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Aug 21, 2017, 09:39 PM
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C of G fuel cell below carburetor level

I've taken a plane and have created a fuel cell that is centered on the CG. It has to go below the wing tube in the fuselage. It is below the level of the carb - an inverted OS .52 four stroke. It is important that the fuel doesn't shift fore and aft. Fuel is contained in a latex bag and is fed by a single tube to the carb. The bag, pick up tube is the type that some heli pilots/ pylon racers use. (silicone fuel tube with multiple holes along the length). The pressure tap off the muffler supplies pressure to the outside of the bag. The bag container is clear plastic. The stopper, bag and pickup go into one end and is air tight, except for the pressure port for the muffler pressure. The design is similar to a Jett tank or Tettra. All of the air is sucked out of the bag, fuel is pumped in. No air is in the bag, just fuel.

The plane starts but has an unstable idle. At over idle, it transitions to full throttle. At full throttle if you lift plane's nose up, it quits.

I tried to use a check valve on the muffler pressure tap tube. It floods the engine if you decrease throttle to idle, drips and makes a mess.

I've read about perry pumps and fuel regulators and will explore this route.

But why wouldn't muffler pressurized fuel just feed into the carb. The tank is clear. I can see the fuel in the bag, no bubbles. It is holding about 7 ounces.

The check valve was a huge fail.

I cannot reposition the tank anywhere else. There is no room for a header tank. A clunk tank will not work in that the fuse is slender and at high alpha the plane will go aft CG.

Any ideas welcome that can make this tank position work.
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Aug 22, 2017, 11:34 AM
B for Bruce
BMatthews's Avatar
Well, for starters muffler pressure is very weak. And you're expecting WAY too much from it. So all the other issues of a long delivery line and a large mass of fuel are dominant over the weak muffler pressure.

If you want this to work you need to shift gears strongly.

First off the carbs on most engines are notoriously weak at drawing fuel. Which is why our usual fuel tanks need to be relatively small and set up very close to the carb.

To work well with a centralized remote tank you need to run a stronger pressure source and use a regulator near the carb. Or run a stronger pressure source and a remote float bowl much like with old car and bike engines. Then the carb on the engine can draw from a small reservoir in the float bowl located just behind the engine.

By a "stronger pressure source" I mean something like an onboard electric fuel pump or a heavier bladder squeezed between two boards with some rubber bands. Muffler pressure is simply nothing in comparison.
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Aug 22, 2017, 06:53 PM
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richard hanson's Avatar
It sounds as if your carb may not be properly adjusted
On a test stand with the cab spray bar and your tank centered , run the engine.
will it easily draw fuel?
wil it transition properly?
Use 20% nitro and a OS four stroke plug.

For muffler pressure setup to work properly , you never use a check valve and the pressure tap should be per stock OS muffler location.
YOUR type tank is then pressurized by using a stock plastic tank, then stuffing the bag and a slotted pickup in the tank

I never liked this setup but it can prevent foaming in some setups
Pressurized setups are in many setups wretched -as was the OS gen one arrangement
The present four stroke setup works well tho.

FWIW the fuel is really never forced into the carb -except on the the bladder type setups such as used on 1/2 A racers .
The correct pressurized setup has pressure to the tank , usually crankcase pressure,-which feeds a
regulaor that is adjacent the carb.
the carb then creates a low pressure which opens the reg ON DEMAND
or as the low pressure signal increases.
This is sometimes tricky but it works

It has been around for 60 years -at least.
Aug 22, 2017, 10:46 PM
Registered User
Bruce, Richard,

Thanks for the replies. I'll pursue the plug , cleaning, valve adjustment and test stand. I bought it used a while ago and just noticed that it had a bent needle holder as if it had been dropped. It never seemed to matter on the last two years I flew it but I want that out of the way.

I also got advice from a hobby shop owner that I probably did not need to get a pump, regulator or use the bag/ Tettra/ Jett tank. This person is well known in the RC pylon racing world. So I'll try it with a clunk tank in plane to rule that out. I won't fly it that way because of the slender tank messing with the CG.

This will all confirm that the engine is running right. Then I can start hunting for stuff for the CG set up. It's been done but there's a lot of claims and conflicting info.


Aug 23, 2017, 08:29 AM
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richard hanson's Avatar
The needle valve orifice should appear perfectly round but lots of use can distort it.
the néedle valve holder should not be distorted and should fit easily .
A quick test by attaching tubing and blowing thru it,should show an even reduction of air as the needle closes.
My favorite setup onthese is 4 strokes is a muffler pressure tank,using stock muffler.
Prop it to easily turn 10000 rpm and run it ever so slightly rich in level flight
Aug 23, 2017, 01:25 PM
An itch?. Scratch build.
eflightray's Avatar
Strange how we flew 60 sized powered aerobatic models back in the 50s - 60s with no pumps, no pressurized tanks, and tanks well in front of the leading edge, often right behind the firewall.

