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RE: GMS Engine Tuning Problem - 2/21/2005 11:07 PM   
Wayne Miller


 

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From: DrumboOntario, CANADA
Status: offline 		Hi XJet,

I expect the muffler pressure will do the trick as well. I'll be interested in knowing the results of your test. I know that when I did my original modifications, and used muffler baffles (posted near the beginning of this thread), I lost a couple of hundred RPM, but the engine ran great. By just drilling the carb brass insert I didn't lose any RPM.

By the way, how do you measure the muffler pressure? I'd like to do the same.

Look forward to the results of your tests.

Fly4Fun,

Wayne

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RE: GMS Engine Tuning Problem - 2/22/2005 1:57 AM   
downunder



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Status: offline 		The easiest way to measure muffler pressure (and still use it to pressurise the tank) is to put a T piece in the pressure line, run a length of clear tubing from the T and drop the end into a tall glass of water. Start the engine and run it at whatever revs and you'll see the level of water in the clear tube drop. If the tube is taped to a ruler you could measure the exact amount the level drops. Now 30 feet of water is near enough 15psi so 1 foot is 0.5psi and 1" is about 0.042psi. You'll most likely see it drop about 6" which would mean the muffler pressure was 0.25psi. Another way is to just raise the tube out of the water until you start to see bubbles coming out the end.

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RE: GMS Engine Tuning Problem - 2/22/2005 2:41 AM   
Wayne Miller


 

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From: DrumboOntario, CANADA
Status: offline 		Hi Downunder,

Thanks for the explaination, I have used capillary action depth gauges a long time ago when SCUBA diving. This is basically the same principle except instead of forcing water into a blocked tube and compressing the air, you are pushing air down into water. The more the water is displaced by the compressed air, the more pressure you have.

I think I've got it! Thanks.

Fly4Fun,

Wayne Miller

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RE: GMS Engine Tuning Problem - 3/24/2005 3:49 AM   
Flyer Freq



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Status: offline 		Hey Guy's!

I hadn't heard anything from this thread or any others on RC Universe in a long time. I decided to check and make sure there weren't any other remarks on any other forums about the GMS .47. Imagine my surprise, when I found that discussions were still going on in this thread and I wasn't getting notification. I hope RC Universe didn't black ball me or something .

I ordered my replacement carb body from Tower, and received it a few weeks ago. The new body had the hole in the spray bar in the same place as my first spray bar. It is slightly off center, but none of the holes on either spray bar is hidden by the wall edge, as was the one in my friends bad runner. I have not run mine since putting all my old carb pieces on the new body, as I have retired my SE and ordered a 4D. I hope to get everything going in the next 3 or 4 weeks.

Wayne, by the way, thanks for giving me credit for solving the turbulance issue. I am eager to hear others that try replacing the carb body. I don't believe the hole has to be dead center, but you should definately be able to see the entire hole when you remove the nipple from the carb body.

Talk to you later!

Andy

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RE: GMS Engine Tuning Problem - 3/24/2005 4:01 AM   
Wayne Miller


 

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From: DrumboOntario, CANADA
Status: offline 		Hi Flyer Freq,

Funny you just posted, I saw Spicoli's name in another thread and just PM'd him to ask how he made out.

Since my last post I've modified 3 GMS .47's by just drilling out the intake nipple and making the hole in the brass insert as large as possible directly under the intake nipple - as shown in the previous pictures.

Keep us updated.

Fly4Fun,

Wayne Miller

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RE: GMS Engine Tuning Problem - 3/24/2005 9:56 AM   
XJet


 

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Status: offline 		Just another datapoint, after getting our club's GMS47 engines running okay with different mufflers, I went back to the original GMS mufflers and added a stainless steel restrictor/extension to the tailpipe, which is also a little longer than the original. I also welded the two muffler halves together (they never seal properly otherwise and we've had at least one through-bolt break so welding them solves both problems).

The pressure nipple was also replaced with one that allowed a 0.5mm larger hole.

I also ended up making inserts for the mounting threads because the original screws stripped out on two of the mufflers. I made the thread-inserts of brass with an M5 outside thread and M3 inside.

Now the engines run great, the planes don't get covered in black gunge from the two muffler-halves vibrating against each other, and you can torque down the mounting bolts without fear of stripping.

One of our guys is now actually able to use his GMS47 on a 3D ship and since we modified and re-fitted the original muffler, he's never had a single dead-stick and the engine runs/transitions perfectly despite all those G's and wild throttle control inputs.

So we've got them running great -- but is it worth it?

