Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Sequence of shimming vs nitro

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Old 11-19-2009, 05:28 PM
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mk1spitfire
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Default Sequence of shimming vs nitro

Thought you busy guys could explain what tale tale signs indicate whether a shim is needed or change in amount of nitro and fine tuning this?

cheers
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Old 11-19-2009, 05:51 PM
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Default RE: Sequence of shimming vs nitro

What kind of engine, exhaust and nitro content are you talking about.
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Old 11-19-2009, 06:17 PM
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Default RE: Sequence of shimming vs nitro

If you are talking about a piped screamer, approach it like you would if you're tuning a diesel...start with a compression that barely runs and subtract shims from there. If you aren't happy with the expected results, then raise the liner a couple thousandths. Always monitor the glow plugs, there is no excuse for obliterating them on every run, but a well tuned engine on high nitro will consume a plug about every run. With 30% or less nitro you should get several runs per plug. Weak engine mounts will eat plugs, too.
Engines with low nitro chambers shouldn't be run with much higher nitro than what is called out.
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Old 11-19-2009, 07:43 PM
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Default RE: Sequence of shimming vs nitro

Great.

My weston 36v1 has been munching plugs.
I have changed the type of fuel and gone higher nitro 16% from 10%. I prefere this newer fuel with edl as its contains 18% oil and more oil than recommended.

I was unsure if this was because the higher compressed engine and higher nitro was causing this or its lean.
Trying to understand how to concude which one causes the blown plug,1st time around.

My high speed needle seems to be half a turn out to get on the pipe,yet also unsure if shorter manifold contributes too with overheating.

Couple this with hot/cold plug to suit the nitro creating variables. Cold used.

A plug every run, is what I want to advoid. My 50v1 can go 10 runs at least. I had no idea to expect regular plug blows were common with well tuned engines

So guess the only way to see how well the engine is behaving is monitor the new plug after the first run as subsequent runs after could be missleading due to their immenent destruction from repeated use.?

So drop the nitro instead of shimming for nitro for higher compression engines at the expense of maybe more power?

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Old 11-19-2009, 08:25 PM
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Default RE: Sequence of shimming vs nitro

We're assuming that you know how to sneak up on the best needle setting over the course of several runs...
assuming that the pipe is tuned for the prop being used, at least roughly....
assuming you are running a bubble free fuel system...
assuming stout engine mounts...
assuming the prop isn't too much for the recommended rpm. Diameter and pitch numbers don't tell the whole story

..heat range? What's THAT? Just get a plug in there with a heavy duty [thick] element and use the throttle just for an engine kill, not for throttling.
The rest is pretty simple, use a tach, and good notes to arrive at the best all around setting that you can live with. I don't know if Weston changes the stock head, but I always thought Webra was a 5% nitro engine?
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Old 11-19-2009, 10:33 PM
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Default RE: Sequence of shimming vs nitro

They say run no more than 10% and my .50 has always seemed happy on that. I get lots of flights on a plug so long as I don't spaz the needle setting, but in truth probably no more than 15-20.

MkI, I would say 16% has taken you over the line with stock compression. I've seen a few "Euro compressed" engines go balky at 15%. The power difference between 10% and 16% is nothing to get excited about. I believe your options are straightforward - stick with 10%, or add a shim. The change is subtle, chances are good it would be a trouble-free escapade.

"Shorter" manifold? Shorter than what, did you change that too? Or before?

As a quick and dirty fix you could try dropping the prop load a bit.

MJD

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Old 11-19-2009, 11:13 PM
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Default RE: Sequence of shimming vs nitro

ORIGINAL: mk1spitfire

Great.
A plug every run, is what I want to advoid. My 50v1 can go 10 runs at least. I had no idea to expect regular plug blows were common with well tuned engines

Common with high nitro, which I consider to be 40% or more.

If the element is consumed to a chalky grey/white, the plug died with honor. If the element is missing and all there is left is a smoking hole, then that is your indicator that something is wrong.
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Old 11-20-2009, 08:04 AM
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Default RE: Sequence of shimming vs nitro

Cheers.

Ha, the plug died with honor.

In the quest for speed,I've been foolish to assume that the nitro is the quick way for a power boost to give me some more get and go with no trouble.

Trying to reduce different types of fuel/nitro content, I was hoping this could cover more types of my engines. humm More 10% needed.

I use my ear and get my engines singing on full tank and then back off a couple of clicks, then launch, its never rich lauching, its on song, so it powers out of my hand.
If It dies to early into flight, I rich click more. If it guggle at all in flight its rich., so lean click a few or less.

