From Model Builder, February 1989, Volume 19, number 205, “R/C Soaring” column by Bill Forrey, pages 41, 86 and 88 The following input is provided by Fred Hacke of Bethalto, Illinois.

His letter briefly hits on an idea that has been successfully tried many times by modelers over the years. The idea is to give the flat-bottom airfoil Paragon sailplane (a kit from Pierce Aero Co.) a boost in performance by decreasing its mean camber line to a more modern Eppler-like airfoil. This improves top speed, ground covering ability in wind, and perhaps even increases maximum L/D or glide ratio.

“I have enjoyed your articles for some time regarding airfoils and interpretation of data. Your efforts provide a means of education for people like me.

“I know you cannot afford to invest much time in the various questions that people contact you about, but hopefully you will be able to take a couple of minutes and make a comment on the following using the SASE enclosed.

“A couple of years ago, I discussed a modification to the Paragon win with Ed Slobod (the Paradon’s designer. ­wrf). At his suggestion, I built a modified Paragon wing. The leading edge is sheeted, and the bottom airfoil is curved up from the bottom main spar to the leading edge ­ Phillips entry. The modified wing has a thinner leading edge than the standard Paragon. I enjoy the way it flies. It has good penetration on windy days and still retains good lift on calm days.

“I am enclosing a copy of the typical cross section of the airfoil. What is your opinion regarding the modified airfoil? Does it relate to any current wing section that is being reported on at present?

“Thank you for your time and continued effort on the articles you write.” My thoughts about this modification are mostly positive. I have seen many thermal competitors use this type of modification to good effect here in California. The Paragon in its stock form in my opinion is the best trainer, best-handling sailplane, best all-around light air thermal ship ever designed. It has a name that really fits. Ed Slobod has been trying for years to come up with a thermal design that he likes better than the Paragon, but he has not yet found it.

The reason so many guys like to modify the Paragon is they like the way that it flies, but they need a sailplane that is more aggressive in searching for lift. The stock Paragon is not a very fast ship. The Phillips entry approach cuts down the lift and the drag that the wing produces making it faster. Most flat bottom designs are producing much more lift than is really necessary anyway, so the decreased total lifting power of the resultant semi-symmetrical airfoil is not really missed, but the decreased drag results in a faster, flatter glide which is desired.

I went through my Eppler book, MTB 1/2, and tried to match up a similar Eppler section to this modified Paragon airfoil. The nearest I can come up with is either a E195 or E197. Both have good thermalling ability, wide drag buckets, and gentle stalling characteristics, but also have higher profile drag because of their greater thickness than the more common E193 or E205.

I have redrawn with pen and ink the pencil-sketched ribs that Fred has supplied for the reader’s benefit. Not all of the Paragon’s ribs are drawn because wing tips ribs W-10 through W-19 are the same design as W-9 with decreasing chord lengths.

The Paragon’s relatively thick (12-percent) flat-bottom airfoil lends itself easily to this kind of modification. Other similar kit sailplanes may also benefit from the Philips entry mod. However, I don’t believe that flat-bottom sections less than 10-percent thick have as much to gain from this procedure. They are already low enough in mean camber that they already produce less lift, and their thickness is such that their drag values are already pretty low. Also, if not done carefully and minimally, the sharp leading edge that would result from trying to keep the same wing chord would produce a sharp stalling section that would not be as much fun to fly.