With the flying season coming up, I have decided to present a truncated version of an article I have been passing along via emails. Some prefer a recipe instead of a story.

The original is still available on this site: http://stunthanger.com/smf/index.php?topic=80.0

Or send an email to dlr111845 at yahoo dot com, asking for Universal Stunt Machine attachment.


My best,

Dan


The O.S. 20FP ABN, as Best Applied to a Wide Range of 35-sized Profile Models, Specifically for use in CL Stunt

Current: April, 2006

Written by: Dan Rutherford
dlr111845 at Yahoo dot com


Brett Buck, third in Open Precision Aerobatics at 2004 AMA Nationals, was first to recognize power, and excellent speed regulation, of O.S. 20FP ABN, and he first did so clear back in 1994, publishing a brief mention in 1999. Reference: March/April 1999 Stunt News, page 19.

For two years I have been promoting this engine--and a very specific tune-up for same--as it is a fact of life that in any hobby activity, the very best way to increased levels of participation is identification and promotion of a package which inexpensively offers high levels of performance and reliability.

The 20FP ABN
1) Use absolutely stock ABN engine. There are no modifications which are known to    improve it.
2) Use stock, unmodified muffler, this being O.S. "E-2030" unit.
3) Use cone-shaped insert in muffler. This piece originally furnished with all E-2030    mufflers.
4) Use stock, unmodified O.S. 20FP/25FP CL venturi.
5) Use stock O.S. "front mount" O.S. needle valve assembly.
6) Use APC 9-4 prop.
7) Use 3-ounce Hayes plastic "RC" clunk tank. Stock plumbing.
8 Use muffler pressure.
9) Use Bru-Line fine (green element) air filter.
10) Use Thunderbolt RC long glow plug.
11) Use SIG 10% Champion fuel.
12) Set engine to 11,400 rpm and fly.

Exceptions to above: I am tempted to say there are none, as I very much want one and all to first fly what is--with absolute certainty--known to work superbly, this having been proven many, many times, before going off the res with a bunch of stuff Brett, myself and others already know do not work to desired results.

Taken in that light, air filter is nice as it leads to increased engine life, and chokes power back just slightly, leading to slower lap times. But it is not an absolute requirement.

There are other plugs which will work well, just be sure it is a "hot" plug, known to be suitable in a wide variety of CL Stunt motors. While not common in CL Stunt, the O.S. #8 plug is excellent.

Cited SIG fuel is nice as it is a known quantity, widely available, very consistent, makes good power. Basic SIG formula of 10% nitro, 10% Klotz, 10% castor, balance methanol can be used to mix your own fuel.

There are no exceptions to items 1) through 8. To the point where if you need help in trouble-shooting your 20FP ABN and have, for whatever reasons, chosen to not follow very specific instructions, there will be no reason to talk.

Detailed package is a system, quite unlike many of us have encountered in the past. Change just one thing and you must also change other components. Don't do it!

Locating a 20FP ABN
These engines are no longer sold new. You'll have to find one, along with differentiating between the older "steelies" as we have never been able to obtain the same results with a steelie as with ABN versions. Parts listing on the Tower Hobbies site pictures the ABN-style case quite nicely. The older and newer 20FP cases are interchangeable.

There are lots of 20FPs out there. They are available all the time on eBay. Don't overlook the many 25FPs; they are easily converted to a 20FP.

Parts for 20FP ABN
All parts for the 20FP ABN are readily available. There are a few part numbers which are troublesome to track down. I can fix that.

O.S. Needle Valve Assembly, referred to as a "front-mount" unit. Part Number 22311000. Fits 20FPs to 40FPs.

O.S. Remote Needle Valve Assembly. I prefer older unit, Part Number OSMG7172.

O.S. Remote NVA bracketry. Again, these are older units. Part Number OSMG2904.

O.S. Nozzle. Part Number OSMG7325. This is spray bar fitted to case and venturi. Required to complete remote NVA installation.

Used engines will generally show wear on Teflon pads at ends of wrist pin. Pads are not separately available, you must order complete pin, Part Number OSMG7644.

For correct E-2030 muffler, supplied with cone-shaped insert, order Part Number OSMG2869. As noted above, this is the same muffler as used on the O.S. 25LA.

Fuel-feed System:
The Hayes 3-ounce clunk tank has become favored tank for 20FP ABN. These tanks are available from the Hobby People mail-order firm, 1.800. 854.8471.

It looks weird, but mounting this tank to the left side of fuselage is a good option.

