2 vs 3 blade props

[Ted Waltman]

Barry,

So I've read or been told, the 3 blade has more thrust (perhaps as much
as 20%?) for takeoff. Also a 3 blade should give you a lower tip speed,
meaning lower noise footprint. I would think the 2 blade is more
efficient in cruise.

I'm sure there are some aerodynamicists out there who will set me
straight on the above opinions...

On my Moose with the standard Murphy tailwheel, I have the 3 blade
Hartzell prop. I have slightly over 19" of ground-prop clearance.

Regarding maintenance issues, there might be (probably is) a difference
in the # of hours between recommended blade/prop governor overhaul hours
between the 2 blade and 3 blades, particularly between the composite MT
props and the metal Hartzell's.

Ted

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of
Barry Haines
Sent: Wednesday, January 19, 2005 11:14 AM
To: rebel-builders@dcsol.com
Subject: Hello from Ohio & a prop question......


Hello everyone! I have been lurking on this list for awhile now and
have
gleaned a great deal of information - am a low time pilot with a strong
interest in building a Moose in the next 18 months or so (finishing a
Master's degree in nursing at the moment). Trying to imagine what it
must
have been like to build an airplane before the internet!!! The
experiences
and advice shared is probably worth more than the price of the kit??? I
look
forward to someday joining the fraternity of builders.....
A question: What are the differences between spinning a 2 blade vs. 3
blade
prop? Ground clearance?, Performance? Noise? Maintenance? (in terms of
the
M14P on a Moose).......

Thanks,


Barry Haines, SRNA
PS. Thanks again to Scott A. for all the info you sent awhile back ; ))

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[bransom]

Barry, i don't think so, at least not theoritically. I'm no aerodynamicist,
but thrust goes up with diameter, and diameter goes up with fewer blades.
Propellors, like wings, benefit from high aspect ratio. Ever see a short
winged or biplane glider? :) The propellor complication is that the engine
power and desired speed of the plane come into play too. If just the
diameter is increased, you soon run out of ouumph to make it go round and
round (this is the aerodynamicist in me talkin) unless you flatten the pitch
or reinvent physics some other way to reduce drag. Okay, assuming we settle
for flattening pitch on that choice, soon we'll get *lots* of thrust but our
flattened pitch would limit max forward velocity to, oh, say 30 mph before we
red-line the engine. Or, if we don't care about the engine, we go way past
red line, but with that big diameter prop, the tips have by now exceeded
Mach .9 and we're really starting to make as much noise as thrust. BTW, as
for the choice of reinventing physics somehow to reduce drag -- maybe you
could say the experiments with a *single* blade (other side counterbalanced
by a weighted stub) were along that line of thinking. I think it was Steve
Whitman that played with that idea.

Other than all that, I'm clueless wrt what's good for an M14/Moose.
-Ben

Barry,

So I've read or been told, the 3 blade has more thrust (perhaps as much
as 20%?) for takeoff. Also a 3 blade should give you a lower tip speed,
meaning lower noise footprint. I would think the 2 blade is more
efficient in cruise.

I'm sure there are some aerodynamicists out there who will set me
straight on the above opinions...

On my Moose with the standard Murphy tailwheel, I have the 3 blade
Hartzell prop. I have slightly over 19" of ground-prop clearance.

Regarding maintenance issues, there might be (probably is) a difference
in the # of hours between recommended blade/prop governor overhaul hours
between the 2 blade and 3 blades, particularly between the composite MT
props and the metal Hartzell's.

Ted

-----Original Message-----
From: mike.davis@dcsol.com [mailto:mike.davis@dcsol.com] On Behalf Of
Barry Haines
Sent: Wednesday, January 19, 2005 11:14 AM
To: rebel-builders@dcsol.com
Subject: Hello from Ohio & a prop question......


Hello everyone! I have been lurking on this list for awhile now and
have
gleaned a great deal of information - am a low time pilot with a strong
interest in building a Moose in the next 18 months or so (finishing a
Master's degree in nursing at the moment). Trying to imagine what it
must
have been like to build an airplane before the internet!!! The
experiences
and advice shared is probably worth more than the price of the kit??? I
look
forward to someday joining the fraternity of builders.....
A question: What are the differences between spinning a 2 blade vs. 3
blade
prop? Ground clearance?, Performance? Noise? Maintenance? (in terms of
the
M14P on a Moose).......

