I've done several searches and can't seam to find an awser to this question. Maybe I'm just not holding my tongue right?

So, what size tank do you guys prefer on a 75cc engine?

I don't usually fly more than 8--10 minutes. Mostly IMAC type flying, but I do intend to try some 3D with this plane. A 20oz tank fits real nice--but I'm afraid it's a bit small. If I have to go to a larger tank--it'll have to get moved from the CG up into the engine box area. [:'(]

I know my other planes with G-62 and 24oz tanks will run 30 minutes EASY if I'm just loafing around and doing basic IMAC areobatics. Just for reference.

Thanks

20oz will be plenty, if you ask me.

20 is fine for 10min flights just time it

on my 3w 85 I can fly 19 safely on a 32oz.

Dick once posted the rule that an engine uses it's displacement in fuel per minute at full throttle. This is slightly above my actual observed consumption, so the rule should be safe to determine minimum tank size requirements.
75cc x 10 = 750cc. One Oz has ~28cc so:
750/28 = 27 Oz

It's gona depend a lot on how hard you fly it, but then you already knew that didn't you????

I'll give you what I get for fuel burns on my ZDZ-50 and ZDZ-100 based on the type of flying you're talking about. These are estimates based on "eyeballing" the fuel level in the tank after a flight.

The 50 will go through about 1oz. per minute and the 28% WildHare Extra that it's mounted in has the 20oz. tank. I'm usually right about 10 minutes per flight and turn on the timer just before the takeoff roll and it burns about 1/2 that 20oz. tank. The 100 is in an H9 33% Edge and flown the same way using a 32oz. tank, it appears to go through about 1 1/2oz. per minute.

Now that's very little full throttle use practicing IMAC sequences, uplines only and on the Edge even uplines don't require anything near full throttle while I do use full throttle on the WildHare. I usually try a bit of 3D stuff each flight but not much.

Hope that helps but I'm afraid I'd be nervous on a 75cc engine and anything smaller than a 24oz. tank...

It does help. I'm nervous about the 20oz tank. I can stuff a half-gallon tank up in the front--behind the firewall. But, thats not the goal here, is it? I'm trying to keep my tank on the CG. The 20oz fits right on top of the wing tube with 3/8" to spare for putting my canopy on. By the time I pad the tank with foam on the bottom and velcro it in--canopy barely fits.

The only other option is to use 2 smaller tanks--with a combined capacity equaling 24-28oz. I can stuff two 12oz or 14oz tanks UNDER the wing tube, but that means running them in series--piggyback style-- and I really don't want to go through the hassle of all that plumbing. Just more places to spring a leak.
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How many G's do you think we can pull with these airplanes?

I'm trying to also determine how strong to build my tank support. Actually, how LIGHT can I build it?

Gas weighs about 6lbs per gallon. If I run a 24oz tank, that should weigh about 1/4th of the 6lbs-- or 1.5lbs when full. If you pull 10G's that tank now weighs 15 pounds. 20G's and it could weigh as much as 30lbs when full.

The difference in construction for a 15lb load VS a 30lb load is quite a bit different.

I currently have the 20oz tank mounted on top of the wing tube. It sits about 1/2" above the tube. There is a small shelf built with balsa for the tank. It is braced by cutting 2 lite-ply ribs to fit directly over the wing tube. Each rib has a 1/2 moon shape cut out to swallow the top of the tube. These ribs hold the bottom of the tank shelf. There is a side-to-side brace in the front. It runs across the front and ties into both sides of the fuse. Thats all I have now. It is extremely strong and very light.

I have no problem with this mount. I know I'm not going to push the tank through the wing tube with any up elevator inputs. The likelihood of me pulling more than 10 or 15G's negative and peeling the suport off the top of the tube is minimal. I don't have the thumbs to do a full throttle outside wall. [:-]

But, if I moved the tank to a location under the tube, there is nothing there but a few balsa stringers. 20G's positive could shove a tank reight through those stringers. Can you imagine the look on my face if I do a wall maneuver and my tank comes flying out the belly of the plane? [X(]

So, how much reinforcement does a guy need to contain (potentially) 30lbs of weight on top of a couple stringers? The simple answer is to span the whole width of the fuse with some lite-ply and be done with it. But, thats not exactly light. [] Not at all.

Thanks for the help guys.