Figure AC 29.427-1 (View Looking Forward)
АС 29-2C 9/30/99 Figure AC 29.427-2 (View Looking Forward) Page C -32
SUBPART C - STRENGTH REQUIREMENTS
GROUND LOADS AC 29.471. § 29.471 GROUND LOADS - GENERAL.
a. Explanation. This regulation specifies that limit ground loads must be considered which are:
(1) External loads caused by landing (ground) conditions and by ground taxiing loads as specified in § 29.235.
(2) Loads considering the rotorcraft structure as a rigid body.
(3) Loads in equilibrium with linear and angular inertia loads.
(4) The critical center of gravity “must be selected so that the maximum design loads are obtained in each landing gear element.”
b. Procedures.
(1) The standards to be considered are specified in §§ 29.473 through 29.511.
These associated standards cover landing gear arrangements, landing conditions, and ground handling conditions.
(2) Drop tests are required for determination of landing load factors. See paragraph AC 29.723.
(3) The application of the design loads derived from the landing load factors will be as specified for each element affected by landing or ground handling loads.
(4) During the applicant’s flight test program, the ground, landing, and taxiing load factors may be monitored to assure the design load factors used are adequate.
See paragraph AC 29.235 for § 29.235 guidance.
AC 29.473. § 29.473 (Amendment 29-3) GROUND LOADING CONDITIONS AND ASSUMPTIONS.
a. Explanation. The rotorcraft is to be designed for the maximum weight. A rotor lift of two-thirds of the design maximum weight may be used. The minimum limit landing load factor is determined by the drop tests of § 29.725. Provisions are made for supplementary energy absorption devices that have triggering mechanisms.
b. Procedures. Loads for the landing conditions are derived considering mass (equal to the maximum weight) and rotor lift (equal to two-thirds of the maximum weight) acting through the center of gravity throughout the landing impact. Unbalanced external Page C - 33
loads resulting from asymmetric loading conditions are reacted as specified in the individual subparagraphs.
NOTE: If supplementary energy absorption devices are used, neither they nor their triggering devices may fail under the loads established by the limit drop tests or the reserve energy absorption drop tests.
AC 29.475. § 29.475 TIRES AND SHOCK ABSORBERS.
a. Explanation. This section specifies the tire and shock absorber position to be used in ground load derivations.
b. Procedures. Ground loads are to be derived with the tires in static (1g) position and the shock absorbers “in their most critical position.” The determination of the “most critical position” for the shock absorbers generally requires a load versus deflection test or analysis of the shock absorber system and a determination of the effect of both load and deflections on the shock absorber, attachment structure, and substructure designed by ground loads.
AC 29.477. § 29.477 LANDING GEAR ARRANGEMENT.
a. Explanation. This section specifies the individual standards to be used for ground load conditions for rotorcraft having two wheels aft and one or more wheels forward of the center of gravity.
NOTE: § 29.497 gives ground loading conditions for landing gear with tail wheels, and § 29.501 gives ground loading conditions for landing gear with skids.
b. Procedures. The ground loading conditions of §§ 29.235, 29.479 through
29.485, and 29.493 will be used for rotorcraft having two wheels aft and one or more wheels forward of the center of gravity. This includes forward wheels on separa te axles.
AC 29.479. § 29.479 LEVEL LANDING CONDITIONS.
a. Explanation. This section provides explicit level landing load criteria for landing gear with two wheels aft and one or more wheels forward of the center of gravity.
(1) Level landings--
(i) Each wheel contacting the ground simultaneously; and
(ii) Aft wheels contacting the ground with forward wheels just clear of the ground.
(2) Application of loads-- Page C - 34
(i) Maximum design vertical loads applied alone;
(ii) The maximum design vertical loads applied with a drag load of at least 25 percent of the vertical load (applied at the ground contact area); and
(iii) The vertical load at the instant of peak drag load in conjunction with the peak drag load. A ground speed and load application is specified.
(3) A 40 percent/60 percent load distribution between wheels for configurations having two forward wheels including quadricycle. This distribution between wheels on a common axis is to be applied for the conditions of vertical loads only, and for vertical loads combined with drag loads of 25 percent of the vertical loads. Section 29.511 concerns a 60 percent to 40 percent ground load distribution between multiple-wheel units. See paragraph AC 29.511 for dual wheels on a common axle or axis.
