| AHRS |
Attitude and Heading Reference System — fuses accelerometer and gyroscope data to estimate pitch, roll, and heading |
| AOA |
Angle of Attack — the angle between the wing chord line and the relative wind. Also referred to by the Greek letter alpha (α). Determines lift and proximity to stall. In OnSpeed firmware and config, the variable named AOA is body angle (fuselage-to-wind), not wing AOA — see Body Angle below. |
| alpha_0 (\(\alpha_0\)) |
Zero-lift body angle — the body angle (DerivedAOA) at which the wing produces zero lift. Typically negative because most aircraft have wing incidence: the fuselage points nose-down when the wing is at zero AOA. Varies with flap setting. |
| alpha_stall (\(\alpha_\text{stall}\)) |
Critical body angle — the body angle at which the wing stalls (maximum lift coefficient exceeded). Essentially constant for a given configuration, regardless of weight or load factor. |
| Body Angle |
The difference between pitch attitude and flight-path angle (\(\text{Pitch} - \text{FlightPath}\)). The angle the fuselage makes with the relative wind. What OnSpeed actually measures and calibrates against — wing AOA is a different quantity that OnSpeed never computes directly. The two are linearly related (body angle = wing AOA + wing incidence + small terms), so body angle is a faithful proxy for the pilot. Equivalent to DerivedAOA. |
| CAS |
Calibrated Airspeed — IAS corrected for position error in the pitot/static system |
| CL |
Coefficient of Lift — a dimensionless parameter describing how much lift an airfoil produces relative to the surrounding airflow. Directly proportional to AOA over the normal operating range. |
| Cp |
Coefficient of Pressure — the ratio of AOA probe differential pressure to pitot (dynamic) pressure. Varies with angle of attack. |
| Critical AOA |
The angle of attack at which the wing stalls. Essentially constant for a given flap configuration, regardless of gross weight or load factor. An airplane can stall at any airspeed and in any attitude, but critical AOA remains the same. |
| DerivedAOA |
SmoothedPitch - FlightPath. The fuselage-to-wind angle computed from the AHRS and vertical speed. Used as the reference AOA for calibration. |
| Directive Information |
Information that directly calls for pilot action, requiring no interpretation. An AOA tone is directive — it tells you to push or pull. Contrast with descriptive information. |
| Descriptive Information |
Information that must be perceived, interpreted, and then acted upon. A conventional flight instrument is descriptive — the pilot must see it, understand it, then respond. |
| Effective Power |
The difference between thrust and drag, normalized by effective weight. When positive, the airplane can climb, accelerate, or sustain maneuvering. When negative, the airplane must descend or decelerate. |
| Effective Weight |
Actual weight multiplied by load factor. In a 2G turn, a 1500 lb airplane has an effective weight of 3000 lbs. |
| EFIS |
Electronic Flight Instrument System — glass panel avionics (Dynon, Garmin, MGL, etc.) |
| EKF6 |
6-state Extended Kalman Filter — an AHRS algorithm that estimates pitch, roll, AOA, and 3 gyro biases |
| EMA |
Exponential Moving Average — a smoothing filter that weights recent samples more heavily |
| FlightPath |
The angle between the aircraft's velocity vector and the horizontal. Computed as arcsin(VSI/TAS). |
| Fractional Lift |
How hard the wing is working relative to its maximum capability, expressed as a fraction from 0 (zero lift) to 1 (stall). Mathematically identical to NAOA. The percent at which each tone region transitions varies per flap by calibration — there are no fixed-percent band edges. |
| FreeRTOS |
Real-Time Operating System used by the ESP32 firmware for multitasking |
| I2S |
Inter-IC Sound — digital audio interface protocol used for the stereo audio output |
| IAS |
Indicated Airspeed — airspeed as measured by the pitot system, uncorrected for position error or density |
| IMU |
Inertial Measurement Unit — a sensor package containing accelerometers and gyroscopes |
| Instantaneous Turn |
A turn where the pilot pulls beyond ONSPEED, borrowing energy. Turn rate increases briefly but the condition is unsustainable — airspeed decays and AOA continues to increase. |
| K parameter |
The lift sensitivity constant in the equation DerivedAOA = K/IAS² + alpha_0. Related to weight, wing area, and lift curve slope. |
| L/DMAX |
Maximum Lift-to-Drag ratio — the AOA at which the aircraft achieves the best glide ratio. Where this lands on the fractional-lift scale varies per flap by calibration; the audio cue (start of fast tone) is what the pilot follows, not a fixed percent. Corresponds to best-glide speed and maximum range. |
| L/DMAX pip |
