File D 52.56 / 17
AIR SERVICE IN
(AVIATION)
PUBLISHED BY THE CHIEF OF AIR SERVICE, WASHINGTON, D. C.
Vol. IV February 15, 1922 No. 303
DISCUSSION OF AIRPLANE TIRES
AND WHEELS
(MATERIAL SECTION REPORT No. 1 SO)
Prepared by Engineering Division, Air Service
McCook Field, Dayton, Ohio ~~~
September 8, 1921 ~\"I>~
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WASHINGTON
GOVERNMENT PRINTING OFFICE
1922
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CER'l'IFICA'l'E: By direction of the Seeretary of War the matter contained herein is published as administra.tive
information and is required for the proper transaction of the public business.
(2)
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DISCUSSION OF AIRPLANE TIRES AND WHEELS: ·..._y
WHEEL DESIGNS.
In general, the clincher wheels are of two types: The
English or Palmer type and the standard clincher type.
The English or Palmer type has a special rim contour,
a special type of spoke lacing, and the wheel hub is offset
rather than centered. All of these features can be seen
in the accompanying diagram.
The tires and wheels used on the undercarriage of airplanes
present a somewhat different problem from automohile
tires and wheels. The wheel on an airplane is
simply used in taking off and in landing. In the air it
acts as a parasitic resistance and affects adversely the
performance of the airplane. The airplane wheel must
have a high strength-weight ratio, must be of sufficient
diameter to permit landing on rough ground, and large
enough to sustain the impact load·dne to average pancake
landings. This has led to the exclusive use of R wire wheel. I
The standard clincher type has a rim of the S. A. E.
contour, has the hub centered, is not cross laced as in the
above, and the spokes are tangential to the hub.
'l'he tire should also have a high strength-weight ratio,
together with a high resistance to side thrust to ·prevent
rolling off of the rim. The wear, which is an important
consideration in automobile tires, is not so important in
airplane tires, as the actual mileage is very low. There- ·
fore the tread is thin, but the carcass must be strong and
light weight. The function of an airplane tire is also
that of a shock absorber, and the tire defiects considerably
under load, which causes severe rim cutting if the tire
is not properly designed.
When the American program of aircraft construction
was started in 1917, there was only meager information
and knowledge available as to the requirements and
performance of the several wheels and tires to be used
on airplanes of different weights. The chief source of
information was the reports of actual service performance
in Europe during the aeronautical development resulting
from the World War. In view of the desire and need for
interchangeability of parts with English and French
planes, the current European practice was literally
copied without any testing in this country. While this
procedure was warranted in the face of the needs of the
time; nevertheless it forced an " undesirable " on the
aeronautic industry of the United States in the shape of
clincher tires and wheels.
Due to low scale of production of airplanes and, there.
fore, lack of funds for experime-q.tal purposes, the clinchers
have persisted, although changes are under way at present
to adopt straight side tires and wheels throughout, or at
least on all except the smallest sizes. This movement
was in large part due to the Tire and Rim Association,
with the active cooperation of the Engineering Division
of the Air Service.
At the present time the clincher wheels and tires
most commonly used are : 26 by 3 inches ; 26 by 4 inches;
650 by 75 millimeters; 700 by 100 millimeters; 750 by 12fi
millimeters; 800 by 150 millimeters; 900 by 200 milli_
meters.
It will readily be seen that the European wheels, as
indicated by the metric dimensions, were standardized
with respect to the wheel diameter. This may or may
not be of advantage in the field, but it certainly has the
advantage of standardization, which is of considerable
assistance to both the wheel and tire manufacturer. The
American sizes, indicated by the English units, have not
been standardized in any way, clue probably to the lack
of activity or development along aeronautic lines np to
the time of America's entry into tl1e war.
,P/f'tJ/'tJJE~ ST!PA/lfHT S/0.F /i'/#
ENGLISH OR PAJ./'?EH /i'JH
FIG. 1.-Sections or wheel rims and tire beads.
RIMS.
