Being rather a novice with regard to 4stroke power, I have een doing quite some reading on this forum and in particular this 4stroke board. What strikes me is the discussion on the tank type to use. I have the feeling that people are mixing up standard, uniflow, suction, muffler pressure and show a limited understanding on the way a uniflow system is supposed to work.
For some reason "uniflow" is put opposite to "suction", while these are 2 completely different attributes of a tank setup. Uniflow can perfectly be suction.
A uniflow setup is in fact a Mariot bottle on its side. Earth gravity becomes centrifugal Gs. A very simple system. Its purpose is to avoid that the mixture becomes leaner as fuel is consumed.
I am wondering how a non uniflow system is coping with this leaning out effect. Is the effect not noticable when running four strokes with a non uniflow tank?
On this forum I often notice that people are putting the uniflow exit and the feed pipe in the tank very close together. Less than 1/2 inch or so. I think this poses a serious risk of bubbles entering the feedpipe. If this is the case, engine runs will be erratic, especially with four strokes are they are not as forgiving in fuel supply interruptions as 2 strokes. There is absolutely no need to have the 2 pipe exits close together. If the uniflow exists at the front of the tank and he feed at the rear, the uniflow principle remains the same and risks of bubbles in the feedpipe is nihil. For 40 years I have been helping rookies and bubbles in the feedline was the main issue they had wrt a decent engine run. Mainly fuel foaming and the uniflow exit too close to the feedpipe. Please don't do it. Also the double clunk principle is inferior to the 1 clunk system with a fixed uniflow tube for the same reason. Perhaps I am missing something?

  What some guys are calling "suction" tanks, I think , are really standard vent tanks with a pick up tube, fill and over flow tube. The over flow is capped off for flight and muffler pressure applied to the fill tube. Search ot threads by Bob Reeves, and another fellow whose name I just can't pull up, about tanks and how to run a four stroke in the low RPM mode. I chased my tail for two seasons trying to get some consistency from my Saito .56 in a Top Flite Score and didn't get anywhere until I found the articles that Bob Reeves wrote and used his suggestions. It's basically a small diameter venturi or a modified carb with a block installed to replace the throttle barrel, and running bigger diameter props with higher pitch and keeping RPM to around 8,000 or lower. Four strokes are all about fuel flow, and getting it as good as possible is key. Higher nitro content lets you open the needle valve more f or more fuel flow, but you still need a smaller venturi to provide as much crank case vacuum as possible for good flue flow. Adding muffler pressure completes the deal. The model may speed up a tiny bit towards the end of the pattern but it's not too much and sometimes depending on weather I can hear the engine speed up but can't see it on my lap times. You really have to forget everything you know about running two strokes because the four stroke is such a different animal It makes great power, but it's a different power, more of a pulsing power. They make great torque and that takes advantage of the bigger props and more pitch at low RPM. Trying to run at a higher RPM with a flatter pitch prop and uniflow tank just over heats the engine, and is hard on push rods and other internal parts. I like flying them because they are so interesting but I didn't start selling off my Super Tigers and OS two strokes either!! On my Score with the Saito .56 I usually get a pattern in comfortably on 3.75 ounces of fuel or less sometimes. The best advice I can offer is to read up on everything you can find and make your own decisions, then fly them and evaluate.
  Type at you later,
  Dan McEntee

Thank you for the quick feedback. As I mentioned in my first post, I went trough all the four stroke posts and I read the inputs of Bob Reeves and Mike Alimov and Bob Zambelli etc. very interesting stuff. I know the venturi systems they are using. I have been in contact with the Beringers here in Europe. This post is not about venturis nor carburetors. I am trying to understand why a simple tank setup as a uniflow tank is not working for 4 stroke engines and why the clear disadvantage of a non uniflow tank does not seem to affect 4 stroke engines. I can imagine that the muffler pressure is so high that it is the main governing pressure so that the effect of the static pressure of the fluid column has very little effect. I will be measuring the pressure of the muffler as soon as I can. Might be a lot higher than for 2 stroke engines. This being said, I am trying to get my engine working without muffler pressure because I hate the backward starting. I do hand starting. No electric starter. And I know I am stubborn now and then.. :-)
 

I have 2 CL models with FS engines. An OS 26 Surpass on a Sterling Yak-9, and a Saito FA 40 on a Brodak P40 ARF. I have them both set up exactly the same and both engines run great.  Pics are of the Yak 9. My particular setup:

