AC 29-2, CHO 1 11/19/84 * b. Procedures. Modern rotorcraft designs have generally employed a fixed or a wholly movable, not split or divided, stabilizing or control surface.
(1) Design Loads.
(1) Limit loads of 15 pounds per square foot will apply up to approximately 90-knot design airspeed. Above 90 knots design airspeed (Vd), the coefficient (CN = 0.55) imposes higher limit loads on the surface.
highest limit loads on the control surfaces of helicopters. This is attributed to the increase in speed (horizontal gust) and to the change in angle of attack and change in airspeed (vertical gust). Imposing the horizontal gust (30 feet per second or 17.8 knots) on the surface in combination with 130-knot design speed results in a 30 percent increase in the design load. The gust conditions cause a significant increase in design loads due to a change in angle of attack, with a change in resultant airspeed, or due to the increase in airspeed.
(111) The applicant may choose to derive the limit loads using maximum aerodynamic coefficients for the surface under consideration at the maximum design airspeed combined with a 17.8-knot gust. This would be acceptable provided these design loads exceed the minimum loads derived from a CN = 0.55 at design airspeed or exceed 15 pounds per square foot load on the surface.
(2) The load distribution on the surface should closely simulate actual pressure distributions.
(1) The spanwise load may be rectangular or other acceptable conservative distributions may be used. The method developed by 0. Schrenk in NACA IM 948, 1940, 1s an acceptable method for approximation of spanwise distribution.
NOTE: The method is valid for aspect ratios of 5 to 12 and for rectangular planforms such as used on rotorcraft, other planforms may be acceptable as prescribed in the TM.
(11) The chordwise distribution appropriate for the aerodynamic shape should be used.
(111) The flight load survey conducted under § 29.571 may be used to confirm design parameters and possible load distribution data. On controllable surfaces, the pitching moment (control loads) is measured for fatigue substantiation of the control system. The control stabilizing surfaces are subject to loads measurement and possible fatigue tests for fatigue substantiation
(3) Proof of the structure for the required loads is specified in §§ 29.301, 29.303, 29.305, and 29.307. Tests or analysis may be used as presoribed by the rules of $$ 29,625, 29,621, and 29.623 will be required in the prescribed. If analysis is used, fitting factors and other appropriate factors analysis.
160.-169. RESERVED.
Chap 2 258 (thru 272) Par 159
SECTION 10. GROUND LOADS * 170. $ 29.471 (through Amendment 29-19) GENERAL.
a. Explanation. This regulation specifies that limit ground loads must be considered which are:
(1) External loads caused by landing (ground) conditions and by ground taxiing loads as specified in § 29.235.
(2) Loads considering the rotorcraft structure as a rigid body.
(3) Loads in equilibrium with linear and angular inertia loads.
(4) The critical center of gravity 'must be selected so that the maximum design loads are obtained in each landing gear element."
b. Procedures.
(1) The standards to be considered are specified in $$ 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 298 of this document.
(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 97 of this document for § 29.235 policy.
171. § 29.473 (through Amendment 29-19) 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 loads resulting from asymmetric loading conditions are reacted as specified in the individual subparagraphs. • Chap 2 Par 170 213
AC 29-2, CHG 1 11/19/84 * 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.
172. § 29.475 (through Amendment 29-19) TIRES AND SHOCK ABSORBERS.
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 oritical 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.
173. § 29.477 (through Amendment 29-19) LANDING GEAR ARRANGEMENT• a • Explanation. This section specifies the individual standards to be used for ground load conditions for helicopters 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 helicopters having two wheels aft and one or more wheels forward of the center of gravity. This includes forward wheels on separate axles.
174. § 29.479 (through Amendment 29-19) 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--
(1) Each wheel contacting the ground simultaneously; and of the ground. (11) Aft wheels contacting the ground with forward wheels just clear * Chap 2 274 Par 171
(2) Application of loads--
(1) Maximum design vertical loads applied alone;
(11) 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
(111) 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 No. 182 of this document 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.
(1) The aft center of gravity condition with the forward gear clear will normally be critical for the aft gear and gear supports.
oritical for vertical loads. fort enty of avity to ditign with a far olomenta
(111) 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 sucoessfully correlate with the results of a previous well-instrumented test program.
Chap 2 Par 174 275
AC 29-2, CHG 1 11/19/84 * 175. § 29.481 (through Amendment 29-19) TAIL-DOWN LANDING CONDITIONS.
a. Explanation. This section provides the criteria for tail-down landing conditions, 1.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 absorbtion 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.i Where questions exist as to the critical attitude, both level landing and tail-down landing attitudes should be used in drop tests required by § 29.725.
176. § 29.483 (through Amendment 29-19) 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.
177. § 29.485 (through Amendment 29-19) LATERAL DRIFT LANDING CONDITIONS.
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--
(1) Only the aft wheels in contact with the ground; and
(11) All wheels contacting the ground simultaneously. * Chap 2 276 Par 175
(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.
(1) One side load condition 1s 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.
(11) 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
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.
