§ 27.501 Ground loading conditions: landing gear with skids. ( a ) *General.* Rotorcraft with landing gear with skids must be designed for the loading conditions specified in this section. In showing compliance with this section, the following apply: ( 1 ) The design maximum weight, center of gravity, and load factor must be determined under §§ 27.471 through 27.475.
( 2 ) Structural yielding of elastic spring members under limit loads is acceptable.
( 3 ) Design ultimate loads for elastic spring members need not exceed those obtained in a drop test of the gear with— ( i ) A drop height of 1.5 times that specified in § 27.725; and
( ii ) An assumed rotor lift of not more than 1.5 times that used in the limit drop tests prescribed in § 27.725.
( 4 ) Compliance with paragraphs (b) through (e) of this section must be shown with— ( i ) The gear in its most critically deflected position for the landing condition being considered; and
( ii ) The ground reactions rationally distributed along the bottom of the skid tube.
( b ) *Vertical reactions in the level landing attitude.* In the level attitude, and with the rotorcraft contacting the ground along the bottom of both skids, the vertical reactions must be applied as prescribed in paragraph (a) of this section.
( c ) *Drag reactions in the level landing attitude.* In the level attitude, and with the rotorcraft contacting the ground along the bottom of both skids, the following apply: ( 1 ) The vertical reactions must be combined with horizontal drag reactions of 50 percent of the vertical reaction applied at the ground.
( 2 ) The resultant ground loads must equal the vertical load specified in paragraph (b) of this section.
( d ) *Sideloads in the level landing attitude.* In the level attitude,and with the rotorcraft contacting the ground along the bottom of both skids, the following apply: ( 1 ) The vertical ground reaction must be— ( i ) Equal to the vertical loads obtained in the condition specified in paragraph (b) of this section; and
( ii ) Divided equally among the skids.
( 2 ) The vertical ground reactions must be combined with a horizontal sideload of 25 percent of their value.
( 3 ) The total sideload must be applied equally between the skids and along the length of the skids.
( 4 ) The unbalanced moments are assumed to be resisted by angular inertia.
( 5 ) The skid gear must be investigated for— ( i ) Inward acting sideloads; and
( ii ) Outward acting sideloads.
( e ) *One-skid landing loads in the level attitude.* In the level attitude, and with the rotorcraft contacting the ground along the bottom of one skid only, the following apply: ( 1 ) The vertical load on the ground contact side must be the same as that obtained on that side in the condition specified in paragraph (b) of this section.
( 2 ) The unbalanced moments are assumed to be resisted by angular inertia.
( f ) *Special conditions.* In addition to the conditions specified in paragraphs (b) and (c) of this section, the rotorcraft must be designed for the following ground reactions: ( 1 ) A ground reaction load acting up and aft at an angle of 45 degrees to the longitudinal axis of the rotorcraft. This load must be— ( i ) Equal to 1.33 times the maximum weight;
( ii ) Distributed symmetrically among the skids;
( iii ) Concentrated at the forward end of the straight part of the skid tube; and
( iv ) Applied only to the forward end of the skid tube and its attachment to the rotorcraft.
( 2 ) With the rotorcraft in the level landing attitude, a vertical ground reaction load equal to one-half of the vertical load determined under paragraph (b) of this section. This load must be— ( i ) Applied only to the skid tube and its attachment to the rotorcraft; and
( ii ) Distributed equally over 33.3 percent of the length between the skid tube attachments and centrally located midway between the skid tube attachments.
[Doc. No. 5074, 29 FR 15695, Nov. 24, 1964, as amended by Amdt. 27-2, 33 FR 963, Jan. 26, 1968; Amdt. 27-26, 55 FR 8000, Mar. 6, 1990]