Found this on another forum it might help
The suggestion by Futaba to stand the rx on its side has the effect of almost doubling the surface area for convective heat dissipation. So any rx generated heat can get away quickly rather than add to any other heat being absorbed.
The components that are getting over heated are (I am told) on the underside of the rx so this also gives a better route for the heat to escape rather than having to migrate up through the upper PCB and out through the top.
Read more: http://www.rcuniverse.com/forum/m_7807297/mpage_5/key_/tm.htm#ixzz0leacHEeh