1.: In Yuri's Revenge, dropping "helicopters" work, because they enabled animated voxels for the Spy Plane - i.e. the HVA will be read as normal. If you don't believe me, download my mod.
2. ComradeJ is right; a dropped tank's HVA is screwed up. The tank appears to be below it's shadow.
3. LordShay: He pondered about a Soviet Mission where tanks were dropped by planes. Missions are usually played on maps. Considering this fact, my previous post totally makes sense.