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Engine basics => Engine set up tips => Topic started by: Motorman on April 09, 2017, 07:06:27 PM

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Post by: Motorman on April 09, 2017, 07:06:27 PM
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Title: Re: Cross Flow on a Profile
Post by: Tim Wescott on April 10, 2017, 12:42:40 PM
It sounds like your tank is too low.  Have you tried raising it?
Title: Re: Cross Flow on a Profile
Post by: Target on April 10, 2017, 12:56:10 PM
He said he raised it 3/8" already and it ran the same upright and inverted that way....

I'm curious about this as well. If all baffle piston engines run this way, then the solution seems simple, don't run a baffle piston engine in it, right?

.281 sounds like a fairly large venturi, depending in what size the NVA is.

Vr,
Chris
Title: Re: Cross Flow on a Profile
Post by: Brett Buck on April 10, 2017, 01:00:51 PM
I run a few different baffle piston engines on profile planes and they all seem to do the same thing. Down elevator means a surge of fuel that tries to snuff the plug out. Some call this the Fox burp but they all do it.

    They don't all do it, and the issue is not generally "snuffing out the plug". It's an internal gas flow issue that affects a few engine types and has varying solutions. There's only one where it is pretty clear that the plug getting blurped on is the issue (ST60), and something similar can happen when it is mounted inverted, as well. The Discovery-Retro has a shield over the plug to prevent the issue.

     Brett
Title: Re: Cross Flow on a Profile
Post by: Tim Wescott on April 10, 2017, 01:39:14 PM
He said he raised it 3/8" already and it ran the same upright and inverted that way....

Dangit, missed that.

    They don't all do it...

Does this mean that MM has the un-luck to have his set of "a few" engines all from the ones that do, or could he be suffering from some other issue?

Thre solutions that I can think of are to either use different engines (Schnuerle-ported, or ask for a list of ones that do work well), or mount the engines upright or inverted.  Depending on what local rules you fly under you may or may not be able to use engines that aren't side-mounted -- I think it's OK in most places, although it does complicate a profile build to do so.
Title: Re: Cross Flow on a Profile
Post by: Steve Helmick on April 10, 2017, 03:50:08 PM
Never seen reference to an "8011", but it appears to be the same as a "4011" except for the barstock head...which I recall was a squishband design with slot for the piston baffle. I used to pit a 4011...can't recall if it was FI or RV now, but it had a cast wedge head. I think I'd look for one of those heads in somebody's junk barrel.

I ran a "4055" for awhile in a 50 oz. Humongus (underpowered). It had a relatively huge single bypass in the case, but was a 3-port cylinder/flat piston. It ran ok, other than lacking the power of a .46LA. Heavy damned thing. 

I'd suggest changing heads and then maybe blocking the bypass down at least 50% with JB Weld. Otherwise, I think 5.1 sec. laps is just fine. Get used to it!  D>K Steve     
Title: Re: Cross Flow on a Profile
Post by: Chris Wilson on April 10, 2017, 09:37:03 PM
Try mounting the engine inboard instead of outboard, in theory this should throw any condensed high fuel charge against the crank and not the head.
Title: Re: Cross Flow on a Profile
Post by: Brett Buck on April 10, 2017, 10:04:04 PM
Try mounting the engine inboard instead of outboard, in theory this should throw any condensed high fuel charge against the crank and not the head.


   That was the trick Frank Williams found for the problem with the ST60. Fuel would pool in the upper part of the bypass at very low revs/rich settings, and then it would, apparently blurp itself onto the plug. The Yatsenkos fixed it with the plug shield, Frank mounted the cylinder inboard so that the fuel would not pool in the bypass.

    Note that he discovered the Fox 35 bypass stuffer fix when he tried the inboard cylinder trick on a Fox, reasoning that it was the same problem. It didn't work, in fact, all it did was make it start burping on *insides*. That was the key observation, that it had to do with the disruption of the internal gas flow due the imposed acceleration, not fuel pooling and getting on the plug. Put a bit of spruce in the bypass to take up most of the space, and voila, fixed for good.

      Brett
Title: Re: Cross Flow on a Profile
Post by: Chris Wilson on April 10, 2017, 10:41:53 PM
Brett ......... you seem to gotten  a lot older all of a sudden.

