Has anyone ever successfully converted a recumbent bicycle or tricycle to hand-powered? How? What are the major obstacles to overcome?
Do you want this as an able-bodied person to play with, or as a disabled person because your legs don't work? If the latter, (two wheeled) bicycles are out of the question unless you're young, fit, and slightly insane (I say this because it's certain you will fall off a two-wheeler, probably more than once, and if the guy I watched/helped is any guide, you are likely to also fall off at speed at least once).
The problems differ based on the type of bike or trike. The most suitable IMO for conversion are SWB ASS bikes (short wheelbase, above seat steering) and trikes. The "hand-pedal" part is the same in all cases. Look at any handcycle, but essentially you mount standard handgrips on bicycle pedal axles, then add brake and gear levers. The cables from those need to be long as they flex as you pedal. The pedals mount to standard bicycle chainring(s) and a standard "bottom bracket" (even though it's not at the bottom any more). That bottom bracket mounts to the top of the steerer tube and the chain runs to a standard bicycle rear wheel.
The trike will be easier to ride and harder to fall off, especially up hills. Handcycles go up hills very slowly. Most handcycles for wheelchair users are trikes, normally delta trikes (often with with negative trail). Negative trail makes them easy to build, hard to ride, and unstable at speed. But since you're already looking at a few thousand dollars, easy to build helps to keep the cost down. The more expensive ones are usually better designed for speed (however I have been very unimpressed with the ones I've seen from a rideability point of view). The conversion can be very easy - you "just" build an arm with a wheel at one end, pedals at the other, and a steering pivot in the middle. You can even buy those premade as wheelchair accessories (http://www.riomobility.com/en/manualhandcycle/index.htm). If you can do that, scratch building the seat/rear wheels shouldn't be too hard. "Conversion" now means "chopping up random bikes to get bits" rather than "add hand-pedals".
But very few foot-pedal trikes are delta trikes, and the ones that are are slow. That's because deltas are inherently unstable, especially when braking and turning (very common in racing or in an emergency). A second hand recumbent tadpole is also likely to be easier to find. Search for "recumbent trike" and you will find a lot of tadpoles, few deltas. Converting a tadpole you'll need to insert a bicycle headset into the frame in front of the seat/behind the cross-member, extend that up, then build your pedal/steering unit on top. The chain will twist when you steer, but that's ok (world record setting speedbikes commonly do this). The chain goes to the rear wheel, and you'll need two idler cogs to bend it up to your cranks. http://www.TriSled.com.au in Australia have built at least one of these but it was very expensive (take a $4000 trike, add a couple of weeks work to convert it to a handcycle). It did, however, work very well :)
Converting a SWB bike is likewise fairly easy. Remove the existing stem and handlebars, build your hand-pedal setup to plug into/onto the existing steerer tube, weld on a couple of idler cogs, and you're done. Don't paint it yet! First try riding it. You will fall off, and may decide to revise how the chain idlers work. The first mount you build may well not be strong enough, especially.
In both cases you will probably find that having the pedal unit pivot forward makes getting on and off much easier.
One other option is a rowing bike - Thys make them commercially, and apparently they're not bad to ride (but they definitely work your legs as well as your arms)
2 wheeled handcycles are not an impossible thing. They have been made in the past successfully using landing gear on a linkage. It's basically folding training wheels that allow the rider to get up to speed then pull them up. They are not for emergency deployment and must be planned in advance according to riders I have spoken with.
Handcycles are not slow, some riders are though. In rolling terrain a handcycle can do very well as they scream downhill and keep their momentum due to superior aeordynamics. Climbing is similar to a tandem bike in my experience as I have enough mile on both machines to compare.
Yes you can make a SWB recumbent a handcycle. I suggest a Rans Rocket so you can cut the front boom off and clamp it on the stem. Front wheel drive is the only way to go to keep things simple. This is a project I am currently working on which is why I found this.
I am a person with a spinal cord injury and some say somewhat insane, I had a two-wheeled handbike and rode it many hundreds (thousands) of miles for many years before foolishly selling it thinking I was going to build a better one which I have never gotten around to.
In any case a few things. Mine was custom fabricated from the wheels up. So it had some bugs. But it worked great. And yes I was clocked at well over 40mph on the thing and I have a few scars to remember the times I fell off the thing, but it was worth every second.
Gotta have landing gear on both sides.
Only forget to put landing gear down ONCE :}
Still trying to figure out how to drive rear wheel on steep grades I could spin the front wheel due to lack of traction. I was riding in the hills with everything I owned hanging off the front of the bike to get more weight on the front wheel.
Found out you can't down shift on a steep grade with standard shifters, gotta keep both hands on crank to go straight. Newer battery operated shifters would be priceless.
Cables break at the most inconvienent time: always carry spares.
Shoulders were NEVER meant to be used as hips and you will feel it for sure.
Make sure you have a flag on the thing; idiots will run you off the road.
I had an internal Sturmey-Archer hub gearbox/brake on the front wheel, back pedal and get front wheel brakes. Had three speeds internal. Then I had 10 speeds on hubs so total 30speed, much duplication. Low was a real grandmother for climbing hills and sometimes if you ride cross country you will need it, now I would have a hubmotor to help. Rear wheel brakes were standard caliper and the handle was mounted under the seat.. quite convienent
crank handles have to be at same point can't be reciprocal if you want to steer but kind of obvious.
Any questions, not sure how often I will check back here. I really don't have any decent photos of it scanned so I could put them on line but is pretty similar to those already posted.