The Furias 96
By: Steve Moon
As my good friend John Grigsby and I left Muncie, IN after another fun and eventful National Championships in 2008 our conversation quickly turned to plans for 2009. This, of course, led to a lengthy discussion of what plane I should fly for the next years CL Stunt wars
For the last four years I have flown the Bob Hunt designed Saturn powered by an OS 52 FS. And, not coincidentally, I have had my four best finishes at the Nats. I have steadily improved, achieving my best finish in 2008; 13th place. My current Saturn has three full flying seasons on it and the wear and tear have begun to show. What to do for 2009? Should I stick with the tried and trusted Saturn/OS 52 combo, or should I try something new?
I have flown many different designs over the last 20 years: Paul Walker’s Impact, Bob Gieseke’s ’97 Bear, Berenger’s Sportster, Hunt’s Saturn, my own designed Shaman, and many more. One constant that I have learned from all this is that I prefer a plane with about 650 sq. in. of wing area, such as the Bear or Saturn. To me, these planes are just right the size: not too big to ‘overpower’ the flying sphere we use, and not too small appearing either.
After many miles and many hours of discussion, John and I decided that it was time for me to try a new design. Now, the question had become: Which one? Naturally, this question took many, many more miles and hours to come up with an answer. What I was looking for was something fairly similar to what I had been flying, but something that would also allow me to improve my flying ability even further. I am not one to try and improve someone else’s design either. I don’t really see myself making a plane that Gieseke, Hunt, or Walker has designed any better than they have by changing it around. That’s just not my style.
As we considered and discarded design after design we kept coming back to one in particular, though not too many people have seen it: the Furias, designed by my brother Doug Moon. Doug completed his original Furias in 2003, and it was an outstanding flyer right off the board. Doug designed it specifically to use the Saito 56 or 72 four stroke motors. The wing is a slightly modified Geo/Bear wing with a tail section very similar to Bob Gieseke’s Geo/Bear. The fuselage, though, is wider and deeper than usual and has a very aggressive appearance with a short nose moment and long tail moment.
You may be asking yourself why Doug called this plane the Furias, when it isn’t actually a scale or semi-scale model of the Furias Sea Fury racing plane. The answer is simple: Doug really liked the paint job on the Furias racing plane, so he put it on his plane and just kept the name. With a Saito 72 in the nose the Furias was a decent flyer right from the start. But, Doug had to add 1 oz. of tail weight due to the size of the Saito 72.
After quite a bit of harping from me, he decided to try a Saito 56. Out came the tail weight and the plane now weighed 63 oz. versus 66 oz.; and this is with over 700 sq. in. of wing area. The Furias came alive! The plane flew beautifully, with a hard, yet stable corner and no odd tendencies anywhere in the pattern. Doug was really onto something here. And then, tragedy, as a suicidal dove decided to end two lives at once one evening by flying into Doug’s lines as he was flying level laps getting ready to land after completing a pattern. We couldn’t believe it, the plane probably had less than 50 flights on it and now it was destroyed. Doug immediately started building another one. But, with three children born in the next five years the Furias took a way, way, way back seat to life in general. It only took Doug a little over five years to build the second Furias, but it is also a great performer with the new Saito 62 in the nose.
I know exactly what you are thinking: Steve just said he likes to fly a plane with about 650 sq. in. of wing area and the Furias has over 700 sq. in. of wing area, what gives? After telling Doug mine and John’s thoughts he said it would be no problem to pull up his Furias plans in his CAD program and redo it a little to come up with the size plane I wanted. He told me he had a few thoughts on ‘massaging’ the numbers anyway and we jumped right into the redesign; thus the Furias 96 was born.
We simply started with a 96% scale version of the original Furias. After drawing this plane we then slightly modified the nose and tail moments to meet the specifications that I believed would yield a great flying plane using the OS 52 FS engine, or in this case the new OS 56 FS-A. The OS 56 FS-A is a newer more powerful engine that fits right into the same space as the original OS 52 FS. The nose moment for the Furias 96 is slightly longer than the Furias and slightly shorter than the Saturn. The tail moment is slightly shorter than the Furias and slightly longer than the Saturn. After redrawing the fuselage profile several times and standing back and taking a look at each one Doug and I were quite happy with the final product. The Furias 96 has a good looking, aggressive profile view. After downsizing, the wing came out at 650 sq. in. This was turning out to be exactly what I was looking for.
