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Fox 15 Schnuerle for DDD conversion

Old 11-12-2010, 11:20 PM
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Lou Crane
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Default Fox 15 Schnuerle for DDD conversion

I saw a thread asking comments on this DDD conversion, but am not sure if it is in here or some other site...

In any event...

I HAVE flown a few models on DDD converted Fox 15 Schneurle engines. In general, on decent fuel, the conversions start, set and run as desired. Not satisfied with my memory, I dug out one such engine, so converted. Turned out that the castor-gum monster had settled in on the specimen. Boiled it out in a crock-pot, in anti-freeze, overnight.

Further caveat: I'm now brewing my own fuel based on John Deere Starting ether, Shell #2 auto diesel, and SIG Castor, with ~1% AMSOIL Diesel Ignition Improver.

The purified engine went back together delightfully! The "crunchy" BB's cleaned up perfectly, on reassembly with Marvel (red) Mystery Oil and Rislone (green) oil supplement. (Some WD-40 was used - after the crock-pot - to rid the interior of rinse water.)

Benching followed, using fuel of 30% JD ether, 25% SIG Castor, 1% AMSOIL DII, balance Shell #2 auto diesel. It was a bit reluctant to make the first start, at roughly estimated initial needle and comp settings, but eventuallly lit off - rich and undercompressed. Tuning approached 'clean' running on the first tank (20cc fuel). Cooling and subsequent runs more closely approached a good running setting. ( NB: I live at about 5,000 ft above above mean sea level.)

Noted in this process: this is a thirsty beast. Porting is as stock Fox BB Schneurle for glow operation. The engine seemed hardly to warm up! (Local temperature ~ 65┬░ F.) The exhaust was visibly wet, but at comp settings, near optimal - (lean fade evinced at leaner needle settings).

Also noted in this procedure: the spinner nut for the 10-32 shaft stud cannot be tightened enough to prevent kicking loose on a hard 'bump.' A hex nut, tightened with an adjustable wrench (monkey wrench) does the trick.

After 4 or 5 more"re-seating" runs - each followed by full cooling - further tweaking brought RPM and solid running up nicely. Restarting - due to the little warming observed - was a toss-up between more comp for cold starting and less for hot starting.

On an APC 8-4, last observed RPM was ~ 12,000 RPM, still with excess oil and liquid exhaust output. Time per 20cc was about half that for "designed-as" diesel engines on this fuel blend, and the engine was still not hot on shutdown.

This engine has a 3- or 4-bypass sleeve (the 'boost' bypass, opposite the exhaust, is split with a rib in the sleeve) which affects the excess delivery of fuel/air to the combustion chamber, and thus most likely, the 'wet' exhaust and high consumption. Planned: block off the 'boost' bypass sleeve ports, so that both bypass 'effects' are corrected: less raw fuel/air delivery to the combustion chamber, and higher velocity charging through the smaller port volume.

The engine has given excellent service in glow operation for various high-performance and high-RPM applications. It IS sturdy! Fortunately, the crankcase casting has a 'bay' for the excessive 'boost' port flow, which can hold a balsa 'plug' in position to obstruct boost port flow.

More on that as things evolve.

To date, I can say the DDD head makes a very apt conversion, although the engine layout itself is subject to enhancement. A strong and easily operated engine results from a simple conversion, but better results may be accessible when the boost bypass is removed from the operation. Years ago, I DID block off the boost ports, in the interest of greater economy. That worked well, including a 45 minute flight on 2 fl oz fuel, in a CL model. ( I learned much from that effort - I was investigating an approach to a CL endurance record, when AMA event rules required a max of 4 fl oz fuel, an .09 or larger engine, and minimum flight radius of 52'.) One thing I learned was that my knees did not take kindly to such an extended flight duration! Another thing was that weather can vary - EXTREMELY - over an hour or so! ( A dust-devil - mini-equivalent of a tornado - passed through my circle during the 45 minute flight!)

Things like this cause me to consider why the designed-as and converted-to-diesels - #1 - DO work, and #2 - may need some help to do so for best results.

More, as promised, as it emerges...
Old 11-14-2010, 04:24 PM
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Default RE: Fox 15 Schnuerle for DDD conversion

Lou,
As usual you write beautifully and I can well see the story unfold as you progress into it. Nice work mate.

