High Speed Integral

In short: HSI keeps the helicopter's attitude steady in fast flight and on big collective changes. The default works for most helicopters; raise it if the helicopter balloons, dives or bobbles in fast passes; lower it if it bobbles at the stops of tic-tocs.

What it does

A rotor disc moving fast through the air produces more lift on the advancing side and the front than at rest. In fast forward flight with positive collective the helicopter tends to pitch up; with negative collective, the opposite. The effect depends on both airspeed and collective -- and it reverses when the collective does.

The HSI offset (the O term) is an integral that works in proportion to the collective. It builds up the correction needed for the current flight condition, and because it scales with collective, the correction reverses automatically when you reverse the collective. On other flybarless systems you may have seen this as the swashplate "arching" when you move the collective with the helicopter tilted.

Settings

On the Profiles tab, for roll and pitch:

Setting	Default
HSI Offset Gain (O)	50	How strongly the offset builds.
HSI Offset Limit	90°	The most offset allowed.

Tuning

You see	Try
Pitches up or dives in fast forward flight	Raise HSI gain
Attitude jumps on big collective changes at speed	Raise HSI gain
Bobbles or restless at high collective	Lower HSI gain
Bobbles at the stops of tic-tocs	Lower HSI gain

A good test: tilt into fast forward flight at high collective and jab the elevator. Raise the gain until it starts to bobble, then back off.

Why tic-tocs are tricky

HSI works best when the correction needed reverses together with the collective. In tic-tocs, a small error also reverses at every stop -- so a high HSI gain can start to treat it as a correction, and bobble. Rotorflight transfers HSI into the normal I-term when large cyclic inputs are given, which limits this; if it still happens, lower the gain.