As I began to gather all the necessary accessories to scratch build another plane I began to think of exactly how I wanted to build this plane. This would be my first scratch built contest plane in several years, as my last few have been kit built. I began to think of ways to combine modern technology and building methods with some old school methods in order to yield a lightweight yet easy and quick to build plane. The older I’ve gotten the less tolerant I have become of sanding dust. Just thinking about hollowing a balsa cowl or top block gets me to coughing. I like to call what I’ve come up with a ‘Conglomeration of Technologies’.
I decided to use standard balsa/ply/hardwood construction on the fuselage with molded balsa top and bottom blocks, a fiberglass cowl, a foam wing core for the wing structure, solid balsa flaps with carbon fiber blade spars, a foam core, balsa sheeted stabilizer with a carbon fiber blade spar, and balsa elevators and rudder. This conglomeration of technologies makes for an easy and quick to build plane.
The fuselage, at 2.75” wide is ¼” smaller than the original Furias, but is still larger than most CL Stunt planes and presents well in the air. I’ve used a standard motor crutch with hardwood maple motor mounts and ply F1 & F2 formers, so I won’t go into too much detail here. The top and bottom blocks are made from molded 3/32” balsa. The molds are made from white foam and sanded to shape. Heavy balsa is then glued to the bottom of the foam to give it some strength. Cut the lightweight 3/32 balsa to a rough approximation of the shape needed to cover the molds and then soak it in the bathtub in a mixture of water and ammonia for about an hour. The water/ammonia mix gives the balsa good flexibility once it has soaked in. Wrap the balsa around the molds using an Ace bandage and let sit for several hours. I usually let mine sit overnight to make sure the balsa is good and dry. Once the water/ammonia mix has dried out the ammonia that has soaked into the wood helps to ‘lock’ it into place. Once the basic structure of the plane is in one piece the top and bottom blocks are easily glued into place.
I decided to use a triple cored foam wing core from Crist Rigotti for the wing structure. After years of building wings with laser cut ribs I just could not fathom the thought of stacking and sanding a set of ribs. Crist’s foam wing cores are beautifully done, and with triple coring are very light. This isn’t the first time I’ve used Crist’s service, and I’ve always been more than satisfied with the results.
To be totally accurate, I decided to also order a set of leading edge bucks from Crist. Using a molded leading edge leads to a very accurate airfoil shape. It is much easier to achieve than trying to sheet the wing to the leading edge point, or sanding a balsa leading edge to shape. The leading edges are molded in the same manner as the fuselage top and bottom blocks.
I sheet my foam wings using Dave Brown Products Southern Sorghum. This is an easy to use product designed especially for this purpose. Over the years Bob Hunt has written many an article pertaining to sheeting wings so I won’t go too deeply into detail here, as I have no earth-shattering revelations to add that Bob hasn’t already discovered. The Furias ’96 does use wing mounted landing gear, so be sure to glue into place the necessary half ribs and landing gear mounts before sheeting the wings. The wing cores came from Crist with the cuts already in them for the location of the half ribs. Make these ribs out of 1/32 ply and epoxy them to either side of the small piece of wing core that results from these cuts. Once this is dry, carefully cut a slot where the wing gear mount will be epoxied into place. Once this is dry, epoxy this and the other leading edge piece into place. Now, make the bellcrank mount out of 1/8” plywood and epoxy into place in the wing cores. The wing is now ready for sheeting.
In an effort to again keep sanding to a minimum I usually construct my wingtips using the ‘Morris’ method. I call this the Morris method, because I learned it from Tom Morris. This method is fast, easy and very light; without having to hollow out balsa blocks. First make a frame of the tip with ¼” balsa that is about ¼” wide. Next, make a 1/16” balsa rib the size of the outboard wing tip. Put a wide slot in this tip piece. Now make two small ribs from ¼” balsa that extend from the high point of the tip rib. Lay the tip rib flat on the workbench, stand the ¼” tip frame up on the rib and glue the ¼” balsa ribs in place. The final step is to sheet the tip with 1/16” balsa. Use a piece of cross grain balsa and glue the sheeting to the tip frame with thin CyA glue. Once the tips are sheeted they can be sanded and glued into place on the wing. Don’t forget to add an adjustable leadout guide to the inboard wingtip. Combined, the tips usually weigh about ½ oz.
The flaps are made of balsa, but to add more stiffness I have added a carbon fiber blade spar. First, cut a flap as shown on the plans out of 3/16” balsa. Now, cut the 3/16” flap in half spanwise. Between the two resulting pieces add a piece of .030 x .310 flat rod carbon. This is a fairly common item at hobby shops now. Glue the two pieces of 3/16”balsa with the carbon spar sandwiched between them together with thin CyA glue. To make it easier to sand to shape later I left the carbon rod about 1” short of the end of the flap on each end and inserted balsa. Next, I add a lite-ply ‘lucky box’ near the inboard end of the flap to house the flap horn. Use 1/8” lite-ply with 1/32” balsa glued to each side of it. Cut a slot in the 3/16” balsa flap core and glue the lucky box into place. To finish the flap sheet it with 1/16” balsa. This method yields a straight, flat, and incredibly strong flap that is quite easy to build.
