submikester

My Feedback: (6)

No it doesn't. I'm not inexperienced (I'm not perfect either but don't tell my wife I said that).

In fact, my standard fuel tank setup is a simple - two line setup. One for the fuel feed and fill and one for the vent. So my engine cannot be flooded by me fueling it up since it is completely disconnected during that event.

I use OS F plugs, sometimes I use on board glow and that can help but it is usually only a mask to the problem so if the tank is still too high you will still likely end up with either an idle that is too high or too rich and unreliable.

I run inverted engines all the time; I wish I could spend some time with these folks at GP and actually what they are doing because I have only been consistently successful by putting the tank in the right place or going to a pumped/regulated engine. I actually don't own any of these at the moment (I have a regulator I have used in a drawer and have sold the engines I had with these features but not because of them).

So - my message is simple: Put the tank in the right place and things will work better more consistently. Engineer it right from the start - not wrong as most of their products are in this area.

opjose

I'll grant you that on a plane which was REALLY designed for an inverted engine, they COULD have done something about the tank height from the start.

But that said, there's no real issue with the way things are now EXCEPT if you are not used to dealing with inverted engines.

As others have stated, all you have to do is get the engine tuned and make sure that you keep the throttle at idle or cut off prior to starting the plane, to avoid hydro-lock.

On most inverted I use a cut off or fueling valve that prevents fuel flow to the carb until I'm ready to start the plane.

Once you to to start the plane there is no problem what-so-ever.

No unreliable idle, no high idle, no overly rich mixture.

If the MINOR .5 to .75 tank height difference on an inverted engine were to be a huge problem, we could never fly our planes upside down, let alone pull uplines nor downlines.
In an upline the tank height difference of my Giant U-Can-Do is well over 8-9 INCHES, and it does just fine.

BTW: the pressure differential over that distance is not even measurable by all but the most highly accurate instruments...

I run a Giant U-Can-Do with an inverted Magnum 1.80 w/o any problems as well as a smaller one with a Saito.

The Saito comes so slobberingly rich out of the box that it is amazing the thing will even start....

I saw one guy chew up a full tank last week on an inverted 1.20, letting it idle for 3-4 minutes... he had not tuned the engine yet.
After I re-adjusted the engine, the fuel consumption dropped to the point it is now sipping fuel slowly.

submikester

My Feedback: (6)

I believe that those larger engines with larger carbs may be less suseptible to these types of problems than the smaller engines that I normally run. My larges is a 120 4 stroke (I have two).

Still - it isn't pressure that is the demon here it is gravity. If you have two containers, one higher than the other and a tube in between them connected within the level of the fluid of the upper container. Gravity will force the fluid out of the top container until it either reaches a level of equality or the fluid level runs below the tube. In out case the higher container is the tank and the lower container is the carb. Now, in some engines - they need a pump to draw and this gravity can actually help things but up in the air when things are sloshing around and moving inverted the lack of pressure in the tank and drawability from the carb will give poor performance results. Take for example an OS 160 2 stroke. I've got a buddy who has one and it just doesn't draw well; I believe the carb is too big allowing atmospheric pressure to be more dominant than internal vacuum pressure within the system. A pump fixes this.

The quality of the carb can also impact things - I tend to buy used (though I have bought the occasional new when times are good) OS engines. I like their quality and longevity. I also buy Magnums because of their overal value. Good running engines with good life and parts that are easy to come by. Plus - global is pretty local to me and I have a good relationship with some folks there. The OS carbs are leaps and bounds higher quality in my eyes than the Magnum carbs and their engines tend to run better to boot. I have an OS 70 and a Magnum 70 4 stroke. Carbs are different, probably a lot of other differences too but they both perform well enough. The OS however is a power house - has almost as much power as a 91 - in fact I get nearly the same RPM with a 14x6 MAS prop as I do on my 91s.

Still, that OS 70 SII does not like to have a fuel tank that is too high.

There are a lot of different factors that can impact engine reliability and performance from design of the engine, tank placement, needle adjustments (duh!), fuel, plugs, etc...

With so many things that can be wrong - I just don't understand why these plane designers compromise and put tanks in positions that are higher than the level of the carb?

Regardless of your individual results you can't argue with 'best practice' and I think we can agree that 'best practice' is to put the tank on the level of the carb. In the Venus (my ongoing example) the tank center line is a a full inch above the center line of the carb. If you take a look at all of those beginner books that describe these sorts of things they tell you where the tank should be. Yet these guys insist that tank placement is not that important.

I've ranted and raved about this till my keyboard is blue in the keys but I guess the point I am trying to make is that I don't understand why they do it 'wrong' to begin with when it is so easy to provide a better product and do it 'right'.

Your mileage may vary.

opjose

Measure the volume contained in the tubing and then weigh the liquid. The weight of the liquid is so negligible as to not have any real effect on the running engine.

This can be demonstrated ( which I've done ) to naysayers by putting an engine on a test stand and measuring the results as you move the tank 1" up above the carb center or 1" down.

It makes not one iota of difference to the engine, idle or performance, particularly on a tuned engine.

Unfortunately the tank height difference siphoning, is often incorrectly taken by some to equate to poor engine, or problematic engine performance, and that myth is often echoed even by the more experienced.


Not really on both counts, even with cheaper engines there is no difference in the performance over such a small vertical displacement.

I've done this with SuperTigers, Tower, K&B, GMS, OS, Magnum, etc... all unpumped... no difference.


I'd argue that if the plane ( like a U-Can-Do ) is designed with an inverted mount in mind, and a specific recommended engine then yes why not go that extra inch ( heh ) in the design and make sure you get things level.

However many planes are designed for many different engines, and as such the designers need not try to hit every possible combination because this is just not very important.


And I'll insist that too!

