If you use a fixed pitch prop on a slow turning out put shaft you
will go from somewhat stalled prop on your take off roll to
hopelessly stalled. Slow props on fast planes almost beg
for the controllable "high priced spread".

I also will state again - that I have not seen dyno data that supports
increased horsepower at 7000 rpm for a modestly modified
13b engine. Vance J

Yes but it will become unstalled at some point in the takeoff run.
Just because it is stalled does not mean it has no thrust.
Besides, you don't have to use full throttle at the beginning of the
takeoff run. Feed the throttle in slowly.

VJ - Stalled does not mean that you have to reduce power - stalled
means that your thrust per horsepower efficiency is in the pits,
the prop bogs the engine down to limit your produced power
and most of that power is being reduced to wasted turbulance.
At this level it will not unstall on the roll, but if you are lucky
and make it over the trees, it will probably start unstalling at
climb speed.

That sounds plausible to the layman but I don't buy it. When wings
stall they don't lose all lift. When props stall they too
don't lose all thrust. Do you have any numbers to back up
this claim. My guess is a prop blade stalled at 45 degrees has
a lift and drag coef. of one.

Von Doenhoff page 3 says a typical wing stalls at a lift coef
of 1.7 and 17 degrees. The lift coef is still 1.5 at 24 degrees
while the L/D goes from about 13 to 5. I would think
the engine would bog long before the blade L/D got down to 5.

If the blade segment that is two foot out on the prop radius
is running at a stalled angle of attack of 24 degrees (pitch angle)
while the airplane is stopped and the prop RPM is 2000 (418 FPS)
it will run at an angle of attack of about 12 degrees at 60 MPH
(88 FPS) and at the same prop RPM. In other words about 12 degrees
angle of attack less than what it had when the aircraft was
stopped and the blade stalled. Check my arithmetic.

No doubt it is inefficient. That is not going to bog the engine
down to a stop.

When I flew with Ian in Perth with his 200 MPH RV6a and his
2.85 gear box I noticed no decrease in take off performance
compared to Tracy's airplane.

Ian how far do you accelerate down the runway before you
break ground. What is your initial climb rate?

BTW Vance were you not the guy that said there was no advantage
to the constant speed prop? :-)

More discussion of your longer message later.
One thing at a time.

Paul Lamar

Me thinks I need to clarify my mission profile.  While I am the first one
to appreciate in flight adjustable pitch, this is a hot rod on a budget (if
an airplane can be such a thing).

The fixed pitch will be a compromise on each end to be sure.  I'm not one
to give up much on the bottom end so will go inefficient on the top.  My
old Warnke prop acted like most all of the Warnke blades do.  It would turn
2350 static then about 200 feet into the takeoff roll it would hook up and
the rpm would drop to 2100, just like every other RV-n with a Warnke prop
I've seen take off.  This is about what I would expect a 150 hp FP RV to
turn at that speed and is not much different than my metal Sensenich.  With
the O-320-E2G rated 150hp@2700 I could get about 2650 max at 185 mph
indicated with the Warnke.  It's a rough RV-4.

My plan for the 13B is to get the same effect on takeoff as the Warnke
prop.  It'll be stalled for a little while but hooked up well before stall
speed of the airframe.  With the engine turning 6000 making about 160 hp,
the prop turning 2100 for a cruise guestimate of say 175 mph.  I'm planning
a 3 blade prop and hoping I can get away with 72" diameter, maybe not.  If
I can turn 1800 on takeoff and get good performance it should be fairly
quiet (with a good muffler of course).

On the top end, I remember what Everet Hatch said about going fast
--"..just spin it faster, you don't need a constant speed prop.".  With the
13B turning 8500 (my own self imposed redline) the prop would be screaming
(literally) at near 3000.  So much for efficiency!  This is just for fun
stuff like the four amateur races that the Rutan And Canard Event
(R.A.C.E.) guys put on every year way out in the desert where no one will
complain about the noise...8-) .  Now, turning a 3 blade 72" prop 3000 rpm
with 220 hp running over 200 mph ... I think I need to go back to one of
those prop spread sheets and try again ...
Mike

Mike McGee, RV-4 N996RV, O320-E2G, Hillsboro, OR
13B in gestation mode

 
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