On Mon, 15 Dec 2008 18:35:28 -0800 (PST), "
wrote:



Good discussion. Of course, most of it does not apply to Joe Newbie
hacking out his first prop :-)

But it does a good job of explaining what goes in to a well engineered
propeller. What it DOESN'T do is tell the newbie that the efficiency
of his prop will be within a few percentage points of a professionally-
made wooden propeller. Which means Joe Newbie's efforts WILL fly the
plane. And that's the message I'd like to get across.

snip

The most correct answer is that you will probably need THREE
propellers to find the one that most closely matched YOUR combination
of engine and air frame. Yeah, I know: It's built EXACTLY according
to the plans. Good for you (seriously). But there are variations
even among commercially built airplanes. With commercially built
engines such as an O-200, driving a commercially built METAL
propeller, these variations are quite small: One Cub was virtually
identical to another. But there ARE variations and it doesn't matter
if your name is Boeing, Douglas or Lockheed ( indeed, there are some
heroic examples which, unbelievable as they might appear, are flying
the world's skies ). If you had to BUY two -- or more -- commercially
built props it can cost you up to $500 to nail down the 'perfect'
prop.


snipped so that I can comment on these points.

a well made newbie prop can deliver exactly the same performance as a
professionally carved prop. after all they were newbies once
themselves.

my experience relates to a wooden prop on an O-200 powered tailwind,
but it is still relevant to a vw prop although it turns the opposite
way.

I have been refinishing my prop and balancing it for years so I have
experience with paint layer variations in the shape of the one prop.
Over the polyurethane varnish I use the cheapest aerosol paint can
lacquer that I can find. it dries quickly, is easy to apply smoothly
and .... when it is all chipped and scuffed it wipes of with a rag
soaked in either MEK or Acetone.

tiny variations in the paint surface can have noticeable effects on
cruise speed.
when I inherited the prop as a new owner it was all daggy and glass
resin runs. straight and level cruise was 110knots.
when I cleaned off all the dags and sanded the surfaces smooth the
straight and level cruise was 115knots.
with some other fuselage changes I now cruise at 120knots reliably.

on one repaint I used lots of coats of paint around the leading edge.
about 65 coats on one blade I recall.
the paint didnt last too long before chips were eroding the finish but
while it was pristine I achieved 124knots cruise.

try as I might I cant get the shape again. best I can get usually is
121 knots in pristine condition.

the point here is that making a family of props all subtly varied from
each other can find you the crackerjack best prop for your aircraft.
in australia a prop is typically $3200 so making them yourself can
make the exotic quite affordable.

here is a trick for sizing the prop. make it an inch overlength.
in straight and level flight you should not be able to hit redline
with full throttle.
trim 5mm from each end and rebalance and refinish.
fly it again and the top rpm will be slightly higher.
when the revs come just up to redline rpm with full throttle your prop
is the correct size.

Stealth Pilot