When my KR-2S project was in the "boat stage" in

When my KR-2S project was in the "boat stage" in 1994, I became interested in finding a better airfoil for the design. I was helping designer Larry French with his Lionheart prototype, a modern composite staggerwing airplane. This gave me a good feel for the design process, and I started thinking about doing a stability analysis on the KR-2S, a plane famous for its pitch sensitivity. Finding precious little design data (one page) on the KR's RAF48 airfoil, I called Stuart Robinson, who designed the KR-1 and -2 with Ken Rand in the 1970s, to see what information he had. Stu laughed and said he and Ken never had any design information on that airfoil. In building the KR-1 they borrowed heavily from the Taylor Monoplane, including its RAF48 airfoil. This when 1 decided to ditch the RAF48. At about the same time I began an e-mail conversation with an aerodynamicist who frequented the Internet newsgroup, rec.aviation.home built. He volunteered to feed the RAF48 into the "Eppler code," a computer program that extracts lift and drag coefficients (C\ and C D, respectively) from a set of airfoil coordinates. He reported that "the experimental data differs greatly from the theoretical results." It seems the Eppler code didn't match the C L and C D data from the one-page chart I had, and he didn't quite know what to make of it. He said the wind tunnels of the RAF48's era (the 1920s) were pretty bad, and he trusted the Eppler code more. He recommended that I find an airfoil with more accurate 71

numbers and pointed me toward a new airfoil, the NLF(1)0115. The NLF(1)0115 was designed for aircraft flying at Reynolds numbers higher than the KR's, but I had data for this airfoil and I was planning on going pretty fast myself. Larry French fed the NLF's data into his extensive stability analysis program, and out popped the incidence angles for the wing and tail and the expected cruise and stall speeds, among other things. Armed with this information I started building my wing spars. At this time we KR builders were corresponding by e-mail through AOL's KR forum. (KRNet, at www.kr net.org, is where we communicate now.) Having finished my wing tanks, I was singing the praises of my new lower drag wing. After reading my e-mail, Steve Eberhart decided to do a little design work of his own. He asked why I was using an airfoil that wasn't a perfect fit for the plane and why I didn't ask the NLF's designers to create an airfoil specifically for the KR-2S? Before I could answer, Steve said he lived near the University of Illinois at Urbana- Champagne (UIUC), home of the airfoil's designer. "Knock yourself out," I said. Not exactly holding my breath for his success, 1 did stop building my wing, just in case. Steve met with Dr. Michael Selig, who designed the NLF and many more laminar flow airfoils (and the man responsible for the very extensive UIUC airfoil site at http://am ber.aae.uiul.eiht/~rn-selig/ails.htrnl). He said the NLF(1)0115 would work fine on the KR-2S, but he'd really like to run a wind-tunnel test on a model to make sure the calculated data matched reality. Selig was particularly concerned with how bug guts and rain would affect the lift the airfoil produced at stall speed. The University of Illinois has its own wind tunnel, but it stays booked for nearly a year in advance, doing airfoil research for the NASA Glenn Research Center, AeroVironment, Ford Motorsports, Farr Yacht Design, and others. Members of KR- Net had cooperatively financed a few other minor projects, and Steve thought maybe there were enough of us to finance the wind-runnel project. Within 24 hours of posting the 72

call for contributions on KRNet, we had $1,100 pledged for the windtunnel tests! Selig said that would do just fine. Other KRNet contributors paid for the carbon fiber, epoxy, and other materials needed to build the NLF(1)0115 wind-tunnel test specimen, KRNet's John Roffey machined its special 4130 spars, and Steve Eberhart built it. DOCTORAL DESIGN About the time Steve finished the tunnel specimen he visited Selig again. He learned that Selig had a doctoral candidate specializing in airfoil design who was looking for a fun project involving a general aviation aircraft. Designing an airfoil especially for the KR-2S would be just perfect! When he met the grad student, Ashok Gopalarathnam, Steve knew things would go well. Ashok was wearing an EAA T-shirt, was a pilot, and had worked on the design and development of light aircraft in India. (And in 1998, after the KR-2 airfoil project, Ashok took a summer job with Scaled Composites where, among other things, he designed the airfoils for Burt Rutan's new Boomerang II.) For the KR-2S Ashok first designed a 15-percent AS5045 airfoil, using a 180-mph cruise speed and a weight of 850 pounds. The preliminary iterations for this airfoil (the GA19980222A) were displayed on the web. Joining them were comparisons of efficiency and speed improvements over the RAF48 and NLF(1)0115 used in the same KR-2S application. These comparisons are not trivial, and the proof is displayed in detail at http://amber.aae.uiuc.edu/~ashok/ kr2/airfoils/. Make sure you visit the links at the top and bottom of this site; they include airfoil refinements and actual wind-tunnel test results. While much of the data is for the GA1998222A, data for the AS504x series is improved even more. Figure la compares the characteristics of both the AS5045 and the RAF48, and Figure Ib compares the power requirements for equivalent performance from each wing. Participating in the KRNet's airfoil dialogue, Dr. Richard Mole (Britain's Tony Bingelis) had this comment: "Since the C^ of Ashok's airfoil is so low, it would be worth trying for a thicker 18-percent airfoil to get the low drag advantages, but also because the increment in C L max due to flaps is also an increasing function of airfoil thickness, and the spars could be made lighter and stronger. Presto, an 18-percent AS5048 appeared! Being an avid CAD fan, I promptly superimposed the 15-percent airfoil onto the stock spars and determined that we needed a 16-percent airfoil for a truly perfect fit. A few days later we had the 16-percent AS5046! In Seattle, Mark Lougheed, a boat designer and mathematician, fed the new airfoil coordinates and characteristics into a highly massaged CFD (computational fluid dynamics) program, attached to a 3-D CAD model of a KR-2S. The results of his analysis, in conjunction with his stability analysis, pegged the wing and horizontal stabilizer incidences required for maximum efficiency and stability. This analysis also resulted in some spectacular images, like Figure 2, the AS5046 approaching stall at a 16-degree angle of attack. Naturally, we'd need a wind-tunnel test to verify the computer predictions, so we decided we shouldn't waste our precious tunnel time on the NLF and should test the AS5045 and AS5048 sections instead. Because our tunnel time was quickly approaching, Steve quickly ordered another set of spars from John Roffey and cannibalized the NLF specimen for its specially machined 4130 spars. The AS5045 test specimen would bring to tears a body man who specializes restoring fine old automobiles, absolutely perfect and in beautiful carbon-fiber black. In the tunnel the test specimens proved the results predicted by the computer analysis. A new airfoil was born! All we needed was a test pilot... AIRFOIL TEST PILOT After a forced landing on a farm road, Troy Petteway of Columbia, Tennessee, needed to build some new wings for his KR. When the Sun Race winner heard "higher speed," he started building the AS5046 airfoil on the stock KR-2 spar. A corporate pilot who flies a Citation Encore, Troy also is an airframe and powerplant mechanic and certificated flight instructor. He first flew the new wing in July 1999 and reported that his KR climbed and cruised at a higher speed, even without its wheelpants. The airplane was also "10 times 73

