RE: GMS Engine Tuning Problem
 

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 [&o]. In the past, I always ran an 11 X 3 APC, at 16,200 RPM :D. When we broke my friends .47 in on a 12 1/4 X 3 3/4 APC, he was turning 14,800 RPM [8D]:D. 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

RE: GMS Engine Tuning Problem
 

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.

RE: GMS Engine Tuning Problem
 

I suppose it is possible it was a false reading. The only other factor that was different, was that the 14,800 was taken in 20 degree weather, vs 60 degree's with the 12,080 on my engine. I don't think that can account for the 4,720 RPM difference, though

Andy.

RE: GMS Engine Tuning Problem
 

[quote]ORIGINAL: XJet

On the GMS engines with a large-bore carb, the best source of pressure is the crankcase, not the muffler. My GMS76 crankcase puts out more pressure than the YS pressurized fuel systems. So the engine never quits by running lean after it warms up.

RE: GMS Engine Tuning Problem
 

OK....air is a fluid....and can't be compressed....

...you need to go back to science class, son.

FBD. :eek:

RE: GMS Engine Tuning Problem
 

Just follow the links I provided and see for yourself. I didn't invent the science -- don't kill the messenger :-)

RE: GMS Engine Tuning Problem
 

Hi,

Just an update, I just finished fighting a GMS .47 that had the modification.

It would die right after takeoff or prolonged wide open throttle. We checked with my 15% nitro synthetic/castrol blend fuel and it worked fine.

I forgot the obvious - the problem ended up being old fuel.

Just thought I'd pass it along.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

While my engine is running well, I am playing with a concept that might be helpful for the .47. I have set up a uniflo tank, which should help with fuel delivery. I am thinking it will especially be helpful when hovering. I tried it with a check valve between the muffler and the tank, and that was a disaster. While I know they recommend a check valve, so fuel can't flow back to the muffler, the fuel tank pressurized to much. After running at high rpm, the pressure forced too much fuel back through the carb, screwing up the idle and transition. I removed the check valve, and the engine running was consistant. It seemed as though I went through fuel faster though. Not sure how much fuel went through the muffler when throttling back. My buddy and I talked about setting up a tank with some sort of a pressure regulator, to keep tank pressure the same, no matter what the tank level or rpm of the engine. I am figuring it would have to maintain a steady 2.5 to 3 lbs. I have some idea's how to accomplish this, but maybe someone reading this knows of such a device that is already made,....

Andy

RE: GMS Engine Tuning Problem
 

I flew the uniflo tank on my .47 today, and I couldn't believe the difference. It took a good running engine and made it great! I can't put my finger on it, but I think the difference was that the mid range was more linear and faster to respond. I had a whole new confidence in my plane. I would encourage anyone to try this tank configuration, especially if you like to hover!:D

Andy

RE: GMS Engine Tuning Problem
 

Hi Andy,

I'm interested in learning more about this tank - where can I find information on it?

Thanks for letting us know the results.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Try the July 2005 issue of Fly RC Magazine. Good description in there, with a simple drawing.

RE: GMS Engine Tuning Problem
 

Keefer is right, Fly RC Magazine is where I got it. The instant throttle response and consistancy of RPM at different stick positions has me wanting to change all my planes to this setup. If you would like, I will try to describe the hookup. Unfortunatley, the Great Planes tanks, with the two built in nipples and one nipple in the rubber stopper, are not candidates for this hookup. I am bummed:(, because that is my favorite tank.

RE: GMS Engine Tuning Problem
 

Found this thread last night, pulled apart my first gen. .47 motor and found the offset hole just like all the others.

This morning I disassembled and used the nipple as a guide for the biggest drill bit I had that spun freely inside the nipple, drilled, then blew out with brake cleaner and reassembled and ran it, on the ground only so far.

Seems to have worked! Before, to ensure good running with the nose pointed up, both at 1/2 throttle and full required a overly rich mixture, both on low and high speed needles, which of course hurt power. Now, I'm only slightly rich sitting level, ( couple of clicks), and the motor will actually pick up a bit of rpm when nose up from the inherent lean out in this position, just like its supposed to. :)

Importantly, it only took just a few minutes of fiddling to get it running right, where in the past it was much more difficult to get right.

Throttle response is improved as well, importantly as this is installed in a tufflight 4d. It'll be nice to actually try some hovering without worrying about lack of power or a flame out.

I'll report back after a test flight.


Dean in Milwaukee

RE: GMS Engine Tuning Problem
 

Hi Dean,

Thanks for the input, I look forward to your first test flight report!

If you still have difficult adjustments on the carb, you may want to drill the intake nipple and brass insert one size larger than you did.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Hey Dean!

