Even Tracy and Finn got higher RPM's with carburetors, so I highly
     > >> suspect the prop. The
     > >> rpm's max out at just beyond 1/2 throttle setting, and advancing the
     > >> throttle further yields no increase in rpm's. The engine still runs
     > >> smoothly, both at the 1/2 throttle and WOT positions....just
     > >> maxes out the rpm's at 4800 to 5000.

Paul:

Congratulations on being safe, airborne and COOL!!  What with being COOL in the coolant and oil temperatures, you are way ahead of most.  When my friend had his RV-4 w/ 13B, Ross PSRU, Crook ECM and
was getting only 130 mph he was certain his prop was not correct.  His is a Amar-DeMuth of 68 x 73 pitch.  He could only get 4,200 rpm or so as I recall.  After fairing the wheels and gear (15 mph),
changing from a belly board air "scoop" to a P-51 underslung duct (30 mph) he now runs Hi-speed cruise at 5,800+ rpm approx. (185 mph ind.) at our 6,000' MSL pattern altitude. Note: it is a single
seater, so I only get his pilot report after the fact!

Bottom line:  Unfaired gear drag and/or gear "down" consumes big HP.  High drag keeps rpm down with a fixed pitch prop....which keeps HP down.  Yes, it does take HP^3 to increase speed, but with
that fixed pitch prop....rpm and HP will go up really fast as you reduce drag.  After you get 50 hrs. of flying and improvement...especially airframe cleanup and gear up, then think about modifying
your prop.

Note:  In my experience, 78" does seems like lot of pitch for a 13B (at least at 5,000' field elevation here) even on a slick airframe, but each manufacturer's "pitch" designation is highly
dependent upon lots of factors and can vary at least 5% manufacturer to manufacturer.  Van's Aircraft in their newsletter a year or two?? ago, had a very interesting runoff of various props against
each other with very interesting results.  Worth finding a copy and reviewing.

Be safe and make changes slowly and one at a time.  The journey of development turns out to be some of the most fun......and challenge!!

Regards,  Doug Dempsey

 
The Rotary Engine NewsLetter.  Powered by Linux.
ACRE NL web site. http://home.earthlink.net/~rotaryeng/            
Copyright 1998-2004 All world wide rights reserved.