I was reading Tracy's web site and he made a couple of statements
I feel the need to comment on.

Tracy wrote:
"Other conclusions are that anything less than 74" L prop is a waste on the -C drive. 
 More would be better but that's as long as the [RV] -4 prop clearance would allow."  

I disagree with this. If you want lots of static thrust for a bush plane or sea plane
yes a longer prop is the way to go. If you want better top speed you can always add 
blades or width to the existing blade length to absorb the increased HP as a result
of the higher engine RPM  and there will be less transonic drag rise on the prop tips. 
For an extreme example here is a carbon fiber Sensenich direct drive air boat prop 
used with Cadilac 500 ci V8 engines. These props with multiple blades can accommodate 
up to 700 HP.

http://www.sensenichprop.com/sen_html/airboat.html

Tracy wrote:
"The prop limitation was always my only reservation about using the higher ratio.  
I must emphasize that in order for this to work, the prop must not only be long but 
has to have a good profile as far down toward the root as possible with the MUCH
higher pitch.  I worked with Clark at Performance Props and emphasized this.  It 
required a thicker than standard hub to get close to what was needed."

"I still have not run the new drive & prop to top speed.  I attempted to yesterday 
evening but the dreaded SAG (Spark plug Attention Getter as Ed calls it) struck as 
I was approaching 220 MPH.   My best guess is that the zero point (same  performance 
as with -B drive [2.17:1] ) is right at this speed.   Things may improve after I get 
the cuffs built (today's project)."
 
Optimum pitch near the hub can also be achieved by using prop cuffs which Tracy 
wound up adding. I consider prop cuffs essential for IVO props.
For new cowling designs a larger diameter spinner (19 inch) can be used such as 
found on the Nemesis Nxt. This blocks some of the less than optimum pitch
area of the blades.

Paul Lamar
 
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