RC Soaring
Mike Garton, 2733 NE 95th Ave., Ankeny IA 50021; E-mail: mike@iastate.edu
THIS MONTH I'll debut a new One-Design Racing (ODR) Slope Racer/sport kit, and I'll present the updated Midwest Slope Challenge ODR rules that are being used at Midwest and Arizona contests.
I threw in a couple "spy photos" (with permission) of Mike Fox's latest prototype at the Eastern Iowa Soaring Society (EISS) Fall Soar In. There is also a photo of Mike Fox's new 1/3-scale grandson.
ODR was started on the West Coast. The intent of the class is to provide the thrill of Slope Racing without the monetary arms race that naturally occurs in Unlimited racing.
Charlie Richardson and Paul Naton are credited with drawing up the initial set of rules for the Torrey Pines Gulls. Charlie has also produced the Fun One as a kit for the class.
The ODR rules have always been very specific, requiring a constant-taper wing with seven-inch root chord, five-inch tip chord, and 60-inch span.
The airfoil in the old rules was an SD6062. Carbon and fiberglass wing skins are not allowed. A minimum 25-ounce flying weight makes low-tech (and low-cost) structures competitive. A maximum weight of 35 ounces is enforced.
The Fun One was the only game in town until recently.
ODR is big in the Midwest—home of the Midwest Slope Challenge (MWSC)—and it has taken off in Arizona. Both groups are using new rules adapted by Loren Blinde.
Loren has been the Contest Director of the MWSC for the last two years, and he helped Paul Wright run it for many years before that.
In using the old ODR rules at the MWSC, several problems had been identified. To summarize, the only kit produced for the class was not technically legal, some of the rules were unenforceable, and some of the rules constrained the design while serving little useful purpose.
Loren used his experience to draft modified rules that rectify these issues late last summer.
The changes in the rules are subtle. First, the airfoil specification has been expanded. The Fun One kit was not technically legal according to the old rules. The airfoil in the initial rules was the SD6060 thinned to no less than 9.5%; the Fun One uses the SD6062 airfoil.
Loren changed the rules to allow any airfoil in the SD6060-SD6063 series with a minimum thickness of 9.5%. Builders can thin the 10.37% SD6060 or thicken the SD6063. These airfoils are all similar. When plotted at the same thickness, you cannot see the difference without overlaying large plots.
A second change was to eliminate restrictions that do not give performance advantages. The ailerons no longer have to be a specific size. A Fun One built per the instructions did not meet the specification anyway.
The dihedral specification was also removed. Most Slope Racers have 5° or less of dihedral. More than 5° may be beneficial for beginners and some intermediates.
Rudder control is still not allowed.
The rule outlawing carbon or fiberglass wing skins still stands. Legal wing structures include balsa-skinned, obechi-skinned, or solid EPP (expanded polypropylene) foam.
Any internal spar structure is allowed. Experience has shown that it is easy to hit the 25-ounce weight minimum with inexpensive construction methods.
Fuselage structure has been opened up to allow any construction materials. The old rules allowed Kevlar™ and fiberglass.
Loren states in the new rules, "You are free to build an ultra-light composite fuselage and then add 6 oz. of lead to bring it up to 25 oz. ... and then lose it in a mid-air anyway. It's your money ..."
The Hammerhead by Denny Maize of Polecat Aeroworks was released in early November 2001. From the outset, the Hammerhead was optimized for the ODR class. It does everything legal to maximize speed while maintaining good handling.
The Hammerhead looks very different from a Fun One. One difference is the wing; the mild forward sweep is unusual. Besides looking different, the forward sweep has some handling effects on a straight-taper wing.
I used John Hazel's Lift-Roll spreadsheet to calculate the local lift coefficient (Cl) along the wing. The mild forward sweep loads up the center of the wing slightly and unloads the tips some.
For those who aren't familiar with this type of plot, a refined biplane usually has a constant Cl for most of the inner two-thirds to four-fifths of the wing. A low Cl at the wingtips reduces the potential for tip stall.
It is also good to have a smooth Cl distribution. A bumpy distribution causes higher profile drag and higher induced drag. The Hammerhead wing clearly has a smoother, more optimal Cl distribution, with increased margin against tip stall.
