Diamond Airfoils

Gang --

Round-nosed and straight lines connecting the LE to the top and bottom spar used to be used some. These were called "diamond airfoils". Anyone have any experience with these? Any tips? I can't think of specific examples in terms of a name of a design, but I know they existed.

My memory is that at anything over 12% or so, the separation of airflow was not that much different between a "real" airfoil and a diamond airfoil. That is, the separation of laminar flow to turbulent flow was right similar with all thick airfoils, regardless of how pretty the curves or straight lines.

(Edit to delete odd half sentence 6/24)
Larry F

Although no one was particularly encouraging, think I'll try it anyway. Any airfoil over 10% has big burbles, ya think? Big diameter LE, and here we go.

Your memory of the performance of diamond airfoils is correct, but like all things aerodynamic there are several caveats. Way back in 1963 two modelers used the wind/smoke tunnel at Princeton to research model airfoils in order to better understand surface turbulation at low Reynolds numbers, and though they were looking toward free-flight relevancy they tried airfoils in the diamond configuration.

As you might assume, separation occured at the high point. This was similar to the results from testing many conventional NACA airfoils. The big surprise was that the diamond airfoils also achieved reasonable critical angles of attack (stall angle)as long as the Reynolds number was very low. Even a small increase in Rn, though, resulted in airfoils that stalled at even a few degrees angle of attack. I suspect this poor stall resistance caused the lack of 'turn' of the combat planes using it. Using a rounded LE should raise the critical angle a few degrees.

By the way, that same study at Princeton proved a few significant things about TE shape too. As many suspect, a sharp TE has no value whatsoever in reducing drag....as a model-size airfoil never achieves laminar airflow that far back. The study concluded that blunt, rounded TE's were very draggy, but that a flat TE of about 1/16" for a 6" airfoil was as good as any.

Bill

But, as airfoils go, round nosed foils with straight lines from the high point back, are not Diamond, rather were modified combat airfoils which gained popularity during the 60's and can still be found in use today.

Most of them were developed for ease of building a straight wing in a hurry. The big advantage being they do not need the use of a jig, just a flat surface. They seem to fly as well or better than any other 'foil, so the building advantage helps a lot.

The recently released Pathfinder Profile from Brodak's uses such an airfoil for a stunt ship. It always has used this foil. It was chosen because of the performance on Gordan Delaney's Challenger series of combat ships. The Challenger is and was a great combat ship, even winning the first "Bladder Grabber" in the hands of Jom Womack.

What I most appreciate with this style of foil is the ease of building without the need for any jigging as long as you have a flat surface. As attested by the designs competition record, the foil works very well in a stunter, and for a low time builder who pays attention, straight wings are always the result. If attention is also given to the alignments, weight, and balance, a good performing plane is also always the result.

The constant chord wing does suffer slightly with bouncing in high winds, but that's easily controlled and offset by the ease of building compared to a more complicated wing for the less experienced builders.

Best, John