[QUOTE=Mark Vandervelden;11849906]
Hi Fredo
"It has been debated many times before on this thread whether to stay with the twin 1500cc tanks or go with two smaller 800-950cc tanks at the back and one of the 1500cc at the front.
Fact is the Viper fly's "OK" with the 60-70mm + CG shift you get with the whole of its 3lt (6lbs!) of fuel that's some 230mm in front of the CG and its a testament to the airframe to be able to tolerate such a "large" CG shift.
Every one I know that fly's the Viper regularly has noticed a real difference this makes during the flight, particularly at take off were a nose heavy airframe "will" take a little longer to rotate. All 8+ Vipers I've built for myself and others plus a few others I've worked on over the past two seasons now have been upgraded to a three tank systems ether during the build or retrofitted. After all any airframe including the Viper cant fly worse with less or no CG shift that you can achieve with a three tank setup. Add to that its very little work to achieve especially with a new build, I just can not see a down side?.
You also get the added bonus of extra "free" space up front ether side of the front tank and it moving rearward by 40mm into the neck of the inlet duct closer to the CG. You also have the option of an extra 400ml of fuel if the 950cc JL smoke tanks are used and easy access to the front tank as the twin 1500 tanks are at best a little troublesome to remove.
My favourite method of plumping up the 2+1 tank setup is not to use a "T" but to use twin feeds into the main tank, this allows the fuel in the system to flow more easily. This can be easily achieved by using all three brass tubes into the bung, one from each rear tank and one going to the pump via a UAT.
I have recently found ta neater way to "plum up" this is to fit two 3-4mm tank fittings to the rear top of the main tank, this keeps the fuel lines very short and tidy. On my latest Viper I actually managed to fit the UAT along side the main tank, again making for a neat uncluttered install with easy access for maintenance. Felt and paper clunks do work and I often fit a felt clunk in the main tank but are no substitute for a good UAT and it is rare if not unheard of not to use one.
If your running a smaller turbine or don't feel the need to ferry fuel about or fly for 10 minuets at a time there's the option of fitting a smaller front tank, say 1-1200cc reducing the CG shift even further and still give good flight times. Do though no mater what setup you use check and check again the CG in all states of full or empty, I've flown happily with the CG at 50mm behind centre of wing tube but don't recommend going further back than 35mm.
What ever setup you use you will defiantly enjoy building and flying JL Viper, its got to be one of the nicest well behaved and best value sports scale airframes available today "probably" 
Fredo.. Entering the world of Turbines and immediately taking short cuts (the easy way) does not seem to be the right road to a long and successful turbine flying career. I would seriously take the advice of Mark Vandervelden posted above. In my personal opinion a UAT is mandatory no matter what configuration you are using. Turbines do NOT like bubbles and the UAT's sol purpose is to eliminate the possibility of bubbles getting to the engine and causing a flame out. Its a very cheap form of insurance to make sure your engine wont quit.