If you’ve ever bumped into that ex at a party or been disheartened by the fifth rebirth of Rave music, you’ll know some things refuse to disappear. Likewise the corpse of many a ‘dead’ aerospace technology has tiptoed out of the grave to dance with the Michael Jackson of progress. Here are six examples of good ideas that have come back to the sky.
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Lighter than air
Most people believe that the popularity of lighter-than-air (LTA) craft crashed with the Zeppelin, R101 and Akron. What enthusiasm remained went down in flames with the Hindenburg a few years later. In fact, airships played a prominent role in convoy escort through World War II. That same conflict also saw unmanned barrage balloons, tethered blimps, defending London from attack. Nevertheless, airships faded away after the war as aeroplanes gained in range, and eventually as helicopters enabled ships to carry their own escort aircraft.
LTA is back, with a vengeance. Veterans of the American campaigns in Afghanistan and Iraq will remember the omnipresent aerostats*, packed with sensors. Residents of Maryland and Pennsylvania may remember an aerostat breaking free and wreaking havoc like a Portuguese Man O’ War across their states. The errant sky sausage wandered for 160 miles, ripping power-lines to pieces and causing mass blackouts.
*’Aerostat’ is an umbrella term for a lighter than air craft that includes airships and balloons.
When not impersonating Herge’s Thomson and Thompson or gracing page four of every aviation history book (following Icarus is on page one, Chinese kites on two, and the Montgolfier brothers on three) the Wright brothers did something with aeroplanes. Flapless and fancy-free, Orville and Wilbur Wright controlled their Wright Flyer not by ailerons, but by actually twisting their wings to change the shape of the airfoil itself, much like a bird. In Orville and Wilbur’s day, it was just done by pulling on cables which pulled on the trailing edges of wings, changing the shape of the airfoil, altering the amount of lift in order to initiate an angle of bank. As aircraft moved beyond men lying prone upon a few layers of canvas (a stage hammocks never progressed from), wing warping fell out of favour.
Today, its modern incarnation is called ‘wing morphing’. Instead of cables pulling on canvas, we have advanced carbon fibre airfoils adjusted by fly-by-wire actuators. This allows for more efficient airfoils and a reduction in mechanical complexity (though lacks the simple poetry of the Wright’s machines). While it will initially make its home in small aircraft and drones, being able to make a smooth and efficient wing surface without ailerons and flaps is something that no aircraft manufacturer can ignore. Sometimes nature had the best idea all along (and other times she didn’t, just ask the male marsupial mice that are killed by the act of mating).
In World War II, glider-borne forces were essential to the D-Day landings. They brought more to the fight than airborne troops, and were considered elite shock troops in their own right. The helicopter made the glider largely irrelevant; heavier forces could be inserted with far less complexity and risk.
Aerial delivery has come back in a big way in recent years, especially in Afghanistan, where forces operate in small outposts far from major bases, aerial delivery of supplies via parachute has become commonplace. GPS guided supply drops have improved the accuracy of those drops. It’s always bad when supplies land outside the wire: troops don’t want to have to fight their way out just to get to their food. Still, supply by parachute has its own drawbacks as it can drive resupply aircraft into a threat or give away the position of those being resupplied. Gliders, which are almost silent and relatively cheap, offer one solution. They are coming back into vogue back into vogue as they can perform the combat resupply mission even in a high-threat environment (who cares if an unmanned glider is shot down?).
Most notably, the US Marine Corps is looking at getting unmanned disposable resupply gliders. They travel further than parachutes, and are also far less expensive than self-propelled unmanned systems. Teams in the field don’t have to worry about bringing a plywood glider back with them after a mission.
The glider is also extremely survivable in one unique way: Modern ‘heat-seeking’ anti-aircraft missiles are extremely effective, the glider is the only type of aircraft that can boast extremely low observability in the infra-red spectrum.
