Author Topic: The infamous burp. (Read 5160 times)

John Leidle · « **Reply #3 on:** September 10, 2016, 12:19:51 PM »

I'm not sure what you want to discuss here... the burp or running leaner on outsides?
John

Lauri Malila · « **Reply #4 on:** September 10, 2016, 12:42:09 PM »

Brett Buck · « **Reply #5 on:** September 10, 2016, 01:01:31 PM »

Quote from: John Leidle on September 10, 2016, 12:19:51 PM

I'm not sure what you want to discuss here... the burp or running leaner on outsides?
John

   He's talking about run asymmetry, i.e, it runs faster on insides vs outsides despite running the same speed upright and inverted. This is the bane of modern existence, particularly since schneurles came along. It's the reason I run a RO-Jett, because it has essentially none of that.

   A lot of people in the midwest/southeast don't even believe it exists, because it is much moderated in those conditions. You can take an engine that is perfectly OK in this regard in Indiana, and do nothing except transport it here, and all of a sudden, dead lean insides and dead rich outsides.

   I think it is a fundamentally different issue than the Fox Burp. And much more interesting, we only have guesses and more-or-less random luck dealing with this issue - although the Jett has and interesting difference from most that relates directly to Lauri's theory, which seems to be mostly in line with min - differential scavenging or the fact that scavenging is affected by external acceleration.

Brett

Brett Buck · « **Reply #6 on:** September 10, 2016, 01:16:07 PM »

Quote from: Lauri Malila on September 10, 2016, 04:19:41 AM

Hi.

It would be nice to have some discussion about inside/outside running symmetry. I have now worked about 5 years with the subject and I think there was a small breakthrough a week ago in Italy. As usual, I went to a contest with a completely new setup. The contest site is very close to sea level and I allwas run out of venturi sizes there. I would have liked to use 4,0mm venturi instead of the smallest I had, 4,1mm. So the run was a little on rich side.
So I was expecting a little rich burp in outside maneuvres, especially right after intersection of 8-maneuvres.
As my better model is in hospital in Kiev I was using my older model with horizontally mounted engine. From experience I know, that this engine position gives a rich burp in outsides with "normal" prop direction. With a pusher prop the behaviour is opposite, it goes leaner in outsides. (And because of reasons unnececcary to explain here, I know that it has nothing to do with the actual port geometry or engine bottom end in general.)
My better model has cylinder tilted 40 degrees down from horizontal and the run is very symmetric.
Anyway, what I had new last weekend was a cylinder head with asymmetric combustion chamber. There are classical reasons (thermal balance) for offsetting the combustion dome but my Idea was to make the gas flow from ports to head more laminar, with less turbulence when it hits the squish band.
It seems to work. It's not yet perfect, there is a slight burp when needle is set too rich or when plug has gone bad.
The next step is to make even more laminar, asymmetric combustion chamber with the front edge of chamber following closely the front rim of cylinder. And the new cylinders will also have the ports aimed more towards the cylinder head.
So, my conclusion is that, at least in this case, the burp is caused by inefficient scavenging/burning process. Not necessary fuel accumulation in the bypass(es) or elsewhere. The burning in stunt run has to be slightly inefficient (rich) for the 4-2-4 thing but the problem is to make the inefficiency stable in differend temperatures.
I think that one of the main factors is that engine is loaded in differend ways in outside and inside maneuvres, due to the gyroscopic precession of propeller. Heck, if the gyroscopic thing is able to disturb the model's flying, would the flying in differend ways be also able to load and unload the engine in differend ways? Could someone calculate it?

I don't think the precession is the issue, because the translational acceleration (which is also present) is something like 15Gs and the precession is fractions of a G, or less - it's tiny.

I think you are on to something, and it's largely as described here:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=40133&mesg_id=40133

Ted Fancher noted the issue first, as far as I know. This is WAY back there. The Imitation article, which should be required reading for any stunt flier, alluded to testing engines but he mentions that it didn't work out too well. This is why.

Brett

John Leidle · « **Reply #7 on:** September 10, 2016, 02:37:03 PM »

In all my trips to the Nats I've seen several setups where the barrel is pointed inboard to the pilot. Or was it outboard . Frank Williams had this setup in 2015. I dont have the energy to build a plane for this experement. But I see how it might remove the lean insides problem.
I was told a good while ago to ship my tank so the insides & outsides are even . He figured the upright , inverted lap times weren't important . I have been doing this for 15 years & like it.
I agree with Brett on the ROJETT's having their merits.
John

David Ruff · « **Reply #8 on:** September 10, 2016, 05:20:04 PM »

And God said let there be electrics

Lauri Malila · « **Reply #9 on:** September 10, 2016, 05:32:43 PM »

Brett Buck · « **Reply #10 on:** September 10, 2016, 06:36:30 PM »

Quote from: Lauri Malila on September 10, 2016, 05:32:43 PM

That is one part of the problem, John. We don't have enough data to go to the source of a problem, then end up compensating an error with another error. It's easy to see relations where there isn't one, or it is difficult to see an interaction when there is. Both equal laptimes and maneuvre speeds are important.
And as I wrote earlier, with most schnuerle scavenged engines the symmetry error reverses between horizontal and vertical cylinder position, the best position is found somewhere between. And it also reverses if you run the engine in opposite direction.
I think some have found the solution without realizing it, when they have tilted a side exhaust engine in order to hide muffler or pipe inside fuselage. L

Also found in the Imitation experiments done by Ted in the late 70's-early 80s . At 90 degrees (cylinder down) the engine ran went rich on insides and lean on outsides, with the engine at 45 degrees, it went rich on outsides and lean on insides. With the engine at 22.5 degrees, symmetrical. Unfortunately it didn't work the same for the next engine.

