By Mark Poole
Wing Area: 1800 sq. in
Recommended Engines: 4.2-5.8
Engine: BME 6.2 twin
Prop: 3 Blade Mejzlik 24x10
Spinner: 5” Truturn Ultimate cut for 3 blade prop
Actual Weight: 24lbs
Quality of plans, instructions, machined balsa and plywood components
Excellent Flight Characteristics
Some balsa stock was not to the correct size (1/4” square sticks as large as 5/16” on one side).
Incorrect rudder/elevator stock.
Wing cabanes bent unevenly (replaced by Ohio).
Incidence on the horizontal stab (see text)
Note: Ohio RC discontinued operations in 2001. As of February 2002 their kits are still not available, though it has been rumored that a new owner would be announced.
I’m one of those with an unexplainable love for biplanes. I wanted an Ultimate biplane the first time I saw one and soon built the Goldberg Ultimate with a YS120. After seven years of great flying I lost it to a servo failure and knew I wanted another. But this time I wanted the largest one I could find that would fit in the back of my GMC Jimmy with the back seat folded down. I also wanted one that I felt could be IMAC competitive (good flying, strong, and light). After looking around and some careful measurements of my truck I decided on the Ohio 75” Ultimate. This was the largest size that I could fit in my SUV, and discussions with Bob and Mike at Ohio plus some previous builders of the kit convinced me that it could be a competitive IMAC flier.
Bob Ankney designed the original 62” version of the kit in 1991 with the goal of having the best flying Ultimate available. A local friend of Bob’s (who was also an RC pilot) offered to use the Air Force Aerodynamics facility in Ohio to help design the kit. The result was changes to the scale airfoil, incidences, and location of the wings and tail surfaces. Testing for structural integrity included power-on dives with a quick pullout at the bottom. The 75” version followed the smaller version and went through the same design and testing process. The kit and instructions were revised again in 1998.
The first thing that came to mind when the kit arrived was, “where’s the rest of it?”. I’m so used to building planes with foam-core wings that I had forgotten how little space is needed when you’re shipping spars and ribs instead of big foam blocks. The shipping box measures only 12”x12”x49”. Everything inside is wrapped in paper and packed in foam peanuts, with several clear bags (labeled) that contain the machine cut formers and ribs, hardware, and smaller components. The only damage found was a single wing rib that had some slight compression from shipping. The two-piece cowl and wheel pants are of white epoxy/fiberglass construction. The cowl was perfect. The wheel pants had only a few pinholes to be filled. Also included in the kit is the hardware needed for the landing gear, interplane struts, cowl attachment, and firewall reinforcement. There are two full-size plan sheets and a bound instruction manual. The instructions are very well written and clear for the most part, with a few exceptions where more detail would have been helpful. You build the wings first, then tail feathers, then the fuselage.
I used Anchor brand epoxies and CA’s for the first time and really liked them. The epoxies come with different pot life and set time options so you can choose the right one for each application. The CA bottles come with a metal pin in the cap to prevent it from getting plugged, what a great idea! I especially like the Medium and used it the most. It seemed to set faster than other brands and gave a really strong joint.
The top and bottom wings are identical (one piece, 75” span) except for the mounting areas (struts and center section). These differences are clearly identified in the plans and instructions so you don’t make any mistakes. You build the rib/spar/stringer structure over the plans, sheet the leading, trailing, and center sections and then turn the wing over. Then you add the leading and trailing edge stock, cap strips, and tip stock. When this is done you cut out the ailerons and finish the wing trailing edge and aileron leading edge with balsa. There was an error in the instructions when you first start the wings, they instruct you to support the back of the ribs with a piece of 1/4x1/2” balsa that will be used later for the aileron leading edge. There is only one stick of this size in the kit, and it’s not the size that’s used for the aileron leading edge (1/4” x 7/8”). The machine cut ribs fit perfectly around the spars and stringers. The only difficult area was in accommodating the “pinched” airfoil. The outermost rib is thinner than the rest (similar to the full-scale Ultimate) so the sheeting must be bent downward to attach to it. You must properly support this last rib during the sheeting process so no washout/washin gets created. Ohio RC does NOT recommend making the outer ribs exactly the same as the inner ribs, stating that performance would be decreased.
