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STRENGTH AND DEFORMATIONS The primary structure should be capable of supporting the ultimate loads determined by the loading conditions and ultimate factors of safety specified herein, if the loads are properly applied and distributed. The structure must support these loads for a minimum period of 3 seconds.
The primary structure should also be capable of supporting, without detrimental permanent deformations, the limit loads of the loading conditions specified, if the loads are properly distributed and applied. In addition, temporary deformations that occur before the limit load is reached should be such that repeated occurrence would not weaken or damage the primary structure.
DETERMINATION OF DEFORMATION • Detrimental permanent deformations are usually considered as
those that correspond to stresses in excess of the yield stress. The yield is the stress at which the permanent strain is equal to 0.002
inches per inch from standard test specimens. • In determining permanent deformations from static test results,
the effects of slippage or permanent deformation of the supporting jig should be considered. If any deflections under load would change significantly the distribution of external or internal loads, such distribution should be taken into account.
Stiffness.—The structure should be capable of supporting limit loads without suffering detrimental permanent deformations. At all loads up to limit loads, the deformation should not interfere with safe operation of the glider.
Wing drag truss.–Fabric covered wing structures, having a cantilever length of overhang such that the ratio of span overhang to the chord at the root of the overhang is greater than 1.75, should have a double system of internal drag trussing spaced as far apart as possible or other means of providing equivalent torsional stiffness. In the former case, the counter wires should be of the same size as the drag wires.
CABLE LIMITATIONS Multistrand cables should not be used in drag trusses since they stretch excessively.
Loads imparted by safety belts.—Structures to which safety belts are attached should be capable of withstanding the following ultimate acceleration that occupants are assumed to be subjected to during minor crash conditions: Upward ---
-- 4.5 g Forward.--
---- 9.0 g. Sideward.--If the belt is attached to the seat, the structural investigation should be carried through to the primary structure. Also, the above noted accelerations should be multiplied by a factor of 1.33 when applied to the seat attachment to the structure.
Pilot and passenger loads.—Pilot and passenger loads in the flight conditions should be computed for a standard passenger weight of 170 pounds. A minimum ultimate factor of safety of 1.5 should be used in conjunction with the applicable acceleration or maneuvering factor.
Local loads.—The primary structure should be designed to withstand local loads caused by dead weights and control loads. Baggage and ballast compartments should be designed to withstand loads corresponding to the maximum authorized capacity. Concentrated (dead) weights include items such as batteries, radios, seats, et cetera.
Loading equilibrium.-Unless provided for otherwise, the air and ground loads should be placed in equilibrium with inertia forces, considering all items of mass in the glider. All such loads may be distributed in a manner conservatively approximating or closely representing actual conditions.
FLIGHT LOADS The airworthiness of a glider with respect to its strength under flight loads usually is based on the airspeeds and accelerations (from maneuvering or gusts) that can safely be developed in combination. For certain types of gliders, the acceleration factors (specified in terms of load factors) and gust velocities are arbitrarily specified and should be used for these classes. The airspeeds which -can safely be developed in combination with the specified load factors and gusts should be determined in accordance with the procedure specified, and they should serve as a basis for restricting the operation of the glider in flight.
Design airspeed.—The design airspeeds should be selected so that resulting operation limitations will be consistent with the type and intended use of the glider. Minimum values for VQ, V raw and V, are specified in table 1-1.
DETERMINATION OF AIRSPEED VALUES The values of the design airspeeds in table 1-I are minimum values. In certain cases it may be desirable to use larger values for high performance type gliders. In order to provide for a high auto-winch tow placard speed, it may be advantageous to use a higher design gliding speed.
