Manufacturing/design flaws

[Konrad]

I've been looking for an F3F trainer to learn how to fly the course and maybe something to fly at the ISR. [Read; pass (I hope) and not get mid aired]. Now I've noticed a persistent theme to all the used gliders I've been looking at. That is that all with huge wing joiner going through the fuselage have major cracks in the fuselage receptacle for the joiner.

I find it rather puzzling as to why the OEMs, some of them well respected, are making these receptacles with sharp corners. Sharp corners are known as force concentrator. Just what we don’t want in a application subjected to high torsional loads and leverage under normal use.

I'd have thought that since this feature is manufactured by the OEM laying up the fuselage that a corner relief radius could easily have been added during the layup. I would also think that this receptacle would benefit from the addition of Kevlar around the receptacle opening during the lay up, as it is tough!

Anybody know why the fuselage receptacles are the way they are?

 

[2walla]

They are made that way for simplicity and for the most part none were designed and or built by engineers

 

[Motowncali]

They are built and designed to fly, not crash and be beaten to death with fair or poor landings...

 

[Konrad]

They are made that way for simplicity and for the most part none were designed and or built by engineers

That's a damning statement!
I don't agree with it, as I know many of these are designed by engineers.

The product should withstand the environment in which it will be used. Landing is one of the requirements. Take offs are optional but landings are a requirement. By my understanding of the molding process it should add little complication to change the feature to eliminate these stress risers.

Every used glider I've looked at that has rectangle wing joiners has these cracks in the fuselage. Most of the gliders actually looked in very good shape other than the fuselage. I'd think some simple changes to the configuration (geometry) will result in much greater durability. Maybe so much, as to allow these glass slippers to actually survive the anticipated loads of landing.

I'd also like to see carbon, or some fiber hoops added to each end of the radio compartment. The wet seam of the fuselage is not adequate. The old taped fuselage seams held up much better with regard to center line splitting.

 

[Wayne]

I have a couple of planes in my fleet that have round joiners with dihedral. They are a trade off, no stress risers since the corners are gone, but the are a general pain to get the joiner lined up correctly, especially the plane that has 2 round joiners both with dihedral. What a nightmare for wing joining.

The rectangle gives the best spar strength and they are easy to fit ballast. The best solution might be an oval shape, but I have never seen one used. Or round with no dihedral in the joiner. This is what my big ASW has, a single round joiner that is straight. The pockets in the wing set the dihedral angle.

 

[Konrad]

Kinked joiner rods, what a nightmare!

This too is my experience, none of the round joiners show these stress cracks in the fuselages of used gliders. Some might be torn out but as Motowncali said these ships aren't crash proof.

To my way of thinking the issue with the rectangle joiner could easily be fixed by pressing pins into the fuselage mold at the corners. No need to remanufacture molds nor change the joiner. Heck a radiused box might even make it easier to layup the fuselage. As it is now, it is rather difficult to get fibers to flow around sharp corners.

 

[thesteam]

Hi Konrad
Would it be OK to modify the fuse by rounding out the corners after the plane has been built? Also applying some type of reinforcement?

 

[Konrad]

That's a broad question. If the joiner socket is a thin walled design, like the RCRCM Strega (yellow) then I see no reason not to radius the corners. It the joiner socket is a deep walled design like the FVK Brisk (white) I'd look at adding some structure after rounding off the corner try to maintain the integrity of the deep socket.

All the best,
Konrad