SECTION
IV
APPLICATION OF DATA
aircraft performance and airfield indexes. Un-
13. General Application. a. Once the airfield
fortunately, these are not exact correlations
f o r a g i v e n l a n d i n g strip is established,
uniquely relating aircraft performance to air-
its relation to aircraft load, tire-inflation pres-
field index. As soils vary in type and condition
sure, and anticipated number of coverages (a
from site to site, the relation of airfield index to
measure of load repetitions) must be deter-
aircraft performance also varies. For this rea-
mined. Relations of airfield index to these fac-
tors have been established from data gathered
over many years.
formance. These relations have been selected so
that in nearly all cases aircraft performance
wheel or equivalent single-wheel load, tire pres-
will be at least that indicated. However, it
sure, airfield index, and coverages. If the load
should be recognized that on occasions perform-
and tire pressure of the aircraft and the airfield
ance may not be quite as good as that indicated.
index are known, it is possible to determine the
number of coverages that can safely be made
14. Illustrative Problem. To assist the user
by the aircraft by applying information to the
of this technical bulletin, an illustrative prob
nomograph (fig. 2).
lem and its solution are presented as follows:
a. Problem. Can 20 C124 aircraft make
mining the equivalent single-wheel load of
three deliveries each to a site having an airfield
a i r c r a f t h a v i n g m u l t i p l e - w h e e l assemblies
index of 12?
(ESWL).
b. Data, The gross weight of the aircraft is
d. The required airfield index for 1 to 1,000
175,000 lb; the tire pressure is 65 p.s.i. The
passes of some of the more common types of
twin-wheel assembly has 44-inch center-to-
cargo aircraft is listed in table 1. In using the
center (c-c ) spacing.
nomograph (fig. 2) directly, allowable coverages
c. solution.
can be determined. However, this determina-
20 aircraft x 3 deliveries x 2 passes/delivery =
tion must be converted to aircraft passes for
120 passes
ultimate use. The relation of passes to cover-
120 passes = 34 coverages
ages depends on the number, width, and spac-
3-1/2 passes
ing of wheels on an aircraft as well as the
(coverage* )
degree to which the aircraft can be expected
To determine equivalent single-wheel load (ESWL),
the procedure described in figure 3 in used:
to wander left and right of a central position.
Tire-inflation pressure = 65 p.s.i.
In nearly all cases, it is adequate to estimate
Single-wheel load ( SWL) = 39,400 lb (from table 1 )
the pass-per-coverage ratio from the following
Tire contact area (A) = 39,400 = 606 sq in.
tabulation which shows the number of passes
65
per coverage of four aircraft gear configura-
Next, the radius of a circle having an area of 606
tions.
sq in. is determined:
Number of Passes--
1/2
1/2
Gear configuration
per-coverage
(A) (606)=13.9 in.
Single wheel .......................5
Equivalent radius (r) =
Twin wheel...........................3-1/2
=(3.14)
Single tandem ......................2
Knowing the equivalent radius of 13.9 in. and the
Twin tandem ........................2
c-c wheel spacing of 44 in., it is possible to get the
spacing in radii:
Caution: Users of the information in this
44 in.
technical bulletin must be awars of its limita-
c-c wheel spacing in radii = -- -- . = 3 . 1 6 r a d i i
13.9 in/radius
tions. The curves and tabular information in
* From para 13d.
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