more stable" than it had been with the stock wing and the exact same center of gravity. But some of the improved stability no doubt came from increasing the horizontal stabilizer length 4 inches per side and from reducing elevator area. These improvements came without penalty. The KR's stall speed remained the same, with the power-off stall coming at 48 knots. It was gentle with no tendency to drop a wing. The increased climb-out speed (100 mph rather than the previous 80 mph) not only got the airplane to a safe altitude more quickly, it increased engine cooling at the time it needs it most, at full power. Another benefit the new wing provided, but no one had really noticed on paper, is that the airfoil actually has a slightly higher drag than the RAF48 at high wing incidences, such as landing and takeoff. This may not sound like an advantage, but when the KR is a few feet off the runway in ground effect, this increased drag is a welcome brake that reduces float. It increases the takeoff run slightly, but a KR's takeoff run is always two or three times shorter than its landing distance. Troy's plane is more stable and faster than before, but it's still not optimal. He should probably reduce his tail incidence slightly, and he hasn't put the KR's wheelpants back on yet, so it can only get better! As of January 2001, Troy has flown more than 100 test hours and was still quite happy with the new airfoil. He unintentionally flew it in a moderate rain shower and noticed no difference in flying qualities. At a 1999 KR gathering, Ashok was the keynote speaker at the "new airfoil" forum, where he explained how the laminar airfoil had been tested with "trip strips" in the wind tunnel to simulate the effect of rain and bug guts on the leading edge. Test results matched the theoretical calculations, and there was very little effect. ; Troy recently installed a stock C-85 engine in his plane, which gives him a straight-and-level top speed of more than 200 mph. He's convinced that the new airfoil performs better than the RAF48 it replaced in all respects, including fuel economy, because of decreased drag at high speeds. NEW WINGS FOR You? If you're already flying a KR, tearing the wings oft it to use the new airfoil doesn't make sense unless, of course, you need to replace your wings for some reason. But if you're still in the boat stage or haven't started building your wings, now is the time to consider the new airfoil. The new wing's planform and construction methods are exactly the same as the plans call for only the airfoil template shapes are different. You don't need any new plans, just the templates for the new airfoil (see http://hotne.hiwaay.net/ ~langford/as504x.html for airfoil coordinates and template source). The biggest construction difference is that you need to raise the aft spar with respect to the fuselage to lower the wing incidence. This makes the new airfoil "happy" and puts the fuselage in level flight at cruise 74

speed, rather than nose down like the stock KRs. You can use the new airfoil three different ways. For new construction, using the 18-percent AS5048 at the root and tapering to the 15-percent AS5045 at the tip works best. This uses a main spar that is 8.19 inches tall at the root, making it 17- percent stronger than the stock spar. It also increases the wing tanks' capacity almost 20 percent. The materials required are the same as the plans call for, but the vertical spacers between the caps are slightly longer. If your spars are already built, the 16-percent AS5046 is almost a perfect match, requiring only a lamination of 1/8-inch spruce to the upper main spar cap to bring it up to proper dimension. One other change you should make is to reduce the horizontal stabilizer incidence to about -.75 degrees. I'll tell you for sure after my plane flies. It has an adjustable horizontal stabilizer, so 1 can "nail" that number. It will obviously work at zero, because that's where Troy's incidence is set, but that's probably not optimal. There are no wing skins available for the new airfoil, so you'll have to build your wings the old-fashioned way. I built my own skins for the AS5046 airfoil because my spars were already built to stock dimensions. (You can see them on my "Outboard Wings" construction page at http://home.hiwaay.net/~langford/kmarkl.html.) They are about the same weight as plans-built wings, but the skins are several times stronger, even though I'm using very large flaps. This airfoil owes its origins to KR- Net and several folks working for a common goal. Were it not for the Internet, it would have never happened. And they say there's nothing on the web but smut! [Editor's Note: All the websites and links listed in this article are available on the EAA webaite at www.eaa.org. Click on the EAA Sport Aviation cover. Scott.] 75