The blocking off the boost port opposite the exhaust - if this port has a longer duration than the side transfer ports (and in most engines will be the case) then blocking it off will also lower the timing or increase the blow down period, no?

And I agree with your assesment, the stock engine would seem to be passing too much fuel through it and if you can limit this and prolong the captive charge time then it should run 'drier' for you.
And have you restricted the intake with a smaller venturi insert or larger diameter spray bar at all?

Talk soon, Chris.

Old 11-15-2010, 09:19 AM
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Default RE: Fox 15 Schnuerle for DDD conversion

Chris, I am interested in this as well, I have a Fox .15 conversion but have only run it on Davis ABC mix and don't remember it running any wetter than other conversions. I have mixed up some fuel using the new Magic, Clean, Green truck Diesel fuel and it runs wet no matter how you mix it. It may be the fuel Lou is using that causes this. Maybe Lou and I can compare prop sizes, rpm, etc for info purposes. The big pipe is a Mac's .90 sized tuned, quiet pipe and has no effect on the little Fox' rpm. the prop is a Graupner 10.5x5.
Old 11-15-2010, 11:31 PM
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Default RE: Fox 15 Schnuerle for DDD conversion

Thanks, Chris and Hobbsy! You've both contributed much thought-provoking info in this forum - which I relish...

#1 - Hobbsy: The key phrase is that you noted no more 'wet' exhaust compared to other conversions. I was comparing with "designed-as" engines. On my tame, plain-bearing type home-brew, comparable 'designed-as-diesel' engines turning the same prop at comparable RPMs got about TWICE the time per cc!

#2 - Chris: The problem with "excess bypass passage" volume in modern (schneurle ported) engines, and for those venerable Fox 35 Stunt engines which "burp" at severe g-transitions required in CL Precision Aerobatics (my particular first love*) has been attacked by blanking out part of the bypass channels. More recent Fox 35s apparently had overly large bypass volumes, and some fliers experienced the 'burp.' A popsicle stick in the (single) bypass reduced the volume so that flow velocity had to increase through the smaller volume, which itself assisted the fuel air mixture to remain better evaporated (for the volatiles in the fuel) or atomized (for the non-volatile oil fraction.) BTW, the 'burp' was an audible 'sneeze' with little effect on the actual flight. It just sounded 'off' and unsettled the flier more than the judges. (That, however, is PLENTY! - an unnerved flier is less likely to achieve properly-formed figures in an event where the model's track is judged to handfuls of inches, and a pair or three degrees of "line elevation" angles... In CL we are never more than 70'-plus-height-to-flier's-shoulders above ground. WE live in ground chop, less than one mistake high...)

Both - there is another topic in this forum discussing bio-diesel and/or RC throttling a diesel (probably one each...). I did mix some bio-diesel back in the 1970's, per an article in Aeromodeller (UK) magazine. Short summary: all the finicky handling and excess compression setting problems mentioned in the 'RC Throttle diesel' topic. Worse: hotter, less power and drifting-setting problems.

And any bystanders: - The rule of thumb for trying to throttle a model compression ignition 2-cycle (aka: diesel) engine is to start final tuning from a 'tad' or 'tad-and-a-half' leaner needle, and a 'quarter-tad' higher compressed c/piston. Observe response in-flight and tickle it to the best - which won't be far away...

I had a throttled diesel-conversion I was breaking in at my local RC field many years ago which makes an interesting anecdote: When it was apparently ready for flight, I wanted to make sure it wouldn't flame out on rapid throttle changes. So, set up my (portable) break-in stand and got the engine running on a fairly large tank. When warmed up, I set it to idle and wandered away. Idle settings are very fuel economical, if you hadn't noticed... Every few minutes, I'd wander past my test-stand, slam it from idle to (relatively) full-throttle, and back to idle a few times. (That 60%-70% max-practical throttle quirk was alive back then, too.) rumble-rumble - ROAR - rumble-rumble - ROAR- rumble-rumble...

So, it can be done...

