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header length for pattern - max RPM or not

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header length for pattern - max RPM or not

Old 05-27-2013, 10:30 AM
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rcjunkee
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Default header length for pattern - max RPM or not

so in my previous post regarding post flight questions for the Minare Cees mentioned that tuning my pipe for max rpm may not be the best policy for pattern. The idea being I'm looking for consistent pull under load going vertical as well as in a straight line.

Please share with me you insight regarding how to cut my header for pattern.

I have a macs quiet pipe on a Webra 50. It is currently cut to turn an apc 10x7 at 15,500 , max RPM.

I can always however get another header or even put a coupler in to lengthen the header for less RPM / different prop / etc.

Please share thoughts.

thank you
Old 05-27-2013, 11:11 AM
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Default RE: header length for pattern - max RPM or not

Difficult to answer, my experience has only been with long stroke motors at lower (~9.5-10.5K) rpm.

I've found on them that setting the length sort of depends on the pitch and how you fly, but I usually set the pipe to a length where I get a smooth transition from idle to full power (over about 5-10 seconds) on the ground. Set a bit short and it will tend to struggle to get up to top rpm because it'll be a bit rich, then lean bit out as it gets on the pipe. Set a bit long and they seem a bit difficult in the midrange in the air as it tries to get onto the pipe at part throttle especially if it's a clean airframe and running less than 11 inches of pitch.

If the pipe's set shorter I need to flog it a bit ie. get on the power early, let the rpm build then pull the verticals because if I get on the power late and it doesn't pick up that couple of extra rpm it falls off the pipe in the climb. With the pipe set about right you can feed in the power as you pull vertical and it just keep pulling but just a little less strongly.

Buy two headers that way you can keep cutting one until it's obviously too short for the prop you've settled on.
Old 05-27-2013, 11:18 AM
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rmh
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Default RE: header length for pattern - max RPM or not

A couple of points-
a slightly smaller prop may resolve a problem of "peaky power
a larger diameter header may do the same thing .
Books on tuned pipes are relatively worthless because they look at max power tuning
A good setup for pattern works a lot like a control line stunt setup - the power ONLY reaches max when he plane is loading the engine - a vertical climb for example.
adding more nitro also may help - adding another head gasket may help
the point of all this is to broaden or slightly DE tune the engine.
there is no magic right length

start with running the engine with NO exhaust system - and adjust needle for max - power but only for a few seconds!!
now richen the needle a bit - install your exhaust setup -
ideally power will increase a little and the needle will easily adjust to full power or if backed off - easily reduce power .
If power drops -you are way off on length
If power surges violently as you try to adjust needle - too short .
Never do these tests for more than a few seconds at full tilt - you will just overheat things and likely wreck the engine.
It is a learning curve.
Old 05-27-2013, 12:12 PM
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Default RE: header length for pattern - max RPM or not

Paul,

We might be starting to loose track of your threads (at least I am, not that it matters I suppose). Just a suggestion: it might be easier to have an ongoing discussion about your Minare and the engine setup if you concentrate on a single thread. I believe there are now 4 threads with a few posts and things may be getting "lost in translation", so to speak.

Anyhow, Dick's advice offered above is always nice to have. Someone with much experience with this type of engine and this type of pattern design in particular. For reference, Dick Hanson is the designer of the venerable Tiporare - a variation on the Prettner Curare.

Getting back to your engine, I read in your other thread that you "cut your pipe". I'm not sure if this means that you actually cut the pipe inlet (presumably) or just cut the overall length of the exhaust in the header length (although in this thread you suggest that you haven't cut the header). It is always a good idea to cut the header to tune the exhaust rather than the pipe as the latter will affect the expansion chamber volume and resonance behavior of the pipe. Reducing this in an 8.5cc pipe may effectively turn it into a 6.5 or 7.5 cc pipe. I've never done that or seen others do it so I don't really know how a pipe would behave if it were cut substantially at the inlet.

Other than the good tips suggested by Dick above, I am aware of others using the Macs 8.5 QP on an OS 55 AX and commenting that while this pipe resulted in more power (higher rpm), it was less "usable" as the throttle response was rather "peaky" with a less friendly power band. Instead, the 10 cc QP was favored and resulted in the engine behaving much better. The Webra 50 is slightly smaller than the OS 55 but it may respond in a similar way and using the longer 10 cc pipe may work out better.

I've also attached a review of your engine which you may find interesting. You will note that you are getting 1300 more rpm than Clarence Lee did in his tests. This difference makes sense considering his figures are using a Webra muffler and of course your plug, prop and fuel are different. Those differences aside though his tests yielded 14,200 on a Rev-Up 11x6. Given that, it seems that you could stand to loose 300-500 rpm on a slightly longer pipe setup (either different pipe or simply longer exhaust) and have a more reliable engine. At 5 lbs, your model should still have plenty of thrust in uplines on the APC 10x7. I would also try the 11x6 although this will require a longer exhaust setup yet. You might get ~13K+ on that prop when tuned.

