I have problems when putting on undercarriages on most of my models, the always bend outward, is there a way to remedy this?

You could always make a jig from heavy cardboard to hold your model stationary while the glue cures.

Usually when I attach the undercarriage I'll turn the plane upside down and hold it level with a few paint bottle under the wings and nose.

I'll only attach one leg at a time and hold it in position until the glue has dried enough to stop it moving around.

It's always a good idea to give it plenty of time before up-righting the plane and putting pressure on the undercarriage or it will splay out.

plenty of time to cure means at least overnight.

I use a Lego jig and may attach upside down or right way up depending which looks easiest to me. Agree with overnight setting, don't rush it

Thanks for the advice! I normally let it dry overnight, But those are really good ideas!

super glue! always helps