An external support bearing would of course be the best but would add some weight. Done really well it might be as little as 10 lbs.
Lynn may be right about the soft counterweight, I don't know how hard it is. I was going on gut feel and comparing it to the incredibly flakey prop hub to crank setups that I've seen on VWs. Compared to that, a DD rotary is pretty stout. Making a hardened steel counterweight wouldn't be too hard either.
I also wonder about the comparative moment of inertia between the manual trans flywheel (really heavy) and the short prop of a DD rotary. Again just gut feel but I'm guessing that a race car cornering hard may impose as much gyroscopic loads on the crank taper as a short prop.
As for efficiency, remember that prop efficiency is relative to the VOLUME of air per HP, not the diameter. That is why a faster airplane is the prefered application for DD engines. Probably the most efficient homebuilt I ever saw was Charlie Airsman's VariEZ with a DD engine and a short prop. He cruised at 200 mph while burning 4 GPH. BTW, he's building a Cozy with a rotary now.
A long prop going slow and a short prop going fast have the same efficiency. If it were purely a matter of length, model airplanes would not fly.
Tracy
On Tue, Mar 11, 2008 at 3:12 AM, George Lendich < lendich@optusnet.com.au> wrote:
The thrust bearing would need to be a multiple layer plain bearing for thrust and drag. A doubled (at least thrust plate). A bending load on the taper of the crank would soon have you driving the prop with just the alignment key, as the counter weight is quite soft and the taper would let go. An outboard bearing would eliminate that problem...........
And provide a place for some real thrust bearings eliminating the plain bearings. In fact you could use one of Tracy's reduction units without the guts. Then you can put the guts in the redrive. And test back to back outcomes. I saw some huge ducted fan models last weekend going like rockets. Motors turning up 50,000 RPM.
I think it will work.
Lynn E. Hanover
Lynn,
That would be my take on it as well, a properly engineered shaft and bearings without the planetary. Although it would be a pretty small prop running at 3,000 to 5,000 RPM. The efficiency of the prop would be well down as well as the HP and Torque. A PP with high compression rotor would off-set some losses.
Anything can be done.
George (down under)
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