As a point of interest, the recommended engine for the new Austrian Curare 60 ARF is the OS 55 (not the 65). I believe that model weighs around 7 lbs all up so your Webra 50 on a 2 lb lighter (and smaller) model is surely an impressive setup!

David
Attached Files
File Type: pdf
Ge95542.pdf (2.25 MB, 17 views)
Old 05-27-2013, 12:55 PM
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Default RE: header length for pattern - max RPM or not

Paul,

Good action to start a separate thread about tuning of the pipe.
In the future it will be much easier to look back for this information.
You'll also reach more contributers this way.


Cees
Old 05-27-2013, 05:06 PM
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rcjunkee
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Default RE: header length for pattern - max RPM or not

thank you all for the flood of input. I appreciate all of you greatly.

To David, I did in fact cut the header and not the pipe. I bought a few extra in anticipation of using different props. I also bought a length of aluminum tubing from which I cut several couplers so that I can experiment with different lengths. I will try some tests to see if I can get a smoother transition and if not will consider going to a larger pipe. Of interest is the fact that the Macs tuned pipes are of the twin cone type vs flat disk. My Macs pipe is the "quiet pipe" model which has several baffles and I don't know if these function the same as a disc type pipe (such as a hatori which I understand has a broader power band) but was told by the manufacturer that it did in fact have a wider power band than their racing (twin cone) pipes.

To Dick, thank you for taking the time to drop me a line and give me some tips. I work in Information technology and am still amazed at how easy it is to communicate with virtually anyone on any topic. 15 years ago I would never have the chance to chat with all of you on a moments notice. In short thanks for helping a new guy.

to everyone else thanks and happy flying. I;ll post my results after I do some experimenting. I think I have enough to keep me busy for a week or two!

Regards

Paul
Old 05-28-2013, 08:40 AM
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rcjunkee
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Default RE: header length for pattern - max RPM or not

Of course one solution would be to switch to a mini pipe like they have on the "just engines" site...but then that wouldn't be period correct
Old 05-28-2013, 09:09 AM
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Default RE: header length for pattern - max RPM or not

Paul,

By the way, 5 lbs dry all up is very good weight for a piped 50 powered painted glass/foam Minare. Testament to the remarkable low weight of the engine as well as good finishing skills!

Are there retracts on your model?

David
Old 05-28-2013, 09:20 AM
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Default RE: header length for pattern - max RPM or not

Paul,
If we were at a contest and your model ran like you've typed here, we'd do this;
Slide the pipe back a 1/4 to 1/2 inch in the coupler by snipping one tie-wrap off, install new tie-wrap to secure and fly again. Peaky goes away, top end not as much, surge probably settles, fly, fly, fly.
If it just surges going straight up you're almost there, you were just greedy on the lean needle and peaked pipe length.
The vertical level of your tank to your carb spray bar is something to look at while on the bench at home, should be level with one another, shim tank as close as you can get to level with the carb. 
Try to go to the contest on June 8,9 to meet a lot of experienced people to help out. You'll love it.
Chris...
Old 05-28-2013, 09:55 AM
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Default RE: header length for pattern - max RPM or not

The technique I used to use was to shorten the pipe in 1/4 in increments until the max rpm started to drop. Then, as noted in a previous post, slide the pipe 1/4 in out in the coupler. This slightly detunes it, but when you go vertical the engine comes back on the pipe.

Jeff
Old 05-28-2013, 11:07 AM
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MTK
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Default RE: header length for pattern - max RPM or not

Paul,

After making whatever adjustments per all the tips guys have given you,do the following when flying:

From safe height put the model in a fullthrottle dive (shallow 20-30 degree or so) to unload the engine. If the pipe length is correct, your engine will not pick up any rpm or lose any for the prop load you are turning.

If it goes fat, (it will likely burble some).....pipe is long for the prop you are using. Either increase prop load and/or shorten the pipe 1/4" and try again.

If it picks up speed,pipe's too short.....either lengthen 1/2" and/or decrease load.

This technique hasworked well on all sizes of glow type 2 strokes (including the big block 160's-180's), that depend on needle and pipe settings for ignition timing. It assumes you are using a good all around plugsuch as KB1L



Old 05-30-2013, 07:34 AM
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rcjunkee
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Default RE: header length for pattern - max RPM or not

I moved the pipe 1/2" back by putting in a 1/2" aluminum coupler.
If the pipe still acts peaky I'm considering trying a hatori type (flat disK) pipe.
Even though the Macs has baffles it still has a converging cone which may affect powerband.
Old 05-30-2013, 11:22 AM
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Default RE: header length for pattern - max RPM or not

The flat back pipe has a converging cone too. You just can't see it. It is the converging cone that slams the pressure, some unspent and spent fuel back into the cylinder head and supercharging your engine.

You seem to be a little overwhelmed with all the advice at the moment. I am as well.

Tuning an engine with a tuned pipe will take some time to get the feeling for it. Be patient. I agree with Chris... I would pull out like it sounds you have done.

