Fuel Tank & CG

The reason is that the typical two-stroke model engine does not have enough fuel draw. The tank needs to be very close to the engine with the centerline of the tank as close as possible to the level of the spraybar of the carburetor to minimize mixture changes as the fuel level in the tank changes.
If you have an accessory fuel pump added to your fuel system, you can usually mount the tank on the CG.
Good flying, desmobob

Your engine does not have a fuel pump, unless you paid extra for the privilege of having one. Your automobile would have to have its fuel tank right next to the engine if it did not have a fuel pump.
Ed Cregger

Doesn't the pressure line from the exhaust act as a fuel pump??

Not the way you want. It adds pressure, but it doesn't keep it constant. With varying pressure at the needle valve you'll see the mixture getting richer or leaner -- having the tank right up against the engine minimizes this.

So Piper Cubs, Taylorcrafts, early Cessnas, etc., are not 'real' airplanes?

Twist my words...I merely meant fuel tank in wings is the most common configuration.
Apologies to all the Piper Cub, Taylorcraft, early Cessna, etc fans out there who now despise me for not considering such planes as 'real'.:)
Sincerely
Tony

It helps, but it is not as effective as a real fuel pump.
Ed Cregger

Oh, I'll get over it. Don't worry about it, Tony.
Only recently have people been really concerned about shifting balance points during fuel consumption to make an issue of it. In the majority of model aircraft, the simplicity of suction fuel draw with a tank located just behind the firewall has served us well over the decades.
One has to keep in mind that most models meet their demise via a crash. It is just part of flying R/C. Some of us feel that risking the extra money in a better fuel delivery system is worth the money, or did for a while. I'm back to suction feed for the simplicity. I love aerobatics, but their performance does not have to meet strict competition standards, so if the model flies a little differently toward the end of the tank of fuel, it isn't really that important to me. Many others feel the same way.
Pumps/regulators do go bad periodically. It usually has to do with the pump/regulator's diaphragm/membrane failing for one reason or another. Since it doesn't make much difference to me in how the model flies, I forego using them most of the time.
Ed Cregger

Sounds like the forward mounted tank is the way to go (at least for this plane)...Thanks for everyones input.
Tony

On Tue, 14 Feb 2006 13:35:04 +1300, Tony Beswick wrote in :
Welcome to RC and r.m.rc.air!
Some of the pattern guys use pumps or "regulators" that pressurize the tanks with a check valve, then meter the fuel to the carb as needed (e.g., Cline Regular).
Their goal is to avoid having the CG change during the flight.
I think some of the big aerobatic planes do the same thing. I think most of the gasoline engines have carbs that can pump the fuel reasonable well.
Other folks have talked about why the fuel tank is kept close to a non-pumping, non-regulated engine. You should try, too, to keep the centerline of your tank just below the spraybar of your carb so that you get a consistent run from top to bottom of the tank.
Marty

| Only recently have people been really concerned about shifting balance | points during fuel consumption to make an issue of it.
I don't think that's true. It's been an issue for a long time -- it's just that people have adapted to it, taken it as a given.
| In the majority of model aircraft, the simplicity of suction fuel | draw with a tank located just behind the firewall has served us well | over the decades.
That's probably true, but it doesn't mean that people haven't made an issue of the shifting CoG.
| One has to keep in mind that most models meet their demise via a crash.
Is that really true? More of my planes have just plain worn rather out than actually been crashed and never repaired. And accidents like having things fall of shelves have taken out at least one plane as well. :) (though perhaps that qualifies as a crash.)
Hangar rash normally isn't fatal, but given many years or extreme conditions it certainly can be.
I guess it's probably true, but probably not by a particularly large margin. Lots of planes get stripped down and the frame tossed just because the owner is unhappy with the plane rather than it having been crashed ... but sometimes a crash will help convince him of that.
| so if the model flies a little differently toward the end of the | tank of fuel, it isn't really that important to me.
Perhaps it's more important than you're giving it credit for.
For example, when you balance your model, you do it with the fuel tank empty on a puller plane (most planes, with the fuel tank ahead of the CoG) and with the fuel tank full if the tank is behind the CoG (as if often is on a canard or other pusher-plane.) You do this because you want the CoG to never go further back than what you've set it. Further forward is OK, but further back could lead to an airplane that's almost unflyable.
I agree about pumps not usually being worth the trouble, but people do still take into account the shifting center of gravity, even if they don't go out of their way to stop it completely.
Though to be fair, lately I've mostly flying electrics and gliders. Now, if my CoG shifts during flight, it usually means that I didn't anchor my battery and/or wing very well. :)

