WV$⅔ KE 2g (KY + 1B3) where- Vs = vertical speed ft/sec, derived from § 29.725(a) Ky = pitching radius of gyration - it. from pitching axis 1B = distance from most critical CG location to the guard or bumper contact point - ft.
W = gross weight less rotor lift from § 29.473(a) - Ibs.
G = 32.2 ft./sec?
(il) Other, more recent, analytical techniques (most utilizing computer programs) may, of course, be used rather than the ANC-2a means after proper substantiation for applicability and validity.
(iii) The tail rotor guard shall not fail when the limit and ultimate load, which is derived from a combination of the limit kinetic energy and the guard resulting limit deflection required to dissipate the energy, is imposed on the guard and the rotorcraft tail (see § 29.305).
(3) Substantiation of the guard, skid, or bumper for the design loads derived may be accomplished by test or analysis as stated in § 29.307(a).
(4) Several rotorcraft tail rotor guards are installed solely for the protection of ground personnel from the rotating tail rotor. For guards installed for this purpose, the applicant should use prudent and reasonable design loads and features. Such guards should not present a hazard to the rotorcraft because of its design features.
159. § 29.413 STABILIZING AND CONTROL SURFACES.
a. Explanation. Minimum design loads are specified for stabilizing as well as control surtaces.
(1) Paragraph (a) of the rule requires application of minimum empirical design loads, application of critical maneuvering loads, and application of critical maneuvering loads combined with vertical or horizontal gust loads (30 feet per second per § 29.341).
(2) Paragraph (b) requires load distributions that closely simulate actual pressure distributions. Both spanwise and chordwise distributions are intended.
(3) These surfaces are used for stability and control thereby hopefully extending the CG range and increasing the airspeed of modern designs.
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7/30/97/ AC 29-2B
(4) To "closely simulate actual pressure condition" on the surfaces, unsymmetrical loads are also required on horizontal surfaces. An arbitrary distribution, if conservative, may be used.
(5) It is noted § 29.571 requires fatigue substantiation of the flight structure which will include control and stabilizing surfaces.
(6) If the surface is controllable, a proof and operation test of the surface control system is required by §$ 29.681 and 29.683.
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.
Limit loads of 15 pounds per square foot will apply up to approximately 90-knot design airspeed. Above a 90-knot design airspeed (Vp), the coefficient (CN = 0.55) imposes higher limit loads on the surface.
In addition, combined maneuvering and gust loads may impose the highest limit loads on the control surfaces of rotorcraft. 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.
(iii) 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 surtace should closely simulate actual pressure distributions.
(i) The spanwise load may be rectangular or other acceptable conservative distributions may be used. The method developed by O. Schrenk in NACA TM 948, 1940, is 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.
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The chordwise distribution appropriate for the aerodynamic shape should be used.
(iii) 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 also.
(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 prescribed. If analysis is used, fitting factors and other appropriate factors prescribed by the rules of §§ 29.625, 29.621, and 29.623 will be required in the analysis.
159A. § 29.413 (Amendment 29-31) STABILIZING AND CONTROL SURFACES.
a. Explanation. Amendment 29-31 removed this section since the requirements are adequately addressed in §§ 29.337, 29.339, and 29.341.
b. Procedures. The policy material pertaining to this section is retained as supplemental information especially as reference material for Paragraph 139 (§ 29.341) of this document.
160.-161. RESERVED.
270 (thru 272) Par 159
162. § 29.427 (Amendment 29-31) UNSYMMETRICAL LOADS.
a. Explanation. Amendment 29-30 added the standard and Amendment 29-31 amended it. Minimum unsymmetrical design loads are specified for horizontal tail surfaces and also vertical tail surfaces whenever they support the horizontal tail surfaces.
(1) Loads are derived by rational analysis, or for earlier certification bases, the prescribed empirical loads of § 29.413 may be used. Section 29.413 was removed by Amendment 29-31 since the requirements are adequately addressed in §§ 29.337,
29.339, and 29.341.
(2) Rational loads, appropriate for the aerodynamic surfaces, should be distributed according to the standard.
(3) When vertical tail surfaces support the horizontal tail surfaces, the vertical tail surfaces and supporting surfaces are required to support the critical combination of vertical and horizontal surface loads distributed as shown.
b. Procedures. Two basic loading conditions are required by § 29.427 for each of the two basic empennage configurations shown.
(1) Horizontal surfaces supported by the tail boom or fuselage. Structural substantiation should be provided for all six combinations shown in Figure 162-1. All of these empirical loading distributions should be used unless rational analysis shows one or more of each set of conditions to be non-critical or equal or more realistic distributions are substantiated. Rectangular spanwise air load distribution should be used unless more rational distribution is substantiated. If end plates are used, the air loads should be distributed accordingly.
(i) First unsymmetrical loading condition:
(A) 100 percent of the flight load is applied to one side of the plane of symmetry; and 0 percent of the flight load is applied on the other side of the plane of symmetry.
(B) For surfaces with end plates or other similar devices, the load distribution will be changed accordingly.
(ii) Second unsymmetrical loading condition:
50 percent of the flight load on one side of the plane of symmetry acting up; and 50 percent of the flight load on the other side of the plane of symmetry acting down.
Par 162 273
(2) Horizontal surfaces supported by a vertical surface. Structural substantiation should be provided for all six combinations shown in Figure 162-2. All of these empirical loading distributions should be used unless rational analysis shows one or more of each set of conditions to be non-critical or equal or more realistic distributions are substantiated. Rectangular spanwise air load distribution should be used unless more rational distribution is substantiated. If end plates are used, the air loads should be distributed accordingly.
First unsymmetrical loading condition:
100 percent of the flight load on one side of the plane of symmetry; and 0 percent of the flight load on the other side of the plane of symmetry.
(ii) Second unsymmetrical loading condition:
50 percent of the flight load on one side of the plane of symmetry acting up; and 50 percent of the flight load on the other side of the plane of symmetry acting down.
274 Par 162
Figure 162-1. (View Looking Forward) Par 162 275
Figure 162-2. (View Looking Forward) 276 Par 162
163.-169. RESERVED.
Par 163 277 (thru 283)
SECTION 10. GROUND LOADS
170. § 29.471 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 guidance.
171. § 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 284 Par 170
external 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.
172. § 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 positioni 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 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 separate axles.
174. § 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-- Each wheel contacting the ground simultaneously; and Aft wheels contacting the ground with forward wheels just clear of the ground.
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(2) Application of loads-- Maximum design vertical loads applied alone;
(її) 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
(ili) 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.
The aft center of gravity condition with the forward gear clear will normally be critical for the aft gear and gear supports.
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 signiticant 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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175. § 29.481 TAIL-DOWN LANDING CONDITIONS.
a. Explanation. This section 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 ieach landing gear must be tested in the attitude simulating the landing condition that is most critical. 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 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 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-- Only the aft wheels in contact with the ground; and (Il) 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 Par 175 287