Have you read Dean Pappas's article on tuning for max performance, not max RPM? I can get that to you if you would like to read it. It is the most succinct, easy to comprehend, easy to follow write up I have ever read on the subject. It explains how everything works too! But even then, educated trial and error are still your best friend.

I am just beginning to learn all this stuff myself. Jim Oliver is an amazing help on this matter.

Brian
Old 05-30-2013, 12:00 PM
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Jim Oliver
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Default RE: header length for pattern - max RPM or not

MTK's comment is most helpful, the inflight adjustment is THE critical factor. Most of us are stll learning, if we're still brreathing
Old 05-30-2013, 12:41 PM
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rcjunkee
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Default RE: header length for pattern - max RPM or not

well the good news is it sounds like I don't need to buy another pipe

@ Brian...if you could point me to this article I'd like to read it.

thanks guys
Old 05-30-2013, 12:44 PM
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Default RE: header length for pattern - max RPM or not

I think I found it via google e.g. http://images.rcuniverse.com/forum/u...62/Nl30530.pdf
Old 05-30-2013, 01:48 PM
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Default RE: header length for pattern - max RPM or not

great article...can't wait to try the techniques mentioned....

One more thing I thinking about is once the length is correct, where to set needle on the ground.
I'm sure this will be yet another thing that gets worked on in the air but as a starting point common sense tells me to set it where it rich of max rpm with nose up.
Old 05-31-2013, 07:14 PM
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Default RE: header length for pattern - max RPM or not

Went through the steps outlined in Dean Pappa's article and a couple observations:

1) I think the extra .5 inch is ABOUT right. The jump in RPM is not so violent...just a noticeable jump like he says.

2) The amount the needle has to be turned between where it get's ON the pipe and comes OFF is a little more than an 1/8 turn as Dean outlines however I question how literally I should take this statement as different needles move at different rates. I think the length is at a place where I'm ready to fly it. the throttle transitions linearly but still a little rough in the midrange.. (see next item)

3) either my glow plug is too cold (no I don't know what type is in it right now) or my idle needle is still to rich. I have a sneaking suspicion it may be both.

a) the manual specifies 2 - 2 1/2 turns out on the low speed needle and mine is 1 1/2.

b) when I take the battery off the rpm drops noticeably but even with the battery still on the rpm slowly drops until it almost dies.

c) If I open the throttle back up it has to clear itself out again. once it's clear the throttle can be snapped from idle to full sort of cleanly but there's still a hint of clearing out hence my suspicion of the plug

d) I will try leaning out the low speed needle another 1/4 to 1/2 turn (had to stop tonight as it's too late to run any longer)

e) I am going to buy an assortment of plugs to experiment with (this is where you can make your recommendations). I'll probably buy a few temp ranges across 2 or three brands to see if I can get one that works good
Old 06-01-2013, 06:21 AM
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Default RE: header length for pattern - max RPM or not

On most engines, the idle needle position and high speed needle position are somewhat related. It is possible to get the idle needle closed off so far that you can't get the high speed needle rich enough. Sounds like you are at least "in the ball park" even at 1.5 on the idle needle-just gonna have to have the high speed needle farther out.

I like to set the idle mix so that, at idle speed (2400 or so, depending on the engine and weight of the prop), if the fuel line is pinched closed (use your fingers) the rpm should rise not more than two hundred rpm. More than that means the idle mix is too rich, some engines like less rise. If, when the fuel line is pinched, the engines stops the mix is too lean. Some engines want a lean idle mix, to avoid loading up and/or a too rich mid-range. Also, if the engine quits suddenly when the throttle is opened from idle to full speed, the idle mix is too lean.

I like to set the high speed needle rich enough to get a 300-500 rpm rise when the fuel line is pinched at full throttle. My little Webra 50 on muffler likes Cool Power 15% with Xoar 10x7, turns 14,500 but wants a relatively lean idle mix and also lean high speed mix. Almost too fast in the Kaos arf at 4.9 pounds.

Jim
Old 06-01-2013, 05:00 PM
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Default RE: header length for pattern - max RPM or not

Lots of great information here.
Try the K&B 1L glo-plug. I have been running them for 35 years or so. Typically I get about 60 flights per plug, the element of the plug should never be distorted (or deformed) or you are too lean or too short or both. I usually notice the idle getting a bit ragged when it is time to change a plug, the transition from idle to full speed may also suffer (seem rich). I have run several Bully engines (slightly modified Webras - two plug head) in the 1.20 and 1.45 displacement. The factory low end setting was always too rich, on one occasion it was so bad you could not throttle up unless the glo-plug igniter remained connected.
Good luck!
Will B.
Old 06-02-2013, 12:38 AM
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Default RE: header length for pattern - max RPM or not

Paul,

Just to complete the procedure:

http://www.rcuniverse.com/forum/fb.asp?m=11524655

2. of the list, because my impression was, this is not a new engine?


and I told you, my post 5 after reading post 4!!, very informative thread!!
Success,
Cees

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