Patent Publication Number: US-2013248650-A1

Title: Helicopter skid type landing gear

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to European patent application 12 400007.6 filed on Mar. 22, 2012, the content of which is incorporated in its entirety by reference herein. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The invention is related to a helicopter with a skid type landing gear with the features of the preamble of claim  1 . 
     (2) Description of Related Art 
     Landing gears of helicopters have to be designed to allow absorption of the vertical energy of a landing, by elastic deformation during regular landings and by plastic deformation during hard landings. For example: for one helicopter type with a classical skid type landing gear with a forward and an aft cross tube the respective plastic deformations can reach a value up to 360 mm in vertical direction at the aft cross tube and up to 460 mm at the forward cross tube. 
     A landing gear of a helicopter can contribute to the so called ground resonance phenomenon. Especially the stiffness and the damping properties of the landing gear influence the ground resonance. Ground resonance is a hazardous condition that can occur any time the rotor of a helicopter or gyroplane is turning while the aircraft is on the ground. 
     Ground resonance can occur when the spacing between blades of a rotorcraft becomes irregular or the damping system, lead lag damping on the rotor as well as damping of the fuselage and the landing gear, are operating out of limits. 
     Two physical properties are of particular importance for skid type landing gear assemblies: the vertical and the longitudinal stiffness. The terms “vertical stiffness” or “longitudinal stiffness”, as used herein, refer exclusively to the linear or elastic portion of the load vs. stroke curves, because it is in the linear or elastic portion of said curves, in which skid gears function during normal helicopter operation. The vertical stiffness is important to attenuate landing energy. The longitudinal stiffness is important because it is a primary source of frequency response to ground resonance frequency. The damping behavior of a skid landing gear with two skid tubes and two cross tubes is influenced by: 
     Friction of the skid tubes on the ground, because any vertical movement as consequence of the vertical stiffness will have a lateral movement of the skid as a result. 
     Friction between the single parts of the landing gear assembly at bolted joints and/or riveted joints. 
     Hysteresis of any elastomeric components in the landing gear assembly or at landing gear attachment points. 
     There are 3 common ways of counteracting the phenomenon of ground resonance, either (1) to change the damping of the fuselage or (2) to change the damping of the rotor or 3. to change the characteristics of the landing gear. 
     The most economical way to solve the ground resonance problem is to change the stiffness and damping behavior of the skid type landing gear so that no ground resonance occurs. This can be done by adding some diagonal struts in between the components of skid type landing gears at the cost of weight and drag. A specific cross section form and material of cross tube members can influence the stiffness. This is the practicable way but not the most elegant. Sometimes a damping element is added to a cross tube, if the deflection of the tube allows effective damping at a certain point. 
     The document U.S. Pat. No. 5,224,669 A (Guimbal) discloses the use of dampers to control resonance. 
     The document U.S. Pat. No. 4,196,878 A (Aerospatiale) discloses a landing gear for rotary-wing aircraft having two main skids connected to the aircraft structure by two cantilevered arcuate supports. Each skid has at least one flexible element capable of flexing resiliently upon contacting the ground. 
     The document U.S. Pat. No. 4,270,711 A discloses a helicopter landing gear assembly which includes a pair of cross tubes having a pair of struts connected at the outboard ends thereof 
     The documents US 2010/0237190 A and US 2007/0181744 A (Eurocopter) disclose undercarriages having anticrash and antiresonance skids for a rotary wing aircraft. 
     The document U.S. Pat. No. 6,427,942 A (Bell) discloses a skid landing gear for a helicopter, in which the directional stiffness of the cross members of the skid landing gear have been de-coupled from one another, such that the longitudinal stiffness of the cross members may be independent of the vertical stiffness and fatigue life of the cross members. In order to de-couple the stiffness in the skid type landing gear, two approaches are employed. In the first approach, the skid landing gear has non-symmetric-section cross members and/or distribution of different materials within the cross section for de-coupling the vertical stiffness of the cross members from the longitudinal stiffness. In the second approach, mounting devices are employed that provide compliance in selected directions, thereby de-coupling the directional stiffness. 
