The Airplane - Jay Spenser [83]
The Kollsman altimeter thus resembled a clock numbered to 10, which made it easy and intuitive to read. If the big hand was on the 7 and the little hand on the 2, the airplane was flying at 2,700 feet. A third indicator needle could be added to display altitudes above 10,000 feet.
Unlike a barometer on a wall, an aviation altimeter should indicate identically regardless of the day’s weather conditions. Consequently, Kollsman made his altimeter adjustable to compensate for ambient air pressure changes. Using a setting knob, pilots could simply dial the current barometric pressure into an inset window to ensure that the device read accurately. It also worked the other way around; setting the altimeter to show the airfield’s known elevation while on the ground revealed the current barometric pressure in the setting window.
On cross-country flights, pilots of the future would receive by radio the destination airport’s current barometric setting and enter it in their altimeters before landing to ensure accurate readings. Of course, this capability was still many years off. While some airlines and military air services had voice communications technology, its use remained experimental at the end of the 1920s. Years would pass before it became broadly available as a vital adjunct to routine instrument flying, which likewise did not yet exist.
Kollsman was thus a visionary. Had he been forced to wait until the world caught up with his ideas, the company he founded would have gone out of business. Fortunately for him, word of his invention reached the ears of a scientific team at Mitchel Field, less than 15 miles (25 kilometers) away.
After meeting Kollsman and reviewing his invention, Jimmy Doolittle took the young German American aloft several times. The young man sat in the open front cockpit ahead of Doolittle, his sensitive altimeter mounted on a board on his lap. The device did all that he claimed, demonstrating uncanny accuracy landing after landing.
Existing altimeters were no good for blind landings, but this one was. Fortuitously, the Full Flight Laboratory team had stumbled across it all but gift-wrapped, an exquisite piece of technology cobbled together by a serious youth still in his twenties.
Doolittle now had cockpit displays allowing flight control more precise than ever before possible without external visual reference cues. In addition to Kollsman’s altimeter and Sperry’s two new devices, the airplane had an existing gyro instrument called a turn and bank indicator as well as airspeed and rate-of-climb indicators.
These flight instruments were half the blind-flying equation. Operating under the hood, Doolittle would also need to know where he was in the sky, and that required radio-navigational aids. To meet this challenge, Doolittle flew the NY-2 to Boonton, New Jersey, where technicians at the Radio Frequency Laboratory installed two additional collaboratively defined and developed electronic devices in the trainer’s ever more crowded rear cockpit.
The first was a homing indicator that would let Doolittle follow an electronic beam projected along the runway’s extended centerline (today called a localizer). The primitive device in the NY-2 had two vibrating reeds at each side of its display window. If one reed vibrated faster than the other, the airplane was off to that side; when both reeds vibrated at the same rate, it was in the middle, aligned with the runway.
The second electronic aid was a fan beacon with a vibrating-reed display. When Doolittle passed over this electronic marker, the vibrating would stop, letting him know to throttle back for a final descent at a constant shallow rate to the runway. Landing this way meant flying the ship literally into the ground, but the NY-2’s beefed-up landing gear was built to take it.
There was no third beam to tell Doolittle whether he was on the proper glide slope. That would come later.
Jimmy Doolittle, arguably the greatest pilot of all.
National Air and Space Museum,