I let it run out of fuel and land. Maybe the end of the run when the engine leans and dies, does it blow the plug????
Should you land before the tank empties to prelong the life of the element?

Never tried a bubbless tank-I cant find them in uk. I see Mike made his own.

Engine fixed with hex screws into individual plastic mounts.

Prop 8x6, good for it im told. Would like to try 7" diameter. I notice apc 7" hubs get alot smaller than 8". Better to to cut down 8" down to 7"???


Heat range, Trying to understand wether shorter manifolds/pipes ment hotter engines, Ie manifold too short? Power/rpm decrease and hotter engine?

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Old 11-20-2009, 10:04 AM
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Default RE: Sequence of shimming vs nitro

I'd be backing off more than a couple of clicks. When I launch my West .50, it is usually struggling on and off the pipe and somewhat rich. It unloads quickly enough.

From your description, you are sneaking up on the right needle setting from the lean side.. you really want to do that? If you are using a clunk tank and running it dry, the mixture will progress from what sounds like could already be a lean setting to more lean as the fuel level gets low. Although you are making life easier in terms of hand launches, you might be allowing temptation to make you twist the needle when it perhaps shold be backed off a bit more. With a clunk tank chopping it before the tank gets really low is a smart idea, saves plugs and engines.

There is much good reading at www.jettengineeering.com, if you go to this section. It mentions a few of these issues.

Technical Reports & Instructions Engine Operating Instructions

R/C Carburator Set-Up

Bubble-Jett Fuel Tank/Tanker Instructions

Fuel Tank Installation

As to pipe setup - shorter pipe setup simply means you need to prop it to run faster. If the pipe/header setup is too short for the prop being used, however.. then yes, you'll munch plugs and have fits because you'll have a heck of a time finding a needle setting lean enough to launch without cooking the engine as soon as it unloads and wants more fuel.

This business about setting the needle rich has to do with the operation of a venturi - fuel draw increases as the square root of intake velocity, not in linear proportion to it. So a fixed needle setting only provides a controlled ratio of X:Y fuel to air at one rpm. So as the engine jumps up in rpm with the boost from the pipe, the mixture heads lean from the unboosted or partially-boosted needle setting. This is usually paraphrased as "the engine needs more fuel, obviously" - but it's not truly obvious why this is other than it's running faster, unless you understand this.

The higher the boost level, and thus the higher the ratio between unboosted and boosted rpm, the greater the allowance that has to be made. Mildly tuned systems, easy. FAI C/L speed engines - the exact opposite. Setting the needle back a couple of hunderd rpm like you would do with a muffler is not going to work. You need to back off many hundred rpm with a tach. If the needle becomes touchy, falling off into a gurgling rich setting, you are propably overpropped for the pipe setup, or too short for the prop in other words.

If I am tach-less at the field (as opposed to tactless which is a more steady condition) I needle it up onto the pipe with the nose a bit high, then I back off until it starts to waver at falling off again. Power is down at launch, but should be adequate as the only aircraft we're talking about are overpowered, right..? If the setting is off in the air, I kill it and try again.

MJD

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Old 11-20-2009, 10:29 AM
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Default RE: Sequence of shimming vs nitro

Thanks.
You make it sound like an art Mike, which it is. ! I am most dissapointed if the engine is not on song in flight.I feel robbed of the speed fix and adrenaline!
A click either way for optimum is the difference in a truly contented speed freak.!

Out of 211 flights on the magnum, I can maintain excellent rpm from launch to finnish but notice the next day that setting needs a tweak.

My mates make snide comments if the engine isn't on song throughout the flight, claiming it sounds rough when rich and leans at the end.!! But what do those slowboys know! Ha The big girls, dont help me launch, so its down to me and thats why I need the power on tap from the onset. Ha but i can vertially launch vertical on the magnum with 9x7!
Having something to hold on to is a help in self launching, A skid for SD?



lean it till it siezes, was my saying!
Makes me think if castor is good for lean runs then lean herr right out if she can take it!


If the pipe/header setup is too short for the prop being used, however.. then yes, you'll munch plugs and have fits because you'll have a heck of a time finding a needle setting lean enough to launch without cooking the engine as soon as it unloads and wants more fuel.


This could explain why my needle is only half a turn out?

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Old 11-23-2009, 10:40 AM
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Default RE: Sequence of shimming vs nitro


ORIGINAL: combatpigg

If the element is consumed to a chalky grey/white, the plug died with honor. If the element is missing and all there is left is a smoking hole, then that is your indicator that something is wrong.
Didn't someone, somewhere publish a chart, or picture, or text, or something about plug failures, what they look like and what they could be attributed to? (You know, kinda like the old spark plug tuning charts.) I feel like I remember seeing something like that in my Internet travels, or am I hallucinating.

