determine things such as lift or drag. These simulators do a good job of simulating the flight characteristics of the aircraft they were designed to simulate, but are not useful in design work, and do not predict the performance of aircraft when the actual figures are not available. X-Plane reads in the geometric shape of any aircraft and then figures out how that aircraft will fly. 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. These forces are then converted into accelerations which are then integrated to velocities and positions. A wing, for example, may be made up of many sections (1 to 4 is typical), and each section is further divided into as many as 10 separate sections, then the lift and drag of each section is calculated, and the resulting effect is applied to the whole aircraft. When this process is applied to each component, the simulated aircraft will fly virtually like its real counterpart does. This approach allows users to design aircraft on their computer quickly and easily, as the simulator engine will show immediately how an aircraft with a particular design might perform in the real world.

X-Plane goes through the following steps to propagate the flight:

1: Element Break-Down

Done 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. The number of elements is decided by the user in Plane-Maker. 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.

2: Velocity Determination
This is done twice per cycle. 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. Downwash, propwash, and induced angle of attack from lift-augmentation devices are all considered when finding the velocity vector of each element.
Propwash is found by looking at the area of each propeller disk, and the thrust of each propeller. Using local air density, X-Plane determines the propwash required for momentum to be conserved.
Downwash 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.

3: Coefficient Determination
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. Compressible flow effects are considered using Prandtl-Glauert, but transonic effects are not simulated other than an empirical mach-divergent drag increase. 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.

4: Force Build-Up
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. Forces are then divided by the aircraft mass for linear accelerations, and moments of inertia for angular accelerations.

5: Get Back to Work
Go back to step 2 and do the whole thing over again at least 15 times per second.

Blade element theory does have its shortcomings, as it can sometimes be difficult to design an aircraft that performs precisely like the real-world aircraft. However, as the flight model is refined, the simulator can better resemble real-world performance (as well as the aircraft’s quirks and design flaws.)

X-Plane is capable of modeling fairly complex aircraft designs, including helicopters, rockets, rotor craft and tilt-rotor craft. Famous real world aircraft modeled in X-Plane include the V-22 Osprey, the Harrier Jump Jet, the NASA Space Shuttle, and Scaled Composites SpaceShipOne.

Through the plugin interface, users can create external modules that extend the X-Plane interface, flight model, or create new features. One such feature is the Xsquawkbox plugin, which allows X-Plane users to fly on a worldwide shared simulation network. Other work has been done in the area of improving X-Plane’s flight model and even replacing entire facets of X-Plane’s operation. X-Plane is also capable of communicating with other applications via UDP. Through a relatively simple interface, third party developers can control the simulator and extract data regarding the simulation state.

The maps and scenery are also fully editable. While no tool is provided to edit the 3D mesh objects, there are tutorials for using the third party 3D modeler AC3D. Once built, editing landscape elevation and 3D object placement is easily accomplished with the scenery editor. In fact, much of the world’s detail, including detail in airports, such as ramps, buildings, and taxiways, is provided by the end-users.

Map imagery and aircraft paint can be created and modified with any paint program capable of manipulating PNG images. Additionally, Laminar Research has released a 7 DVD “Global Scenery Package” containing imagery of a much higher quality than the default information. This package covers close to 85% of the Earth’s surface. The release of X-Plane 9, in January 2008, has introduced much improved areas of high ground relief (in particular, mountains) and a plethora of other improvements.

RC Helicopter

Japanese Anime Aircraft

Taken and edited from: X-Plane and Wikipedia

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