bjp

I'm just curious, why are the fuel tanks so far in front of the center of gravity? you would think they would be directly under the CG or just slightly in front of if. That way, as you burn fuel your CG wont have changed much from when it was full. Anyone know why this is?

vtol_guy

the closer the tank is to the engine, the easier the fuel is picked up and fed into the carb.

You could always experiement with putting the tank furthur back but this may cause engine problems such as deadsticks shortly after take off

hookedonrc

bjp: There are several threads on RCU about moving gas tanks. Vtol_guy is right, the engine can only pump from so far and then needs assistance. I have had one plane where I moved the tank without adding a pump. It was an Ultra Stick 60 and I moved the tank to just where the back end of the tank was on the CG. This moved the tank about 6-7 inches behind the engine (a OS61FX), but I would not use this as a hard and fast rule. I may have just been lucky.

vtol_guy

as far as i'm aware, bigger engines should be able to pick up fuel a bit stronger so therefore should be able to have longer distances, its proportional im guessing

spooner

here's what i have understood from the guys that help me put my planes together...not sure if this is the answer...when you set the CG you do it with an emepty tank,so if you run out then you are still nose heavy alittle..if you have fuel in tank and dead stick then you are still nose heavy...if tank was at or behind CG then if you dead stick then you might end up tail heavy, which i hear is no fun to fly....i hope this made since....i'm still learningso dont take my word for gospel...spooner

vtol_guy

spot on!

i'd take nose heavy over tail heavy any day

David Cutler

Absolutely right!

Also, if the CG moves slowly back in flight the plane becomes a little more sensitive, which isn't such a bad thing, as by then, the initial shock of getting used to it has died away a bit!

Also, landing becomes a little easier, or slower, as the plane floats a little more when the fuel has gone.

-David C.

Woody 51

Yeah, when the tank is m/t, last thing you want is a tail heavy airplane when trying to land. Makes for some sweaty hands, a lot of entertainment for those watching and raises the possibility of extra building experience for the pilot!

A 4 stroke engine will also draw better than a 2 stroke, so one can mount the tank further back with a 4 stroker up front, if it is required.

sigrun

The definitive answer is that tank placement is the way it is because our model engines are predominently suction feed. As such mounting the tank next to the engine has become an accepted convention with good reason.

You're right about the desirability of having the fuel located over the CofG as it is IRL from a ideal stability standpoint, however in R/C its viability is offset by undesirable economics and unnecessary complexity. The compromise of locating it on a short forward arm is acceptable, and negates the need for or dependency upon a fuel pump.

Given the size of the average 8-10oz fuel tank in a typical sport flyers model, the change in CofG with fuel burn is relatively small. Unless the CofG was set up significantly aft of recommended in the first place, the shift with fuel burn off won't alter the sensitivity of the controls any more noticably than the reduced wing loading does, and can be trimmed out as easily as a changed throttle setting.

As a safeguard against the model's flight characteristics becoming uncontrollably unstable with an empty fuel tank, for this reason we always balance it with an empty tank aka ZFW. (Zero Fuel Weight).

David Cutler

Hmm!

It's true the plane is more sensitive when the CG is further back, but, on the other hand, it doesn't need to be 'flown in' like a nose heavy one when landing. Throttle management is more difficult with a nose heavy plane.

Landing with the weight further back means things happen slower, which can't be bad.

-DC

vtol_guy

true dave but the furthur you go back ,the more unstable it gets in pitch

hookedonrc

BTW: I have had 2 Ultra Sticks and have had the opportunity to try it both ways. I didn't mention that in my first answer. To tell the truth, I couldn't tell the difference. I am on my third now and will place the tank right behind the firewall as instructed. (Yeah my third. I really love the plane, but put two in this Summer. One my fault and the other not.) I am hoping that it is true that 3rd time is the charm.

David Cutler

Yes, true!

At the moment I'm toying with the idea of employing some sort of movable weight in one of my planes, maybe a rack and pinion setup down the center of the fuselage, that is activated by, maybe, the dual rate setting on the elevator, for those periods when full throw on the controls and pitch sensitivity are needed.

-David C.

JohnBuckner

My Feedback: (1)

David that would be an interesting experiment if you were doing it for the purpose of providing different stability levels to match various manuvers. But if you are doing it just to compensate for the rearward march of the CG as fuel is consumed then it would be far simpler just to use a bubbleless tank centered on the CG, that will feed reliably from that position .


John

sigrun

Quite the opposite.

You're confusing the effect on flight characteristics of reduced weight/wing loading with an aft displacement of the CofG from reference datum.

Displacing the CofG aft, (tail 'heavy') will cause a model to be less stable and more sensitive to control input requiring greater anticipation and trim managment with ea throttle change by the pilot. In lay terms, to the operator this makes things seem to happen 'faster' through time relativity. ie: tasks vs time available 'compression' effect.

On the other hand, diminishing weight with fuel burn-off reduces wing loading irrespective of CofG location, and that is what allows a model to be approached safely at a physically lower speed. This is independent of CofG location. Provided the pilot takes advantage of this by setting up a lower speed approach, this is what makes things happen 'slower', both physically, eg: S=D/T, and relatively.

Whilst ideal target approach speed and attitude is a function of angle of attack and wing loading, not CofG location, more frequently actual approach speed is a function of R/C pilot klutz factor.

David Cutler

That may be true if you take High Energy Plasma Physics into account, but if you've got a nose heavy model on finals, you had better prepare for the fact that it's gonna come in nose down, and therefore quicker than you expected.