USE OF K VALUES The K values specified in table 1-I have been determined on the basis of studies of the “cleanness” of current gliders. The values of K for high performance gliders have been set approximately 11 percent higher than the values of K for utility gliders since instrument flying in high performance gliders is permitted and higher speeds are apt to be encountered in recovery from inadvertent upsets. These attitudes are more likely to occur in instrument flying than in normal operations under good weather conditions. Since these constants have been established on a simplified basis, it is possible that they may lead to irrational values of V, when applied to particular cases. In any event, it will be necessary to design high performance gliders to a V, greater than .40 times the terminal velocity or to design utility gliders to a V, greater than .36 times the terminal velocity. In cases where the value of V, is based on terminal velocity in accordance with the above, calculations substantiating the terminal velocity value should be submitted.
Load factors. The flight load factors specified should represent wing load factors. The net load factor or acceleration factor should be obtained by proper consideration of balancing loads acting on the glider in the flight conditions specified in this chapter under "Maneuvering load factors." The net or dead weight load factors should be obtained from balancing computations such as are outlined under "Balancing loads."
Maneuvering load factors.—The limit maneuvering load factors specified in table 1-I should be considered as minimum values unless it can be proved that the glider embodies features of design which make it impossible to develop such values in flight, in which case the proven lower values may be used.
In some cases it may be advisable to use higher values, as when a higher auto-winch tow placard speed is desired.
Gust load factors.—The gust load factors should be computed on the basis of a gust of the magnitude specified, acting normal to the flight
path. Proper allowance should be made for the effects of aspect
k =gust reduction factor (fig. 1-I)
S =W/S, wing loading, p.s.f.
rected for aspect ratio, R.
K - GUST REDUCTION FACTOR
NOTE: K = 1/25'4 = 1/2 (W/S)"
but not exceed 1.0
s = W/ S - WING LOADING, p.s.f.
Figure 1-1. Gust reduction factor.
Factors of safety.—A minimum limit factor of safety of 1.0 and a minimum ultimate factor of safety of 1.5 should be used unless otherwise specified. Also, refer to p. 43 for multiplying factors of safety required.
TABLE 1-1 Minimum Design Airspeeds and Minimum Limit Load Factors For the
Symmetrical Flight Conditions
Class of glider
2. Design Gliding Speed K(s)$
K(s)*2. (V., m.p.h.). 3. Design Auto-Winch Tow 35(8)%.
35(8)33. Speed, Vraw, m.p.h. 4. Design Flap Speed Vf, m.p.h.. nb | 1.67 Vsf.----
1.67 Vsf. 5. Positive Maneuver Load Fac
4.67. tor, n. . 6. Positive Gust Load Factor 3-.. Corresponding to a Corresponding to a 24
24 f.p.s. “up” gust
f.p.s. “up” gust at
at Vo. 7. Positive Auto-Winch Tow
---------Load Factor. 8. Negative Maneuver Load Fac - 2.67-----
Factor, n. 9. Negative Gust Load Factor 5. Corresponding to a. Corresponding to a 24
24 f.p.s. "down" | f.p.s. "down" gust gust at V..
at Ve 10. Design Dive Speed, VD------ Not to exceed 1.2 V.- Not to exceed V..
i The design gliding speed, Ve, shall not be less than the design aircraft tow speed Voe. 3 The following value of K should be used; however a conservative interpolation of the value of K for a particular design will be acceptable:
For gliders of very clean design 61.---
with cantilever wings. For gliders of clean design having
single strut braced wings. For gliders of the utility type hav 51.
ing double strut braced wings and open cockpit.
KUVm : Positive Gust Load Factor: n=1+ =
Factor: n=160 [ 9300 e],
r(Vitam) * Positive Auto-Winch Tow Load Factor: n=
391 where s=des. wing loading, p.s.f.
e=unit wing weight, p.s.f. Vias= Item 3 of table 1- I The factor 391 above is based on a Cų value of 1.0 which experience indicates to be the maximum value likely to be reached when auto or winch towing the glider.
K24 Vom • Negative Gust Load Factor: n=1 -
SYMMETRICAL FLIGHT CONDITIONS
(Flaps Retracted) Basic flight envelopes.—The basic flight envelope or V-n diagram is a locus of points representing the limit wing load factors and the corresponding velocities for the particular design criteria.