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Post  944_Jim Thu Aug 01, 2019 7:44 pm

I've been thinking about my AP powered P-40 lately. And after hearing the .15 Medallion bark once, I started thinking of the 4-2-4 break. I know it is used in bigger engines, and wonder if the same reasons would also apply to the smaller stuff. The P-40 just seems to lose headway and lift while trying to do loops and go inverted.
I started thinking maybe it is going too lean during the harder maneuvers. It sure snaps out of a wingover with authority. I know the simple test would be "fuel it, and sling it running rich," and see how it behaves during stunts beyond a wingover.

What say y'all?
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Post  Ken Cook Thu Aug 01, 2019 8:48 pm

Jim the 4-2-4 is mythical. Even A Fox .35 flying in that mode will fly so slow you will have little to no line tension. Many refer to this phenomena as the 4-2-4  but due to the large bypass the engine sounds like it's switching back and forth.  Smaller engines don't seem to offer this as larger engines do. Modern flat topped piston engines are almost in a constant wet 2 with a slight break when the nose goes high. It sounds to me your having tank feeding issues. This could also be tank height, incorrect needle setting on ground or incorrect prop. If your taxiing the AP to the point it's going overlean, something isn't correct. The AP like Norvel's demands higher and constant rpm's. Not just for speed but also for fuel draw. Keeping the fuel lines as short as possible which means keeping the tank as close to the engine as possible. While I use wedge tanks and I don't recall what you have, the wedge can be very responsible for the same issues your explaining. If mounted profile, the pickup needs to be closer to the centerline of the the engine. This may require one to shim the engine outboard or sink the tank inboard in the fuse.  You could rotate your entire engine having the cylinder mounted outboard which if using a tank like I mentioned above could help due to this application moving the venturi centerline more inline with the tank. Just remember that you need to align the centerline in two planes vertical and horizontal.

         I do recall you have the Jan venturi, this is something that I'm becoming a little suspicious off. If the venturi is too large, some of what your experiencing could be a result of using it. Having it too large is going to cause insufficient fuel draw causing it to go overlean. This is a easy fix, just acquire tubing and slide it into the venturi and drill if needed where the spraybar passes through. You could also jam small pieces of different size fuel tubing into the hole provided it fits tightly just to try a flight.  Generally, a venturi too large will cause a engine to quit or almost quit on launch when it unloads due to insufficient fuel.

        The plane itself has very little wing area. I don't recall if this wing is fully symmetrical or flat bottomed. If flat bottomed, this is not going to fair well in the maneuvers especially inverted. The AP weighs just over 2 oz's and a few grams, the Golden Bee ( thin wall ) weighs approximately the same but it doesn't require a mount like the AP. Grams matter here due to wing loading. Most of the  Scientific design planes generally don't fair well using Cox engines unless one shortens the nose or uses a product engine and mount a tank on the CG behind it. When I spoke to Walt, he said almost every design he made had it's wingspan clipped due to getting more wood from stock sheet sizes. It goes like stink flying level but doesn't maneuver well due to the CG shift forward. It doesn't glide but comes down like a stone when the power cuts. The engine used for most of these were OK's. The .049 weighs 1.5 oz's and it's a little over a 1/2" shorter.
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Post  Cribbs74 Fri Aug 02, 2019 2:12 am

The 4-2-4. I agree with Ken it’s a myth as it’s not actually going from a 2 to 4 stroke. It’s a sound the engine makes when going rich to lean. Personally I like it as it slows things down for me, but as Ken explained more modern engines really prefer a wet two or in reality a slightly leaner run.

The Cox will do a rich/lean run, but it kills power on level laps and is not really effective.

As to your engine I would default to what Ken wrote.


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