I have a 91 AC that I'm wanting to do 3D with in either a U Can Do or a Funtana 90. Which would work better an APC 16 X 4 or 17 X 4? I was thinking the 16 X 4 might be a little on the small side.

The 16x4 will probably be about right unless you have an exceptional 91. Then maybe a 16x6.

dear dave,
i have an old ys 91 ach i hardly used in my heli.
can i use it on my u-can-do 90?
have you any idea where to find the manual?
thank you
yoram

Sorry, no manuals here. It should work fine on a Ucando.