ED Decision 2003/15/RM
(a) Loads and equilibrium. For limit ground loads –
(1) The limit ground loads obtained in the landing conditions in this Subpart must be considered to be external loads that would occur in the rotorcraft structure if it were acting as a rigid body; and
(2) In each specified landing condition, the external loads must be placed in equilibrium with linear and angular inertia loads in a rational or conservative manner.
(b) Critical centres of gravity. The critical centres of gravity within the range for which certification is requested must be selected so that the maximum design loads are obtained in each landing gear element.
CS 27.473 Ground loading conditions and assumptions
ED Decision 2003/15/RM
(a) For specified landing conditions, a design maximum weight must be used that is not less than the maximum weight. A rotor lift may be assumed to act through the centre of gravity throughout the landing impact. This lift may not exceed two-thirds of the design maximum weight.
(b) Unless otherwise prescribed, for each specified landing condition, the rotorcraft must be designed for a limit load factor of not less than the limit inertia load factor substantiated under CS 27.725.
CS 27.475 Tyres and shock absorbers
ED Decision 2003/15/RM
Unless otherwise prescribed, for each specified landing condition, the tyres must be assumed to be in their static position and the shock absorbers to be in their most critical position.
CS 27.477 Landing gear arrangement
ED Decision 2003/15/RM
Paragraphs CS 27.235, 27.479 to 27.485, and CS 27.493 apply to landing gear with two wheels aft, and one or more wheels forward, of the centre of gravity.
CS 27.479 Level landing conditions
ED Decision 2003/15/RM
(a) Attitudes. Under each of the loading conditions prescribed in sub-paragraph (b), the rotorcraft is assumed to be in each of the following level landing attitudes:
(1) An attitude in which all wheels contact the ground simultaneously.
(2) An attitude in which the aft wheels contact the ground with the forward wheels just clear of the ground.
(b) Loading conditions. The rotorcraft must be designed for the following landing loading conditions:
(1) Vertical loads applied under CS 27.471.
(2) The loads resulting from a combination of the loads applied under sub-paragraph (b)(1) with drag loads at each wheel of not less than 25% of the vertical load at that wheel.
(3) If there are two wheels forward, a distribution of the loads applied to those wheels under sub-paragraphs (b)(1) and (2) in a ratio of 40:60.
(c) Pitching moments. Pitching moments are assumed to be resisted by:
(1) In the case of the attitude in sub-paragraph (a)(1), the forward landing gear, and
(2) In the case of the attitude in sub-paragraph (a)(2), the angular inertia forces.
CS 27.481 Tail-down landing conditions
ED Decision 2003/15/RM
(a) The rotorcraft is assumed to be in the maximum nose-up attitude allowing ground clearance by each part of the rotorcraft.
(b) In this attitude, ground loads are assumed to act perpendicular to the ground.
CS 27.483 One-wheel landing conditions
ED Decision 2003/15/RM
For the one-wheel landing condition, the rotorcraft is assumed to be in the level attitude and to contact the ground on one aft wheel. In this attitude:
(a) The vertical load must be the same as that obtained on that side under CS 27.479(b)(1); and
(b) The unbalanced external loads must be reacted by rotorcraft inertia.
CS 27.485 Lateral drift landing conditions
ED Decision 2003/15/RM
(a) The rotorcraft is assumed to be in the level landing attitude, with:
(1) Side loads combined with one-half of the maximum ground reactions obtained in the level landing conditions of CS 27.479(b)(1); and
(2) The loads obtained under sub-paragraph (a)(1) applied:
(i) At the ground contact point; or
(ii) For full-swivelling gear, at the centre of the axle.
(b) The rotorcraft must be designed to withstand, at ground contact –
(1) When only the aft wheels contact the ground, side loads of 0.8 times the vertical reaction acting inward on one side, and 0,6 times the vertical reaction acting outward on the other side, all combined with the vertical loads specified in sub-paragraph (a) ; and
(2) When all wheels contact the ground simultaneously:
(i) For the aft wheels, the side loads specified in sub-paragraph (b)(1); and
(ii) For the forward wheels, a side load of 0.8 times the vertical reaction combined with the vertical load specified in sub-paragraph (a).
CS 27.493 Braked roll conditions
ED Decision 2003/15/RM
Under braked roll conditions with the shock absorbers in their static positions:
(a) The limit vertical load must be based on a load factor of at least:
(1) 1.33, for the attitude specified in CS 27.479(a)(1); and
(2) 1.0 for the attitude specified in CS 27.479(a)(2); and
(b) The structure must be designed to withstand at the ground contact point of each wheel with brakes, a drag load at least the lesser of:
(1) The vertical load multiplied by a coefficient of friction of 0.8; and
(2) The maximum value based on limiting brake torque.
CS 27.497 Ground loading conditions: landing gear with tail wheels
ED Decision 2003/15/RM
(a) General. Rotorcraft with landing gear with two wheels forward, and one wheel aft, of the centre of gravity must be designed for loading conditions as prescribed in this paragraph.
(b) Level landing attitude with only the forward wheels contacting the ground. In this attitude:
(1) The vertical loads must be applied under CS 27.471 to 27.475;
(2) The vertical load at each axle must be combined with a drag load at that axle of not less than 25% of that vertical load; and
(3) Unbalanced pitching moments are assumed to be resisted by angular inertia forces.
