ORIGINAL: opjose
ORIGINAL: submikester
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.
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.
ORIGINAL: submikester
The quality of the carb can also impact things..
Still, that OS 70 SII does not like to have a fuel tank that is too high.
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.
ORIGINAL: submikester
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.
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.
ORIGINAL: submikester
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.
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...