-Joe


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Old 11-23-2009, 11:20 AM
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Default RE: Sequence of shimming vs nitro

Here's my entry. Out of the West .50 when the header cracked in flight and eventually failed. The small pressure drop as the crack was developing drove it lean but not too lean to tip dumdum on the Tx off until the exhaust sound started to noticably change. I had launched my usual rich so wasn't immediately able to grasp the idea that it might be in need of attention.

What you are talking about sounds like something I'd find in an older MB article.. hmm.

MJD
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Old 11-23-2009, 12:33 PM
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Default RE: Sequence of shimming vs nitro

Duke Fox used to run a lot of informational ads. They were like mini tech articles that the mag should have paid HIM for. He might have posted some info about reading plugs.
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Old 11-23-2009, 12:52 PM
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Default RE: Sequence of shimming vs nitro

Yeah, those sidebar ads.. I was reading one in the can this morning (really, I have a stack of '60's-70's mags in the rack). I'll snoop around on my next "sortie".

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Old 11-23-2009, 01:07 PM
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Default RE: Sequence of shimming vs nitro

Hi!
The symptoms are the same for every engine!
Hard to set high speed needle and non exact feeling (engine not rewing up as crisp) with sudden stoppages of the engine (engine not willing to rew)...too large combustion volume!

Remedy: Remove one or two head shims ...or go up in nitro content...or use a hotter plug.
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Old 11-23-2009, 09:55 PM
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Default RE: Sequence of shimming vs nitro

Dub will tell you this as well as I ......

Blown plugs come (97% of the time) from a lean run.
What caused the lean run ...... that is what you are looking for.
MOST of the time when people can not see any other cause, it can be traced to the fuel system and very likely a fuel foam issue.

A head shim or two really will not prevent blown plugs. The head spacing and glow plug combine to define the "ignition timing".
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Old 11-23-2009, 10:12 PM
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Default RE: Sequence of shimming vs nitro

I lost two plugs this weekend - one each run, on a piped ST .45 (on 5% nitro). These didn't look like the one MJD posted, but instead the element looked normal except for it was broken about half way down (or further). I'm guessing this is due to vibration as the "tune" was fairly conservative.

I could swear I saw somewhere where it talked about the way the blown element looked was an indication of pipe length, preignition, lean run, vibration, etc...

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Old 11-23-2009, 10:26 PM
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Default RE: Sequence of shimming vs nitro

I've flown Fast Combat for over 20 years and know the difference between a lean run and an engine that is using too much nitro or is overcompressed. An overcompressed engine will trigger lean runs because that is what they do, they COMPLETELY burn the mix and THEN SOME. During the middle of a real good run, you can hear the plug "go away" and the diesel action takes over from there, with just a slight drop in rpm. Decrease the compression [or the nitro] and the tendency for the mixture to end up too lean decreases.
There are many more reasons for a run to get too lean, but no one has ever hurt an engine by lowering the compression a little to see what happens.
I would not run a high rpm piped engine without a hand full of different thickness shims and a variety of props to solve the blown plug issue. This assumes that all the other basic "engine hygene" issues are already met.
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Old 11-24-2009, 08:23 AM
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Default RE: Sequence of shimming vs nitro

ProBro - which ST is that?

On the plug I posted, the element was in the process of being barfed out the hole when I removed it, but luckily it had not come off yet to play let's chew the cylinder. It's still sort of in there but the picture is just a point-and-shoot quicky.

bob27s and DJ are very vocal proponents of harshly scrutinizing fuel systems, and no freaking wonder. Anytime I start to have any kind of issue, nearly invariably [excepting disintegrating headers etc..] it is fixed by a run through of the fuel system. I've always "known" the value of a good test stand for clearing out the airframe variables, but I am very happy now that I got off my duff and set myself up with a solid new test stand rig early this year, versus anything I kludged together in the past out of bubble gum and scrap lumber.

CP - here's an off the cuff question, just looking for a broad answer - say your setup on a typical strong .40-.50 is working well and maybe close to ideal. Add a .005" shim - how much rpm drop would you expect to see? 2 shims? Is it much? Not competing myself, I've never toyed with compression limits.

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Old 11-24-2009, 11:17 AM
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Default RE: Sequence of shimming vs nitro

Mike, that was on a ST GS-45 with a Macs 2620 header, 8.5cc muffled tuned pipe, an APC 10 x 7 and 5% Omega (with a dash of added Benol) I'm getting 14,300 on the ground with the full length header. Wave-O-Scope says she's unloading to about 15,400 in the air, so we're not talking crazy RPMs here.