Thanks,


Barry Haines, SRNA
PS. Thanks again to Scott A. for all the info you sent awhile back ; ))

_________________________________________________________________
Express yourself instantly with MSN Messenger! Download today - it's
FREE!
http://messenger.msn.click-url.com/go/o ... direct/01/




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[Angus McKenzie]

Some early Taylorcraft used a single blade prop on floats and skis. Several
pictures of these planes are in a soft cover book about the Taylor
story........... Angus


----- Original Message -----
From: <bransom@dcsol.com>
To: <rebel-builders@dcsol.com>
Sent: Thursday, January 20, 2005 12:20 PM
Subject: RE: 2 vs 3 blade props

Barry, i don't think so, at least not theoritically. I'm no

aerodynamicist,

but thrust goes up with diameter, and diameter goes up with fewer blades.
Propellors, like wings, benefit from high aspect ratio. Ever see a short
winged or biplane glider? :) The propellor complication is that the engine
power and desired speed of the plane come into play too. If just the
diameter is increased, you soon run out of ouumph to make it go round and
round (this is the aerodynamicist in me talkin) unless you flatten the

pitch

or reinvent physics some other way to reduce drag. Okay, assuming we

settle

for flattening pitch on that choice, soon we'll get *lots* of thrust but

our

flattened pitch would limit max forward velocity to, oh, say 30 mph before

we

red-line the engine. Or, if we don't care about the engine, we go way

past

red line, but with that big diameter prop, the tips have by now exceeded
Mach .9 and we're really starting to make as much noise as thrust. BTW,

as

for the choice of reinventing physics somehow to reduce drag -- maybe you
could say the experiments with a *single* blade (other side

counterbalanced

by a weighted stub) were along that line of thinking. I think it was

Steve

Whitman that played with that idea.

Other than all that, I'm clueless wrt what's good for an M14/Moose.
-Ben

--
Internal Virus Database is out-of-date.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.6.10 - Release Date: 1/10/2005




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[bosdes]

My daddy allus taught me that the most efficient propeller was single blade. That's because it's always biting into clean air, whereas multiple blades disturb the air and degrade efficiency. There is currently an Italian motorglider, the Silent IN, that flies with a counterbalanced single-bladed prop.

My glider club recently changed the two-bladed prop on our L-19 to a four-blader in order to reduce noise. Everyone complains about the reduction in performance. I talked to the Air Cadets in Nova Scotia (the government is converting all their towplanes to four-bladed props) and they had the same complaint.

Ted Hauri 577E

-------------- Original message from "Angus McKenzie" <angus@lweb.net>: --------------

Some early Taylorcraft used a single blade prop on floats and skis. Several
pictures of these planes are in a soft cover book about the Taylor
story........... Angus


----- Original Message -----
From:
To:
Sent: Thursday, January 20, 2005 12:20 PM
Subject: RE: 2 vs 3 blade props

Barry, i don't think so, at least not theoritically. I'm no

aerodynamicist,

but thrust goes up with diameter, and diameter goes up with fewer blades.
Propellors, like wings, benefit from high aspect ratio. Ever see a short
winged or biplane glider? :) The propellor complication is that the engine
power and desired speed of the plane come into play too. If just the
diameter is increased, you soon run out of ouumph to make it go round and
round (this is the aerodynamicist in me talkin) unless you flatten the

pitch

or reinvent physics some other way to reduce drag. Okay, assuming we

settle

for flattening pitch on that choice, soon we'll get *lots* of thrust but

our

flattened pitch would limit max forward velocity to, oh, say 30 mph before

we

red-line the engine. Or, if we don't care about the engine, we go way

past

red line, but with that big diameter prop, the tips have by now exceeded
Mach .9 and we're really starting to make as much noise as thrust. BTW,

as

for the choice of reinventing physics somehow to reduce drag -- maybe you
could say the experiments with a *single* blade (other side

counterbalanced

by a weighted stub) were along that line of thinking. I think it was

Steve

Whitman that played with that idea.

Other than all that, I'm clueless wrt what's good for an M14/Moose.
-Ben

--
Internal Virus Database is out-of-date.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.6.10 - Release Date: 1/10/2005




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[David Ricker]

Just like Ted said, the Nova Scotian L-19s have switched to the to the 4 blade with less performance. They also put a muffler on them for noise reduction.

Isn't the three blade option used to get more blade area in the wind so to speak for a given diameter? Maybe more blade area (like a biplane) and then reduce diameter a bit for float plane ops presuming the compromise still performs better than a two blade.

Dave
ELite583.cjb.net

Ted Hauri wrote:

My daddy allus taught me that the most efficient propeller was single blade. That's because it's always biting into clean air, whereas multiple blades disturb the air and degrade efficiency. There is currently an Italian motorglider, the Silent IN, that flies with a counterbalanced single-bladed prop.

My glider club recently changed the two-bladed prop on our L-19 to a four-blader in order to reduce noise. Everyone complains about the reduction in performance. I talked to the Air Cadets in Nova Scotia (the government is converting all their towplanes to four-bladed props) and they had the same complaint.

