The LLRV (Lunar Landing Research Vehicle) was used to develop the LLTV (Lunar Landing Training Vehicle), but were there design differences between the two? Were any major lessons learned from the LLRV that resulted in design changes for the LLTV?
Essentially there was no difference – the LLRV was the "prototype", the LLTV the "production" version.
Bearing new names, the LLTVs were nearly identical to the Dryden vehicles except for a few minor improvements to more closely match the LMs.
As is not uncommon with prototypes, they made modifications to the LLRV after delivery, most prominently:
Meanwhile, to make the Dryden LLRVs more comparable with the real Apollo lunar modules, controls for the reaction control systems were incorporated into a single three-axis side arm controller and the control panels were also relocated from the center of the pilot stations to the right sides. Later, the cockpits were enclosed and the window visibility was restricted to match the LMs.
Initially the controls were like those of a helicopter: a center stick, a side stick (cyclic), and rudder pedals. Within three months, however, engineers and mechanics at the FRC had replaced these with two side sticks that better emulated the controls of the emerging lunar module. The left stick still controlled the lift thrusters and the jet engine (when not in lunar simulation) while the right stick now controlled all the maneuvering thrusters, eliminating the rudder pedals
And these changes were then incorporated into the LLTV:
Changes to the LLRV were incorporated into the plans for three LLTV, or Training Vehicles, which Bell was asked to construct and deliver to the Manned Spacecraft Center in Houston, starting in 1966.
(Ditto, page 14)
LLRV no. 1 was modified into LLTV and sent to Houston for astronaut training; although modified likes its sibling, LLRV No. 2 it remained at the FRC where it was cannibalized for parts.