[WIP] airfoil workbench

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Chris_G
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Re: [WIP] airfoil workbench

Post by Chris_G » Tue Dec 17, 2019 9:56 pm

hammax wrote:
Tue Dec 17, 2019 3:40 pm
Chris_G's CurvesWB has a tool "DiscretizedEdge" with different parameter options.
One of them is curvature controlled with a special algorithm (???) by angular and curvature properties.
I simply exposed to the user what is offered by FreeCAD Edge.discretize() python API.
And this angular and curvature discretizing algorithm is offered by OCC :
https://www.opencascade.com/doc/occt-7. ... ction.html
hammax wrote:
Tue Dec 17, 2019 3:40 pm
How to harvest those points? => PointsWB => convert to points => export points
The "discretized_edge" object has a hidden Points property that contains all the points as a vector list.
In FreeCAD 0.19, if you right-click in the property view, you can enable "Show all", and you'll see all the hidden properties.
show_all.jpeg
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montagdude
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Joined: Tue Jan 08, 2019 9:04 pm

Re: [WIP] airfoil workbench

Post by montagdude » Wed Dec 18, 2019 12:04 am

Zolko wrote:
Tue Dec 17, 2019 11:16 am
looo wrote:
Tue Dec 17, 2019 10:29 am
A different number of poles for each airfoil of a wing needs some recomputing of the splines to generate spline-surfaces
yes, this is a good point: fitting coordinates so that all airfoils from different families are treated equal in FreeCAD is a good idea.

looo wrote:
Tue Dec 17, 2019 10:29 am
So the user selects a start configuration and the optimizer should find a better design.
I think you're joking: you pretend to make a tool with which any random user can make better airfoils than the Seligs and Epplers and NACAs and RGs and HQs and Hepperles have done ? An optimizer only optimizes what the user has input, it's not some magic that will always give good results.
My airplane used a custom airfoil produced by an optimizer.

http://forum.freecadweb.org/viewtopic.php?t=39790

Besides being able to tailor the design to specific operating conditions, another advantage is that I could fix the trailing edge thickness based on the wood that would be used to build it. Of course, that's not to say that anyone could just push a button and end up with something good. It's garbage in, garbage out, and there are a lot of potential pitfalls that require experience to avoid. In the end, an optimizer is just another tool that may be useful for certain purposes.
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looo
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Re: [WIP] airfoil workbench

Post by looo » Wed Dec 18, 2019 7:41 am

montagdude wrote:
Wed Dec 18, 2019 12:04 am
Besides being able to tailor the design to specific operating conditions, another advantage is that I could fix the trailing edge thickness based on the wood that would be used to build it. Of course, that's not to say that anyone could just push a button and end up with something good. It's garbage in, garbage out, and there are a lot of potential pitfalls that require experience to avoid. In the end, an optimizer is just another tool that may be useful for certain purposes.
Yes, I agree. If you don't know what you are doing, better to use some airfoil available. Also most of the time optimisation doesn't make sense as the differences between real airfoil and the airfoil used for the computer design differ too much.
Std Wings which have basically the same conditions for every sections (despite induced aoa at the wing tip) are something already optimized to a point where not much gain in performance can be expected.
One use case I have in mind is a horten wing. These kind of wings have different aoa at every section. So it makes sense to use also different airfoils at each section. And at the wing tip some airfoils can help to reduce the negative yaw momentum (at least this is what I experienced with 3 prototypes.) As this airfoil is acting at low aoa you might want to optimize towards low drag at low cl and high drag at higher cl's. The problem with negative yaw momentum is introduced when the airplane rolls which leads to higher aoa at the side which moves downwards. This leads to higher lift pointing forward compared to the center of the wing. The same thing happens on the other side but in the opposite direction. This results in the effect of negative yaw momentum.
There are some ways to fight this. Negative aoa at the wing tip is what works best but is not good for performance. Using winglets or find is the traditional way (tailless wings). Optimizing the tip airfoils drag to introduce positive yaw momentum might help to reduce the negative aoa needed at the wing tip.
So that's my motivation for the optimizer. I know what worked best in reality (testing 3 different airfoils). So the start geometry is known. Looking at the conditions when a turn is introduced it might be possible to find a better design which helps to reduce the geometric twist (performance) or decrease the measure of stability (higher max lift).
please help with my conda-packaging efforts: https://liberapay.com/looooo/
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