     The document US 2011/0133378 A (Nanokas Aviation) discloses a landing gear damper which allows for appropriate ground resonance performance, while reducing the needs for maintenance and enhancing performances with a combined spring and damper element with disc springs inside the damper cylinder. The disadvantage is that the combination of both leads to high spot forces in case of a crash landing. Even moderate hard landings may deform the cross tubes to a certain extent. Said cross tubes consequently need to be exchanged regularly, thus causing customer dissatisfaction. Cross tubes of skid type landing gears on helicopters of the state of the art operating on ships need to be replaced quite often as they reach soon their limits in setting. The designs of skid type landing gears of the state of the art require experience/analysis and trial and error to find the correct stiffness and damping to avoid ground resonance under all possible landing attitudes. 
     The document U.S. Pat. No. 3,144,223 A (Nichols) discloses a skid-type landing gear comprising cross beams being each arranged essentially transversal relative to a longitudinal main axis of the helicopter, the skids of the landing gear being mounted to lateral ends of said cross beams. 
     The document U.S. Pat. No. 4,519,559 A (Logan) discloses a landing gear of the undercarriage of a helicopter with skids connected to upwardly extending cross tubes pivotally connected to an airframe structure. Dampers have a first end pivotally connected to said cross tubes and a second end pivotally connected to the airframe structure. Hydraulic cylinders in the respective dampers are interconnected through hydraulic restoring devices. Hydraulic restoring devices yieldable urge the pistons in each of the hydraulic cylinders to an equilibrium position, thereby restoring the length of each landing gear to an equilibrium length following a disturbance. The respective dampers are not suitable to absorb any vertical energy at landing of the helicopter. Stiffness to absorb the vertical energy of the landing is provided by the cross tubes and skids of the landing gear of U.S. Pat. No. 4,519,559 A. The teaching of U.S. Pat. No. 4,519,559 A does not address ground resonance problems of helicopters. 
     BRIEF SUMMARY OF THE INVENTION 
     The objective of this invention is to avoid or reduce ground resonance problems by means of a helicopter&#39;s skid type landing gear. 
     The solution is provided with a helicopter skid type landing gear with the features of claim  1 . Preferred embodiments of the invention are presented with the subclaims. 
     According to the invention a helicopter is provided with a fuselage and a skid-type landing gear mounted to said fuselage. Said skid-type landing gear comprises cross beams with skids being each disposed in an essentially longitudinal direction laterally on a respective side of a longitudinal midplane in elevation of the landing gear. Each of said skids is mounted on one respective side of the longitudinal midplane to a lateral end of at least one of said cross beams. 
     The cross beams are designed as a supporting structure to transfer forces and moments between skids and fuselage. Each of the cross beams is provided with an inner end next to the fuselage. Fixed bearings and floating bearings of the inventive helicopter skid-type landing gear are provided at the fuselage. Torsion bar springs are provided between said fixed bearings and floating bearings with each of said torsion bar springs being fixed by means of at least one of said fixed bearings whereas said torsion bar springs are allowed to rotate around their longitudinal axis relative to said floating bearings. Said respective inner end of each cross beam is attached to said torsion bar spring at the floating bearing in such a manner that forces and moments are transferred from the respective inner end of each cross beam to said torsion bar spring, i.e., the cross beam becomes a pivotable cantilever cross beam as the cross beam may rotate (pivot) with the torsion bar spring it is mounted to relative to the floating bearing while transferring all its forces and bending moments into the torsion bar spring. 
     All of the transversal or any longitudinal forces from the torsion bar spring are supported in the floating bearing. The fixed bearing withstands all moments from the torsion bar spring. At least one discrete damper is provided for any of said pivotable cantilever cross beams, said discrete damper being connected with one end to the respective pivotable cantilever cross beam and with another end to the fuselage. 
     The inventive helicopter skid-type landing gear provides remedy to ground resonance problems by discrete means for elastic deformations of the landing gear and for the damping while fulfilling among others the regulations FAR/CS-27, FAR/CS-29 concerning energy absorption. The dampers and the torsion bar springs of the inventive helicopter skid-type landing gear are two distinct elements, not automatically aligned. Instead of two cross tubes the inventive helicopter skid-type landing gear is provided with four pivotable cantilever cross beams each supported at the fuselage on respective torsion bar springs. The torsion bar springs are attached at one end to the fuselage structure with a fixed bearing principle for transfer of torsion moments from the torsion bar springs into the fuselage structure via the fixed bearings. At their opposed end each of the torsion bar springs are attached to the pivotable cantilever cross beam via a floating bearing principle for transfer of forces into the floating bearing and transfer of torsion moments into the torsion bar springs. The torsion bar springs provide for elastic stiffness of the inventive helicopter skid-type landing gear to absorb vertical energy at landing of the helicopter together with contributions to said elastic stiffness from the cross tubes and the skids. 
