why is the fuel tank so far in fronf of CG?  
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All Forums >> RC Airplanes >> Beginners >> why is the fuel tank so far in fronf of CG?
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why is the fuel tank so far in fronf of CG? - 8/23/2003 10:30:26 PM   
bjp


 

Posts: 14
Joined: 8/1/2003
From: augusta GA
Status: offline 		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?
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why is the fuel tank so far in fronf of CG? - 8/23/2003 10:34:15 PM   
vtol_guy


 

Posts: 897
Joined: 1/13/2003
From: Driffield, UNITED KINGDOM
Status: offline 		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


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why is the fuel tank so far in fronf of CG? - 8/23/2003 10:56:03 PM   
hookedonrc



Posts: 2181
Joined: 12/30/2002
From: Broken Arrow, OK, USA
Status: offline 		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.

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why is the fuel tank so far in fronf of CG? - 8/23/2003 11:04:34 PM   
vtol_guy


 

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From: Driffield, UNITED KINGDOM
Status: offline 		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


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why is the fuel tank so far in fronf of CG? - 8/24/2003 1:02:22 AM   
spooner


 

Posts: 426
Joined: 1/1/2003
From: las vegas, NV, USA
Status: offline 		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

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why is the fuel tank so far in fronf of CG? - 8/24/2003 1:43:20 AM   
vtol_guy


 

Posts: 897
Joined: 1/13/2003
From: Driffield, UNITED KINGDOM
Status: offline 		spot on!

i'd take nose heavy over tail heavy any day


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why is the fuel tank so far in fronf of CG? - 8/24/2003 1:50:28 AM   
David Cutler



Posts: 2163
Joined: 9/13/2002
From: Los Angeles, CA, USA
Status: offline 		[QUOTE]Originally posted by justgotr
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 [/QUOTE]

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.

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why is the fuel tank so far in fronf of CG? - 8/24/2003 2:54:37 AM   
Woody 51



Posts: 1159
Joined: 6/21/2003
From: GeelongVictoria, AUSTRALIA
Status: offline 		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.

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why is the fuel tank so far in fronf of CG? - 8/24/2003 3:21:22 AM   
sigrun



Posts: 1023
Joined: 7/18/2003
From: Dunnunda, AUSTRALIA
Status: offline 		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).

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why is the fuel tank so far in fronf of CG? - 8/24/2003 8:41:10 AM   
David Cutler



Posts: 2163
Joined: 9/13/2002
From: Los Angeles, CA, USA
Status: offline 		[QUOTE]Originally posted by Woody 51
Yeah, when the tank is m/t, last thing you want is a tail heavy airplane when trying to land. [/QUOTE]

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

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why is the fuel tank so far in fronf of CG? - 8/24/2003 4:34:50 PM   
vtol_guy


 

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Joined: 1/13/2003
From: Driffield, UNITED KINGDOM
Status: offline 		true dave but the furthur you go back ,the more unstable it gets in pitch


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why is the fuel tank so far in fronf of CG? - 8/24/2003 5:45:54 PM   
hookedonrc



Posts: 2181
Joined: 12/30/2002
From: Broken Arrow, OK, USA
Status: offline 		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.

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why is the fuel tank so far in fronf of CG? - 8/24/2003 9:12:42 PM   
David Cutler



Posts: 2163
Joined: 9/13/2002
From: Los Angeles, CA, USA
Status: offline 		[QUOTE]Originally posted by vtol_guy
true dave but the furthur you go back ,the more unstable it gets in pitch [/QUOTE]

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.

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why is the fuel tank so far in fronf of CG? - 8/25/2003 3:26:47 AM   
JohnBuckner



Posts: 4047
Joined: 12/23/2001
From: Kingman, AZ, USA
Status: offline 		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

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why is the fuel tank so far in fronf of CG? - 8/25/2003 4:24:33 AM   
sigrun