Flight Modeling and Aircraft Creation.....

Flight modeling is of course the heart of any flight simulation and this is an area where X Plane is totally unique. Rather then tuning a flight model to fit published performance data, X Plane predicts performance based on geometry, foils, weight and power using a technique known as blade element theory. BET is based on the idea of breaking up an object such as a prop or a wing into a number of discreet segments, predicting lift and drag for each segment and then integrating the results to come up with the final figures for the object of interest. X Plane applies this approach to the entire aircraft in order to decide what it should be doing at any given moment. The main advantage to this approach is that it greatly simplifies the process of creating a plane and because it's predictive in nature, you can use X Plane to test new design ideas. Because of it's bottom up approach it's also a great platform for learning about basic aerodynamics.

The heart of the aircraft creation and modification process is the included Plane-Maker application. In X Plane the only parts of the aircraft that you actually need to shape by hand are the fuselage, wheel pants and engine nacelles, if needed. The fuselage design screens are shown below.

The wings, control surfaces. props and landing gear are all automatically generated in 3D by the program itself based on user provided data. As you create your design you can monitor your progress with a fully user adjustable 3D view or rotation animation in wire frame or textured rendering. This view can also be animated with moving parts and painted with the aircraft's texture set.

Of course the heart of any aircraft are the lifting surfaces. It's the cross-sectional shape of these elements that to a large extent determines the flying characteristics of the aircraft. In X Plane each lifting surface can be assigned a unique airfoil shape. X Plane comes with a small library of airfoils and the included application Foil-Maker provides a facility for the creation of new ones from standard data as shown below.

All in all this makes for a powerful aircraft design package for replicating existing designs or for creating your own. Doing so however does require a certain level of aeronautical knowledge. If you don't know how to balance a design or are unfamiliar with basic terms, the learning curve will be very steep. The upside is that once conquered, the creation process is fairly quick as witnessed by the huge number and variety of available user created aircraft.

The acid test for all this is of course, how does it fly? In general the answer has to be "very well". The first thing that hits you when you fly X Plane is just how smooth and realistic the simulation feels. It's very convincing and it's easy to see that the fight modeling system is getting a lot of things right. If you have accurate data to feed into the Plane Maker application you'll end up with performance in the critical areas of top speed, climb rates and stall speeds that are within a few percent of published values. However, the price X Plane pays by taking the bottom up - physics based approach is that it needs to take all forces into account and everything must be in balance. If any forces are ignored or incorrect then the flight behavior won't be totally correct.

Because of the bottom-up approach there's also very little scope for tuning the performance of a given design unless you're willing to deviate from real-world values or do some creative hacking of the aircraft files in order to get a design to meet a certain performance benchmark or behavior. That's a common practice when working with other types of flight models but a questionable approach with a flight model like X-Plane's. This also points out the fact that if you're using X Plane as a serious design tool you need to know exactly what the flight model calculates, what it ignores and what it doesn't quite get right. The important thing to remember is that the FM is in a constant state of adjustment and this sometimes causes problems. It's not unusual to have a plane flying perfectly only to have everything change in the next release. That said,  the general trend is towards the positive side so it's just something users need to be aware of. 

In summary it has to be said that taken as a whole, the flight modeling system in X Plane is absolutely brilliant. While it's not a virtual wind tunnel, it's bottom up, physics based approach along with its included set of aircraft creation tools and fluid feel make it unique in the world of desktop flight simulation.

An X Plane flight test and video by certified pilot and X Plane aircraft creator Jason Chandler.