In 1958, a small California company run by an engineer named Edmond R. Doak Jr. built an aircraft that did something the V-22 Osprey wouldn't reliably do for another four decades: it took off vertically, transitioned to horizontal flight without drama, and landed again. The Doak Model 16, designated VZ-4DA by the U.S. Army, used a configuration that has since been quietly rediscovered by half the eVTOL industry — tilting ducted fans on the wingtips.

The airframe was almost absurdly simple. A conventional-looking high-wing monoplane with a single Lycoming YT53-L-1 turboshaft producing 840 shaft horsepower in the fuselage, with shafting running out to two 4-foot diameter ducted propellers mounted on the wingtips. The ducts pivoted 90° from vertical (hover) to horizontal (cruise). A small reaction-control jet at the tail handled pitch authority in hover. Empty weight: 2,300 lbs. Max speed in cruise: 230 mph. Hover ceiling: roughly 6,000 feet.

Doak first flew it on February 25, 1958. It hovered. It transitioned. Test pilot George Edenborough made the first full conversion to horizontal flight by May 5, 1958. The Army accepted delivery in 1959 and flew it at Edwards AFB and later Langley by NASA. Over roughly 50 flight hours, it never had a serious accident. Pilots reported the transition was smoother than the helicopters they'd flown.

So why did it die?

• Doak Aircraft was tiny. Edmond Doak sold the design to Douglas Aircraft in 1961, where it disappeared into a portfolio of competing projects. Douglas had no business case to pursue it.
• The Army wanted bigger. Counterinsurgency doctrine in Vietnam demanded squad-sized lift, not a 1-pilot demonstrator. The Bell X-22 and Curtiss-Wright X-19 got the budget.
• Transmission technology wasn't there. The cross-shafting that synchronized the two ducted fans was heavy, finicky, and the single failure point that scared engineers.
• The aircraft itself was donated to the Smithsonian in 1972 and sat in storage at the Paul E. Garber Facility for decades.

Why it's viable now: Every reason it failed has evaporated.

• Distributed electric propulsion kills the cross-shaft problem entirely. Each duct gets its own motor; fly-by-wire handles synchronization in software. Joby, Archer, and Beta Technologies are all flying variations of this concept right now.
• Composite ducts are 60% lighter than the 1958 aluminum versions, with better acoustic shrouding — solving Doak's noise problem.
• Modern turboshafts like the GE T901 deliver 3,000+ shp at half the specific fuel consumption of the YT53. A scaled VZ-4 with a single T901 could lift 4 passengers at 280 knots.
• Hybrid-electric architectures let a single small turbogenerator feed four wingtip duct-fans, giving you Doak's planform with quad-redundant propulsion.

The most damning evidence that Doak was right is that Bell's V-280 Valor — the $1.3 billion winner of the Army's FLRAA program in 2022 — uses tilting nacelles on wingtip pylons. It is, conceptually, a 72,000-lb great-grandchild of a 2,300-lb prototype that flew before the Beatles formed.

The VZ-4 sits today at the U.S. Army Transportation Museum at Fort Eustis, Virginia. Visitors walk past it without a second look.