Hi Volfy, from the front view, your thrust line is way off center. I'll be more concerned on that than the downthrust. The chin tunnel is basically to facilitate airflow path over the cylinder head in inverted position. I will be reinforcing my fuse in various areas by using 1/16" maple dowel pinning and ply doubling and good old epoxy, without adding too much weight. I tested the glue used throughout the fuse and wing structures, and I'm sure it's hot glue, just like my other GP ARFs. Put your finger thru the firewall hole and press on the glue fillet with your fingernail, and you'll know it's not epoxy. Good luck........