-Brodak oval profile uniflow tank (but I had Brodak custom move the tank fuel pickup tube to the bottom of the tank)
-Ram air pressure for the tank (not using muffler pressure)
-Ram air tube on the model inside to get clean air from the prop
-Tank overflow tube plugged during flight (This is necessary for a uniflow tank to work properly)
-Profile mounted engine
-Stock carb on both engines, but I have mine set up with a manually adjustable throttle so I can start the engine at low speed (about 1/3 throttle opening) for a brief warmup, then set to wide open full speed before launch. (Also, I find the engines easier to start at 1/3 throttle compared to full speed if a venturi was to be used. Just my personal preference.)
- Centerline height of the tank is at the centerline of the carb/intake. (I am getting equal lap times both directions)

The FS engines on both of my models run rock solid for the entire flight. I set the ground rpm where I want, and I don't notice (audibly) any rpm change during the flight. Power is constant during any maneuver. I have not noticed any fuel draw issues. And I have been hand-flipping forward to start my engines.

Colin

Looking good, Colin. Thanks for sharing your setup. So a uniflow tank can be made to work. Great!

Quote from: Paul Van Dort on February 24, 2026, 02:02:18 AMThank you for the quick feedback. As I mentioned in my first post, I went trough all the four stroke posts and I read the inputs of Bob Reeves and Mike Alimov and Bob Zambelli etc. very interesting stuff. I know the venturi systems they are using. I have been in contact with the Beringers here in Europe. This post is not about venturis nor carburetors. I am trying to understand why a simple tank setup as a uniflow tank is not working for 4 stroke engines and why the clear disadvantage of a non uniflow tank does not seem to affect 4 stroke engines. I can imagine that the muffler pressure is so high that it is the main governing pressure so that the effect of the static pressure of the fluid column has very little effect. I will be measuring the pressure of the muffler as soon as I can. Might be a lot higher than for 2 stroke engines. This being said, I am trying to get my engine working without muffler pressure because I hate the backward starting. I do hand starting. No electric starter. And I know I am stubborn now and then.. :-)
 

    If you are in Europe, and can get direct information from Gilbert Berringer, you have the ear of the man that brought the Saito four strokes to the fore front of stunt. There is YouTube video out there of his flying his twin Saito powered model at the 2004 World Champs at Muncie, plus some others. I used to see him at Oshkosh every year when he started coming to set up there for his aircraft brake, wheel, and hydraulic business, and he and his wife Veronique would fly demonstrations at the KidVenture circles. I consulted him on venturi sizes and props, and I had him fly my Score during one mid-day demo and he was very complimentary about how mine ran, as it was really pretty much what he ran I came to find out. I don't think you can get better help than you can get from him.
    Type at you later,
    Dan McEntee 

Quote from: Paul Van Dort on February 24, 2026, 02:20:33 PMLooking good, Colin. Thanks for sharing your setup. So a uniflow tank can be made to work. Great!

There is nothing magic about a uniflow fuel tank. It has to work if set up properly whether 2 or 4 stroke. Its only real purpose, as the name implies, is to maintain a relatively uniform fuel supply throughout the flight, vs a standard vent tank which will slowly lean out throughout the flight (which some pilots like as the engine offers increasing power for the late maneuvers in the pattern).

If you are not getting a good engine run, there must be something else going on with your particular setup. In my case both of my models are profile-mounted engines and my particular experience has been good. I can't speak for up or down cylinder arrangements on 4-strokes.

Maybe share your particular FS setup so forum members can offer their opinions on possible solutions. Pictures would also help.

BTW, I had a damaged Brodak wide wedge uniflow tank and out of curiosity I opened it up to see how Brodak arranges the tank internal tubing. To my surprise the uniflow tube end in the tank was very close to the fuel pickup at the back of the wedge. Say 3/16" away from the pickup, maybe even a bit closer. I thought being so close that air bubbles would short-circuit to the pickup. I run mostly Brodak uniflow tanks and if there was any air bubble short-circuiting going on I sure have not notice it. Engines (both 2 and 4 stoke) run fine.

Colin

Quote from: Colin McRae on February 24, 2026, 04:19:35 PMBTW, I had a damaged Brodak wide wedge uniflow tank and out of curiosity I opened it up to see how Brodak arranges the tank internal tubing. To my surprise the uniflow tube end in the tank was very close to the pickup. Say 3/16" away, maybe even a bit closer. I thought being so close that air bubbles would short-circuit to the pickup. I run mostly Brodak uniflow tanks and if there was any air bubble short-circuiting going on I sure have not notice it. Engines (both 2 and 4 stoke) run fine.