Yours is a good post, Doug. And I had pretty much the same thoughts as you when I read the letter but I remember when starting out (as the original letter addressed). I remember the several plateaus we all wanted to reach. One of the big ones was going home after a day's flying and having no repair to do. All we had to do was clean the model up a little better and put it where we could charge it before we flew again. Before that day came, I might not come home with what I left the house with. Now, I have airplanes I would not mind crashing if I knew why they did. With power planes, we will often destroy them when we are bored with them and start trying to make them do things they were not designed or trimmed to do. (Speaking from experience here. ) With sailplanes, I have lost more models from mid-airs and flutter than the ground. I once had a wing sawed off from another's launch line. It got a beautiful Horner wing tip caught in the gear mechanism of a powered station wagon window that had no inside panel. I have had winch batteries slide and damage models. Many models are damaged on the ride to or from the field. And I have broken the same model twice putting it in or taking it out of the same car. There are a lot of ways of damaging or destroying a model, with new ones being invented every day.
They are called "tractors."
I have the shifting battery problem in two models and I keep little hard foam spacers cut to the proper sizes for what sized battery I might be using that day. I wonder...if my battery is a little off the CG, should I check the CG with the battery charged or discharged? Like if ahead of the CG, check the CG with the battery discharged, and if behind the CG, check it with the battery charged?
Ken

| I wonder...if my battery is a little off the CG, should I | check the CG with the battery charged or discharged? | Like if ahead of the CG, check the CG with the battery | discharged, and if behind the CG, check it with the battery charged?
Well, according to Einstein, a charged battery would weigh more than a discharged battery, so if it's ahead of the CoG you should check with the battery discharged, and if it's behind, with the battery charged. If you don't heed my warning, your CoG might very well end up an angstrom or so off ...

Actually, a charged battery and a discharged battery would weigh exactly the same thing. When charging the battery, all you are doing is moving the battery's electrons from one side of the battery to the other side of the battery. You're not adding any electrons.
Ed Cregger

Ah, but you are adding energy, which has mass.
When you put uranium into a nuclear reactor and get energy out the initial reactions don't change the total number of protons, neutrons or electrons -- yet you get energy out, and the total mass reduces. If you take beta decay into account then you _still_ aren't changing the total number of quarks + leptons, yet the mass reduces even further.
Charging a battery is the same thing, but instead of breaking and making bonds within atomic nuclei you're breaking chemical bonds which are much weaker -- so the mass change is minuscule compared to the total mass of the battery. I doubt that it could even be measured.
Lessee -- (11.1V)(1.6 amp-hour)(3600 sec/hour) = 64kJ.
E = mc^2, so m = E/c^2, so m = (64kJ)/(300*10^6 m/s)^2 = 710pg (that's pico-grams, for anyone keeping track).
A 3-cell 1.6 amp-hour Li-Po pack weighs 120 grams . So you see a change of 6 parts per trillion. This is, perhaps, something that you could measure one day if you were feeling puckish and worked at a major metrology lab, but the change in mass from the connector wear would probably be far greater than the change in mass from the charge.
So a charged battery _does_ have a different weight than a discharged one, but only if you're a smart ass.

| Ed Cregger wrote: |
... | > When charging the battery, all you are doing is moving the | > battery's electrons from one side of the battery to the other side | > of the battery.
That's not a very accurate description of the chemical reactions that are involved in the charging of a battery :)
That's more of how a capacitor works, and even then it's a bit on the overly simple side. (In a side note, there's been a few articles about ultracapitors lately, like this one -- If they can ever make this work, we may be able to replace our batteries with capacitors. Alas, this is many many years away.)
| A 3-cell 1.6 amp-hour Li-Po pack weighs 120 grams | . So you see a change of 6 parts per | trillion.
Larger than I would have thought. But I didn't bother to do the math, so ...
| This is, perhaps, something that you could measure one day if | you were feeling puckish and worked at a major metrology lab, but the | change in mass from the connector wear would probably be far greater | than the change in mass from the charge.
I was thinking more of any outgassing that happens during each charge or discharge being a larger factor, but there is that too.
| So a charged battery _does_ have a different weight than a discharged | one, but only if you're a smart ass.
Oh, the mass change has nothing to do with me being or not being a smart ass :)
In any event, I wasn't really trying to be a smart ass -- I was just going with what Ken started --
| >>| >> | >>| I wonder...if my battery is a little off the CG, should I | >>| check the CG with the battery charged or discharged? | >>| Like if ahead of the CG, check the CG with the battery | >>| discharged, and if behind the CG, check it with the battery charged?
... and I'm sure he knew the answer to his question already :)
And I did quantify the sort of difference that charging a battery might make in your CoG --
| >>If you don't heed my warning, your CoG might very well end up an | >>angstrom or so off ...
:)
(I originally was thinking of a micron, but it seemed too large. But if the difference really is 6 parts in a trillion, perhaps microns would be a good deal closer.)

Nope Time, Laws of Conservation of Matter/Mass and Energy - None can be created or destroyed but their forms may be changed. Energy is *not* added rather some mass/matter is converted/changed to energy. The universe has a finite amount of energy/matter and that is a constant no matter what happens to change the forms of energy/matter.

Yeah...what you said. If battery is in front of CG, discharged; if behind, charged. I am glad we have finally cleared this up.
Ken

Actually, "real" aircraft C/G is affected by fuel burn both longitudinally and laterally.. I'm a helo pilot and the difference in C/G between full and nearly empty tanks is not huge, but it's significant.
The tank is up close to the engine to minimize the fuel flow fluctuations that occur in our models. Unless you've got a pump equipped engine, you need to rely on gravity and muffler pressure to TRY and maintain even fuel flow.
The C/G difference isn't that bad as fuel burns off and models (like the full scale types) will have a C/G envelope they can fly in.