Hi, I am building a 4o size twin trainer called a Gemini from plans. Its fairly big with a 70" wing and around 900 sq. inches. The plans call for nylon wing mounting bolts. I am wondering if this would be stronger than a 1/4 ply piece going across the fuselage with the typical blind nuts. It would probably be easier to mount but I am not concerned about that. Any thoughts?? Below is a pic of the plane and the mounting parts that is on the plans.

Not familiar with the design.  Hopefully it has a solid dowel(s) socket forward and the wing bolts just keep the wing from pivioting up.  If this is so the Nylon bolts are fine (just don't overtighten them and weaken them).  I have two Nylon bolts holding the wing on a 15-1/2 pound sport model with no problem.

Yes, the wing has 2 dowels in the front and the bolts are for the back of the wing. I am just not sure if the nylon mounting is stronger than than a ply piece across the fuselage.

The weakest point should be the nylon bolts so that they shear before pulling the guts out of the fuse. They will stretch when you apply torque to tighten the wing. They will snap if you carwheel the plane or plant a wing (I personally know of such things [:'(]).

I'm with you in your thinking that reinforced (with tri-stock) 1/4" ply with embedded 1/4x20 blind nuts epoxied will be more secure. You gotta decide if you can do a competent "upgrade" given the design of the plane's guts. If you can't then the nylon threaded bushings are as good as it gets.

I think that you'd have nightmares of stripping the threads out of those lil black thingies ...

How far along are you in the kit? Have you heard of "kit bashing"? See attached kit bash concept for your plane.

Hi Mr67Stang, As stated in my original post, there is only a set of plans from RCM, there is no kit. I have decided to go with the ply and blind nuts mounting. I have the wing shapes built. That is the ribs were drawn out, cut, and glued to the spars for both wing halves. The fuselage sides and wing doublers have been cut out as well as the 5 formers. I have the vertical stabilizer, horizontal stabilizer, rudder, and elevator cut out. There are 2 marks on the wing doublers for mounting holes. I checked to see what the holes are for. They are for the steel bolts to mount the nylon bits. I haven't seen these before and that's why I was asking about them. I also built a 60 size Cloud Dancer from plans a few years ago so I have some experience.

Hi Fastsky long time no see. Cool to see you are doing some multi stuff.

The manufacturered plastic type wing hold downs are fine in themselves, the theoretical weakpoints should of course be the nylon bolts but another posible weakpoint with those could be the screws into whatever structure you attach them to. This in much the same way that the threaded (or 'T' nutted plywood fuselage cross plate often has a weak point where it is epoxied to the fuselage sides.

The real bottom line is either system could work just fine and either can be compromised with poor workmanship so it is really up to you.

Now just another point and it is a hard fact that no one considers. This wing to fuselage joint in a multi engine airplane where the engines are wing mounted actually receives less stress than a conventional single engine airplane of a similar size and weight. The greatest percentage of aircrafts total weight is already in the wing and not transferred throught this fuselage to wing joint. This I pointed out to allow you to be a bit more comfortable and it does not of course to mean you can be lax with the building practices.

Perfect illustrations of this are my Quad Kaydets, 12.5 pounds or 15.5 on floats virtually all this weight is in or on the wing with the fuselage being even lighter than the the stock ARF Kaydet fuselage. Even more extreme example of this is my Six Pack Telemaster.

Just some food for thought, Cool twin

John

Hi John, You are right and I should have considered that. Thankx for the insight!