Almost every conceivable way of taking off and landing vertically has been tried at least once, and most were judged impractical at the time. But technology marches on, and growth in the unmanned-aircraft market has brought with it resurgence in interest-and invention-in vertical flight. With no need to carry a pilot, fly supersonically or transport passengers, the VTOL barrier to entry has been lowered for unmanned aircraft systems. And the benefits to operational flexibility of being independent of runways-or launch and re- covery systems-can be compelling when the penalties in payload and endurance can be minimized, or accepted.
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