Use of Semi-empirical Relations

DATCOM (U. S.) and RAE data sheets (U. K., recently replaced by ESDU) have served many generations of engineers for more than a half century and are still in use. Over time, as technology has advanced, new tools using computer-aided engi­neering (CAE) have somewhat replaced earlier methods.

Semi-empirical relations and graphs cannot guarantee exact results; at best, error-free results are coincidental. A user of semi-empirical relations and graphs must be aware of the extent of error that can be incurred. Even when providers of semi-empirical relations and graphs give the extent of error range, it is difficult to substantiate any errors in a particular application. Other methods could provide better results.

If test results are available, they should be used instead of semi-empirical rela­tions and graphs. Tests (e. g., aerodynamics, structures, and systems) are expensive to conduct, but they are indispensable to the process. Certifying agencies impose mandatory requirements on manufacturers to substantiate their designs with test results. These test results are archived as a databank to ensure that in-house semi­empirical relations are kept “commercial in confidence” as proprietary information. CFD and FEM are the priority, before semi-empirical relations and graphs. The consistency of CFD in predicting drag (see Chapter 14) has to be proven conclu­sively when semi-empirical relations and graphs are used extensively. This also is true for weight prediction.

This book does not include many of the DATCOM/ESDU semi-empirical rela­tions and graphs. Inclusion will prove meaningless unless their use is shown in worked-out examples. Typically, their use during conceptual studies can be post­poned until the next phase of study (see Chapter 2), which is beyond the scope of this book. It is important for instructors to compile as many test data as possible in their library of resources.