The plugs (OS #8 and Fox std. long) weren't obliterated, just "open" far down in the hole... The Phoenix F20 I'm flying it on has a fairly springy engine mount, so I'm guessing it could be a combination of things such as the pipe being too long, vibration, lean, compression, etc... This is my first attempt at running a true pipe, so I expect there to be a learning curve.

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Old 11-24-2009, 12:51 PM
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Default RE: Sequence of shimming vs nitro

MJD, when I'm shimming the head I'm not looking for a peak RPM difference, I'm looking for an engine behavior difference. You simply can not have an engine that chews up and spits out the elements. The combustion chamber and piston should have a sooting glaze finish, not sandblasted or pitted. The rod ends, crank pin and bearing get hammered from too much compression. A tuned pipe effectively raises the compression br cramming more air into the chamber. If you are running a racing engine with maxxed out breathing ability and unloaded rpm potential, you want to work the compression from the lowest that will barely run up to a point that seems worth trying. From there, you can almost throw the tach away and only pay attention to the health of the glow plug and the times you are getting from the plane.
For the topic of this thread which is about the sequence of adding nitro and shims, [not basic engine running hygiene], it's the same deal...start out with enough shims to make the glow plug survive a bench run or 2, then go fly it and take your tools with you to the field for further fine tuning.
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Old 11-24-2009, 04:28 PM
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Default RE: Sequence of shimming vs nitro


ORIGINAL: combatpigg

MJD, when I'm shimming the head I'm not looking for a peak RPM difference, I'm looking for an engine behavior difference. You simply can not have an engine that chews up and spits out the elements. The combustion chamber and piston should have a sooting glaze finish, not sandblasted or pitted. The rod ends, crank pin and bearing get hammered from too much compression. A tuned pipe effectively raises the compression br cramming more air into the chamber. If you are running a racing engine with maxxed out breathing ability and unloaded rpm potential, you want to work the compression from the lowest that will barely run up to a point that seems worth trying. From there, you can almost throw the tach away and only pay attention to the health of the glow plug and the times you are getting from the plane.
For the topic of this thread which is about the sequence of adding nitro and shims, [not basic engine running hygiene], it's the same deal...start out with enough shims to make the glow plug survive a bench run or 2, then go fly it and take your tools with you to the field for further fine tuning.

Thats intersting your not looking for rpm difference.

engine behavior?? what do you mean here?






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Old 11-24-2009, 04:45 PM
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Default RE: Sequence of shimming vs nitro

Destroyed plug elements, pitted, sandblasted piston and head.
Out of control heat build up confused for simply being too lean.
Quickly hammered out con rods.

All this is considered good in many forms of motor sports if you are fully sponsored.
Nitro drag racers toss the pistons and all other heat exhausted parts after every run.
Decide what is right for you.
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Old 11-24-2009, 05:16 PM
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Default RE: Sequence of shimming vs nitro

Ha, so speed cost money to do it right.!!

I want a engine that will last a season, with the ****e flown out of it, am i dreaming? Hence this is why I've upped to edl 18%oil on the 50v1. To be honest I think this has helped prelong its life for the season. The weston pro synth fuel is an unqualified amount of oil. They wont tell me, so i Changed fuels.

But the 36v1 on the same 18%oil 16% nitro behaves differently, a plug a flight, difficult to tune, i mean the needle is only a 1/4 turn out. Maybe she is more compressed. I know when new she is very tight to turn over but sweet when running.


So if I want to maintain the new fuel for all my engines, do you think the shim is better way to go than resort back to lower nitro?

You see Im confused to wether a highly compressed engine on high nitro is as damaging to a plug than a geniune lean run at end of tank and knoweledge to know the difference.!!

Plugs are expensive,if you pop em every go.

Then you get on thickness of the platinum thats reacts with the nitro. More nitro, bigger element(colder plug) to react with less methonol and the choice of transition from very cold to cold plug



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Old 11-24-2009, 05:22 PM
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Default RE: Sequence of shimming vs nitro

If the needle starts to get real peaky, or it is hard to get a good setting that holds up in the air, you may be over compressed. As CP says, you can't hurt the engine with too little compression (except for running lean). Start off with light props (smaller diameter, lower pitch) and work with them and pipe length.

The other thing to consider, is every day is different. The atmospheric pressure, temperture, and humidity from day to day will change what the correct amount of shims are for the day.

Nitro - 65-70% is racing fuel, below 40% sport fuel, below 15% weed killer.
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