FLY RC MAGAZINE  PROJECT PITTS

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HALF-SCALE

Pitts Challenger

by Bill Hempel PHOTOS BY RICK ADAMS

I modified the original kit slightly to improve performance and reduce weight. One of my main goals was to keep the airplane as light as possible without compromising integrity or strength. I also sought scale accuracy and a pleasing appearance.

Build this airplane light and do not add extra weight while finishing. While a large scale model like this would look great covered with fabric and paint like its full-scale counterpart, I used Monokote to save a lot of weight and time. I also recommend sheeting the forward section of fuselage with lightweight contest grade balsa to save additional weight, rather than using the fiberglass panels of the kit. This alone saved one to two pounds. Monokote adheres to the wood well, while the fiberglass would need to be painted. Keeping off the extra ounces is important even on a plane this large.

Because of the size of the finished airplane, I also modified the kit for ease of transport. The horizontal stabilizer is now removable. Each half of the stab is fitted with a Gator R/C wing adjuster so the stabilizer incidence on each side can be fine-tuned for optimum tracking and trim.

The motor box was also made to be removable so that the engine and exhaust system can be removed as one integral unit. I can now pull the engine and exhaust in just 15 minutes. With a removable motor box, the minimum length of the fuselage is reduced by an additional 18 inches. This also helps shrink the overall size of the shipping crate.

SMOKE AND HORSEPOWER

I needed the most powerful and reliable engine possible for this project. The kit manufacturer recommends a 150cc displacement engine. Since I am fortunate enough to be sponsored by 3W, I soon had them on the phone discussing the new four-cylinder 3W-212 engine. Imported by Cactus Aviation, this four-cylinder engine is basically two 3W-106s put together, and it puts out over 21hp! The engine can run up to a 36x10 propeller, but for better speed and performance I fly with a 34x14 3W two-blade propeller. The Pitts has an impressive 2:1 thrust-to-weight ratio with this engine and prop.

Because of the size of the airplane and engine, I was quite worried about the noise it would make. I chose to install the 3W exhaust canisters since many RC clubs now impose noise abatement standards. The airplane had to be modified extensively to accommodate these large canisters. A custom insulating box was installed to prevent the heat from penetrating through to the fuel tank and radio compartment. I made this compartment from 1/16-inch balsa and fiberglassed it for additional strength.

A CLOSER LOOK AT KIT DETAILS Exclusiv-Modelbau from Germany has accurately reproduced Sean D. Tucker’s world famous Pitts Challenger in this 50% scale RC version. Desert Aircraft in Tucson, Arizona is the US importer for this and several other unique kits from Exclusiv-Modelbau. The airframe is all plywood and balsa wood construction with fiberglass wheel pants, cowling, landing gear, side panels, canopy hatch, and I-struts. The wings have lite-ply ribs and balsa sheeted leading edges. They are built as four separate wing panels to reduce the overall size for transportation and for scale accuracy. Both the top and bottom wings have mounting tubes, and the streamlined flying and landing wires provide additional strength to keep the wings in place. All ailerons are built and hinged in a scale manner with a gapless hinge. The ailerons can be removed by pulling a long wire hinge pin that goes through each hinge. Each wing panel requires two aileron servos, for a total of eight servos in the wings alone! The fuselage is built with lite-ply construction and a balsa sheeted turtledeck. The forward section of the fuselage is “skinned” with 10 pre-made fiberglass panels to give the airplane a very scale appearance of simulated sheet metal. These panels are designed to be glued in place and then painted. All the fiberglass parts have a white gel-coat finish to eliminate pinholes and are ready to paint with very little additional work. The canopy hatch frame is also fiberglass. The canopy is mounted by installing screws through the canopy into the hatch and the hatch is secured to the fuselage with four bolts. FINISHING I finished my Pitts with Top-Flite Monokote. It took six weeks of spare time and 125 feet of Monokote (five 25-foot rolls) just to cover the airplane. The cowling, wheel pants, landing gear, propeller, and struts were all painted using PPG’s base-coat and clear-coat finish system. After the airplane was painted and covered, I added over 450 No. 1 screws to simulate the realism of the removable panels on the real Pitts Challenger. While these screws did add two ounces, the finished look was well worth the extra weight.

The four cylinder 3W-212 is a perfect match for this airframe. Note again the engine baffling, another implementation of full scale practice.

DUAL IGNITION

The 3W-212 has two ignition units, supplying spark to two cylinders each. If one failed, you could still safely land the airplane on the remaining cylinders. This engine also uses two carburetors–one for each set of cylinders. I installed a separate 32-ounce Du-Bro fuel tank for each set of cylinders. In theory, if I was running out of gas, I would probably lose one set of cylinders before the other. Though I don’t want to test it in the air, this should give me an opportunity to get the airplane on the ground before losing all power.

