They are (should be) determined for each aircraft during Phase 1 testing. There are so many variations in builds that numbers may be similar but won?t be the same. My Vx with a light plane and fixed pitch prop will not be the same as somebody with an io360 and constant speed.
Chris
Yep, part of the fun/education of the Experimental world. Just to throw my Vy number out there, it's 82 KIAS, which also happens to be best glide speed at max gross. My -9A has an IO-320-D1A and a FP Catto prop, 70x70.
I typically climb out around 90 KIAS for better engine cooling, and then above TPA transition to a 100 KIAS cruise climb to altitude.