178. § 29.493 (through Amendment 29-19) BRAKED ROLL CONDITIONS.
a • Explanation. This section provides two loading conditions for ground braking operations.
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 acoomplished 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.
Chap 2 Par 177 277
AC 29-2, CHG 1 11/19/84 * 179. § 29.497 (through Amendment 29-19) 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:
(1) All wheels (main and tail) contacting the ground simultaneously, as well as only forward main wheels contacting the ground.
(11) Maximum design vertical loads applied alone.
(111) The maximum design vertical loads combined with a drag load of at least 25 percent of the vertical loads for both conditions.
(2) Noseup 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 oenter of gravity condition is used for each gear and gear support structure.
(1) The forward center of gravity condition with the tail gear clear will normally be critical for the forward gear and gear supports.
Chap 2 278 Par 179
(11) 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.
(111) 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 noseup landing procedures use $ 29.481. The reference to "extremely remote" in $ 29.497(d)(2) predates current 99 25.1309, 29.1309, and AC
25.1309.l. This phrase has been used to require consideration of noseup landings unless features of design are present which prevent noseup landings or where such landings are unlikely during the life of the rotorcraft. See paragraph No. 175 of this dooument.
(4) Use § 29.483 for one-wheel landing procedures, paragraph No. 176 of this document.
(5) Use § 29.485 procedures for side load conditions, paragraph No. 177 of this dooument.
(6) Use § 29.493 procedures for braked roll conditions, paragraph No. 178 of this document.
(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 looked, 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 rotororaft 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 No. 97, of this document.
180. § 29.501 (through Amendment 29-19) 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:
under limit loads is allowed. (1) Structural yielding (plastic deformation) of elastic spring members, Chap 2 Par 179 279
AC 29-2, CHG 1 11/19/84
(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.
(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 degrees from the horizontal axis:
(1) Distributed among or between the skids;
(11) Concentrated at the forward end of the straight portion of the skid tube; and
(111) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
(9) A concentrated 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 attaohments are to be considered in determining the critical center of gravity condition.
* Chap 2 280 Par 180
(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 gafety 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),
181. § 29.505 (through Amendment 29-19) SKI LANDING CONDITIONS.
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 centerline of the pedestal bearings.
NOTE: Where P is the maximum static weight on each ski and n 1s 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 statio 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.
See § 29.737, paragraph No. 305, for information on ski design standards.
TSO-c28 concerns, in part, standards for aircraft akis.
Chap 2 Par 180 281
AC 29-2, CHG 1 11/19/84 * 182. § 29.511 (through Amendment 29-19) 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).
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; 1.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.
183.-192. RESERVED.
SECTION 11. WATER LOADS
193. § 29.519 HULL TYPE ROTORCRAFT: WATER BASED AMPHIBIAN AND LIMITED AMPHIBIAN. (RESERVED)
194. § 29.521 FLOAT LANDING CONDITIONS. (RESERVED)
195.-204. RESERVED.
Chap 2 282 (thru 314) Par 182
SECTION 12, MAIN COMPONENT REQUIREMENTS * 205. § 29.547 (through Amendment 29-19) MAIN ROTOR STRUCTURE.
a. Explanation. This regulation requires the main rotor structure to be designed to the static load requirements of $$ 29.337 through 29.351 (vertical maneuvering loads, vertical and horizontal gust loads, and yawing maneuver loads). In addition, the main rotor blades, hubs, and flapping hinges are specified to be designed for impact forces of each blade against its stop during ground operation and for specified limit torque at any rotational speed including zero. The torque forces (from the drive system) are distributed to the rotor blades as specified.
b. Procedures.
(1) Substantiation in compliance with this standard is accomplished by application of the flight loads of §§ 29.337 through 29.351 and the torque loads of § 29.361 to the rotor structure by stress analyses and/or static tests. The use of wind tunnel data as well as flight loads survey data may be used to generate and/or check the external load magnitudes and distributions.
(2) Where new materials are used in the main rotor structure, such as composites containing plastics, the effects of temperature and humidity are to be considered in accordance with § 29.603, and the effects of uncertainties in manufacturing processes or inspection methods are to be considered in accordance with § 29.619.
(3) The design impact forces of each blade must be imposed against its stop or stops. Appropriate monitoring of the blades, hubs, flapping hinges, and stops during laboratory tests, ground endurance tests, and flight tests should ensure that the stops are sufficient for ground operation loads. The design torque loads are derived as prescribed.
206. § 29.549 (through Amendment 29-19) FUSELAGE AND ROTOR PYLON.
a • Explanation. This regulation requires that the fuselage and rotor pylon (including the tail fin, if any) be designed to withstand the flight loads of §§ 29.337 through 29.351, the ground loads of §§ 29.235, 29.471 through 29.497, sk1 loads of § 29.505, water loads of § 29.521, and rotor loads of § 29.547(d) (1) and (e) (1) (1). The ski and water loads pertain to optional features.
(1) Consideration is also required of--
(1) Auxiliary rotor thrust;
(ii) The torque reaction of each rotor drive system; and
(111) Balancing air and inertia loads.
• Chap 2 Par 205 315