Chieftoo
 

In 1960 I was 15 I built a Veco Chief flew it with a Torpedo 29. Last winter I built 2 Ringmasters and bought a new Fox .35. The 35 runs good except it won't keep running with out side "G"s I have tried all kinds of stuff including putting a stick in the bypass but it still wont do a horozonal 8 with out quitting on the back side. With the stick I can go inverted fly a lap and then come out with another end with a half inside loop. Well I want an airplane with the engine up right. For nostalgia sake I am decided to build a Chieftoo. http://www.outerzone.co.uk/plan_details.asp?ID=466. I have cut out some ribs.http://www.rcuniverse.com/forum/atta...mentid=2059846http://www.rcuniverse.com/forum/atta...mentid=2059850http://www.rcuniverse.com/forum/atta...mentid=2059851

Looking ahead I am thinking about the wing span. I would like to make the wings in one peace 48" wood is not long enough. I am wondering about building in 2 peaces and joining them with fiber glass. The plans don't show a method to build the wings in one peace unless you can get 50" balsa wood. It will be difficult to join the wing leading edges and still keep a nice curved leading edge. Any ideas?

What do mean "it won't keep running with out side "G"s"? Are you sure you're not sending it up too lean or have tank issues?

I have been fighting this for over a year now. The folks over at stunt hanger gave me some help. It seems the fuel collects in the low side of the engine when the engine is in a profile. When you introduce negative G's it floods the engine. It will do inside loops with out a problem. Figure 8ts when it starts up the back side it stops. They say to increase the velocity in the crank case you put a peace of wood the the bypass. I did that now it will do a half loop to inverted and will fly inverted as long as you want and climb and do an inside half loop to recover. According to them this is real common to the Fox 35s. But the Fox 35s run just fine mounted up right.

I understand what you are saying. But I had a Fox .35 Stunt on a Sig Banshee and never encountered problems you mentioned. Are you running any pressure from the muffler to the tank? Also I used a Uniflow tank, a Thunderbolt RC plug and 10% fuel. Something doesn't seem right. Fox .35 Stunts are not my favorite engine, but I ran enough of them to know that they are pretty dependable as long you feed them right. I'm just wondering if something in your setup is really causing your problems. When I was flying the Banshee, I could do a pretty crappy pattern, but I couldn't blame the engine, it ran just fine. That engine should not be cutting out on a horizontal eight. It's one of the easiest maneuvers you could do with it. Don't take this wrong, I don't mean to upset you, I just think something is wrong. Could you be running it too rich?

Joe,
The problem most likely is "relative tank height."

For many years now, it's been practice to raise the centerline of the tank about 1/4" above the engine centerline on Fox 35 powered profiles. The comments you read on Stunthanger are good, but it sounds like they described the cause of the problem, not a solution.

There is a fairly simple method to come closer to an equal run upright and inverted...

You can simulate the direction of forces on the model in flight. Consider - the centrifugal force on a typical CL sport model is about 3 g, straight out from the handle. Gravity is always with us, so there's another "component" of 1 g straight at the ground. "Resolving" these forces - finding a single force that has the same effect as the CF and gravity - is simple. Sketch a rectangle 3 units wide and 1 unit high. The diagonal of the rectangle represents the equivalent single force resulting from CF and gravity. That's about 19.5° out and down. The actual value, 3.16 g, is less important than the angle.

In upright flight, that angle is out and towards the wheels; inverted it is out and toward the "pilot's head." Simple enough? We can't simulate that 3.16 g force, but we can simulate the effect of its angle

With the lines off the model, start the engine and set it rich enough you can hear any effect of different "fuel head" when you hold the fuselage centerline horizontal, outboard wing vertically down. Keeping the fuselage centerline level, roll the wings to about 45° towards upright and listen to the engine or, preferably, tach the RPM. Now roll it to about 45° the other way and tach it there.

Fuel head relates to whether the engine needs to draw fuel uphill, or it gets fed fuel by gravity. If the tank is 'lower' in one rolled position, the engine is - relatively - drawing fuel uphill, compared to being - relatively - gravity fed in the other rolled position.

When you have it clear in your mind which 'bank' causes the engine to lean put, stop the engine. Nose straight down and inboard wing up usually puts the fuel pickup out of the fuel for a quick, safe stop. Adjust the tank height to raise it from where it was when the engine 'leaned out' slightly. (The engine should have richened slightly at the other banked attitude, due to fuel having gravity assist it.)

It may take more than one adjustment to place the tank where there's little or no RPM change between the two 45° banked positions. Once you get close, the fuel's "natural" feed to the spraybar is the same upright and inverted. If the model is severely warped, or top- or bottom-heavy the vertical CG may cause the model to bank a bit. That could require another small vertical tank height adjustment. Once you have it, the tank feed situation is 'centered' - different loads, like inside and outside loops, change from the level flight 3:1 condition, but they vary the same way and with equal effect both ways from "center."

There are many descriptions on the model sites describing making tank height adjustment rigs. They are simple, and easiest on profile models.

Somewhat related, but not necessary to know. Modern ukies most often use "uniflow" vented tanks. These hold needle setting much more consistently for the length of a flight, but they are more sensitive to tank height. Get it right and it's a joy - not hard, just takes a little fiddling, then you're set.

Luck!

What Lou says,
I had a couple of Fox 35 powered profiles when i was learning to fly the Pattern and found the same need to bias the height of the tank relative the spray bar.
Chris...