Author: Frank Granelli
Edition: Model Aviation - 2013/03
Page Numbers: 49,50,51,52,53,
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GREAT PLANES DIRTY BIRDY ARF

Frank Granelli [email protected]

A classic performer for modern times.

During the last 37 years, the Dirty Birdy has earned its place as a competition aircraft. How it fits into today's complex RC sport/aerobatics world needs exploration. Because this ARF has few parts and assembles quickly, the building portion of this review will be short. The Dirty Birdy was meant to outfly anything in the air, then and now, making the flying part of this review longer than usual.

Joe Bridi designed this airplane for Precision Aerobatics (Pattern) competition in the mid-1970s as an improvement on his winning Kaos series. The most obvious improvements included increased wing sweep and area, a straight trailing edge, retracts, increased frontal fuselage side area, and an airfoil stabilizer.

The result was an aircraft that rolled precisely, flew straight in knife-edge, required less rudder correction, and had few bad habits. The Dirty Birdy was an easy-to-manage Pattern airplane that sport pilots could make perform on any level, and it remains so today.

Unlike many of its contemporaries, the Dirty Birdy will not snap out of tight turns when flown slowly, must be forced to tip stall, goes precisely where directed, and will gracefully perform any Pattern maneuver asked of it. It always was, and is, a pilot's airplane.

The new Great Planes ARF version is also a pilot's airplane in an easier-to-build fiberglass/foam format. Its airframe is nearly identical to my 1976 wood kit. A comparison showed that the wing's root airfoil and airframe dimensions have only minor improvements.

The engine is side-mounted on a modern fiberglass mount for easier tuned-pipe installation and better looks. The fiberglass cowling is removable rather than being part of the fuselage, making engine and nose wheel installation easier. Twin aileron servos are used for more precise aileron control.

Although the original Dirty Birdy was primarily designed for Masters Class AMA Pattern competition, it probably served as one of the better sport airplanes at many local flying fields. Choose it for everyday flying, and the Dirty Birdy will quickly become your favorite. Even with fixed landing gear, the airplane's precision, smoothness, piloting ease, great looks, and sheer gracefulness will keep your other sport airplanes gathering dust in the hangar.

The airplane will perform precise snap rolls and spins on demand, yet exhibits none of the "snappiness" that is common with scale aerobatics airplanes in this size range. Although the landing speeds are higher than those of a low-wing sport aircraft, they are slow enough that sport pilots will hardly notice the difference.

The Dirty Birdy is also an extremely capable competition aircraft in both modern Old-Timer and non-turnaround-style Pattern classes. The two different forms of Pattern competition are the Senior Pattern Association (SPA) and the Classic Pattern Association (CPA).

Aircraft in SPA competition must have fixed landing gear and cannot use a tuned pipe. Although the Dirty Birdy was designed for retractable landing gear, it will fly the wings off any Hershey Bar (constant chord) 1960s SPA Pattern aircraft. Because of its inherent precision, great slow-speed performance, and gentle handling, it is one of the better-flying 1970s airplanes, even with fixed gear. CPA rules allow the Dirty Birdy to fly as conceived using both retracts and a tuned pipe.

Because the rules for each organization are so different, now might be the time to choose whether your Dirty Birdy will fly for Sport, SPA, or CPA competition—or maybe do all three? But first, we have to assemble the airplane. Because the Dirty Birdy arrived with fixed gear, I'll detail the SPA version.

Construction

The 45-page instruction booklet has 155 photos detailing standard and optional construction steps. That's quite an improvement over the 16-page, text-only directions from 1976, although those were better than the usual, "Here's the box photo. Make it look like this," directions of the day.

The directions start with building the wing. Before installing the aileron servos, cut a small groove in the foam around the servo opening for the wire. The fit is tight.

The spar is constructed by laminating two plywood pieces to an aluminum center using 30-minute epoxy. Mark the spar's center. Insert the spar (without adhesive) and the nylon anti-rotation pin (glue it first with thin CA or it will sink into the wing) in one wing half and test the assembly. When satisfied, apply 30-minute epoxy inside each spar slot, then open the anti-rotation pin hole on one wing root. Join the wing halves as shown in the instructions.

