Brodak 40 and fuel tanks

First off I mistakenly posted in another thread that my Brodak 40 wouldn't safely finish the pattern on 4 ounces of fuel. I just pulled and measured the fuel tank in the Omnium, lo and behold it is only 3 1/2 inches long which equates to 3 1/2 ounces for a standard metal wedge. I could have swore I put a 4 ounce tank in the silly thing, isn't getting old a bitch

Anyway, I am building a new tank and would like some opinions on how far in front of the pickup tube the uniflow vent should end. What I am looking for is a clean shut off when the tank is empty. The existing tank is about 3/8 of an inch and I wasn't real happy with the way the engine shut off. It would burp a few times before it actually stopped. Any hints would be appreciated....

>Anyway, I am building a new tank and would like some
>opinions on how far in front of the pickup tube the uniflow
>vent should end. What I am looking for is a clean shut off
>when the tank is empty. The existing tank is about 3/8 of an
>inch and I wasn't real happy with the way the engine shut
>off. It would burp a few times before it actually stopped.
>Any hints would be appreciated....

That 3/8 inch location is about par. The problem with the not-so-clean shutoff is not in the location of the vent tube but probably a function of yaw in the airplane. With the engine running, the nose yaws out and fuel flows forward, draining the pick up tube (not the vent tube) sooner. When the engine "stutters", attempting to quit, the nose yaws back in, the fuel sloshes to the rear, and voila, engine runs again. Several of these instances take place until it is truly out of fuel. If you can angle the tank slightly or build a tank with the rear wider than the front (the wedge tapered) you can solve this problem. (Thanks to Brett Buck whose fuel tank solved a nasty problem similar to this on Matt's "Jaws" Stuka).

I am really surprised the B 40 would not make it with room to spare on 3.5 oz. I found mine very thrifty needing only 3.0 oz of 7.5/22 fuel with a good setting.
I had a 3.0, all tubes forward in a slip in mode in my last Oriental as I didn't have a 3.5 or 4 at the time and never had to change it out when I got a larger tank in stock.
Only real problem with that set up was I almost always got several false shutdowns at the end of the tank.

Hi Jim..

Don't know what to tell you, I was running 5% nitro 22% 50/50 lube and couldn't get through the full pattern on 3 1/2 ounces. Were you running an air filter?

>Were you running an air filter?

Nope.
Interesting that I stuck the Brodak 40 on a Brodak Zero today and went out in perfect conditions to a no turbulence field. This Zero has had 3 different OS35S engines, all stock, on it and is light, been flown a lot, and is well trimmed. Fuel was 7.5/22, 4 oz chicken hopper uniflow. 10-5 APC, same as I have generally be running on the OS35S, same tank and all. I do use different, higher oil fuel in the OS35S' of course.

I noticed an immediate gain in grunt at the same 5.0 lap times on 60'center to center lines, lots more overhead tension, nothing at all funky in the runs(which has not always been the case with the various OS35S). First run was two cycle all the way and thought it would never shut down, next two were perfect 4-2 breaking. Fuel consumption was 3.25 oz for pattern plus 10-20 laps. Shut down was immediate and with absolutely no warning on this tank. Goes off dead wet on the ground to a nice 4 cycle in level flight when it hits the air.

Only gripe, if you will, is that I notice this engine is the most easily flooded I have ever seen. A little too much choke and it is absolutely dead, no bump no nothing until the plug is removed and all dried out. This is as true on the profile mount as it was on the inverted mount. First hot restart, I did not clamp fuel line when filling, gave it one choke and it was flooded bad. Second restart, I clamped fuel line, gave it a tiny baby choke, just enough to get fuel to venturi and it one flipped. Proceedure was similar on the inverted mount Oriental.

It is possible that the apparent overall improved torque, overhead tension and sense of power without runaway with the B 40 as compared to the various 35S is due to today's perfect flying conditions but I don't think so.

>It is possible that the apparent overall improved torque,
>overhead tension and sense of power without runaway with the
>B 40 as compared to the various 35S is due to today's
>perfect flying conditions but I don't think so.

Yep I saw the same thing with the Omnium, don't know how to rate the difference but the increased "power" was very obvious.

Thanks Len,

What you say makes sense, can't cock the tank, just barely fits in the fuselage and I am re-pluming an existing tank so building in a bubble isn't feasible. Guess I will have to live with the burping on shutoff or build a complete new tank.

>Thanks Len,
>
>What you say makes sense, can't cock the tank, just barely
>fits in the fuselage and I am re-pluming an existing tank so
>building in a bubble isn't feasible. Guess I will have to
>live with the burping on shutoff or build a complete new
>tank.

Here is a tapered tank that should be sufficient. Print out 1:1, use 3m77 to stick it to some K&S easy solder tin. Cut along lines (leaving tabs to solder, bend, solder.

My Dad is just finishing a Chipmonk with a 4oz tank and was getting a little nervous about the fuel run as it is a tight fit for a 4oz tank and it is fitted already!!
Thanks a ton for the update, Bob
Mike Ogren

I suspect you will never know 'til you go. 4 oz is normally a gracious plenty for Fox 35's, OS 35S, OS 35/40 FP's. Marginal on SSW 35/40FP's. Insufficient on OS 46LA. Note Brodak 40 comments above-a wide discrepancy in two individual engines.

I use low nitro, 5% to 7.5% which increases run time. Also needle setting, venturi size, airplane size, weight and drag, prop, etc. all have to come into the formula.

More power, whether from displacement, venturi, nitro, engine tweaker's style of rework, etc. requires more fuel.