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Author Topic: Fuel, Tanks, and CG shift.  (Read 1285 times)

Offline Peter Nevai

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Fuel, Tanks, and CG shift.
« on: October 28, 2011, 11:04:23 AM »
While figuring out if the 8 oz sullivan tank I got would clear the Exhaust tuned pipe header, I was struck by the volume of the liquid that sloshes back and forth in a tank especially of that size and the huge CG shift that a volume of liquid would cause. I mean several ounces of fluid sloshing back and forth more than 4 inches for and aft.

With all of the developments and technologies over the decades I wonder why some one had not come up with a way to mitigate this. I can not imagine that centrifugal forces pin the fluid consistently at a fixed position within the tank. Flexible bladder tanks within a rigid container probably are best for keeping the fuel static but bladder tanks have their own issues as they put fuel under pressure that lessens as the fuel volume goes down, not to mention that some sort of shut off valve has to be implemented in full fuse planes to keep the fuel from squirting out of the venturi.

This is one of the advantages of electrics as batteries are not allowed to flop around.

I mean 5 or more ounces of fuel falling toward the front and then to the back of the tank during maneuvers has to have some type of impact.
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Offline Jim Thomerson

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Re: Fuel, Tanks, and CG shift.
« Reply #1 on: October 28, 2011, 03:45:54 PM »
I don't know that that has been thought about.  There is an urban legend that the stunt pattern was designed to have the harder maneuvers last, when the CG was moved back due to less fuel in the tank, and the airplane became more sensitive. 

 S?P I once raised this on an electric forum.  Should electrics be designed such that the batteries move back during the pattern?  Or is it a matter that the batteries lose enough weight to take care of it?  One of the Fermi Lab guys calculated the weight loss between a charged and discharged battery in stunt application. The difference is a micro-nanno-jillionth of a gram.   :o
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Offline Bootlegger

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Re: Fuel, Tanks, and CG shift.
« Reply #2 on: October 28, 2011, 06:47:09 PM »
 S?P    Ty, I too have seen Bob Hoover does those tricks, and my eye's are blue... #^
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Offline Howard Rush

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Re: Fuel, Tanks, and CG shift.
« Reply #3 on: October 28, 2011, 07:24:09 PM »
Bob Reeves made a wonderful video of fuel in plastic tank in flight.  You can see what it does. 
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Offline Russell Shaffer

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Re: Fuel, Tanks, and CG shift.
« Reply #4 on: October 28, 2011, 10:16:13 PM »
Can this video be accessed?  Sounds very interesting.
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Offline Rafael Gonzalez

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Re: Fuel, Tanks, and CG shift.
« Reply #5 on: October 29, 2011, 06:36:47 AM »
Forces on the fuel in a tank are centripetal and Earth's gravity. Force vectors at 90 Dgrs are independent from each other. On our flying circle, the only time when the vectors are independent is when we are flying on a level path. Fuel stays at the back of the tank when we climb even if we are "stopped" due to gravity (9.8 ms(sq). When we dive, we are accelerating to counteract the Earth's gravity and that keeps the fuel also at the rear. If we don't accelerate or at least equate 9.8 ms(sq), the fuel will fall away from the pick up tube. The best tanks for the stunt pattern are the homemade tanks where  the wedge  is tapered/angled out towards the rear thereby using the centripetal forces to keep the fuel at the rear.

In R/C pattern, 3D or any aircraft where the transfer of weight by the fuel is critical for the performance, the tank is mounted near the center spar of the wing/CG and a pump or pumped engine is installed.

 H^^
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Offline Bill Little

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Re: Fuel, Tanks, and CG shift.
« Reply #6 on: October 30, 2011, 03:27:37 AM »
Quote from: Ty Marcucci on October 28, 2011, 04:47:24 PM
(snip)Ever see the video of Bob Hoover flying with a full glass of tea on the instrument panel. The tea does not move andhe is oing rolls and loops.  H^^But then my eyes are brown. H^^ LL~

One of the highlights of my aircraft ventures was meeting Bob Hoover and getting a "stand alone" call out by his announcer (the field's PA went out and we were beside him on the tarmac).  If I had not witnessed his flight in person,"up close and personal", I would have never been convinced that he could actually DO that show with the P-51 AND the Aero Commander .

Bill
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Offline Chris Wilson

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Re: Fuel, Tanks, and CG shift.
« Reply #7 on: October 30, 2011, 10:17:47 PM »
Quote from: Peter Nevai on October 28, 2011, 11:04:23 AM
.......... the huge CG shift that a volume of liquid would cause. I mean several ounces of fluid sloshing back and forth more than 4 inches for and aft.

With all of the developments and technologies over the decades I wonder why some one had not come up with a way to mitigate this.

Probably its the soft cell bladder technology that could counter this, but with the penalty of some form of pressure needed to form that collapsing bladder constantly around its contents.

Combat bladders with their internal pressure come to mind as do Jett bubbleless tanks with their external pressure.

But I agree, the fuel shift makes a mockery over some of the pin pointed CG arguments.
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