Another OBTW: There are RC throttle equipped, "designed-as" diesels available, at least, from either PAW (UK) or Dr Diesel (Eric Clutton - PAW USA rep) who advertises in Flying Models mag. Ed Carlson, Phoenix, AZ, also carries several RC-throttle-equipped diesels in many sizes, from many parts of the world. His site-master eloped a while back so there's been little updating, but he is available on the telephone and enjoyable to chat with.

* - RC is fine, but I do not feel connected with the model in flight. CL - for me at least - is tactile. If I had that kind of money, I wouldn't buy a top-line Ferrari for someone else to drive for me: I'd want to drive it and feel all the sensations through my fingertips and toes, not just through the seat-belt restraints...

We are free to do what lights our fires. For me that is the direct muscular involvement of control-line flying. It's a hobby, something we do of free choice because we like to do it. ENYA engines have a website, and several recent engines types, which - as I recall - include many RC Throttle versions. (If nothing else, properly treated ENYA engines will provide solid, dependable service for a very long time. It's up to YOU to treat them as they deserve, to get the best from them.)
Old 11-16-2010, 09:24 AM
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Default RE: Fox 15 Schnuerle for DDD conversion

Lou, you may have touched on why the LA series OS engines are such fine Diesel conversions. They have no rear transfer port, only the two side ports This LA .65 is very clean throttling and in turning a 13x8 three blade at 8,800 is powerful.
Old 11-16-2010, 06:24 PM
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Default RE: Fox 15 Schnuerle for DDD conversion


ORIGINAL: Lou Crane



#2 - Chris: The problem with "excess bypass passage" volume in modern (schneurle ported) engines, and for those venerable Fox 35 Stunt engines which "burp" at severe g-transitions required in CL Precision Aerobatics (my particular first love*) has been attacked by blanking out part of the bypass channels. More recent Fox 35s apparently had overly large bypass volumes, and some fliers experienced the 'burp.' A popsicle stick in the (single) bypass reduced the volume so that flow velocity had to increase through the smaller volume, which itself assisted the fuel air mixture to remain better evaporated (for the volatiles in the fuel) or atomized (for the non-volatile oil fraction.) BTW, the 'burp' was an audible 'sneeze' with little effect on the actual flight. It just sounded 'off' and unsettled the flier more than the judges. (That, however, is PLENTY! - an unnerved flier is less likely to achieve properly-formed figures in an event where the model's track is judged to handfuls of inches, and a pair or three degrees of "line elevation" angles... In CL we are never more than 70'-plus-height-to-flier's-shoulders above ground. WE live in ground chop, less than one mistake high...)

Hi Lou,
I don't know if this slipped through the cracks before but part of the advantage of blocking off the boost port in a schnurle engine is lowering the timing of an engine since that port tends to be angled and timed higher than the two remaining side transfer ports.

If you completely block that port off you will increase the total blow down time (the difference between the exhaust and first opening of the transfers) and then of course increase the velocity of the charge coming into the head by restricting it to the remaining two ports .... but some engines are reliant upon the boost port being in operation to make good power. This is usually the case when using methanol based fuels and at higher revs.

The Fox 'burp' thing I know full well about and is based around the crankcase charge being subject to G forces and thrown into the head upon doing certain manouevers.

I have MVVS's dieselised that are prone to the same thing by virtue of having a fourth unused vestige of a transfer port that collects fuel when running and then throws it out again when the G forces can get to it!

Cheers.

Old 11-17-2010, 05:51 PM
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Lou Crane
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Default RE: Fox 15 Schnuerle for DDD conversion

ORIGINAL: Recycled Flyer
... part of the advantage of blocking off the boost port in a schnurle engine is lowering the timing of an engine since that port tends to be angled and timed higher than the two remaining side transfer ports.

If you completely block that port off you will increase the total blow down time (the difference between the exhaust and first opening of the transfers) and then of course increase the velocity of the charge coming into the head by restricting it to the remaining two ports ....
Chris, a few nattering points?

I've measured the 'sleeve timings' on several glow-schneurle layout engines, of many makes. For starters, I agree that the 'boost port(s)' is/are 'aimed' to assure the desired flow patterns at high RPMS. However, their 'port-opens timing' begins after the primary bypass ports have cracked open, by several shaft degrees. The angled shape enhances flow, yes, but it does not increase the duration, or advance the 'time' in each revolution when that port opens.