It's not gonna be a floater no matter how close Mars is.



-David C.

sigrun

Althjough I appreciate basic aerody probably seems like proverbial rocket science to some, the fact is that R/C flight dynamics are as unaffected by facetious comments about the distance from Mars and everything to do with basic aerodynamics and mathematics.

It's not gonna be a floater no matter how close Mars is.

Float has nothing to do with whether a model is nose or tail 'heavy', ie: Cof G location, and everything to do with excessive approach speed and low wing loading (now there's a clue) when in ground effect.

Phone a friend?

David Cutler

I'm glad you took my posting as a joke, as it was meant to be.

I do, however, still contest what you say.

Try moving the CG back on, say, an Edge or Extra, and as well as it getting more pitch sensitive, (and increasing the chance of a snap) it also 'digs in' more on landing (and, incidentally, is more difficult to take off for similar reasons)

My slight facetious comment was really to highlight the fact that theory (which, if you look at my previous postings I am all in favor of!) has its limitations when experiencing in real time.

Weight forward means having to 'fly it in' and takes up much more runway, as opposed to floating and stalling in when the weight is further back.

If you haven't experienced it that way, then we must have different definitions of the words we are using.

-David C.

David Cutler

Agreed John.

My thoughts about it were more to do with the fact that I seem to plan to try moving the CG back for the next time I fly, just to see how the handling is affected, but never seem to get round to doing it! Having a system that can be adjusted on the spot might help that situation.

I was thinking of doing it with my Edge, with a YS110 in it, so the fuel tank position, with respect to fuel suction isn't really an issue in the first place.

-David C.

canardlover

Hi guys, was intrigued by Johns statement that a bubbleless tank would feed the engine reliably from farther back than a standard clunk tank. Recall having seen this written before but I´m still puzzled as to how this can be so.....please confirm and try to explain...The reason I ask is that I have just ordered my first bubbleless tank(s) so I´m new to the use of these tanks. Will mount them in a push-pull canard similar to the one in my avatar and would be helped by moving tanks around a bit. Thanks in advance for your input....Cheers/Harald

JohnBuckner

My Feedback: (1)

Hi canardlover

I generally don,t come to this site anymore but spend it at more comfortable ones but will try to answer your question.

The bubbleless type tank was developed by the pylon folks maybe ten years ago and no serious racer would be without now. The purpose was to be able to position the tank on the CG without resorting to pumps, crankcase pressure or pressure bladders, all of which are not allowed by current rules in this country. They fit the rules because they still only use muffler pressure just as normal with a conventional system. They are able to function in the extreme environment because the only thing introduced to the internal fuel bladder is the fuel itself. No air or exhaust gasses can be mixed with the fuel. The exhaust pressure itself is introduced between the inner bladder and a hard outer shell. They can never mix. No air or no gases, no bubbles thats why they are called bubbleless tanks. No foam is needed and it is normal to hard mount the tank.

A tank clunk is not used, the pickup is directly in the center of the tank and it will feed in any attitude untill the fuel is completly used. The delivery characteristics of a bubbleless is far superior to the conventional system and I now use them not just for the racers but an occassional problematical airplane. One example is my vintage Graupner delta pusher with a 6 oz Tetra bubbleless on the CG.

The downside is you really need to use a syringe to fuel them with this allows removing the air from the tank before pushing the fuel in. The Tetra tanks come with instructions for fueling with a pump but at best this does allow some air in. Best to use the Syringe like the 'Jett' Tanker or I think Sig makes a large syringe.

John[8D]

FHHuber

Part of the fuel tank location is convenience (its fairly simple to find a space right behind the engine for it)

Part of it is tradition. Before it was found that you could pressurize the tank from a muffler (because mufflers weren't used much) you had to have the tank right behind the engine to have a chance to get fuel to the carb if you pointed the nose up.

With muffler pressure, yes 4 to 6 inches back won't be a big problem. Tune the engine a tad richer and then the long durration nose-up attitudes won't be a problem. If you put a check-valve in the muffler pressure line you can nearly double the effective fuel pressure... and can have the tank even further from the engine. (would require retuning the engine for the higher fuel pressure...)

It is common for Pattern and 3D competition (larger models... not .40 size) to have the fuel tank on the CG. These planes will use a fuel pump and possibly a pressure regulator to give reliable fuel pressuer at the carb.

You just don't want the fuel tank behind the CG... unless you plan to balance the airplane with a full fuel tank. You always balance the aircraft with the fuel load which puts the CG to the rear. If the tank is in front... empty. If its inback.. full. Some pusher designs are balanced with a full fuel tank.

Flypaper 2

Biggest problem with the tank far back is the pressure difference between nose level and nose straight up. Even with good tank pressure to pump the fuel from, say eight ins. from the engine when the nose is high, and the carb set for that position, it will be very rich when you bring the nose down to level with that much fuel pressure.

MinnFlyer

My Feedback: (4)

That's why I like my YS engines. I put my tank at the CG with no problems.

canardlover

Hi again and thanks for input, from your answers I sense that placing the tank some 8 in. behind the firewall will be problematic using muffler/pipe pressure regardless of tank being std. clunk or bubbleless...right..?
Using a check valve will not change the relative performance of the two tank types, overall pressure goes up for both types..right..?
So in summary bubbleless tanks are NOT less sensitive than std. clunk tanks regarding their placement in relation to the carb position...right..?
FHHuber...I never make BIG MISTEAKS....
John..sorry to drag you into this thread again but thanks for your good explanations
Gord, long time no see.....
..........sack time now in Sweden...Good night...../Harald