Improved Cost Effectiveness Of Helicopters Through A Two-Point Design Criterion

A helicopter designed only to meet a given set of stringent primary mission requirements will be a minimum size helicopter for this mission. A helicopter, with the same power plant, but with some additional structural capability provided beyond the primary mission requirements, will be slightly heavier, but may well be significantly more cost effective. Such an increased capability can be identified by a second-design point altitude and temperature to which the various helicopter components are sized. A method was developed for determining the most cost effective second-design point. The method uses a series of mathematical models which include performance relationships, mission analysis, statistical and analytical weight predictions, mission effectiveness (including payload utilization data and environmental statistics), and fixed and operating cost estimation. First a helicopter is sized to meet primary mission requirements. Then by the use of an analytical weight model the helicopter is grown to meet the selected second-design point requirements, and the cost effectiveness of the resulting design is determined. The repetition of these calculations for several second-design points leads to the determination of the most cost effective second-design point. The method is applied to a utility type helicopter, with substantial improvement in cost effectiveness. A unique computer program was developed to implement this and any other analytical models. The program is very general and treats the mathematical models as data. The program is easily usable by engineers without progranrning experience. A “single-point-design” helicopter is one which has been designed to take off at a high altitude and ambient air temperature with a given mission payload. These specifications, along with other mission requirements, size the power plant and the structure for the helicopter. While it is admittedly an over-simplification to describe present helicopters as pure single-point-design vehicles, this criterion is certainly a major factor in their final configuration. A “two-point-design” helicopter is one which in addition to having to meet minimum mission requirements at specified conditions, is designed to make full use of the additional engine power available at some other but less stringent altitude and temperature condition. A single-point-design helicopter has the advantage of being the lightest vehicle capable of performing the required mission. Its initial costs will also be the lowest. On the other hand, its design limits can easily be exceeded. For example, the transmission design torque of such a helicopter can be exceeded during more than 90% of world-wide daylight operations. The two-point-design helicopter will be heavier and have higher initial costs. It will, however, be able to take advantage of the additional engine power under less stringent ambient conditions. It will be able to carry greater loads safely and will have greater endurance. Because it is less susceptible to abuse, its maintenance costs will be reduced. Depending on what is selected as the second-design point, the particular operating conditions, and the appropriate effectiveness criterion, this vehicle can be significantly more cost effective. Several studies (Refs. 1-4) have been conducted to select single-point design criteria, i.e., hot-day hover requirements for minimum missions. This paper presents an optimization technique for selecting a second-design point. A mathematical model is developed and implemented using a computer program especially designed for use by engineers conducting design and optimization studies. Details of this program, called ZODIAC II, are given in the Appendix. The use of the mathematical model is illustrated by the determination of a second-design-point for a utility mission helicopter.