The ailerons are factory hinged. Make sure the servo arms are on the outside when installing the control horns, because that is where the plywood mounting plates are located. There is extra space for the fixed landing gear mounts. Use the front block for alignment when drilling the screw holes.

The entire wing-mounting system is factory installed and perfectly aligned. Some covering must be removed before mounting the two belly pans. Use the scraps to protect the area around the visible bolt holes, which are slightly exposed under the fairing.

There is little work to do on the fuselage. The supplied adjustable mount fits nearly every .60-size, two-stroke engine made. I tested eight brands and they all fit. The chosen engine is the O.S. Max .65. It is well suited to both SPA and CPA competition and is great for sport flying. The firewall mounts are factory made. Fit your engine with its thrust washer 4 5/8 inches from the firewall and drill the mounting holes.

The cowling must be cut out to fit the engine and nose wheel strut. The straight openings don't require any special techniques.

Position the cutout cowling 1/8 inch behind the spinner backplate, align it, and drill the four mounting screw holes. The fixed nose wheel mounts inside the motor mount and Great Planes has already filed the "flats" in the strut for steering and mount retention security.

The stabilizer mounts onto two carbon-fiber spars and is glued to the fuselage using 30-minute epoxy, achieving stabilizer incidence and wing alignment. This great feature's importance cannot be overstressed.

The vertical fin is part of the fuselage, and the rudder and elevators are factory mounted. The only real work required is mounting the control horns. Control rod installation is simple and Great Planes provides full-size templates where needed.

Although standard servos such as the Futaba S3004 will work, a competition Dirty Birdy will benefit from the precision performance of Futaba S3151 digital servos on the control surfaces. Before mounting the servos, reinforce the servo tray/fuselage mounting joints with fillets made from five-minute epoxy and microballoons. Mounting plywood servo trays to curved fiberglass fuselage sides can prove problematic over time, making this precaution a good idea.

Completing the model with fixed gear will take roughly 10 hours. The retract version requires three extra hours. The small time difference is because Great Planes included all of the extra parts needed for retract installation, including axles and the servo mount. Great Planes also installed a plywood mounting plate in the wing for the tuned-pipe installation.

Flight Tests

The Dirty Birdy is a versatile airplane that can perform three separate missions: sport, SPA competition, and CPA style. How you equip yours depends on the mission you choose. This airplane flew more than 54 test flights. Most were flown in sport/SPA configuration, the rest with retracts and tuned pipe (CPA style). When you laterally balance this airplane (a must), leave any balance nails protruding from the wingtip until the flight tests are complete.

Sport flying the airplane is nearly a dream come true. The sport configuration is the same as SPA: muffler and fixed gear.

Several propellers were tested in this setup, but the best was the APC 12 x 6 propeller (11,600 rpm maximum). The 11 x 7.5 standard and wide propellers reached 11,900 rpm, but the climb suffered.

A 13 x 5 propeller (10,900 rpm maximum) provided the highest airspeeds, but with reduced climb performance. The 13 x 5 propeller produced excessive torque and required rudder correction in the climb.

The 12 x 6 APC propeller provided the most climbing ability, while needing the least amount of rudder correction in either the takeoff or verticals. The airplane flew more smoothly with this propeller.

There isn't any non-3-D maneuver you can ask of the Dirty Birdy that it won't fly with a smooth grace that will surprise even its pilot. When this airplane was designed, having computer transmitter mixing would have meant you were from outer space. The airframe was designed for good flight characteristics.

Knife-edge flight is straight in both directions. The airplane has little tendency to pull out of vertical dives. Point it and it goes straight. There is even some tendency to lock onto a 90° vertical up-line should the pilot be 2° or 3° off during the initial pull up (if throttle is slowly applied during the upward quarter loop).

The airplane is faultless in its sport handling ease. Hold full up-elevator in low-speed, steeply banked turns, and the airplane flies slow circles with no tendency to snap out in either direction.

Inverted flight requires some down-elevator. Full aileron and rudder must be used for inverted spins or the airplane will stop rotating. It doesn’t get any easier to handle than that!