Well I've just ordered two more Thunder Tiger 46Pros -- they run sweet straight out of the box without all that messing around and the one on my 3D ship just keeps getting stronger and stronger (about 20 hours on it now). The rest of our club members are also buying TT engines -- life's too short not to :-)

< Message edited by XJet -- 3/24/2005 10:00 AM >

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RE: GMS Engine Tuning Problem - 3/24/2005 2:02 PM   
Flyer Freq



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Status: offline 		Hey Wayne!

Curious to see the overall feedback from elongating that hole. It resembles the rectangular cut made in the spray bar of some of the Super Tiger engines that had midrange issues. ST went back to a round hole now, from what I have heard. I don't think the shape of the hole is as important as where, on the needle, it falls. This may be a good fix. I can't wait to get back to flying again!

Andy

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RE: GMS Engine Tuning Problem - 3/24/2005 2:11 PM   
Flyer Freq



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Status: offline 		XJET:

I have looked at a few of the latest engines from GMS. It looks to me like they have corrected the hole alignment problem. While the hole is not dead center, it is not so far out that the fuel has to make a 90 degree turn to get through it. I am back to recommending the GMS again. With the problem solved, they make a great 3D engine.

Just a word of caution on the TT engines. There are some quality control issues and poor customer service issues with ACE RC. I hope I can say that without getting slapped. Our local hobby shop discontinued the entire TT line, because they couldn't deal with ACE anymore. You can read on many threads within RCU to back this up. I have always liked the TT line also, but they have their issues, just like everyone else, I guess .

Andy

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RE: GMS Engine Tuning Problem - 3/24/2005 2:57 PM   
Wayne Miller


 

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Status: offline 		Hi Flyer Freq,

Thanks for the information on the hole alignment on the new engines. It looks like they are agreeing with the fix.

Although I doubt if it will ever be acknowledged, I bet we had a lot to do with getting the problem fixed.

Fly4Fun,

Wayne Miller

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RE: GMS Engine Tuning Problem - 3/24/2005 4:49 PM   
Flyer Freq



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quote:

ORIGINAL: Wayne Miller

Hi Flyer Freq,

Thanks for the information on the hole alignment on the new engines. It looks like they are agreeing with the fix.

Although I doubt if it will ever be acknowledged, I bet we had a lot to do with getting the problem fixed.

Fly4Fun,

Wayne Miller



I would like to think so!

Andy

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RE: GMS Engine Tuning Problem - 3/24/2005 11:49 PM   
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Status: offline 		Wayne, I have a gms .40 and am having the exact same problems! The only difference is that I have a pitts muffler on mine. I found today that Tower recommended the wrong muffler for this motor. If you put your finger over the muffler exhaust partially covering the outlet it would rev up fine and not stall out. So I called them. The muffler that I have does not give enough pressure back to the tank to pressurize the tank due to it having 7/16" tubing for the exhaust as the correct muffler has 3/8" tubing for the exhaust. They told me I could return the muffler but it has already had the tubes cut to fit my corsair. So I would have had to buy a new/correct size muffler. WRONG! Not for another 40 bucks! I am going to slip some copper tubing inside the too large of exhaust tubes and cut the exhaust down a bit. This motor was starving for back pressure. Yours sounds like the muffler you are using is causing yours to starve for back pressure also. You might want to try what I said about partially closing off the exhaust with your finger as someone holds the plane and you run it up to full throttle.

Rick

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RE: GMS Engine Tuning Problem - 3/25/2005 1:10 AM   
Wayne Miller


 

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Status: offline 		Hi Rick,

Thanks for the input.

You will notice at the beginning of the thread, (there are pictures) I had experimented with putting baffels in the muffler body to increase back pressure, this worked well. However, in the end we found that if we fixed the fuel draw by drilling out the carb input nipple and brass insert (see previous picture in an earlier poast), that we could elliminate the baffles. Since then I have done several engines just by drilling out the carb fuel nipple and carb brass insert - all worked well after that.

Since the time when I had inserted the baffles, I was looking at some of my older K&B engines and I noticed the muffler pressure nipple is on the extension between the engine and the muffler I was wondering if this may be a better tap for muffler pressure?

I have never played with the GMS .40, so I'm not sure what would fix it. Please keep us up to date when you get the problem fixed.

Fly4Fun,

Wayne Miller

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RE: GMS Engine Tuning Problem - 3/25/2005 6:40 AM   
XJet


 

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quote:

ORIGINAL: Wayne Miller

Since the time when I had inserted the baffles, I was looking at some of my older K&B engines and I noticed the muffler pressure nipple is on the extension between the engine and the muffler I was wondering if this may be a better tap for muffler pressure?


No way -- that's exactly the *wrong* place to put a pressure nipple.

The exhaust stack has gases flowing at very high speed through it during most of the engine's operating cycle.

Mr Bernoulli dicovered that the faster gases travel through a tube, the lower the pressure becomes inside that tube.

