Dirty Birdy What’s Up Front: The OS Max .65AX B
reaking in the OS Max .65AX was re- ally easy. The instructions are the best I’ve seen, particularly on basic operation and break-in. They are well worth your time to read. As I turned the engine over by hand, I felt a definite tightness as the pis- ton passed through top dead center. In this type of ringless engine, the tightness is an indication of a well fit engine that is going to have good power and long life. The engine started easily on the needle valve settings it came with. The tightness lessened after the first run as the piston fits itself perfectly to the sleeve. After a couple of runs the engine was clearly ready for flight. It has great compression and runs strong. It will hand start easily, although with sharp edged props, it’s not recom- mended. The carburetor has a swept back
needle valve design that keeps your valu- able flying fingers a bit further from the prop.
After a few flights on the Dirty Birdy the
OS .65AX showed peak rpm of 11,500 on an APC 12–6 prop, with idle at 2,500. I used flight settings of about 11,000 and 2,700. The resulting performance is great—good high speed, and plenty of pull for vertical maneuvers.
A critical part of an easy starting engine is a properly lighted glow plug. I like a sys- tem that provides not only a strong glow, but gives me an indication that the glow is there. Seeing the needle in the green does that. So I need only to crank the engine, and get fuel to it for a quick start. The Ni starter is available at
www.sonictronics.com or by phone at 888-721-0128.
gine mount, then tank, then engine. I’d sug- gest tank first; that will go a little easier without the engine mount on the firewall. As the manual notes, a two- or three-line tank system can be used. I prefer the two- line system as it is a bit less weight, and one less joint to become a possible leak. I recom- mend using different color fuel tubing for the fill/carburetor line, and the overflow/ muffler pressure line. Blue for the carb line, and red for the muffler pressure is my stan- dard choice of colors. The 260cc tank (about 8.8 ounces) fits into pre-installed formers. It took a bit of jiggling to work the tank into position. I added a bit of silicon adhesive at the tank to firewall joint.
PHOTOS: KAREN LOCKHART
Ron contemplates the original Great Planes Dirty Birdy kit plans from the 1980s (above) with the current day DirtyBirdyARF. The top (below left) and bottom (below right) color schemes are dramatically different; this is a great aid to maintaining pilot orientation in aerobatic maneuvers.
Then continue with the engine mount, engine and nose wheel strut per manual instructions. The nose gear fit into the mo- tor mount was tight; I loosened it by gen- tly running a 5⁄32-inch bit through the holes. Holes and T-nuts for the engine mount are already in the firewall. Down and right thrust is built into the firewall as well. Not my favorite job, but fitting the cowl is next. It can be a pesky process to get the cutouts for engine head, needle valve, muf- fler, fuel lines, and such in the right place. The manual suggests using templates taped to the fuselage to mark cutout loca- tions. I used the template process and find it very helpful. Manila folder stock works well for template material. It will probably take several iterations of rough cutting the template, trial fitting and marking for fur- ther cuts: cut some, trial fit again, etc. Make some reference marks on the fuselage and the template, to aid in getting the tem- plate back onto the firewall in the same lo- cation each time. I found a hole in the tem- plate for the nose gear with an allen wrench placed through into the nose gear mount was a handy aid to re-locating the template.
When you have a template done, use it to mark cut lines on the cowl. Cut openings a little less than you may need and cut further after trial fitting the actual cowl. Plug ex- haust and carb to keep fiberglass dust out of the engine.
Just one more step finishes the cowl work: installing the mounting screws. I made a
58 NOVEMBER 2012
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