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edge load similar to that induced by reverse airflow on a wing. _(See fig. 1-XII(3.) The design airspeed should be V = 10 VW/S +10 m.p.h.; Cų = -0.8.

Figure 1-XII(3). Ground gust load distribution, Slab Tail.

VERTICAL TAIL SURFACES Maneuvering.—The minimum average limit pressure specified in fig. 1-XIV should be applied in either direction and distributed in accordance with fig. 1-XII. (Above comments for horizontal surfaces also apply, in general, here.)

Damping (Vertical surfaces).—The total limit load acting on the fixed surface (fin) in the maneuvering condition should be applied in accordance with the load distribution of fig. 1-XI acting in either direction. The load acting on the movable surface in the maneuvering condition may be neglected in determining the damping loads. (Above comments for horizontal surfaces also apply, in general, here.)

Gusts (Vertical surfaces).—The minimum average limit pressure should not be less than that corresponding to a 15 f.p.s. sharp-edged gust at the design gliding speed, V, The gust should be assumed to act normal to the plane of symmetry in either direction. For the purpose of determining the slope of the tail lift curve, the aspect ratio should not be taken as less than 2.0. The chord distribution should simulate that for a symmetrical airfoil, except that the distribution of fig. 1-XI may be used where applicable.

AILERONS Maneuvering.—The minimum average limit pressure specified in fig. 1-XVI should be applied in either direction and distributed in accordance with fig. 1-XV.

Wing flaps.-Wing flaps should be loaded in accordance with the provisions under "High-lift devices" p. 10. In any case, the average limit prssure should not be less than 9 p.s.f. and should be considered uniformly distributed unless a more rational distribution is used.

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Compliance Suggestion

FLAP DESIGN The critical loading is usually obtained when the flap is completely extended. The above recommendations apply only when the flaps are not used at speeds above a certain predetermined design speed. A placard is required to inform the pilot of the speed which should not be exceeded with flaps extended. Reference should be made to current NACA Reports and Notes for acceptable flap data. (See NACA Report No. 824 and Technical Note No. 690.)

HINGE

Figure 1-XV. Aileron load distribution.

Special devices. Special design recommendations should be obtained in connection with the design and analysis of surfaces equipped with unconventional ailerons, auxiliary airfoils and similar devices. Requests for special recommendations should be accompanied by suitable drawings or sketches of the structure in question, together with general information and an outline of the method by which it is proposed to determine the structural loading conditions.

Compliance Suggestion

DESIGN OF SPOILERS In lieu of wind tunnel data, it is recommended that the design of spoilers and their attachment structures be premised on the limit loading obtained from the following formula: Wsp = .0052 V8p? Where Wsp = the limit loading, p.s.f.

Vsp = the IAS at which max. operation

of the spoilers is assumed, m.p.h. It should be assumed that the load is uniformly distributed over the surface.

Dive brakes. The operating mechanism and supporting structure for dive brakes should be designed for critical loads occurring in the brakes extended flight condition, with the brake configuration in any position, from fully retracted to fully extended. When an

automatic brake-limiting device is employed, these parts may be designed for critical combinations of airspeed and brake position permitted by the device. Brake installations should be fitted with a “lock open” device to maintain the unit in the extreme open position. NACA Technical Memo No. 926 contains information on dive control brakes. (Wing flaps may be employed as dive brake units.)

CONTROL SYSTEM LOADS All control systems should be designed using as a basis at least the limit forces hereinafter specified, unless a more rational method is used. Unless otherwise specified, a minimum limit factor of safety of 1.0 and a minimum ultimate factor of safety of 1.5 should be used. See also table 1-III for multiplying factors of safety required in certain cases. See chap. 2, p. 51, "System and Components Tests” for the operation requirements for control systems. The forces in the control system members, cables or push rods operating the movable surfaces should be computed and their effect on the rest of the structure should be investigated and allowed for in the design of such structure.

Compliance Suggestion

DETERMINATION OF LOADINGS The control forces recommended are of an arbitrary nature; hence they may prove to be somewhat irrational in certain cases. In general, however, they represent simplified requirements which will result in satisfactory control systems. If he so desires, the designer may use a more rational loading for the design of the control system. The following loadings are considered satisfactory. The control systems may be designed for limit loads 25 percent greater than those corresponding to the limit loads specified for the control surfaces to which they are attached, assuming the movable surfaces to be in that position which produces the greatest load in the control system except that the loads should not be less than those listed below:

(a) Elevator: 75 lb. fore-and-aft
(6) Rudder: 100 lb. on one pedal only and

200 lb. on each pedal simultaneously
(c) Aileron: 50 lb. laterally or as part of a couple applied to the

control wheel The control forces specified should be applied to the entire control system, including the control surface horns. The multiplying factor of safety of 1.15 need not be applied to the fittings in the control system.

