Page images
PDF
EPUB

GUIDES TO SUPPORT BEAM AGAINST ROLLING AND LATERAL BUCKLING

DEFLECTION BOARD

SUPPORTING FIXTURE

HYDRAULIC CYLINDERS APPLY AND MEASURE LOAD

Figure 2-11. Example of use of single defection board.

WING TESTS

Rib tests. A. Selection of ribs.1. Wing of uniform chord.At least two ribs should be tested

for each of the two loading conditions. 2. Tapered wing.Considerable judgment should be used.

Where a series of ribs of varying sizes is involved, the largest

rib of each series should be selected for testing. 3. Where tapered wings incorporate either two or three similar

series of ribs, if two series are used, the tests should be made on two each of the largest rib in each series, or a total of four ribs. If there are three series, it would probably be sufficient to test two identical ribs of the largest size used and one each

of the largest, in each of the other series. 4. Usually, no more than a total of four ribs need be tested.

However, in all cases, it is essential that representative ribs

be tested. B. Test loadings.1. The rib tests should at least cover the positive high angle of

attack condition (Condition I) and a medium angle of attack condition. The total load to be carried by each rib should equal 125 percent of the ultimate air load over the area supported by the rib. For the medium angle of attack condition, the load factor should be taken as the average of the

ultimate load factor for Conditions I and III. 2. The leading edge portion of the rib may be very severely

loaded in Conditions II and IV. An investigation of the maximum down loads on this portion should be made when V, exceeds 125 m.p.h. If this is not applicable, it should be demonstrated that the rib structure ahead of the front spar is strong enough to withstand its portion of the test load acting in the reverse direction. A test for this condition usually is advisable in the case of a rib which appears to be

weak. 3. The following loadings are recommended for two-spar wing

construction when the rib forms a complete truss between
the leading and trailing edges.
a. For the high angle of attack flight condition, ribs of

chord length greater than 60 inches should be subjected
to 16 equal loads at the load points given in tables 2-IV
or 2–V. In order to determine which set of load points
is applicable to the particular airfoil used, it is first neces-
sary to determine the following airfoil characteristics:
(1) PD (Pressure Distribution) classification—this is

expressed by a capital letter followed by a two digit

number such as C 10, B 11, D 12, et cetera. For the present purpose, only the number portion of the classi

fication need be considered. (2) Cmac-moment coefficient about the aerodynamic

center. (3) Camber—in percent chord. (This is necessary only

in the case of airfoils having a 12 pressure distribution

classification.) These characteristics are readily obtainable for most airfoils from N.A.C.A. Technical Reports Nos. 610 and 628. For airfoils in the 10 or 11 classification, the load points should be taken from table 2-IV, using the line corresponding to the Cmac value of the airfoil. (Table 2-IV should also be used for ribloading points in cases where the P.D. classification is not available, or in cases where the designer does not wish to determine it.) For airfoils in the 12 classification, the load points should be taken from table 2-V, using the line corresponding to the Cmac, and the camber of the airfoil. In cases where the actual position of load number 1 is less than 12 inch from the leading edge, loads 1 and 2 may be combined into a single load (of twice the unit value) and applied at their centroid. For ribs having a chord of less than 60 inches, 8 equal loads may be used, their arrangement being such as to produce shears and moments of the same magnitude as would be produced by the application of 16 equal loads

at the locations specified above. b. For the medium angle of attack condition 16 equal loads

should be used on ribs of chord of 60 inches or greater, 8 equal loads for chords less than 60 inches. In either case the total load should be computed as specified. When 16 loads are used, they should be applied at 8.34, 15.22, 19.74, 23.36, 26.60, 29.86, 33.28, 36.90, 40.72, 44.76, 49.22, 54.08, 59.50, 65.80, 73.54, and 85.70 percent of the chord. When 8 loads are used they shall be so arranged as to give

comparable results. 4. When the lacing cord for attaching the fabric passes entirely

around the rib, all of the load should be applied on the bottom

chord. 5. When the covering is to be attached separately to the two

chords of the rib, the loading specified in paragraph c below should be modified so that approximately 75 percent of the ultimate load is on the top chord and 50 percent on the

bottom, the total load being 125 percent of the ultimate load. 6. The aforementioned load distribution also is applicable for

ribs attached directly to the spar of single spar wings.

[ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Any value.-

[blocks in formation]

1 - 10 or greater ----

5.5 8.2 11.4 14.818.6 22.7 27.3 32.2 37.5 42.9 49.6 57.5 72.0 20.0 |

1 Shown as C 10, B 11, etc., in data tables of N.A.C.A. reports 610 and 628. 2 Expressed as percent chord. 3 Airfoils with values of Cm are classified with those having a Cm

ing a Cmo.c.=0.

TABLE 2-V-Rib Load Points for High Angle of Attack

Load points in percent chord

PD Classification

Camber

[blocks in formation]

13.1

16.3

20.1

24.4

28.9

34.3

41.0

49.0

72.0

90.0

0.0 to 2.9.

-.02 to -. 0399.---

2.5

4.5

6.4

8.7

11.3

14. 1

17.5

21.3

25.5

30.5

36.2

43.2

51. 1

72.0

90.0

PD
Classification

(2)
Camber

10 and 111---------

12...

[merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

.8 2.5 4.5 6.5 8.7 11.0 13.5 16.4 19.7 23.6 28.0 33.5 39.7 47.7 22.0 30.0 |

2.6 5.0 7.2 3.6 12.0 14.7 17.9 21.5 25.6 30.1 35.5 41.8 40.7 72.0 20.0

-.02 to -. 0399.-------|

14

C. Test methods.-Standard procedure.-
• The ribs should be attached to short spar sections to simulate

conditions in the actual glider. The spar sections should be
supported in such a manner that they will not prevent free
deflection of the rib. It is satisfactory to mount the spars so
that their edges rest directly on the supporting structure but
they must not be restrained from rolling or twisting. See
fig. 2-III.
To simulate the lateral bracing effect given a rib in the actual
wing assembly, it is permissible to employ vertical guide blocks
along the sides of ribs which are tested singly. These guide
blocks should leave the ribs free to deflect in the direction in
which the load is being applied, should have faces bearing
against the rib which are not wider than 12-inch, and, for metal
covered wings, should be spaced at least 8 inches apart. For
fabric covered wings these lateral supports should not be closer
than twice the stitch spacing, or the length of the individual
rib chord members, or 8 inches, whichever is the greater. In
any case, the lateral supports should simulate, as slosely as

practicable, the actual conditions represented in the glider. • In order to avoid local failures of a type not likely to be en

countered in flight, it is permissible to use small blocks not more than 1-inch long to distribute the load at the loading points.

WING PROOF AND STRENGTH TESTS A. Test loads.—The loads to be used and their distribution over

the wing will depend upon the particular condition for which the test is conducted. In general, when tests are made to prove the strength of the entire wing, four tests should be conducted corresponding to the four basic flying conditions of positive high angle of attack, negative high angle of attack, positive low angle of attack, and negative low angle of attack. In some cases, involving cantilever wings, the wings are sufficiently symmetrical structure wise that only those tests involving the

maximum loadings in the positive direction need be conducted. B. Test methods.1. General.The procedure outlined elsewhere for the prepara

tion of the manufacturer's test report is also applicable to

reports of wing tests. 2. Mounting. For the tests, the wing will usually be mounted

on a jig. The method of attachment should simulate the method of attachment in the actual glider. In particular, the method of attachment should be such that the strength and rigidity of the jig is not imparted to the wing thereby leading to erroneous conclusions regarding the strength of the wing adjacent to the attachment fittings. See figs. 2-IV through 2–XI.

high ang/ding to the foewing, four te

« ՆախորդըՇարունակել »