Colyn,
As the 2 blade prop reaches horizontal, the descending blade has a
higher AOA and the descending blade has a lower AOA with respect to the relative
airflow. In climb there are two airflow vectors to consider - vertical
relative to climb rate and horizontal relative to forward speed. The
higher AOA creates more lift - that is why you hold right rudder in the climb
with a clockwise prop rotation.
With a three blade prop and when one blade is descending through the
horizontal, the other two are ascending, not at the opposing
horizontal minimum AOA. Thus, the thrust is more even, the blades are
usually shorter and the tip vortice induced drag may be less because of reduced
tip speed. When a blade is ascending and at the horizontal, the
other two are descending, but not at max lift AOA.
It seems that 3 blades are smoother and a good match for 6 cylinder engines
when the prop is properly indexed. That is the engine power pulses are
more even and the three blade lift curve is also smoother - even in
cruise. With modern prop airfoils, the loss in cruise may be very
small.
Now you can think about 4 or 5 blades in climb and perhaps eight or twelve
cylinders or even two rows of 9 cylinders in a radial.
Grayhawk
Grayhawk, could you please expand on that climb performance
argument a little?
Here is more to think about (rather than just efficiency).
Blades > 2 = better climb performance - consider the relative air
(AOA) to the prop chord for both the ascending and descending blade
for a 2 blade versus longer arcs, better bites for more than 2
blades. Don't confuse this with level flight where all blades
see the same AOA.
Blades > 2 can produce the same thrust as Blades = 2 but the prop
diameter for more blades can be smaller, thus allowing for higher rpm whilst
still avoiding the tips going supersonic. I.E. The further the tip from
the hub the faster the tip is moving at a fixed rpm.
Momentarily consider the weird 2-blade Hartzell CS prop for the 320
- an 84 inch diameter prop cut down to 70 inches. Most props
deliver max thrust about 2/3 out from the hub. What did that mean for
the enormous chord and pitch for that prop?
Finally, consider the corkscrew path of each blade tip and its
path separation (interference) based on airspeed. You'll be
surprised - odds are the bird will hit the windshield and not a prop blade at
cruise speed.
Hmmmm.....
Grayhawk
PS Computations left to the reader and EXCEL.
I
am interested in this subject, because I purchased a partly built kit some
years ago, (which I am still building!) which came with an MT 3 bladed
constant speed prop. In my conversation with a builder in South Australia
recently, he mentioned that he had swapped out his 3 Blade MT prop for a 2
blade prop, and increase his cruise speed by 7 knots. (Can’t recall if that
was 7 Kts indicated, or 7 Kts TAS.) Not sure how this fits with the graphs
which indicate the 3 blades are more efficient! It was my understanding that
because a 3 bladed prop generates 3 tip vortices against only two on a 2
bladed prop, it is less efficient.
Rob
Stevens
Perth,
Western Australia.
From:
Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
Charles Brown
Sent: Monday, 30 June 2014 7:38 PM
To:
Lancair Mailing List
Subject: [LML] Re: Two-blade or three-blade
prop for I-550
I've enclosed the spreadsheet provided to me by Les Doud
of Hartzell. In 2009 his phone number was Phone:
937-778-4262 . He believes that the 3-blade is more
effiicient than the two blade even in cruise. I have a hard time
believing that, and my airplane with 3 blades has not lived up to the
prototype Legacy's performance with the 2-blade as documented in the CAFE
report; and the 3-blade is heavier and more expensive to buy and overhaul.
Of course, my airplane is probably not as clean as the
prototype.
But the 3-blade prop sure looks
cool.
=