From the earliest days of helicopter development, it was known that blade taper improved hovering performance by unloading the tips to achieve a more uniform
FIGURE 10.5 Twist Modification at Tip
Source: Arcidiacono & Zincone, “Titanium UTTAS Main Rotor Blade,” JAHS 21-2, 1976.
induced velocity distribution across the disc Because of this, many early helicopters had tapered blades. Generally, these blades were built like airplane wings with a spar, many ribs, and a covering of plywood and fabric. When blades began to be made of sheet metal, it was easier to use a constant chord and to rely on twist to make the induced velocity distribution uniform. The next innovation in blade construction was the use of fiberglass or another composite material. For these blades the fabrication engineers had no difficulty with taper, so it о ice again became feasible and has been used on several programs.
FIGURE 10.6 Twist Modification at Root
Source: Fradenburgh, “Aerodynamic Design of the Sikorsky S-76 Helicopter,” JAHS 24-4, 1979.
Several factors, however, should be considered before deciding whether to use taper. For small helicopters, taper may drive the tip chord to such a small value that the tip airfoil will suffer penalties in drag and in maximum lift coefficient because of low Reynolds numbers. A second consideration has to do with tip weights. Most rotor blades have tip weights either to improve the autorotational capability or to control dynamic characteristics by placing the frequency of the second blade bending mode below three times per revolution (3/rev). Planform taper, especially when combined with thin tip airfoil sections, may not provide enough physical volume for the required weights. Finally, a tapered blade needs more area than a straight blade to produce the thrust required by a high load factor maneuver. Thus it may weigh more, which will subtract from whatever payload advantage resulted from its increased aerodynamic performance in hover and vertical climb. (Some preliminary studies indicate that perhaps inverse taper holds some promise in this regard.)