A discussion of the merits of the two types of wheel
design would be concerned first with a consideration of
rim contours. The Palmer type supports the tire bead
only at the toe and the upper part of the heel. There
are numerous claims made regarding the superiority of
the performance of such a rim design when there is a Ride
load on the tire and wheel. The fact that the heel of the
exe bead is not supported, is supposed to permit of some
flexibility to the bead when the tire is under a side load,
and thereby lessen the chance of blow-out due to rim
cutting. Nothing has ever been shown to indicate that
such an effect really occurs, and it is the opinion of the
writer that there is not a great deal of reliability in such
claims.
87800- 22 (3)
SPOKES /'/OT CHOSS·Lf1C£D
FOR .SIDE T!ili'U.ST
;;
4
FIG . 2.-Standard clincher wheel.
$POKES Cl?tJJ.f- l.H C £"a
rDli' SIDE rHHVS'T
FIG. 3.-Eng!ish or Palmer wheel.
'Ti&iNt;ENTll'IL.. SPoJ<E
/flTT,#ICHMEHr .4T lftlB
1'/IID!l9l SPOI(£
/tfTT~CH/1'1£/VT l'IT tiUB
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The standard clinch er rim permits of a firm seat of the
bead from the bead heel to the toe at all points. This
head seat seems to be a much more reasonable arrangement
of a tire bead and certainly ·admits of a maxi.mum
in bead support . In the case of sudden side loading, it
would be more satisfactory if the bead could be so seated
on the rim as to i11Sure more or less freedom, but it i~ a
question if such a condition has been attained in the
Palmer type of rim.
WHEEL LACING.
The Palmer type of cross lacing of the wheel spokes is a
feature which should be incorporated in airplane wheels.
It permits of strong bracing for side thrust, which, in
the case of airplane wheels, is very necessary. In the
American sta ndard clincher wheel there is no cross lacing
of the spokes, and the desirable feature of bracing for
side thrust is neglected.
SPOKES C/10.SS L/11C£D
5
J:'Pli' S IDE TH If 11.f__T ___
in the case of automobile wheels. If an airplane wheel
were driven- that is, if movement to the plane were
transmitted by gearing through the wheels, there might
be some just ification for the use of the tangential method,
but such is not the case, and, in addition, this type of
attachment prohibits any possibility of cross-lacing for
side thrust, therefore, all things considered, it is believed
that for use on airplanes, the radial type of spoke attachment
is superior to the tangential method.
The Palmer type wheels all have a considerable offset
to the hub, the idea being, it is presumed, to increase the
bracing effect for side thrust. No doubt the construction
is theoretically correct, but a stress analysis of several
wheels of this type showed a considerable eccentricity
which would not add to the general efficiency of the wheel
in service. In the case of a wheel with the hub centered,
there would be no eccentricity, but the effect of bracing
for side thrust would be slightly less than in a wheel of the
Palmer type.
RADIAL SPOKE"
ATT,qCH/'1£"NT /117 .HVS
FIG. 4.-New straight side wheel:
HUB ATTACHMENT.
There are two general methods of attaching the spokes
to the hubs, radially and tangentially.
The radial attachment is self-explanatory in that the
spoke is attached directly to the hub flange and extends to
the rim in a straight line. vVhen a load is applied to such a
spoke, it functions as a column with both ends fixed, and
the load normal to the column axis, or as a straight tension
member depending upon its position in the wheel.
In the tangential attachment the spoke extends from
the wheel rim to a point tangent to the hub. The tangential
type of lacing makes the wheel heavier than the
radial type, because this method necessitates a 90° bend
in the spoke at the point of attachment to the hub. This
type of lacing is similar to the method of hub attachment.
87800-22-- 2
TIRES.
Airplane tires differ from automobile tires in weight and
tread construction. The carcass is composed of plies of
frictioned cord fabric laid in the usual way. There is no
cushion or breaker strip used, and the tread is of highgrade
tread stock of t inch thickness. The side walls and
the bead are constructed in essentially the same manner
as automobile tires. ·
PHYSICAL TESTS OF WHEELS.
