Center of Gravity

When making a flyable model for flight sim it is important to establish what is referred to as its "Center of Gravity" or "CofG". This serves basically two purposes. The first is that when viewing the vehicle in any external mode. it is the point to where the "camera" or viewers eyepoint is centered. The second is a bit more technical as it serves as a point of reference for the aircraft.cfg and *.air files. It is the point that the vehicle rotates around when climbimg (pitching the nose up or down, rotating around the lateral or X axis), turning, bank and yaw (banking, dipping the wing, rotating around the longitudinal or Y axis, yaw, rotating around the verticle or Z axis). It is also used to position the vehicle when it is loaded in the simulator.

The screen shot images here were taken from gMax, but the same principles apply reguardless of what 3d modeling program you are using.

For the most part, the position in which the CofG is located should be at the 3d modeling programs World Origin or 0,0,0 in relation to all of the geometery or shapes. If you want your aircraft centered relative to the "spot plane camera" you will have to offset your Reference Datum Position from the world center, 0,0,0. Usually moving it fore/aft relative to the 0,0,0, point in the aircraft.cfg file. This, of course, will require you to do some math and may lead to some hair pulling when adjusting things in the aircraft.cfg and *.air files. It doesn't matter what you are building.

If you are going to make a static vehicle, then none of this applies, as you would think of 0,0,0 as the ground plane. The geometry must be rotated around the X axis so that the wheels all rest squarely on the ground (X,Y) plane.

Most vehicles generally are symetrical, meaning the left and right sides are just mirrors of the oposite side, then the location of the lateral, or Y axis is down the center of the vehicle. Easily seen in either the top/bottom or front/back viewports.

Next is the location of the longitudinal or X axis. The standard placement of aircraft shapes are such that the leading edge of the wing is 1/4 of the wings cord back from the leading edge of the wing. So if you have a wing that has a cord of 12 ft the distance from the CofG to the fore most part of the wing would be 3 ft. and the distance from the CofG to the rear most part of the wing would be 9 ft. Oh, and remember, that the measurements are based on what are called the Y-intercepts. In other words, if you project the leading edge of the wing thru the Y axis, this is where the 3 ft.measurement should be and not the point at where the wing simply meets the fuselage because the fuselage is actually considered to be part of the wing. This is best seen in the top viewport.

Last is the verticle placement of the aircraft shape on the Z axis commanly referred to as the waterline. In aircraft models, this is based on the verticle center of the engine(s) or prop(s). If you are building an aircraft with four eingines you might want to "split the difference" between the inboard and outboard engines.

Once you have the vehicle shape positioned it is very easy the take measurements from your model and put them into the aircraft.cfg and *.air file or into a configuration calculating spreadsheet.

Static pitch is the angle between the main gears lowest point and the nose gear or tail wheels lowest point in degrees when at rest on the ground (+=Up, -=Dn). A DC-3 has a positive pitch (the waterline is angled up) and a DC-9 has a negative pitch (the waterline is angeled down).

Static_CG_Hight measurement is based on the gear extended at animation frame 150.

When you have your gear heights and static pitch finally set right, look at your vehicle switching between the slew and normal modes. There should be very little change in the vehicles attitude between the two modes and it should not go blasting off to the next state.

Main and Center Gear Static Compression also affects how the vehicle sits on the ground. Max/Static Compression Ratio in conjunction with the Damping Ratio affects how the gear reacts to rough terrain . Start out with figures from a similar vehicle. Wrong settings can cause uncontrollable bouncing and eventual crash. Max/Static Compression Ratio = total animation distance divided by the Static Compression distance.

Scrape Points work together to form a vitual perimiter around the vehicle. These can be determined from the extremities of the vehicle and their tangents. If the vehicle has crash detection and an outside object comes into this zone, then you have a crash. It is extremely important that none of these points are lower than the gear, otherwise you have a ground crash right off the bat when you change vehicles on the ground and if it is a plane you will always crash when landing if you have changed aircraft inflight.

Other examples