Performance and Mission Calculation
In the main body of the book, methods have been described which enable the calculation of the power necessary to allow a helicopter to operate at a given weight and at a given speed. The results of these calculations, when combined with appropriate engine data, enable the fuel consumption to be determined and a mission can then be ‘flown’ in a computer. The ability to perform these calculations now enables a project study to be carried out on a proposed helicopter design. The purpose of this appendix is to collate the various theories of the momentum/actuator disc into an overarching calculation scheme. Earlier chapters highlighted the fact that momentum theories are the simplest available and, in order to make the results more realistic, a scheme of factoring will be required. The simplicity of these methods makes them easy to implement but, in addition, prevents them from being used for calculations where the rotor is operating close to any limitations of the flight envelope. For normal operations, this limitation should not be necessary and the simplicity of these methods will permit parametric studies to be performed with speed and economy. The modern personal computer, and the software available, make these methods, described in this appendix, readily implementable. In this way, an overall picture of the proposed helicopter configuration and its ability to complete a given mission can be readily assessed.
The following sections describe the practical use of these methods of determining the power required and the consequent rate of fuel consumption for a helicopter of given weight and speed. The calculations use the momentum method and, because of this, should only be used for a general investigation of helicopter performance. They are unsuitable for investigating helicopter performance when the aircraft is approaching its flight envelope. The methods, as presented, are formulated for a single main and tail rotor configuration. However, because of their inherent simplicity, they may readily be adapted for other rotorcraft configurations such as the tandem.
The description of the various methods is arranged in separate sections. Each section deals with a particular aspect of the helicopter.
1 Portions of this appendix have been taken from The Foundations of Helicopter Flight, by Simon Newman, Elsevier, 1994.
Basic Helicopter Aerodynamics, Third Edition. John Seddon and Simon Newman. © 2011 John Wiley & Sons, Ltd. Published 2011 by John Wiley & Sons, Ltd.
A summary of the components of the calculation is:
1. Using the helicopter’s drag and weight to determine the attitude of the main rotor disc and the thrust by balancing the force components.
2. Determination of the main rotor, induced, profile and parasite powers implementing the appropriate factors. These powers are summed to give the total power required to drive the main rotor.
3. The main rotor power is then converted to the equivalent torque which fixes the value of the tail rotor thrust necessary to trim the helicopter in yaw.
4. With the tail rotor thrust and forward speed now determined, again implementing appropriate factors, the induced and profile powers of the tail rotor can then be calculated (note: no parasite power).
5. The total helicopter power required can then be determined by summing the main and tail rotor powers together with that required to drive auxiliary services.
6. The losses in the transmission are then included as a multiplying factor which gives the power required of the engine(s).