Even the otherwise ideal 40VF tends to some run asymmetry if you let it run rich, which is why you adjust the venturi to get the right speed right at the bottom of the two-stroke region.

It is still evident in many airplanes even at the NATs. One particular individual, who shall remain nameless but is a Top 10 flier, had his engine running so that it was loading up rich on insides and screeching lean in the outsides, particularly in the outside part of the square 8. It took something like 2/3 the time to get through the outside versus the inside. I asked him about it and he said it wasn't noticeable and launched into a histrionic defense of the engine.

Brett

Motorman · « **Reply #11 on:** September 10, 2016, 10:03:46 PM »

Offset plug, looks like Duke Fox was right all those years ago.

MM

Brett Buck · « **Reply #12 on:** September 10, 2016, 10:58:01 PM »

Quote from: Motorman on September 10, 2016, 10:03:46 PM

Offset plug, looks like Duke Fox was right all those years ago.

Actually. not really (or maybe coincidentally). The issue is not where the plug resides but how you control the internal ballistics of the gas flow. As illustrated by the RO-Jett 61 BSE which had the plug dead center and does nothing like this.

The other observations early on had no good explanation (and lots of really absurd explanations), but Frank Williams showed the way, first with the inboard cylinder ST60 and then the Fox bypass stuffer. That got some of us thinking about internal ballistics, and afterwards people started addressing it.

Just like the Fox Burp, there are many people who will argue that run asymmetry is either not real, can be fixed by shimming the tank, etc, and/or doing what it supposed to do.

Brett

John Leidle · « **Reply #13 on:** September 11, 2016, 01:00:11 AM »

It sounds like I'm putting band aids on my engine runs by shimming for equl running in the 8s . I would think my upright & inverted laps are not the same but I rarely notice. However for me to think I'm able to custom head as Lauri did is over the top. I have a nice lathe & mill but about 10% of the skills to do so. John

Chris Wilson · « **Reply #14 on:** September 11, 2016, 03:05:20 AM »

Hi Laurie,

Wouldn't it be a goal to have all your transfer streams collide and slow slightly 'before' coming into contact with the head geometry?

I consider that the Schneurle type of porting uses the curve of the cylinder wall as a deflector plate just as surely as any other.

But your 'old' diagram seems to intimate that the first real 'deflection' reached is the squish band at the front of the head, not the cylinder wall.

Have you considered inviting chaos to your transfer streams a lot earlier than the top of the cylinder by flattening out the direction of flow?

Good luck with this.

P.S.
Try this link mate, its only a tease and may not apply but it was penned by a very smart guy (The 'Secret life of Squish' and 'Squish and new Combustions' specifaclly.)
http://aerosports.net.au/LS/Model_Aircraft_Stuff/Squish_and_New_Combustion-Space_Designs.html

Lauri Malila · « **Reply #15 on:** September 11, 2016, 04:14:51 AM »

Chris Wilson · « **Reply #16 on:** September 11, 2016, 05:06:14 AM »

Quote from: Lauri Malila on September 11, 2016, 04:14:51 AM

No Chris, quite opposite.

Its just that when you posted the 'old' picture ( that has quite a vertical transfer angle) I assumed that was representative of your start point.

And your 'new' picture where you were aiming towards with the only visible difference being the bowl position - hence my thoughts on flattening the streams.

Lauri Malila · « **Reply #17 on:** September 11, 2016, 05:35:43 AM »

Lauri Malila · « **Reply #18 on:** September 27, 2016, 12:47:07 PM »

Steve Fitton · « **Reply #19 on:** September 28, 2016, 06:06:41 PM »

Quote from: Lauri Malila on September 27, 2016, 12:47:07 PM

Hi again.

It seems that my latest modifications, that worked perfectly in allmost 40C Italy, won't cure the symmetry issue in colder weather. I found out that last weekend in 8C morning temperature that went up to 20...25C during the day.
So, the issue is strongly temperature-related, I would like to install a temperature sensor to the engine to find out the magical temperature above which the problem disappears.
I don't mind running the engine very hot, it seems that our metallurgy is very stable for that. It just causes following problems:
-Engine cooling time between flights is longer.
-with such efficient burning (2,6oz fuel per flight) taters are formed to plug quite quickly. But I may get around it with correct oil.

Other things that may lower the critical temperature, are:
-More efficient scavenging.
-A hotter plug (now I have Enya#3). But my experiments with hotter plugs have not been good, as the engine operates so close to detonation that many plugs fail because of high compression/mechanical stress. Perhaps a 2-plug head a'la Merco would be worth trying.
-Also a titanium plug shield a'la Retro stabilizes engine run, both by shielding the plug from raw fuel droplets and by acting as an additional glow element.

