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Both Al and I had been flying swept forward trailing edges (and double forkin'flap horns) for many years without problems that seem to have given Keith and other builders the soft "flappity-n'-flexity' flap issues.
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Hi Don,
I am not sure if you are referring to me in the above. However, I do not have nor do I believe I have ever said that I have "flapity-n'-flexity flap issues" with my Bearcat. I have often explained that flaps that flex excessively are to be avoided like the plague. This results from a flexing flap horn (because of too small wire or too soft wire for the horn) or what can result if there are two horns.
Al abandoned the split horn approach some time ago. Yes, his published Bearcat plans show two horns but he does not do that anymore. Even with his later Mustangs that have more dihedral and hinge line sweep than the Bearcat, he uses a single horn with the hinges lashed to the trailing edge of the separate TE's, using the flex in the wire to act almost like a universal. The horn busings are not on the same center line. The horn wire is not bent, but flexes between the bushings. You would think that this would cause excessive drag on the controls. Yes, they are not completely "loose" but the controls are free. It is surprising and you have to see it to understand how well it works.
On my Bearcat, as any of my semi scale models with dihedral/flap hinge line sweep, I now use a single horn with the horn bushings on the same centerline. (In other words, the horn wire is straight.) With minimal dihedral/hinge line sweep, this works. This has also been used by Bill Werwage on several of his designs with swept forward flaps. One would think that there would be a bind in the control system as soon as the flaps deflect up or down. zi think this works because there is enough flex in the flap hinges and in the horn bushings and in the wood holding the horn and in the horn itself to allow complete freedom of flap movement. In this system, the horn wire hinge line needs to be carefully located such that it aligns with the flap hinge lines where the flap horn elbows go into the flaps. The horn hinge line must accommodate for the dihedral and/or flap hinge line sweep in this manner.
Split horns, no matter how complex the system is to interconnect the two will result in more flex between the two flaps than is necessary. Any flex is undesirable. More flex than necessary is to be avoided. In cases with extreme dihedral and/or hinge line sweep, use lucky boxes.
Hope this makes sense. If what I tried to explain above does not come across clearly, let me know and I will try again.
Keith