     The discrete dampers connected with one end to the pivotable cantilever cross beams and with their opposed end to the fuselage structure allow controlled damping of the inventive helicopter skid-type landing gear and the discrete dampers provide velocity dependent energy dissipation to absorb vertical kinetic energy at landing of the helicopter. The forces acting at landing of the inventive helicopter are split in two force components: one component is fully reversible and acts as torsion moment at the torsion bar springs and the second component are forces to the damping. 
     The invention allows controlled energy absorption during landing and controllable and tunable stiffness- and damping behavior of the inventive helicopter skid-type landing gear to avoid ground resonance. Another advantage of the invention is to allow elastic energy absorption instead of plastic energy absorption, thus avoiding or reducing a regular exchange of conventional cross tubes after hard landings being especially an advantage for skid type helicopters operating on ship decks as usually the cross tubes of skid type landing gears of helicopters operating on ships need to be replaced quite often as they reach soon their limits in setting. The total weight of the inventive skid-type landing gears is in the same order of magnitude as the weight of current skid-type landing gears. The discrete dampers and torsion bar springs of the inventive skid-type landing gears can be easily replaced in case of damage. 
     According to a preferred embodiment of the invention the pivotable cantilever cross beams are connected to one end of the torsional bar spring mounted in a floating bearing. 
     According to a further preferred embodiment of the invention said pivotable cantilever cross beams on either side of the fuselage are symmetric with respect to the midplane. 
     According to a further preferred embodiment of the invention the discrete dampers are provided with separate means for attachment to the pivotable cantilever cross beams and to the fuselage. 
     According to a further preferred embodiment of the invention up to 4 pivoted cantilever cross beams are attached to 4 torsion bar springs in combination with 4 dampers to absorb required energies elastically/reversibly. 
     According to further preferred embodiments of the invention the at least one discrete damper is of the tunable and/or self-regulating liquid-type. The combination of elastic torsion bar springs with discrete dampers of the inventive skid-type landing gears allow precise fine tuning of said energy absorption with said discrete means whereas with the designs of skid type landing gears of the state of the art it is a matter of experience/analysis and trial and error to find the correct stiffness and damping to avoid ground resonance under all possible landing attitudes. 
     According to a further preferred embodiment of the invention two pivotable cantilever cross beams are attached to two torsion bar springs in combination with two dampers on either side of the fuselage preferably symmetric with respect to the midplane of the helicopter. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Preferred embodiments of the invention are shown with reference to the following description and drawings. 
         FIG. 1  shows an isometric view from outside of one side of a helicopter&#39;s skid type landing gear according to the invention, 
         FIG. 2  shows a detail of  FIG. 1 ; 
         FIG. 3  shows a different view of  FIG. 2 ; 
         FIG. 4  shows an cross sectional view of a floating bearing of the invention; and 
         FIG. 5  shows a diagram with repartitions of dissipated energies in a helicopter&#39;s skid-type landing gear according to the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to  FIG. 1  a left side of a helicopter&#39;s skid type landing gear  10  comprises two aligned torsion bar springs  1 . Each torsion bar spring  1  has two ends. A first end is mounted into a fuselage fitting  6  with an integrated floating bearing  7  for each torsion bar spring  1 . A second end of each torsion bar spring  1  is mounted to a fuselage  8  of the helicopter (not shown) by means of a fixed bearing  9 . One torsion bar spring  1  going all the way through the fixed bearing  9  to the floating bearings  7  may replace the two aligned torsion bar springs  1 . The floating bearings  7  are opposed to each other with respective distances relative to the fixed bearing  9  in between. The torsion bar springs  1  have an essentially cylindrical cross section. The torsion bar springs  1  are made of metal, such as steel. 
     Two bow shaped pivoting cantilever cross beams  3 ,  4  are mounted with their respective inner ends to the torsion bar springs  1  by special means, e.g. splines (not shown) at the respective floating bearings  7 . The two pivoting cantilever cross beams  3 ,  4  are attached essentially rectangular at their respective outer lateral ends to a common skid  5  with the pivoting cantilever cross beam  3  being attached next to a forward end of skid  5 . 
     The cross beams  3 ,  4  are each disposed in an essentially transversal direction relative to a main longitudinal axis of the helicopter (not shown). The skids  5  are mounted laterally, each on a respective side of a longitudinal midplane in elevation of the landing gear  10 . 