Accurate information regarding the performance of the
wheels used, both in the field and in the laboratory, was
necessary and essential for the general development of
new 'Yheels and tires, and more important still for the
information of the designer. It was with these facts in
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view that extensive laboratory tests were run on all the available
clincher wheels and tires, as well as all available sizes
of the new straight side type of the proposed new schedule.
The physical tests in the lab9ratory were confined largely
to static loading to determine the wheel deformation under
load and the ultimate load of the wheel.
The testing machine and the nature of the test rigging
were about as follows:
An Olsen 50,000-pound capacity test machine was used .
A 4-inch dirt surface in a suitable container was placed on
the stationary head of the test machine and the wheel to
be tested was placed upon it in a vertical position. The
tire, deflated , was allowed to remain mounted on the wheel
during the test. An axle was motmted in the huh and
the load was applied to the wheel from the pulling head
of the test machine through stra.ps to the axle in the wheel.
This test rigging is shown in figme 5.
There are several very good reasons wlJy the wheel was
mounted on a tightly packed dirt surface. The earlier
methods of wheel testing required that a wooden block be
fitted to the wheel rim at the load point. In this case, the
wheel failure would evidence itself by the rim bending
at .the ends of the fitted wooden block. The breaking load
of the wheel varied with the length of the block fitted to
the rim. In the case where the wheel, without the-tire,
was simply mounted on the head of the test machine,
the first failure was in a distortion of the rim contour
and finally a rim failure at the load point. Neither
of these fail mes was typical of a wheel failure in service,
and it was with this point in mind that the final
test rig was decided upon. With the tire on the wheel,
but deflated, and the whole supported by a tightly
packed dirt surface, a failure was obtained which resembled
in every way .the failure which was observed under
service conditions.
In obtaining information regarding the wheel defonnation
under load, radial measurements were made from the
hub to the rim at various stages of the static test for breaking
load . In general, the distortion of the wheel tends
from a circle to an ellipse, with a slight lowering of the hub
below the intersection of the main axes of the ellipse.
This tendency is slight but will probably explain why all
of the spokes, except those immediately in the plane of
the load point, are under a tension load.
The main determinations made in this series of tests are
indicated in the following table: FIG. 5.-Sta Lic test rig foe a irplane wheels and tires.
Physical features of airplane 1cheels.
Gap;c ofri1n ... . ...... _ ....... _ ... ... . .. .. . ..... .. .... . .... ___ ... inches._
Gage of spokes .... . ..... . .. . . . ..• .. . . .... .. ........... . ......... do .... ~;~f i?f i-f :;i;l~):·:-i: ::-iiiif ff j iii j i:·:·:·:·:·:·i:.i:·i:.iiiii:·:-/tult~ ~
~~~ ~!~~t~~uii?ciei.·.:::::::::::: :: : : : : : : : : :: : : :: :: :: : : : : : : :i~~~~~::
·Method ollacing . ............. ........ . . . . .. •... . ..... .. ... _____ ...... .
Type of wheel. .. . ............. . .. . • . . ... _ .. _ ............... ___ ....... .
DESIGNERS' LIMITATIONS.
In the design of airplanes to Government specifications,
a certain factor of safety is prescribed. This factor varies
with the part of the plane and also with the type of plane;
26 by 4 inch 28 by 4inch. m7i5l0l ibmye 1te2r5. m8010l lbimy e1t5e0r. 136 bv, si·nch. 44i nbcyh 1. 0
0.080
0.112
40
8
7.500
9. 5
6. C
1. 5
Tangential
Standard
clincher.
o.095
0.105
52
10
i.500
ll
5.G
1. 5
Ra.dial.
Straight
side.
0.092
0.105
64
12
10.000
12
7.2-5
2.38
Radial.
Palmer.
0. 125
0.105
6-1
15
13. 000
12
7. 25
2. 38
Radial.
Palmer.
0.125
0.135
80
25
18. 000
33
6. 75
2.56
Radial.
Straight
side.
