I have a question for someone who is knowledgeable of the above titled issues. I have an OS FT-120 twin-cylinder engine. This particular engine has the carb positioned horizontally, facing forward and on my plane is open directly to the airstream through the propeller. Here is a Tower Hobbies picture of the FT-160 engine which is identical with just a bit more displacement, http://www3.towerhobbies.com/cgi-bin...?&I=LXBY68&P=7 . This carb is strictly the venturi type. It has no pressure line from the exhaust to the tank and no pump. It draws fuel simply by some gravity(when the tank is full) and mostly suction(carb venturi). The tank height is centered on the carb intake. Now the question??? When in flight, since the carb is facing forward directly into the airstream, will the mixture go more lean or rich as the airspeed is increased? Or will there be no effect?

The major impact on the rpm in flight will come from the prop unloading. That has way more effect than anything the airstream does.

Ram air effect is rather insignificant. It's been tried by many, many modelers. The prime mover of the air that goes through an engine is atmospheric pressure. Unless of course, you hang a supercharger on the sucker.

The mixture will go leaner unless you can pressurize the tank somehow. I'm sure the engine is designed to run great as is, just wouldn't run it too close to the ragged edge.

The density of the fluid has great effect in Bernoulli's equation. Another factor not in the equation is friction loses. Because the fuel has greater density and viscosity, it needs more pressure to get the same ratio of flow between air and fuel, because of this the mixture leans out as the air velocity increases. Ram air will improve induction, however as mentioned the dynamic pressure is not huge, but any improvement will also cause an improvement in flow so the pressure is negated by the reduction of static pressure. Automobiles used to get around this by using metering rods and extra jets activated by the increase in manifold pressure.

Manifold pressure [vacuum] decreases with demand. The spring loaded metering rods are held in the lean position by high manifold pressure when demand is low. The main jets are enabled when the manifold pressure is low, then this circuit is allowed to flow as much fuel as the venturi can siphon from it.

The OS 120 has a simple barrel carb

I ran one once in a 8 lb pattern plane I designed specifically for the engine - worked very well
The only thing really required is to keep the fuel tank centre on the centreline of the carb - which is a trick because the carb sets quite low.
With a nice oily high nitro fuel - the engine is dependable and very smooth tho not very powerful
the ram air -is not worth mentioning on this setup.
Brer Noolie laws -well the effective low pressure in the carb/manifold tract is pretty meager on these old four cycle setups
.

I said manifold pressure, that is the opposite of vacuum, not the same thing. As manifold vacuum goes up manifold pressure goes down. A manifold vacuum of say 30" of vacuum ( a perfect vacuum as that is as high as it will go) is a lower pressure than 10" of vacuum. The atmospheric pressure is pushing the metering rod in because the pressure in the manifold is low. A spring is balancing these forces so that there is equal force on bothe sides of the meter diaphram. As the manifold pressure goes up the increased pressure and spring to gether push's the metering rod up causing the fuel mixture to be enriched.

I usually convert all my pressures to absolute pressure before starting calculations, that way there is less confusion.

I think the answer is yes and no. Yes because ram effect is present. No because everything is small and not designed to take atvantage of it. If you built a scoop that crammed air in you would have problems. Most 2 strokes have the carb facing forward as you know; we as a hobby would be having all sorts of fits if it was a problem. I would hate to have to have a plenum that would have to equalize any supercharging effect or multi jets to control the mixture at different speeds.

Yes it is the same, but it is a negative pressure.
Whatever you posted yesterday, you corrected today, but not entirely.
Intake manifolds never do see positive pressure, so unless you are talking about a supercharged engine, you leave that term out of the conversation.

I've never seen any engine acheive 30 inches...I'll bet there is an exponential relationship between 25 inches [normally produced] and 30.

I don't think the carb on the four-stroke OS FT-120 twin-cylinder engine is this advanced. It is just the regular two/four stroke style carb with a rotating barrel. My only reason for posting this question was my curiousity whether or not the high speed needle when set on the ground would be affected by the air pressure going toward the forward facing carb opening. Most other four-strokes have the carb at the rear of the engine facing downward. Most two strokes have the carb at the front of the engine facing upward or with a small amount of forward tilt. This carb is positioned horizontally with the venturi/opening facing directly into the airstream.

