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Old 12-11-2016, 10:35 AM
Join Date: Oct 2004
Location: Arroyo Grande, CA
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In ibuild's defense, I should state up front that my post #14 was incomplete. I started to include "the rest of the story", but decided that the post was already longer than desirable and stopped with the first half of the story. If you'll bear with me, I'll try to round out my thoughts now.

Measuring airfoil characteristics by use of wind tunnels goes back at least to the days of the Wright Brothers. This was the the era of cut-and-try, in which candidate shapes were built mostly by eye and then performance checked in a wind tunnel. Unfortunately, the small size and low speed of the Wright's tunnel wasn't accurate in predicting the performance of their larger and faster full scale aircraft.

Ludwig Prandtl (in the WWI time period) began to explain the scaling effects of size and speed, and opened our eyes to the need to test in the regime that the full size airplane would be flying.

As research marched on between WWI and WWII, people gained much greater understanding of airfoil technology, always with an eye to gaining efficiency and going faster. Then came the challenges of transonic flight and supersonic flight. Newer wind tunnels were optimized for the problems of the day in full scale aeronautics, leaving our model airplane regime far behind.

In more recent times, Computational Fluid Dynamics has come along to augment experimental wind tunnel testing. CFD isn't perfect, but it does facilitate study of airfoil shapes much less expensively than construction and operation of a dedicated wind tunnel just for the benefit of science in the model airplane regime.

Only in recent times have some world class engineers (Dr. Michael Selig immediately comes to mind) come along to combine their talents and interests with tools like CFD and wind tunnel in their study of low speed aerodynamics.

Going back now to the Original Poster's question, I would suggest that if you want to taper from one root airfoil to a different tip airfoil to improve stall behavior, then Dr. Selig's work would be much more useful than reliance on wind tunnel data from the 1920's or 1930's. As an example, it's my understanding that the root and tip airfoils for the Top Flite P-47 were recommended by Selig with the specific intent of making a model with gentle stall characteristics.

If you prefer to use one of the early airfoils such as 2415 or even Clark Y, then the time honored wing twist method will probably give your more predictable results. Twist is pretty much a slam-dunk solution to wing drop in the stall. And don't be bashful about a little more twist. About 20 years ago I built a Comet 54" Taylorcraft for SAM 1/2A scale competition. I covered it with silk and, unfortunately, the wing developed a nasty warp on one side. I tried all the usual tricks to remove the excessive twist, but to no avail. In desperation I finally warped the opposite wing to match, assuming that the model was a throw-away at that point. But the model flew very nicely and went on to win many contests. And, boy, was it stall proof. How much was the twist ? 10 degrees. Go figure.

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