Bobby answered the "how much HP" question with the HP chart

 

Thanks for the chart Bobby. 

 

  - Looked like 112 HP at 4000 to me which is the RPM I would target.  That was also with a stock manifold.  With an intake manifold optimized for 4000 and adding a few ponies for a less restrictive exhaust system, I think 120 is do-able.  And I think it could come in at least 50 lbs lighter than a Lyc O - 320.  Another 5 or more pounds off the prop too.  And that's normally aspirated.  Turboing to 150 HP would be conservative.

 

Only down side is the ~ 56 - 58" prop you would have to use.  That would cost some takeoff & climb performance. 

 

This is all interesting, but I still think the geared single rotor is a better choice for the 100 HP range.  It will definitely be lighter than the direct drive two rotor, and will allow standard aircraft prop RPM's.  It too can be turboed to 150 HP with ease if that's what you need.  There's only that nasty torque reversal to deal with, but certainly that can be worked out.  Once above 3000 rpm, it's smooth as silk assuming proper balancing.  

 

When thinking about the weight, don't forget that you don't give up the full weight of the redrive.  You will still have to have an external thrust bearing and prop hub, as well as a way to hold the starter.  I'm guessing you might only save 30 lbs over the weight of a redrive, but you'll be losing half the power.  Doesn't seem like a very good deal to me.  

 

Tracy  (why the hell am I talking people out of gear drives :>) 

 

I'm not sure you "have" talked anyone out of a gear drive yet :-)

 

Rusty (thawing in the hotel)

 

Good points Rusty, it's probably all wrong for you but humor me while I ponder the problem.

  Part of the D-D appeal is my Scottish tendancies.  As counter intuitive as it sounds, a single rotor is a lot more expensive to build than a 2 rotor.  That, along with the redrive is at least a $5000 advantage for D-D right off the bat.  

Thrust bearing is a real consideration but not a show stopper.  I'm thinking along the lines of a 3rd gen or Renesis thrust bearing (larger than the 2nd gen part)  and bolting the prop with a short extension directly to the counterweight.  The rotary's crankshaft is about the only one I would consider doing this with.  This would still mean changing the thrust bearing every 500 hours or so.   Not very expensive and you don't have to crack the engine core to do it. Remember that the thrust of the Ross drive input shaft was many times higher than the prop thrust.   Mounting the starter drive on the other end of the engine would be an extra expense but not that big a deal.  I might even consider hand proping this setup to eliminate the weight.