Virtual Creation

When my KR-2S project was in the "boat stage" in. 1994, I became interested in finding a better air- foil for the design. I was helping designer Larry. French with ...
2MB taille 15 téléchargements 212 vues

net newsgroup, rec.aviation.home built. He volunteered to feed the RAF48 into the "Eppler code," a computer program that extracts lift French with his Lionheart prototype, a modern and drag coefficients (C\ and CD, respectively) from a set of airfoil coorcomposite staggerwing airplane. This gave me a good feel for the design dinates. process, and I started thinking about doing a stability analysis on the He reported that "the experimental data differs greatly from the theoKR-2S, a plane famous for its pitch sensitivity. retical results." It seems the Eppler Finding precious little design data the KR-1 they borrowed heavily from code didn't match the C L and C D (one page) on the KR's RAF48 airfoil, the Taylor Monoplane, including its data from the one-page chart I had, and he didn't quite know what to I called Stuart Robinson, who de- RAF48 airfoil. signed the KR-1 and -2 with Ken This when 1 decided to ditch the make of it. He said the wind tunnels of the RAF48's era (the 1920s) were Rand in the 1970s, to see what infor- RAF48. mation he had. Stu laughed and said At about the same time I began an pretty bad, and he trusted the Eppler he and Ken never had any design in- e-mail conversation with an aerody- code more. He recommended that I formation on that airfoil. In building namicist who frequented the Inter- find an airfoil with more accurate

hen 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


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, is where we communicate now.) Having f i n i s h e d my w i n g 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 UrbanaChampagne (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 t u n n e l , 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 KRNet 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

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 and Figure Ib compares the power especially for the KR-2S would be requirements for equivalent perjust perfect! formance from each wing. When he met the grad student, Participating in the KRNet's airfoil Ashok Gopalarathnam, Steve knew dialogue, Dr. Richard Mole (Britain's things would go well. Ashok was Tony Bingelis) had this comment: wearing an EAA T-shirt, was a pilot, "Since the C^ of Ashok's airfoil is so and had worked on the design and low, it would be worth trying for a development of light aircraft in In- thicker 18-percent airfoil to get the dia. (And in 1998, after the KR-2 air- low drag advantages, but also befoil project, Ashok took a summer cause the increment in CL max due job with Scaled Composites where, to flaps is also an increasing function among other things, he designed the of airfoil thickness, and the spars airfoils for Burt Rutan's new could be made lighter and stronger. Boomerang II.) Presto, an 18-percent AS5048 apFor the KR-2S Ashok first designed peared! Being an avid CAD f a n , I a 15-percent AS5045 airfoil, using a promptly superimposed the 15-per180-mph cruise speed and a weight cent airfoil onto the stock spars and of 850 pounds. The preliminary iter- determined that we needed a 16-perations for this a i r f o i l ( t h e cent airfoil for a truly perfect fit. A GA19980222A) were displayed on few days later we had the 16-percent the web. Joining them were compar- AS5046! isons of efficiency and speed imIn Seattle, Mark Lougheed, a boat provements over the RAF48 and designer and mathematician, fed the NLF(1)0115 used in the same KR-2S new airfoil coordinates and characapplication. teristics into a highly massaged CFD These comparisons are not trivial, (computational fluid dynamics) proand the proof is displayed in detail gram, attached to a 3-D CAD model at of a KR-2S. kr2/airfoils/. Make sure you visit the The results of his analysis, in conlinks at the top and bottom of this junction with his stability analysis, site; they include airfoil refinements pegged the wing and horizontal staand actual wind-tunnel test results. bilizer incidences required for maxiWhile much of the data is for the mum efficiency and stability. This GA1998222A, data for the AS504x analysis also resulted in some specseries is improved even more. Figure tacular images, like Figure 2, the la compares the characteristics of AS5046 approaching stall at a 16-deboth the AS5045 and the RAF48, gree 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 t u n n e l time was q u i c k l y 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 a i r f o i l was born! All we needed was a test pilot... AIRFOIL TEST PILOT

A f t e r a forced l a n d i n g 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 a i r f o i l (see ~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 17percent 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 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 KRNet 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 Click on the EAA Sport A v i a t i o n