Air conditions close to the ground and the effect on airplane landings Page: 3 of 13
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AIR CONDITIONS CLOSE TO GROUND AND THE EFFECT ON AIRPLANE LANDINGS
once apparent from an examination of these results.
The average gradient with altitude for each wind con-
dition was determined by averaging the ratios of ve-
locities at various elevations to the velocities at 51
feet. The gradients thus obtained showed somewhat
different characteristics but no consistent variation
with average wind speed. The average gradient for
the entire series of measurements is represented by the
solid line in figure 10. It is interesting to observe that
this curve follows fairly closely a gradient represented
by the expression -V A2 as shown by the broken
line in figure 10. Other results, such as those reported
by Schmidt in reference 2 (see also references 3 and 4),
indicate that the gradient may conform to this type
of expression with exponents varying between 1/3
Of greater importance than the average gradient
with height are the fluctuations with time as regards
bota the magnitude of the velocity and the inclination
to the horizontal. The maximum fluctuations in
magnitude were generally of the order of + 4 miles per
hour from the average at any height, there being no
pronounced dependence on either the height or the
magnitude of the nominal wind velocity, except that
at the highest wind speed there were occasional
fluctuations as great as 6 miles per hour. The fluctua-
tions in inclination, on the other hand, show a pro-
nounced decrease in magnitude with increased wind
speed, as well as a fairly definite tendency for the
fluctuations to decrease in magnitude with decreasing
height. At the three upper stations the range of
variation in inclination was of the order of + 200 to
30' for nominal wind speeds of 8 miles per hour and
of the order of + 100 at 16 miles per hour. This fact
indicates that, as was previously noted regarding
resultant velocities, vertical components are independ-
ent of the average wind speed. Fluctuations in
horizontal components, being of approximately the
same magnitude as the fluctuations in resultant veloci-
ties, were of the order of 4 miles per hour. Vertical
components increased in magnitude with height.
The combination of relatively high wind speeds and
inclinations at a height of 51 feet indicates that at this
elevation fluctuations of vertical components of veloc-
ity were also of the order of 4 miles per hour.
It can readily be appreciated that the conditions
encountered by a landing airplane are not represented
by the conditions that prevail at any instant in a
vertical section of the 51-foot layer of air. In order to
obtain a general indication of the conditions that
might have been encountered by an airplane in a land-
ing approach for some of the cases shown in figures 3
to 9, the following method was used: It was assumed
that the airplane moved horizontally relative to the
air with a constant velocity of 70 feet per second and
regardless of vertical air currents encountered. Con-
ditions encountered at some desired time at the 51-
foot level were spotted on the curve for this level.
For the assumed conditions, the airplane reached the
36-foot level 134 seconds after passing the 51-foot level,
and moved upstream 105 feet relative to the air.
Therefore, the time interval t required for this air
distance was calculated by assuming the average speed
over this interval to be the same as the average at 51
feet, and a point was spotted on the curve for the 36-
foot level at t seconds later than the point at the 51-
foot level. A similar procedure was followed for each
step of the descent, using as average wind velocity
over each increment the average velocity at the start
of the increment. Points obtained by this procedure
for a sample case are shown by the broken line in
Curves showing the horizontal and vertical compo-
nents of the wind speed that would thus be encountered
are shown for several cases in figures 11 to 14.
Although the method is rather inexact, it serves the
intended purpose of giving consideration to fluctua-
tions of wind velocity with time as well as with altitude.
One objection to this method of interpretation is that
the wind front on which the measurements were
obtained has no width and disturbances actually may
not extend over a distance equal to the span of the
The effect on the motion of a gliding airplane that
encounters disturbances such as some of those shown in
figures 11 to 14 would appear to be large. In order to
determine what the effect might be for a hypothetical
case it was necessary to resort to a step-by-step inte-
gration with a solution by trial and error. Data were
used for an actual airplane (Doyle 0-2) for which lift,
drag, moment of inertia, and pitching-moment char-
acteristics were known (reference 5). The elevator was
assumed to be held stationary and the airplane was
assumed to be gliding steadily at the 51-foot level, as
though the conditions prevailing at that level extended
upward indefinitely. For gliding descent in still air,
the airplane was assumed to have a horizontal com-
ponent of velocity of 70 feet per second, a vertical
component of velocity of 10 feet per second, an attitude
angle of 6.30, and an angle of attack of 14.40. The
results of the calculations are shown in table I. It is
interesting to note the increase in vertical velocity to
more than 20 feet per second and a decided change in
attitude to a nose-down condition. Changes in angle
of attack were small.
Similar calculations were made to determine the
effect of the average gradient in a fairly strong wind.
The results of these calculations are also shown in
table I. Here again it was assumed that the gradient
was encountered at the 51-foot level, conditions above
that elevation being constant. The vertical velocity
had a constant rate of descent of 10 feet per second
in this case increased to 16.6 feet per second.
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Thompson, F. L.; Peck, W. C. & Beard, A. P. Air conditions close to the ground and the effect on airplane landings, report, April 3, 1934; (https://digital.library.unt.edu/ark:/67531/metadc66146/m1/3/: accessed April 24, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.