(4) Aircraft pitching moments are to be reacted by the forward landing gear or by the angular inertia forces when the forward landing gear is clear of the ground as specified.
b. Procedures.
(1) The specified loading conditions will be used in load derivations.
(2) The critical center of gravity condition will be used for each gear and gear support structure.
(i) The aft center of gravity condition with the forward gear clear will normally be critical for the aft gear and gear supports.
(ii) The forward center of gravity condition with each gear contacting the ground simultaneously will normally design forward gear elements critical for vertical loads.
(iii) The forward center of gravity condition with the forward gear clear may result in high load factors, angular plus linear, that will greatly affect security of items of significant mass.
(3) The vertical load, at the instant of peak drag load combined with the peak drag component, can be determined from drop tests utilizing wheel spin-up or it can be analytically determined. If analysis is used, it must successfully correlate with the results of a previous well-instrumented test program.
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AC 29.481. § 29.481 TAIL-DOWN LANDING CONDITIONS.
a. Explanation. This s ection provides the criteria for tail-down landing conditions,
i.e., “the maximum nose-up attitude allowing ground clearance” with ground loads acting “perpendicular to the ground.”
b. Procedures.
(1) The tail-down landing condition will be used to check (by analysis or test) for criticality of landing gear or support structure. This attitude generally creates the highest forward loads on the landing gear in combination with vertical loads.
(2) The tail-down landing condition may be the critical condition for both landing load factor and for energy absorption by the main gear. Section 29.725 requires that “each landing gear must be tested in the attitude simulating the landing condition that is most critical.” Where questions exist as to the criti cal attitude, both level landing and tail-down landing attitudes should be used in drop tests required by § 29.725.
AC 29.483. § 29.483 ONE-WHEEL LANDING CONDITIONS.
a. Explanation. This section gives the condition to be used for one- wheel landing conditions. Only the vertical load condition of § 29.479(b)(1) is required.
b. Procedures. The one- wheel landing condition is generally critical for the landing gear-to-fuselage attachments and the landing gear elements between the attachments. Unbalanced external loads are reacted by rotorcraft inertia. Large items of mass located radially from the center of gravity (aircraft centerline may be used) should also be structurally substantiated for the combined rolling (angular) and linear accelerations of this loading condition.
AC 29.485. § 29.485 LATERAL DRIFT LANDING CONDITIONS.
a. Explanation.
(1) This section provides the loading conditions which impose side (and vertical) loads on the landing gear. A level landing attitude is specified. Two main conditions required are--
(i) Only the aft wheels in contact with the ground; and
(ii) All wheels contacting the ground simultaneously.
(2) Loads. The vertical loads to be applied with the side loads are specified as “one-half of the maximum ground reactions of § 29.479(b)(1).” These vertical loads are the level landing loads considering both contact and noncontact with the ground by the forward wheels.
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(i) One side load condition is specified as “0.8 times the vertical reaction acting inward on one side and 0.6 times the vertical reaction acting outward on the other side” when only the aft wheels contact the ground.
(ii) The other side load condition (for all wheels contacting the ground) specifies the 80 percent inward/60 percent outward distribution for the aft wheels and
0.8 times (80 percent) the vertical reaction for the forward wheels.
b. Procedures. The loading conditions, as specified, are applied to the landing gear and attaching structure. The loads are applied at the ground contact point, except for full swiveling gear which has the load applied at the center of the axle. In other words, full swiveling gear is considered to have swiveled to a static position under the side load before the design vertical and side loads are achieved. The landing gear backup structure, as well as the landing gear itself, will be substantiated for these side load conditions.
AC 29.493. § 29.493 BRAKED ROLL CONDITIONS.
a. Explanation. This section provides two loading conditions for ground braking operations. Specific vertical loads in conjunction with drag loads (due to braking) are to be considered. The limit vertical load factor is 1.33 for condition of all wheels in contact with the ground, and 1.0 for condition of aft wheels only in contact with the ground and nose wheel clear. The drag load on wheels with brakes is 0.8 times the vertical load or the drag load value based on limiting brake torque, whichever is less.
b. Procedures. The braking loads are calculated from the specified criteria with the shock absorbers in their static (normal) positions and with the drag loads applied at the ground contact point. Structural substantiation of the affected structure may be accomplished by test or analysis. If tests are used, the wheel and tire assembly is commonly replaced with a test fixture so the limit loads and static deflections specified can be more accurately controlled. The test specimen should be complete enough to assure that the landing gear structure and the attach and backup structure are adequately substantiated.
