DarZeelon

Hugh,


Again, read my posts #66 and #71...

What 'bubbling pressure differences" are you talking about now?

DarZeelon

Dave,


In a properly designed model, the fuel tank would be located so its center-line is at the same level as the carburettor spray-bar is designed to be, according to the building instructions that come with that model.


....In a real, high-wing Cessna (152, 172,...) the fuel tanks are about 3' above the carburettor...

But this Cessna and for that matter, most other 'real' airplanes are not supposed to fly inverted, are they?
Those that do have pumps...

Try saying that when your tank is mounted at crankshaft level, you're flying on the last 1/4-1/3 of the tank and you pull an extended, high positive G maneuver and your engine leans out and dies.

...Or the same model at the beginning of the flight, with a full tank and you pull an extended, high negative G maneuver and your engine is 'dunked' in fuel, blubbering, or even dieing rich.


You can compensate with adjustments for either situation, but not for both simultaneously.

If the design placed the fuel tank center-line at spray-bar level, you would just have to adjust the engine a bit rich, to compensate for fuel level going down, but behavior will be identical; upright and inverted.

Sport_Pilot

Dar,
On most tanks the muffler vent is at the top of the tank at or above the fuel level, when it is upriht there is nothing impeading the muffler pressure and the tank pressure is the same as the muffler pressure. When inverted the vent is now below the fuel level and the muffler pressure must now bubble up through the fuel, the tank pressure is now reduced by the head of the fuel, or in a case of a 2" tall tank it will be reduced by 2" when inverted. If the tank is 4" thick it will be reduced by 4", 6" by 6", etc. The best way to get around this is with a uniflow arangement were the vent is at the same level as the pickup. Otherwise you will not have equal pressure till the fuel level is equal to the carb spraybar. As someone mentioned (Dick?), with sufficient muffler presure you may not notice the differance.

DarZeelon

Hugh,


I read what you repeatedly wrote here, in several of your posts.

Take that 'standard fuel tank' and turn it on its side (leaving all the innards as they were)...


Now, where is that vent/pressure tube?


So as you can see, there is no difference between upright and inverted fuel pressure.

There would, however, be a difference between left 'knife edge' and right 'knife edge' flight...

Sport_Pilot

A bit nitpicky I guess. But this is yet another arguement, not the one we have been discussing. At any rate the high G manuver would not make a huge differance. A negative manuver with a normal R/C setup would make an engine lean out, as well as a high g positive manuver. Unless you have a uniflow setup.

Flyboy Dave

My Feedback: (21)

DarZellon....do your planes exhibit these problems ? If they do....you simply do not have your
planes set up correctly....or, you do not have enough fuel pressure.

I'm not going to argue these simple facts....take them, or leave them.

Take this Ultra Stick as an example of fuel tank mounting. This example will trump any
argument you can make. There is only one place to mount the fuel tank in this plane, right
in the middle of the fuselage. There are only three positions to mount the engine....upright,
sideways, or inverted.

The whole thing comes down to fuel pressure. The undeniable litmus test can be
proven using the example of the YS engines. These engines run the same....regardless of
the position of the fuel tank, and regardless of the position of the engine. The amount of
fuel in the tank makes no difference, nor does the direction or the amount of a positive or
negative "G" maneuver. It all depends on the fuel pressure.

The same applies to the pumped OS two stroke engine.s...like the Hanno .61 types.

No matter how you mount the engine in the Ultra Stick....as long as you have proper
fuel pressure....it will not have the problems you describe in your "Doomsday
Improper fuel tank mounting scenario".

The fact is....no matter how tight you pull a positive "G", and no matter what the fuel
tank level is....it will not lean out enough to make much difference, much less
kill the engine....if you have enough fuel pressure, plain and simple.

The tank position is nearly always in the standard "center" position....and this is not
a problem....unless you have an upright mounted engine, and you do not have enough
fuel pressure.

Did I mention that fuel pressure is important ?

DarZeelon

Dave,


I absolutely agree with you that fuel pressure is important.
I also agree that a pump/regulator pressurized fuel system, whether is it Perry, Iron-Bay, Cline, or YS is superior to any suction and to any exhaust pressure system.

I also agree that a side-mounted engine is preferable to an upright, or inverted installation, whenever a 'normal' fuel system is used.

It is this advantage of the side-mounting, that makes one think; "Why the he!! do all ARF manufacturers design their planes with anything but a side mounted engine?"

With crankshaft center-line 'universal' mounting, the variations are too great no to notice, especially with low nitro fuel.


My H9 UltraStick Lite has a side mounted engine, but it needs the pump nevertheless, because the muffler pressure is insufficient to insure adequate fuel supply, when you pick the nose up, unless the needle is opened to a very rich, nearly four-cycling setting, at level attitude.

Do all engines need a pump? Maybe not, especially smaller ones, with small tanks and smaller fuel level variations.

But all large, aerobatic ones do need it, unless 'dead-sticking' your way out of a 6' high hover is one of your favorite maneuvers...

Flyboy Dave

My Feedback: (21)

....you made my point....

