"A Comparison of Four versus Five Blades for the Main Rotor of a Light Helicopter"

One of the major design decisions in the preliminary design of a new helicopter is the selection of the number of blades for the main rotor. This paper compares the selection of four versus five blades for the main rotor of a light helicopter. The results of a dynamic/vibration analysis are presented which show major advantages for five blades relative to four blades. As a result, a five-bladed rotor can probably meet the requirements of Specification ADS-27 without needing vibration-attenuating devices such as pendulums, dynamic absorbers, higher-harmonic control (HHC) etc., such as would probably be required for a four-bladed rotor. Thus there would be a significant saving in weight and cost for the five-bladed rotor. The superior vibration characteristics of a five-bladed rotor over a four bladed rotor have been demonstrated in actual designs. Weight-data correlations show additional weight advantages for the five-bladed rotor over the four-bladed rotor. The blade weight, hub weight, and controls weight are all lower for a five-bladed rotor than for a four- bladed rotor of the same diameter and total solidity. Thus there are additional savings in weight and cost. There is some minor structural advantage in the rotor hub for a four-bladed rotor over a five-bladed rotor. The four-bladed rotor also has a minor advantage over the five-bladed rotor from the standpoint of blade vulnerability to high-explosive (HEI) threats. The maintenance characteristics of the two rotor systems are believed to be approximately equivalent. For a light helicopter with a disc loading of the order of 6 to 8 lbs per square foot and a rotor solidity of the order of 0.10 , either four or five-bladed designs result in reasonable values of blade aspect ratio. Based on all the above considerations, it is concluded that a five- bladed rotor is superior to a four-bladed rotor for a light helicopter.