By Javier Rollon · 2020-11-05
People ask me all the time: "What makes X-Plane different?" My answer is always the same three words. Blade element theory. And then I watch their eyes glaze over. So let me try to explain this without turning it into a physics lecture. Because it's genuinely the most important thing about X-Plane, and it's the reason I develop exclusively for this platform.
Most flight simulators — including MSFS — use lookup tables. The developer says: "At 10 degrees angle of attack, this wing produces X amount of lift and Y amount of drag." The sim reads the table, applies the numbers, done. It works. It's fast. And it's fundamentally limited.
Limited because a lookup table can only contain scenarios the developer anticipated. What happens at 47 degrees angle of attack in a 30-knot crosswind with one engine out and the flaps at 15 degrees? If nobody programmed that exact combination into the table, the sim guesses. Sometimes it guesses well. Sometimes the aircraft does something physically impossible.
X-Plane divides every lifting surface — wings, tail, propeller blades, even the fuselage — into small sections. For each section, at each moment, the sim calculates the local airflow angle, the local airspeed, and applies basic aerodynamic equations to determine lift and drag. Then it adds up all the sections to get the total forces on the aircraft.
Simple concept. Profound implications.
When I build the Boeing 747 for X-Plane, I model the wing shape. That's it. I don't tell the sim how much lift the wing produces. X-Plane figures it out from the geometry. If I get the airfoil right, the sweep angle right, the dihedral right — the aircraft flies correctly because the physics is correct.
This means unusual attitudes work. Knife-edge flight works. Spins work. Not because someone programmed spin behavior, but because the airflow over each wing section is calculated independently, and when one wing stalls before the other — which is what happens in a real spin — the asymmetric forces naturally create rotation.
For me, blade element theory is the difference between modeling an aircraft and faking one.
When I developed the SIAI-Marchetti SF-260, the aerobatic behavior emerged from the geometry. I didn't have to hand-tune how the aircraft behaves during a snap roll. I modeled the wing accurately — the airfoil, the planform, the control surface deflections — and the sim produced snap roll behavior that pilots who've flown the real SF-260 immediately recognized.
Try doing that with a lookup table. You'd need hundreds of entries for every possible combination of control input, airspeed, and flight attitude. Or you do what most sims do: you approximate. The snap roll looks "close enough." But close enough isn't what simulation is about.
Blade element theory gets its name from propeller analysis. Each propeller blade is divided into elements from root to tip. Each element sees a different airflow velocity (faster at the tip, slower at the root) and a different angle of attack. The sim calculates thrust and torque for each element independently.
This is why the T-34C Mentor's propeller behaves differently at sea level versus 15,000 feet. The air density changes, the blade efficiency changes, the torque effect on the airframe changes. I didn't program any of those differences. I modeled the propeller blade shape and X-Plane did the rest.
In a table-based sim, the developer has to manually specify "prop efficiency at FL150 = X." In X-Plane, it's an emergent result of correct physics applied to correct geometry. When the real aircraft's pilot manual says "expect 3 degrees of torque correction at cruise power," and my sim shows exactly 3 degrees without me specifically targeting that number — that's when I know the model is right.
Blade element theory costs CPU time. Calculating forces on dozens of wing sections, multiple propeller blade elements, control surfaces, and the fuselage — every frame — is computationally expensive. This is one reason X-Plane has historically been harder to run than other sims.
But CPU power keeps growing. What was expensive in 2010 is trivial in 2020. And the payoff — physically accurate flight behavior that emerges from geometry rather than being hand-tuned — is worth every CPU cycle. It's why professional training organizations use X-Plane. It's why I build for X-Plane. And it's why, when someone asks me "what makes X-Plane different," I don't talk about graphics or features. I talk about the physics. Because that's where truth lives in a simulator.
Javier Rollon is the developer behind JRollon Planes, creating aircraft add-ons for X-Plane since 2010. Follow on Twitter.