Would love to see MM use a radial mount that could rotate to any desired position and see what the results might be.
Title: Re: Cross Flow on a Profile
Post by: Lauri Malila on April 11, 2017, 03:24:48 AM
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Title: Re: Cross Flow on a Profile
Post by: Brett Buck on April 11, 2017, 10:21:15 AM
Brett ......... you seem to gotten  a lot older all of a sudden.

Would love to see MM use a radial mount that could rotate to any desired position and see what the results might be.

    Check out this thread:

http://stunthanger.com/smf/engine-set-up-tips/the-infamous-burp/

    As far as I know, Ted was the first one to do it, but that was for "schneurle effect", not baffle-piston woes.
 
     Brett
Title: Re: Cross Flow on a Profile
Post by: Lauri Malila on April 11, 2017, 11:39:44 AM
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Title: Re: Cross Flow on a Profile
Post by: Lauri Malila on April 11, 2017, 11:40:56 AM
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Title: Re: Cross Flow on a Profile
Post by: Brett Buck on April 11, 2017, 01:04:44 PM
Also, the side where gas flow hits should be curved, something like r=baffle height.

  Just like a 1946 McCoy...

   The baffle height certainly matters. Back in the late 70's-early 80's, that was one of the many "stunt lore" tricks to "tame" various engines, like the STG21/40. Of course what it led to is the stunt flyers maxim, if a little is good, a lot is better, so that was one of the early examples of people ruining perfectly good stunt engines with barnyard hacks. Like the many people who thought it was a really good idea to "soften up" an ST46. While all the guys actually competing with it were trying to put in all the nitro they could get, and if I had owned a welder, I would have RAISED the baffle to get more power rather than lower it to soften it up. That was before everybody jumped on the "if one head gasket is good, 12 are better" plan, which is at least reversible.

    Brett

   
Title: Re: Cross Flow on a Profile
Post by: Chris Wilson on April 11, 2017, 01:35:54 PM
One possibility that just came to me is from Kieth Renecles dieselised MVVS 49's, look for ANY cavity in the lower crankcase that could harbour fuel and fill it with JB weld, bring the backplate in as far as it can go to eliminate areas where fuel can condensate.
Even consider sleeving down the crank tunnel AND stuffing the bypass .

Sometimes a slug of fuel can break loose during manevours and be thrown towards the transfers.

Just as an aside here, side mounted diesels work well because of their leaner fuel/air mix and smaller transfers - glows do better invert mount and I suspect that if this engine was peaked lean the burp would disappear.

Good luck.
Title: Re: Cross Flow on a Profile
Post by: Brett Buck on April 11, 2017, 02:58:12 PM
One possibility that just came to me is from Kieth Renecles dieselised MVVS 49's, look for ANY cavity in the lower crankcase that could harbour fuel and fill it with JB weld, bring the backplate in as far as it can go to eliminate areas where fuel can condensate.
Even consider sleeving down the crank tunnel AND stuffing the bypass .

Sometimes a slug of fuel can break loose during manevours and be thrown towards the transfers.

Just as an aside here, side mounted diesels work well because of their leaner fuel/air mix and smaller transfers - glows do better invert mount and I suspect that if this engine was peaked lean the burp would disappear.

   In general, the problem is caused primarily by trying to use supersaturated or overly-rich mixtures. It's just like the 4-stroke thing from the other thread, we are running the engines extremely rich, extremely cool, and far slower than they were designed for in most cases, in addition to wasting large amounts of fuel through poor scavenging. The Fox burp is caused by the "too slow" part, as in "too slow for decent gas velocity in the bypass". The ST60 issue was the extremely rich part.

    Brett
Title: Re: Cross Flow on a Profile
Post by: pmackenzie on April 12, 2017, 04:14:26 AM


Sometimes a slug of fuel can break loose during manevours and be thrown towards the transfers.



This video shows the  oil buildup in the crankcase, more than I would have thought.

https://www.youtube.com/watch?v=eu8X8QgRcO4
Title: Re: Cross Flow on a Profile
Post by: Target on April 12, 2017, 10:10:14 AM
Now if we could just get that bolted to a profile, running on a 4-2-4 break, in flight with a go pro, it could visually represent what folks are subscribing here.