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
I made a few calculations for different static RPM's.
A worst case scenario is the prop pitch is 106 inches
or a blade angle of 25 degrees and the prop RPM does
not increase at 60 MPH. In the real world the prop
RPM will probably increase slighty and these angles of
attack will become slightly greater. The prop is still
probably stalled at 15 degrees but it is well on it's way
to becoming unstalled. However if the prop RPM does increase
more power will be put into the prop and probably the
thrust will increase slightly over the static value
at 60 MPH.
These are the angles of attack of a 2/3 prop radii
segment at 60 MPH. Seems reasonable to me. Check my trig.
I could be wrong.
Vance if you have some other numbers I would be glad to see them.
*******************************************************************
Six foot dia. Pitch 106". 2/3rd (2'r) blade angle = 25 degrees.
RPM Tang Vel FPS Blade angle at 60 MPH (88 FPS)
1500 300 9
1600 320 10
1700 340 10
1800 360 11
1900 380 12
2000 400 13
2100 420 13
2200 440 14
2300 460 14
2400 480 15
2500 500 15
Paul Lamar
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