Hi - My Super Chipmunk kit states a 12 oz tank - I happen to have a spare 14oz that will fit the space will fitting a bigger capacity tank be ok? - thanks in advance

Yeah, sure. Two ounces won't make that much difference. You do the CG with an empty tank so it won't matter with weight. The only consideration is space and if you want to fly any longer than a 12 ounce would do for you..

CGr

What size engine? It will be fine.

Folks are always trying to cram more tank capacity than what the designers had originally intended and yes as stated you can do it.

My tendency is to use no larger than intended and frequently smaller tanks and the reason is you will get a more uniform engine run throughout from start to empty than if you use grossly oversized tanks.

John

You can use that tank but, I am warning you... YOU will have to suffer through the slightly longer flight time

You can also fill the tank and then back off (pump out) a few ounces to have the same capacity if fuel weight is an issue. No law says a tank has to be full.

Aw Charlie P you are breaking one of the cardinal rules of aviation....with lesson number one....the 4 useless things in aviation.....
1. runway behind you
2. altitude above you
3. airspeed you don't have
and finally
4. fuel in the fuel truck, or in our case fuel in the fuel jug......LOL.....

As everyone has said an additional 2 oz in tank size will be ok....the problem with bigger tanks is weight of the fuel, but more important is airspace that needs to be filled with exhaust...the engine will only put out so much exhaust.....if the tank is so big that you don't fill the void you can have fuel flow problems....causing dead sticks

good flyin to all

could you clarify more please, this goes against all my education as follows
1)an engine has no clue as to the size of tank delevering the fuel, nor does it matter
2) as long as an engine receives the four basic elements, fuel, air, compression and ignition, it will run
3) as long as an engine is running, it will produce exhaust
4) as long as exhuast is being produced, the tank will recieve pressure

as for weight, if my math is correct, with 2oz of fuel, you would add, 1oz or 15grams of weight

Actually Ray what Jetmech05 has said is quite accurate. The size of the tank has a direct effect on the headpressure coming out of that tank dispite the pressure from the muffler.

Take a one gallon pail and poke a hole in the bottom side and do the same to fifty gallon drum and measure the pressure at these holes, I beleve you will detect quite a difference. That is head pressure and it will change dramatically from a full bucket to an empty one and this is one of many varibles in our fuel system beyond just muffler pressure that must be delt with.

Its a fact that grossly oversized tanks very often can present running problems and its a fact that newbies that tend to cram grossly oversized tanks into their airplane will very often have running problems or at the least never able to utilize the full capacity before the engine stops.

This is most certainly not to say that one cannot do it but it a very poor practice to suggest such to new folks as it often presents problems.


John

Hi!
Well said!!!!

hi richard - i'll be using an os 91 fs

John, I don't think anyone is suggesting "grossly oversized tanks", just that in this case, 14 ounces vs. 12 ounces is not going to cause any appreciable issues.

However, you are absolutely correct to point out, though, that it's not a good practice to suggest grossly oversized tanks. Your point is well taken.

CGr

john; still confused help if you can, I fully understand head pressure, I gallon of water wieghs 8.3 lbs, vs 55 gallons weighing 456lbs, but if you were to put only 1 gallon in the drum, your pressures would be the same at the holes, the question would be, could an engine provide enough muffler pressure to the drum so that the amount of flow from the drum matches the bucket?

Its just not that critical! Put the 14oz tank in there and go fly.

John,

If it were that critical your "pony tank" setups that you have most graciously shown us wouldn't work and you have most likely doubled the fuel capacity in the pictures shown.

In the extreme setup it maybe a problem, but just how much positive pressure do you need? Most of these engines can be setup then tuned to run without positive tank pressure anyway. We ran without pressure or with pressure for years flying U-Control not a big deal.

I Did note in my first post that no the two ounce differance in itself may not be a big issue but it is common for new folks to get carried away on tank size and thats the point I wished to raise.

Ray your figures do make the point yourself that the larger a tank and the wider the spread is from full to empty then the headpressure change is even greater than with a smaller tank. The needle valve must deal with this change from full to empty or there will be running problems.