My only modification to the wings was to move the servos one bay inward to better center the servo across the aileron length and between two hinges. I drilled ¼” holes through the ribs to accommodate my soda-straw servo wire tubes. Building the wings was enjoyable as everything fit really well.
The horizontal stab and vertical fin are built from balsa and spruce stock and then are sheeted with 1/16” balsa, making them a total of 5/8” thick. This is a full ¼” thicker than tail surfaces on other planes of this size, which, along with the spruce components, explains why Ohio states that no flying wires are needed. I wanted the option to mount them anyway so I placed ½” dowel hard points where I could mount the wires. The rudder and elevators are also stick construction and went together easily. Two pieces of stock, however, were the wrong size (width too small) and had to be replaced. After completing and sanding the (very large) rudder to shape I noticed that I was easily able to twist it so I sheeted both sides with 1/16th balsa. This made a huge difference in stiffness.
Each fuselage side is built from four pieces of fairly heavy 1/8” balsa sheeting that must be edge-glued together. I figured it out, but a diagram indicating how this is to be done would have been helpful. Then 1/8” lite plywood doublers are laminated into the wing and engine box area. The firewall, fuel tank floor, and engine box area is built separately to ensure straightness and then is attached to the fuselage sides. The fuselage was placed upside down on top of the plans to keep it aligned while the formers and remaining spruce and balsa stringers were added. The wing cabanes are held in place with plywood mounts between the top formers. I found that my cabanes were bent unevenly, with one side 3/8” longer than the other. A call to Ohio quickly had a new set on the way. The turtle deck and area in front of the canopy are constructed of formers, stringers, and 3/32 balsa sheeting.
My modifications to the fuselage were weight saving efforts and fuel tank location. I cut 2 1/8” diameter holes in the 1/8” lite ply fuselage doublers and the ¼” plywood firewall/engine-box area, and ½” holes in the tank floor. The ¼” plywood firewall was also lighted by removing the corners from the box and drilling a large hole immediately behind the engine. I also moved the fuel tank back from the engine area to the CG by building a 1/8” light ply base under the plywood cabane mounts. The tank (32 ounce) hangs from this mount, centered on the CG and thrust line.
The landing gear are made of ¼” aluminum, weighing 18.4 ounces out of the box, 19.6 ounces with the axles attached. Based upon experience with previous kits I was comfortable removing 3/8” from the back edge and some additional material from the mounting area. I had a local machine shop do this for me for $15. The weight savings was 5.3 ounces.
The epoxy glass wheel pants were wider than most of this scale, giving me plenty of room to install the mounting hardware. The kit includes ¼” plywood mounts, 3/16 axles and wheel collars, and screws for mounting the pants to the gear.
Finishing touches to the fuselage included the instrument panel and Haigh style tail wheel offered by Ohio.
The two-piece epoxy-glass cowl fits against the front former on all sides, with slits cut at the bottom to accommodate the landing gear. I chose to change this so that the cowl butted up against the front of the landing gear and used a 3/8” balsa block under the landing gear to fill in this area. This will allow me to remove the gear without removing the cowl. An additional benefit is that if I ever do a hard landing and pull the gear out the cowl won’t be ripped up as well. There was no information in the instructions indicating how to attach the top part of the cowl to the bottom. I mounted 1/8” light ply and #4 Ohio Superstar “All Threads” inserts in the bottom section and used ½” #4 button head screws to attach the top to it. Lastly, to please my fellow critical club members, I added some 3/16 balsa to the front spinner ring and tapered this to have 2 degrees of right thrust so my gaps would be equal on both sides of the spinner.
At this point the tail feathers were attached and the top wing aligned with the bottom wing and fuse. The horizontal stab was attached first and then marked where the vertical stab would go. Then I took some scrap 5/8” stock and tack-glued it in the vertical stab position. The two pre-cut tail blocks were tack glued in place and then carved/sanded to match the turtle deck. This makes for a perfect fit of the blocks around the vertical fin and is a lot easier that any other method I’ve tried. The vertical fin and blocks were then attached.
