ArduPlane: fix quadplane stopping speed equation

[eppravitra]

I'm not sure if you are aware of this, @tridge, but I think this is the correct equation to calculate the stopping speed. From my test, I don't notice any difference. I only test using SITL with Gazebo though.

[IamPete1]

I have done a geogebra of the change.

https://www.geogebra.org/calculator/frtg2bry

The current equation gets to 0 speed at distance threshold, the new equation gets to target speed at the distance threshold.

The change certainly makes the equation do what the comment says it should.

[tridge]

@eppravitra thanks! we just need to do some testing both in RealFlight and in some real vehicles, @peterbarker and I can test on 2 quadplanes at SpringValley on a weekend, likely in 2 weeks time (I will be away this weekend)

[tridge]

@eppravitra I'd also note that this code is also bad if you'd like to fix it - should not depend on wp_speed

        // limit target speed to a the pos1 speed limit, which starts out at the initial speed
        // but is adjusted if we start putting our nose down. We always allow at least twice
        // the WP speed
        target_speed = MIN(MAX(poscontrol.pos1_speed_limit, 2*wp_speed), target_speed);

[tridge]

tested on a MakeFlyEasy Hero, no issues

[eppravitra]

Hi Andrew @tridge,

The ship landing test keeps failing. I wonder if it has to do with this change.

I quickly looked at plane_ship_landing.lua, and I still can't spot anything obvious.

Your help would be greatly appreciated.