Finder, Feeler, Sounder

From the naval reserve base at Floyd Bennett Field on Long Island one morning last week, a small motor truck containing a radio transmitter set off for an unannounced point an hour's drive away. Hour or so later an American Airlines transport ship took off from the same field. Mounted horizontally just behind the throttles between the pilot and co-pilot was a circular dial face marked off in degrees like a compass. Over this swung an indicator hand. A little tuning picked out the truck's signal, and the hand froze like a pointer on the bearing. Following this bearing, the plane chased over villages and farms, finally passed over Valley Stream Airport. As it did, the tell-tale needle swung full around, pointed backwards, spotted the truck parked behind a hangar.

Chief purpose of any direction finder in transport flying is not alone to locate ground points but to help determine the plane's position in flight. After a ground station is tuned in on the ship's radio receiver in this new Sperry-RCA apparatus, a loop antenna suspended beneath the plane rotates automatically until it is at right angles to the source of the signal, registering the bearing on the dial. Where two or more such bearings intersect is approximately the plane's position.

Most direction finding is "180DEG ambiguous," i.e., shows the bearing of the radio wave, but not the source. In the Sperry-RCA device this ambiguity is eliminated by placing two antennas in an electrical relationship making it possible to single out the source and keep pointing at it despite any maneuver the ship may make.

To the military-minded, the demonstration proved that even a mobile enemy radio unit would have to be extremely laconic to avoid detection; to the airline safety-minded, it meant that a pilot off his course could orient himself almost as fast as he could tune in radio bearings.

> United Air Lines and Bell Telephone Laboratories also were at work on safety. One afternoon last week United Air Lines' flying laboratory, went up from Newark Airport carrying a new type of altimeter mounted beside a regular barometric altimeter. Up the Hudson River it flew at 800 feet, both dials registering alike. But as it crossed towering George Washington Bridge, the reading on the new altimeter dropped to 500 feet. Few miles farther on the plane banked sharply, headed for the Palisades, still flying at 800 feet by the barometric altimeter. As it approached the sheer bluff the other needle quivered, then dropped to 250 feet as the ship passed over.

What did these tricks was a "terrain clearance indicator,"'first practical development of a radio-run absolute altimeter. From a transmitting antenna on one wing, radio waves are bounced groundward, caught on the rebound by a receiving antenna on the other wing. The elapsed time interval is measured in millionths of a second at radio's speed of 186,300 miles per second, is translated into feet on the altimeter.

Mainspring of the device is a newly-developed radio tube capable of generating for practical use an ultra-high frequency wave of 500 megacycles that is not affected by static conditions, has a cork-centre bounce, has a frequency that can be measured at distances as low as 50 feet off the ground. Sensitive at present to 5,000 feet, improvements in the tube and transmitter can extend the altimeter's effectiveness to 15,000 feet. As feeler for possible obstructions dead ahead, the present 5,000-foot range would be inadequate because it would give a pilot flying three miles a minute less than 20 seconds in which to climb out of danger.

> When an airplane climbs so steeply that its wings lose lifting power, it stalls, falls. Last week Langley Field engineers introduced a gadget that senses the loss of lift, blows a horn to warn the pilot.

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