If you have proper fuel pressure it doesn't matter where the tank is mounted, or how the engine
is mounted....it will run perfectly, even in a 6' hover.

To say ARF's and kits aren't designed properly because the tank is mounted in the ideal location
is a silly statement....at best. [sm=75_75.gif]

Gizmo-RCU

My Feedback: (27)

Dave I doubt that you get it. Why must we be forced to buy all the extras to make the engine run properly when it would be so easy to design a plane to avoid those problems. I like reliability and consistancy along with not having to invest in pumps, regulators, etc. to obtain satisfactory performance.
It seems that some enjoy doing things the hard way.

buzzingb

My Feedback: (2)

Pumps, regulator, valves and etc. are expensive and make a bad situation more complicated. I know some of these things work but I vote for properly designed airplaines to start with. If you try to make an engine run on an improperly designed fuse can ruin it. The only way some people can understand this is to experience for themselves. There are a host of problems come with an inverted engine with the tank above the carb like fuel running out of carb, hydrolocked engine, erattic running engine, etc. Same is true with the engine above the fuel tank but some of the problems are different.

downunder

By doing this the tank acts like a uniflow until the fuel level drops to half full and then uncovers the end of the vent line. From then on the tank is normally vented and the engine will feel the reducing fuel level.

But as Jim Thomerson said earlier, uniflow does not compensate for height of an incorrectly positioned tank, it only eliminates the affect of a changing fuel head.

Even so, a uniflow can help with a badly out of position tank when two things are understood. First, any time the vent is covered with fuel then the pressure inside the tank is lower than with an uncovered vent. Second, while the vent is submerged the engine (carb) thinks the fuel level is at the end of that vent.

Consider a tank that's set too high (let's say the engine was inverted and the tank left where it was). In this case you'd bend a fixed vent down near the bottom of the tank as close as possible to the level of the spray bar. In upright flight or when tuning it's a uniflow so the pressure in the tank is lowered. Fly inverted (making the tank now too low) and the vent gets uncovered which raises the pressure inside the tank offsetting the lowered position. For those who read this properly you'd have noticed that instead of bending the vent to the bottom you could simply invert the tank itself.

For a tank that's set too low then leave it as it is with the standard venting because the moment you go inverted (tank now too high relative to the carb) it becomes a uniflow which then drops the pressure inside the tank to lessen any richening that might have been felt.

It's not a cure-all but it can help.

Sport_Pilot

I find that plumbing the tank for a uniflow setup works just fine for a tank position that is too high. However, you do have to use hemostats to keep the fuel from driping into the carb till it is started.

AERORICH73

Well fellows, I just install the tank that will give me the quantity of fuel I want that can be located properly on its' centerline behind the firewall. I suspect this is what every is doing, right? This fuel tank location problem came to me through the scale people having trouble installing fuel tanks due to the space available in their chosen models to install the tanks behind the engine, or engines. Did not realize it to be a problem in sport flying. I have flown only one ARF model so far, and it had flight control problems. The engine operated normally. Because of this thread, I will double check the fuel tank installation in the models now on both workbenches, and the ARF's yet to assemble(note I did not say built ARF's). Yes, I got over involved on e-bay, and now have three of these things.

Rich S.

NM2K

My Feedback: (14)

------------------


Stop thinking of your old Kaos and other Seventies models and look at today's 3D ARFs.

The new models have inverted engines with the fuel tank center line far above the carb spraybar. To add insult to injury, the morons then actually encourage the builder (I use the term loosely) to permanently afix the fuel tank neck into and through a hole in the firewall, thus guaranteeing fuel foaming.

THIS is what Dar Zeelon has said and I agree 1000% with him. Most new models have been designed by embiciles that have never flown a model and they are not familiar with the problems that their ignorant designing efforts create for the consumer of their products.

Happy Freaking New Year! <G>


Ed Cregger

Flyboy Dave

My Feedback: (21)

OK, ED....since you called me out....where would you mount the tank ??

If you say "anywhere other than directly behind the engine"....I'm puttin' the BULL PUCKY
right back on ya'.

The fact is....even in 3D terms....(read this carefully, ED )....

....If an airplane is hovering....it doesn't matter which way the engine is mounted....
upright, sideways, or inverted....it doesn't matter if the tank is above, below, or where the
centerline of the tank is mounted....the only thing that matters is the distance the tank is
from the carb, and the fuel pressure.

This doesn't change between planes....whether it is a Kaos, an U Can-Do 3D, or a C-130.

Another thing....mounting the fuel tank through the firewall hole and sealing the hole with
silicon sealer doesn't have much to do with fuel foaming. I have many of my firewall/tank
holes sealed with silicon. Fuel foaming is caused by vibration.

If your engine is running properly, with the correct fuel, plug, ect....and your accessory parts
(prop, spinner, backplate) are balanced you will not have fuel foaming....period.

Granted, there may be engines that produce excessive vibration that need every consideration
available....including "non contact of fuel tank with airframe"....however, I don't have any of these
engines. The fact of the matter is....if you eliminate the causes of vibration....you will not have
to worry about fuel foaming....even if you bolted the fuel tank to the engine, and that's a fact.

Happy New Years back to ya' .

FBD.