A second beneficial effect of the sweep is that the Hammerhead has maximized the distance between the wing aerodynamic center and the tail. This makes the tail more effective for a given size. Forward sweep is not the answer for every design.
Usually when aircraft designers want a longer tail-moment arm, they increase the length of the fuselage. The Hammerhead is already at the maximum 36 inches allowed by the ODR rules.
Forward sweep can have a slight negative effect on yaw roll coupling. A left yaw induces slight right roll. This would be an issue in slow thermal turns. The airplane would need more dihedral and/or more aileron differential. It is not an issue for a Slope Racer.
No strange yaw behavior has been observed with the Hammerhead. This may be in part because of a large, high-aspect-ratio vertical stabilizer.
One could argue that Denny has added a design trick that violates the spirit of the class. Paul Naton did explicitly try to discourage design experimentation in the class.
As a counterpoint, forward sweep does not add to the cost of the airplane.
The biggest benefit of the planform is probably that it looks different. It will be easier to tell which model is yours coming out of the turns in a four-airplane heat.
Many of the parts in the Hammerhead kit are prebuilt. The wing is pink foam with balsa skins vacuum-bagged on at the factory. Carbon darts under the wood skin provide spar strength.
Basswood leading and balsa subtrailing edges are in place and rough-sanded flush with the skins already. The builder just has to radius the leading edge to finish.
Wingtip blocks are preglued in place and sanded flush. Servo holes are prerouted for Hitec HS-81 servos.
The stock wing bolts on with two nylon bolts. Denny already crashed one and was happy to report that the bolts sheared with absolutely no damage to the fuselage.
The Hammerhead's fuselage has also been optimized within the rules. It has minimum allowable cross-section area.
The cross-sections are circular to minimize surface area and maximize structural rigidity. Flat spots in a fiberglass fuselage would be more susceptible to buckling on harsh slope impacts.
Wing hold-down blocks are already in place.
Denny put a nose-cone-style access hatch on the Hammerhead for ease of access. The ODR rules do not permit wing ballast. There is room under the wing for ballast.
Notice in the picture that Denny did not cut holes for servos. I hate it when a fiberglass fuselage comes with a large rectangular gap under the nose cone.
Ideally, the builder can cut his or her own holes that fit the servos exactly.
I make a round or elliptical hole for battery access. This eliminates the need for a servo tray and maximizes the strength of the fuselage. Pushrods and full hardware are supplied in the kit.
The Hammerhead tail is contest balsa sheet that has already been sanded into an airfoil. The tall, very effective vertical stabilizer keeps the airplane tracking straight. A smaller tail might give a tiny decrease in drag, but handling would be compromised.
The kit comes with a carbon tube piece that fits over the rear end of the fuselage. This gives the builder an option for a removable tail. Simply glue the tail parts to the tube.
At the field, the tube is slipped in place over the fuselage, the stabilizer linkage is hooked up, and a small nylon bolt is screwed through to fix the tail in place.
A two-piece-wing option is in the works and will probably appear on Denny's Web site. Nonswept wings are available.
The "spy photos" of Mike Fox's latest Hand-Launch Glider (HLG) were taken at the EISS Fall Soar In. With the help of a gyro, he finally got the hang of discus-launching. He has also learned to design structures for discus.
Mike won the contest at which I saw him, and he won HLG at the Tulsoar contest.
Mike said that the prototype was not yet named. It is not a Focus because it has an all-new fuselage, wing, and tail changes.
This one has a thinned MH32 airfoil, it weighs 8.5 ounces, and it has a slip-on nose cone.
Tail surfaces are fiberglassed balsa. Mike said that lighter tails kept blowing off under the extreme discus-launch loads. MA
Sources:
ODR Slope Racing rules and Midwest Slope Challenge info: mwsc@alltel.net www.alltel.net/~mwsc/
Arizona Slope Fest: GHANSON@SWLINK.NET www.swlink.net/~ghanson/ Site%20Banner.htm
Hammerhead: Denny Maize (717) 503-9859 denny@polecataero.com www.polecataero.com/
Fun One: CR High Performance Sailplanes Charlie Richardson (760) 451-0056
Transcribed from original scans by AI. Minor OCR errors may remain.
