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CFD, wind tunnel, and GA aircraft.

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Mac790

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Discussion moved from here https://www.homebuiltairplanes.com/forums/general-experimental-aviation-questions/7962-new-canard-designs-3.html#post78436

Sorry for delay with my response, but I'm very busy at the moment.

Orion said:
Since I'm the one who's said something similar in the past, maybe I should clarify.
It's true that you used very similar words in the past, but others weren't far off, that's why I wrote most people here, not some people here. Orion you said many times, that CFD is not a design tool , and I believe it's clear for everybody, but since you decided to clarify it a little bit, I have to do same :).

...it does require one's clear and in-depth understanding of its capabilities and limitations.
Of course it does require not only skills, but also extensive knowledge in many areas.

Extensive knowledge and understanding of Fluid Dynamics, (and I'm not talking about basic knowledge like what is a difference between stagnation and separation point, or what is boundary layer:)).

Knowledge about type of meshes (grids), in CFD we have structured, and unstructured types, it's important to know difference between them, and when we should use structured, when unstructured etc.

Knowledge about turbulent models, like k-epsilon, Spalart–Allmaras, etc.

Etc, etc etc.


Furthermore, it is an analysis tool, not a design tool, although it can be an aid within the design process. And most important, it is only a tool, not a substitute for design experience.
Of course it's an analysis tool not a design tool by definition, we have CFD which means Computational Fluid Dynamics, for example softwares like Solidworks, Catia, etc, belongs to CAD software which means Computer Aided Design, so there are design softwares by definition.

If a designer has said experience it is unlikely that he needs the CFD tool to aid in the design of a GA product, unless of course he's designing something unique and/or out of the ordinary where past experience may not be sufficient and some verification is necessary to quantify the design.
As a corollary, given the amount of data out there, an experienced designer really wouldn't need a wind tunnel either. Recently I've ran into several development programs where the organization has developed a relatively conventional configuration but goes on in great lengths about the wind tunnel and/or CFD modeling they used in the process.
Yes, and no.:)
Just a few examples from our forum.

We had a discussion about blended winglets a couple of times at HBA, for example here https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/4200-another-day-another-new-idea-3.html#post29003 in theory blended winglets should make a "difference" in bigger airplanes (not necessary in small ones), but in practice even small airplanes can improve their top speed. I know that you've seen this report http://www.cozybuilders.org/Oshkosh_Presentations/2007_Blended_Winglet_Eracer.pdf but it clearly shows difference between two configurations.
Some people tried to copy the idea, pix 1,2, but I got the feeling that it would be simpler to use CFD for it.

We had also a discussion about "fastbacks" https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/5136-fastback-fuselage-shapes.html in theory plane without fastback, has smaller wetted area, so in theory should be faster, but in practice not necessary, I've seen somewhere comparison of two versions of a same plane with and without fastback , the plane with the fastback was a little bit faster, smaller wetted area in theory but probably better pressure distribution and/or potentially smaller drag in practice. There are some Reno Racers P-51 modified as fastbacks. The CFD analysis could give an answer for it.

Another example, https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/7320-optimising-cooling.html#post68532 maybe it not so important for conventional airplanes, but really important for pushers. If you take a look at some modified Berkuts/ Long Ezs, etc. Some have cooling inlets above a wing pix3, some under pix4. Both sides claims that their solution is the best, it may be another place where CFD could show usage.

The shape of the tail section for pushers, pix 5 if I remember correctly this mod gave a few mph. You can test it in flight, or you can make a few virtual configuration test them, and chose the best one.

Potential problems wind h-stab blanketed, by a wing for T-tail and not only sections https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/4204-razorback-construction-thread.html#post29774

Etc, etc.

...which essentially tells me that they didn't really know what they were doing and someone simply took their wallet for a ride.
Honestly, I don't feel sorry for people like that, maybe that taught them something. If you want hire someone for some kind of analysis, you really should know what you are looking for. In short if someone look for an answer, he need to know a question first.

Of course there is a question about costs, I would say if someone is going to hire a company for those analysis, only to gain a few miles, it doesn't make any sense, unless someone want to beat a Guinness Record in the most expensive modification in GA category:gig:.

There are also people who has access to those softwares in work for example, why they shouldn't give it a try? Special if they know what they are doing.

There are also some "cheap" programs I gave a few links here https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/5025-what-software-do-you-use-14.html#post69445 seems that one of our members use them https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/8043-jerry-blumenthals-concepts-5.html#post76331
there is also free OPEN-FOAM, I didn't try it yet, but it looks "nice" https://www.homebuiltairplanes.com/forums/aircraft-design-aerodynamics-new-technology/5025-what-software-do-you-use-14.html#post69460.

btw,
me said:
Did you notice noticeable difference between those results?
That was a rhetoric question, CFD and wind tunnel result should match otherwise it means that someone *%^ CFD analysis, but it's a common practice to verify CFD results in wind tunnel. Of course I might be wrong here, we don't have wind tunnel.

Seb

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