How to calculate Aerodynamic Imperfections?

Hello, for the past few days I have been wondering how aerospace engineers calculate for the Aerodynamic Inefficiencies like things that cause more parasitic drag(ex - screws, bolts, bumpy surfaces, grooves, dents, etc). Is there a way to estimate the system of error when building the aircraft to tell how much drag will be added on top of the simulation before building the aircraft? For example, if you were designing an RC plane, ran a CFD simulation, and got 1 newton of drag. Now, you go to build your RC aircraft it is almost certain if the CFD simulation is accurate, that it will have more parasitic drag than the CFD simulation predicted because of Imperfections on the wings, fuselage, etc. Can someone please explain to me how to estimate for these imperfections? :thinking:

Thank you for your time, effort, and knowledge,
Frank Lucci

Hey Frank, that is a good question, tagging our @power_users here. :smiley:

Best regards,

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Hello Frank,

interesting question indeed.

Please note that I do not work for aerospace industry so it is only a guess of mine, not a firm fact. Others may correct me with actual insights.

The first thing is that in most cases CFD results have more qualitive value than quantitive. The most important is to see how the flow behaves around the body with different angle of attacks and speeds to estimate flight characteristics. Of course, an approximation of forces and moments may be important too but I think for the first calculations and validations the imperfections can be neglected. Later, if required additional wind tunnel tests or flight tests can be conducted. There should be a significant margin of thrust for airplanes to cope with any additional drags anyway.

There is an additional tool in the hands of serious aerospace companies, the data of their previous designs. It is called heritage engineering I think.
If you have a previous plane with similar size and similar assembly method (e.g. riveting), and it was measured that the actual forces were 5% more than the calculated ones, then you could simply introduce a coefficient and multiply the current CFD force with it (x1.05). This way you can estimate the performance better in advance.

Regarding RC planes, it is quite easy to achieve high thrust to weight ratios so I’d suggest to run CFD on it, and simply identify the areas where it will be “draggy” and try to optimize those areas. Also, select a fitting motor and when building, try to make the surfaces as smooth as possible.
There are a lot of great tools online helping with motor selection and basic dimensioning, I’ve designed a plane based on these rules of thumb and flies quite neat despite it looks ugly as hell. :wink:

Have a nice day,