Must have been because it was before ......... 'climate change'

Or perhaps 2 strokes were just better at 'sucking'. ?
Aug 23, 2017, 06:26 PM
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richard hanson's Avatar
Originally Posted by eflightray
Strange how we flew 60 sized powered aerobatic models back in the 50s - 60s with no pumps, no pressurized tanks, and tanks well in front of the leading edge, often right behind the firewall.

Must have been because it was before ......... 'climate change'

Or perhaps 2 strokes were just better at 'sucking'. ?
OK lets be serious
Most of these old 2 strokers hard to rev to produce power and the carbs were rudimentary to say th least - Been there started with back port OR and ATOM.
The 4 stroker were often setup badly at first simply because they really have different fuel and carb size requirements.
Usually overpropped to boot .
I tested all th e early Saito models which came to US some of the carb setups and rec fuels were wretched.
I also ran first OS models as well as al the earlier YS- rebuilt a bunch of em for friends.
Eventually it all got sorted out by the mfgrs and customers
There were some awful 4 stokers, webra and sone early Chinese types
The OS series of simple non pumped 4 strokers (Surpass) are great engines ran all of em
for a sport setup -these Surpass setups can't be beat
just use right plugs,prop,fuel and correct muffler pressure .
Aug 23, 2017, 11:21 PM
B for Bruce
BMatthews's Avatar
I'm still a little puzzled about why you think you need the fuel tank to sit at the CG. If you want to run a grossly larger than typical amount of fuel for very long durations I can see it. But the fuel needed for a typical 8 to 10 minute flight has always been carried with ease by a suitable size tank mounted in the nose of the model.

The procedure is you balance the model with an empty tank. Then you fill it and fly. The weight of the fuel does make the model a touch nose heavy at first. And occasionally that requires a click or two of up trim. Then later in the flight you remove the click or two when you notice the model seems to want to climb a touch more easily.

Messing around with a bladder tank in a pressure bottle located right at the CG is just making your life more complicated than it needs to be. You've run into some of the issues already and posted about them here. But I'm sure you're going to run into more as you go along.

You'll be far happier with how the engine runs in every way if you just go with the proven formula and do it like the rest of us have done for close to 100 years of model flying history.
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Aug 24, 2017, 10:53 AM
Registered User
Originally Posted by BMatthews
I'm still a little puzzled about why you think you need the fuel tank to sit at the CG. If you want to run a grossly larger than typical amount of fuel for very long durations I can see it. But the fuel needed for a typical 8 to 10 minute flight has always been carried with ease by a suitable size tank mounted in the nose of the model.
For this plane it is a conversion. It is a 3D Hobby Shop Super Vyper 48. The design from firewall back is tailored to batteries and includes obstacles like a UC cage, battery tray and such. For the engine rails I used the original thrust lines and used the motor box cage that is integrated with the inner fuselage. The battery tray is a structural part of the airframe, great design. It is blocking the center line where a tank would go. The area for a tank would hold about 4 oz. at best. I'd have to gut the tray.

Below the tray is a space spanning from the UC box to the rear servo tray that has a good amount of room to slip in a slender tank. That's where I have the tank. It is equally over the CG fore and aft, but about 2" below the carb. The engine is inverted.

Indeed I wanted an 8 oz tank but the max I could get for a bladder is coming out to a little over 7, so I might get around 8 min of flight.

It is a plane capable of 3D flight and sport pattern, so it is important to me that it does not shift CG much. The tank is so long and slender that the clunk won't flop to the front on downlines, and the fuel will be far back from the GC if the thing does 3D hover. That's just my guess though, although I could see the clunk not flopping. I may try thinner walled tubing.

It's common for large gassers to have the tank on the CG but those folks have carbs that can pull the gas up to the engine.

Really appreciate the help and advice. I'm going to give it a shot to test it. The tank stopper is 3" from the firewall. I'll shorten the extra fuel line length and try it after it get the part for the carb.
Aug 24, 2017, 11:33 AM
B for Bruce
BMatthews's Avatar
Ah, that helps make your "motivation" more clear.

For the size of tank and distance muffler pressure is a non starter that I've already mentioned. It's simply way too weak and not at all consistent due to changing wildly as the throttle opens and closes. A better source of pressure from the engine itself is to add a pressure tap to the crankcase. These pressure taps typically use a very small hole so they work sort of as a one way valve. If you can't find a fitting with a suitable hole then an inline one way valve along with a larger bore fitting would work as well.

