Overall Design Requirements

Both the conceptual preliminary and detailed design processes comprise a highly interactive effort among aerodynamicists, structural dynamicists, aeroelasticians, material specialists, weight engineers, flight dynamicists, and other specialists. Prouty (1986) gives a good overview of the basic helicopter design process, particularly for military machines. The helicopter design must start with a clear set of specifications, which are defined based on the needs of a potential customer, or more so in the case of military machines, the needs to meet a specific mission requirement. Design technology for the civilian market is driven mostly by customers who emphasize reduced acquisition and operating costs, increased safety, reduced cabin noise and increased passenger comfort, and better overall mechanical reliability and maintainability. Because many of the helicopters in civilian use will operate from heliports and in populated areas, there is also an increasing emphasis on design for reduced external noise. The military have somewhat different requirements. They tend to demand much more in the way of flight performance, speed and maneuver capabilities, and damage tolerance, so the design of military helicopters is often much more difficult and expensive. Military planners also constantly emphasize the need of operational flexibility and adaptability and the need for long operational life with components that can be continuously upgraded. Vulnerability of the helicopter and the survivability of crew and passengers in a combat situation are also issues important to the military. Today, increasing emphasis is being placed on the dual use of military and civilian technology, which is simply the efficient integration of these traditionally separate design technologies. This has benefits for both the customer and the manufacturer.

The general design requirements for a new helicopter will include (not in any order of priority): 1. Hover capability, including both in and out of ground effect operations; 2. Maximum payload in different types of roles or missions; 3. Range and/or endurance un-

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5. Climb performance, both vertically and with forward speed; 6. “Hot and high” perfor­mance, icing, and other environmental effects; and 7. Maneuverability and agility (for mil­itary helicopters). One general objective for the manufacturer will be to design the smallest and lightest helicopter to minimum cost. A challenge in minimizing costs is to lower the design cycle time, and this is where the role of analysis and improved mathematical models becomes useful. However, the design must proceed on the basis of many constraints, which may limit the number of design choices. These constraints may include (not in any order of priority): 1. Maximum allowable main rotor disk loading; 2. Maximum allowable overall physical size of the helicopter; 3. One engine inoperative performance; 4. Autorotative ca­pability at maximum gross weight; 5. Maximum allowable noise (both civil and military);

6. Various civil certification or military acceptance requirements; 7. Crashworthiness and survivability requirements; 8. Maintenance issues; 9. Radar cross section and detectability (for military helicopters); and 10. Vulnerability (for military helicopters).

The various requirements for a new helicopter design will be initially specified by the Customer. ThcSC are then negotiated With the manufacturer аПи Written ІПІО a Sales СОП – tract. Often the “customer” will be the military forces, which will invite various competing manufacturers to respond to a “Request for Proposal” or RFP. Less often, the manufacturer will risk its own resources to develop a new design in anticipation of a production contract. In the design of the new helicopter, performance guarantees will be made to the customer based on various agreed metrics such as hover capability, payload, range and endurance, and cruise speed. In addition, the performance of the machine with one engine inoperative may be included in the guaranteed performance. Methods for determining compliance with the specified performance by means of predictions, analyses and flight testing will be detailed in the contract. Because any failure of the manufacturer to achieve the negotiated perfor­mance may result in substantial cost and other penalties, the manufacturer needs to have high levels of confidence that the performance guarantees can indeed be met.

The basic procedure that a manufacturer will follow in establishing a performance guar­antee is based on statistical confidence levels of results obtained from both mathematical models and flight tests. A good summary of some of the key flight testing activities con-

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Confidence values can be assigned to predictions based on the established accuracy of a given mathematical model. Typically, most models will have been in use by the manufac­turer for some time and will be well-validated based on correlation with idealized laboratory experiments, ground and wind tunnel tests of sub – and full-scale rotor systems, as well as flight tests. This will allow the manufacturer to establish good statistical bounds on the confidence levels for the predictions. For example, methodologies validated with reference to a prototype or a similar helicopter will allow a high confidence level to be accessed. In contrast, a completely new helicopter with an advanced rotor design or new blade tip shape may have more uncertainties in the design, and confidence levels in any predictive

methodology will be lower. This, however, is where the benefits of fundamental research and development become useful, and significant payoffs can be realized by producing a much more advanced and competitive helicopter design.