BH Models Katana ARF

Oct 14, 2008 No Comments by

By David Vaught

(See pictures in slideshow below)

Organic is the word Pietro Terzi uses to describe his design for the Katana T-30. Having gained experience in the Zlin 526F, Dr. Terzi designed the T-30 with the intent to produce an aircraft that was more efficient at high incidence angles. BH Models has integrated that element and many more into their 71-inch wingspan balsa and lite-ply constructed fully aerobatic model. Weighing in at nearly 12 pounds and with Oracover covering, mid-wing Katana features the signature large wing fairings that mitigate the disruptive joint between the wing and fuselage with a reduced drag interface that seamlessly merges the wing with the fuselage.

Couple the Katana’s big airframe with a Thor 45cc CDI gasoline engine and a 20×8 Zinger prop and you have an unlimited aerobatic airframe designed for expert pilots, but with a gentle feel between maneuvers indicating possibilities for advanced-intermediate pilots wanting a large scale plane to build their large scale or 3D skills.

The Kantana kit provides wheel fairings and cowling that are fiberglass. Since the plane design uses individual wing panels the build is limited to the tail using typical building methods and installation of the ailerons, rudder and elevator halves using fiber hinges. Control linkages are carbon that is fused to the clevis with CA giving a secure link to the single bolt control horn. The canopy is installed onto the forward deck, that when removed, provides huge amounts of space for electronic installation.

Unique features

One of the many features I liked about the Katana is while it is big, it is not too big. With a 71-inch wingspan and length of 67 inches one is likely drawn more to the thickness of everything than just the overall size. For example, at the wing root, enhanced by the fairing built with the wing, the thickness is 3.75 inches and the wing tips are 1.25 inches. The fuselage measures 7.5 inches across and the vertical and horizontal stabilizers are both large and provide a symmetrical airfoil. So everything about the Katana says aerodynamic lift.

Running the full length of the wing the ailerons attached cleanly. The leading edge of each aileron is steeply angled for maximum movement. The aileron servos are located slightly inboard with a short distance between them and the aileron control horn. The elevator halves and rudder leading edges are also designed for full throws. The elevator servos are close to the tail and the rudder uses a pull-pull system.

In this class of aerobatic aircraft, split elevators are expected. Improved mix opportunities and better trim are benefits of this setup. Control horns install through pre-drilled holes under the covering. End caps are provided to disperse the tightening load, but I recommend before you install the horns you harden the opening with CA. As well, once you have your final setup, be sure to apply some thread lock to the linkages and lock the linkage with the supplied stop nut.

For this project the Thor 45cc CDI engine was selected. Keep in mind if you use a gasoline engine break-in is necessary and at a minimum you will run through 1 to 2 gallons of premix fuel. This should be done on an engine test stand. Early runs of the Thor produced significant vibration, but after a gallon of fuel that settled down. As well the CDI provided with the engine began to produce engine cut-outs so I switched to a CRRC ignition that included auto-timing. Be sure to line the CDI mounting surface with aluminum tape to shield any interference. The CDI easy mounts to the firewall or engine standoff.

Either before or after the engine break-in, mount the engine. The kit includes a mount for a 1.60 2-stroke or 2.0 4-stroke. The Thor engine has a motor mount built into the engine block. You have two necessary elements in mounting the big Thor: Get it in the center of the firewall and cowling and get it far enough from the firewall to meet the prop. Neither were a problem.

The hole in the firewall for the fuel tank tubes is the firewall center. The template provided for the Thor was incorrect so I made my own. I inverted the engine and once centered it perfectly lined up with the cowl opening. I did need to remove areas for the cylinder head, spark plug, muffler and intake manifold in the cowling. The fuel tank included in the kit is fine for nitro fuel but did not work well when I needed to install a different stopper in the tank for gasoline. The neck was too short to hold the stopper. I instead opted for a rectangular Sullivan 12-ounce tank and gasoline stopper with their stainless steel wires ties to secure the fuel lines to the copper tubes.

Final setup requires you move a few things around to be sure you have a CG at 160mm at the wing root. I used the ProMax 6.0 volt 3000 mAh ignition battery to get enough weight in the tail. I added a plate to the interior airframe and placed the ignition battery under the turtle deck. I ran my wires opposite the receiver to avoid any interference. Finish by installing the decals.