Now I was ready to mount the top wing for the first time and finally get a good view of what the completed Ultimate would look like, a moment I look forward to during the entire building process. I was incredibly surprised to find that the wings had the exact same incidence along their entire length. There was no trimming/fitting needed to get them aligned! Prior to starting construction I had bought a damaged 2” thick solid-core door from Home Depot and rebuilt my workbench around it, making sure it was absolutely flat no matter how I measured it. The effort was certainly worth it. I told my wife that there was a zillion people in this world that couldn’t care less about my straight-wing result, but to me it meant a lot. She was one of the zillion.
I made my tail wires with .070 carbon rods from CST Composites. I used medium CA to glue a threaded 2/56 Sullivan clevis on one end, and a 2/56 threaded coupler on the other end. Another 2/56 clevis is screwed onto the threaded coupler. I used new Sullivan mounting straps to attach the wires (rods) to the surfaces. Although I didn’t use wire, these straps are great because they have a brass insert in the attachment hole to keep from wearing traditional flying wires. My carbon rods and clevises made for a very clean and easy installation.
The struts are primarily one large piece of ¼” lite ply, which Ohio recommends lightening by drilling four 2 1/8” diameter holes on 3” centers. After fitting to the wings you attach additional plywood bracing plates and 1/8”x1/4” balsa trim on both sides. To save weight, I omitted using the plates and trim, putting these components in my bag of unused or removed material. The struts are then attached to the wing with 6/32 bolts, two on the bottom and two on the top of each strut.
I chose to use a BME 102 twin engine based upon the years of fantastic performance I’ve had with my lightweight BME 5.8, the original version of this engine. I haven’t been surprised at all how quickly the giant scale and IMAC crowd adopted it and its successors. I used 1” diameter dowel rod to space the engine off of the firewall, tapering each 2 degrees for the proper right thrust offset. I wanted to keep noise to a minimum so I chose to use a 24x10 3-blade Mejzlik prop, fitted with a 5” Truturn spinner cut for a three-blade prop. The BME just fits inside the cowl with only about 1/16” clearance for the left spark plug. I did have to cut some eye-shaped holes for the front of the mufflers and for carburetor access, and a large area on the bottom of the cowl for cooling airflow. I placed a piece of ½” triangle balsa along the front of this opening to direct air down and away from it, which aids in pulling air through the cowl. I also installed baffles in the cowl to better direct the airflow for cooling.
I used 6 JR4721 servos, one for each aileron and elevator, and a Multiplex Jumbo Speed on pull-pull for the rudder, mounted in the center of the fuselage. The throttle servo is a Hitec 545. I originally planned to use two 1500mah 5-cell NiCad packs and two switches for the receiver and one 1500mah 4-cell pack for the ignition. This has been a standard setup for me for quite a while. The total weight of the batteries for this combination is 14.5 ounces. Wanting to keep weight down, I compared this with Duralite batteries and found that three 1600mah packs weigh only 7.2 ounces. The price for these is pretty steep but the weight savings, non-discharging characteristics, lifetime, and reputation convinced me to give them a try.
After years of hearing about them I finally bought a Don Harris smoke pump. It doesn’t require a separate battery pack because it has been built and isolated such that your standard receiver pack can be used. This saves the weight of an extra pack plus no extra charge jack and balancing issues to mess with. It also has the reputation of being able to pump a large volume of oil without loosing prime in high-G maneuvers. Several of these have been used in recent TOC competitions. I wrapped the pump in foam and taped it to my 20oz Dubro smoke tank.
The Ultimate was covered with White, Dark Red, Sky Blue, and Royal Blue Monocote. The cowl and wheel pants were primed and painted with Lusterkote paints, sanded smooth with 600 wet sandpaper, then clear-coated with automotive clear polyurethane. The match of the blues and red were fantastic. The white paint (after clear coating) was a little darker than the covering. I was lucky to get the covering finished and plane fully assembled just a few days before our annual Mall Show. There were seventy planes entered. As usual, scale master Dave Platt won first place in the People’s Choice award. I was proud to get second-place.