The stabilizer has a foam core with a carbon fiber blade spar embedded. Once the foam core is cut to size it is then cut in half width wise and the carbon spar is epoxied in between the two foam core halves. A frame of 3/8” balsa is then built around the foam core and epoxied into place. For the trailing edge I like to build a trailing edge with the hinge slots already built into it. Cut two strips of 3/8” balsa that are just slightly less than 3/16” thick. Lay one piece on the workbench and mark the locations of the hinges. Glue pieces of 1/32” balsa onto this piece of wood, except where the hinge locations are marked. Next, glue the other piece of almost 3/16” balsa into place and you now have a trailing edge with the hinge slots already in place; and this method yields a much stiffer trailing edge piece. Once the frame has been glued into place, sheet the stabilizer with 1/16” balsa using 30 minute epoxy. Place the stabilizer on a flat surface and cover it with a piece of glass or wood and weigh it down with about 30 pounds of weight. I actually let this sit for about 24 hours before removing the weight.
This plane also uses the ‘stablets’ that Bob Gieseke has made popular over the years. Doug and I have been flying with Bob for almost 20 years now, and we have always seen these on his planes. The theory is that the ‘stablet’ plate at the end of the
stabilizer ‘captures’ the air coming over the stabilizer and forces more air over the elevators. This enables the use of a slightly smaller stabilizer that is still just as effective as a larger one. The rudder is made of solid balsa. Before final assembly I usually do as much pre-finishing of the parts as possible. Before installing the wing into the fuselage I pre-finish it with Nitrate dope and Polyspan. I do the same with the flaps. I pre-finish the stabilizer and rudder with Nitrate dope and .02 carbon fiber veil. The carbon fiber veil is an excellent filler for balsa wood. The fuselage, stab, elevators and rudder will not have any covering, except for the carbon fiber veil.
At this point all of the subassemblies are completed and the plane is ready to final assembly. Start by making two angled cuts in the bottom of the fuselage and removing the resulting piece. Check the fit of the wing into the fuselage. Once you are satisfied with this fit, the wing can be glued into place. Be sure to do this on a flat surface and use multiple measurements to be sure the wing is flat and straight in the fuselage. Next, check the fit of the stabilizer. Once satisfied with this fit glue the stabilizer into place with epoxy. Once again, use multiple measurements to be certain the stab is flat, straight and properly aligned with the wing.
Before adding the top and bottom blocks to the fuselage it is necessary to make and install the pushrods. I always use carbon fiber pushrods with threaded ends from Central Hobbies, with ball links and an Ultra Hobby Products Ultra Clevis. When making up the pushrods I always use 24 hour J-B Weld and build the rods one end at a time. Due to this, it actually takes several days to construct the pushrods. I will do one end of each rod and allow it to dry while I am working on another part of the plane. Once the plane is in one piece the other end of the elevator pushrod can be made. To ease adjustment I added hatches to the rear of the fuselage underneath the stabilizer. By putting a hatch on each side I am now able to be very precise when making any adjustments to the pushrod and clevis. I can turn the clevis as little as ½ turn when adjusting.
Having finished the pushrods and necessary adjustments the fuselage top and bottom blocks can be added. There are four pieces, and it doesn’t really matter what order they are glued into place. Just be certain to check and recheck the fits before gluing into place. The better the fit the less glue is required. This yields a stronger yet lighter model. Add the rudder, and we now have a nearly complete plane. Yet, this has been accomplished with a minimum of sanding and little wasted wood.
As my brother did on his Furias, I have decided to make a fiberglass cowl. This is actually a quick and easy process without all the balsa dust generated when hollowing a balsa block to make a cowl. To begin, tack glue 1/32 ply to the fuselage. This will yield a 1/32 ply frame when finished. Next, use Styrofoam brand Gentle Grip Standard green foam to make a mold of the cowl by gluing it directly to the ply frame and sanding to shape. Be careful when sanding as the green foam sands very easily and quickly. And, while it does yield a fine dust, it is heavier than balsa dust and will just settle onto the work surface. I vacuumed it up several times while sanding. I also wore latex gloves while sanding. Once the foam is sanded to shape add a layer of ¾ oz. fiberglass cloth using Pro-Set 24 hour epoxy. Then add a second layer of ¾ oz. fiberglass cloth. I then added one layer of ½ oz. fiberglass cloth. The ½ oz. cloth has a tighter weave and is easier to finish than the ¾ oz. cloth.
