fancman

My Feedback: (16)

It's been a long time since I actually thought about it but I always set my needle valves on an engine when the fuel tank is half full which puts the fuel level closer to the center of the carb in almost all cases. The post where the gentleman eptied out half his tank to get his engine running right is right on point. Once I needle my engine with a half full tank I don't worry about it going a bit rich with the tank full. Setting the needle at half a tank gives me the best run overall. It doesn't get to rich when full and doesn't get to lean when the tank is near empty. This practice was common place and common knowledge way back when. I do it all the time but never really thnk about it. Give it a try.

rainedave

My Feedback: (1)

This is what bothers me. There's fifty years of r/c experience at the average flying field, but newcomers are still re-learning all the same old tricks and techniques the hard way that others have been doing for decades. I hope newcomers realize what a valuable asset they have under their noses in the form of the older, more experienced modelers at their clubs. (That's assuming the Geezers want to pass on their knowledge.[X(])

Don M.

My Feedback: (5)

I have a Venus II with an INVERTED OS 120 NON PUMPED 4 stroke. I did lower the tank as much as possible but the carb is still level with the bottom of the tank ( tank completely above ) I have no problems at all. I ran an inverted Saito .56 for years with 3/4 of the tank above the carb, again no problems. Just lucky I guess.

downunder

I suspect the reason the hole in the firewall is put where it is is because every engine mount I've seen has got a dirty great hole in the middle of it so you might as well make one hole match the other. If the firewall hole for the tank was raised to match the general height of a carb barrel above the mounts then it'd be right in line with the top part of the engine mount. That'd be cause for complaint so it's a no win situation for the designer. I've never had an ARF or even looked closely at one but I'll agree that if some effort hadn't been made by the designer to give a proper tank position for either an upright or inverted engine (according to the original design) then he should be fired.

fadi

Dar, this seems the common advice I read on this forum, and I think it makes sense to be half way from full or empty...
But I rechecked again the manual of my OS 46AX and the diagram on page 12 doesn't suggest that at all.

Have a look if you have access to that particular manual; they say explicity, the top of the tank should be 5mm to 10mm above the level of the needle-valve. This is really the distance from top of tank at wich fuel level reaches max before overflowing from the vent to the muffler. From which I concluded that the top fuel level when the tank is full needs to be at the same height as the carburettor

Anyway, we will not argue over this, my engines are running fine and that's what matters in the end

Charlie P.

That's a point I've made about the computer simulators. They teach you nothing about engine tuning or performance. No wonder electrics are growing so. Most new pilots want the same "plug and play" as they learned with the simulators.

And how often do we really fly perfectly horizontal so that the tank fitting to carb relative level remains constant? It is a good guideline, but there is some lattitude for installation. IMHO a nice short and straight fuel tube run, even if angled up or down, lessens the "sink-trap" effect of a hump that will lean you out in a high G maneuver.

Sport_Pilot

Unless you are using a uniflow tank that is flat wrong! The tank center line should be below the carb, how much depends on the muffler pressure and carb suction.

DarZeelon

Not true, Hugh.

The choice of the fuel-tank center-line as the carburettor venturi level, is used because it is supposed you would fly until your fuel tank is nearly empty and also to make the engine behave identically, whether the model is flying upright, or inverted.


I had a model that if adjusted perfectly for upright flight, would become so rich when inverted that the engine would begin to four-cycle and would even die rich, if a high negative-G maneuver was added to boot.

You guessed it!
The fuel-tank center-line was about 1.5" lower than the carburettor spray-bar and with the tank nearly full, the engine would simply be drowned when performing/attempting an outside-loop...

The muffler pressure has absolutely no bearing on fuel tank level, since at full throttle it adds a given, small number of psis to the fuel pressure, whether the tank is full, half-full, or empty.
The exhaust pressure addition does not change between positive and negative-G flight, since the pressure addition remains the same whether the pressure tube is bubbling in the fuel, or spewing in the free gas over the fuel.

The mixture will become leaner as fuel level declines and as the nose is raised, but the tank level is not supposed to compensate for this.

Matthew Allen

Dar,

Your point is a good one - this is a problem with an amazing proportion of models, most of which are ARFs. The two I own, a trainer and a H9 Twist both have it, and although they fly well, the mixture changes in aerobatics and the effect on priming are clearly observable.

I don't suppose anyone's listening though... Perhaps someone should talk to a well known designer like Mike McConville, who is behind some of the most popular models on the market right now.