Colin

Apparently the Brodak tanks are made without fully understanding the uniflow principle. Basically the feedpipe and uniflow pipe are not related. the uniflow is the pressure reference. The feedpipe needs to be submerged in the fuel at all time. Thats all. The first uniflow designed by Bob Palmer had the uniflow tube at the front of the tank. if you want to reduce fuel pressure at the venturi when going vertical, you can move the uniflow more to the back, but the effect is minimal. I have made tanks where I could play with this distance. 

Quote from: Paul Van Dort on February 25, 2026, 01:24:45 AMApparently the Brodak tanks are made without fully understanding the uniflow principle. Basically the feedpipe and uniflow pipe are not related. the uniflow is the pressure reference. The feedpipe needs to be submerged in the fuel at all time. Thats all. The first uniflow designed by Bob Palmer had the uniflow tube at the front of the tank. if you want to reduce fuel pressure at the venturi when going vertical, you can move the uniflow more to the back, but the effect is minimal. I have made tanks where I could play with this distance. 

Not sure I agree. As I mentioned I cut into a damaged Brodak uniflow tank and it is designed as it should be. The fuel pickup and uniflow tubes need to both be submerged in the fuel at the same elevation for any uniflow tank to operate as uniflow.

The Brodak uniflow tanks I use are 'wedge' design. Due to inflight centrifugal forces, the fuel is directed to the rear-outboard edge of the tank. The fuel pickup and uniflow outlet ae both submerged in the fuel at the same relative elevation. The uniflow tube outlet being so close to the pickup at the rear wedge of the tank ensures uniflow operation until the fuel is almost gone. On my models, I am getting uniform engine runs through the entire fuel charge except for maybe 2-3 final laps where the uniflow tube finally gets 'unsubmerged'. On the final 2-3 laps the engine leans out which is proper and normal.

If the uniflow tube is farther away from the pickup (say at the front or halfway down the tank, the tank still operates as uniflow until the uniflow tube gets unsubmerged. It just happens sooner in the flight fuel cycle. Once the uniflow tube is uncovered the tank basically just becomes a standard vent tank.

The Brodak tanks advertised as uniflow are designed and operate properly.

Colin

Quote from: Colin McRae on February 25, 2026, 09:35:14 AMNot sure I agree. As I mentioned I cut into a damaged Brodak uniflow tank and it is designed as it should be. The fuel pickup and uniflow tubes need to both be submerged in the fuel at the same elevation for any uniflow tank to operate as uniflow.

The Brodak uniflow tanks I use are 'wedge' design. Due to inflight centrifugal forces, the fuel is directed to the rear-outboard edge of the tank. The fuel pickup and uniflow outlet ae both submerged in the fuel at the same relative elevation. The uniflow tube outlet being so close to the pickup at the rear wedge of the tank ensures uniflow operation until the fuel is almost gone. On my models, I am getting uniform engine runs through the entire fuel charge except for maybe 2-3 final laps where the uniflow tube finally gets 'unsubmerged'. On the final 2-3 laps the engine leans out which is proper and normal.

If the uniflow tube is farther away from the pickup (say at the front or halfway down the tank, the tank still operates as uniflow until the uniflow tube gets unsubmerged. It just happens sooner in the flight fuel cycle. Once the uniflow tube is uncovered the tank basically just becomes a standard vent tank.

The Brodak tanks advertised as uniflow are designed and operate properly.

Colin

     Uniflow tanks have been around a LONG time, and were in use before we discovered them here in the US. The concept actually goes back to the times of early steam power development . The late Jim Thomerson did a great write up on it explaining the concept and his posts may still be in the site archives. I began to notice that uniflow tanks were in a lot of British designs when going through old Aeromodeller magazines as I collected them. Palmer toured Europe in the 1950 flying stunt and that may be where he picked up the idea. I ended up doing as Paul mentions, putting the uniflow end almost anywhere it's easy and I can get a solid connection at the end. These have worked the best of any that I have used previously. Fuel doesn't do what you think it does as the load goes down, unless you angle the back side out, or even the front side if you are flying speed or a race airplane. Some one posted video from a tiny video camera mounted on the wing of a profile model, and they used a transparent Sullivan tank so you could seethe fuel load as the airplane went through the maneuvers. The fuel is pushed to the outside and really doesn't move around that much. Many guys worry about their clunk tanks and that the clunk needs to move to the top and bottom of the tank but that just isn't necessary. The clunk actually moved very little, because if you are flying smooth enough it's just a small mass in motion and goes along for the ride. Fuel is used up from front to back as the tank finishes out. If I'm using a plastic tank, I just put the uniflow tube at the same spot I would if it was a metal wedge, any where from the front to half way back. Anywhere it's easy , really. You can go to the Outerzone plans site and search out British stunt models and most of them will have the pattern for the tank on the plan  to illustrate things. And there has been countless threads on Stunthanger about uniflow tanks also, so you can read all about it. What you have may be working because the engines are smaller displacement, and from your photos, you do have the tank very close to the engine for minimal fuel line run, and that always helps.
  Type at you later,
  Dan McEntee

