Vertical climbing tank/rover
Hate to throw cold water on this, but I too doubt you'll ever get it to work. I really don't like being the guy that says, "It'll never work", because I also believe there's a solution to every problem but....
A 5X3.5 prop is about the right size for an .049 engine, but the .049 will NOT provide sufficient thrust to hold the tank against the tree and give it the necessary traction. In order to get that type of thrust, I'd guess you'd need more like a bearing type .40 or .46 at least. Then you're looking at props in the 9" to 11" diameter range. (Maybe an 8X7", 3 blade prop.) But now you'll never get something this size up a tree, "so densly congested you can't see through them."
I actually doubt this would climb a smooth concrete wall very well (if at all). You're relying on prop thrust to push something horizontally with enough force to overcome gravity AND provide traction. You'll also need to consider the prop trying to "suck" air from around the vehicle.
At the very least, the engine would have to have some degree of "down thrust" to overcome gravity.
Wish I was a physicist. I'm sure there's mathmatical theory that would give a better indication of the possiblity.
Dennis-