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

Most of this argument dovetails with the prop article in the latest Contact
magazine.  It also is very similar to the new blade profiles that Whirlwind
Props are selling for the RVs.  All the logic for the blade planform stacks
up.  It is an unusual looking prop.

It amounts to a very large cord at the root with an extreme taper out to
the very small tip.  Large diameter spinners not required.  Paul
Lipps,  the guy writing the article, has some pretty impressive comparisons
with other props and his incremental performance improvements to his
Lancair 235.  It sounds like he is working with Craig Catto to carve and
layup his design.  He also worked with Klaus Savier on various
projects.  He's a retired engineer/rocket scientist, Vance probably knows
him.   P-)

You can do a lot worse than Catto for a prop maker based on comments I've
gotten from those running his props.  http://www.cattoprops.com/  He has
several running at Reno every year.

It would be really interesting to take the pitch numbers from Tracy's prop
and have one of Lipps' props cut in a three blade version for a 200 hp rotary.

Lipps also seems to know what he's doing in regards to efficient cooling
ducts.  Look how small these are on the nose of his Lancair.  Granted he's
only cooling 123 hp but he's only using 12 sq inches of opening.  That's
another Contact article entirely.

An article in the RVator (Van's Aircraft factory newsletter) a few months
back talked about the philosophy behind the new Whirlwind prop blade
shape.  These props don't look quite as radical as Lipps' profile, but,
they're still unusual looking.  We have a few RVs here in the Portland area
that are running Whirlwind 200RV props. http://www.rv-8.com/Prop.htm  The
new Whirlwind 151 blade profile follows this same cord distribution.

Perry- have you seen these two articles?  I can scan them for you if you
want to read them.  I don't want to publish them to the group without
permission from the publishers first.  The scans would be pretty big to
read all the pages anyway.

If you guys aren't subscribed to Contact magazine I highly recommend it.
http://www.contactmagazine.com/

Well, that got long winded...

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode, RD-1C, EC-2

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