Matthew

Sport_Pilot

That would only be true with the uniflow tank when the vent line is always at the same level as the clunk. With a normal configuration the vent line is unrestricted when up right so the tank pressure is equal to the muffler pressure. When inverted the tank pressure is less than the muffler pressure by the total head of fuel the muffler gas must bubble through. For this reason the tank centerline should be below the carb spraybar. The amount depends on the thickness of the tank, the muffler pressure, and carb suction.

From my experiance for most engines with plenty of muffler pressure moving the tank from the top of the tank equal to the carb, the bottom of the tank equal to the carb, is about equal to a click or two on the needle valve. Try moving the tank up and down on a test bench. With no muffler pressure there is a much greater differance. Normally the ideal placement is from 1/4 to 1/2 inch below. With a small bench you can even test this.

MJD

My Feedback: (1)

I've seen first hand the act of R&D personnel testing prototypes of flying models, those models going into production and out to the consumer, with no beta testing, limited production model testing etc. Problem is, if the designer builds a prototype, chances are "fair to middlin'" that a pretty meticulous job of building is going on, and in addition many of these subtle little details are automatically taken care of due to the builder's experience. Another factor is pressure to go to market, depending on who the company is and who runs the show.

Yet another issue is feedback - without feedback problems often never get fixed. I beg our customers and dealers to tell us everything so we can make an effort to correct (no, we're not in the ARF business ).

Glowing magazine reviews of mediocre products sometimes don't help either, I was on about that one in another thread recently.

In order to stop purchasing problematic products, the chain has to respond; from the consumer to the retailer AND manufacturer, from the retailer back to the distributor to the manufacturer. Again, there can be some catches to that idea in this non-ideal world.

MJD

DarZeelon

Hugh,


The head of the fuel is determined by its level and if you have 3" of fuel level above the vent, with the specific density of model fuel being 0.7918, you will have 0.009 Bars (0.088 psi) of pressure differential.

This number is very small, even when compared with normal muffler pressure. It is like flying at ~34 ft.

NM2K

My Feedback: (14)

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I suspect that your estimation of where the tank was in relation to the engine is off by a gross amount. It is easy to do this when you cannot see the tank.

It is impossible for an unpumped/regulated engine to run properly with the fuel tank elevated above the carb's spray bar. Physics do not vary from model to model.

Now, a kink in the fuel line that one is unaware of can act as a restrictor. It is even possible for this set up to permit the engine to run somewhat acceptably, but it is the exception, not the rule.

Humans cannot breathe underwater without help. Glow engines on suction do not run well with the fuel tank elevated well above the carb. There must be other factors influencing the fuel flow if the engine does manage to run in a situation such as the one that you and another poster have described. As much as I would like to believe, I do not believe in magic. With a fuel tank set up at the proper height in my pattern ships, I had to use forceps to block off the flow of fuel to my engine. Otherwise the upper half of the tank would attempt to drain itself in my engine.

When flying pattern, you do not have enough time to fuel your tank just before your flight on the flight line. It must be filled before, or you run the risk of timing out and forfeiting your flight. This is why forceps were so popular before competition engines began using pumps/regulators on a regular basis.

I am not questioning your honesty or intelligence, just your observation.


Ed Cregger

Sport_Pilot

This is proven wrong every time a model makes a dive. When correctly adjusted the model will run slightly rich in either a power dive or while idling. Yet it will be reliable and the differance between flying straight up or straight down is barely noticeable. So obviously you can get the engine to run well with the tank elevated, the problem is when you fly inverted and the engine leans out.

wiz310

Hey ya Iguanna Bwana. good to see you hang here too. just something about the way you say something gives you away.

downunder

I don't follow, now you're saying that even a 3" difference in head won't make much difference.
BTW, the SG of methanol is .8, nitro is 1.13 and oil is near enough to 1.

buzzard bait

In a dive, as long as the plane is accelerating, the fuel is not experiencing the full force of gravity.

I don't know if our airplanes spend enough time in terminal velocity dives to fully test the effect on an engine run. It's more likely to test the structural integrity of the airframe.

Of course, after the usual needle valve adjustment test with the nose up, you could do the opposite and hold the plane straight down for awhile. I've never seen anyone do that.

Jim

DarZeelon

Brian,

I corrected it for methanol.



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Jim,


For the fuel to be smack against the rear of the tank, the plane would have to accelerate down longitudinally in the dive, at more than 1 G.
It will take a second, or two, for this not to be true any longer, as the plane reaches a terminal velocity, from which speed is nearly constant. This is determined by the prop's pitch-speed and its RPM.

At terminal velocity, the fuel behaves as if the plane is being held with its nose down...

Fuel will be at the front of the tank, for most of the dive.

