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Fox 15BB conversion -transfer volumes

Old 12-13-2010, 03:17 PM
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Lou Crane
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Default Fox 15BB conversion -transfer volumes

The excess flow capacity in the transfer passages of glow ignited engines, with regard to converted glow-to-diesel engine operation has been discussed at length in this forum. Mainly, comments confirm that (many) converted glow engines seem to reach max RPM at partially open throttle - between 2/3 and 3/4 open.

I mentioned, possibly in a post somewhere in here, that a converted Fox 15BB ran very nicely, but was excessively thirsty. Also, that the exhaust spewed considerable droplet-containing content. I mentioned my intent to "stuff" the 3rd, or boost, bypass port in the sleeve.

Which I did about that time. I finally found opportunity to run the engine again. After a few juicy and low-compressed runs to start restoring running fits, I observe that the engine has a much less juicy exhaust, can be set to higher comp/leaner needle running, and that duration per unt of fuel volume is much increased.

The engine is still fairly "low-time since overhaul," so I'm not pushing for ultimate settings just yet. Starting and setting sre at least as simple as before the boost channel was blocked off. RPM, running easily on an APC 8-4 sport prop, was almost 1,000 RPM above what I got running in stock-case condition. So, blocking the 3rd transfer channel seems to, at least, do no harm... As I add run-time on the engine, I may pass along the results - good, bad, or not worth th effort...
Old 12-13-2010, 03:58 PM
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Default RE: Fox 15BB conversion -transfer volumes

Lou, any percieved benefits on 'partially' blocking of the boost port with a 'V' shaped piece of wood?

This shape should increase the velocity of the gas as it rises towards the head due to a narrowing of the passage the further the gas travels.

Has been suggested before as a 'Fox 35' burp fix before this and I have always thought that (here) restricting a design was better than denying it completely. So here perhaps a narrowing of all the arteries may work better with all the ports since the engine was designed the work best in its original form with all three transfer ports working in concert, so maybe the boost port (even in a lesser form) is essential to it?

I mean if you purpose built that engine from the start as a diesel you would in fact simply reduce all the ports down in size, rather than just block one off completely, as the this is the theoretically correct way to do it.
Thoughts?

Cheers.
Old 12-15-2010, 04:12 PM
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Lou Crane
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Default RE: Fox 15BB conversion -transfer volumes

I am considering trying a partially blocked boost port. So far it is just aquick and dirty first shot.

Fox 15 Schneurles have a slight rectangular recess in the crankcase outboard of the boost port, probably to incease 'flow volume' for glow use. I merely trimmed a piece of soft 1/16" balsa to lie in that recess, and, frankly, mashed the wood until the sleeve could be reinserted. ...Awkward working in the close confines... ...but it has been unaffected in about 15 minutes on the bench. No loosening, no discoloration, no lost chips...

I'd done similar many years back, when I was experimenting toward extreme high fuel-economy. Our AMA had/has a category for CL Endurance. Engines from .09 cu in to something like 0.40. Line length and diameter specs. flying, outdoors only, with a limit on flier ranging from the marked center of the circle. 4.0 fl oz fuel. These rules have changed, but the record on the old rules was above 2 hours... Current rules specify much the same, except fuel is limited to 1.00 fl oz.

I modified a Fox 15 schneurle and an OS (possibly MAX, or S) 10, using Bob Davis' heads and fuel. Fox choke was plugged except for a 1/16" diameter hole. Plug was above the standard spraybar. Hey, it worked! Got a 45 minute flight on the Fox, using less than half the fuel in the tank. Expecting a lot of nose-out yaw, I'd put the fuel pickup well forward in the tank. I didn't realize how much angle of attack was needed at the slow speeds on the limited power available... Learned, however, that my knees did not care to go stumping around backwards quite that long.

Benching the OS10 showed a rate of fuel consumption which could have provided about 60 minutes per fl oz, if it could be realized in the air. Mods were mostly exterior, but I did restrict the bypass volume some. Externally? The intake choke was 1/32" diameter, again, above a standard spraybar. Exhaust was covered with a plate drilled for a 1/16" diameter opening. Prop a drastically lightened and thinned Rev-Up 12-6... 3000 RPM +/- a small handful, depending on ambient wind (and I imagine forward motion of flight would have played with that.) Static thrust at that RPM on that prop was surprisingly present, if not stormingly powerful.

It did so run!

Never flew a model on the OS mod. My knees had convinced me by then.
Old 12-16-2010, 03:39 PM
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Default RE: Fox 15BB conversion -transfer volumes


ORIGINAL: Lou Crane
I am considering trying a partially blocked boost port. So far it is just aquick and dirty first shot.
Hi again mate,
But do you think that partially blocking off 'all' the ports could be a workable solution here?