0.175
0.150
80
66
33. 000
74
9. 63
3.19
Radial.
Straight
side.
thus, the factor for the wheels on a training plane is 7 .0,
while the factor on the wheels of a two-seater pursttit
plane is 4.6. To a&•ist the designer in this phase of bis
problem, the following table was compiled:
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WHEEL LOADING. Maximum
allowable
plane weight
per wheel
Size. (pounds). 44 by 10 inches.......... . ...................... 6,500 ~;ob{: 2~~c~~~li~~t~~J . . . . . . . . . . . . . . . . . . . . . . . . . 4, ooo
SOO by 150 millimeters . . . . . . . . . . . . . . . . . . . . . . . . . . 2, 500
750 byl25millimeters..... .. .. . . ................ 1, 800
-
2
io
8
obby
4
l?O nuh·llimeters}- __ . _. _____ ... _ .. _ . . __ . _. l , 200
y inc es ... .. . .
26 by 4 inches.. ... ...... . . . . . . . . . . . . . . . . . . . . . . . 1, 000
If the above maximum allowable weights are closely
followed by the designer, satisfadory performance is
assured, as these limits are indicated not alone in the
laboratory, but in the field as well.
EQUIPMENT IN SERVICE.
It is interesting to note how closely the above table for
maximum wheel loading checks with the load per wheel
on a number of service airplanes.
The following table was compiled in determining if the
various sizes of wheels and tires were overloaded:
grees of tire deflection, and the contour of the tire at these
points is plotted. From a study of these contours, it is
possible to check the tire and rim construction to obtain
the maximum in tire performance. ·
Attached are load-deflection cmves for all of the present
sizes of tires, at pressures of 50, 65, and 75 pounds, and
from these one can safely interpolate for any intermediate
pressure.
One point to bear in mind when using load-deflection
curves is that the tire deflection, when the plane is resting
normally on the ground, should be about 20 per cent.
This figure was arbitrarily chosen as a result of recommendations
made by the various tire manufacturers with reference
to tire life for different degrees of normal tire deflection.
In designing undercarriages for airplanes, it is necessary
to use the load-deflection curves in conjunction with the
table showing the maximum allowable plane weight per
wheel, if the greatest efficiency is to be obtained from the
combination of wheel and tire.
CLINCHER VS. STRAIGHT SIDE.
A discussion of the relative merits of the above types has
been exhaustively treated in connection with the manu-
Name. Type. Weight. Wheels.
Weight
per
wheel.
Tire s ize.
Recommended
weight.
------------------1--------- - ------ - - - ---------1--- -
Standard E-1.. •.. .... .... .... . . ..... .. ........ .... I ·single Seater ................... . ..... .
Pounds. Pounds. Inches. Pounds.
1,190 2 595 26 by 3 600
Thomas Morse S4C ........ , ... . ....... . .. . ..•..... l. .... do ............ . . . .. . . . . ... . .... . . . 1, 374 2 *687 26 by 3 600
~~~gJ;J~.f~~~~~e_._-_·_: :: : : : : : : : : : : : : : : : : : : : : : : : : J ~'~~d~~~~~r_._-_:::::::::::::::::::::::::
2,600 2 1,300 26 by4 1, 000
2,050 2 *l,g~ 26 by 4 1, 000 t~;;~::~:~:~;B//\\//'.'.// J;~t:~~tt'.//'.\'.///\
~ifL IF!EI + !
1,960
2,060
2,600
2 350
4;000
4, 800
2, 645
3,660
2,140
2
2 * 1,030
2 * 1,300
2 1, 175
2 *2,000
2 * 2,400
2 1,325
2 *l,~O
2 1, 070
26 by 4 1, 000
26 by4 1,000
]i{illimeters.
700 by 100 1, 200
700 by 100 1, 200
750 by 125 1, 800
750 by 125 1, 800
750 by 125 1,800
750 by 125 1; 800
750 by 125 1,800
Le Pere Triplane ..... . ........................... ·1 Multi-seater . ............ .. . ..... . .... .
