this is a little story. I bought this and built it as analog first, expecting to convert to HD later. so I installed the 20x20 stack and built it as normal. it's a very powerful quad as built on 3 inch props. video was great as analog and the frame is pretty dam bullet proof. so I expected to be able to put the vista air unit behind the stack, but that was an incorrect assumption. the intended strategy is actually to use an AIO and mount the vista on top of that. as I had already built this, I didn't want to change all that. so I disassembled it, marked a new 20x20 mount pattern and drilled a new mount in my mill. so my stack is about 3mm forward of its original mounting position. I spoke with Gank Multirotors, and Matt was happy to hear the feed back. he may make a revision on the HD frame later.