A wind turbine's rotor only faces the wind squarely when the nacelle is yawed to match the wind direction. When the rotor plane is tilted away from the wind by a yaw error γ, less of the wind's momentum flux passes through the swept disc, and the power output falls off following an empirical power law widely used in wind-energy engineering:
P(γ) = P(0) · cos^p(γ)
P(0) = ½ · ρ · A · v³ · Cp (capped at rated power)
A = π R², ρ = 1.225 kg/m³, R = 40 m, Cp ≈ 0.45
Field and wind-tunnel studies put the exponent p between about 1.9 and 3 depending on turbine design — the slider lets you compare a shallow loss curve against the textbook cos³ law. Because available power grows with the cube of wind speed, even a modest yaw error costs real energy at high wind speed — the top-down disc view visibly narrows to an ellipse as γ grows, showing exactly how much less swept area faces the flow.
The active yaw drive only intervenes once the error exceeds a deadband — real drives don't chase every gust, both to save wear on the yaw gear and because a small error costs little power. Once triggered, the nacelle turns at a limited yaw rate toward the wind, always along the shorter rotation, and stops once it's back inside the deadband. Switch to Manual and drag the disc (or use the slider) to fix the nacelle heading yourself and watch the power collapse as the wind direction wanders away from it.
- Top-down rig panel — the rotor disc foreshortens with cos(γ) exactly like the real swept area facing the wind; drag it in Manual mode to set the heading directly.
- Power strip chart — rolling history of output (orange) against the aligned maximum at the current wind speed (dashed), so you can see the drive clawing power back after a gust.
- Polar loss-law plot — cosp(γ) traced all the way around the compass for the current exponent, with a marker at today's actual yaw error.