No it is not. Ask a full scale pilot about their manifold pressure gauge, it reads absolute pressure starting at 0, though some start at a higher pressure because it is impossible to have 0 pressure. The gage sits around 30" sitting on the ground with the engine off. Ask a mechanic about his vacuum gauge, the needle sits at 0 when not used, and can go above 0 and down to no more than 30" of vacuum.

Sorry but there is no such thing as a negative pressure except relative to atmospheric. To a properly trained engineer there is no negative pressure so the manifold is always under positive pressure. Your thinking is pre space age.

I simple gave the perfect vacuum pressure. Did you not read between parenthsis? I did not claim it is a typical engine pressure. 30 is impossibe because no engine can create a perfect vacuum, so I guess itis exponential, never thought about it. I have worked on projects as low as 5 microns of absolute presure, and that is far from the best that can be done. But that had nothing to do with engines.

And others said that there will be little or no effect. Ram air effect is low, it is measured in inch's of H2O because it is so low. I doubt it is more than a half inch of Hg. They do this in drag racing because they are after any advantage they can, it may be worth only a few thousanths of a second and not noticebale at all, but a few thousandths of a second could be the differance of winning or losing, and its cheap so they do it.

There is more written about negative pressure than what 1000 "properly trained engineers" could read in a lifetime.
It doesn't even have to be relative to atmospheric to be considered negative, relative to surrounding pressure is good enough to qualify as negative.
Even I know that and I'm not properly trained.

Look you are now simply repeating what I have said. In my original post I said manifold pressure not vacuum or negative pressure. That term automatically means I am referring to absolute pressure not vacuum or negative pressure. So based on that reference my statement is exactly correct. I was simply stating I prefer not to think in terms of vacuum. I do all my calculations referencing absolute pressure then I calculate the resulting difference from atmospheric pressure after the fact. I am not saying there is no such thing as a term negative pressure, only that it is a different reference from what I was using.

Not repeating what you've said, merely correcting what you've said.

I won't clutter this thread up any more with acres of quotesam, but you did also say that "the manifold is always under positive pressure".
Nope, almost never...the carburetor would cease to function.
The only positive manifold pressure spikes are seen during the valve overlap period and the net result of that is a bouncy vacuum reading, rough idle, but the manifold is still under a negative pressure with respect TO THE ONLY REFERENCE POINT THAT MATTERS, which is atmospheric pressure.

It is. From 5" Hg to just below 30" when the throttle is popped open. It is only negative relative to atmospheric pressure which is 29.92 at sea level on an average day. Engineers are now taught to think in terms of absolute pressure. There is no negative pressure in terms of absolute pressure. Your way of expressing this is not wrong but it is outdated. Using atmospheric pressure as a reference point can lead to errors in calculation and thought process of how this actually works...

BTW this also how it is taught to student pilots learning how to set a constant speed prop. The manifold pressure as shown on the manifold pressure gauge increases when the throttle is opened. You then set the prop pitch based on manifold pressure.

how many of you guys actually ran and setup the OS 4 strokers?

Now I like that question. It might even lead to experiences referrenced in the original question when this thread started.

I ran 56 gallons of fuel through the FT 120 gemini II before the right jug blew off of the crankcase due to metal fatigue.
The carb on mine was the old design airbleed for the idle mixture. The idle mixture was set just slightly rich which worked fine at 458 ft. ASL. Never had a flameout or a dropped cylinder in the 100 plus hrs of run time.

I also ran 2 of the 160 twins which make enough power for a pretty good sport motor. One had the old airbleed carb while the other had the newer twin needle unit. I ran 10 and 15% Omega fuel in them summer and winter and could tell little difference in the carb performance between the two.

The 120 Gemini II and the FT160 are good reliable smooth not too powerfull engines that require very little maintenence or needle valve adjustment once they are set up. Adjust and lube the valve train a couple of times per year and change the plugs out about every 10 gallons of fuel whether they need it or not and turn the needle valve in or out about 5 or 6 clicks summer to winter and you are good to go at these altitudes.


I still have a new in box 120 Gemini II that has been here for years since I slid over into gas back in the 90s.

