The recent letters on fixed pitch props noted that metal props are

more

efficient at the tips because they can have thinner sections.
Flying thru rain favors metal props as well.
What would be involved in making an aluminum prop?
I see the pre WWII props were sometimes twisted from a comparatively

thin

piece of stock, then profiled thinner toward the tips.  Today props

appear to be

aluminum forgings.
Could a piece of aluminum bar stock be CNC milled, then hand

finished at a

competative price?
What effects determine the alloy and heat treat of the bar stock?

(toughness

vs hardness etc.)
What harmonic frequencies would be an issue, and how do you tune the

prop to

avoid them?
What other questions should be asked?

The problem with aluminum is the low fatigue life.
Back in WW II they made hollow props out of pressed sheet steel brazed
or welded at leading and trailing edge. Heat treated 4130 steel sheet
has four or five times the fatigue stress limit as aluminum.
Some go as far to say that aluminum will eventually fracture no
matter how low the stress if it is allowed to flex.

I think the best material to use is carbon fiber.

Paul Lamar

American Flyer wrote:

I have wondered about this same subject. In fact, I intend to play
with making an aluminum prop using CNC. GA Planes have been using
aluminum props for at least 50 years. They do occasionally crack,
 but more often go ten thousand hours or more without incident.
Carbon fiber may very well be superior, but can you machine it
on a mill? As a point of reference for cost, the raw material to make
an aluminum prop to fit a Cessna 172 would be in the neighborhood of

$150.

Harrison Young

NC'd billet is not as good as forged as far as fatigue strength is

concerned.

On the other hand it might be OK as the rotary has a much less severe
torque pulse than a O-320 or O-360 four cylinder. Unlike the four cylinder
the rotary torque pulse does not go negative and it has the benefit
of passing though Tracy's torque damper.

It takes a long time to mill out a 3D shape particularly if you want a
smooth finish. If you were paying a legitimate NC shop to do it I would
guess thousands of dollars. Perhaps the appropriate use of the NC mill
would be to make female molds for carbon fiber props. My next door hangar
neighbor John Lynch used to use his 3D router to make wood cores for

Klause's

hand covered and finished  carbon fiber props. Unfortunately Klause no

longer

offers these props.

That reminds me Perry, as soon as you get a real good design I could do
a female mold in aluminum if you wish.

Paul Lamar

I would like to add a resounding DO NOT DO IT to making aluminum props.
They have been actually out lawed in the F -1 planes for good reasons.
Aluminum has low damping and low endurance limits, and even the pro's
have serious problems with many configurations. Wood and composite,
and combinations have a much safer and happier future - Vance

 
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