Ted Hauri 577E

-------------- Original message from "Angus McKenzie" <angus@lweb.net>: --------------

Some early Taylorcraft used a single blade prop on floats and skis. Several
pictures of these planes are in a soft cover book about the Taylor
story........... Angus


----- Original Message -----
From:
To:
Sent: Thursday, January 20, 2005 12:20 PM
Subject: RE: 2 vs 3 blade props

Barry, i don't think so, at least not theoritically. I'm no

aerodynamicist,

but thrust goes up with diameter, and diameter goes up with fewer blades.
Propellors, like wings, benefit from high aspect ratio. Ever see a short
winged or biplane glider? :) The propellor complication is that the engine
power and desired speed of the plane come into play too. If just the
diameter is increased, you soon run out of ouumph to make it go round and
round (this is the aerodynamicist in me talkin) unless you flatten the

pitch

or reinvent physics some other way to reduce drag. Okay, assuming we

settle

for flattening pitch on that choice, soon we'll get *lots* of thrust but

our

flattened pitch would limit max forward velocity to, oh, say 30 mph before

we

red-line the engine. Or, if we don't care about the engine, we go way

past

red line, but with that big diameter prop, the tips have by now exceeded
Mach .9 and we're really starting to make as much noise as thrust. BTW,

as

for the choice of reinventing physics somehow to reduce drag -- maybe you
could say the experiments with a *single* blade (other side

counterbalanced

by a weighted stub) were along that line of thinking. I think it was

Steve

Whitman that played with that idea.

Other than all that, I'm clueless wrt what's good for an M14/Moose.
-Ben

--
Internal Virus Database is out-of-date.
Checked by AVG Anti-Virus.
Version: 7.0.300 / Virus Database: 265.6.10 - Release Date: 1/10/2005




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--
David A. Ricker
Fall River, Nova Scotia
Canada






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[Ken]

At climb speeds and above, even 4 blades will be biting into clean air.
Blade to blade interference effects generally don't start to get
noticeable until more than 4 blades are used.

As has been mentioned, a larger diameter means more mass of air is
accelerated less which means more thrust per hp at low speeds. Rebels
are low speed Recall Force = Mass x Acceleration. I believe it is
fair to say that the energy that goes into accelerating the air is an
exponential function of its speed and that all that energy is wasted as
that air slows down and dissipates that energy in turbulence/heat. So
ideally we want to keep the mass flow high and acceleration low for high
efficiency (high thrust per hp) and a large diameter prop does exactly that.

Mass flow through the prop goes up linearly with speed so a high speed
(not a Rebel ;) aircraft can still have reasonable efficiency with a
smaller diameter prop. Higher forward airspeed also increases the
helical tip speed of course and at some point it still may make sense to
consider a gearbox to slow the prop and increase the prop diameter.

More than two blades becomes necessary to absorb more power when you
don't have enough ground clearance to increase diameter or when the tip
speeds are getting high enough to significantly reduce efficiency.

Another complication is the advent of thin composite blades such as the
Warp blades that generally can not handle more than 50 or 60 hp per
blade and that may have a maximum diameter. Obviously that type of prop
will need at least 3 blades to handle a 150 hp engine. Fortunately
though the thin blades (especially near the tip) tend to be more
efficient than older thicker blades so the performance may not suffer
from using an extra blade. I think it is also safe to say that these
newer props are quieter for a given tip speed. The Ellipse prop with
very thin and narrow tips is an interesting development.

A counterbalanced single blade prop is sure not kind to everything it is
connected to though as the thrust shifts from side to side everytime it
goes around ;( I'd call that taking the theory a bit too far for
most applications...

Ken

David Ricker wrote:

Just like Ted said, the Nova Scotian L-19s have switched to the to the 4 blade with less performance. They also put a muffler on them for noise reduction.

Isn't the three blade option used to get more blade area in the wind so to speak for a given diameter? Maybe more blade area (like a biplane) and then reduce diameter a bit for float plane ops presuming the compromise still performs better than a two blade.

Dave
ELite583.cjb.net

Ted Hauri wrote:

My daddy allus taught me that the most efficient propeller was single blade. That's because it's always biting into clean air, whereas multiple blades disturb the air and degrade efficiency. There is currently an Italian motorglider, the Silent IN, that flies with a counterbalanced single-bladed prop.

My glider club recently changed the two-bladed prop on our L-19 to a four-blader in order to reduce noise. Everyone complains about the reduction in performance. I talked to the Air Cadets in Nova Scotia (the government is converting all their towplanes to four-bladed props) and they had the same complaint.

Ted Hauri 577E

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