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last frontier VTOL, The

Flight Journal, Jun 2003 by Kress, Robert

The Ups and Downs of Vertical Takeoff and Landing

This is my insider's look at the sometimes frustrating world of creative aerodynamics and the difficulties associated with getting a new design concept adopted. Two decades or so ago, Grumman had an idea that we thought would fill a national need, but it was not to be. Politics doesn't always understand the rationale behind technical progress. Today, the idea in question is still viable. The national need for it is still unsatisfied. So, this old engineer soldiers on.

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Since the laws of physics refuse to bend for anyone, we engineering types are reaching the point of diminishing returns with much of our design work. For instance, the progression from the Wright Flyer to Spads to Mustangs to Tomcats represents enormous strides, but since the F-14/F-15-type aircraft came on the scene, we've been bumping our heads against technological ceilings. Does the absolutely state-of-the-art F-22 represent the same progression as we saw from Mustang to F-86? Of course not. Does it offer anything hugely different from its predecessors? No, it doesn't. It does, however, improve the breed, but it doesn't redefine it.

So, have we seen all there is to see with respect to the development of flying machines? Here, too, the answer is no because at least one frontier hasn't been efficiently conquered: vertical takeoff and landing (VTOL). VTOL represents the last frontier primarily because, although we've poked our toes into those aeronautical waters, with the exception of helicopters, we really haven't waded all the way in and learned to swim. For the most part, we have retreated to the shore when we found the going tougher than we expected.

VTOL

A ton of VTOL aircraft (not helicopter types) have been invented and some were even tested as full-scale prototypes. Seth Anderson of NASA-Ames constructed a wonderful "pinwheel" chart, later named the "wheel of misfortune," that presented all of the VTOL configurations tested up to about 1980 (there haven't been many since then). Some flew without crashing. Some were inventors' dreams. Some attempted to defy the laws of physics and suffered the consequences. However, with the exception of the Harrier and the XV-15 (V-22 prototype), nothing came of them.

WHY?

When you study most of these aircraft knowing what we know now, it's obvious that they were technically flawed. The most obvious problems included inadequate control authority in hover, mechanical complexity, poor structural integrity, erratic stability and control during transition and that they had to try to lift too much weight with too little thrust.

It's important to understand the history of VTOL aircraft in terms of the principles of aerodynamics. Thrust-per-shaft-horsepower in VTOL aircraft is determined by the amount of horsepower applied to the tube of air being accelerated by the powerplant. Power could come from a reciprocating engine ducted fan, a jet engine or a turbofan. As long as the horsepower is there-it could be generated by a rubber band-the column of air doesn't care.

At the low-disc loadings typical of helicopters (that is, the weight the disc is required to lift measured in pounds per square foot of disc area), the thrust produced per input-shaft horsepower is very high. Aircraft in this class lift large loads, have long hover duration but not enough downwash to chew up the ground as a jet does. Their speed, endurance and ceiling are generally much lower than the higher-disc-loading vehicles. A good modern example of low-disc loading was the XV-15 (which became the V-22 Osprey). At the other end of the disc-loading spectrum is the lowbypass turbofan AV-8 Harrier. This thing is really loaded! The Harrier made the concept work but not without paying a price: high fuel flow in hover. Airshows excluded, Harrier pilots don't want to hover for long, or they'd better hover right over to a gas station.

If all of the VTOL concepts, those that have been proposed and those that have flown, are compared, it would be noticed that there is a distinct gap between the lightly loaded birds and those that carry a lot of weight on a small disc. This gap indicates that little effort has been expended on developing VTOL turbofan transport-type aircraft, with one exception-Grumman's Design 698 Tilt Fan VTOL. The goal of Design 698 was to fill this gap with a Learjetclass, high-speed, long-endurance and high-altitude aircraft that would also have USN surface-combat applications. A lot of work was done on the 698 and, considering that the niche for which it was designed still exists, more needs to be done on this and other promising turbofan VTOL concepts. If we could just get our VTOL act together, the payoff could be huge in terms of highly versatile civil and military aircraft.

Grumman started blasting away at the last frontier in the early '70s. I was the primary player along with a couple of other fugitives from the lunar module (LM) program. Our first concept was the Nutcracker.

IN THE BEGINNING THERE WAS THE NUTCRACKER

last frontier VTOL, The

Flight Journal, Jun 2003 by Kress, Robert

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