Elevator systems.-In applying the recommendations for elevator systems, a control force of 150 pounds should be assumed to act in a fore-and-aft direction and should be applied at the grip of the control stick, or should be equally divided between two diametrically opposite points on the rim of the control wheel.

Rudder systems.-In applying the recommendations for rudder systems, a control force of 150 pounds should be assumed to act in a direction which will produce the greatest load in the control system and should be applied at the point of contact of the pilot's feet with the control pedal. As a separate condition, two forces of 200 pounds magnitude should be assumed to act simultaneously at both points of contact of the pilot's feet with the control pedal.

Aileron systems.-In applying the recommendations for control system loads, it should be assumed that the ailerons are loaded in opposite directions. A control force of 60 pounds should be assumed to act as part of a couple equal to the specified force multiplied by the diameter of the control wheel. Suitable assumptions should be made as to the distribution of the control force between the ailerons.

In regard to the distribution of the control force between the ailerons, the following assumptions are considered suitable:

• For nondifferential ailerons, 75 percent of the stick force or

couple should be assumed to be resisted by a down aileron, the remainder by the other aileron; also, as a separate condition, 50 percent should be assumed to be resisted by an up aileron,

the remainder by the other aileron. • For differential ailerons, 75 percent of the stick force or

couple should be assumed to be resisted by each aileron in either the up or down position, or rational assumptions based

on the geometry of the system should be made. Flap and auxiliary control systems.—In applying the recommendations for control system loads, suitable minimum manual forces should be assumed to act on flap control systems and other similar controls.

Compliance Suggestion

MINIMUM LIMIT FORCES It is recommended that the following limit forces be used as minimum values for the design of flap and auxiliary control systems: (1) Flaps

50 lb. * (2) Spoilers.-

50 lb.* (3) Hand operated brakes.--

75 lb. * (4) Foot operated brakes.---- 100 lb.* It should be noted that the flap position that is most critical for the flap proper may not be critical for the flap control mechanism and supporting structure. In doubtful cases the flap hinge moment can be plotted as a function of flap angle for various angles of attack within the design range. The necessary characteristic curves should be obtained from reliable wind tunnel tests.

*The force used for the design of flap and spoiler control systems should not be less than 1.25 times the force corresponding to the limit load used for the design of the surfaces.

Towing and launching (release mechanism) control systems.-In applying the recommendations for control system loads, a control force of 75 pounds should be assumed to act in a direction corresponding to that normally used by the pilot, and should be applied at the grip of the control handle or lever.

To aid in assuring emergency release it is recommended that a suitable placard or painted instruction be placed adjacent to the glider release hook to indicate that tow rope size (strength) in excess of the limit load of the release mechanism should not be used.

Also, for auto or winch towing, provisions should be made for a back load trip release mechanism to preclude any possibility of failure to release the towline.

GROUND LOADS The following conditions represent the minimum amount of investigation recommended for conventional landing gear. For unconventional types, it may be advisable to investigate other landing attitudes, depending on the arrangement and design of the landing gear members. Consideration will be given to a reduction of the specified limit load factors when it can be proven that the shock absorbing system will positively limit the acceleration factor to a definite lower value. 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, see table 1-III, for multiplying factors of safety required in certain cases.

Compliance Suggestion

CONVENTIONAL LANDING GEARS Insofar as the requirements of ground loads are concerned, landing gears will be considered conventional if they consist of:

• A single wheel or double coaxial wheels located on the bottom

of the fuselage and directly below (or nearly so) the center of
gravity of the glider, together with auxiliary skids attached to
the bottom of the fuselage. One auxiliary skid running from
the wheel forward to the nose, the other running aft to a point
below the wing trailing edge (approximately). The rear
auxiliary skid may be replaced or supplemented by a suitable
tail skid (see fig. 1-XVII), or
A single main skid on the bottom of the fuselage which extends
from the nose to a point below the wing trailing edge (approxi-
mately). This skid may be supplemented by a tail skid.

NOTE.-Wing tip skids may be employed if desired. Level landing.The glider should be assumed to make contact with the ground while in a level attitude. The basic vertical component

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