Conclusion

When you more to larger aircraft and especially when you chose a gasoline engine your time to complete will be extended exponentially. Not that the build is more difficult, you just have many more elements to attend to and with this large of model you might add some extra effort to secure your investment. As an example, the throttle pushrod must not be metal and you should not connect the servo with any metal linkages or clevises. I noted the throttle arm on the Thor was too short to get a good rotation from the throttle so I removed the throttle arm screw and reattached a plastic servo arm over the existing metal arm. I then used this to connect to the plastic clevis. These kinds of details take time to ensure you have no vibration or interference generated that might corrupt your reception.

Your biggest investment in time will be the engine mounting and subsequent cowl fitting. There is so much room in the interior to place everything and if you wanted, plenty of room for a smoker system.

IN THE AIR

Grass and or dirt are my favorite landing field surfaces and I had no problems with ground handling. Takeoffs were straight and little rudder response was needed. Landings, because of the 12 pound mass, require some runway length. I recommend no less than 50 yards and perhaps a little more if on a hard surface.

Throughout the throttle response I was impressed with the flight characteristics. The Big Thor 45 is a torque engine that spools up plenty of power but provides speed too. Rolls, loops, knife edge and inverted flight within the compliment of normal aerobatic flying were generally no problem and on the 3D side harriers, hovers and axial rolls were no problem but you must retain your airspeed and engine RPM. Stalls are quick and rolls induced by the rudder are even quicker at slow speeds. There is plenty of throttle response to pull you out of any trouble, but getting low to the ground requires your concentration. When you do stall the wing tip drop is quick. Add nearly 50mm of rudder throw and you will be on your back as quick as you can say Katana.

I quickly realized I did not need as extreme of control settings as recommended especially with the rudder. I reset my low rates to just under half of the higher rates specified at 30mm. That provided a more comfortable flight envelope. The recommended CG proved to be perfect and during the initial flight, dialing in small amounts of trim, was all it took to get a nice level flight both normal and inverted.

In the air the big Katana is easy to see and the powerful Thor beats a strong rhythm while swinging a 20 x 8 prop. The bottom side of the Katana is solid red so no issues with distinguishing the top from the bottom. Because I had switched to a smaller 12-ounce Sullivan tank I got about 10 to 12 minutes of flight time depending on power curve requirements. For me that is plenty of concentration time.

General Flight Performance

Overall you get a great flying plane that follows your inputs, but requires you pay attention to airspeed and engine RPM. I flew much of the time at half throttle and had no problems with control. When I dropped below that I found myself reacting and correcting with haste if I was at a low altitude. The Thor is a great engine and other that the bad ignition I have absolutely no problems in the air. I had power-on-demand and throughout the big loops, rolls, inverted flight and hovers I felt the combination was a good choice. Power-off glide as well was stable with some airspeed.

I also used the Airtronics RDS 8000 2.4 GHz transmitter and receiver and had no problems with ignition noise

Pilot Debriefing

I believe the Katana will teach the seasoned pilot to fly better. I noted the rolls were straight and little nose drop occurred after a 360 degree roll. Loops were straight and there was no indication of the Katana wanting to roll out of the loops. Inverted flight was as solid as the normal level flight. I thought the knife edge had some early roll coupling that I worked to remove, but found I needed a lot of throttle to keep the Katana straight and level. In all aspects of flight the Katana and Thor 45 are a joy to fly.

CONTROL THROWS

Elevator – 30mm

Ailerons – 30mm

Rudder – 50mm

Highlights

· Easy Build

· The Thor 45 is the perfect engine choice

· Strong rigid airframe capable of high stress maneuvers

· Huge interior for electronics and CG adjustments.

Gear Used

Radio: Airtronics RDS8000 2.4 GHz with 8 Channel Receiver

Servos: Airtronincs 94162 MG BB

Engine/motor: Thor 45cc CDI

Fuel/Battery: 25:1 synthetic premix for break-in and 40:1 general flying

Prop: Zinger 20×8

Specifications

Model: Katana ARF

Manufacturer: BH Models

Distributor: Hobby People (hobbypeople.net)

Type: Mid-wing fully aerobatic

Wingspan: 71inches

Wing area: 832 sq/in total

Length: 67 inches

Weight: 12 pounds

Wing loading: 33 oz/ft sq

Radio req’d: 6-channel with dual elevator capabilities

Power req’d: 1.60 2-stroke, 2.0 4-stroke or 45 to 50cc gas

Price: $279.99

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