by David Zurawski

The Flight Model

These simple words seem to summon the desire of every simulation advocate and nay-sayer to muster their personal wealth (or dearth) of accrued information and passion regarding the accuracy and validity of any given flight simulation's true worth. In the process, they heap accolades on developers or crucify them as a group.

This raises the question, "What is up with the flight models today and what makes them the center of such debate?"

"Into the breech…"

My decision as to whether to base this article on jet simulations or prop simulations was an easy one. Aside from the odd zealots determined ranting, jet simulations benefit from the access to complete and recent specification documentation. Prop simulations on the other-hand suffer from their age of inception, resulting in a scarcity of complete and accurate documentation.

In preparation for this article I structured a questionnaire and sent it to many of the most recent and upcoming simulation developers with the hope of "picking their brains." Surprisingly, I received few responses (one of with was rendered mute due to Sierra's cancellation of Desert Fighters). Those who did respond provided me a wealth of information that I believe will shed a great deal of light on this subject.

"Enter the players…"

• Chris Sherland representing Playnet and their WW2Online simulation using their "as of yet" unnamed flight loop technology.
• Constantine Hantzopoulos and Mike Valdez representing Looking Glass and Flight Combat based on the Flight Unlimited series Flight of flight models.
• Andrew Walrond representing Wayward Design and B17-II the Mighty Eighth using an all-new engine developed by Wayward Design.
• Michael Harrison representing Parsoft / Inertia and Fighter Squadron, using the Openplane engine.

"And the Question Is…"

Firstly to better understand the state of flight models, I thought it best to allow the developers to clue us in as to what kind of flight model engines they are currently using.

Q 1.) Describe your flight model engine and how it works within its virtual environment.

Q 2.) What advantage do you feel your engine has over existing simulations?

Playnet

Playnet Q1) Our flight model looks at each vehicle in components. First the plane is measured and divided into logical sections. Each section is given accurate weight, roll inertial values in all axis, and sub components such as control surfaces, fuel and oil tanks. All the sub components share the same level of detail as well. The airfoil's sections are all calculated for lift/drag/stall through 360 degrees of AOA.

That is simply obsessive, but it gets us much more dynamic interaction with airflow at severe performance regimes, and it's fun to claim. After all that is set we send the model through a wind tunnel program that was written in house. Here is where we can see how close the performance values are, right off the bat. Then it's a matter of detailing the control surface dynamics and entering dedicated detail and oddities to the individual model.

Click to continue

Playnet Q2) We feel our advantage is that our flight engine works with real-life numbers, weights, pressures, HP, drag, lift, and velocities etc. All that the flight "modeler" needs to do to create a model in our engine is to accurately transfer the dimensions, weights, and sub components of an aircraft into flight model data.

I'm not sure that we are making any huge leaps forward in flight modeling in general, but we are certainly increasing the overall fidelity and upping the level of detail on flight regimes for the industry standard. I'd say the big advantage in our model is that it's all based on real numbers, and it's very easy to check your specs that way. This translates into more accurate depiction of the actual performance of the aircraft with less guesswork.

Looking Glass Q1&2

Q1) Our flight model is what we call a piecewise force-based model. Let me explain these separately. "Force-based" means that we calculate the forces on the plane (lift forces, for example), then use Newton's First Law (F = ma) to calculate the resulting motion of the plane, that is, its new position and velocity.

I believe that most flight Sims today use the force based approach, and with good reason. Since nature is "force-based", it is no surprise that modeling a plane this way would result in the most natural and realistic flight behavior. "Piecewise" means that we do not model the forces on the plane as a single, indivisible piece. Instead, the plane is made up of separate pieces (up to 70) which individually calculate their force contributions and then all these contributions are added together to give the net force on the plane.

I'm pretty sure that few flight sims use the piecewise approach, which has advantages as described below. Within the simulation our flight model is merely one of several physics models that can "take control" of an object. The player plane is always under control of the flight model, and at certain times, other planes are switched from a simpler model to the full flight model.

LGlass Q2) I believe that relatively few flight sims use the piece-wise approach described above, which I think is the biggest strength of our model. This is because it captures the behavior of a plane with missing or damaged pieces almost automatically - we just need to damage or remove the corresponding piece in the model, and the model just starts behaving accordingly.

For example, if the left wing tip breaks off, then we just remove that piece from the flight model. The remaining pieces of the plane keep providing their force contributions and the plane starts tailing to the left because there is no longer that lift contribution from the left wing tip.

Go to Part II: Wayward Design