This means that a pressure nipple placed at the point of highest gas velocity will produce *very* poor results and may actually end up sucking as much (if not more) than it blows.

The *best* place for a pressure nipple is at the point in the muffler where the gasses are travelling at the lowest velocity (and therfore the pressure is highest). This is typically (although not always) at the point where the nuffer is widest.


< Message edited by XJet -- 3/25/2005 6:41 AM >

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RE: GMS Engine Tuning Problem - 3/25/2005 1:17 PM   
Wayne Miller


 

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From: DrumboOntario, CANADA
Status: offline 		Hi XJet,

Thanks for the lesson, I didn't know that! When I think about it, it makes sense.

Fly4Fun,

Wayne

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RE: GMS Engine Tuning Problem - 3/25/2005 4:15 PM   
Flyboy Dave



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Status: offline 		Who's Mr Bernoulli ? I don't think he was an aero modeler, because if he was
he would know that putting a pressure fitting in the header flange is OK. Read
this tutorial from Macs Pipes about the subject....

http://www.macspro.com/pressure.asp

FBD.

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RE: GMS Engine Tuning Problem - 3/25/2005 9:30 PM   
rlbrobst


 

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Status: offline 		So would partially blocking one of the exhaust tubes on the pitts muffler help or hurt this situation? I partially blocked one of the tubes on my gms 40 and now I have great fuel line pressure, idles great and rips up to full speed like a speeding bullet. Not sure if this causes a hardship on the motor though. Motor seems to be doing fine. This was all caused by Tower sending me a .47 pitts for my .40 motor. The .40 pitts has smaller exhaust tubes (3/8" od) and the .47 pitts hast 7/16" od. Caused motor to lose power at high rpms due to lack of fuel.

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RE: GMS Engine Tuning Problem - 3/26/2005 2:14 AM   
Flyboy Dave



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Status: offline 		It seems to me that even one 3/8 outlet on a .40 size engine would result in
low back pressure. I think one 5/16 would be ok....

....I'm just guessing.

FBD.

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RE: GMS Engine Tuning Problem - 3/26/2005 3:37 AM   
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Status: offline 		It did have two 7/16" OD (3/8" id) so I plugged one and then opened it to 3/16" id and the other is 3/8" id. Seems to run great now. ramps up rapidly. I am not sure whether I really like these pitts setups or not!

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RE: GMS Engine Tuning Problem - 3/26/2005 7:53 AM   
XJet


 

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quote:

ORIGINAL: Flyboy Dave
Who's Mr Bernoulli ? I don't think he was an aero modeler, because if he was
he would know that putting a pressure fitting in the header flange is OK. Read
this tutorial from Macs Pipes about the subject....
http://www.macspro.com/pressure.asp


You fly model airplanes and you've never heard of Bernoulli?

As they say on the webpage you referenced, "The most consistent pressure is found where the pipe first reaches its apex in width" and as I said, the pressure will be greatest at the widest point (where the flow is slowest) -- so Macs and I agree.

And, while with the Macs pipe it appears that you will get *some* usefl pressure if you tap the stack, there's no guarantee that this will also be the case on other muffler types/designs.

I have a friend who toasted his heli motor after fitting a tap on the exhaust stack. I warned him that this would cause the engine to run excessively lean at high RPMs but he went ahead and did it -- it cost him a piston, ring, liner and head.

This is simple CFD stuff that any first-year colleged student is taught.


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RE: GMS Engine Tuning Problem - 3/26/2005 10:13 AM   
Flyboy Dave



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Status: offline 		XJet....

quote:

You fly model airplanes and you've never heard of Bernoulli?


....no sir....I have never heard of the man....(Bernoulli)....did he invent something
special ? And yes....I have been flying r/c for 35 years, and have NEVER heard
of the man.

quote:

As they say on the web page you referenced, "The most consistent pressure
is found where the pipe first reaches its apex in width" and as I said, the pressure will
be greatest at the widest point (where the flow is slowest) -- so Macs and I agree.


The widest point of an "expansion chamber" may provide "the most consistant
pressure"....but in fact, it is the lowest "pressure area" of the pipe. When the exhaust
charge flows into the largest "expansion" of the pipe....the pressure drops.

Yes....the widest point of the pipe has the least pressure....albiet one of the most
"consistent" points. However, the fact of the matter is....when all we need is approx
one and one half pounds of pressure to the fuel tank....

....any....yes, any point along the exhaust pipe will do.

quote:

And, while with the Macs pipe it appears that you will get *some* usefl pressure if you tap the stack, there's no guarantee that this will also be the case on other muffler types/designs.

I have a friend who toasted his heli motor after fitting a tap on the exhaust stack. I warned him that this would cause the engine to run excessively lean at high RPMs but he went ahead and did it -- it cost him a piston, ring, liner and head.