XB-1-A ..... . ............. . .... . •. . . . .. .. ..... .... Two Seater. .. . . ... .. • .... . •.. ...• .. . .
Armored Triplane .. .. ... . ... . ..................... , Multi-seater. ..........................
1
7,130 4 1,780 750 by 125 1, 800
2, 994 2 1, 497 750 by 125 1,800
9,740 4 2, 435 900 by 200 4, 000
::::::::::'.:::: :: :: ::: ::: ::: : : : : : :: ::: : J ::: ::::::::::::::: ::: :: :::: : ::: : : : : 14, 450 4 3,610 900 by 200 4,000
10,000 4 2,500 OOJ,,~r.~oo 4, 000
10, 000 2 1 5, 000 44 by 10 6, 500
I .
NoTE.-The (*) indicates that the wheel is overloaded with respect to the recommended maximum wheel load. The most serious of these are
the DH- 4, the D-9-A, and the Le Pere, all of which are at present using a larger size tire than indicated in the above.
PHYSICAL TESTING OF TIRES.
'fhe laboratory test of tires is concerned chiefly with
determining the most satisfactory operating load and
pressures. The tests involved in determining the quality
of the material used in a tire are sufficiently well known
to require little explanation. The quality of the tread
stock , t ire cords, and cements, the construction of the
heads, including wire reinforcements, are all indicated
to insure a tire of high quality, so that the chief problem
to the designer is one of satisfactory and efficient tire
operation.
In testing for " load-deflection" data, the test rigging is
essentially the same as in determining the breaking load of
the wheel. The wheel and inflated tire are mounted
directly on the stationary head of the test machine.
Stations are marked on the tire at the load point and along
the side wall of the tire to the rim in the plane of the load.
The location of the stations is determined at various de-facture
of automobile tires. The conditions are essentially
the same for the airplane ti.res, but they are much more
aggravated than in the case of automobile tires.
The chief cause of failure in ser vice of clincher tires has
been blowouts due to rim cutting. In every case of blowout,
due to reasons other than an airplane crash, more than
90 per cent of the failures are typical blow-outs. Another
feature well worth mentioning is that in the case of a blowout
of a tire motmted on a Palmer wheel with the offset
hub, the blow-out occurred on the side of the wheel on
which the hub is offset. Of course, the chief cause of rim
cutting is under-inflation, and as practically all airplane
tires must be slightly under-inflated, this condition will
always obtain as long as clincher tires continue to be used.
The peculiar feature about Palmer tires, mentioned above,
is believed to be partly due to the eccentricity in the
wheel, shown by the stress analysis of a wheel of this type,
al though the chief cause h ere is also u nderinflation /
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- -- -- -
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LoAD-0.EFLEcrtor;
rO/f
IJJRPL/1!'1£ ll.RES
ST11/'IOAR1JSizes
lltR PJrESSVRE - so#
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- - " ... .... I.II 1.
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~ ,.. .. .. ,. .. ,,. - r .... - - - - .... .,. - ... - ~ ·L• - I; ,... . ~ ,.. .. - - -- - ,. ... _· - ........ - - - - , .... ........ ·- ,-. ... .... - -
JO 10 30 ~ so 6(1 -,o 80 90
TIHE DEFLECT/ON- PER Cr#r '"o
FIG. 6.
9
0'90-0£FLcCTI OIY
rOR
/11 RPL/1/YE "liKcS
Si~IVDl9RD S1z£.s
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.... ,. .. .,. , ,.... ....
.,
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c;, - - .
IJ t...
~~ - ... - I; - ... - ~ -
/0 20 J() 4() .50 60 70 80 9o /CO
TIRE DEFL EC TIOIY-/ltrCcnr
FrG . 7. /
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...:I
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Lo~o DEFLECT/Off
FOR
ri1RPLIU1E li"HES
S TAHDAl?D £zE.s
r/11? PR£.S.S()/?£
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l/ , ....
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~ , ., ., V ....