Stock plumbing works best. Run muffler pressure to upper fitting on tank, opening up lower fitting, use this for fueling, capping it off for flight. During fueling operations, pressure line to muffler functions as a vent. It is best to mount Hayes tank such that protruding "tongue" is low, widest side of tank against fuselage. Do not "lay it down" to emulate hard tanks mounted in full-fuselage models. Kick rear of tank outboard with 1/8" shim.

Details:
With needle valve assemblies I sometimes prefer frequently shunned remotes for profiles. I am using old-style pieces, not newer ones supplied with "LA" series of engines.

Earlier O.S. part number for older style (metal) brackets was cited, and these normally work pretty well, although they can complicate your life if much vibration is transmitted to the NVA. So when dealing with a remote, I intentionally leave NVA to simply flop around, held in place only by tubing from tank to NVA, tubing from NVA to inlet at venturi.

This works!! NVA is nicely isolated from vibration, is tucked down and out of the way, comes readily to hand when needling the mix as the knob on needle faces to the rear. Try it!

There are a few APC props--of several sizes, certainly 9-4s--coming through with drilled hole for prop shaft not centered. However, .375-diameter molded depression on back side of APC 9-4 props is centered. Fabricate a drill guide from hardware store-sourced .375 x .250 brass bushing. Drill bushing to 9/32", using it as a guide in drilling hole for prop shaft over-size and centered. Install prop bushing cut from 9/32" O.D. brass or aluminum tubing. A drop of thin CA will make it permanent.

The mufflers like to come adrift, and best fix I have found to date is to look for a Du-Bro rack of metric fasteners. You want 3.0mm x 30mm cap screws, Du-Bro number 2127.

Still with muffler, there is a really good reason why O.S. installs that long through-bolt with locking compound in rear cone and a jam nut external to rear cone.

The ARF Flite Streak:
While there are a great many 35-size profiles--and even at least one full-fuselage model, the Super Combat Streak--which are suited to power and power characteristics of a 20FP ABN, by far easiest and quickest model to get flying is the ARF Flite Streak.

The next step is to determine how much effort you want to put into modifications to basic model. Suggestions:
A.CONTROLS:
1) Leadouts of .027 stranded material, bushed with 1 1/2" length of 1/16" brass tubing.
2) Replace stock leadout guides with metal tubing if there is any binding.
3) Use inner of three holes on stock bellcrank for output to pushrod.
4) Fabricate stiff pushrod.
5) Hinge surfaces with over-under film hinges fabricated from MonoKote.
6) Use .015" x 60' lines, centerline to centerline.

B. ENGINE MOUNTING:
1. Temporarily mount Hayes 3-ounce tank. Tank should be vertical, pressure fitting up,    protruding "tongue" mounted low.
2) Stock motor mount sticks are pretty soft, to the point of being only weak area of consequence in this model. With position of engine located such that tank is cleared, drill engine mount holes. Drill again to 1/4". Plug holes with 1/4" hardwood dowels. Use epoxy. Trim dowels flush. Drill mounting holes again.
3) No motor offset. Be careful there is no left thrust, but any significant right thrust is counter-productive. With prop blades level, measure from the tips to leading edge in determining left/right thrust setting.
Also of concern is up/down thrust. With prop blades vertical, measure from tips to trailing edge of wing.
4) Use 1 3/4" x 1/2" pads of 1/16" or 3/32" hard aluminum under engine mounting lugs.

C. AEROLIGNMENTS:
1) No rudder offset.
2) Stock tip weight located on top of right tip. Dig out the bird shot, fabricate a weight box on lower side of tip.
3) Balance 1 5/8" from leading edge of wing. It is strongly suggested that your ideal balance point will not vary more than 1/8".

