EARLY INSTRUMENT FLYn^G DOOLITTLE 351 



best, to the nearest 50 or 100 feet. It would be of great value to have 

 an altimeter that, near the ground, would measure to 10 or even 5 feet. 

 The Kollsman Instrument Co. developed such an instrument, and I 

 was very pleased, on August 30, 1929, in the second 02U — a more 

 modem version — to take Paul Kollsman and his new instrument up 

 on its first test flight. Mr. Kollsman held the sensitive altimeter in 

 his lap during the flight and it performed perfectly. Here was another 

 important addition to flight instrimientation. We promptly obtained 

 one and installed it in the N Y-2. 



This instrument had two hands and a multiplication factor of 20 

 between them. Actually the fast-moving hand made one complete 

 revolution for each 1,000 feet change in altitude, which meant a move- 

 ment of about %2 inch for a change of 20 feet in altitude. This was 

 more than one order of magnitude more accurate than earlier altim- 

 eters. Although the Kollsman altimeter provided a very consider- 

 able advance in instrumentation, it still measured the barometric 

 altitude or height above sea level. An instrument which would meas- 

 ure the height above the gromid regardless of changes in barometric 

 pressure would be of great value. 



With this idea in mind, several companies were encouraged to work 

 on a sonic altimeter. One built by Sperry was tested out in the 

 NY-2. A note on a frequency of about 950 cycles was directed at 

 the ground from a megaphone on the bottom of the airplane and 

 picked up by a detector on the airplane after having been reflected 

 back from the gromid. The elapsed time interval was measured, as 

 in a marine "fathometer," and the altitude above the ground thus 

 determined. The concept was theoretically sound, but in its initial 

 form the equipment caused considerable drag and was unduly large, 

 heavy, difficult to install, and complicated. It also appeared that a 

 radio altimeter measuring the phase difference of radio waves re- 

 flected back from the ground offered more promise. Several radio 

 altimeters were under development — with fund encouragement — and 

 the sonic altimeter experiments were therefore abandoned. 



FOG-DISPERSAL EXPERIMENTS 



Although major emphasis was given in the Full Flight Laboratory 

 to flight tests designed to permit flying safely and reliably despite fog, 

 the fund and the laboratory were also interested in experiments on the 

 penetration of fog by light rays and on the dispersal of fog. 



Studies on fog penetration were made by Dr. Anderson of the Uni- 

 versity of Washington and Dr. Barnes of Bryn Mawr College under 

 fund sponsorship, and by Dr. Julius A. Stratton of the Massachusetts 

 Institute of Tecluiology for Col. E. H. L. Green. The visual, infra- 

 red, and ultraviolet portions of the spectrum were carefully explored. 