Variable speed propellers
When aeroplanes first debuted, the speed of an aeroplane’s propeller changed with the speed of the engine. While more commonly called ‘fixed pitch* propellers’, they could conversely be described as ‘variable speed propellers’. The technology did not yet exist to change the pitch of the propeller, so getting more thrust just came down to making the propeller spin faster.
*The pitch of the propeller is the angle it presents to the air it chops through. Angles of pitch – like different bicycle gears- are appropriate for different phases.
Unfortunately, aircraft engines work most efficiently over a narrow range of speeds. If changing the speed of the prop is the only way to change the speed of the aircraft, then the engine has to work over a broad range of speeds, and thus has to work much harder.
If instead of just turning faster, one could change the pitch of the propeller, one could then keep the engine turning at a consistent speed. Over time, variable pitch and eventually constant speed propellers did just that. The majority of modern aircraft, including helicopters, have some kind of mechanism to keep rpm constant.
But aeroplanes and helicopters have different optimal blade speeds. Tiltrotor aircraft, like the Bell-Boeing V-22 Osprey, have to deal with both regimes, and thus have to turn their blades at different speeds in different modes of flight. The Osprey has selectable rotor speeds–roughly 84 or 100 percent in aeroplane mode, and 100 or 104 percent in conversion or VTOL mode. That lets the aircraft use the most efficient rotor speed for the flight regime, and make the best use of blades with different sections optimised for aeroplane and vertical flight.
Varying the speed of the proprotors will become more common as tiltrotors proliferate. Even more traditional appearing rotorcraft designs will start to employ some type of mechanism to select rotor rpm in order to improve aerodynamic and mechanical efficiency. Boeing’s (formerly Frontier’s) A160 Hummingbird UAS demonstrator used a two-geared transmission, switching between high and low gears, to improve efficiency, and thus endurance. While still without a prototype, Karem Aerospace claims it will someday be able to put similar technology to work in a tiltrotor aircraft, allowing the engine to work at its optimum speed regardless of the mode of flight.
Whatever the mechanism, eventually future aircraft will have the ability to change their rotor rpm as needed throughout their flight envelopes, which in conjunction with varying pitch will get the most out of their drive systems.
A space capsule is a wingless spacecraft.
When the shuttle Columbia took flight in 1981, it seemed to end the reign of the space capsule. That had been the vehicle of manned spaceflight since Yuri Gargarin first orbited the earth in 1961. Why wouldn’t it? It was reusable, and came back like an aeroplane, under control, not hanging under parachutes to land wherever the winds blow.
The Space Shuttles never lived up to their original billing as a cheap way to lift cargo into space. Costs remained high–$1.3 billion per sortie and $10,400 per kilogram taken to space, by some estimates (this makes an Etihad flight from New York to Abu Dhabi look pretty good value). Sortie rates remained much lower than originally projected (think F-22s maintained by manic depressives).
The Russians and Chinese continued using capsules to lift cosmonauts and taikonauts ( Chinese space dudes and dudettes) into space. After the Shuttles’ retirement, the US was even forced into the embarrassing position of having to hitch rides on Russian capsules to the International Space Station.
While commercial operators are proceeding with several winged designs, the next generation of NASA spacecraft is going to be a capsule model. The Orion spacecraft will be reusable, but its mission profile would look very familiar to astronauts of the 1960s.
My money, in the long run, is on the return of the ornithopter, a flying machine that uses flapping wings for propulsion and control. As Carl notes above, nature often gets things right: the future ornithopter would include wing morphing with complete variable geometry wings for optimum efficiency. Flapping winged machines have been attempted for centuries, but with today’s lightweight materials and engines it has become viable. Only very recently has this method of flying been reliably demonstrated. Remarkably, human ornithopter was demonstrated in 2010:
Hey- why didn’t you mention supersonic airliners (like the very cool Convair concept at the bottom)? Because I’m not convinced they are set to return just yet. There’s 10 incredible cancelled airliners here (something to consider as you sit in your snoozy 737). Ten most boring aircraft can be endured here.
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