Blow-down. I had first approached the shaft┬░ sleeve timing effects as you described. George Aldrich saw some comments I'd made in a newsletter, or possibly even in an early Stunt News, and took me to task about that. Actually, I did know better: my high school had an excellent engine-test shop course. We studied combustion engine operational cycles, various test procedures (even including a leather-shoe Prony brake, and other types, for torque/horsepower measurements!), efficiency, heating/cooling and much else.

Blowdown, in a 2-cycle, is (or was 50 years ago) defined as the difference in shaft┬░ between the total durations of exhaust and bypass port-open conditions. A different term is needed to describe the shaft┬░ lead/lag between ports opening and closing. The exhaust port leads, i.e., opens earlier than the bypass port, and lags (closes later) than the bypass, obviously. Example?

I 'retime' Fox Stunt 35s to EX=130┬░;BY=120┬░ duration when the individual sample engine permits.

EXOpens 115┬░ ATDC; BYOpens 120┬░.

Blowdown is 130┬░-120┬░ = 10┬░.

Port 'lags' are 120┬░-115┬░ = 5┬░. (Fox Stunt 35s have been symmetrical for several decades...)

In a symmetrically laid-out 2-cycle engine, sleeve port lead and lag are equal, and each is half of the total blowdown.

DeSaxe engines, with the cylinder axis and shaft axis not intersecting, - "cylinder is offset from shaft axis" - have non-symmetrical timing and piston accelerations! Determination of TDC is also not simple. Dwell in the vicinity of TDC is different, too. Power stroke duration (in measured time) is briefer than bypass/charging (measured, not shaft┬░) time if the offset places the con-rod more nearly vertical during the power stroke. I suspect the lower rod angles during the power stroke were the key idea behind the DeSaxe layout - less side-force rubbing the piston against the sleeve...

Amazing what we can do with only "three" moving pieces: shaft, rod, piston (granting that there is a piston pin and usually some form of restraint to keep it where it belongs)!

Simple sometimes isn't...


Old 11-17-2010, 06:55 PM
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Default RE: Fox 15 Schnuerle for DDD conversion


ORIGINAL: Lou Crane

I've measured the 'sleeve timings' on several glow-schneurle layout engines, of many makes. For starters, I agree that the 'boost port(s)' is/are 'aimed' to assure the desired flow patterns at high RPMS. However, their 'port-opens timing' begins after the primary bypass ports have cracked open, by several shaft degrees. The angled shape enhances flow, yes, but it does not increase the duration, or advance the 'time' in each revolution when that port opens.
Ok then, I must have confused the hieght of which ports.

Thanks for the update mate.

Old 01-11-2011, 11:38 AM
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Default RE: Fox 15 Schnuerle for DDD conversion

Hi Lou,

You did nice work, let me throw a few added facts about for you. The very large ports were designed for 50 nitro were a very large fuel flow was needed. That's why fox 15's did so well in combat. We worked many years a go with a model by the name of John Sullivan and his dieselized Cox Conquest's which employed a Cox .010 spraybar assembly and I belive a 16" prop. It won a freeflight event which was a tribute to the Good brothers radio development. Flight time was very high. As for the port thing the original schnurle porting was just the two angled ports the boost was added later to a racing .29 TWA engine for geater fuel flow thus more power. Some modeler plug the boost on the fox .61 and .74 on glow by turning the gasket 180 for more constant fuel flow during violent maneuvers.
Old 01-12-2011, 12:39 PM
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Lou,

You should change to K 1 kerosene rather than use #2 diesel.
Old 04-15-2023, 12:51 PM
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I have tried this combination over the years and I have yet to get it to start. It runs great as as a glow engine but nothing as a DDD conversion.

Compression is great, fuel is great and the electric starter i great.

Comments/ suggestions welcome!

Franchi

Old 04-15-2023, 02:29 PM
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Priming and starting compression setting is critical. Also to note; some DDD conversion heads note to include the factory head shim under the diesel head. If there is excess head clearance AND you include the shim, the compression setting even at maximum may not be sufficient to get it to start. A thinner head shim or no shim at all may be necessary. Some careful measuring may be in order.

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