Since the Pitts Challenger is to be used for freestyle competition and 3D flying demonstrations, a full smoke system had to be installed. A Sullivan Products electronically controlled SkyWriter smoke pump was chosen for its light weight and miniature microprocessor-controlled electronic pulse drive speed control. This allows you to set the smoke rate with your transmitter trim and mix the smoke with the throttle channel if desired. This pump works with 6v to 12v battery packs. The pump is being run from the receiver battery without problems. The pump weighs only 3.9 ounces and pulls smoke fluid from a Du-Bro 40-ounce tank.

RADIO WIZARDRY

Multiple servos must be used to drive the big control surfaces of an airplane this large. To reduce the risk of flutter and lack of authority or “blowback”, high quality coreless, ball bearing servos were used. I chose Futaba’s new S9351 digital servos for elevator and aileron control, and three Futaba S5050 digital servos to drive the rudder. Each of the S9351 produces 180 ounces of torque with a speed of .15 seconds per 60 degrees of travel. A single S5050 produces 264 ounces of torque with a 60 degree speed of .20 seconds. Eight aileron servos, four elevator servos, three rudder servos and one throttle servo: a total of sixteen servos are needed to control the 50% Pitts Challenger.

I control all this gear with the Futaba 9Z WC2 series transmitter and a Futaba 149DP 9-Ch PCM receiver. In order to plug 16 servos into the receiver, I am using an Emcotec Dual Power Servo Interface unit. This unit allows up to 26 servos to be plugged into a single receiver. It also accommodates dual battery back up.



The DPSI monitors the status of each battery and electronically switches between the two as needed for power. The unit will ignore a failed battery and continue operating the system with the good battery. Both airborne batteries supply power to the DPSI board and receiver through a heavy-duty failsafe switch. The switch is designed to maintain power to the electronics should there be a mechanical failure in the circuit. The DPSI board supplies 4.8 regulated volts of power to the receiver. The servos are regulated to 6.0 volts for additional power and speed.

It is very important to minimize the voltage drop on long leads. Every servo in the Pitts Challenger uses a Hobbico 18- gauge servo extension to connect directly to the DPSI. This heavy gauge wire helps reduce the voltage drop normally seen in long servo leads. There are no “Y” connectors in this plane.

All wiring in the Challenger passes through the Emcotec DPSI-2001. 26 servos could be controlled with a single receiver, though only 16 are needed for the Pitts!

FIVE ONBOARD BATTERY PACKS

I decided to use the Powerflite Lithium-Ion battery system with variable rate regulators. I can adjust the nominal 7.4 volt output of the batteries to suit the different requirements of the radio and ignition system. There are five battery packs in this plane. Two 4400mAh Powerflite Lithium-ion batteries, regulated to output 6.5 volts, supply receiver and servo power to the DPSI. Two 2200mAh Powerflite Lithium-ion batteries, regulated to output 5.5 volts, power the ignition units. A nine volt transistor battery handles the on-board digital tachometer. The Powerflite battery system will allow up to 12 fifteen-minute flights per day. That is over 180 minutes of flying–a great way to maximize practice time!

ONBOARD VOLTMETERS

I chose the i4C Products C-volt digital voltmeters to verify all is well with the batteries prior to takeoff. These digital voltmeters are very lightweight and have large displays. Simply plug the unit into any open slot in the receiver to view voltage. I installed five of these units to monitor the condition of the batteries and DPSI regulator. By connecting two into the charge lead of the receiver batteries, I am able to view the unregulated voltage, a true indication of battery health. These two C-volts are mounted on the rudder tray next to the batteries. Two more C-volts are mounted next to the ignition switch near the engine cowling to monitor the ignition unit power. As you fly multiple flights you can actually see from the voltage drops how much power you are consuming. If you were to have the C-volt display the regulated voltage you would never see a change in output voltage until the battery was almost completely discharged.

A last C-volt is installed a little differently. It is mounted on top of the fuselage in front of the canopy. When the aircraft power is turned on, this unit will display regulated voltage from the DPSI board for the servos. Once I start the engine, it turns into a digital tachometer displaying engine RPM! This unit has a hall-effect sensor at the front of the engine. It utilizes the engines ignition magnet on the hub to display engine RPM. After the engine has been shut down, the unit displays the max RPM recorded during the flight. After 30 seconds the unit will revert back to a voltmeter. The C-volt is a great safety enhancement in this application since it allows me to monitor RPM’s without getting near a very large rotating propeller. I strategically placed this unit for maximum visibility when performing a run up prior to takeoff.