AC 29.497. § 29.497 GROUND LOADING CONDITIONS: LANDING GEAR WITH TAIL WHEELS.
a. Explanation. This section provides the loading conditions for landing gear designs with tail wheels.
(1) Level landings are to consider the following:
(i) All wheels (main and tail) contacting the ground simultaneously, as well as only forward main wheels contacting the ground.
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(ii) Maximum design vertical loads applied alone.
(iii) The maximum design vertical loads combined with a drag load of at least 25 percent of the vertical loads for both conditions.
(2) Nose-up landings with only the rear wheel or wheels initially contacting the ground must be considered unless shown to be extremely remote.
(3) Level landings on one forward wheel only are to be considered. Drag loads are not required.
(4) Side load conditions are imposed on the main wheels and tail wheels for level landing attitudes. Criteria for full swiveling and locked tail wheels are included in this standard.
(5) Braked roll conditions are specified for the level landing attitudes.
(6) Rear wheel turning loads are also specified for swiveling and locked tail wheels.
(7) Taxiway condition loads for the landing gear and rotorcraft are those that “occur when the rotorcraft is taxied over the roughest ground that may reasonably be expected in normal operation.” The aircraft design load factors should not be exceeded during the evaluation. Section 29.235 contains an identical standard that applies to all types of wheel landing gear.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity condition is used for each gear and gear support structure.
(i) The forward center of gravity condition with the tail gear clear will normally be critical for the forward gear and gear supports.
(ii) The aft center of gravity condition with the tail gear clear should be checked for criticality of security of large mass items located forward of the center of gravity. Vertical and angular accelerations are additive under this landing condition.
(iii) The aft center of gravity condition with each gear contacting the ground simultaneously will generally design tail gear elements critical for vertical loads.
The other conditions are generally less severe but must be proven.
(3) For nose-up landing procedures use § 29.481. The reference to “extremely remote” in § 29.497(d)(2) predates current §§ 25.1309, 29.1309, and AC 25.1309.1.
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This phrase has been used to require consideration of nose-up landings unless features of design are present which prevent nose-up landings or where such landings are unlikely during the life of the rotorcraft. See paragraph AC 29.481.
(4) Use § 29.483 for one-wheel landing procedures, paragraph AC 29.483.
(5) Use § 29.485 procedures for side load conditions, paragraph AC 29.485.
(6) Use § 29.493 procedures for braked roll conditions, paragraph AC 29.493.
(7) For rear wheel turning loads, swiveling of tail landing gears is allowed as in basic side load conditions. The side load is applied at the axle, or if the wheel is locked, the load is applied at ground contact. Rear wheels are loaded with the critical vertical static load in conjunction with an equal side load to substantiate the tail gear.
(8) Since the rotorcraft is to be designed for load factors that will not be exceeded during taxi tests or other conditions, an instrumented taxi test program will be necessary. Use § 29.235, paragraph AC 29.235.
AC 29.501. § 29.501 (Amendment 29-3) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. This section provides the ground loading conditions for landing gear with skids. The loading conditions are similar to those for wheeled gear except for the following criteria which are unique to skid gears:
(1) Structural yielding (plastic deformation) of elastic spring members under limit loads is allowed.
(2) Design ultimate loads for elastic spring members need not exceed the loads obtained in a drop test with a drop height of 1.5 times the limit drop height. The rotorcraft and the landing gear attachments are subject to the prescribed design ultimate loads.
(3) The gear must be in its most critically deflected position (similar to § 29.475).
(4) Ground reactions are rationally distributed along the bottom of the skid unless otherwise specified. Paragraph (f) concerns specific “concentrated” and arbitrary load conditions.
(5) Drag loads are 50 percent of vertical reactions rather than the 25 percent for wheeled gear.
(6) Side loads are 25 percent of the total vertical reaction rather than the 60-80 percent for wheeled gear.