Here my new F2B design I want to have ready and trimmed for the Wch of 2028 in France. Building is just starting.
The uniflow tube can be extended or shortened inside the tin tank. I have noticed that uniflow tanks can cause the engine to go slightly richer as the flight progresses. I was never able to explain why, to my frustration. But to cope, I made the uniflow end 1/2 an inch from the external side of the tank. With the new tank design I can adjust this distance to my liking. A tat leaner for the overheads or Cloverleaf does not harm :-) 

Quote from: Paul Van Dort on February 25, 2026, 11:53:05 AMHere my new F2B design I want to have ready and trimmed for the Wch of 2028 in France. Building is just starting.
The uniflow tube can be extended or shortened inside the tin tank. I have noticed that uniflow tanks can cause the engine to go slightly richer as the flight progresses. I was never able to explain why, to my frustration. But to cope, I made the uniflow end 1/2 an inch from the external side of the tank. With the new tank design I can adjust this distance to my liking. A tat leaner for the overheads or Cloverleaf does not harm :-) 

  And to add to the mix, your new model will be a full fuselage, meaning that the tank will be right on the centerline. On a profile model, the tanks sits outboard. Even a little bit makes a difference. on my profile tanks I started to route the pick up line closer to the inboard side of the tank. Centrifugal force affects fuel flow if it has to turn into the center to get to the engine. It's a speed fliers tuning tool to richen or lean out an engine run by moving the tank out or in from the centerline. Look for drawings of profile speed model coffin tanks and how they are constructed. Most are uniflow with the pick up close to the bottom of the front of the tank and the uniflow just above it. The tank tapers out towards the front in a coffin style shape and again, centrifugal force will push the fuel outboard and then the slant of the tank makes it flow toward the pick up. This is all different from stunt tanks but the principles and the physics behind them are the same. Keep us posted on things as you progress, and if you try any experiments in a test model, please post the results.
  Type at you later,
  Dan McEntee

Quote from: Dan McEntee on February 25, 2026, 12:59:56 PMAnd to add to the mix, your new model will be a full fuselage, meaning that the tank will be right on the centerline. On a profile model, the tanks sits outboard. Even a little bit makes a difference. on my profile tanks I started to route the pick up line closer to the inboard side of the tank. Centrifugal force affects fuel flow if it has to turn into the center to get to the engine. It's a speed fliers tuning tool to richen or lean out an engine run by moving the tank out or in from the centerline. Look for drawings of profile speed model coffin tanks and how they are constructed. Most are uniflow with the pick up close to the bottom of the front of the tank and the uniflow just above it. The tank tapers out towards the front in a coffin style shape and again, centrifugal force will push the fuel outboard and then the slant of the tank makes it flow toward the pick up. This is all different from stunt tanks but the principles and the physics behind them are the same. Keep us posted on things as you progress, and if you try any experiments in a test model, please post the results.
  Type at you later,
  Dan McEntee

Hi Dan, indeed the further the tank is off center to the outboard, the lower the fuel pressure will be at the venturi in flight. This is because of the position of the uniflow tube. Not of the pickup. If the uniflow exit would be at the center or relative height of the venturi/ spray bar, the centrifugal Gs would have no effect, until the uniflow is uncovered. From there the pressure will start to drop until the tank is empty. The uniflow position dictates everything. The pick up just needs to be submerged. That's all that matters. We have been experimenting with a clunk tank with fixed uniflow at the inboard side of a profile model. It was the Skyray of Dirk Van Doninck. The engine was adjusted super lean on the ground. When it took off, it immediately went rich due to the increased pressure at the venturi, because the of the horizontal distance between the uniflow and the venturi/ spraybar. It was flying slow, but when the centrifugal force decreased in the maneuvers, the engine speeded up giving sufficient power for the stunts. Back to level flight it was slow again. The effect was in fact too strong to be practical for F2B but it confirms your observations. It is interesting to assess what are the changes in engine behavior between static run and being airborn . Centrifugal force affects the fuel, the load on the engine goes down, the prop becomes more efficient (It is partially stalled near the hub on the ground), there can be pitot effects on the uniflow entry .... In attachment the model I am trimming now with a Saito 72. My latest design is based on this one but with extra insights and lessons learned.
Exciting!