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,/ ~ "' / ...
l,1 " ., ... ., ...
.,; .,
Lr - ,i,, t..,, ,,.. - - - -- - ~ - - t..,, , - - - -- L, ... - -
to 1"'J JOE 4Q SO 60 70 80 90
,,H DEFLECT/ON- PEH CE/'IT
FIG . 8.
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Straight-side tires will remedy this trouble due to rim
cutting, and the only other objectionable phase would be
that of rolling the tire off the rim, in the case of a sudden
or heavy side load. This condition can be overcome by
the use of a good steel in the rim, and by the proper design
of the straight-side bead with steel wire reinforcing.
Another consideration in favor of the adoption of straightside
tires is that of ease of handli~g. A clincher ti.re is not
easy to mount even on the smaller. sizes, and in the larger
sizes it is extremely difficult. It has been noted that usually
four men are necessary to properly mount a 900 by 200
millimeter ti.re on a rim. In the case of the straight-side
tire, one man very often can completely mount even the
largest size ti.re of the straight-side type. No case requires
more than two men.
PROPOSED STRAIGHT-SIDE SCHEDULE .
In view of all difficulty experienced in the maintenance
of the clincher tires and wheels in the fi eld, a program of
straight-side wheels and tires was laid out, which is believed
to incorporate the good featmes of both the clincher
and straight-side types, @cl eliminates the bad features
of both. In laying out the program, t he following were
borne in mind:
RB :IS.
A one-piece rin{ with a drop center, as shown in figure
1, was chosen for lightness, simplicity, and ease of
manufacture.
LACING OF SPOKES.
A type of lacing should be used which would permit of
cross bracing for side thrust as far as possible.
HUBS.
The hub should be centered to overcome ·any eccentricity
due to offsetting the hub in bracing for side thrust.
The sizes included in the pro.,.ram are the 3 4 5 6 7
8, 10, and 12 inch sizes, and th
0
e wheel dian1~te;. i~ k~pt
constant at 20 inches. The resulting tire sIZes are: 26 hy
3 inches, 28 by 4 inches, 30 by 5 inches, 32 by 6 inches,
34 by 7 inches, 36 by 8 inches, 40 by 10 inches, and 44
by 12 inches. In view of the difficulties consequent to a
complete schedule change, it was decid ed to attempt to
change only those sizes with which the greatest trouble
was experienced. With t his end in view, the 28 by 4
inch, the 32 by 6 inch , and the 36 by 8 inch sizes have been
selected for preliminary tests. The 28 by ,1 inch and the
36 by 8 inch have successfully passed laboratory and·
service tests, and from the data obtained from these tests
it has been possible to make any necessary changes in the
wheel design of the remaining wheels in the proposed
schedule.
One new whee\ worthy of special mention, is a 44 by 10-
inch wheel and tire used at present on the Glenn Martin
bomber, of the type which has only two wheels in the
undercarriage. The wheel is laced in essentially the
same way as the proposed type, but the rim embodies the
truck wheel feature of the removable side ring for ease
of mounting and removing the ti.re. In fact, the rim is a
truck rim which has been machined down to a thickness
of 0.175 inch. Its other featuree are indicated in the foregoing
table of " Physical features of airplane wheels." '
0
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File D 52.56 / 17 / Addendum McCOOK FIELD REPORT, SERIAL No. 1699
AIR SERVICE INFORMATION CIRCULAR
Vol. IV
(AVIATION)
PUBLISHED BY THE CHIEF OF AIR SERVICE, WASHINGTON, D. C.
September 15, 1922
(MATERIAL SECTION REPORT)
Prepared by Engineering Division, Air Service
McCook Field, Dayton, Ohio
July 27, 1922
WASHINGTON
GOVERNMENT PRINTING OFFICE
1922
No. 303 (Addendum)
{ .... ' l~ ~·:,·; ;
CERTIFICATE: h ~'.frfr~ction of the Secretary of War, the matter contained herein is published as administrative
information and is required for the proper transaction of the public business.