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(7) Side loads are applied to one skid only (inward acting and outward acting) with resulting unbalanced moment resisted by angular acceleration.
(8) A ground reaction load of 1.33 times the maximum weight is to be applied at 45° from the horizontal axis:
(i) Distributed among or between the skids;
(ii) Conc entrated at the forward end of the straight portion of the skid tube; and
(iii) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
(9) A concent rated vertical load equal to one-half of the design limit vertical load is to be applied at a point midway between the skid tube attachments.
b. Procedures.
(1) The specified loading conditions are to be used in load derivations.
(2) The critical center of gravity conditions are to be used for each gear and gear support structure. Asymmetry of the skid tubes, cross tubes, and gear attachments are to be considered in determining the critical center of gravity condition.
(3) The rotorcraft and landing gear attachment must be substantiated for ultimate landing loads by either test or analysis utilizing an ultimate load factor of 1.5 in accordance with § 29.303. The elastic spring members may be analyzed or static tested for ultimate loads (and deflections) using either a factor of safety of 1.5 or one associated with an “ultimate” drop height of 1.5 times the limit drop height.
Substantiation by “ultimate” drop tests may be used provided all combinations of critical parameters are included in the total substantiation effort. This method will require a series of tests using several test specimens, or a limited number of drop tests plus further substantiations by static tests or analyses for additional critical conditions not covered by the drop test(s).
AC 29.501A. § 29.501 (Amendment 29-30) GROUND LOADING CONDITIONS:
LANDING GEAR WITH SKIDS.
a. Explanation. Amendment 29- 30 relaxes previous requirements in two cases by:
(1) Allowing the total sideload of § 29.501(d)(3) to be distributed “equally between skids” rather than being “applied along the length of one skid only;” and, Page C - 40
(2) Allowing the concentrated load of § 29.501(f)(2)(ii) to be distributed over
33.3 percent of the skid (between skid tube attachments) rather than being “concentrated at a point midway between the skid tube attachments.”
b. Procedures. The previous procedures (through Amendment 29-19) continue to apply to Amendment 29-30 except for the use of the new load distributions.
AC 29.505. § 29.505 SKI LANDING CONDITIONS.
a. Explanation. This is an optional requirement for ski operations. The regulation specifies vertical loads, side loads, and torque loads (MZ) to be applied to ski installations. The four loading conditions to be applied at the pedestal bearings are:
(1) Simultaneous application of Pn, up load, and Pn/4, horizontal load.
(2) Up load of 1.33 P.
(3) Side load of 0.35 Pn.
(4) Torque load of 1.33 P (in foot-pounds), about vertical axis through the centerline of the pedestal bearings.
NOTE: Where P is the maximum static weight on each ski and n is the limit load factor obtained from drop tests. The load factor obtained from wheel or skid landing gear drop tests may be used.
b. Procedures. Structural substantiation may be accomplished by static test or analysis using the specified loads. Skis generally have a limit load rating. The design loads derived for this standard must not exceed the rating. TSO -c28 concerns, in part, standards for aircraft skis.
AC 29.511. § 29.511 (Amendment 29-3) GROUND LOAD: UNSYMMETRICAL LOADS ON MULTIPLE-WHEEL UNITS.
a. Explanation. Two loading conditions are provided to account for unsymmetrical loads on multiple-wheel units due to landing and normal operations over crowned runways and taxiways and to account for deflated tires. They are:
(1) Sixty percent of total ground reaction applied to one wheel of a dual wheel unit and 40 percent to the other.
(2) Sixty percent of the “specified load for the gear unit” is applied to the wheel with an inflated tire when the other tire is deflated (the 60 percent load may not be less than the 1g static load).
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NOTE: The 60:40 distribution also applies to nose wheel units as noted in § 29.479(b)(4).
b. Procedures. Structural substantiation may be accomplished by static test or analysis using the specified load. As provided by the standard, the total load on the gear units may neglect the transverse shift of the load centroid due to unsymmetrical load distribution; i.e., the external load for each gear may be calculated considering the same load centroid as with symmetrical wheel loads, and then the external load for each gear is divided in accordance with the distributions of § 29.511(a) and (b) between the wheels.
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