A PHOTOGRAPHIC DETERMINATION OF THE FORM 
OF AN AIRPLANE LOOP, AND THE DY- 
NAMICS OF THE LOOP 
L. P. SIEG 
Introduction. During a greater part of the years 1918 and 
1919, the writer was engaged at Langley Field, Virginia, on ex- 
perimental work connected with the determination of bomb tra- 
jectories. The work was under the direction of Dr. A. Wilmer 
Duff, as Engineer-in-charge, and the writer as assistant engineer. 
The credit for the development of the photographic method, 
described below, is due to Doctor Duff. The details connected 
with this paper, have been worked out by the writer as an inter- 
esting and important side issue of the main problem. 
Method and Apparatus. In order to gain information as to the 
actual path of an airplane during any manoeuvre, for example 
the loop, it is necessary to have some method of accurately sur- 
veying its positions in space as a function of the time. Whatever 
method could be used for accurately surveying the path of a 
falling bomb, could obviously be used for the present problem. 
If the flying is done at night, if there is a light on the airplane 
sufficiently bright to affect a photographic plate at a distance of, 
say two miles, and if, to record the path of this light, there are 
two cameras which are situated at the ends of a measured base 
line, are raised to known elevations, and are pointing in known 
directions ; then it is possible to reduce the two plates so as to give 
the X, Y, and Z co-ordinates of the path of the plane, referred 
to any arbitrary origin. If in addition one has a means of ob- 
scuring the camera lenses simultaneously at regular intervals, 
say every second, he has in addition the necessary time element 
to completely define the path of the plane as a function of the 
time. Such apparatus was installed at Langley Field for the 
regular program of trajectory determinations, and it served of 
course equally well for the present problem. There is neither 
space nor occasion for an elaborate description of this apparatus, 
nor of the labor involved in meeting the practical difficulties 
one by one until accurate results were possible. But the briefest 
description, therefore, will be given of the apparatus and method. 
