Hi -
I just finished installing a tigre .61 on a Hangar 9 f22 raptor (it had a .46 before) I left the same tank that was there before and it's an 11 oz tank. What size tank should a .61 have?

I know that the smaller the tank the shorter the flight, and that it all depends on how I fly, but as a general "rule" what should it be?(for a .61)

The std for years for the 61 two stroke was a 12 oz

An appropriate tank size for a forty, fortysix eight to ten ounces and for a Sixty one is ten to twelve ounces. Everyone is always trying to stuff bigger and bigger tanks in and forgets that using oversize tanks on suction/muffler pressure systems for a given size engine will reduce the fuel delivery capability.

In other words using an oversize tank unless it is a pumped system will hinder the ability to have a uniform engine run from full to empty tank. Its not unusual to start experiancing shorter flights with a bigger tank.

John

The bigger the tank the more weight you are hauling around

I would keep the same tank. I've got 8oz tank for my .46 and I get very long flights. I've got a 14oz tank for my 1.20 and get a good 8 minutes or more.

isn't a .61 engine too big for that plane? [X(]

Yes. but it should go faster

I hate the "a guy at my field has..." comments, but, a friend at my field has that plane (had two but an instructor lost one) and it's been outfitted with the Super Tigre G61. The plane seemed to 'fall out of the sky' with the .46, even after prop changes and engine mods. It's a 'jet' and with the .61 it actually flies like one

Correct me if I'm wrong, but that's because the engine doesn't create enough muffler pressure to adequately fill the airspace in the larger tank, so the available muffler pressure is spread out across the larger (and ever increasing) airspace, thereby resulting in lower overall fuel delivery pressure?

Would the same be true if you rigged the tank in a "uni flow" setup? With this setup the muffler pressure is restricted by the pressure of the fuel in the tank, and as the fuel is exhausted the restriction is eased, thus allowing a more even fuel pressure delivery across the entire flight which allows you to tune for the lower (but more constant) pressure.

John,

I'm confused. the pressure in the tank remains the same no matter how big it is. Given that there is pressure and a bigger tank won't take the pressure away, fuel flow should be indepedant of the tank size. How would a larger tank reduce the fuel delivery capability?

FYI - We have a 12 oz in a 40 sized trainer, combined with a 2200mA battery, so the beginners can fly until they are tired and not have to turn the controls back to an instructor to come in to refill => more stick time. It works out perfectly.

Jack

I cannot argue physics and certainly not something I am qualified to do however I do beleve that head pressure from the full to near empty certainly does change and this is more noticible when a larger tank is used with no increase in suction/muffler pressure from a larger engine.

This became very evident to me when a few years ago when experimenting with various large tank setups for a .45 powered x-country effort, up to fifty ounces.

Since this is the beginner forum I would suggest to anyone that staying within normal protocol for tank size is very good advice indeed.

John

I agree 100%.

Jack