So in full, restrict the venturi, main side transfers, bypass and exhaust because this is what a purpose built diesel would do.

And I am most curious as to what a tapered wedge of timber would do if used in all 3 transfer ports instead of simply blocking the boost port off completely.
My hunch is that leaving the gas flow pattern as intended by the manufacturer but to a lesser extent would be the way to go IF you can get those ports partially blocked in a permanent manner.

Itoo have used a piece of spruce carefully shaped as to jam tightly into the entire boost port and my fear is how would one tightly jam anything into part of the port?
A partial piece is not going to be as supported as a full piece of timber and it could move around in use, so perhaps JBweld glue to the rescue?

Talk soon, Chris.

Old 12-22-2010, 11:56 AM
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Lou Crane
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Default RE: Fox 15BB conversion -transfer volumes

It's been a while to get back on this...

I haven't had opportunity to bench run this engine further. Some thoughts on restoring the intended flow patterns to a restricted OEM configuration:

(Sport and everyday) diesels seem to be much less fussy than glows RE: so many things, so this simple butchery may work quite well.

In most schneurle engines, the third bypass, often called the "boost" bypass, opens after the mains. It may - probably does - complement the charge flow, but the basic pattern and timing is established by the two which open earliest and close latest. Again, the simple full-block may provide enough restriction to simplify a very satisfactory conversion.

The specimen engine, Glow Fox 15BB Schneurle, is capable of quite high RPM on enough nitro and a small-enough prop. That suggests to me that optimal crankcase interior flows were developed for RPM well above, say, 17,500. Are flow patterns in those conditions suitable for a diesel conversion for sport use?

In this instance, I did not want a permanently attached 'block,' for several reasons. One, of course, would be further shape and area variations in all three bypass channels. I shaved a balsa segment for the boost channel because it would be least likely to press on the exterior of the cylinder harmfully. Fox sleeves, like most, are susceptible to distortion. At least, this one is steel; brass is likely more so. You've inspired a thought or two more. Two pieces of 1/32 balsa, sandwiched, might allow some boost flow by shaping channels in the one against the sleeve. The boost port 'window' has ample height for a continuous piece across the top to fill the gap... Might try this, time and occasion permitting.

Of course, repeated disassembly and reassembly will likely disturb fits. That means several additional "re-seating" runs after each, with additional wear against the compression sealing surfaces...

Of course, a lot of these speculations cannot be answered without more rigorous experimentation and comparison. I have flown converted Fox15BB-Sch's with this simple mod, and without, at times over the past few decades (the engine first appeared in about 1972/73...) Any problems I had can easily be called fuel related. It is only in the past few years I've settled on a home-brew method that does work well and dependably. (Not knocking DDD, Aero, or Clutton fuels - they are very good - but HAZMAT charges being what they are here, we might pay more for the shipping than the contents.)

Don't know if you have anything similar - sounds like it in recent comments on difficulty obaining fuels and aircraft dope. Our HAZMAT fee is a shipping surcharge for flammable liquids, and other "HAZardous MATerials." There have been a few 'incidents' involving such. The fee supposedly insures the carriers against future 'incidents.'

Edited to add: The engine's main bypass ports are a bit harder to mod evenly. The backplate extends up to the bottom cooling fins, and that bypass channel is primarily IN the backplate. Shaping, fitting and securing a filler volume in it would be much less certain. If front and back fill is uneven, results would be unreliable and harder to identify...
Old 12-22-2010, 01:18 PM
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Default RE: Fox 15BB conversion -transfer volumes


ORIGINAL: Lou Crane
Don't know if you have anything similar - sounds like it in recent comments on difficulty obaining fuels and aircraft dope. Our HAZMAT fee is a shipping surcharge for flammable liquids, and other "HAZardous MATerials." There have been a few 'incidents' involving such. The fee supposedly insures the carriers against future 'incidents.'
Hi Lou,
here in Oz I am sure that we have similar transport issues as you have, as well as diethyl ether being used by 'crack' factories and attracting a great reluctance to over the counter sales but at the moment if you can show the seller a genuine use for model diesel fuel chemicals then there seems to be no problems as long as you know the right people to talk to.

I haven't seen diesel fuel in hobby stores for years now and most flyers obtain the seperate ingredients through club contacts.

Interestingly the most common excuse not to sell you ether is that it is on the dangerous chemicals list and when I ask the seller to show me where on the list ether is they can not do so as it is not there (at least it wasn't there the last time I looked!)

Just had a look at a picture of the offending engine mate, and it seems to be a bit of a bugger with modifying the rear main transfer port. But all you really need to do is affect the very top of port or its opening in order to restrict and shape the flow - kinda looks to be too much trouble though.

Nice talking to you.

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Old 01-11-2011, 11:32 AM
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ddd
 
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Default RE: Fox 15BB conversion -transfer volumes

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.

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