In 1912, aviation experts with the US Army in College Park, Md., successfully tested a virtually noiseless biplane engine. They were also experimenting with camouflage paint that could make planes extremely difficult to spot when more than 200 feet overhead. These breakthroughs, enthused the magazine Aerial Age at the time, ``opened up a wonderful field in aviation, making it possible for a biplane or monoplane to sail over cities unheralded and unseen.''
Seven decades later the US military is still trying to devise an undetectable aircraft. The aim this time isn't to fool people on the ground but the ``eyes'' of enemy radar and sensors.
The Pentagon is spending billions on applying ``stealth'' technology to bombers, jet fighters, and cruise missiles.
Yet the interest in making airborne objects less detectable extends well beyond this, to everything from helicopters to satellites.
``Stealth is the darling of the [military] R&D world,'' says Jerry Cantwell, an aerospace-defense analyst at First Manhattan Company, an investment firm.
The thrust reflects what is emerging as a significant military theme in the 1980s: making armaments of all kinds -- from submarines to foot soldiers -- more hideable from the enemy.
While the progress in stealth technology remains shrouded in secrecy, enough details continue to leak out to offer some glimpses of the shape of aircraft being developed.
Recent reports have surfaced, for instance, indicating that the ``flying-wing'' concept is being revived as the leading design for the Air Force's stealth bomber.
There has long been speculation that the radical design was being considered for the warplane, which is to succeed the B-1 bomber in the 1990s. But details of the program have been kept under wraps.
Now evidence continues to build suggesting that the Northrop Corporation, the prime developer of the bomber, is pursuing a flying-wing design similar to one the company had built in the 1940s. The shape is known to be ideal for helping evade enemy radar, since it has no high tail or fat fuselage to reflect incoming signals.
The early versions suffered stability problems and never were pressed into service. But some engineers believe these problems can now be overcome with advanced technology, though questions lurk about the range and payload capacity of flying wings.
There are several ways to hide aircraft from radar:
Shape. Radar acts like a searchlight: Radio waves are beamed out and reflected off the target, revealing a plane's distance and position. Sharp edges are highly reflective. Thus stealth engineers want to make aircraft as smooth and curvilinear as possible.
This explains the allure of the flying wing: Blending the tail and fuselage into a thin-profile plane would, in theory, leave only a speck at most on the enemy's radar screen.
But engineers are known to be looking at other design tricks as well.
Because the intakes of jet engines are highly reflective, engines can be buried in the body or moved to the top of the plane, which also helps shield their hot exhaust from heat-seeking missiles. Weapons on future stealth fighters will probably hug the underbelly of the aircraft or be carried inside.
To help foil heat-seeking missiles, engineers are also exploring such things as shaping engine outlets to create smaller exhaust plumes.
Materials and coatings. Radar waves bounce off metal like a superball. So engineers are working with special plastics and composites -- fiber-reinforced plastic materials -- that absorb or deflect radar signals. No plane will likely be all plastic-skinned. But certain surfaces, such as wings and radar-reflecting edges around engine enclosures, are emerging as good candidates for employing such materials.
Radar-absorbant coatings are another way to elude detection. One type of stealth ``paint'' already in use on first-generation stealth craft like the Lockheed SR-71 reconnaissance plane, for instance, contains microscopic iron particles that deflect signals in different directions.
The snag here, however, is that some coatings only work for a few radar frequencies. Thus multiple coatings have to be applied to thwart multiple signals -- adding to a plane's weight.
Electronic countermeasures. Electronics can be used to fool or jam enemy radar. One method is to transmit delayed radar pulses in the opponent's frequency band to create ghost images on his screen. Another is to send out flashes of radio energy that blind his radar.
All this means that a mix of technologies, not just one elixir, will go into any future stealth aircraft.
It also means, says Kosta Tsipis, director of the Massachusetts Institute of Technology's Science and Technology for International Security program, that an invisible plane is beyond reach. Instead, he says, stealth will just enhance the ``survivability'' of aircraft.
A chief technical challenge is incorporating the different design changes into an aircraft that will be aerodynamically fit and militarily sound. While the flying-wing design, for instance, may be radar ellusive, some experts wonder whether it can be built to carry enough weapons or have the necessary range of a strategic bomber.
``The more stealthy you make an aircraft, the less efficient will be its performance,'' says Anibal Tinajero, a defense analyst with the Congressional Research Service.
There's also the growing concern about money. At present, the scope of the Pentagon's overall stealth program remains a mystery. Wall Street analysts estimate that Northrop is receiving $1 billion annually to develop the stealth bomber, officially known as the Advanced Technology Bomber (ATB). But there's also Lockheed Corporation's work on a stealth fighter (now thought to be in early production), development of stealth cruise missles, helicopters, and a proliferating group of other ``low-observable'' technologies.
The ATB remains the most politically divisive, locked in an enduring funding battle with the B-1 bomber.
Stealth supporters have prevailed in holding off attacks on stealth funding in the fiscal 1986 budget. But few observers expect the Air Force to terminate the production of B-1s -- the most recent of which have plenty of radar-eluding gimmickry of their own -- at the 100 approved by Congress.
Beyond that, however, there is growing anxiety among some legislators and public-interest groups over the increasing number of secret programs related to stealth, which critics feel is hampering Congress's ability to oversee the defense budget.