Flight Model

The core philosophy of the flight model is to be of a realistic and reasonable approximation of flight that is easy to understand and prototype on. All aircraft elements will be defined in text documents whenever possible.

There are four forces involved with flight: lift, drag, thrust and gravity. As of right now, the model for lift is complete and diagrammed but not yet implemented. The equations for thrust are mostly complete but not diagrammed or implemented. Drag and gravity are planned but have not been started yet – stay tuned as we update!

Force 1/4: Lift

The primary philosophy for me, as an artist, is to make something easy to modify. I do know quite a bit about how airplanes work, but even so I’m no aerospace engineer and I need something that can be modified easily and tinkered with. Therefore right now as a version 1 I want everything to be easily modifiable via text files. For version 2 I’d love to have a user interface, but for right now everything should be easy to modify and written in plain English, preferably in a formatted XML document.

Looking at the image above you can boil it down to this:

Everything gets fed into the Lift Equation at the top of the screen. Starting with the aircraft:

The aircraft’s Center of Gravity (CoG) is defined by location and weight in the Aircraft Definition file. The Center of Gravity is what moves through the air and orients the entire aircraft and it’s what the entire aircraft rotates around.

Wings are defined as panels with dimensions (also done in that same Aircraft Definition file). From those dimensions the wing’s Center of Lift (CoL) is determined. The CoL is the averaged location of all upwards lift force for that wing panel. From the CoL, we create an upwards vector – lift.

The amount of lift is given in kgs. If you have more lift kgs than weight kgs your plane flies!

Lift itself is dependent on the wing panel’s airfoil. In our case, the Aircraft Definition file might just say “NACA 2212” and we will have a separate text file that has all the performance information for that airfoil – like it’s lift and drag curves. Below are some examples of airfoils:

An airfoil is the shape of the wing cut in half and looked at from the side. At different angles relative to the airflow (called the “Angle of Attack”), the wing will create more or less lift.

The Angle of Attack will tell us how much lift the airfoil will develop, and after doing a lot of math that “amount” has been boiled down into the “Lift Coefficient“, which is a dimensionless number used in the Lift Equation. Generally the Lift Coefficient is expressed as a related to the Angle of Attack. See below for more details.

Above we have an example of a Lift Curve for a given airfoil. In this example, if you were going forward and were to pull your aircraft’s nose up by 5° your Coefficient of Lift would be 1. At 10° it would be 1.5. That gets fed into the Lift Equation and combined with the size of the wing, the aircraft’s velocity squared, and the density of the atmosphere. What we get at the end is the amount of Lift in a given weight unit.

Notice though where the Lift Curve dips? That’s when your airfoil reaches its maximum angle of attack, and the wing stalls. Until the angle of attack is reduced your wing will remain stalled. Basically, the wing is at too high of an angle so the air is just shoved into it and can’t flow smoothly around it. See the video below for what this looks like in real life.

Coming up next…drag and thrust!