Sport_Pilot

Dar,
I believe you are an engineer, right? I just went through some static calculations which may prove we are both wrong. Basically it proves that with our standard RC set up you can never have the engine run at the same mixture upside down as right side up, except when the Top of the fuel level is at the same level as the carb. So basically tank placement would be if you want this to be right shortly after take off (top of tank just over the spraybar), when half full (tank centered on carb), or running not for from empty, (tank somewhat lower than the spraybar). If you don't agree or don't understand what I am trying to say I will try to make some sketch's and run through some calculations.

DarZeelon

Hugh,


From the aspect of my university degree, I am a bachelor of economics, but my trade is engineer of software testing.

I thought about the previous post, regarding the difference with the different placement of the vent, in level and in inverted flight.

There is absolutely no difference in fuel pressure!

Although the relative pressurization through the vent is at a slightly lower pressure differential, because of the pressure of the fuel above the vent, to this lower pressure you must add back the pressure of the fuel level above the vent, since it still exerts the same pressure there and at clunk level....

If the tank is positioned with its center-line at the pray-bar level and you lean the engine to its absolute peak, in upright attitude; i.e. one click leaner and it will begin to sag and then you invert the plane; the mixture strength will remain the same and the engine will not sag as a result of the inversion.

As fuel level is reduced, it will eventually show equal signs of a lean situation, whether inverted, or upright.

Sport_Pilot

That is not what I meant. What I was trying to say was that if the fuel level is above the carb, then the engine will run (getting confused here) ~~~~ leaner when upside down than upright, once the fuel level drops below the centerline of the spraybar it will run richer inverted. This happens regardless of tank position. However, if the tank is mounted so the top of the tank is even with the spraybar it will run at the same mixture inverted right after startup but after that it will leaner upright but leaner inverted and get worse as it goes. If the tank is mounted with the bottom even with the carb, (and you manage to prevent fuel from siphoning into the carb) then it will run lean out inverted, but this will get better as the fuel is burned off.

Not sure about this, more later.

DarZeelon

Hugh,


I know that this was not exactly what you meant, but I indirectly addressed it.

The only way the fuel will be the same distance higher, or lower than the spray-bar, going from upright to inverted flight, or vice-versa, is when the tank-center-line is level with the spray-bar.

If the model has an asymmetrical wing profile, forcing it to be more nose-up in inverted flight to maintain altitude, the tank center-line should be a bit lower than the spray-bar, to compensate for this difference.

So, in a flat-bottom wing trainer, the tank center-line should be 1 cm (3/8") lower than the carburettor spray-bar.

Sport_Pilot

Consider the following sketch.

When upright the pressure to the carb is the muffler pressure plus X minus Y. For example if the muffler pressure is 10 Inch's and X is 4!QUOT! and Y is 3!QUOT! then the pressure is 10" +4-3" or 11" of fuel.

But when inverted the pressure is 10" - 4" + 3" or 9" of fuel. When the fuel level is equal to the carb or X = Y then the pressure will be the same either way. This will be true regardles of tank height or elevation.

downunder

First thing, who's Hugh? Oh, of course, Hugh's on first.....

Second thing...you're absolutely wrong, there absolutely is a difference in fuel pressure between inverted and upright with a normally vented tank. Forget about muffler pressure for a moment and consider a vent that's open to ambient pressure. When upright the vent is at the top of the tank above the fuel level so the air pressure inside the tank is ambient (there's no restriction to the flow of air into the tank). Now go inverted. The vent outlet is now at the bottom of the tank (relative to the ground) and is completely submerged in the entire head of fuel in the tank. For air to get into the tank now, it first has to overcome the head of fuel inside the vent line (the vent line wants to fill with fuel to the level in the tank). It does this because the engine continues to draw fuel from the tank for a short while before any air can get in to replace it. This gradually reduces the air pressure inside the tank above the fuel. This reducing pressure eventually is enough to overcome the head of fuel inside the vent line and air can now bubble through to replace any further fuel drawn into the engine. But this reduced pressure acts against the fuel draw from the carb venturi (there's less difference in pressures) so the engine runs slightly leaner.

In other words, when a normally vented tank is inverted it then acts the same as a Uniflow tank and leans the mixture slightly. With a full Uniflow tank the engine is tuned for this slightly reduced pressure inside the tank and the needle would need to be maybe a click or two further out. But it'll hold that tune no matter which way up the model is flying. Now there's absolutely no difference in fuel pressure. And that means all the way from full tank to empty.

NM2K

My Feedback: (14)

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We are saying the same thing from a different perspective.

Ed Cregger