(2)
ADDENDUM TO INFORMATION CIRCULAR, VOL. IV, NO.
303---DISCUSSION OF AIRPLANE TIRES AND WHEELS.
PURPOSE.
To rletennine tbe load p er square in ch o[ tire contact on
the supporting surface and its relation to the supporting
lo:1d and t ire inflation.
CONCLUSIONS.
The load which a tire will carry depends upon the area
of contact and the inflation pressure, and is eq ual to the
product of the a rea in square inch es by the inflation
pressure in pounds per square inch. The area of con tact
is the same for eq ual tire de fl ections regard less of the
inflation pressure, b ut the load carried by the tire will be
greater for the same area of contact as the pressure is
increased .
The de fl ection of the tire is principally a flattening of
the tread, and bears a simple mathematical relation to the
area ·of contact. The area of conta.ct is n.n ellipse equal
to 1- rAB. . A B . I . 4--, 1n which n.nd n.rc the rn n.1or n.nc mrnor axes
of the ellipse. The length o[ these axes can be approximn.
ted closely from the form"t1 la for the chord subtended
by the arc of the segment of a circle. The length of the
chord is · equal to twice the sq uare root of the deflection
multiplied by the difference between the diameter and
the deflection, or
A=2.Jh (d-h)
d for the major axis is equal to the outside diameter of the
wall and tire. d for the minor axis is equal to the diameter
PROCEDURE.
The wheels wilh i nflat ed tires moun ted on lh cm were
mounted on the sta tionary head o[ an Olsen tesling mach
ine. Soapslone was scattered on pa.per between the
support ing surface and the tire to record the diagram of
tire contact \vith supporting smface. The contact diagram
of each t ire was obtained for deflections of 25 and 50 p er
cent at inflation pressure of 50 and 65 pounds. The area
of the tire contact diagrams was obtained by means of a
planimeter , and the pressure p er square inch on the
diagram was obtained from the load supported by the
tire and the area of the diagram. ·
RESULTS OF TESTS.
Table 1 gi ves the resul ts of tests and calculations obtain
ed in this series of ex periments.
In the case o[ airplane tires s ubj ected to load there is
always an increase in t be inflation pressure as a result of
tire de fl ection. \Vhi le this i_n crease is n ever very large,
it is a rn e:::.surable quantity, and the fo llowing table shows
the general tendencies of pressure change in_ the several
tires tested :
Size. Initial
pressure.
Presstue at 50 per
cent deflection.
28 X 4 .. ..........................•.... . ........ . ..
of the t ire. 32 x 6 .... ., ••. .... • • . . .. . •• ... . . . • . . . . ..• • . .. . . .•.
fi5
uo
50
60
65
70
65
55
65
70
The loads for the 54 by 12 ti re calculated on this basis 3~~L:: ::::: :::::::·· ···· ······ ··· ·::: .. ...... .
for a 50 per cent de fl ection o[ the tire would be 10,000, 4,1 x 10. · ·· · • · ·· · · · · · · · ·· · · · · · · · · · · · · · · · · · ·· · · · · · ·
12,000, and 14,000 pounds for 50, 60 , and 70 pounds p er I -------- --------'--------squa
re in ch iufl at.ion p ressures, resped ively .
APPARATUS.
The standard 11-h eel and t.ii·e test rig 11·as used in all cases,
with the fo llowing change : A sheet of paper was p laced
between tbe tire a nd the supporting sur[ace. Powdered
soapstone was scatte red 0 11 the paper to pe rmit o[ obt.ain iug
the con tact diagram of the t.ire when the load was
applied. These contact diagrams were obtained for inflation
pressu res of 50 and 65 pounds and fo r t ire de fl ections
of 25 and 50 per cent.
MATERIALS.
The wheels and tires used were the new straight-side
type of the following sizes: 28 by 4, 32 by 6, 36 by 8, and
44 by lO.
