on one side of the undercarriage to restrict the speed of movement of that side and arranging for both shoes 6 to return to their starting positions when either of them reaches the limit of its travel thus one shoe moves a shorter distance than the other in each cycle of operation the controls can be either inside the helicopter or outside it for operation by a man walking beside the aircraft where he may have the best possible view of obstacles the asymmetry of the arrangement as seen in fig2 about a vertical plane perpendicular to the paper should be particularly noted if the construction were in the form of a mirror image about the vertical pane at the position of the return spring 24 and the positions of the bags 8 and 10 on for instance the right of the spring 24 were interchanged the resultant lift force would not act through the centre of the skid and this would increase the tendency of the skids to lift at one end when the centre of gravity of the helicopter was near an extreme position it in some cases is desirable to make provision for cutting off or reducing the pressure in bags.