838 PHOTOGRAMMETRIC ENGINEERING 
SOME EXPERIMENTS 
DEFORMATIONS OF A MODEL AIRPLANE WING 
The deformations of an airplane wing 
on a model were to be determined under a 
number of different loadings of the wing. 
A Zeiss phototeodolite, f=192 mm. and 
image size 13X18 cm., was used for the 
photography. The average distance from 
the lens to the wing was around 150 cm. 
and consequently the average image scale 
was 1:8. A number of polished steel balls 
with a diameter of 4 mm. were fixed in the 
places where the deformations were to be 
determined. Also on the wall behind the 
model, a number of balls were placed to 
serve as a check of the orientation. See 
Figure 3. The average distance from the 
wing to the flash was around 1.8 m. 
To get the average sharpness in the im- 
age as good as possible, a suitable ancillary 
lens and a small aperture were used. 
The images of the reflected flash were 
excellent for the comparator measure- 
ments; these were performed with the 
greatest of care in order to avoid the influ- 
ence of systematic changes in the stereo- 
scopic measurements. From a great num- 
ber of repeated measurements the stand- 
  
t „ The wall 
steel ball 
The wing 
steel bal 
  
  
  
Camera 
Flaskligh é 
Fic. 3. The arrangement of the photography 
of the wing for deformation measurements. 
  
Fic. 4. The arrangement of the phototheodo- 
lite (left) and the stereocamera (right) on the 
bridge of the ship. 
ard error or one measurement was found 
to be of the magnitude 0.0015 mm. This 
high accuracy is doubtless a result of the 
excellent definition of the points, the ex- 
cellence of the stereoscopic vision of the 
operator, the high quality of the photo- 
graphic material, and the good condition 
of the comparator. 
After small corrections to the measured 
parallaxes had been made from the ob- 
served parallaxes in the fixed points, the 
vertical deformations of the object were 
computed with the aid of the scale factors. 
These were determined by comparing dis- 
tances in the object parallel to the corre- 
sponding distances in the picture, and 
from direct measurements from the lens 
to the balls. 
A direct check of the results of the pho- 
togrammetric determination of the defor- 
mations was obtained from the readings 
of measuring gauges in some of the points. 
The average difference in deformation 
was around 0.03 mm. 
The results were regarded as having 
considerable value for the purposes. The 
measurements and computations were 
performed by P. Hjelstrom.! 
The principal advantages of the photo- 
grammetric method in comparison with 
! A detailed investigation of the accuracy of 
the measurements has been reported in Hjelm- 
strom, P.: Noggrann deformationsmütning med 
fotogrammetrish metod. Manuscript. Also the 
results of another similar experiment have been 
reported in this interesting paper.