After the epoxy has dried for over 24 hours cut off the excess cloth with a Dremel tool and cutoff wheel. Next, begin sanding the cowl with 120 or 180 grit sandpaper. Now, hollow out the green foam with a knife, sandpaper, and finally with a Dremel tool and sanding drum. The green foam is very easy to remove and this step should go quickly. Now you can complete the sanding of the cowl. At this point my cowl weighed 5/8 oz.
Something else I’ve decided to use on this plane is a remote glow driver. For several years now I’ve seen Bill Suarez and Noel Drindak use remote glow drivers when I’ve launched their planes at the Nats, and I think this is a wise item to use due to the safety factor. It eliminates the need to reach underneath the plane to remove the glow driver with a spinning prop only inches from your fingers. I used the Hangar 9 remote glow driver, part #HAN3025. This is a simple product to use and I am very happy with the results.
Doug and I have used auto paint on our planes for at least 15 years now, and I decided to stick with our same tried and true process for this plane. I start by brushing on two coats of nitrate dope on the entire plane and sanding it with 320 grit sandpaper. I then dope on .02 oz. carbon fiber veil on the wood surfaces: fuselage, stab, elevators and rudder. I brush on two coats of dope and then sand this with 220 or 280 grit sandpaper. Now, I spray on two coats of dope on the entire airframe and sand with 320 grit paper.
Next, I spray on one coat of sandable auto primer, let dry thoroughly and wet sand with 400 grit paper.
At this stage the plane is ready for paint. We like to use the ‘Forward Masking’ method of painting. For this plane I painted the white areas first with no tape on the plane. I then taped off the areas that would be staying white and sprayed the blue areas. I then taped off the blue areas (with the tape covering the white areas still on the plane) and what was left exposed were the areas that were to be painted red. After painting the red areas I added the white checkerboards. After letting dry a couple of hours (actually more than necessary) I removed all the tape and made any necessary touchups and added the decals. This entire process took just two days, and we have actually accomplished this task in one day on several occasions.
The final step is the clear coat. Auto paint clear coat actually consists of: Clear coat, activator and reducer. When done right auto clear coat is very shiny and very fuel resistant. I start by lightly spraying over the decals and all the edges of the plane. Next, I spray the fuselage and actually go out onto the wing and stab about 2- 3 inches while spraying the fuselage. I do this so that when I spray the wing and stab I won’t have to go all the way to the fuselage. This helps cut down on the amount of overspray. I also spray the clear with lower pressure and move the gun slower than when applying the color. Once the entire plane has been sprayed with clear, I let it sit for about six hours before wet sanding with 600 grit sandpaper. I now add a second coat of clear in the same order as before, but I lower the pressure just a little more and adjust the gun for a slightly wetter spray. When done correctly, this will yield a shiny, slick surface. After letting this dry for at least 24 hours final assembly can be done: epoxy the flaps and elevators into place, install the motor and tank, install the landing gear, wrap the leadout lines, install the remote glow driver and LET’S GO FLYING!
What I had hoped to accomplish with this plane was a little more ‘corner’ while having the same stability as some of my planes of the past. I can safely say that has been accomplished. The Furias 96 has a nice, sharp, crisp corner; yet no bobbles. Nothing makes me crazier than a nice, hard corner with a big ‘bounce’ or bobble at the end of it. To me, this is just as big a mistake as a corner that is too soft. The Furias 96 has a hard, crisp corner, yet it is easy to control and the plane just settles in when returned to neutral. From the center of the flying circle the plane appears to be turning right around the center of gravity. The OS 56 FS-A motor also has plenty of power. It has a claimed 20% more horsepower than the previous generation OS 52 FS, and I don’t doubt this. The increase in power is quite noticeable, both on the ground when launching the plane and in the air when flying it.
Having watched hundreds and hundreds of CL Stunt planes fly over the years I have noticed that there are many, many differences in the appearances of how these planes turn a corner. Some appear to ‘throw the tail’, while others appear to ‘lift the nose’, and others look as though they are on a pivot around the center of gravity. One type isn’t necessarily any better than another; it is up to flyer preference. The Furias 96 flies in the manner that I was hoping for. Overall, I am very happy with the results of this project and hope that anybody out there who takes it on will also be satisfied. Feel free to ask me any questions at: firstname.lastname@example.org.
LIST OF SUPPLIERS
Balsa wood, plywood, hardwood:
Ultra clevis, carbon fiber pushrods, 4-stroke conversion kit:
Ultra Hobby Products