Granted this is only one of the involved varibles effecting an engine run with the muffler pressure also a varible as well as flight loads and the fuel tank spray bar relationship.

Don,t get me wrong although for most of my sport glow flying I use smaller tanks than most for the more uniform runs from full to empty that can be the result but I do on occassion have some silly setups. Just this morning I completed a two hour and three minute flight with a .70FS powered airplane and most of 56 ounces of fuel.

John

Care to share what you talking about and what a ''pony tank'' setup is Or what pictures I may have posted that caused such anger. Hmm controlline, yes I still fly controlline and I use Tetra bubbless type tanks. Hope that does not anger you futher.

John

just to add more confusion on myself[sm=bananahead.gif] anybody know just how much muffler pressure an engine produces? I know it's a loaded question, a lot of variables, but lets say a os 55ax 2 stroke

And yet, when I was learning full-size, they sold me a nomograph to figure how much fuel to load in order to maintain a safe take-off weight. ;-)

Pony Tank setup

For instance I am putting together a ducted fan. I am putting a small 3 oz tank just a couple of inches fro the engine so the engine can have a reserve I also have two eight oz wing tanks on each side of the fuse, which are the main tanks.

The mains feed the "pony tank" or "feeder tank" so I do not have to push fuel a great distance to the engine from the beginning.

In some planes with retracts you put two tanks in one close to the engine and another behind it. (most cases on the CG) so you can carry more fuel.

A friend of mine has a Sig Kougar with retracts and runs two eight ounce tanks feeding his ASP 61. Because space is a premium inside the fuse.

The vent line of the "feeder" goes to the fuel line of the main tank and the pressure line of the second tank goes to muffler or tuned pipe for pressure. This pushes fuel out of the second tank into the "feeder". Very easy to do and you can increase your fuel capacity without using a larger tank.

Hope that helps.

John,

Not angered all!

If you recall you were kind enough to show pics of you adding an additional tank (pony tank) to your airplanes on certain setups which is what I am referring to essentially doubling the fuel capacity. Looking at your setup and plumbing diagrams supplied by RCKen or Minnflyer the muffler pressure would be the same if you have one or two tanks involved. The pressure is supplied to the second (pony) tank to feed the primary fuel cell which in turn supplies the engine.

My point is this is not a big deal in most cases if the setup is sufficiently close to the muffler to provide adequate pressure. Otherwise a pump maybe required depending on the distance and location of the tank(s).

It makes no difference to the engine how large or small the volume of the fuel cell as the muffler back pressure provides X amount of constant pressure depending upon the rpm. The pressure would certainly drop as the volume decreases if it were a closed system like a compressed charge of air as with shallow well water pressure tank system. In that system as the water volume drops so does the pressure until the air volume control senses the allowable drop to start the pump to recharge the system. We don't have that type of setup. We have fairly constant muffler back pressure to provide positive tank pressure as long as the engine is running, even though the pressure chamber or the tank's volume of air increases as burn off occurs.

If you were using a 91 2 stroke I'd go with the 14 oz. tank. Since you will be using an OS 91 4 stroke I'd use the 12 oz. The 4 stroke will give you more MPG than the 2 stroke will.

At least my OS 91 4 stroke does.

Hi!
That's the kind of knowledge you put in the file of "Unimportance things" . Just like "How many amps does a glow plug draw"?

The answer is that that silencer pressure is not enough...! The weight of the fuel inside the tank cannot be compensated for by just using silencer pressure. That's why we must set our engines a little rich at the beginning of the flight to compensate for the loss of fuel pressure when the tank empties.

Setting the needle valve rich allows for better lubrication of the glow engine, set too lean it will often quit from overheating thereby increasing additional wear to components.

The engine experiences ram air effect in flight into the air intake which has the effect of leaning the engine, a slightly rich setting is needed for that purpose as well.

The engine on its own draws fuel which is certainly assisted by the pressurized tank setup or perhaps a pump if needed provides positive fuel flow pressure in all attitudes of flight regime. Otherwise certain maneuvers ( high G negative or positive or perhaps inverted flight) would most likely case cause insufficient fuel flow through the spray bar to maintain combustion.