Dragqueen II

Paul: As you and many others know, I have been tinkering with four stroke engines for quite a while.
I've owned every size from .20 to .90.
Built/converted around 30 different planes both for myself and others.
Smallest was the ultra light Sterling Profile Mustang powered by an OS FS 20, the largest was the Mega Flite Streak built by Dan Banjock and Ted Hienritz (sp?) and powered by an ENYA 90 FS.
Having had that experience, it gave me to opportunity to experiment with a large variety of fuel systems (mainly tanks). Clunk, uniflow and all sorts of pressure and venting.
After all that time spent, I have come up with a bottom line preference: A simple non-uniflow metal tank. Single vent to the top front inboard area of the tank.
Muffler or atmospheric pressure? This seemed to depend mostly on the aircraft/engine combination.
The smaller OS engines seemed to run better with atmospheric venting while the larger OS worked better with muffler pressure. SAITO and ENYA engines followed no particular pattern - I just tried what worked best for me.
My Little Cherokee, powered by an inverted SAITO 30, runs best with muffler pressure while the same engine worked better on a profile with atmospheric pressure.

Bottom line? Find what works best for you. I've tried some uniflow but found no advantage. Some clunk tanks worked well but, in my opinion, not as well as a simple metal tank.

That's my story and I'm stickin to it!

Bob Z.



 



 

The critical matter- in fact, the most important thing we ever learned about 4-strokes - is that you must use a clunk tank. It works miraculously better than anything else. Just buy a tank and assemble it just like it says for the RC instructions.

   You can certainly get it to run with other tanks and that is how people around here started out. But the few 4-stroke runners around here had nothing but issues, and in particular, had extremely poor overhead performance, to the point that Jim Aron was running 4.6 seconds a lap just to make it through the overheads. We eventually guessed it was probably going lean.  At the 2003 or so NATs, Brad Walker suggested he get a clunk tank. Jim went over to the on-site hobby shop, got a 4 ounce Sullivan tank, put it together just like the instructions said, and immediately it ran perfectly overhead, to the point that he was running as slow as 5.4 with massive line tension and no problem with overheads.

  When we got back, passed this information around, and every 4-stroke runner reported similar results, from significant improvements to near-miracles like Jim's. The apparent effect was fuel starvation, which led to a bunch of experiments along the same lines with fuel system drag, including oversized lines on conventional tanks, and reduced viscosity fuel, also with solid improvements, on large 2-stroke engines. It's also probably why YS 20/20 seems to work so well - it has lower  viscosity compared to similar fuels using lots of castor.

    Brett

You Stated: "The critical matter- in fact, the most important thing we ever learned about 4-strokes - is that you must use a clunk tank. It works miraculously better than anything else. Just buy a tank and assemble it just like it says for the RC instructions".

In my opinion: TOTAL NONSENSE.
Take, for example, the Barringers. They probably know more about four strokes than anyone on the plane.
They sent me one of their tanks and it's METAL. So basic it's ridiculous.
When I was working in Italy, I had the pleasure of visiting and flying with some of their best. Maggi, Semmoli, Cappi and Campostella (European Champion). All flying four cycle engines and using metal tanks.

Don't try and insult people's intelligence with a statement like "that you must use a clunk tank". Total bull$hit in my opinion. So, clunks work for some people. Metal tanks also work for MANY people.
I've tried clunks, uniflow and numerous configurations of metal tanks.
For me, at least, the ones that worked best were the simplest.
HOWEVER, people have different experiences and preferences. Any individual can and should do what works best for him/her.
PERIOD.

My 2 cents.

Bob Z.

Quote from: Robert Zambelli on March 06, 2026, 11:04:37 AMDon't try and insult people's intelligence with a statement like "that you must use a clunk tank". Total bull$hit in my opinion.

  And other people have other opinions.

    Brett

This is the tank that Mr Berringer gave me (though a friend in Italy)
He had it in a Sukoi, powered by a SAITO 56.
Note the position of the pickup tube.
On the front, the tube toward the outboard is a uniflow, the other a vent.
The internal uniflow tube is around 3/4 inch ahead of the pickup tube inlet.
I was told that he tried both and preferred non-uniflow.
As you can see, the overall length of the tank is 6 3/16 inches.
I have never seen Mr Berringer fly but my Italian friends told me that his engine runs were perfect.

Bob Z.