POLAR CALCULATION BY COMPUTER

The widespread availability of micro computers has rendered the working out of aircraft polars very much easier and software is on the market which enables the model flier without much aerodynamic knowledge, or mathematics, to produce a polar curve in a very short time. Before using such software it is wise to investigate the basis of the calculations incorporated in the programming. From the description given above, it is obvious that for a full computation some fairly sophisticated computer programming is necessary, with interpolation from wind tunnel results and allowances for wing taper, planform, and other factors. Much of this information may have to be fed to the machine by hand from the keyboard and the work involved in this is not negligible. If the software package does not call for such input, and if the time taken for the results to appear is very brief, the chances are that the programming is not in fact very thorough and the results will at best be crude in proportion. A program which does the task properly is likely to be quite costly and may take an appreciable time to run on the computer, as well as demanding more attention from the modeller using it

10.8 LIMITATIONS OF COMPUTED RESULTS

Even when the computer has been correctly programmed, the user should not expect the results to be correct in an absolute sense. That is, if the best L/D ratio is calculated at 1:20 at 35 m/s, it is very unlikely that these figures will be achieved exactly in flight There are always too many imponderables such as wind tunnel errors, faults in model construction and finish, variations of engine power, etc. which render the results more or less doubtful. What may be safely inferred from the calculations is that comparisons will remain valid. In other words if the computer indicates that this or that aerofoil or wing planform will yield an improvement in performance compared with another, this will probably be true and will show up in flight. The actual achieved L/D ratio or top speed may not be as calculated, but there should be an improvement if the new wing is built, finished and flown to the same standards of accuracy as the old one.