I measured the CG with everything ready to go except for the two 1600mah Duralite receiver packs and found it was 1/8” behind the location shown on the plans. A quick call to Ohio RC revealed that this was perfectly OK, many people have flown it at a full inch behind the plan location. I was aware of one other Ohio Ultimate with nearly the exact same engine and servo configuration as mine that resulted in a CG that was ½” in front of the plan, so my efforts to lighten the fuselage (especially the nose) had really paid off. My total dry weight of 24lbs was the lightest I’d heard of. My bag of removed materials weighed 12 ounces, 5 of which was the aluminum removed from the gear, the remaining 7 ounces coming from removed or omitted wood. When you add the 7 ounces saved by using Duralites, my Ultimate is 19 ounces lighter than it would have been if built stock and powered with nicads.
I balanced the Mejzlik 24x10 3-blade prop and found one blade required several shots of clear paint to get it balanced. The next step was to fire up the BME and run a couple of tanks through it. With the ignition off I set the choke, throttle at full, and turned the prop over until I saw gas dripping from the carb. I then removed the choke, set the throttle to a high idle, and turned on the ignition switch. The BME started on the second flip. I observed a maximum high-end rpm of 6800, then settling to 6700 if I left it there. I was able to idle all the way down to 1100 rpm before running out of servo travel. This is VERY low for a brand new engine, and I hadn’t even touched the needles yet. I set the idle at a safe 1500rpm for my initial flight. Acceleration is very quick and the engine is smooth throughout the rpm range. The three-blade prop is also noticeable quieter than other two-blade props I’ve used. Lastly I set the engine at idle and held the plane nose high and nose low to ensure reliability at all attitudes.
I had been using my RealFlight simulator to fly a scaled-up Ultimate, anticipating how mine would handle when I finally got it in the air. After a thorough inspection and radio checks at the field I was ready to see if all the stories of great flight performance were true.
It taxied really well with no tendency to tip over on our grass runway. The takeoff occurred at 1/3rd throttle in about 50 feet. I needed four clicks of down trim and two clicks of right trim to get hands-off level flight. After that it was a big trainer, very easy to fly around and very controllable at all speeds. Rolls are very smooth and require very little rudder at knife-edge to keep the nose up. The recommended throws seemed about right for these first flights. Snaps start and stop immediately, but happen quicker than my monoplanes. I over snapped the first two times I tried. It climbs in knife-edge with only the slightest amount of rudder applied, certainly the best knife-edge performance I've ever had. Stall turns are way too easy with all that rudder authority. Inverted flight showed that I was still nose heavy, even with my CG 1/8” behind the plan location. It took 8% mix of rudder to aileron, 12% mix of rudder to elevator to remove roll and pitch coupling. Up lines were straight hands off. Down lines showed it wants to slowly pull out, in part due to my nose-heavy condition. Stalls were perfectly straight ahead. The landing was very easy but it required that I keep some throttle all the way to the ground. Like most biplanes, the draggy design slows very quickly when you pull the throttle back so there’s no need for a long approach.
The BME102/Mejzlik 24x10 three-blade combination is a great one for the Ultimate. It never sags on the up lines and accelerates immediately, yet never gets loud. I ended up with 40% expo on the throttle to give a more linear response with the stick. One thing I really noticed about it was the comfortable flying speed. At about 40-50% throttle its flying at a very easy speed to do figures, using more throttle of course on the up lines to keep the speed constant. I just liked the way it felt. On following flights I did some of the IMAC Sportsman sequence and really enjoyed myself, feeling like I had a lot of time to get ready for the next maneuver.
The only flight issue was the amount of down trim required. There was a full degree of effective down elevator even with my slightly nose-heavy setting. I later found that nearly all biplanes have this requirement. Others building this kit should build a degree of positive incidence into their horizontal stab.
The Ultimate looks fantastic both on the ground and in the air. I enjoyed the construction process and am glad I took the effort to lighten it. If I were doing it again I would probably totally remove the stock firewall/engine mount area and build a more traditional engine box, saving even more weight. I would estimate that there is another 4-8 ounces in the airframe that could be omitted without jeopardizing strength. The BME 102 is light, provides incredible power just as expected, and looks fantastic inside the cowl. The exhaust note of the twin is also beautiful to hear. It has to be the one of the best choices for this plane.