Fixed Wing: Compensate minimum sink rate based on weight ratio and air density

[RomanBapst]

Solved Problem

The minimum sink rate (gliding) of a fixed wing is a function of vehicle weight, air density, wing area and the airfoil polars.
While the wing area and the polars are generally constant, the weight and the air density are likely to change.

In PX4 the parameter FW_T_SINK_MIN determines the mapping between commanded sink rate and throttle output. If the commanded sinkrate equals or exceeds FW_T_SINK_MIN then generally TECS will command minimum throttle (since we are gliding or maybe even descending faster).

Since the minimum sink rate changes with weight and air density, it's important to compensate for this, as it affects how well TECS tracks desired energy rate.

For both the vehicle weight and the air density there is a square root relation to sinkrate. This makes compensation based on these two factors straightforward.

$V_{sink} = \sqrt{2mg \over {\rho S}} * f(C_A, C_D)$

where $S$ is the wing area.

Solution

Compensate minimum sink rate based on weight ration and air density.

Changelog Entry

For release notes:

Feature: Compensate TECS minimum sink speed for weight ratio and air density.
Documentation: Need to clarify TECS tuning.

Test coverage

• Unit/integration test: ...
• Simulation/hardware testing logs: https://review.px4.io/

Context

Related links, screenshot before/after, video

[tstastny]

Are you still working on this one? - looks like geMinimumSinkRate() isnt actually called anywhere..

[tstastny]

thanks for writing a brief - can you list the conditions that are considered, and can you add units to the @return?

[tstastny]

same comment

[tstastny]

also you can replace this with your new encapsulated weight ratio method

PX4-Autopilot/src/modules/fw_pos_control/FixedwingPositionControl.cpp

Lines 466 to 469 in d6a770a

	if (_param_weight_base.get() > FLT_EPSILON && _param_weight_gross.get() > FLT_EPSILON) {
	weight_ratio = math::constrain(_param_weight_gross.get() / _param_weight_base.get(), MIN_WEIGHT_RATIO,
	MAX_WEIGHT_RATIO);
	}

[RomanBapst]

Closing in favor of #22091