The issue with both muffler and crankcase pressure in a bladder and outer bottle setup is that oil or raw fuel will enter the bottle and make a mess that needs to be cleaned out on a regular basis.

But if you just use a clunk tank then the fuel from a crankcase tap simply mixes back into the tank fuel. So perhaps try a test bench mock up with a regular tank.

To keep the clunk line from flying forward and pinching the line a common "fix" is to use a shorter section of flexible line and an extension to the clunk weight that is made from stiff plastic or metal tubing. So the clunk weight can still flop around easily but you can't fold it over. Try this with roughly half flexible line and have stiff tubing and see if that's enough to fix the ramming forward issue.

If direct crankcase pressure isn't enough to do the job then look into a pulse pump such as used to be found on some engines or as stand alone items. The stand alone version used crank case pulses to operate a diaphragm which along with a couple of one way valves pumped fuel directly to the carb. Some had a built in pressure regulator and some were sized to work directly with a specific engine. The idea was that the fuel arrived at the carb with a consistent pressure.

To work with this style of constant pressure you'll need a carb which has some way of altering the idle mixture setting. It may also need to be one of the carbs that cams open and closed to alter the mixture evenly through the throttle range. The carbs that do that are easily seen because they are the ones with the throttle barrel that moves in and out slightly as it rotates from the servo.

That's all I recall of the old tech stuff. Hope there's a gem or two in there to help you.
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Aug 24, 2017, 06:49 PM
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richard hanson's Avatar
In my opinion ,crankcase to tank pressure on this engine is not a good setup
Saito did this on their first versions and getting the adj correct was not easy.
The basic problem was the tank would pressurize , then if the carb was shut down, the pressure in the tank would flood the carb
Same thing happened with the early OS crankcase pimps ,plus those pumps broke their assy bolts from vibration
As simple a thing as using a bit more nitro to broaden the needle mix often fixed "fuel delivery" problems
Anyway I would avoid any setup other than a simple small clunk tank and stock muffler pressure on this setup. It is the easiest setup to get "right".
We ran FAI pattern with the 91 OS using an 11 oz tank. for 8 min flights
.a 6-8 oz tank should work with the 52.
again propped correctly the mix issues are further reduced
Over propping four strokes is a common problem
WE ran early ENYA four strokes at 14000 with no problems whatsoever.
The performance was unreal.
I made small tanks using lotion bottles and reshaping with heat as needed .
Way way back in time (1958) , we set up a 283 Chev we would run to 8000 on the low to second gear shift - 100 in the quarter mile.
Slow now - fast back then.revs don't hurt if you setup the engines for it.
------- My " old folks" Venza will run in the 90s times change.
Last edited by richard hanson; Aug 24, 2017 at 07:00 PM.
Aug 24, 2017, 07:40 PM
AndyKunz's Avatar
I'll be coming into an Enya 90 FS in a couple weeks. I'm going to have to e-mail you when it gets here.

Aug 24, 2017, 08:21 PM
Registered User
richard hanson's Avatar
Get your Dremel tool ready
These early 4 strikers were typicallly over compressed
The Enya and first series OS were typical
OS learned fast and the Surpass series were golden.
The 1.2 supercharged O S was a glaring example of supercharging done Wrong
We dug out all the chamber we could on these, then ran em on a nice 15% oily mix
The stock setup was a mess
Running a GMC blown Olds on the street when I was a kid, was a priceless learning experience on supercharging.
Last edited by richard hanson; Aug 24, 2017 at 08:28 PM.
Aug 25, 2017, 12:05 AM
Registered User
These are great stories. I actually shopped in Wal-Mart for the tank shown. It was a dog deodorizer spray. It actually smelled good to just spray in the house. The rest went in an empty flower pot.

I actually rebuilt a couple of air cooled engines. No way I could afford to do it now. Wasn't cheap back then.

I got the part in today and was the other one flat on one side. I tried to wallow it out and saw it was also cracked.

On hurricane watch here so I might not get to bench test this weekend.

Thanks for the advice.
Aug 25, 2017, 02:21 PM
B for Bruce
BMatthews's Avatar
Good point on the crankcase pressure story Richard. I'm used to carbless racing and free flight engines where we don't need to throttle up and down. I figured on a problem which is why I suggested a carb with separate idle mixture control. But from what you're saying that would not allow for the variation as the pressure bleeds off over some amount of time following a throttle chop.

I suspect that what is really needed is a regulator up near the engine's carb which puts a more or less low but constant head pressure in the fuel line that feeds the carb. I know that regulators of this sort used to be available. But it's an electric world out there these days so parts like this which were a niche item even during their heyday are likely not easily found.
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