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Guy Montagu-Pollock

Flight Model

There are two approaches to flight simulation: either an aircraft's flight characteristics can be made to obey tables of values, which guarantees a level of accuracy for the user; or an aircraft's behaviour can be simulated by calculating the forces generated by its shape, mass and thrust. The advantage with the second, more dynamic, approach is that the model can react naturally to its simulated environment, and it can be used to test wholly new or heavily modified designs to see how they fly. In practice software manufacturers use a combination of the two, but X-Plane leans heavily towards the second approach. This makes it an excellent educational tool and highly rewarding for developers. The disadvantage is that the simulation can only ever be approximate, because there simply isn't enough number-crunching power to simulate the real world; but this is improving in leaps and bounds.

The modeller's objective is to find the right balance between accuracy in critical situations and reasonable accuracy at other times. An airliner will have different priorities compared with a fighter: it is not necessary for an airliner to be aerobatic, but it is important that it behaves realistically during take-off and landing, and that real routes can be flown without running out of fuel.

It took years to collect enough information: dimensions, weights, aerofoils. Flying manuals contained graphs on performance, the Operating Data manual contained climb, cruise and descent tables for every permutation of weight, altitude and temperature, and a Rolls-Royce manual showed the power and fuel consumption curves for the Avon engines. The elusive figure was the drag caused by the engines being in the wing root.

Therefore I arranged a test schedule to derive a realistic drag figure from known performance.

Test Properties
1. Stall Lift      
2. Unstick Lift Thrust Drag  
3. Climb Lift Thrust Drag Fuel
4. Cruise Lift Thrust Drag Fuel
5. Descent Lift Thrust Drag Fuel
6. Approach Lift Thrust Drag  

Stall tests established the maximum lift from the aerofoils and flaps, while unstick and landing tests confirmed them in a practical situation. There was no single test where drag could be isolated, so a reasonable compromise was made between climb, cruise and descent.