I bought the pumped 1.20 when it first came out. It ran perfectly when the pump was working. To wake up the pump I had to push and pull fuel through the system with a syringe. I never had it apart, all it ever needed was to get something inside the pump loose at the start of the flying season, then just run it at least weekly after that. That engine will pull 9.5 pounds straight up to Jupiter with a 15x6 prop.
I understand that the YS is King of the Hill now, in this size range. If so, they must be truly awesome...
I never had to "set up" a quality model engine that was brand new.

Thanks for the response. I now have a just at two gallons through my NIB FT-120 Gemini II. It starts and runs great. My observation while flying it was that it seemed to lose a bit of power/rpm in flight. On the ground I tune the high speed needle to peak and then back it off about 400-500 rpm. This may be a bit much to back off but for now I'm trying to ensure it doesn't run lean. Anyway, if flight, the engine just sounds as if it were running a bit rich. It doesn't sound as if it develops the rpm that it does on the ground. The carb on this installation sticks out below the cowl and faces directly into the airstream. I know that absent wind tunnel testing, one could aerodynamically argue that the carb might go rich, lean or neither. I would imagine that since there are not a significant number of these engines in use and probably only a small number of those with the carb exposed that the experiences of others may be hard to come by. I probably should have posted this question in the glow forum, but I kinda thought a modeler aerodynamicst among us might have the answer. If, at this point, I had to pick an answer, I would say the effect, if any, was slight.

We are discussing the OS FT-120 twin cylinder four-stroke engine. It doesn't have a pump and is strictly venturi fuel draw without muffler pressure to the tank.

As long as the question is about the venturi and airflow around it then this forum isn't a bad place to discuss it. But let's mostly confine the discussion to this aspect.

Something to consider is that short mouths on carbs of any sort do odd things when air flows past them. And when you add a cowl near to the intake the issues compound greatly. Remember the old 60's muscle cars? Many of them had hood scoops. Then some bright wig got the idea of raising the opening on a standoff so that the "turbulent boundry layer" along the hood was not the source for air but the supposedly smooth flow about an inch and a half above the hood. Then another bright wig actually did some tunnel testing and found that even BETTER boost could be hand by having the hood scoop turned around so it could draw air from the high pressure zone at the base of the windshield. Something that was completely counter intuitive to many of the folks out there that lacked the understanding to know why the darn hood scoop was mounted on backwards!

Then there's the opening turbulence issues. The aerodynamics of the carb opening on most carbs is not all that clean. Adding a velocity stack venturi may help this and with some tuning you may find that it may make the engine run more smoothly. Even in my motorcycles if I open up the airbox over the 4 carbs there's short venturis inside the airbox. The function of these being to smoothen the air "stack" being drawn into the injector bodies or carbs. Otherwise the air tends to be drawn in from the sides as well as the front. And when it passes over the fairly abrupt lip of the typical model engine carb there's going to be some turbulence that tends to "jam up" the opening. The idea of a properly shaped venturi is that the flared lip acts a bit like an airfoil's leading edge to smoothly bend the air over the nose of the airfoil without creating any separation. The function of the venturi is the same but it smoothens the sideways inflow of the air by smoothly bending it down so it lines up with the direct center column.

Play with it if you want. Adding a nicely flared out venturi to the mouth of the carb may allow it to run more evenly at full throttle. Or depending on lots of issues you may not notice a difference at all.

I will vote for
no detectable difference.
I was running three carbs on flathead Fords -long ago -found out that a simple bonnet on each Stromberg 48 carb was as good as any setup.
when I switched to modified Chev V8's -Iused same setups and blew of the popular"dual four barrel setups .
The reason was that I had NO flat spot on my setup
It pulled hard from 3000 to 8000 (283 cu in) my Ford was 274 cu so I thoght the flow was enough and it was.
Actual experience on carbs beats theory all to hell .
Why?
too many variables
even the Carter /Holley/Rochester guys had more mods to dump tubes andpower valves etc., than Carter has pills.
model airplane engines are super simple setups after wadingthru this other stuff.
Especially as they are essentially ONE speed devices- no real load except full power.
Totally different as compared to engines drivingbikes karts boats etc..
Anyway the OS just needed a constant fuel head -and it ran like a champ - an anemic champ but a champ.