OUTLINE OF STEPS IN THE PRELIMINARY DESIGN PROCESS

Although every preliminary design team will have a different process for achieving its goals, the following steps are typical and can be used as a guide.

1. Guess at the gross weight and installed power on the basis of existing helicopters with similar performance.

2. Estimate the fuel required using a specific fuel consumption of 0.5 lb/ h. p. hr for piston engines or 0.4 Ib/h. p. hr for turbines applied to the installed power.

Fuel = sfc x h. p.installcd x Mission time

3. Calculate the useful load:

U. L. = crew + payload + fuel

4. Assume a value of the ratio U. L./G. W. based on existing helicopters and trends. Use Figure 10.1 for guidance.

5. Estimate gross weight as:

U. L./G. W.

and compare this value with the original estimate. Modify the estimate of installed power and fuel if the two gross weights are significantly different.

6. Assume a disc loading at the maximum allowable value or at the highest deemed practical, and lay out the configuration based on the rotor radius corresponding to this disc loading and to the estimated gross weight.

7. Make first design decisions for main rotor tip speed, solidity, and twist based on maximum speed or maneuverability requirements.

Year FIGURE 10.1 Historic Trend of Ratio of Useful Load to Gross Weight

14. Continue with layout and structural design. Modify group weight statement as the design progresses.

15. Make detailed drag and vertical drag estimates based on drawings and model tests if possible.

16. Maintain close coordination between the team members to ensure that design decisions and design compromises are incorporated in the continuing updating of the various related tasks.

If the new design is to be fairly conventional, the first thirteen steps can be programmed on a computer to yield a good starting configuration even before the designer puts a sheet of paper on his board. Figure 10.2 slightly modified from reference 10.1 shows one such scheme.

To find the design gross weight, the input gross weight is varied and the program is used to calculate the weight of fuel required to perform the mission and the amount of weight available for fuel from the equation:

Fuel = G. W. – (E. W. + Payload + Crew + Unusable fuel)

FIGURE 10.2 Block Diagram of typical Computer Program for Initial Steps of Helicopter Preliminary Design

The gross weight that makes the fuel available equal to the fuel required is the design gross weight as shown in Figure 10.3.

As a fallout of this process, the difference in the slopes of the two lines of fuel weight versus gross weight yields the growth factor—the change in gross weight that is forced by a 1-pound increase in the payload or the structural weight. The growth factor, G. F., is:

where the two slopes are taken at the design gross weight. The denominator is always less than unity, so the growth factor is always greater than unity. Determining the magnitude of the growth factor in preliminary design gives the engineers an indication of the feasibility of their design. A growth factor of over 2 is an indication of serious trouble at this stage. After the helicopter is built, of course, a 1-pound increase in payload or structural weight is accepted as a 1-pound increase in gross weight, with a corresponding decrease in performance.

THE MAIN ROTOR

Although a program like the one just outlined is very useful, with or without a computer, it cannot be used to make all the necessary engineering judgments. The selection of some of the configuration parameters is influenced by considerations

not easily quantified. For this reason, it is necessary to discuss the individual parameters and their qualitative effect on performance, size, weight, cost, and operational suitability. As a backup to these discussions, a tabulation of configuration parameters for a number of current helicopters will be found in Appendix B.