RE: Why Are Nearly All Kits and ARFs Incorrectly Designed?  
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All Forums >> Glow Engines, Gas Engines, Fuel & Mfg Support Forums >> Glow Engines >> RE: Why Are Nearly All Kits and ARFs Incorrectly Designed?
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RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 1:54:33 PM   
fancman


 

Posts: 1232
Joined: 11/21/2002
From: N. Charleston, SC, USA
Status: offline 		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.

(in reply to DarZeelon)
       Post #: 26

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 2:11:41 PM   
rainedave



Posts: 5028
Joined: 7/21/2005
From: Greensboro, NC, USA
Status: offline
quote:

ORIGINAL: fancman
This practice was common place and common knowledge way back when.


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. )

(in reply to fancman)
       Post #: 27

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 2:16:19 PM   
Don M.



Posts: 1775
Joined: 3/2/2004
From: Halifax, NS, CANADA
Status: offline 		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.



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       Post #: 28

RE: Why Are Nearly All Kits and ARFs Incorrectly Designed? - 11/7/2006 2:46:37 PM   
downunder



Posts: 3171
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline
quote:

ORIGINAL: DarZeelon
the tank is mounted so the fuel tube(s) exits the firewall in the middle of the engine mount, which for most engines means the tank center-line is at the crankshaft level; about 1.5" below the carburettor.

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.

(in reply to DarZeelon)
       Post #: 29

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 3:13:00 PM   
fadi81



Posts: 268
Joined: 9/11/2006
From: Luxembourg, LUXEMBOURG
Status: offline
quote:

ORIGINAL: DarZeelon


quote:

ORIGINAL: fadi81

We all agree that the top fuel level when the tank is full needs to be at the same height as the carburettor, or am I missing something?



Fadi,

You are missing something...
The fuel jet, center of the carburettor barrel must be at the same level as the fuel-tank center-line; not the fuel-tank's top!

You should have the correct average fuel level, so since you usually land with some fuel left in the tank, the center of the carburettor can be a bit higher than the fuel-tank center-line, but not more than 3/8" (10 mm) higher.

Never should it ever be lower than the fuel-tank center-line.



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

_____________________________

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(in reply to DarZeelon)
       Post #: 30

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 3:19:11 PM   
Charlie P.



Posts: 2259
Joined: 2/25/2003
From: High Above Upstate, NY, USA
Status: offline
quote:

ORIGINAL: rainedave


quote:

ORIGINAL: fancman
This practice was common place and common knowledge way back when.


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. )


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.


_____________________________

Charlie P.
"Gravity is weak but it is persistant" AMA 747089/IMAA 30723

Oh Lord, your earth is so hard and my planes are so frail.

(in reply to rainedave)
       Post #: 31

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 4:38:17 PM   
Sport_Pilot



Posts: 7652
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline
quote:

The fuel jet, center of the carburettor barrel must be at the same level as the fuel-tank center-line; not the fuel-tank's top!


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.

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(in reply to DarZeelon)
       Post #: 32

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/7/2006 5:27:46 PM   
DarZeelon



Posts: 6993
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: offline
quote:

ORIGINAL: Sport_Pilot
...The tank center line should be below the carburettor, how much depends on the muffler pressure and carburettor suction.


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.


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(in reply to Sport_Pilot)
       Post #: 33

RE: Why Are Nearly All Kits and ARFs Incorrectly Desi... - 11/7/2006 5:31:49 PM   
Matthew Allen



Posts: 160
Joined: 9/6/2005
From: Bofferdange, LUXEMBOURG
Status: offline 		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

(in reply to DarZeelon)
       Post #: 34

RE: Why Are Nearly All Kits and ARFs Incorrectly Desi... - 11/7/2006 8:25:06 PM   
Sport_Pilot



Posts: 7652
Joined: 1/21/2002
From: Acworth, GA, USA
Status: offline
quote:

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.


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.

< Message edited by Sport_Pilot -- 11/7/2006 9:32:14 PM >

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       Post #: 35

RE: Why Are Nearly All Kits and ARFs Incorrectly Designed? - 11/7/2006 9:43:54 PM   
MJD


 

Posts: 1563
Joined: 5/27/2003
From: Orangeville, ON, CANADA
Status: online
quote:

ORIGINAL: BillS

Maybe it’s possible that there was no design effort and most of the ARF’s have never been test flown.

The manufacturers will not get smart until we stop purchasing improperly designed junk.

Bill



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

(in reply to BillS)
       Post #: 36

RE: Why Are Nearly All Kits and ARFs Incorrectly Desi... - 11/8/2006 4:00:08 AM   
DarZeelon



Posts: 6993
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: offline
quote:

ORIGINAL: 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.



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.


< Message edited by DarZeelon -- 11/8/2006 2:36:44 PM >

_____________________________

Dar Zeelon - ISRAEL - ddzeelon@gmail.com
MVVS - Jett - Nelson - Bolly - Mejzlik

(in reply to Sport_Pilot)
       Post #: 37

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 11/8/2006 9:40:07 AM   
Ed Cregger



Posts: 7646
Joined: 1/31/2002
From: Ringgold, GA, USA
Status: offline
quote:

ORIGINAL: Don M.

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.



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


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