Trying to hang a porch swing but the overhead structure isnt conducive. Joists are all 2×6 that run perpendicular to the direction I need to hang the swing. I’m thinking of putting a couple 4x4s in between the joists and drilling into the 4x4s from below. But Im concerned about ability of the 2x6s to bear the weight. Any thoughts on using face mounting joist hangers to hang a 4×4 (and in-turn the swing)? And the best way to do it. House is 98 yrs old so I dont want to screw anything up. Attached picture shows the overheas structure of the porch. The red circles are the two points where I need to drill in from below. Thanks for any assistance. 
Porch swing Supports
hangingjoists
Related Solutions
The problem with the pipe idea is that unless you're going to run it at an angle, you'd never be able to get a pipe through several consecutive rafters.
You may actually be OK with lag screws - a properly installed 3/8 lag screw should provide an enormous amount of withdrawal resistance. Here's a sample chart from Masco:1
I wouldn't be horribly concerned about the roof members, as long as you are attaching where they have full thickness and minimize the distance of the cantilever.
Your most likely points of failure are going to be the chains (see this answer), or the hardware on the swing itself. In general, working with rated fasteners installed they way they were intended to be used is a better idea than "winging it".
Note that I have no idea of the condition or construction quality of where you intend to hang your swing to, and I am not a structural engineer.
1 Not to be taken as a product endorsement or as representative of other similar products. You will always want to check the ratings of the hardware that you actually intend to install.
According to the table in the 2012 IRC code a exterior deck needs to handle a 60lb per sq ft (40 lb live + 20lb dead load). Most treated lumber that I am aware of is #2 southern yellow pine, using that criteria on the table in the code book, your joists can carry a 10'-9" span.
The biggest failure in deck construction has not been the posts or the joists, but the connection to the house. I believe code now requires through bolts into the band joist, or the joist of the house, or posts at the house is cool too.
The length of the post will dictate whether they need to be 4X4 or 4X6. Its a deflection (bowing) thing. IMO 4X4 for a 10' span is not overloading the post, a 16' span may...
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Best Answer
You need a structural analysis that considers the load on the roof, ceiling load, and the proposed swing load. Way too many variables to address here. If you can’t handle the structural analysis, you can do it the old fashioned way. Put a piece of plywood across the ceiling members and add 50 lbs and measure the deflection. Keep adding weight to 1.5 times the weight you plan on the swing. Don’t exceed 3/8” deflection at the center of the room.
My gut tells me that your roof won’t support the load without truss support to spread the load.