Landing places the only demands on a sport pilot. Landings are faster than the usual low-wing sport model, but are much slower than needed by most scale aerobatic aircraft. Keep the airplane in a level attitude during the approach with speeds in the 25-mph range. The airplane will land itself slightly nose high.

Although I have been flying Masters turn-around Pattern, I competed in non-turnaround AMA Pattern back in 1980 and did so until it was phased out. I remember those days and look forward to flying this style again.

As a test, I flew the Dirty Birdy through the Expert SPA Pattern. Flying the slow roll, four-point, eight-point, and multiple horizontal rolls was simple. The airplane stayed on line, needing little piloting.

The Dirty Birdy maintained axial horizontal rolls (they must have been slightly off axial, but I couldn’t see it). However, it did politely ask for retracts during the Top Hat, Figure M, and 180° turn vertical rolls by being slightly out of true axial rotation.

This was not because of aileron differential; the entire airframe was slightly off axial because of the fixed gear. The airplane exhibited no adverse yaw during slow-speed tests, showing that differential isn’t the problem.

Even with fixed gear, the Dirty Birdy maintained sufficient airspeed. No special handling was needed when flying the Double Immelmann or Running Eight maneuvers. The airplane’s excess rudder authority made stall turns beautiful.

This airplane should be popular in SPA events in the coming years. Its performance is a generation ahead of the 1960s aircraft, yet because of its minimal wing sweep and low wing loading, its aerobatic abilities at medium, fixed-gear airspeeds are amazing.

Low airspeeds are not a factor in CPA competition. CPA aircraft usually fly in the 100-mph range, using retractable gear and tuned pipes. When equipped with $20 Hobbico mechanical retracts (HCAP400), the Hobbico Command CS-63 retract servo ($28), and an extra Futaba S3004 servo ($14), the Dirty Birdy becomes a different performer.

Add the MACS Products #1060 Quiet Pipe and the #2861 Long Header to provide an extra 1,000 to 1,500 rpm, converting the Dirty Birdy into the world-class Precision Aerobatics performer that it was designed to be. The instruction booklet contains detailed instructions for the conversions.

The results are amazing. On takeoff, the airplane leaps ahead as the gear is retracted. It gains at least 20 mph in the climb. When leveled out, with the engine turning more than 13,500 rpm (Zinger 11 x 7.5 wide propeller) on the pipe, the airplane not only exceeds 100 mph, but grooves as if it were riding a monorail.

Rolls are perfectly axial in all attitudes, especially the vertical. At these aerobatic airspeeds, the airplane actually climbs in knife-edge while holding a perfectly straight line.

Any maneuver, especially loops, all rolls, Vertical 8s, stall turns, and all snaps, becomes effortless. Maneuvers can be as large or small as the pilot commands. They are always on track and need little correction.

The 6-ounce weight penalty of the retract/pipe installation goes unnoticed by the airplane, and stall and landing airspeeds seem nearly the same. The Dirty Birdy always maintains its great slow-speed handling. As a CPA airplane, its performance is as outstanding as it was in the 1970s, and flying the current CPA Master’s Pattern is enjoyable.

SPA competitions are popular and fun. The same is true for CPA events. What is a budding Old-Timer Pattern pilot to do? If you know me, you know the answer is to do both! Great Planes has made this easy by supplying all of the equipment needed for both configurations.

Use the supplied fiberglass hatch cover when flying SPA rules. Make your own from 1/32-inch plywood with cutouts for retracts. Main gear hatch covers are made from 1/8-inch light plywood.

Instead of cutting up the supplied fiberglass nose-hatch cover for the retractable gear, copy its shape and make a 1/32-inch plywood version with the necessary cutouts. Make both a screw-on installation as shown in the photos. Make a template from cardboard and create two 1/8-inch light plywood covers for the retract wheel openings and screw them into 1/4 x 3/8-inch spruce blocks. The Dirty Birdy’s red matches Top Flite MonoKote True Red, so cover the three “hatch” covers in it.

Removing the retract gear and replacing it with fixed struts took 17 minutes. Extending the nose gear and installing the stock fixed-gear hatch required another 4 minutes (cut a small notch to clear the nose strut).

