Friday, 2 May 2014

Better airfoils using 6mm depron

After the poor performance of my first 2m motor glider and great performance of the slim wing and symmetrical wing I though it was time to investigate airfoils more thoroughly.

My standard building materials are 6mm depron and 7.5mm Skyshark P4X spars while Experimental Airlines and Flite Test use 5mm Dollar Tree foam so I needed to tweak their methods for my designs.

For inspiration I looked at the Bixler 2 wing which glides beautifully, and the widely used Clark Y airfoil.

The key features are ... maximum thickness is around 11.7% of the total chord and positioned 30% back from the leading edge.

My mission was to come up with a technique to replicate these features using my materials.

Wing section construction

My standard wing building method is to cover one side of the 6mm depron with packing tape. I have used PPS and Scotch tapes and prefer the cheaper PPS.

The inside of the bottom trailing edge is tapered to 1" to conform with the top layer for gluing. I found it was better not to sand all the way down to the covering tape but to leave about 2mm thickness at the trailing edge. This allows the aileron to better blend in to the overall airfoil shape.

I make a slight dent line (with the other end of a pencil) along the inside of the leading edge position to give an accurate fold. Don't make the dent too deep or you will have a sharp leading edge.

I made up airfoil sections ranging in chord length (including ailerons) from 6.5" to 9.5", spar placement 20 to 30% back from the leading edge line with 20mm formers glued either side, and zero, half or a full thickness former on top of the spar.

The combinations that most closely matched the Clark Y airfoil were:

A. 7.0" chord (5.5" plus 1.5" aileron) Spar 53mm back with no extra former
B. 8.0" chord (6.5" plus 1.5" aileron) Spar 61mm back with an extra 3mm former layer
C. 9.5" chord (7.5 plus 2" aileron) Spar 76mm back with an extra 6mm former layer

To my eye these sections looked much nicer than my previous builds and much closer to commercially produced RC gliders and planes. Depron will kink rather than bend smoothly over the formers if the curve is too great but it's not a problem with these slimmer sections.

These have become my standard wing chord designs now. I would add larger 2" ailerons for aerobatic models, especially slope soarers. Varying degrees of airfoil section symmetry can be built-in by varying how much you press the wing down flat while gluing up.

A symmetrical section gives better inverted and aerobatic performance while a flat bottomed section gives more lift and better gliding performance.

I used the B section in my next project, the wonderful 2m slope soarer.

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