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 Desi... - 11/25/2006 4:41:30 AM   
DarZeelon



Posts: 7104
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: online
quote:

ORIGINAL: Sport_Pilot

This is clearly wrong! For your example the tank is 2" thick so when upright the pressure to the carb is 1" of fuel plus muffler pressure.
When it is inverted it will be the muffler pressure minus the 2" that it has to bubble through + the 1" that the fuel is above the carb. That is a loss of 2". This may not be enough to make an engine with plenty of muffler pressure to sag, but you might detect a small drop in RPM.



Hugh,


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

quote:



To overcome the minor pressure difference that can result from the pressure/breather tube level that you stated, you can mount the tank on its side, to have the pressure/breather tube also at tank center-line level...
You will, however, have to hold the model on its side when filling the tank with fuel, so the pressure/breather tube will be at the very top of the tank.



quote:



With the breather/pressure tube also at the eventual tank center-line level, there is nothing to create a difference between upright and inverted attitudes.



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


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(in reply to Sport_Pilot)
       Post #: 76

RE: Why Are Nearly All Kits and ARFs Incorrectly Desi... - 11/25/2006 5:03:49 AM   
DarZeelon



Posts: 7104
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: online
quote:

ORIGINAL: Flyboy Dave

Now, where would you put the tanks in these so called "Improperly designed" planes ?



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...

quote:



I say they are just fine where they are.



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.


< Message edited by DarZeelon -- 11/25/2006 5:08:43 AM >

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       Post #: 77

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 5:08:35 AM   
Sport_Pilot



Posts: 7728
Joined: 1/21/2002
From: Acworth, GA, USA
Status: online 		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.

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(in reply to DarZeelon)
       Post #: 78

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 5:16:16 AM   
DarZeelon



Posts: 7104
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: online 		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...

< Message edited by DarZeelon -- 11/25/2006 5:18:31 AM >

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

(in reply to Sport_Pilot)
       Post #: 79

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 5:18:31 AM   
Sport_Pilot



Posts: 7728
Joined: 1/21/2002
From: Acworth, GA, USA
Status: online
quote:

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.


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.

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       Post #: 80

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 7:22:22 AM   
Flyboy Dave



Posts: 13462
Joined: 3/20/2002
From: San Bernardino County, CA, USA
Status: online
quote:

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.


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 ?





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A mechanic says..."Oh yeah, watch this".
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(in reply to Sport_Pilot)
       Post #: 81

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 11:09:22 AM   
DarZeelon



Posts: 7104
Joined: 4/9/2003
From: Rosh-Ha'Ayin, ISRAEL
Status: online 		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...

< Message edited by DarZeelon -- 11/25/2006 11:57:18 AM >

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(in reply to Flyboy Dave)
       Post #: 82

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 7:12:45 PM   
Flyboy Dave



Posts: 13462
Joined: 3/20/2002
From: San Bernardino County, CA, USA
Status: online 		....you made my point....

quote:

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.


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.

_____________________________

An engineer says.... "That won''t work".
A mechanic says..."Oh yeah, watch this".
"Old Age, and Treachery will overcome youth and skill".
Revver Bro #4.

(in reply to DarZeelon)
       Post #: 83

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 10:22:20 PM   
Gizmo-RCU


 

Posts: 976
Joined: 9/17/2002
From: Athol, ID, USA
Status: offline 		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.

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Gizmo

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       Post #: 84

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 11:54:51 PM   
buzzingb


 

Posts: 1512
Joined: 9/4/2002
From: Bruce, MS, USA
Status: offline 		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.

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Buzz

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       Post #: 85

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/25/2006 11:55:29 PM   
downunder



Posts: 3198
Joined: 10/10/2002
From: Adelaide, AUSTRALIA
Status: offline
quote:

ORIGINAL: DarZeelon
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.

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.


(in reply to DarZeelon)
       Post #: 86

RE: Why Are Nearly All Kits and ARFs Incorrectly Des... - 11/26/2006 12:30:10 AM   
Sport_Pilot



Posts: 7728
Joined: 1/21/2002
From: Acworth, GA, USA
Status: online 		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.

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       Post #: 87

RE: Why Are Nearly All Kits and ARFs Incorrectly Desig... - 1/1/2007 12:49:09 AM   
AERORICH