Add the uniflo tank to that equation and you will even get better consistancy. The uniflo tank allows you to set the carb for best running on the ground. That setting is not supposed to change in the air, because gravity, for all practical purposes, is removed from the equation. Gravity affects the force of fuel flow without muffler preasure. The more fuel in your tank, the more gravity feed. If you get your preasure line and carb line at the same reference point, gravity is no longer the factor it once was. I set mine up on the ground to scream, and expected it to go real lean when I pointed the nose up. To my surprise, there was no noticeable difference. I didn't tach it, but my calibrated ear:D didn't notice anything.

RE: GMS Engine Tuning Problem
 

"Hey Dean!

Add the uniflo tank to that equation and you will even get better consistancy"


I will probally try that out too. Anything for better throttle response during a hover.

Now if I can only get some more stiffness into my 4d's tail control linkages to stop the flutter at medium speeds. Maybe a pull/pull setup will do it.


Dean in Milwaukee

RE: GMS Engine Tuning Problem
 

Dean:

I am putting a 4D together right now, also. The only way you will get rid of the flutter is to make the high speed modifications they show on the Tuff Flight website. It is basically the addition of counter balance weights for the elevator and rudder. The setup appears easy.

Andy

RE: GMS Engine Tuning Problem
 

Thanks for the tip Andy, its too late for me to move the surfaces a couple of flutes without cutting new surfaces, but I was able to easily add counterweights by inserting heavy nails in the leading edges facing forwards. I did'nt put them out on the edges but closer to the root to hopefully not snag objects as easily.

I needed to add tail weight anyways, so hopefully this extra mass will raise the speed the surfaces will flutter to reasonably high speed.

Looks like good weather tommorrow morning, should be able to get a couple of flights in before work. Maybe I'll have both a good running motor and a flutter free tail. :)


BTW, I had mostly solved the elevator flutter problem by the simple expedient of CA'ing the elevator 1/2s to the carbon shaft forcing them to stay locked in perfect alignment with each other. This means that for any elevator flutter to occur, both elevator surfaces must flutter together, and, both elevator linkages must flex together as well, effectively doubling the force it requires to deflect each elevator surface individually.

If you do this, the only trick is to put grease inside the brass tube so that the CA won't wick inside locking the whole assembly solid. :eek: Don't ask me how I found out that one. :(


Dean in Milwaukee

RE: GMS Engine Tuning Problem
 

Ooops! Yea, I forgot about the different position for the axle on the tail feathers. I guess i am in the same boat you are. Did you have any trouble with the wing being a little warped? I have one side with a little dip in the last 10 inches or so. It isn't a twist, but an actual dip. Not sure what to expect out of it.:eek: Thanks for the tip on the carbon rods. I am not that far yet. The dip in the wing tip stalled me[X(].

Andy

RE: GMS Engine Tuning Problem
 

The wing was easy to straighten just by using the tried and true " twist it while using the heat gun " approach. This works on mine even with me using zagi tape for covering instead of monocote.


Dean in Milwaukee

RE: GMS Engine Tuning Problem
 

I was a little afraid to get too close with the heat gun, for fear I would melt the foam. I will give it a try! Thanks!

Andy

RE: GMS Engine Tuning Problem
 

Hey Guys,

Your hijacking my thread here on the GMS .47, if you don't mind, how about starting a new thread to discuss the 4D. Thanks.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Sorry Wayne, as I am sure you are aware, it is easy to have it happen.[X(]

I hope you find the info on the uniflo tank helpful!

Andy

RE: GMS Engine Tuning Problem
 

Hi Andy,

I did some reading on the uniflo tank setup and I can only see a couple of dissadvantages. One, when the tank is full, and the plane sitting in the sun, fuel would be pushed into the carb and muffler when the fuel/air expands in the tank. When on the ground one might have to pince the lines with clothes pins or something. Two, when the tank is pressurized at full throttle, then the throttle decreased, the extra pressure in the tank will force fuel into the muffler/carb until the pressure is reduced - I don't know how much fuel would be lost in the muffler from this. One very good thing about this is the automatic pressure release at low throttle.

I like the idea of both the muffler pressure line and the fuel pickup line being controlled by one cluck (both lines attched) but you would have to have a very flexible line to do this.

I'll put some thought into it and will have to give it a try.

Thanks for the idea.

Fly4Fun,

Wayne Miller

RE: GMS Engine Tuning Problem
 

Those blatter tanks are used a lot in racing planes, where the pilot doesn't
want to lose fuel pressure in the high "G" corners....and keeps a steady
pressure in flight....keeping his needle valve setting close to the same as
possible. ;)

FBD. :D