Summary of Civil Aircraft Design Choices

This section summarizes some of the information discussed in Sections 4.5 through 4.10. Readers will have a better appreciation after completing the sizing exercise in Chapter 11. The seven graphs shown in Figures 4.5 through 4.11 capture all the actual aircraft data from the Jane’s All the World’s Aircraft Manual and other sources (acknowledged in the preface of this book). These statistical data (with some dispersion) prove informative at the conceptual design stage for an idea of the options that can be incorporated in a new design to stay ahead of the competi­tion with a superior product. It is amazing that with these seven graphs, the reader can determine what to expect from a basic customer (i. e., operator) specification for the payload range. Readers may have to wait until their project is completed to compare how close it is to the statistical data, but it will not be surprising if the coursework result falls within the statistical envelope. Civil aircraft layout method­ology is summarized as follows:

1. Size the fuselage for the passenger capacity and the amenities required from the customer’s specification. Next, “guesstimate” the MTOM from Figure 4.6 (i. e., statistics) for the payload range.

2. Select the wing planform area from Figure 4.9 for the MTOM. Establish the wing sweep, taper ratio, and t/c for the high-speed Mach-number capability.

3. Decide whether the aircraft will be high wing, midwing, or low wing using the customer’s requirements. Decide the wing dihedral or anhedral angle based on wing position relative to the fuselage. Decide the twist.

4. Guesstimate the engine size for the MTOM from Figure 4.10. Decide the num­ber of engines required. For smaller aircraft (i. e., baseline aircraft for fewer than 70 passengers), configure the engines aft-mounted; otherwise, use a wing – mounted podded nacelle.

5. Estimate H-tail and V-tail sizes for the wing area from Section 4.5.6.

The industry expends enormous effort to make reality align with predictions – it has achieved performance predictions within ±3% and within ±1.5% for the big aircraft. The generic methods adopted in this book are in line with the industry – the difference is that the industry makes use of more detailed and inves­tigative analyses to improve accuracy in order to remain competitive. Industry could take 10 to 20 man-years (very experienced) to perform a conceptual study of mid­sized commercial aircraft using conventional technology. In a classroom, a team effort could take at most 1 man-year (very inexperienced) to conduct a concise con­ceptual study. There may be a lower level of accuracy in coursework, yet learning to design aircraft this way is close to industrial practices.

It is interesting that no two aircraft or two engines of the same design behave identically in operation. This is primarily due to production variances within the

manufacturing tolerance allocations. The difference is minor: The maximum devi­ation is on the order of less than ±0.2%. An older aircraft would degrade in per­formance: During operation, the aircraft surface would become deformed, dented, warped, and/or contaminated, increasing viscous drag, and so forth. Manufacturers consider actual problems of operational use by maintaining a record of performance of all aircraft produced. Manufacturers’ comments cover average aircraft degrada­tion only up to a point. In other words, like any product, a brand new aircraft gener­ally would perform slightly better than what is indicated in the pilot’s manual – and this margin serves the operators well.

If a new design fails to reach the predicted value, who is at fault: Is the shortcom­ing originating in the aircraft or the engine design or from both? Is it a bad aircraft or a bad engine (if a new engine design is incorporated)? Over time, the aerospace industry has successfully approached these issues. As mentioned previously, some aerospace stories could be more exciting than fiction; readers may examine some old design cases. Today, engine and aircraft designers work cooperatively to identify the nature of and then repair shortfalls. In general, it is convenient for the shaping of external nacelle mould lines to be the responsibility of airframe designers and the internal shaping (i. e., intake duct and exhaust duct) to be that of engine designers.

The compressibility effect of the airflow influences the shaping of an aircraft. Airflow below Mach 0.3 is nearly incompressible – in a regime, all aircraft are propeller-driven (i. e., piston engine). From Mach 0.3 to Mach 0.6, the compressibil­ity effect gradually builds up; however, turboprops are still effective up to Mach 0.5. Above Mach 0.6, the aircraft component geometry caters to compressibility effects. Jet propulsion with reactionary thrust becomes more suitable above Mach 0.6. Therefore, the aircraft component configuration is divided into two classes: one for flying below Mach 0.5 and one for flying above Mach 0.6. A carefully designed turbo­prop can operate at up to Mach 0.6, with the latest technology pushing toward Mach 0.7. Lifting-surface geometries are those that are affected by compressibility. The fuselage being cylindrical (i. e., axi-symmetric) makes is easier to address the com­pressibility effect.

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