Hummmm … when I see the military
planes with props like the E2C Hawkeye with more than 7 blades I have to think
more is better for some. When the hp is high and clearance is tight, more
blades is obviously good. Same for our lawndarts – so why not an 8 blade
for the LNC2? It’d be short likely - $$$$. No need to be that short.
The Navy’s stuff:
“In 2004, the E-2C's
propeller system was changed; a new eight-bladed propeller system named NP2000
was developed by the Hamilton-Sundstrand company to replace the old
four-bladed design. Improvements included better
fuel economy as a result of increased efficiency, reduced vibrations
and better maintainability as a result of the ability to remove prop blades individually
instead of having to remove the entire prop and hub assembly.”
http://en.wikipedia.org/wiki/E-2_Hawkeye
Increased efficiency? Who needs that?
Petrol is only $7/gallon. Cheap if you say it fast.
Now what to believe is best for your
rocketship or which is most efficient, effective, I dunno cause I ain’t
no aerodynamicist or “cool” appraiser. Over my pea head. I paddle
when things get tough, flap and other stuff.
Jim
From: Lancair Mailing
List [mailto:lml@lancaironline.net] On Behalf
Of Sky2high@aol.com
Sent: Monday, June 30, 2014 10:14
AM
To: Lancair Mailing List
Subject: [LML] Re: Two-blade or
three-blade prop for I-550
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.
PS Computations left to the reader and
EXCEL.
In a message dated 6/30/2014 9:39:14 A.M.
Central Daylight Time, stevens5@swiftdsl.com.au writes:
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.