[Fig. 26] shows a T frame made from a hollow spar, having the ends splayed out to give the required support to the elastic skein. No propeller bar is used but there is a tension wire from bearing to bearing to prevent the splayed section of the spar from spreading when the rubber is wound up. E shows a section of the spar. It should be pointed out that the greatest width of a spar should be placed vertically. Never use a square-sectioned spar. Moreover, the bearing centres should only exceed the propeller diameter by ½ in., since, apart from consideration of weight, greater rigidity is obtainable from short spars than from long ones. It is in details such as this that not only is a considerable saving in weight effected, but also a material increase in strength and efficiency.
The T frame adapted to a “rise-off-ground” fuselage is shown in [Fig. 27]. A hollow spar should be used for preference, but the spar cut from the solid is shown, as most amateurs will not be in possession of a joiner’s plough, which is the tool required for this job. F is a section on the vertical line of the spar.
Fig. 27
Fig. 28
Fig. 29
Fig. 30