So Shoji Kawamori is known to be a mecha designer with meticulous attention to detail, and indeed that's one of the reasons his Variable Fighters are so alluring as they are. Nonetheless, with Kawamori's more recent designs I think he may have forgotten an important aspect of the transformation sequence: how GERWALK mode propels itself.
From the VF-1, VF-11 up to machines like the VF-19, variable fighters are shown to have a second set of engines that appear during GERWALK mode, which handily explains where they get the forward thrust to move while their actual engines are pointed straight down to the floor, or in the case of the YF-21/VF-22 their engines simply don't change orientation at all.
On the other hand, designs like the VF-25 and the VF-31 are most definitely lacking in visible secondary engines for GERWALK propulsion. While Macross Frontier TV Episode 2 does have a scene where we see the VF-25's GERWALK mode come out of the rear of the hull centerline (just in front of the place where the Shield would normally be stored in fighter form), this location in many of the VF-25's physical manifestations like the DX figures do not show any such engines at that location. The VF-31 supposedly place their secondary engines at a similar spot (at least according to the VF-31AX Master File), though again neither the figures nor the show reflect this. The issue of GERWALK propulsion goes away when you equip them with SUPER Packs, though that seems quite the unfortunate kind of solution to this issue. For further inconsistency, designs like the VF-27 and the YF-29 don't have this issue due to having very large wing-mounted secondary engines.
Where am I going with this? Dunno. Just felt the need to point them out.