It depends on the type of craft and how you are going to operate it. I don't think a pilot's license is required for some ultra-light aircraft. If your craft doesn't go very far or very fast and never gets very high - e.g. a human-powered experimental craft or pehaps a ground-effect craft, you can probably test it in a remote area without running afoul of any laws. If this is a conventional, flies faster than about 50 mph, goes higher than about 100 ft, weighs more than a couple hundred pounds kind of airplane, count on needing that pilot's license. I believe your craft must be FAA inspected and test-flown by a certified FAA rep before you can fly it. _________ Added 11-21-05 SafetyGuy: According to the information I have found, "Operating an ultralight vehicle under FAR Part 103 requires appropriate training, but does not require an FAA Sport or Private Pilot certificate." I have seen no indication that home-built hovercraft or over-water ground-effect vehicles require pilot training. All the information I have found indicates my original answer is right. It does depend on what type of "aircraft" the question has in view. If you have different information, please post your own answer with details..