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Throttle limiting

Summary

Throttle limiting caps a drone's maximum speed by scaling the throttle range, in Betaflight via throttle_limit_type = SCALE on a switchable rate profile. On libdrone it serves two purposes: a gentle, forgiving mode for a first-time pilot learning to fly, and a calibrated speed ceiling for any operation where a maximum ground speed must be held. The cap is per-profile and transmitter- switchable, so full-speed capability is preserved on another profile. This is the first of a planned set of approachability and safety features that bring a beginner-friendly flight envelope to an open platform; where a speed ceiling is held for a regulatory reason, the regulatory question itself lives in legal-and-regulatory.

Concept

What throttle limiting does

Betaflight's throttle_limit_type = SCALE reduces the effective throttle range from 0–100% to 0–N%, where N is the configured limit percentage. At full stick, the motors receive only N% of their maximum PWM signal. throttle_limit_percent sets N. The result is a lower top speed with the full stick range still usable — the pilot uses the whole throttle travel, but the ceiling is lower, which makes control finer and overshoot smaller.

The limit is profile-specific and switchable from the transmitter, so a single aircraft carries both a capped mode and a full-speed mode and the pilot chooses between them in flight. On libdrone this maps to the SB rate/speed switch (see betaflight-profiles): position 1 = capped, higher positions = progressively faster.

Why it matters for a beginner

A capped top speed is the single most useful aid for a first-time pilot. It does not change how the sticks respond, only how far the aircraft can get away before the pilot reacts — which is exactly the margin a learner needs. Combined with soft rates and self-levelling Angle mode, throttle limiting turns an otherwise twitchy quad into something a novice can keep over a field. It is the libdrone equivalent of the "beginner mode" on a commercial drone, achieved with a single open Betaflight parameter rather than locked vendor firmware — and it is the first entry in a planned set of approachability features in that spirit.

What it does not guarantee

Throttle limiting is an operational tool, not a hard ceiling. Wind assistance, dive angles, and motor differences can push actual ground speed above the nominal cap. It reduces speed; it does not enforce an absolute limit, and it is not a safety certification of any kind. Where a specific maximum speed must be held for a regulatory reason, that requirement and whether it applies to your operation is a matter for legal-and-regulatory — decide for yourself from the single source of truth there; this atom only describes the mechanism.

Reference

Betaflight CLI (per profile)

profile 2

# Enable throttle scaling
set throttle_limit_type = SCALE
set throttle_limit_percent = 55   # starting point; tune to taste or target
save
  • Beginner / training start: ~55%, lower if the aircraft still gets away.
  • For a measured speed target: calibrate against GPS ground speed (below).

OSD

Enable the gps_speed OSD element to read real-time ground speed, and the profile-name or number element to confirm which profile is active before flying.

Transmitter mapping

Throttle limiting follows the rate/speed profile on the SB switch per edgetx-model and betaflight-profiles: position 1 is the capped profile, higher positions are uncapped or faster.

Procedure

Calibrating to a measured speed target

When a specific maximum ground speed must be held (confirm whether and why such a requirement applies to your operation in legal-and-regulatory):

  1. Enable GPS and the OSD ground-speed element.
  2. In the capped profile, set throttle_limit_percent to a conservative start.
  3. Fly a straight pass at full throttle in calm conditions (< 2 m/s wind); read peak GPS ground speed.
  4. Adjust throttle_limit_percent by ±5 points and repeat until peak speed sits a clear margin below the target.
  5. Confirm on three separate passes.
  6. Record the value with battery cell count and prop size in the build log; recalibrate if either changes, or if payload weight changes.

Rationale

A switchable rate-profile parameter was chosen over a dedicated flight mode or a firmware modification because it needs no flight-controller code change — it is a pure Betaflight parameter compatible with any Betaflight version, reversible from the transmitter, and immediately visible to the pilot. GPS-based speed limiting in firmware was rejected: it depends on features not present in standard Betaflight releases and an outer GPS-rate loop incompatible with tight PID timing, adding complexity without improving the operational result. Keeping the mechanism this simple is also what lets it double as a beginner aid — there is nothing to misconfigure beyond one percentage.

Connections

requires: - betaflight-profiles - betaflight-setup related: - edgetx-model - gnss-gps - legal-and-regulatory leads_to: - betaflight-profiles - first-flight