Anyway, I got the engine running quite well by using all my O-rings and stuffing some paper towel in cowling intake.

I think that in the end there is 2 separate reasons for the problem, and in certain cylinder position they balance each other. I found that with cylinder 40 degrees down from horizontal, my engine run is very symmetric in both cold and hot weather. The reasons are:
-The fuel ballistics inside cylinder. It's quite a logical explanation in cylinder down -situation when a typical fault is leaning in outside maneuvres. The final proof would be running my engine in cylinder down -position with a pusher prop. If it is the ballistics, the symmetry issue wouldn't reverse. But I don't have a model to do the test yet, so it has to wait..
-Brett may disagree but I would still want to look at the load differences that originate from gyroscopic action of propeller. It would be the most logical reason for my findings with horizontally mounted engine behaviour. It is true that we are talking about fractions compared to other G-forces present in the system but by shimming the fuel tank up and down you will see that not much is needed.
What is a little strange is that (with normal propeller direction) the engine goes richer only after an intersection, when I start outside loop right after an inside loop. I cannot detect the problem if I just do loops from level-or inverted flight.

L

Couldn't you defeat all the problems by running a RoJett?

Brett Buck · « **Reply #20 on:** September 28, 2016, 06:56:34 PM »

Quote from: Steve Fitton on September 28, 2016, 06:06:41 PM

Couldn't you defeat all the problems by running a RoJett?

Yes, but how impolitic can you get?

Seriously, people can enjoy this event in a lot of different ways. Only one person is champion at a time, so there had better be something else beyond trophy-counting. Like exploring how stunt engines really work, instead of 50's bullshit barnyard theories on how they work (which is the vast majority of what you read). Which for 99.99999% of the participants, might as well be magic.

Brett

RandySmith · « **Reply #21 on:** September 28, 2016, 11:47:47 PM »

Hi Lauri

If you want to test out a hotter head..inside... make a head button out of an insulating ceramic material, They run quite hot on the inside and quite cool on the outside. I would try SiAlON or Alumina , try to keep it accurate as some of them are brittle, these two are not as ultra hard as some of the other materials like silicone carbide

Randy

Lauri Malila · « **Reply #22 on:** September 28, 2016, 11:56:40 PM »

RandySmith · « **Reply #23 on:** September 29, 2016, 12:02:16 AM »

I would not make a head, make a head button that is a nice tight fit into the head, use the head as a clamp for the ceramic button.
You will use more fuel, not less

Randy

RandySmith · « **Reply #24 on:** September 29, 2016, 12:27:22 AM »

Did you buy the 915 MACOR?

RANDY

Tim Wescott · « **Reply #25 on:** September 29, 2016, 12:31:16 AM »

Quote from: Brett Buck on September 28, 2016, 06:56:34 PM

Which for 99.99999% of the participants, might as well be magic.

It's technology that's sufficiently advanced that I do not understand it. Therefore, it is magic.

Tim Wescott · « **Reply #26 on:** September 29, 2016, 12:34:44 AM »

Doesn't making a head button mean making a glow plug? Or did you mean to make a button with a threaded hole for the plug?

What about just making a smooth aluminum head, with no fins? Or, if you're already there, make the head flat on top, make an insulator out of FR-4 with bolt holes to match the head, and bolt it down with that.

Howard Rush · « **Reply #27 on:** September 29, 2016, 12:38:09 AM »

Quote from: Lauri Malila on September 28, 2016, 11:56:40 PM

Yes Randy, I have bought some Macor ceramic, just have to figure out how to turn it into a head. Especially the plug area causes lots of head scratching. L

If you are scratching it, you are using the wrong tool. Hope this helps.

Lauri Malila · « **Reply #28 on:** September 29, 2016, 01:39:54 AM »

Chris Wilson · « **Reply #29 on:** September 29, 2016, 01:53:15 AM »

The "button' is simply an insert that serves to house the combustion and the glow plug - it all internal.

Externally the 'head' is simply a finned clamp.

Much like MVVS used to be years ago to swap out for compression changes.

But here I am sure that Laurie could make a button and still have the thread for the plug still in the head.
This makes the most sense.

Lauri Malila · « **Reply #30 on:** September 29, 2016, 03:50:05 AM »

Lauri Malila · « **Reply #31 on:** September 29, 2016, 07:30:11 AM »

RandySmith · « **Reply #32 on:** September 29, 2016, 10:34:59 AM »

Quote from: Tim Wescott on September 29, 2016, 12:34:44 AM

Doesn't making a head button mean making a glow plug? Or did you mean to make a button with a threaded hole for the plug?

What about just making a smooth aluminum head, with no fins? Or, if you're already there, make the head flat on top, make an insulator out of FR-4 with bolt holes to match the head, and bolt it down with that.

Hi Tim

No the head button is the entire combustion chamber, and the Aluminum finned part is separate and acts as a clamp to hold the head button in place, the head button is just machined and tapped for a glow plug

Randy