This is simple CFD stuff that any first-year colleged student is taught.


What is it you are warning us about, XJet....some evil placements of the pressure
nipple ?

FBD.




< Message edited by Flyboy Dave -- 3/26/2005 8:07 PM >

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RE: GMS Engine Tuning Problem - 3/26/2005 3:15 PM   
rlbrobst


 

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Status: offline 		XJET,
This still does not answer my problem listed below:
On my Pitts Muffler it did have two 7/16" OD (3/8" id) so I plugged one and then opened it to 3/16" id and the other is still 3/8" id. Seems to run great now. ramps up rapidly. I am not sure whether it is causing harm to the engine or not. When I rev her up to full speed it smokes so I am assuming that it is not running lean and the exhaust is oily. I am not sure whether I really like these pitts setups or not!

If you could and know alot about pitts setups just email me directly.

Thanks,
Rick

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RE: GMS Engine Tuning Problem - 3/26/2005 8:14 PM   
Flyboy Dave



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Status: offline 		Daniel Bernoulli

--------------------------------------------------------------------------------

Born: 8 Feb 1700 in Groningen, Netherlands
Died: 17 March 1782 in Basel, Switzerland

Bernoulli's Principle states that as the speed of a moving fluid increases, the
pressure within the fluid decreases.
________________________________________________________

Our friend Mr. Bernoulli experiments pertain to fluids, which of course do not
compress....and unfortunately....he passed away long before the invention of
the airplane.

Attachments
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RE: GMS Engine Tuning Problem - 3/27/2005 12:42 AM   
XJet


 

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quote:

ORIGINAL: Flyboy Dave
Bernoulli's Principle states that as the speed of a moving fluid increases, the
pressure within the fluid decreases.
________________________________________________________

Our friend Mr. Bernoulli experiments pertain to fluids, which of course do not
compress....and unfortunately....he passed away long before the invention of
the airplane.


I suggest those interested take a look at the following links for more information (and note that air *is* a fluid that is often considered incompressible at subsonic speeds)

http://www.bbc.co.uk/dna/h2g2/A517169

http://142.26.194.131/aerodynamics1/Basics/Page3.html (or why Jumbo jets fly)

http://www.answers.com/topic/bernoulli-s-principle

Therefore -- where the gasflows are fastest in an exhaust system, the pressure will be lowest.

Since the exhaust stack is usually one of the narrowest parts of an exhaust, there will be far less pressure at that point than at a point where the gases have slowed.

It's just science!

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RE: GMS Engine Tuning Problem - 4/12/2005 6:01 PM   
Flyer Freq



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quote:

ORIGINAL: Flyer Freq
I ordered my replacement carb body from Tower, and received it a few weeks ago. The new body had the hole in the spray bar in the same place as my first spray bar. It is slightly off center, but none of the holes on either spray bar is hidden by the wall edge, as was the one in my friends bad runner. I have not run mine since putting all my old carb pieces on the new body, as I have retired my SE and ordered a 4D. I hope to get everything going in the next 3 or 4 weeks.


I got to fly my .47 with the rebuilt carb last Sunday. I threw it back on my old SE, since I have hardly started my new 4D. I put taller tires on, so I could swing a 12 1/4 X 3 3/4 APC. The engine ran very well, with no transition problems. The wind was really bad, so I only put in two flights. I came in with the engine running both times. Power was very good! The thing that surprised me though, is that when I tached the engine, I was only turning 12,080 . In the past, I always ran an 11 X 3 APC, at 16,200 RPM . When we broke my friends .47 in on a 12 1/4 X 3 3/4 APC, he was turning 14,800 RPM . Does an engine get worn in a certain way, with a certain prop, and then not handle a prop size change well??? My compression is great, and the idle is low and rock solid. Any idea's guy's??? The performance is more than ample, to 3D my 4D, and in a way, it is nice to not be breaking any eardrums, and getting that crack! from the prop tips going supersonic, but I expected better.

Andy


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RE: GMS Engine Tuning Problem - 4/12/2005 11:51 PM   
Harry Lagman


 

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quote:

ORIGINAL: Flyer Freq


The thing that surprised me though, is that when I tached the engine, I was only turning 12,080 . In the past, I always ran an 11 X 3 APC, at 16,200 RPM . When we broke my friends .47 in on a 12 1/4 X 3 3/4 APC, he was turning 14,800 RPM


I recently tached a couple of GMS .47s with stock exhausts with the 12 1/4 x 3 3/4 APC prop while helping set them up. We got between 11,700 and 12,000 at sea level. Your 12,080 seems about right based on my experience with these engines. The 14,800 must have been a false reading.

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