Ive been really following this post with a big interest, ive been tempted to reply many times to other postings, but due to work pressures, a small mobile phone, and on the go, i waited until the next day, that never gave me the chance.
Its been a really fascinating topic, and a topic i have gleaned a lot of academic knowledge from.
Two things that have really made me think, a while back, someone posted the "Brute force and common sense approach" to this sort of engineering problem. ie take an engine that is maybe 25% bigger then the 2 motors used to fly a device like this, make it as best as you can, then sort out the short falls later, this is the brute force approach, and as a 'seasoned' engineer, used to playing with things bigger then this, this was my 'gut' feelings from the start.
The second was the 50 watts per pound to lift a UAV. Thats not an impossible reality, ive seen adverts and data sheets for small alternators that fit inside the nose cone of an RC prop, will generate from 150 watts to over 5Kw, the tiny one will cost you $2500
Somewhere along the line here, i might be mistaken in thinking, we might have missed some of the basics. I think more of the brute force approach, and less of the maths might get a more reasonable result.
Again, from my dim and distant days, when i wasted my time in Uni, and then worked in the Electrical generating field, a good rule of thumb was...expect about 80% efficiency, 85 if you wanted a bonus on your salary! Scaling things down makes the job easier, as you can factor in other quantities as well.
Please dont abandon this topic, im learning as well!
I think you've managed to get at least a few of us behind your concept, Dr. L. I think your brute force method may end up working, albeit with some trouble at first. What are your ideas for the airframe? Also, does it necessarily have to be a UAV to study the hybrid angle?
Im on very much a learning curve here, my RC world experience is very limited, but engineering very diverse.
This story started over a year ago in Indonesia, after wrecking a 450 class heli, many times, it evolved into a single coaxial UAV and an interesting learning curve.
When i adopt the brute force thinking methods, and gut feel engineering principles, i look at a small heli, 450 class (and bigger) with an internal combustion engine. Now add a second coaxial set of rotors, the power delivered from the engine will need to twice the amount for double the payload (im still talking gut feeling here, and not mathematics) now for ease of control, we want to control the 2 sets of rotors by motor. We will keep it inefficient but simple, DC brushed motors and we need to feed these from a DC source. Batteries, no matter how good, wont give you the duration, so it has to come from a generator driven by an internal combustion engine. Add in a poor efficiency conversion factor for the power train, lets be generous, and say the best efficiency we can convert from an IC engine is going to be around 65% the 50 watts energy required, to lift 1 lb, and with a UAV of say 20+ lbs you are heading towards the 2Kw mark, there are many small IC engines which will deliver this. (theoretically)
One of the biggest payloads is the alternator/generator followed by the fuel.
At the moment, the airframe is undecided, very likely to be aluminium to try the idea's out, and see what sort of lift/payload we can realistically generate. If we have enough margin in payload, then maybe look along the monocoque shell construction using carbon fibre.
If im wrong with any of my assumptions, or gut feelings, let me know, im always happy to have a re-think.
100 watts per pound is the generally accepted number that works well. Yes you can do it for less but wing loading will have to reduce somewhat.
What are these brushed motors of which you speak? Perhaps I heard they powered the ark?
You can get twin motors on the same shaft axis so an electric power solution is out there. They are just expensive
So, I think you have a decent idea now of what hardware is available and feasible. My best advice, as a fellow engineer, is to take the time proven approach to designing something like this: Define your problem thoroughly (pretty much already done); Define your constraints (budget, environment, size, etc.); Decide on the components that you will be sourcing, rather than making (i.e. engines, motors, etc.); Finally, design your problem's solution around the constraints present both from external (money, environment) and internal (pre selected components) factors. I'd suggest picking an engine first, and then working your way down the list. As always, we're here to help if you hit any snags along the way.