Ju Table J t.he column next to la8t, is headed " Load
recommended in designers' handbook. ,, This Yalue is
obtained from 1 he load-de fl ecti.on cun·es for each size tire
at the clefl.ecti on and inflation pressures noted . The last
column is t,he load p er square in ch calculated from area
ol cont.act di agram and the recommended loads. It r an
be seen that. the calculated or ser vi C'e pressures a re almost
id entical to those obtained in the laboratory, and these
pressures are · not excessive and arc well within the working
lirni ts of the tire:
DISCUSSION OF RESULTS.
It will be seen from Table 1 that the load per square
-inch of contact on the supporting surface, as determined
by the contact diagram, is the same as the inflation pressure
of the tire. The load per inch of tread width is con-
5477- 22 (3)
siderably higher for high inflation pressures than for low
pressures and varies with the different sizes of tires. It
is not a satisfactory basis for calculating the load which
a tire will carry.
The last two columns in Table 1 are included for general ·
consideration. Previous to the laboratory determinations
as obtained above, it was felt that it was necessary to
indicate a " maximum allowable plane weight per wheel "
for the guidance of designers. These limits were chosen
4
from the load-deflection curves previously developed in
this report, after deciding in a more or less arbitrary way
that a normal tire deflection of approximately 20 per cent
would per.mit of satisfactory tire performance.
It will be seen that the load per square inch of contact
diagram area calculated from these maximum loading
limits for designers checks very closely with t h e .loading
per square inch of contact diagram area as determined in
the laboratory.
TABLE 1.
'fire.
Air Deflec- L d
pr~ssure tion °~ '. (pounds) . (per cent). (po1.1., ds ).
.Area
contact
diagram
i~~ih;)~
Short
a.xis of
diagram
(inches).
Load per
L1t~fer square
tread inch
width co:..l!ct
(pounds). (poll.llds).
Load
recommended
in designers'
handbook.
Load per
square
inch,
recommended.
---------------------------- - --- ---- - ------ - -
28 X 4 ... .. ..... ......••....... •....................
28 X 4 .... ... . ... . . . ........... ......... . ......... . .
28x 4 .. ·····- · ·· 28 · ·· ·· · · ···· · ·· · · · · ·· ····· · · · · ····· · X 4 •................ . • .................•.........
32 X 6 ....... ...... . .•..... . .............•...•...•.•
32x6 ...... . ..... .. - .. . ................ . ...... . ... .
32 X 6 . .... - •.• • .• .•. •• •. · · ··-· ······· · ·· · ··· • ••••• •
32x6 .................. . ... . ............. . ........ .
36x8 ...................... . ..................... ..
36x8 ......... . ... . ........ . ....... . .............. .
36 X 8 ..... . . . . . . . .... . ...... -······ · -······ · - ·· · · ··
36 x8 .......... .... ........ .. .... .. .. : .......• .. . • .
44x 10 .............................. . ...... . . .... ..
44 X 10 .................... . ................ ..... .. .
50
50
65
65
50
50
65
65
50
50
65
65
50
50
25
50
25
50
25
50
25
50
25
50
25
50
25
34
0
1,300 23. 4
2,400 45.1
1,560 23.1
2, 900 44. 0
2,200 43. 0
4,700 87. 0
2, 600 39. 3
6,000 86. 3
3,600 69. 4
8,000 136. 9
4,900 73. 5
10,000 140.6
5, 000 93.4
8, 320 150. 2
3.0 433 55. 5 1,200. 51. 5
4.6 520 53.5 2,400 53. 5
3. 4 462 67.5 1 600 69.0
4. 5 645 66.0 3;400 77. 0
4.4 503 50.0 2,000 46. 5
6.9 684 54. 0 4, 500 52. 0
4.3 612 66. 2 2,500 63. 0
6. 4 942 69. 5 5 500 64. 0
6.6 544 52.0 3:500 50.5
8.9 900 58.5 7, 200 52. 5
6.6 740 66. 6 4,000 55.0
9.0 1,120 71. 2 8, 750 62.0
7.1. 704 53. 5 6, 000 65.0
9.1 913 55.5 9, 000 60.0
L 3
Air Service Information Circulars