With retracts and a pipe, performance becomes locked on and extremely precise, while slow-speed handling remains gentle.

Replacing the pipe installation with the muffler required 9 minutes. Changing propellers was another 8 minutes. Total conversion time is 38 minutes.

With 38 minutes of work, my Dirty Birdy can fly as one of my best sport aircraft and also be a world-class SPA competitor. Or, it can again become one of the best non-turnaround Pattern airplanes of all time as CPA events grow in popularity.

Only one real question remains. Why wasn’t I smart enough to be flying a Dirty Birdy in my early competition days? I am going to enjoy this gentle, high performer in the years ahead. —Frank Granelli

AT A GLANCE ...

SPECIFICATIONS

  • Model type: Sport/Old-Timer Pattern competition ARF
  • Skill level: Intermediate builder / intermediate pilot
  • Wingspan: 64.5 inches
  • Wing area: 690 square inches
  • Wing loading: 24.2 ounces per square foot (fixed gear)
  • Airfoil: Symmetrical
  • Length: 56 inches
  • Weight: 7.25 pounds (fixed); 7.6 pounds (retract/pipe)
  • Engine: .60 to .65 two-stroke
  • Radio system: Four- or five-channel, standard-size servos
  • Construction: fiberglass and foam
  • Retail price: $299.97

TEST-MODEL DETAILS

  • Engine: O.S. Max 65 AX muffler or MACS Products #1060 quiet pipe
  • Propeller: APC 12 x 6 (with muffler); Zinger 11 x 7.5W (with pipe)
  • Radio system: Futaba T7C transmitter; Futaba R617FS receiver; four Futaba S3151 servos on flight controls; two S3004 throttle/nose retract; and Hobbico CS-63 main retracts (Y harness on retracts)
  • Flight duration: 11 to 12 minutes
  • CG: 5.875 inches
  • Control movements: Ailerons: 1/2 inch, 35% exponential; Elevator: 9/16 inch, 30% exponential; Rudder: 2 inches, 60% exponential

PLUSES

  • Fast, accurate build.
  • Everything fit well and is competition grade.
  • Extreme precision aerobatic performance.
  • Gentle, but outstanding, sport airplane.
  • Hardware included for sport, SPA, or CPA versions.
  • Fun and impressive to fly.
  • Detailed instruction manual.

MINUSES

  • This excellent ARF was not available in 1980!

Adding a tuned pipe

Two-stroke engines usually react positively to the installation of an adjusted tuned pipe. How much extra power is gained depends upon the engine. The “tuned” pipe chosen for the Dirty Birdy was a MACS Products #1060 Quiet Pipe. MACS has been a trusted, quality manufacturer of these types of products for as long as I can remember.

Choose your propeller. Use all safety precautions (including hearing protectors) and enlist experienced help.

  1. Attach the tuned pipe and line from the manifold to supply tank pressure. Note that the maximum rpm will be higher than when it is equipped with the muffler. (The engine will have roughly 700 rpm more without tuning.)
  2. Remove the pipe. Use a small pipe cutter to remove 1/4 inch from the header only. Install the pipe and note the maximum rpm.
  3. Continue removing 1/4-inch segments until the rpm no longer increases. Stop if the rpm no longer increases, regardless of the rpm gain achieved. Removing too much from the header can make your engine sensitive to mixture settings. I usually stop cutting when a 1,200 rpm gain is achieved. That’s enough for me, even if more power might still be available.

With the APC 12 x 6 propeller, the O.S. Max 65 AX gained 900 rpm in excess of the muffler’s 11,600 rpm. Removing 1/2 inch from the header resulted in a maximum rpm of 12,900. The 11 x 7.5 propeller raised the rpm from 11,900 to 13,300, while the extra pitch resulted in true precision airspeeds.

—Frank Granelli

MANUFACTURER/DISTRIBUTOR: Great Planes Model Manufacturing Box 9021 Champaign, IL 61826 (800) 637-7660 www.greatplanes.com

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Transcribed from original scans by AI. Minor OCR errors may remain.