The two small white dots on the indexer's index-bar edges. Aerodynamic reference cue. Slides smoothly from the cleanest detent's L/DMAX percent to the most-deployed detent's OnSpeed-band center as the lever moves; intermediate detents are intentionally ignored. The pip lines up with the bottom chevron's edge only at the cleanest detent — at deployed flap settings the pip sits inside the donut while the audio threshold (chevron edge) stays at the active detent's calibrated L/DMAX percent. Per Vac's design rule (ld_max.pdf §8): "L/Dmax pips are aerodynamic references." See Indexer Spec. |
| LittleFS |
Little File System — flash-based filesystem used by the ESP32 for configuration storage |
| Low-tone threshold |
The body angle (or its percent-lift representation) at which the audio low tone begins playing — also the lower edge of the bottom green chevron on the indexer. Operational cue. Snaps to the active detent's calibrated L/DMAX body angle at every detent transition, in lockstep with the audio path. The chevron and the audio fire from the same source of truth; if a tone is on, the chevron is green. Per Vac's design rule (ld_max.pdf §8): "Fast tone is an operational limit cue." See Indexer Spec. |
| Load Factor |
The ratio of lift to weight, commonly expressed in G units. Denoted by engineers as n. In level flight, load factor is 1G. In a 60° bank turn, approximately 2G. |
| Madgwick |
A complementary filter algorithm for AHRS using quaternion mathematics. The default OnSpeed AHRS. |
| NAOA |
Normalized AOA — AOA expressed as a fraction of the usable range: (AOA - alpha_0) / (alpha_stall - alpha_0). 0.0 = zero lift, 1.0 = stall. Mathematically identical to fractional lift. |
| OAT |
Outside Air Temperature — measured by EFIS or DS18B20 sensor. Used for TAS and density altitude corrections. |
| ONSPEED |
A specific angle-of-attack condition, not an airspeed. Corresponds to ~60% fractional lift, balanced effective power, maximum sustained turn rate, VREF, best angle of climb, and maximum endurance. The airspeed associated with ONSPEED varies with weight, configuration, and load factor. |
| OneWire |
A serial protocol used by the DS18B20 temperature sensor |
| OTA |
Over-The-Air — firmware updates delivered wirelessly via WiFi |
| PPS |
Pulses Per Second — the rate at which the audio tone pulses on and off |
Pressure Altitude (PALT, Palt, PAlt) |
Altitude derived from the static pressure sensor against the ISA standard atmosphere (1013.25 hPa / 29.92 inHg sea-level reference). Every altitude OnSpeed reports — on the M5 display, the live view, the SD log columns (Palt, Altitude), and the display-serial wire — is pressure altitude. No Kollsman / QNH / baro-set correction is applied, so the value matches the panel altimeter only when the altimeter is set to 29.92. The CSV's Palt column is the raw 50 Hz reading; the Altitude column is the same quantity smoothed by a 3-state Kalman filter. |
| PSRAM |
Pseudo Static RAM — 8MB of additional RAM on the ESP32-S3 module |
| SPI |
Serial Peripheral Interface — the bus protocol used for sensors and SD card |
| Sustained Turn |
A turn flown at ONSPEED where available power balances drag. AOA and airspeed are stable, and the turn can be maintained without energy loss. Maximum sustained turn rate occurs at ONSPEED. |
| TAS |
True Airspeed — IAS corrected for air density (altitude and temperature) |
| Unload for Control |
The fundamental principle for maintaining positive aircraft control near the aerodynamic limits. Reduce AOA and load factor to regain aerodynamic margin. An airplane cannot stall at zero G. |
| VA |
Maneuvering Speed — the speed below which the airplane will reach the aerodynamic limit (stall) before exceeding the structural G-limit. Changes with weight. |
| Velocity Vector |
Where the airplane is going and how fast. Not necessarily aligned with where the airplane is pointing. AOA governs wing workload but does not determine flight path. |
| VFE |
Maximum Flap Extended speed — the maximum speed at which flaps may be deployed |
| VNO |
Maximum Structural Cruising Speed — the speed above which flight should only occur in smooth air |
| VREF |
Reference approach speed — typically 1.3×VS at maximum gross weight. Corresponds to the ONSPEED AOA condition. |
| VS |
Stall Speed — the minimum speed at which the aircraft can maintain level flight at a given weight and configuration. Varies with the square root of load factor. |
| VSI |
Vertical Speed Indicator — rate of climb or descent, typically in feet per minute |
| V--n Diagram |
A plot of load factor (n) vs. airspeed (V) showing the airplane's flight envelope. The curved left boundary represents the aerodynamic limit (stall). |
| WebSocket |
A protocol for real-time bidirectional communication. Used by the live view page (20 Hz updates). |
| ×VS |
"Times stall speed" — a way to express approach speed as a multiple of stall speed (e.g., 1.3×VS = 1.3 times the stall speed) |