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CoG and fuel tank positioning

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Old 01-19-2013, 08:48 PM
  #1
ghosted_24
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Default CoG and fuel tank positioning

I recently bought a 'great Planes- extra 300' and am now stuck with fuel tank position and CoG paranoia, from many sources including my LHS they say CoG is paramount and I've learnt this very well with all my electric flyers, and it's easy with electric coz you got a static weight to work with, my planes instructions says to place the fuel tank in front of the CoG and to balance the plane with an empty fuel tank (fine, simple enough, then the paranoia sets in), for me this is my thought process, if I have the plane perfectly balance as per instruction (neutral with an empty fuel tank) as soon as I start putting fuel in to it the plane would be nose heavy, therefore I'll take off nose heavy and land neutral or even tail heavy if I get things wrong, wouldnt it be a good idea to put the tank on top of the CoG to keep consistent CoG fron take off to landing?
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Old 01-20-2013, 12:51 AM
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Default RE: CoG and fuel tank positioning

It's not like you're balancing it on the head of a pin, you have a range of CG to work with. Putting the tank on the CG is ideal, but not possible in many aircraft. What's more important IMO is having the fuel tank height correct for reliable engine runs. The centerline of the tank should be at the carb centerline in a level attitude. 

The manufacturer has done all the work, so put the plane together and enjoy. Unless you change something, the CG will probably be very close to what's recommended when you're done building. Leave the battery and servo install til last, then you can move them around to fine-tune the CG. 
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Old 01-20-2013, 06:49 AM
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Default RE: CoG and fuel tank positioning

Ed is absolutely right - and even if you could put the tank at the CG, the fuel line might be too long for the engine to draw from. To add a little bit to EddieC's comments, a lot of pilots notice that their planes handle a bit differently from the begining to the end of a flight as the fuel (and weight) gets burned off... Not drastically (unless it's a poor design) but a bit.

Rule of thumb... balance a plane with the tank empty unless it's a pusher engine... then balance with the tank full.

Just my $.02

Bob
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Old 01-20-2013, 08:31 AM
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Default RE: CoG and fuel tank positioning

With Great Planes kits and ARFS, if you build them as they tell you to, the CG is usually right on the money. Put the fuel tank up front, finish the plane and then balance it. I have the Extra 300 kit and mine has 2 ounces in the nose to balance it. Yes it is a little nose heavy on take of because of the fuel load and the CG will move backward as the flight continue's, but you will not notice it at all. Good Luck, Dave
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Old 01-20-2013, 09:05 AM
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Default RE: CoG and fuel tank positioning

Oh, and welcome! I have a buddy in Oz.
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Old 01-20-2013, 12:04 PM
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Default RE: CoG and fuel tank positioning

When a kit is finally offered to the public it has been built and test flown a number of times and the CG location for best over all flight is marked. After you start flight trimming you can move things around to get the plane set as you like but as shown on the plans and instructions will get you a nice flying plane. As Ed pointed out, the tank is positioned up front for a number of reasons. I do place my fuel tanks over the CG when I can but all my glow engines except one are YS and pumped so the tank location doesn't mater to the engines. I have one plane with the tank too far back with an un-pumped engine and as Dave has mentioned I have had a heck of a time getting the engine to draw fuel and run correct, I got it after a lot of work and fooling around but if I could have gotten the fuel tank in the correct location the engine and plane would have been better off. Don't worry about it, just go with the instructions and all will be fine, it's an outstanding plane and well designed.
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Old 01-20-2013, 07:30 PM
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Default RE: CoG and fuel tank positioning

If running gas, tank position really wont matter, and the tanks dont weigh much when empty, so if you run gas and can fit the tank on the CG, do it, if not running gas, put the tank where the manual tells you to.
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Old 01-21-2013, 12:45 AM
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Default RE: CoG and fuel tank positioning

Thank you all for your responses, bloody helped calm the nerves a hell of a lot, but it'll all come back on test-fly-day
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Old 01-21-2013, 11:18 AM
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Default RE: CoG and fuel tank positioning

If this helps, I fly Extras, I have built and flown all types and brands of them. I do have a favorite but to date I have only flown one that had any bad habits and no one else will ever fly this one. It was a proto type and only two were ever built. They were 30% and sent to Great Planes for testing when they were going to put out the Patty and not chosen. One had crashed and the one I had was too heavy. I had to use a G-62 for power and the plane would tip stall without any warning if you were flying too slow. I ended up giving the plane away, not for that reason though. I got back into flying smaller planes. Today I am still flying the old CG Extra kit built plane and it is the better of the Extras I have flown. It just tracks a bit better then the GP Extra so it is a bit smoother then the GP kit you have. Your plane in this size is my second choice and is a fine plane with no bad habits. Built to the plans it's a perfect plane. The tank location is just fine behind the fire wall and you won't ever notice the change from take off to landing. It's a nice kit too with some pretty good wood. Don't over think a problem that it doesn't have. There are a lot of build threads on that kit right here on RCU.
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Old 01-22-2013, 03:10 AM
  #10
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Default RE: CoG and fuel tank positioning

Cheers grey beard!
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