FAA section 333 provides permission to fly in the NAS with an airworthiness certificate. Obviously, the design would need to be perfect with extremely redundant control schemes. Going through the red tape, could an amateur build a UAV the size of, say, an RV-8 and legally fly it like a normal airplane?
You sure can,
The FAA provides a special airworthy cert for this kind of thing. There are a few types that you can apply for depending on what you are trying to fly. You can find the relevant info here
You can find the full Special Order that covers it here.
- Purpose of This Order. This order establishes procedures for issuing special airworthiness certificates in the experimental category or special flight permits to unmanned aircraft systems (UAS) and manned aircraft integrated with UAS technology. For the purposes of this directive, manned aircraft integrated with UAS technology are referred to as optionally piloted aircraft (OPA).
In your specific example if it was an RV-8 (built as it usually is) with the ability to remote pilot it added in, it would be classified as an "optionally piloted aircraft"