     A tunable and/or self-regulating liquid-type damper  2  is mounted with its cylinder side to the forward pivoting cantilever cross beam  3  and with its piston side to the fuselage fitting  6  to absorb energies transferred from the forward pivoting cantilever cross beam  3  and to allow fine tuning with regard to damping to avoid ground resonance. The cantilever cross beam  3  is provided with means, e.g. brackets  11  to attach said discrete damper  2 . The orientation of the damper  2  is slightly inclined with regard to a principal direction of the pivoting cantilever cross beam  3 . 
     A further tunable and/or self-regulating liquid-type damper  2  is correspondingly mounted with its cylinder side to the rearward pivoting cantilever cross beam  4  and with its piston side to the fuselage fitting  6 . 
     According to  FIG. 2  corresponding features are referred to with the references of  FIG. 1 . The torsion bar spring  1  is mounted into a circular opening with gearing of the floating bearing  7 . The forward pivoting cantilever cross beam  3  is mounted to the floating bearing  7 . The floating bearing  7  is mounted in the fitting  6  attached to the fuselage  8 . 
     The floating bearing  7  is shown in more detail in  FIG. 3 . The forward pivoting cantilever cross beam  3  is able to rotate with the gearing  12  of the floating bearing  7  relative to the fitting  6 . The damper  2  is driven with its cylinder side  13  by the forward pivoting cantilever cross beam  3  relative to the fuselage fitting  6 . 
     According to  FIG. 4  corresponding features are referred to with the references of  FIG. 1-3 . The floating bearing  7  is mounted inside the fitting  6  with an essentially u-shaped rectangular casing  14 . The torsion bar spring  1  is coaxially supported by journal-, taper- or roller bearings  15 ,  16  mounted into coaxial circular openings of the fitting  6  and the torsion bar spring  1 , such that the torsion bar spring  1  is rotatable coaxially with the bearings  15 ,  16 . 
     The pivoting cantilever cross beam  3  is provided on an inner circumference of its opening with the gearing  12  and the torsion bar spring  1  is on an outer circumference provided with a coaxial gearing that fits with the gearing  12  on the inner circumference of the pivoting cantilever cross beam  3 . The torsion bar spring  1  carries the pivoting cantilever cross beam  3  relative to the fitting  6 . 
     According to  FIG. 5  there are two curves: a lower one for the energy absorption at the forward cantilever cross beam  3  and an upper one for the energy absorption at the rearward cantilever cross beam  4 . 
     The energy absorption at the forward cantilever cross beam  3  is about 10 000 J for a load of 30-40 kN and a stroke of 300-400 mm at the landing gear  10 . The energy absorption at the rearward cantilever cross beam  4  is about 14 000 J for a load of 50-60 kN and a stroke of 300-400 mm at the landing gear  10 . 
     The Young&#39;s modulus of the torsion bar spring  1  is from 200 000 N/mm 2 -220 000 N/mm 2  at a Poisson&#39;s ratio of about 0.3 and a shear modulus of 80 000 N/mm 2 -81 000 N/mm 2 . 
     The torsion bar springs  1  are solid with a length between 900-1000 mm and a diameter of 45-48 mm. Hollow torsion bar springs  1  have inner diameters of 40-44 mm and outer diameters of 50-54 mm resulting in moments of inertia between 440 000 mm 4  and 480 000 mm 4 . 
     The energies absorbed elastically by the torsion bar springs  1  for the forward cantilever cross beam  3  corresponds to the area below the lower curve EEf. The energies absorbed by the damper  2  for the forward cantilever cross beam  3  corresponds to the shaded area Def above lower curve Eef. 
     The energies absorbed elastically by the torsion bar springs  1  for the rearward cantilever cross beam  4  corresponds to the area below the upper curve Eer. The energies absorbed by the damper  2  for the rearward cantilever cross beam  4  would correspond to the shaded area Der above upper curve Eer, if a damper  2  would be mounted to the rearward cantilever cross beam  4 . 
     The energies absorbed by the dampers  2  are about 5-15% of the energies absorbed by the torsion bar springs  1 . 
     REFERENCE LIST 
       1  torsion bar spring 
       2  damper 
       3  cross beam 
       4  cross beam 
       5  skid 
       6  fitting 
       7  floating bearing 
       8  fuselage 
       9  fixed bearing 
       10  landing gear 
       11  brackets 
       12  gearing 
       13  cylinder side 
       14  casing 
       15  bearing 
       16  bearing