Wednesday, June 11, 2008

Comparing different configurations and plotting fuselage cross sections

Each configuration is a different compromise. I have been thinking hard which would work out the best. This may need to be proven to do a design for all the different alternatives as follows:

1. Laminar body fuselage with prop in rear. Boom tail. Front free of protruding elements until the laminar-turbulent transition point. Rotax 914 might fit into the rear of a rotated NACA 66-030 with no (or at least not long) extension shaft needed.
2. Laminar body fuselage shape with prop in the front, potential for laminar flow lost because of the prop disturbing air in the front. Like Stemme S6.
3. Laminar body fuselage with prop in the rear of the tail. Requires extension shaft which is structurally challenging.

Each design would need to be identical (fuselage pod length in Reynolds number should be equal) and the objective would be to investigate which one produces best compromise for low drag and is structurally the best solution (without unacceptable risk of in-flight failing parts (extension shaft in any circumstances must not fail)).

Measuring the difference actually is quite difficult because of the difference in the Reynolds number of a model aircraft and a full size aircraft because it affects quite heavily the laminar low drag area and where the transition to turbulent flow occurs. Also airfoil which is proper for full size aircraft would not work on a model. The NLF414F I discovered earlier does not work with low Reynolds number, it has nasty stall characteristics with low Reynolds number.

What interests me most in this is that how much drag the two tail booms would add. Would the penalty be more than the benefit of achieving laminar flow in the forward fuselage? Is the extension shaft the only way to achieve laminar flow without sacrificing the benefit?

I have been thinking possible concept for a model: try out the boom tail configuration as specified above. Fuselage would be rotated NACA 66-030 with propeller in the rear. Wortmann FX38-153 profile might work with the target Reynolds number range (the wing span and fuselage length would be determined by the interior size of our car, must be able to be disassembled to a size that fits inside for transportation, using a trailer for moving a model aircraft would be overkill). Target aspect ratio could be around 12-14 for main wing. I haven't done any calculations yet though.

I want to also create a plotting program for the fuselage. Martin Hollman's book has a Basic language program listing for a such thing. I am not sure if it is useful actually, I have been thinking how to parametrize a fuselage cross section (often it is not circular but rather boxy with rounded corners or it might have entirely different airfoil shape in horizontal and vertical axis), how to modify the shape of the centerline where the fuselage cross sections are referenced to and how to make the cross section follow a airfoil coordinates, possibly using the same data files that work with X-foil. Making circular or elliptical (LH-10 cross section for example seems to be elliptical) cross section plots from nose to tail for a rotated airfoil wouldn't be that impossible task to do and visualization could be even quite reasonable to do with OpenGL. Before doing the visualization, I however, need to determine how to parametrize it, in other words, how to make it easy to produce differently shaped fuselages. Rhino3D does all this, but I don't have Rhino3D, and this task is not that complicated, it should be doable with some little C++ work.

Any advise on the math and how to make the fuselage design easy would be great, feel free to add comments if you invent something or know something already.

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