D. DETAILS:
1) Fabricate single landing gear strut from 3/32" hard aluminum sheet. Bolt to inboard side of fuselage. Try to keep axle of wheel in suggested location per kit parts.
2) Cheap tip skid using stock control horn.
3) Hayes 3-ounce tank should be installed such that outlet nipple on tank is 1/8" low in relation to centerline of engine.
No matter tank chosen, I have found ability to shim tank up or down in an accurate and repeatable manner to be invaluable. We're talking about increments of 1/32" to 1/64" here...
Position Hayes 3-ounce tank on fuselage with fuel outlet centered on engine, locate positions for (2) upper hooked rods to hold tank in place. Drill to 13/64". Pound short lengths of Pylon Gold-N-Rod (the 4-40 size) into these holes. Glue with thin CA. Trim flush.
Lower tank to have fuel outlet 1/4" below centerline of engine, repeat above for (2)    lower hooked rods.
While at the hobby shop buying Gold-N-Rod pick up some of those metal rods which are threaded 4-40 on one end.
Bend a hook in rods. Don't get tricky in bending what looks like a cup holder or    rubber-power prop shaft! I want the rods to come straight out of embedded    receptacle, stay straight to the hook, which is simply rod bent up and then back on itself, clipped to length, filed smooth.
This leaves pretty stout, straight rods projecting from fuselage, handy for accurately locating tank and then holding it in position.
Thread rods into Gold-N-Rod pieces embedded in fuselage. If above was not clear, with hooked rod fully threaded into fuselage side, I like to see about 1.25 inches of rod extending from fuselage to end of hook.
With the lower rods, in some installations there is not room for full-length rods in side of fuselage. In this case I will embed Gold-N-Rod receptacles in lower side of fuselage. Much shorter hooked rods are used. Again, these are bent from 4-40 threaded-one-end rod. Or, as effective and cheaper, SIG sells packs of 4-40 threaded hooks which work well for lower attach points.
In order to securely and reliably attach tank, form a loop from medium silicone fuel tubing. Close loop by inserting short length of all-thread 4-40 rod in open ends, wrapping tubing to rod with serving wire.
Install tank, positioning it such that fuel outlet on Hayes 3-ounce tank is about 1/8" below centerline of engine.
Fabricate a shim which almost fills space between upper rods and tank. Ideally, shim will be a piece of plywood approximately 1 inch wide by 2 inches long. Fabricate a few shims to same dimensions from 1/64- and 1/32-inch plywood. Balsa shims tend to get mashed, allowing tank to shift around over time, something we are trying hard to avoid.
With slightly under-size ply shim between upper rods and tank, silicone band strapping it in place, fabricate an over-size shim or block from hard balsa, maybe even a chunk of rubber, inserting it between bottom of tank and silicone band. The band will "give" enough to allow this, pressing tank up and against upper rods and shim.
At the field it is simple to fly, insert shim(s), fly again. Or if you missed on guessing certain dimensions, to replace upper shim with a stack of 1/32" and 1/64" shims.

Choices in Models:
This is a subject which could encompass a lot of models, especially considering the great many 35-size, 400-square-inch profiles out there.

A kit- or plans-built Flite Streak would be quite nice. Barry Baxter has excellent plans he has drawn in CAD, he can be reached at 3292 Greenleaf Drive, Brea, CA 92823. Better, go to barenekd@aol.com for a look at his offerings, ordering information.

The Super Combat Streak is a really sweet little model. Upright engine, built-up fuselage, Classic-legal, very competitive in PA, you just can't go wrong here. No kits are available that I know of, but Barry has truly excellent plans.

The "Ukey" series of models are well suited to the 20FP ABN. The Ukey 35 is a natural choice, although if it were me, I would go with the Ukey 40. Ukeys are available from Jim Pearson, 4112 Devon Wood Drive, Marietta, GA 30066. Or phone 770.516.0398.

SIG Skyray 35 is model which started all this enthusiasm for 20FP ABN and Brett's model is stock, the only exception being wing ribs. First time it went in, the relatively heavy lite-ply ribs just kept going forward, snapping wing in half. Model was rebuilt with balsa ribs to stock configuration, has since hit the grass around two dozen times with no or little damage aside from a broken prop.

I have taken a good idea to the max in fitting a 20FP ABN to my Zilch X OTS model. This is not a small or particularly light model, 50-inch span, 600 square inches, 37-38 ounces depending upon trim for balance point.

A caution: The Ringmaster would be nice as it is legal for OTS, Classic, P-40 and PA. However, typically it will be too fast in level flight when fitted with a 20FP ABN. It's a lot better with a 15FP. Keep this in mind when evaluating other designs.

At the Flying Field:

If the engine is new, simply nail model to ground with a stooge, fill tank, start engine. Yes, with muffler in place, flying prop, flying fuel, etc.

Set needle valve to run engine at approximately 10.5K to 11K. There seems to be no advantage whatsoever in short runs, rich runs, fully-peaked runs, all the other blah, blah, blah frequently loosed upon us concerning engine run-in.

With all prepped for first flight, start engine. Peak revs to Full Kill setting, building some heat. Back off to 11,400 rpm.

If concerned, short-tank the thing with an ounce or less, this option being an attractive feature of clunk tanks.

Assuming a well-behaved model, you are looking for a "wet" or "medium" two-stroke in level flight. Admittedly, this is not the easiest thing to hear. A muffled 20FP ABN gives off confusing vibes compared to a great many engines.

So while experience equals increased accuracy in nailing the setting, for now you do not want any four-stroke running at all, nor do you want to see a very fast, lean two-stroke setting.

Enjoy yourself!


Dan