Anyone who has problems with tank height placement in terms of getting an engine to run reliably needs to look elsewhere for the problem.

In every case where I've had direct contact with someone INSISTING that they are having tank height issues, I've been able to fix the real problem WITHOUT re-adjusting the tank height....

Case in point was an inverted Saito I dealt with last week... the pilot had changed the thank TWICE and insisted that everything was correct.... but his engine behaved strangely, sometimes cutting out at throttle up, surging, or not idling properly no matter what we tried with tuning.

I redid his tank myself with new tubing, put on wire ties ( both internally and externally ) re-installed the tank, cleared all of the nipples, sprayed hi pressure air through the fittings, then fired up his engine to a perfect and reliable run for the very first time...

I made four converts that day on how unimportant this "issue" truely is....

Siphoning PRIOR to engine start is another matter altogether however, and even that can be cured by proper pre-start technique...

submikester

My Feedback: (6)

The weight of the liquid makes no difference. It isn't the weight that does - it is its presence and the quantity.

I've done my own testing to validate my own experiences too. Inverted OS 70 on a test stand - properly tuned with the engine at the centerline of the carb with the engine inverted. It idles well until the cows come home. If I place a block under the tank it starts to load up and via the pinch test at idle you can demonstrate a longer 'run up' indicating a richer fuel environment while with the tank lowered back to the proper level the engine behaves back to a proper 'needle' setting.

Gravity does not care how much something weights - simply that it has mass is enough. A simple test of dropping a bowling ball and a baseball in a vacuum can demonstrate that. They will drop at the same speed - their actual mass has no bearing on the fact that they fall.

If you follow my container's analogy though and maybe I wasn't clear - once the second container is full of course the flow will be stopped as things have then equalized as much as they can. In our case we're talking about a closed barrel of a carb or an open one. In the case of the open one of course the fuel will continue to flow.

Bottom line here is that we will have to agree to disagree. I don't see you're 'reason' and you don't see mine.

opjose

What it's "dark matter" now?

Weight has everything to do with it as the fuel must be brought up through the tubing against gravity.

Overly long runs can be problematic for that very reason... and this is why we perform nose up/down tests too.



If offsetting the tank 1" higher or lower is making such a difference on your engine, you have other problems.

Look elsewhere.

If I were there I'd find and fix the problem for you.


Which has NOTHING to do with what I said or this problem...

The liquid has mass and weight, the amount of both is negligible and does NOT affect the flow of fuel to the carb over a 1" or so displacement.

If it did our engines would cut out in flight every single time the plane goes nose up, due to the then MUCH larger offset between the tank and the carb.


Disagree if want. If I had your engine in front of me, I could prove to you, that you are quite wrong. I'd find and fix the real problem for you and you would end up with a much better performing engine as it sounds like you have something amiss.

Inverting the engine offsets the fuel level a typical .5 to .75" no more and that simply has NO effect on the engine, it's idle or it's performance in any way what-so-ever.

Move the tank A LOT further and you start to see the effect of the weight of the fuel in the tubing, on the engine's performance... e.g. a nose up/down test, but even then, once tuned PROPERLY the effect is negligible.... though the displacement is well over 5+ inches or more....

A nose up ( veritical ) flight condition is also a "worst case", where the engine is also experiencing the greatest loads too.

All of my engines run just fine at idle nose up with that big offset to the fuel level. Don't yours?

I own well over four dozen different engines... I have a pretty good sampling to go by.

None of my O.S.'s react when inverted as you've indicated. They don't seem to care.

submikester

My Feedback: (6)

As I stated earlier: We will have to agree to disagree.

I've read it from Clarence Lee in many magazine articles that he has authored specifically on inverted model engine tuning. The tank height should be around 1/4" above the centerline of the carb.

I believe him, I trust him (have for 30 years now) and I will continue to follow his suggestion.

Oh, and this too:

http://en.wikipedia.org/wiki/Pascal%27s_Law

opjose

Good ol Clarence has been wrong some times. He's not infallible.

He also made the "fuel stays in the back of the tank on downlines" statement which like the "inverted engine / tank height" problem are both myths that have resulted from erroneous extrapolation of potentially real conditions ( aka: junk science ). I e-mailed him on that linking him to the video disproving the "fuel in back of tank" myth.


I would have no problem fixing your engine and making you a believer.

BTW: Quoting non-relavent information doesn't really establish anything.

Here is one for you: http://en.wikipedia.org/wiki/Air_pressure since you quoted Pascal's law, note the section on pressure increase/decrease due to altitude applied to the fluid.


That .5" height difference that you are SOOO concerned about is simply unimportant to a running engine.

If it were so dramatic to the engine, our engines would cut out or misbehave even with a slight nose up/down state, which lowers/raises the engines FAR in excess of a tank height difference caused by running an engine inverted. What is not to understand?

Tune the engine properly, eliminate plumbing problems and there are no issues at all... almost all of my engines run inverted without a hiccup. I've never bothered moving the tank either.

I guess I just figured out how to do this while some have not.

submikester

My Feedback: (6)

joshfreund

My Feedback: (1)

saito .82 inverted on flip 3d tank is 1/2" above spraybar. A little tougher to tune but runs perfectly. idle through a tank of gas at 1900rpm , full throttle(static) 9600 rpm. tank in stock location but im about to move it over the C.G. dubt it will even need a pump. Saitos' suck!

opjose

Saito's are WONDERFUL!

The pilot's who own them need to learn a bit more about the engine though.

Saitos will run almost completely out of tune.

Because of that people will attempt to fly them usually far far too rich, and then complain about poor transition, idling problems, etc.

Unlike other engines they WILL run tuned so horribly... but once you have them set correctly, they sip fuel lightly, produce a lot of power for their weight and are extremely reliable.