(c) Level landing attitude with all wheels contacting the ground simultaneously. In this attitude, the rotorcraft must be designed for landing loading conditions as prescribed in sub-paragraph (b).
(d) Maximum nose-up attitude with only the rear wheel contacting the ground. The attitude for this condition must be the maximum nose-up attitude expected in normal operation, including autorotative landings. In this attitude:
(1) The appropriate ground loads specified in sub-paragraphs (b)(1) and (2) must be determined and applied, using a rational method to account for the moment arm between the rear wheel ground reaction and the rotorcraft centre of gravity; or
(2) The probability of landing with initial contact on the rear wheel must be shown to be extremely remote.
(e) Level landing attitude with only one forward wheel contacting the ground. In this attitude, the rotorcraft must be designed for ground loads as specified in sub-paragraphs (b) (1) and (3).
(f) Side loads in the level landing attitude. In the attitudes specified in sub-paragraphs (b) and (c) the following apply:
(1) The side loads must be combined at each wheel with one-half of the maximum vertical ground reactions obtained for that wheel under sub-paragraphs (b) and (c). In this condition the side loads must be:
(i) For the forward wheels, 0.8 times the vertical reaction (on one side) acting inward, and 0.6 times the vertical reaction (on the other side) acting outward; and
(ii) For the rear wheel, 0.8 times the vertical reaction.
(2) The loads specified in sub-paragraph (f)(1) must be applied:
(i) At the ground contact point with the wheel in the trailing position (for non-full swivelling landing gear or for full-swivelling landing gear with a lock, steering device, or shimmy damper to keep the wheel in the trailing position); or (ii) At the centre of the axle (for full swivelling landing gear without a lock, steering device, or shimmy damper).
(g) Braked roll conditions in the level landing attitude. In the attitudes specified in sub-paragraphs (b) and (c), and with shock absorbers in their static positions, the rotorcraft must be designed for braked roll loads as follows:
(1) The limit vertical load must be based on a limit vertical load factor of not less than:
(i) 1.0 for the attitude specified in sub-paragraph (b); and
(ii) 1.33, for the attitude specified in sub-paragraph (c).
(2) For each wheel with brakes, a drag load must be applied, at the ground contact point, of not less than the lesser of:
(i) 0.8 times the vertical load; and
(ii) The maximum based on limiting brake torque.
(h) Rear wheel turning loads in the static ground attitude. In the static ground attitude, and with the shock absorbers and tyres in their static positions, the rotorcraft must be designed for rear wheel turning loads as follows:
(1) A vertical ground reaction equal to the static load on the rear wheel must be combined with an equal sideload.
(2) The load specified in sub-paragraph (h)(1) must be applied to the rear landing gear:
(i) Through the axle, if there is a swivel (the rear wheel being assumed to be swivelled 90° to the longitudinal axis of the rotorcraft); or
(ii) At the ground contact point, if there is a lock, steering device or shimmy damper (the rear wheel being assumed to be in the trailing position).
(i) Taxying condition. The rotorcraft and its landing gear must be designed for loads that would occur when the rotorcraft is taxied over the roughest ground that may reasonably be expected in normal operation.
CS 27.501 Ground loading conditions: landing gear with skids
ED Decision 2003/15/RM
(a) General. Rotorcraft with landing gear with skids must be designed for the loading conditions specified in this paragraph. In showing compliance with this paragraph, the following apply:
(1) The design maximum weight, centre of gravity, and load factor must be determined under CS 27.471 to 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 CS 27.725; and
(ii) An assumed rotor lift of not more than 1.5 times that used in the limit drop tests prescribed in CS 27.725.
(4) Compliance with sub-paragraphs (b) to (e) 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 sub-paragraph (a).
(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% of the vertical reaction applied at the ground.
(2) The resultant ground loads must equal the vertical load specified in sub-paragraph (b).
(d) Side loads 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 sub-paragraph (b); and
(ii) Divided equally among the skids.
(2) The vertical ground reactions must be combined with a horizontal sideload of 25% 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 subparagraph (b).
(2) The unbalanced moments are assumed to be resisted by angular inertia.
(f) Special conditions. In addition to the conditions specified in sub-paragraphs (b) and (c), the rotorcraft must be designed for the following ground reactions:
(1) A ground reaction load acting up and aft at an angle of 45° 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 in sub-paragraph (b). This load must be –
(i) Applied only to the skid tube and its attachment to the rotorcraft; and
(ii) Distributed equally over 33.3% of the length between the skid tube attachments and centrally located midway between the skid tube attachments.
CS 27.505 Ski landing conditions
ED Decision 2003/15/RM
If certification for ski operation is requested, the rotorcraft, with skis, must be designed to withstand the following loading conditions (where P is the maximum static weight on each ski with the rotorcraft at design maximum weight, and n is the limit load factor determined under CS 27.473(b)).
(a) Up-load conditions in which:
(1) A vertical load of Pn and a horizontal load of Pn/4 are simultaneously applied at the pedestal bearings; and
(2) A vertical load of 1.33 P is applied at the pedestal bearings.
(b) A side-load condition in which a side load of 0.35 Pn is applied at the pedestal bearings in a horizontal plane perpendicular to the centreline of the rotorcraft.
(c) A torque-load condition in which a torque load of 1.33 P (in foot pounds) is applied to the ski about the vertical axis through the centreline of the pedestal bearings.